Polymer–Fullerene Bulk-Heterojunction Solar Cellsfotonica/teaching... · 2011. 5. 19. · solar...

www.advmat.dewww.MaterialsViews.com

REV
Polymer–Fullerene Bulk-Heterojunction Solar Cells
IEW
By Christoph J. Brabec , * Srinivas Gowrisanker , * Jonathan J. M. Halls, * Darin Laird , Shijun Jia , and Shawn P. Williams
Solution-processed bulk heterojunction organic photovoltaic (OPV) devices have gained serious attention during the last few years and are established as one of the leading next generation photovoltaic technologies for low cost power production. This article reviews the OPV development highlights of the last two decades, and summarizes the key milestones that have brought the technology to today’s effi ciency performance of over 7%. An outlook is presented on what will be required to drive this young photovoltaic tech-nology towards the next major milestone, a 10% power conversion effi ciency, considered by many to represent the effi ciency at which OPV can be adopted in wide-spread applications. With fi rst products already entering the market, suffi cient lifetime for the intended application becomes more and more critical, and the status of OPV stability as well as the current understanding of degradation mechanisms will be reviewed in the second part of this article.

1. Introduction

Organic photovoltaics (OPV) have become a highly popular research topic during the last 10 years. Between 2000 and 2007, the number of publications in the fi eld of organic solar cells grew very rapidly, with more than 10% of all photovoltaic publi-cations dealing with OPV. [ 1 ] OPV is not a precise defi nition, but typically summarizes the third generation of PV technologies which contain at least one organic semiconductor or molecule in the active light absorbing layer. [ 2 ] The most popular tech-nologies are: (i) the wet electrochemical as well as solid state dye-sensitized solar cells (DSSC) pioneered by Graetzel, [ 3–5 ] (ii) the hybrid solar cells, either consisting of inorganic nanoparti-cles dispersed into a semiconducting polymer matrix [ 6 – 9 ] or by

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheiAdv. Mater. 2010, 22, 3839–3856

[∗] Prof. C. J. Brabec Institute for Materials in Electronics and Energy Technology Friedrich-Alexander-University Erlangen-Nuremberg D-91054 Erlangen (Germany) E-mail: [email protected] Prof. C. J. Brabec Bavarian Center for Applied Energy Research (ZAE Bayern) D-91058 Erlangen (Germany) Dr. S. Gowrisanker , Dr. D. Laird , Dr. S. Jia , Dr. S. P. Williams Plextronics, Inc. 180 William Pitt Way. Pittsburgh, PA 15238 (USA)E-mail: [email protected] Dr. J. J. M. Halls Solar Press Ltd 2 Royal College Street, London, NW1 0NH (UK)E-mail: [email protected]

DOI: 10.1002/adma.200903697

inorganic nanostructured semiconductor templates such as ZnO or TiO2 fi lled with organic semiconductors [ 10–15 ] and (iii) and all-organic solid-state approaches. The all-organic technologies themselves again are distinguished into several areas: the small molecule, gas phase deposited solar cells on the one hand [ 16–19 ] and the solution processed organic solar cells on the other hand. The term solution processed organic solar cells includes the (i) all polymer cells, [ 20 , 21 ] (ii) the small molecular organic solar cells, [ 22 , 23 ] and fi nally, as the largest and most important division, polymer/fullerene based solar cells.

During the last two years, progress in polymer/fullerene based solar cells was so rapid that new effi ciency records frequently

news articles or web pages before full pub-
were published vialications could follow, as seen with the recent 7.6% effi ciency world record announced by Solarmer. [ 24 ] These performance developments have been highlighted on multiple occasions in reviews, [ 25–43 ] special book chapters in edited books on organic electronics, [ 44 , 45 ] special journal issues [ 46–51 ] and books. [ 52–56 ] In contrast, this review of the fi eld of organic solar cells has a slightly different motivation than these numerous and excellent reviews. In this article, we revisit the fi eld of organic solar cells from an industrial perspective, focusing on the key milestones that have marked the path from the original conception to prac-tical solutions, enabling today’s fi rst commercial products.
In the fi rst part we will review the progress in OPV perform-ance during the last 15 years. Progress has been continuous, resulting from successive developments in material design and synthesis, and control of the morphology of the bulk hetero-junction composite. With the fi rst publication of offi cial certifi -cates for OPV cells (full certifi cates as well as the notable excep-tions listed in [ 57 ] ) from the NREL, Fraunhofer ISE, and AIST, OPV have become accepted by the PV community as a serious contender among third generation PV technologies, having the greatest potential for low costs among other next generation PV technologies.

In the second part of this article, we shall discuss the stability of OPV. Until recently, the search for higher effi ciency OPV materials and device architectures has been of greatest interest in the OPV community, with stability of OPV devices being largely ignored. However, this focus is now changing, largely driven by the recent critical need to understand and optimize the stability and reliability of OPV based products. With the fi rst accelerated as well as real-world outdoor lifetime data for

m 3839

3840


REV

IEW
OPV devices and modules becoming available over the last few
years, OPV lifetime studies have become of increasing interest to the research community.

The majority of OPV research activities carried out over the past 17 years have focused on the donor/acceptor bulk het-erojunction approach, using a conjugated, semiconducting polymer as the donor, and a fullerene as the acceptor. We have therefore chosen to focus this review on this material platform, which by necessity has required us to omit important devel-opments in other “species” of OPV, such as the DSSC and vacuum deposited organic solar cells. We acknowledge that sig-nifi cant advances have been made in all these areas which have positively impacted progress in polymer/fullerene OPV—as have more fundamental PV studies of more generic device structures, that have probed charge generation, charge trans-port, numerical simulation of j – V curves, modeling to correlate material properties with device performance as well as fi nancial forecasts and life cycle analysis studies.

2. Effi ciency Progress in OPV Over the Last 10 Years

Signifi cant research efforts were devoted to the development of photovoltaics during the late 1970s and early 1980s. During this time the fi rst activities were also established in the fi eld of organic semiconductors, a new class of materials that offered the promise of low cost and facile processing combined with the electronic properties normally associated with inorganic semiconductor materials. The recognition that organic semi-conductors could potentially offer a low cost approach to large area solar cells led to the growing fi eld of OPV, stimulated also by the interest in exploiting the materials used by nature in pho-tosynthesis, and the growing use of organic photoconductors in xerographic reproduction. At that time, good chemical stability combined with strong optical absorption was regarded as the main challenge for organic semiconductors. For this reason some of the fi rst studied compounds included merocyanine [ 58 ] and phthalocyanine dyes, [ 59 ] which can be readily deposited as a thin fi lm on various substrates (including fl exible materials) either by vacuum evaporation, or by solution casting of com-posites consisting of grinded crystallites dispersed in polymeric binders. Power conversion effi ciencies of such devices were about 1% at best at that time.

It was recognized that to truly achieve the aspiration of large area, low cost, conformable, fl exible OPV modules, directly solution processable organic semiconductors are required. Among the various material systems that held the promise for such a revolutionary technology, solar cells based on polymeric semiconductors were generally regarded as the most promising and powerful technology. The fl exibility of chemical tailoring of desired properties, favourable solution rheology as well as the cheap production technologies closely met the demands for cheap photovoltaic device production.

Not unusual for revolutionary or disruptive ideas, the effi -ciencies of the fi rst polymeric solar cells reported in the 1980s, based on conducting polymers (mainly polyacetylene), were discouraging. [ 60 ] The situation began to improve with the emergence of the fi rst generation of highly soluble polymer

© 2010 WILEY-VCH Verlag G

semiconductors (including polythiophenes (PT), and polyphe-nylenevinylenes (PPV) and their derivatives), though the quality, reproducibility and purity of these polymers at that time was generally too low to allow good device performance to be achieved. However, the main reason for the low effi ciency of these early OPV devices became evident when a greater degree of understanding of the nature of photoexcited states in organic semiconductors was attained. Detailed studies of the photo-physics of these materials indicated that photoexcitation did not lead to the formation of free charges, but rather to the genera-tion of bound neutral excitonic states, with substantial binding energies of around 0.1 eV to 0.5 eV. Only a small fraction of these bound states separate in a simple OPV device, leading to charge generation effi ciency in pristine organic semiconduc-tors in the order 0.1% or lower. The separation of these excitons was found to be driven by the presence of defects or impuri-ties. [ 61 ] Identifying methods of encouraging the separation of these bound states became a major focus of OPV research. Ini-tial studies built on observations that coating the active layer with a stronger electron acceptor increased the probability of charge separation. In this scenario, charge transfer of the elec-tron from the active absorber (donor) to the deeper LUMO level of the acceptor provides the energy required to overcome the exciton binding energy. Typical electron acceptors used for this heterojunction approach included the fullerene C60, deposited by vacuum sublimation onto the polymeric donor. [ 62 ]

Building on this work, a major breakthrough was the obser-vation that mixing the donor and acceptor materials together to form distributed charge generating interfaces led to a higher charge generation effi ciency. This arose from the fact that the diffusion length of excitons in organic semiconductors is typi-cally of the order of 10 nm, around a tenth of the thickness of the active layer required to absorb a high proportion of the inci-dent light. As a result, the majority of the photogenerated exci-tons in a planar heterojunction system decay before reaching the active interface. [ 62 ] This design concept of composite active layers with high charge photogeneration effi ciency and ambi-polar transport properties was classed the bulk heterojunction composite (BHJ) architecture. [ 63 ] The fi rst BHJ architecture devices showed solar energy conversion effi ciencies of around 1%. [ 64 , 65 ] These early reports from 1995 triggered signifi cant interest but also built confi dence that a printing-based produc-tion of effi cient organic solar cells would become feasible.

Although small molecule OPV devices have had an approxi-mately 10 year head start compared with polymer OPV, the gap in performance has closed very rapidly, and polymer solar cells are now more advanced in commercialization than small molecule based systems. Nevertheless, the most recent reports on small molecule solar cells have presented very encouraging performance and lifetime data, [ 66 ] and it can be anticipated that these two organic technologies will continue the race for the next generation PV technology over a longer period.

Although solution processed BHJ photovoltaic cells were fi rst reported in 1995, [ 64 ] it took another 5 years before the sci-entifi c and industrial community realized the potential of this technology, and after 2000 the number of publications in the fi eld started to rise rapidly. Stimulated by this success, fi nan-cial investment into OPV from a variety of sources followed, including from venture entrepreneurs, institutional investors,

mbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 3839–3856


REV

IEW

public grants as well as research spending from established photovoltaics and chemistry companies. While the best effi -ciency reported eight years ago barely reached values higher than 1%, certifi ed effi ciencies beyond 7% are state of the art today. [ 24,57 , 67 – 72 ]

2.1. The Invention of the Bulk Heterojunction Solar Cell

Energy conversion effi ciencies of photovoltaic cells made from pristine conjugated polymer were typically of the order of 0.001 – 0.1%, heavily limited by the excitonic nature of conjugated semiconductors [ 60 , 61 ] and orders of magnitude too low for real-istic commercial applications. The discovery of photoinduced electron transfer in composites of conjugated polymers and fullerenes provided a molecular approach to achieving higher performances from solution processed systems. [ 62 ] Such a bulk heterojunction composite not only has internal quantum effi -ciencies close to unity for photon to electron conversion, but also offers ambipolar transport in the bulk. This is because the donor and acceptor materials may be tuned for effective trans-port of holes and electrons respectively. In the fi rst report of solution processed BHJ solar cells, Yu et al. [ 64 ] used a highly soluble fullerene derivative, phenyl-C61-butyric acid methyl ester (PCBM), [ 73 ] to achieve suffi cient percolation of charges through both components. Remarkably, even after 15 years of acceptor material research, PCBM is still one of the best per-forming acceptors, and certainly the most popular electron acceptor for OPV devices. The original device architecture and operating scheme for the BHJ device proposed by Yu et al. in 1995 [ 64 ] is shown in Figure 1a , while the dark and illumi-nated current-densitry–voltage j – V characteristics for a pristine

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 3839–3856

Figure 1 . a) Schematic drawing of the photoinduced charge transfer proPCBM composites together with a scheme for the heterojunction compositethe photovoltaic cell used by Yu et al. is shown at the bottom of the fi gure. bols) and illuminated (full symbols) j – V characteristics of indium tin oxideCa (A) and of ITO/MEH-PPV/PCBM/Ca (B). Reproduction with permission.American Association for the Advancement of Science.

poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as well as for a 1:4 MEH-PPV/PCBM device is shown in Figure 1 b.

Yu et al. conducted an extensive device study to optimize the BHJ architecture. They investigated the impact of (i) dif-ferent fullerene loadings (from 1:4 to 3:1), (ii) different solvents (xylene vs 1,2-dichlorobenzene) and (iii) different top electrode materials (Al vs Ca) and identifi ed that the morphology of the D–A network can be modifi ed by ink formulation and variation of the process parameters. Their systematic approach pioneered the industrial organic PV material investigation strategy, which remains largely unchanged today.

There are two further aspects of this paper that are note-worthy: First, the authors discussed fi rst indications that the open circuit voltage of BHJ composites did not follow the simple metal–insulator–metal (MIM) concept well established from OLEDs. The origin of the Voc in BHJ cells was only fully understood signifi cantly later. [ 74 ] Second, their solar cells had very low dark currents under reverse bias, of the order 100 pA cm − 2 at − 2 V. Such low dark currents are unusual even today, despite the use of highly selective electron/hole blocking interfacial layers (e.g. PEDOT).

Yu et al. reported monochromatic external quantum effi cien-cies of 30% at light intensities of app. 20 mW cm − 2 . Monochro-matic power conversion effi ciencies were 2–3%, while AM1.5 effi ciencies were probably below 1% (not measured in the orig-inal paper). Nevertheless, this performance marked a signifi cant increase over (i) single polymer cells, as well as over (ii) alternative BHJ concepts based on polymer/polymer blends, which became popular in the same year, [ 65 , 75 ] and (iii) matched the performance of the small molecular evaporated donor – acceptor solar cells invented by Tang in 1979 [ 76 ] and later on published in 1986. [ 17 ]

bH & Co. KGaA, Weinh

cess in MEH-PPV/s. The structure of

b) dark (open sym- (ITO)/MEH-PPV/ [ 64 ] Copyright 1995,

2.2. Understanding Morphology: Overcoming the 1% Hurdle

It was more than 5 years later before Shaheen et al. [ 77 ] took the next signifi cant step in the OPV performance roadmap. During that time, materials had signifi cantly improved in purity, molecular weight control as well as in reproducibility. Nevertheless, PPV based materials were still the dominant class of polymers for OPV. The majority of academic and industrial materials research in organic electronics was directed towards OLED appli-cations, and most OPV materials were taken from OLED development activities. However, progress was made on the design of the side chains that control the physical properties of polymer semiconductors, which had a two-fold impact. On the one hand, chiral side chains prevented polymer–polymer aggrega-tion, and, on the other hand, these side chains improved the solubility of the polymers, allowing molecular weight to be increased: (i) had a negative impact while (ii) had a posi-tive impact on OPV device performance.

3841eim

3842


REV

IEW

Figure 2 . a) j – V characteristics for devices with an active layer that is spin coated from a toluene solution (dashed line – J sc ∼ 2.33 mA/cm 2 , V oc ∼ 0.82 V, FF ∼ 50.5, η AM1.5 ∼ 0.9%) and from a chlorobenzene solution (full line: J sc ∼ 5.25 mA cm − 2 , V oc ∼ 0.82 V, FF ∼ 0.61, η AM1.5 ∼ 2.5%). J sc = short-circuit current density, V oc = open-circuit voltage, FF = fi ll factor, η AM1.5 = power effi ciency under 1 simulated sun with air mass 1.5. The overall device structure was glass/ITO/PEDOT/MDMO-PPV/PCBM (1:4)/LiF/Al. The active area of the devices was typically 10 mm 2 . Data are for devices illuminated with an intensity of 80 mW cm − 2 with an AM1.5 spec-tral mismatch factor of 0.753. The temperature of the samples during measurement was app 50 ° C. b,c) AFM images of the surface topography of toluene cast (b) and 1,2 dichlorobenzene cast (c) MDMO-PPV/PCBM fi lms. Reproduction with permission. [ 77 ] Copyright 2001, American

s.

Shaheen et al. showed that the power conversion effi ciency of an OPV cell based on a conjugated polymer (MDMO-PPV)/methanofullerene (PCBM) blend can be dramatically affected by modifi cation of the molecular morphology. The use of alternative solvents (chlorobenzene instead of toluene) allowed the composite to become self-organ-ized in a more intimate mixture with smaller phase segregated methanofullerene domains [ Figure 2 ]. Simultaneously, the degree of interaction between the conjugated polymer chains increased. Overall, this resulted in a power conversion effi ciency of 2.5% under AM1.5 illumination representing nearly a threefold enhancement over previously reported values for BHJ devices.

The authors speculated that the choice of the solvent may also positively impact the hole mobility of the polymer, similar to what had been found in OLEDs, in which the

Figure 3 . Electron μ e and hole μ h zero-fi eld mobility in blends of MDMO-PPV/PCBM as a function of PCBM weight percentage, at room temper-ature (295 K). The mobility values were calculated from SCL currents. Reproduced with permission. [ 83 ]

choice of solvent controlled the degree of interchain interac-tions. However, this picture was later revealed to be more com-plex. Transport investigations of pristine MDMO-PPV revealed that the polymer mobility was approximately ten times too low to give such high fi ll factors and photocurrents. [ 78 ] Intui-tively, one would expect that “dilution” of the PPV with PCBM would lead to a further reduction of the hole transport proper-ties, due to a reduced percolation pathway. But Pacios et al. demonstrated for a different polymer that addition of PCBM to a conjugated polymer can actually result in an increase in mobility. [ 79 , 80 ] The origin of the increase was an enhanced inter-molecular interaction between the polymer chains driven by addition of a “phase incompatible” fullerene molecule, leading to an increased degree of aggregation of otherwise well separated polymer chains. [ 78 ] Based on these fi ndings, an enhancement in hole transport in MDMO-PPV was also demonstrated and is shown in Figure 3 . A gradual increase of hole mobility with increasing fullerene concentration is observed from 33 to ∼ 70 wt%. For fullerene concentration larger than 67 wt% the hole mobility sat-urates. It has been observed that phase separation, resulting in pure PCBM domains surrounded by a homogeneous matrix of 50:50 wt% MDMO-PPV/PCBM, is established for concentrations of more than 67 wt% PCBM. [ 81 ] As a result, the hole mobility in this homogeneous matrix of 50:50 PPV/PCBM is indeed expected to saturate, consistent with the experimental observations.

Several important improvements followed this record OPV performance cell report, which have been detailed in numerous excellent review articles. [ 82 , 83 ] In addition to developing a better understanding of morphology and transport, a number of spe-cifi c material developments supported further performance progress. A new synthetic route for MDMO-PPV delivered much more well defi ned polymers with ultrahigh molecular weight and an extremely low defect density. This was prob-ably one of the fi rst times when the OPV community realized the importance of controlling the defect levels in the organic semiconductors. [ 84 ] Based on these low defect density polymers, the subsequent optimization of electrodes [ 85 ] as well as side chains [ 83 ] yielded effi ciencies of about 3%. [ 84 ] Finally, alternative

Institute of Physic


fullerene acceptor molecules were assessed by Wienk et al., [ 86 ] who replaced [ 60 ] PCBM with [ 70 ] PCBM, because of its signifi -cantly higher absorption in the visible down to 700 nm. Since the performance of MDMO-PPV-based devices was limited by its wide bandgap and small photocurrent, addition of [ 70 ] PCBM could further improve performance by expanding the spectral sensitivity into the near IR, and effi ciencies of over 3% were demonstrated. [ 86 ]

2.3. Controlling Morphology of P3HT/PCBM: Towards 5%

After the rapid progress with PPV based materials, interest in this material platform began to fade. The main reason was the relatively large bandgap of the PPV-type polymers in combina-tion with their low charge transport mobility, which limited solar power conversion effi ciencies to 3% at best. This limita-tion was a very strong motivation to identify next generation



REV

IEW

Figure 4 . j–V curves of P3HT/PCBM solar cells under illumination with white light of 800 W m − 2 : as produced solar cells (fi lled squares), annealed solar cells (open circles) and cells simultaneously treated by annealing and applying an external electric fi eld (open triangles) Reproduction with permission. [ 90 ]

polymers. In response, research efforts quickly focused on poly-alkyl-thiophenes, and especially on the hexyl substituted analogue, P3HT. Poly-alkyl-thiophenes were one of the fi rst conjugated systems studied for polymer electronics [ 87 ] as well as for organic solar cells, [ 88 ] but initial results with this mate-rial were relatively poor. In 2002 the fi rst encouraging results for P3HT/PCBM solar cells were published. [ 89 ] The short circuit current density was the largest ever reported in an organic solar cell (8.7 mA cm − 2 ) at that time and resulted from a high EQE, which had a maximum value of 76% at 550 nm. This paper was quickly followed by the fi rst explicit reports that thermal annealing of P3HT/PCBM composites signifi cantly increased the photovoltaic performance, and Padinger et al. reported effi -ciencies up to 3.5%. [ 90 ] Researchers worldwide shifted focus to this material system, making P3HT and PCBM the new work-horse of OPV development. This transition was supported by the excellent supply situation for P3HT by various companies (including Rieke Metals, Merck Chemicals, and Honeywell), since the structure of poly-alkyl-thiophenes was not protected


Table 1. Non-certifi ed performance data of P3HT-PCBM solar cells as repor

Year Ratio to PCBM (weight) Layer thickness [nm] Solvent Max EQE [%]

2002 1:3 350 – 76

2003 – ≈ 110 DCB 70

2005 1:1 – CB 58

2005 1:1 63 DCB –

2005 1:1 ≈ 220 DCB 63

2005 1:0.8 – CB –

2005 1:0.8 – CB –

2006 1:1 175 CB 70

2006 1:0.8 – CB 88

2006 1:1 320 DCB 82

2008 1:1 ≈ 220 DCB 87

2008 3:2 80 CB + NtB –

by patents. Most of the efforts were spent on understanding and explaining the annealing mechanism and in optimizing the annealing conditions for highest performance.

Figure 4 illustrates the typical enhancement in the j – V charac-teristics upon thermal annealing as reported by Padinger et al. [ 90 ]

The mechanism of annealing was revealed by a series of detailed transport and morphological investigations. Annealing enhanced the charge carrier mobility [ 91 ] in combination with changing the recombination behavior from a Langevin type into a non Langevin type recombination mechanism. [ 92 ] X-ray, AFM and TEM investigations enabled a microscopic picture of the annealing process to be developed, [ 93 , 94 ] which is considered to take place in three subsequent steps: (i) annealing softens the P3HT matrix, which (ii) allows PCBM molecules to diffuse out of disordered P3HT clusters and form larger fullerene aggregates, before (iii) the now fullerene-free P3HT matrix recrystallizes into larger fi brillar type crystals, which are embedded in a matrix con-sidered to consist of PCBM nano-crystals and amorphous P3HT.

The effi ciency of P3HT/PCBM cells was quickly increased to the 5% level. Main contributions came from (i) increasing the regioregularity of P3HT, [ 95 ] (ii) optimizing the annealing temperature in combination with the molecular weight of the polymer, [ 96–101 ] (iii) slowing down the drying kinetics of the wet fi lms, [ 102 ] (iv) using processing additives to support phase sepa-ration between P3HT and PCBM, [ 103–105 ] (v) optimizing inter-face losses [ 106 ] and (vi) growing crystalline P3HT fi bers already in solution. [ 107–109 ]

Table 1 provides a non exhaustive survey of reports describing effi cient photovoltaic cells based on P3HT/PCBM blends. Note that obviously inconsistent data has not been listed in this table.

Although there are several literature reports publishing effi -ciencies for P3HT/PCBM with effi ciencies of over 4% and as high as 5%, there is a signifi cant discrepancy between these values and the best values offi cially certifi ed for P3HT/PCBM. Figure 5 shows one of few publicly disclosed certifi cates for P3HT/PCBM solar cells. The cells were submitted by Plextronics and were certifi ed with an effi ciency of 3.8%. Other companies including Sharp and Konarka had P3HT/PCBM cells certifi ed between 3.5–4%. [ 116 , 117 ] The origin of the discrepancy between

3843bH & Co. KGaA, Weinheim

ted in literature.

V oc [V] FF J sc [mA cm − 2 ] Eff [%] Light intensity [mW cm − 2 ) Ref.

0.58 0.55 8.7 2.8 100 [ 89 ]

0.55 0.6 8.5 3.5 80 [ 90 ]

0.61 0.53 9.4 3.0 100 [ 110 ]

0.61 0.62 10.6 4.0 100 [ 111 ]

0.61 0.67 10.6 4.4 100 [ 102 ]

0.65 0.54 11.1 4.9 80 [ 112 ]

0.63 0.68 9.5 5.0 80 [ 96 ]

0.6 0.52 12 4.4 85 [ 95 ]

0.61 0.66 11.1 5.0 90 [ 113 ]

0.56 0.48 11.2 3.0 100 [ 114 ]

0.64 0.69 11.3 5.2 100 [ 115 ]

0.66 10.5 4.3 100 [ 105 ]

3844


REV

IEW

Figure 5 . NREL certifi cate for a small area P3HT/PCBM solar cell. Figure 6 . NREL certifi cate of a polymer/fullerene solar cell with a low HOMO polymer.

certifi ed effi ciencies and values reported in research reports remains unclear.

2.4. Beyond 5%—Novel Donor Materials

The progress with the P3HT/PCBM material platform stimu-lated the whole research community and widened the research activities. The limitations of P3HT based solar cells were quickly clarifi ed. [ 118 ] P3HT is an excellent absorber and trans-porting material. As such, an active layer of only 100–200 nm absorbs most of the light and increasing the thickness beyond 250 nm has little impact on improving the effi ciency. Due to its excellent transport properties and the strongly suppressed bulk recombination P3HT based solar cells exhibit fi ll factors of ∼ 70%. [ 115 ] Nevertheless, the highest Voc reported for well per-forming P3HT/PCBM solar cells was around 0.66 V. [ 105 ] Higher Voc for P3HT based solar cells were reported for bis-substituted fullerenes but will be discussed later. Compared with an active layer bandgap of 1.8 eV, a V oc of only 0.6 V represents a 66% loss compared with the thermodynamic limit. The solution to reduce the energetic Voc losses was found by switching to donor polymers with deeper highest occupied molecular orbital (HOMO) levels. This led to the fi rst certifi ed OPV devices with greater than 5% effi ciency ( Figure 6 ). The polymer used in this device remains proprietary to Konarka, but from the Voc it is clear that the HOMO of this polymer must have been around 250–300 meV deeper than that of P3HT.

Rapid progress has been made with deep HOMO donor poly-mers, and polyfl uorenes (PF) became established as the fi rst deep HOMO, high V oc class. The pioneering work of Andersson and Inganäs and co-workers [ 119 ] paved the way for other groups to optimize polyfl uorenes and demonstrate high device perform-ance. Non certifi ed effi ciencies from 4 to almost 6% were reported for PF based polymers. [ 120–122 ] A few prominent PF structures yielding good effi ciencies are summarized in Figure 7 .

© 2010 WILEY-VCH Verlag Gm

Various side chain modifi cations were assessed in an attempt to fi ne tune the morphology. A promising approach was to replace the C-bridging atom in the fl uorene unit by a Si atom. Si-bridged PF was found to systematically increase the otherwise rather low hole mobility of the polyfl uorene class. [ 121 ] Another promising and related donor material class are the poly(2,7)carbazoles. These materials were pioneered by Leclerc and have been successfully developed for OPV appli-cations. [ 123 , 124 ] Although signifi cantly smaller efforts has been applied to the polycarbazoles compared with the polyfl uorenes, a new certifi ed effi ciency record of 6% was quickly set with the prototype polycarbazole, PCDTBT (see Figure 8 ). [ 125 ]

It is worthwhile to note that the polyfl uorene pendant with the TBT acceptor group (PF8TBT, PF10TBT) gives lower per-formance compared to the polycarbazole analogues. Both, the photocurrent and particularly the FF were found to be larger for polycarbazole based material combinations. Encouraged by such good performance, substantial synthetic efforts were focused on decreasing the bandgap of the polycarbazoles while keeping the HOMO at this favorable low position. The versa-tility of the donor-acceptor (push-pull) chemistry allowed effi -cient modifi cation of the bandgap, HOMO and lowest unoccu-pied molecular orbital (LUMO) levels of the polycarbazole based polymers. Leclerc summarized the material roadmap required to achieve a 10% effi ciency in the 2D contour plot developed by Scharber et al. ( Figure 9 ) to correlate the donor material’s prop-erties with the maximum possible achievable effi ciency. [ 124 , 126 ]

Nearly parallel in time Plextronics published an NREL certi-fi ed OPV device performance with another high Voc material combination. The composition and structure of the photoactive material system (Plexcore PV 2000 ink) remains proprietary to Plextronics, [ 127 ] (Figure 7 , right).

Reviewing the certifi cates and effi ciency records for the large Voc materials, the current densities of these material systems are in the best case between 10–11 mA cm − 2 . This is a low photocurrent compared with other thin fi lm solar technologies

bH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 3839–3856


REV

IEW

Figure 7 . Chemical structure of a few prominent polymers which gave effi ciencies in the order of or above 5%.

nS

NS

N

S*

N

H17C8C8H17

*S

S

X

*

*

N NS

n

X=C or Si

such as a-Si or dye sensitized solar cells (DSSC), which typically exhibit current densities from 15–25 mA cm − 2 . Despite the respectable V oc of 0.8–0.9 V and acceptable FF of 65 to 70%, the overall performance was limited by the rather small photocur-rent due to the wide bandgap, which restricts the portion of the solar spectrum that can be absorbed.

To address this, more attention was paid to lower bandgap systems. The goal with such systems is to achieve a bandgap signifi cantly lower than 1.8 eV in combination with a low lying HOMO, to achieve both a high photocurrent and high Voc. However, the fi rst low bandgap polymers certifi ed again


Figure 8 . NREL certifi cate of a PCDTBT: [ 70 ] PCBM OPV solar cell [ 125 ] (left) PV2000 high- V oc material system (right). [ 127 ]

showed a low V oc , comparable to typical P3HT/PCBM OPV cells.

The fi rst low-bandgap polymers that gave good OPV perform-ance were the class of cyclopentadithiophene-based polymers which combined a low bandgap (1.45 eV) with good absorp-tion properties [ 128–130 ] and excellent transport capabilities with hole mobilities of more than 0.1 cm 2 Vs − 1 . [ 131 ] The prototype structure of the bridged bithiophenes is PCPDTBT [poly[2,6-(4,4-bis-(2-ethylhexyl)-4 H -cyclopenta[2,1-b;3,4-b ′ ]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)], whereas the thiophenes can be bridged by a carbon or silicon atom. (see Figure 7 ).


and a Plextronics

The performance of carbon bridged PCP-DTBT (C-PCPDTBT)/PCBM OPV solar cells was found to be limited by an unfavourable nano-morphology. A solution was found by either using processing additives such as alkanedithioles [ 69 ] or by using Si-PCPDTBT instead of the carbon bridged C-PCPDTBT. [ 132 ] In both cases, effi ciencies beyond 5% were reported, and, in the case of Si-PCPDTBT, cer-tifi ed by NREL (Figure 9 ). The certifi ed device delivered a short circuit current of approaching 15 mA cm − 2 , equivalent to a 50% increase compared with the wider bandgap systems certifi ed so far. This high photocurrent was gained from the high EQE of over 60% in the IR. The FF of 61% and the V oc of 0.58 V were at best “average” performance data, and, as such, limited the overall effi ciency for that material combination to the 5% regime.

Further progress with low bandgap polymer systems quickly followed. Konarka pub-lished a certifi ed effi ciency of 6.4% for a low

3845eim

3846


REV

IEW

Figure 9 . Theoretical material performance for various polycarbazole derivatives. Reproduction with permission. [ 124 ] Copyright 2008, American Chemical Society.

bandgap polymer which had an even higher photocurrent. Close to 17 mA cm − 2 was demonstrated, and only the low Voc of ∼ 0.6 V prevented higher effi ciencies from being achieved.

Looking back at the progress in material development, there appears to be a trend in developing two material classes at the extremes of the spectrum: either with a large Voc and a lower photocurrent (such as with the PPVs, PFs and PCz), or with a low V oc and a large photocurrent (as exhibited by P3HT, C and Si- PCPDTBT).

The fi rst fully functional material class that delivered a low HOMO in combination with a small bandgap was the polymer PTB, developed by Yu et al. [ 72 ] Yu demonstrated that side chains with an electron donating respectively electron withdrawing effect allowed the HOMO and LUMO levels of

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

Figure 10 . NREL certifi cate of (a) Si-PCPDTBT: [ 70 ] PCBM solar cell from Konarka, b) low-bandgacell from Solarmer [LBGII-2/PCBM].

the polymer to be fi ne-tuned without nega-tively impacting its electronic properties. Effi ciencies of close to 6% were reported by Yu, [ 133 ] for a polymer/fullerene com-bination that showed ∼ 13 mA cm − 2 short circuit current, a V oc of 0.74 V and a FF of 61%. Shortly later, Yu et al. reported further device improvements with this polymer class [ 72 ] and Solarmer published certifi ed OPV device data on a newer polymer, syn-thesized in house, with 6.8% effi ciency (Figure 9 ). Their low bandgap polymer LBGII-2/PCBM OPV device gave a photo-current of 13.3 mA cm − 2 , a V oc of 0.76 V and a FF of 66%. [ 134 ] In between, Solarmer announced that the certifi ed performance of their devices was further raised to 7.6%. [ 24 ]

2.5. Beyond 5%—Novel Acceptor Materials

The major focus of the synthetic efforts for OPV has been, and remains, placed on the development of novel donor materials. Progress with alternative acceptor materials has not been as visible as for donor polymers. At fi rst sight it may appear that little progress was made in PCBM research [ 73 ] since the fi rst reports in solar cells applications in 1995. [ 64 ] However, signifi cant industrial progress has been made with this material. First, the quality, purity and industrial supply of fullerenes was established. 10 years ago, fullerenes were regarded as exotic materials, with no clear understanding of the technical and commercial viability of fullerenes for large-scale manufacture of electronic products. Today, supply

heim Adv. Mater. 2010, 22, 3839–3856

p solar cell from Konarka, and c) low-bandgap solar


REV

IEW

Figure 12 . a) j – V curves of P3HT/Lu3N@C80-PCBH (triangles) PCE = 4.2%, V oc = 810 mV, J sc = 8.64 mA cm − 2 , and FF = 0.61 and P3HT/[60]PCBM (squares and dashed lines) PCE = 3.4%, V oc = 630 mV, J sc = 8.9 mA cm − 2 and FF = 0.61 blend devices. Filled symbols show the dark curves and open symbols show devices under simulated AM1.5@100 mW cm − 2 ). b) chemical structure of Lu3N @C80-PCBH. Reproduced with permis-sion. [ 142 ] Copyright 2009, Nature Publishing Group.

of PCBM on a multi-kg basis is not an issue. Second, costs have been dramatically reduced and PCBM is now compat-ible with the bill of materials for an OPV product. [ 53 ] The dominant process developments in PCBM synthesis were driven by Hummelen et al. [ 135 ] and enabled the industrial scale up of PCBM as an electronic material at reasonable costs. Besides establishing the fullerene and PCBM supply, there was also research in the development of improved acceptor materials. So far, acceptors other than PCBM and related fullerenes, [ 136–138 ] among them conjugated polymers, carbon nanotubes, perylenes and inorganic semiconducting nano-particles, [ 139 ] have not satisfi ed expectations. Switching from C60 to C70 based fullerenes has proven to give a sig-nifi cant effi ciency improvement for various donors due to an enhanced spectral sensitization. [ 86 , 125 , 132 ]

The biggest potential to increasing effi ciency will come from tuning the LUMO levels of the fullerenes relative to the LUMO of the donor to minimize thermalization losses. Modifying the fullerene LUMO has proved to be a major challenge. Until recently, relatively small shifts of some 50 meV were reported for PCBM derivatives with electron donating group attached to the carbon cage. [ 136 ] The major breakthrough came when Hummelen et al. successfully demonstrated that fullerene multiadducts give a 100–200 mV higher lying LUMO compared to pristine C60. [ 140 ] Fullerene bis-adducts brought a signifi cant effi ciency increase even for P3HT ( ∼ 15%) due to an open circuit voltage increase [see Figure 11 ], and similar improvements are expected for the high performance polymers discussed in the previous section. Higher multiadducts, such as tris-PCBM were shown to improve the V oc by more than 200 mV, but trapping of elec-trons in the tris fullerenes has so far prevented an overall effi -ciency increase. [ 141 ]

An alternative, highly innovative path to modify the LUMO level of fullerenes for OPV applications was introduced by Drees et al., [ 142 ] who suggested the use of trimetallic nitride endohedral fullerenes. To achieve the required morphology for-mation, Lu3N@C80 methano derivatives were synthesized, as endohedral analogues to PCBM. State-of-the-art OPV devices based on P3HT/endohedral fullerene composites showed a Voc of 890 mV, the highest ever reported open circuit voltage for a P3HT/fullerene solar cell. The short circuit current as well

© 2010 WILEY-VCH Verlag GmbAdv. Mater. 2010, 22, 3839–3856

Figure 11 . a) Current-density versus voltage curves of P3HT/PCBM and P3Hcells under illumination of a halogen lamp with an intensity equivalent to 1.16 PCBM by bisPCBM increases the V oc from 580 mV to 724 mV while leaving thparameters unchanged. b) chemical structure of mono-PCBM and bis-PCBM.permission. [ 141 ]

as the FF were mainly unaffected by the endohedral fullerene, and effi ciencies of 4–4.5% were demonstrated [ Figure 12 ]. As with the bis-fullerenes, high expectations are now set on OPV devices that combine the endohedral fullerenes with one of the novel high performance polymers discussed in the previous sections. Nevertheless, one has to acknowledge that as long as commercial supply of endohedral fullerenes at reasonable costs is not established, this class of promising acceptors will be only of academic relevance. Scale up of bis-fullerenes, on the other hand, is not expected to become a major issue.

2.6. Beyond 5%—Solar Cell Technology

Following this review of largely materials-driven performance enhancement, we will now briefl y consider potential device architecture based developments that will help to further increase the effi ciency of OPV devices. All of these develop-ments set out to reduce the effi ciency limiting loss mechanisms of OPV cells. One of these technologies, the tandem cell archi-tecture, will be discussed. Other methods, including light management, plasmonic coupling, up-conversion materials

H & Co. KGaA, Weinh

T/bisPCBM solar sun. Substituting e other solar cell

Reproduced with

and down-conversion layers are only just emerging and certainly will become the sub-ject of separate reviews.

As mentioned earlier, the two major losses occurring in solar cells are the sub-band gap transmission and the thermalization of hot charge carriers. [ 143 ] One way to cir-cumvent both effects simultaneously is to construct a tandem solar cell, and effi cien-cies of over 40% have been demonstrated for inorganic solar cells based on this concept. [ 57 ] Figure 13 depicts a typical organic tandem cell comprising two distinct devices stacked on top of each other, each of which is based on a donor/acceptor composite. The light that is not absorbed in the fi rst (bottom) device impinges on the second (top cell) which incorporates lower bandgap materials. The

3847eim

3848


REV

IEW

Figure 13 . Schematic of a two-layer tandem cell comprising two semicon-ductors with different bandgap.

thermalization losses are lowered due to the usage of materials having different band gaps. The two cells involved in the device can be connected either in series (two-terminal) or in parallel (three-terminal) depending on the nature of the intermediate layer and on the way the intermediate layer and the two elec-trodes are connected. In the vast majority of reports a series connection architecture is used.

The fi rst organic tandem solar cells with small evaporated molecules were reported as early as 1990. [ 144 ] It took more than 10 years before research on organic tandem solar cells became more commonplace. Tandem OPV architecture is more obvi-ously compatible with vacuum deposited small molecules than with solution processed polymer semiconductors, although cross linking and/or use of polymers with orthogonal solu-bility can be used to achieve multilayer structures. Overall, the tandem architecture has not matured to the status of single junction cells. On the one hand, a reliable, reproducible and loss free interlayer technology is missing, and, on the other hand, highly performing donor/acceptor materials combinations with suitable bandgaps to cover a wide region of the spectrum are only just becoming available. Important work has been done by the following non-comprehensive list of research groups, among them A. J. Heeger et al. at UCSB, S. Forrest et al. at Princeton, K. Leo and M. Pfeiffer et al. at Univ Dresden and at Heliatek, P. W. Blom et al. at the Univ Groningen, R. A. J. Janssen et al. the Univ Eindhoven, and Y. Yang at UCSB. A detailed report on

Figure 14 . a) Drawing and TEM cross section of the highly effi cient tandem cell as reported in. [ 145 ] b) j – V curves of the single cells as well as of the + 6% tandem cell. Reproduced with permission. [ 145 ] Copyright 2007, American Associatation for the Advancement of Science.

the progress in tandem technologies can be found in numerous reviews. [see Ref. [ 1 ] and references therein]. In this manuscript we will limit the review to just two highlights.

An important breakthrough in solution-processed tandem cells was reported by Kim et al., who demonstrated a high effi ciency tandem device with 6.5% power conver-sion effi ciency. [ 145 ] These tandem devices were entirely solution-processed except for the top evaporated electrode [ Figure 14 ]. For the bottom BHJ cell a 130 nm thick layer of PCPDTBT/PCBM was used and the top cell consisted of a blend of P3HT/PC70BM with a thickness of 170 nm. The absorption bands of PCPDTBT and P3HT complement each


other fairly well, allowing good spectral coverage in the 400 to nearly 900 nm region. The half cells were separated by a trans-parent TiO x layer and a highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) layer. The TiO x layer was deposited by means of sol–gel chemistry [ 113 ] and served as the electron selective layer of the fi rst cell. The nature and the function of the TiO x /PEDOT interface in tandem cells was not fully clarifi ed. The performance of these devices was more than twice as high as any other polymer based tandem cell at that time, though the effi ciencies have not been offi cially cer-tifi ed. It took more than two years before other research groups were able to reproduce these pioneering results. [ 146 ]

The second important development in the tandem tech-nology was reported by Janssen et al., who introduced a ZnO/PEDOT:PSS recombination layer. [ 147 ] This layer was mechani-cally and chemically robust and allowed multijunction cells to be produced with up to 5 half-cells. Figure 15 shows a graph comparing the single junction cells with a triple junction cell.

Although the Tandem cell architecture for organic solar cells has been demonstrated to be feasible, the materials for the inter-connection layer, suitable absorber couples as well as the tandem technology itself are just in their infancy. However, due to the signifi cant opportunity to enhance effi ciency, it is likely that tandem cells technology will become a key technology for OPV.

2.6. Outlook and Challenges: Towards 10%

Figure 16 summarizes the effi ciency roadmap of the last 15 years. Within a few years, organic, polymer based solar cells have emerged from non-certifi ed performances of around 1% to cer-tifi ed performance of over 7%. Figure 15 also indicates when OPV technology might be expected to reach the 10% milestone. Comparing the progress in effi ciency with that achieved by other PV technologies such as a-Si, CdTe or CIGS in their early years, shows that OPV is advancing at a comparable speed. Fur-ther, the community agrees that OPV will not face any funda-mental limitation on the way towards 10%. Only for effi ciencies beyond 10% will some of the fundamental limitations of OPV need to be addressed.

The lower and the upper line of the shaded area in Figure 16 are effi ciency predictions based on the performance data from



REV

IEW

Figure 15 . j – V curves of the single cells compared to the j–V curve of a triple cell. Reproduced with permission. [ 147 ] Copyright 2007, American Institute of Physics.

2000–2005. There, a realistic and an optimistic model (both models were non-linear in nature, able to address constant acceleration) were projected based on the last 5 years progress. The certifi ed data over the last few years fi ts reasonably well to the realistic projection. If the performance progress con-tinues with the same rate as over the last years, 10% certi-fi ed cells are expected to emerge in 2011. Similar models project tandem cell effi ciencies of up to 15% within the next few years. These high effi ciencies will open the way for more widespread application of the technology, including grid con-nected applications, than the current ∼ 5% effi ciencies allow.

3. Testing Methods and Lifetime

3.1. Introduction

As we have discussed in the previous section, OPV device effi -ciency has steadily increased over the last few years, reaching


Figure 16 . OPV Effi ciency progress and roadmap over the last years. The sinto the eye, whereas the shaded area is the forecast from a model based onprogress from 2000–2005. All data after 2005 are data taken from certifi cateciency value for the tandem cell.

a performance level that makes initial applications feasible. However, in order for OPV based devices to be commercialized, stability and lifetime aspects must also be well understood. The organic nature of the active and interfacial layers of OPV, combined with the exposure to sun and with heat at ambient conditions makes OPV lifetime a natural concern. Consider-able work has been done so far in understanding the degra-dation mechanisms, stability and lifetime of OPVs by groups led by F. C. Krebs et al. (Risoe National Laboratory, Denmark), R. de Bettignies et al. (CEA, France), M. Niggemann et al. (Fraunhofer Institut Solare Energiesysteme and Freiburger Materialforschungszentrum, Germany) and H. Neugebauer (University of Linz). The results are well summarized. [ 34 ] The review article discusses the chemical, physical and mechan-ical degradation that are predominant in OPV materials and devices. This section of the article reports the fi ndings of life-time related to the degradation mechanisms predominant in polymer-based solar cells.

OPV cell lifetime is determined by extrinsic and intrinsic stability factors. Intrinsic instabilities arise from the bulk of the active layer, active layer/electrode interfaces and choice of electrode materials. Extrinsic instabilities arise from inad-equate encapsulation and weathering conditions and the sen-sitivity of photovoltaic device components to water and oxygen, which may be further catalyzed in the presence of light and elevated temperature and humidity. Complex chemical reac-tion between the different device layers, in combination with moisture, oxygen, light and heat could lead to a degradation pathway and device failure. It is critical to understand these degradation pathways and reduce their impact to enhance OPV device lifetimes.

The lifetime of a device can be defi ned as the time taken for the effi ciency to drop below a predetermined percentage of its initial value. Lifetime of OPVs is commonly evaluated by indoor and more recently by outdoor testing. Accelerated lifetime testing (ALT) and extrapolation techniques may be used to char-acterize OPV stability. With ALT, the device is driven to failure at enhanced illumination and/or temperature to obtain an


gle line is a guide the performance s, except the effi -

acceleration factor and results are correlated to end-use conditions. With the extrapolation approach, results are extrapolated to longer times based on real test data. It is worthy to note that for most inorganic PV technologies lifetimes reported are after stabilization [ 148 ] of performance metrics, which is part of the standard reporting conditions (SRC) for PV lifetime. OPV technology being in the com-mercialization stage does not have a lifetime testing standard or SRC yet, although efforts are underway the way to establish them. (see International Summit on OPV Stability (ISOS) [ 149 ] ).

The list of failure mechanisms of OPV cells is long, and certainly as extensive as for any other photovoltaic technology. Typical degradation mechanisms and their reaction to the specifi c test criteria chosen to accelerate degradation are summarized in Table 2 .

3849eim

3850


REV

IEW Table 2. Mechanism specifi c developmental testing. Summary of various failure modes

(cause/effect) leading to reduction in effi ciency in OPVs.

Stress Response

Mechanical Delamination, electrode failure, packaging failure

Temperature Acceleration, delamination, morphological changes, diffusion

Light: spectral response, total intensity Photochemical oxidation, photobleaching, yellowing, mechanical

failure

Oxygen: humidity, water Donor/acceptor oxidation, electrode oxidation, charge extraction,

change in mobility, TCO etching, interface failure

Coupled effects: water and mechanical,

light and mechanical

Interconnect failures (in addition to above mentioned failures)

Electrical: electric fi eld, columbic charge Localized heating, shorts

Figure 17 . Temperature dependence of the acceleration factor K (full circles) and prediction of the Arrhenius model according to equation (8) with an activation energy of 350 meV. The reference temperature is 298 K. Reproduced with permission. [ 150 ] Copyright 2007, Springer.

0,0025 0,0030 0,00351

10

400 380 360 340 320 300

Acc

eler

atio

n fa

ctor

K

1/T [1/K]

T [K]

3.2. Review of State-Of-The-Art Lifetime Results

It is well known that the increase in operating cell temperature accelerates the aging mechanism. Brabec and co-workers [ 150 ] have discussed the most commonly used models for electronic devices to determine the degradation constant. The Arrhenius model which is the most commonly used is more appropriate for processes with fi rst order kinetics. The degradation constant is given by

kdeg = A exp

−Ea

kBT (1)

where E a is the activation energy in eV, k B is the Boltzmann con-stant, T is the temperature in Kelvin and A is the pre-exponential parameter that depends on degradation mechanisms and is independent of temperature. Failure mechanisms that follow Arrhenius model are typically material related, and may include charge trapping or structural and chemical changes (reaction products) and in general vary as a function of temperature. In the Arrhenius model, temperature is the only stress causing creation of defects leading to the device failure. In order to account for multiple stresses causing defect generation, the Eyring model is used to determine the degradation constant. The Eyring model is given by

kdeg = AT"exp

−Ea

kBT+ B +

C

TS1

(2)

This model includes multiple stress factors S 1 , S 2 etc and parameters B 1 , C 1 , B 2 , C 2 etc corresponding to S 1 and S 2 respec-tively. A more suitable model that is used when the device oper-ates at constant temperature and when current or voltage is the stimulant is the inverse-power model given by

kdeg = AV( (3)

Here, V is the applied stress, A and γ are product specifi c parameters. Brabec et al. determined the degradation constant of PPV/PCBM OPV cells with Ca/Ag and Al as the cathode elec-trode materials and at elevated temperatures ranging from 40 ° C to 105 ° C. In order to determine the degradation constant a linear model was assumed to describe the variation of J sc as a function of time. All devices were encapsulated with glass and a constant illumination of 35 mW cm − 2 was used. It was

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

found that the degradation kinetics were not infl uenced by the choice of cathode material over the temperature range evaluated and the acceleration constant was evaluated using the Arrhenius model according to the equation

K =

kdeg (T1)

kdeg (T2)= exp

Ea

kB

1

T2−

1

T1 (4)

An acceleration factor ranging from 1 to 10 was obtained for temperature ranging from room temperature to 360 K, under hal-ogen lamp illumination ( ∼ 35 mW cm − 2 ) as shown in Figure 17 . From the slope of the linear fi t an activation energy of 300–350 mV was obtained. It was also reported that there

absorption characteristics, photobleaching or
was no change inshifting of the absorption characteristics observed. The degra-dation in J sc was attributed to possible morphological changes in the blend or a polymer in the blend and electronic defects in the solar cell. Based on MDMO-PPV/PCBM the reduction in J sc reported was less than 20% over 1000 h of operation at 85 ° C.
Bettignes at al. [ 151 ] reported a similar temperature study on P3HT/PCBM OPV devices. Two different electrodes were tested, Ca/Ag and LiF/Al double layers. The initial burn-in period observed was similar irrespective of the cathode mate-rial. The effi ciency of the device with LiF/Al continued to decay to less than 0.5% effi ciency in 100 h, while the device with Ca/Ag decayed to 30% of its initial value in 200 h. It was reported that the cell temperature increased from 25 ° C to 60 ° C in 1 h during the measurement process and based on this an acceler-ation factor of 4.45 at 60 ° C was calculated using the Arrhenius model. T85 of J sc at 25 ° C was 1000 h under 100 mW cm − 2 illu-mination. The choice of cathode material and the active layer material are critical to obtaining long lifetimes of BHJ solar cells. The decrease in performance of MDMO-PPV/PCBM devices over time was not found to be dependent on the choice of cathode material in contrast to these observations with

heim Adv. Mater. 2010, 22, 3839–3856


REV

IEW

P3HT/PCBM. This suggests that the degradation in the case of P3HT/PCBM devices is driven by interactions at the active later/electrode interface. However, in both types of systems, the absorption spectrum of the active layer was not found to change.

Tipins et al. [ 152 ] have evaluated indoor lifetime of large area (233 cm 2 ) rigid P3HT/PCBM based OPV modules, with ITO and Ca/Al as anode and cathode electrodes respectively and a proprietary HTL layer. Modules were encapsulated with cavity glass containing desiccant and a UV-curable epoxy adhesive. The initial NREL certifi ed total area effi ciency was 1.1%. The modules were light soaked at an intensity of 100mW/cm 2 by a Xenon lamp and maintained at room temperature. Lifetime in this paper is defi ned as the duration over which the module diminishes to 80% of its stabilized power output normalized by the illumination intensity (to take into account the light vari-ations or decay over a period of time) under ∼ 1 sun at 100% duty cycle. Based on 1320 h of actual data, 50% duty cycle and decay rate of ∼ 5% per 1000 h the lifetime of the module was estimated to be 5000 h using a linear data fi t. Figure 18 shows the normalized output power and j – V measurements at dif-ferent time intervals of the module soaked under the Xenon lamp. Further, these modules performed fairly well with out-door exposure in the environment of Pittsburgh, US (Figure 18 ,


Figure 18 . a) Normalized output power of a module (after stabilization)time under. b) j – V characteristics of the module measured at different timAM1.5 100 mW cm − 2 . c) Normalized output power of 10 modules located aReproduced with permission. [ 152 ] Copyright 2009, Elsevier.

bottom). Following a period of 5 months, only a small degrada-tion of the output power was observed.

Katz et al. [ 153 ] reported outdoor data and long-term stability of encapsulated solar cells fabricated on fl exible substrates using MEH/PPV/PCBM, P3HT/PCBM and poly(3-carboxythi-ophene-co-thiophene) (P3CT)/C60 as the active layers with ITO/PEDOT:PSS and Al forming the electrodes. The outdoor experiment was carried out in Sede Boker, Israel where the clear sky spectrum at noon has a close match to the AM1.5 G spec-trum. [ 154 ] The MEH/PPPV devices were reported to degrade the fastest whereas the devices with P3CT donor showed the slowest degradation among the 3 samples tested. P3HT/PCBM and P3CT/C60 samples showed some restoration or recovery effect of J sc and V oc when the cells were kept in dark irrespective of whether the cells were stored in a Nitrogen or in ambient air. J sc recovered partially every night but continued to decay over a month while V oc recovered almost completely during the test period. It was also shown that J sc and V oc were completely restored when P3CT/C60 samples were shadowed from sunlight during daytime. The time period for the restoration was found to be between 10 and 30 min. The restoration effect was attributed to the disappearance of photoinduced trap sites in the dark.

Hauch et al. [ 155 ] have evaluated temperature accelerated indoor and outdoor lifetimes of P3HT/PCBM based solar

bH & Co. KGaA, Wein

as a function of e interval under

t Plextronics, PA.

cells fabricated on fl exible substrates. These cells and modules were encapsulated with a transparent barrier fi lm with a WVTR rate of 0.03 g m − 2 d − 1 . The outdoor tests were car-ried out in Lowell, MA, USA facing the solar south. After 8 months the module perform-ance started to degrade in a linear fashion. However, when the modules reached 80% of initial performance and were tested under a solar simulator, it was found that the module effi ciency had increased by 3.3% due to an increase in FF . The drop in the output power of the modules measured outdoor was attrib-uted to the shift in the maximum power point. The outdoor module data was also cor-related with the temperature accelerated lab cell data. It was suggested that 600 to 800 h of lab cell data operated under 65 ° C at 1 sun corresponded to 1 year of outdoor module lifetime. Lungenschmied et al. [ 156 ] reported shelf life of up to 6000 h (to 80% of its ini-tial performance) based on ITO/PEDOT:PSS/P3HT/PCBM/Al device fabricated on fl exible ultrabarriers from NOVA-PLASMA, Inc.

J. Hauch et al. [ 157 ] have performed accel-erated lifetime testing on fl exible P3HT/PCBM encapsulated with food packaging barrier fi lms with WVTR of 0.2 g m − 2 d − 1 at 65 ° C/85% relative humidity (rh). The bar-rier fi lms were characterized using electrical calcium tests and lifetime of OPV cells under accelerated conditions were correlated to the WVTR. The OPV cells were exposed to 65 ° C/85% rh (damp heat and dark), 65 ° C (high temperature, dark) and 65 ° C/1 sun

3851heim

3852


REV

IEW
(high temperature under light). The devices were in open cir-
cuit in dark while the samples under light were short-circuited V oc was found to be stable under dry conditions and showed a 5% drop under damp heat conditions. This loss in V oc was attributed to the oxidation/interaction at the electrode/active layer interface. J sc showed less degradation in dark than under light while there was a rapid decay of J sc under light; however the cause for this J sc loss was not known. P3HT/PCBM cells encapsulated with barrier fi lms having WVTR of 0.2 g m − 2 d − 1 at 65 ° C/85% were shown to have at least 1250 h of life-time under accelerated test conditions with damp heat being the worst accelerant. Unlike organic light-emitting diodes that require barrier fi lms with a WVTR < 1 × 10 − 6 g m − 2 d − 1 at 25 ° C/40% rh, [ 158 ] the P3HT/PCBM system is obviously less sensitive to water and oxygen and lifetimes relevant to commer-cialisation of products can be achieved with existing packaging materials.

It is known that prolonged exposure of unencapsulated organic solar cells to air typically results in degradation of the active layer due the permeation of oxygen and moisture through pinholes in the top electrodes and oxidation of reactive low work function cathode metals. [ 159 ] In addition it was also shown [ 160 ] that diffusion of the top electrode metal into the active layer could potentially alter the semiconducting properties of the active layer. Jong [ 161 ] and Greczynski [ 162 ] have also reported mor-phological and chemical instabilities at the PEDOT:PSS/ITO interface. It is clear that eliminating the reactive low work func-tion metal cathode will be benefi cial for stability. For this reason, more recently, inverted architectures have been employed that incorporates more air stable metal/metal-oxides as replacements for low work function metals. [ 163 , 164 ] The progress on inverted device architectures was recently reviewed by Y. Yang et al. [ 37 ]

Glatthar et al. [ 165 ] demonstrated an inverted architecture using Au as the top electrode and a Ti buffer layer between Al and active layer. This device had an effi ciency of 1.4%. White et al. [ 114 ] used a solution-derived ZnO (annealed at 200 ° C) inter-layer between the P3HT/PCBM active layer and ITO, with Ag as the top electrode. An initial effi ciency of 2.97% was reported which decreased to 2.32% in 7 days. Although the effi ciency of the cells increased when ZnO and Ag was exposed to air (due to shift of the effective metal work function away from vacuum level) this process was competing with the degradation of the active layer. Waldauf et al. [ 166 ] used solution based TiO x on ITO as an electron-selective electrode. An effi ciency of 3.1% was reported for the inverted architecture in comparison to 3.6% for normal architecture. This has recently been further improved to 3.6% by incorporating a solution processed polyxyethylene tridecyl ether (PTE) interfacial layer between ITO and TiO x . [ 167 ]

The fi rst environmental stability data for the inverted solar cell architecture appears to justify the expectations. Hau et al. [ 168 ] have reported excellent short–term stability of several weeks for unencapsulated, inverted solar cell that used ITO/ZnO nano particles as the bottom electrode and PEDOT/Ag as top electrode with a PCE of 3.3%. Zimmermann et al. [ 169 ] have evaluated the long term stability of the inverted architec-ture employing P3HT/PCBM as the active layer, a double layer of Ti or Cr/Al as the cathode and Au deposited on PEDOT:PSS as the anode. The device was fabricated on a glass substrate and encapsulated with glass plates. It was shown that after 1500 hrs


of continuous illumination under a sulphur plasma lamp with a light intensity of 100 mW cm − 2 at 50 ° C the devices with Cr/Al cathode retained 90% of their initial effi ciency of over 2.5%. This light dose corresponded to 1.5 years exposure to sunlight. It should be noted that the methodology involved in correlating indoor data to outdoor data was not provided. Auger electron spectroscopy (AES) revealed partial oxidation of the Ti and Cr layers in the inverted architecture and the long term sta-bility was attributed to the semiconducting metal-oxide with a conduction band close to teh PCBM LUMO level for preserving charge transfer in the long term.

3.3. Degradation Mechanisms

The predominant degradation pathway in organic solar cells is the sensitivity of active layer to oxygen. Damn et al. [ 170 ] found, in 1999, that singlet molecular oxygen was the principal reactive intermediate in the photoinduced oxygen-dependent decompo-sition of the phenylenevinylene polymers they were studying. The reaction rate was found to decrease as a function of conjuga-tion chain length and by incorporation of electron-withdrawing susbstituents in the oligomer. Neugebauer et al. [ 171 ] studied degradation and stability of MDMO-PPV, fullerene (PCBM and C60) and 1:3 mixtures using ATR-FTIR spectroscopy under the infl uence of light and oxygen. It was shown that MDMO-PPV polymer by itself degraded much faster than C60 or PCBM. The degradation rate of MDMO-PPV was slowed down signifi cantly in an Ar environment. [ 172 ] However, degradation was slower in the MDMO-PPV/C60 blend. It was hypothesised that the fast electron transfer from polymer to C60 after excitation along with polaron formation on the chain decreased reactivity of the polymer to oxygen and triplet-triplet annihilation reaction with oxygen under formation of reactive singlet oxygen.

Pacios et al [ 173 ] studied the effects of photo-oxidation of MDMO-PPV/PCBM on OPV device performance. The cells were subjected to white light over 20 hours in a dry oxygen environment. Using transient absorption spectroscopy it was shown that formation of deep traps led to a reduction in charge recombination and loss in absorption alone was insuffi cient to explain the loss in J sc . Charge transport measured from steady-state IV and time-of-fl ight measurements showed a reduction in mobility. It was concluded that a change in the distribution of deep trap states reduced the mobility, reducing J sc over time. Loss in absorption was found to be only a secondary factor. Luer et al. [ 174 ] also showed oxygen induced quenching of excited states in P3HT fi lms. The fast reversible fl uorescence quenching was attributed to macroscopic diffusion of oxygen into the bulk of the fi lm. The slow partially reversible quenching was found to be strongly dependent on light intensity and was assigned to a charge transfer complex between molecular oxygen and P3HT.

Krebs and Norrman [ 159 ] have reported a detailed study of the pathway of O 2 molecules into the heterojunction OPV devices with the structure ITO/P3CT/C60/Al. 18 O isotope labeling and time-of-fl ight (ToF) SIMS revealed that O 2 diffuses through pin-holes in the Al top electrode and through Al grain boundaries. It was also shown that oxygen was originating from organo-aluminum species at the Al/C60 interface and the device deg-radation was attributed to oxidation of aluminum electrodes



REV

IEW

and gradual degradation of the Al/C60 interface. The effect of water-induced degradation [ 175 ] was also studied by H 2 18 O labe-ling. Unlike molecular oxygen, moisture seemed to penetrate through Al grains and homogenously degrade all interfaces.

Recently, Manceau et al. [ 176 ] reconsidered the mechanism of photo- and thermooxidation of P3HT thin fi lms using infrared spectroscopy. The chemical modifi cation of P3HT as a result of exposure to light and heat (60 ° C and 100 ° C) in ambience was studied. The mechanism proposed ruled out the possibility of singlet oxygen being the main intermediate in the degradation process. [ 177 ] The main degradation pathway was attributed to oxidation of the side-chains and backbone leading to the disap-pearance of alkyl groups and thiophene rings when subjected to heat and light. This resulted in oxidation species of sulphur and side chains that reacted leading to a decrease in absorption.

Paci [ 178 ] used energy dispersive X-ray refl ectometry to mon-itor the in-situ morphological instabilities of the MDMO-PPV/PCBM active layer in OPV devices under operatio. It was shown that the morphology remained stable in the dark but the active layer/Al electrode interface thickness increased progres-sively under illumination indicating a chemical reaction at that interface. Steim [ 179 ] investigated the formation and impact of “hot spots” on the performance of P3HT/fullerene solar cells on fl exible substrates under reverse bias and under illumina-tion. It was shown that the localized homogenous “hot spots” were formed that became the dominant failure mode. These “hot spots” lead to increased leakage currents under reverse bias and their intensity increased over time. It was shown that increase in leakage current led to a decrease in FF of the solar cell and did not depend on a particular architecture or a mate-rial system.

3.4. Challenges and Outlook: Towards 10 Years Lifetime

In order for organic solar cells to fully mature from research and development into cost effective products, continuous improve-ment in effi ciency and stability must be achieved. It is clear that the organic semiconductors and electrode materials used so far are susceptible to oxygen and moisture. To reduce the degradation of the active layer, oxygen and water vapor barrier coatings become a necessity. Traditionally, the packaging costs have dictated the fi nal product costs in the electronic industry and a barrier fi lm that serves as a good encapsulation layer without adding prohibitive costs has to be realized. The sensi-tivities of the organic layered stack to water vapor and oxygen to realise a certain product lifetime has to be understood. Dif-ferent material stacks might demand different water vapor and oxygen transmission rates. For this to succeed, one needs to understand the degradation mechanism involved leading to the failure of the device.

Other challenges include the intrinsic stabilities to the mate-rials used. It is known that metal-organic reactions can lead to reaction products that can further accelerate device degradation. This degradation is further accelerated in the presence of light or water. The stability of a particular electrode with an active layer may not be universal to all polymers used in solar cells. The effect of impurities on device performance and stability has to be quantitatively studied. A better understanding of the


infl uence of spectral dependence on solar cell performance and stability has to be understood. The effect of electric fi eld and the columbic charge that the device is exposed to on stability and performance must also be understood.

Research and development in the fi eld of OPV is primarily driven by the potential of the new technology to create prod-ucts that are effi cient, cost-effective and long lasting. The OPV market segment opportunities range from on-grid power gen-eration to meet the ever increasing clean-energy demand to everyday consumer electronics. It should be noted that in com-parison to inorganic PV technology, OPV technology provides a wide range of active material and electrode combinations for construction of cells/modules and hence provides many options for improving performance. The effi ciency and lifetime requirements vary based on the application and as well as the encapsulation requirements. Consumer electronics and energy harvesting applications require moderate effi ciency (3–5%) and lifetime (3–5 years) and this could be achieved in the near future. For on-grid power generation, effi ciency and lifetime requirements will be more demanding; organic solar modules used for power generation will need to pass tests similar to IEC and IEEE tests for inorganic solar cells.

4. Conclusion

Polymer/fullerene OPV cells have advanced rapidly in recent years, making the technology a prime candidate for future low cost solar energy generation. Recent developments in materials have brought solar power conversion effi ciencies above the 7% level. If the fi eld continues to develop at the same dynamic rate, expectations are that the 10% effi ciency milestone will be met by 2011. In parallel, there is clear progress in understanding and optimizing the operational stability of OPV devices. First reports on indoor, accelerated and outdoor OPV degradation indicate that the lifetime of fl exible as well as glass-based mod-ules is already good enough for fi rst consumer electronics appli-cation. Indeed, while this review was in preparation, the fi rst commercial OPV product, a bag with a fl exible, fully printed OPV module combined with a standard battery, was success-fully launched on the market. [ 180 ]

Acknowledgements This article is part of a Special Issue on Organic Electronics.

Received: October 28, 2009 Revised: March 10, 2010

Published online: August 17, 2010

[ 1 ] T. Ameri , G. Dennler , C. Lungenschmied , C. J. Brabec , Energy Environ. Sci. 2009 , 2 , 347 .

[ 2 ] B. Kippelen , J.-L. Bredas , Energy Environ. Sci. 2009 , 2 , 251 . [ 3 ] B. O’Regan , M. Grätzel , Nature 1991 , 353 , 737 . [ 4 ] U. Bach , D. Lupo , P. Comte , J. E. Moser , F. Weissörtel , J. Salbeck ,

H. Spreitzer , M. Grätzel , Nature 1998 , 395 , 583 . [ 5 ] M. Graetzel , Nature 2001 , 414 , 338 . [ 6 ] W. J. E. Beek , M. M. Wienk , R. A. J. Janssen , Adv. Mater. 2004 , 16 ,

1009 .


3854


REV

IEW
[ 7 ] W. U. Huynh , J. J. Dittmer , A. P. Alivisatos , Science 2002 , 295 ,
2425 . [ 8 ] I. Gur , N. A. Fromer , M. L. Geier , A. P. Alivisatos , Science 2005 ,

310 , 462 . [ 9 ] D. J. Milliron , I. Gur , A. P. Alivisatos , MRS Bull. 2005 , 30 , 41 . [ 10 ] A. C. Arango , L. R. Johnson , V. N. Bliznyuk , Z. Schlesinger ,

S. A. Carter , H.-H Hoerhold , Adv. Mater. 2000 , 12 , 1689 . [ 11 ] D. Gebeyehu , C. J. Brabec , F. Padinger , T. Fromherz , S. Spiekermann ,

N. Vlachopoulos , F. Kienberger , H. Schindler , N. S. Sariciftci , Synth. Met. 2001 , 121 , 1549 .

[ 12 ] K. M. Coakley , Y. X. Liu , C. Goh , M. D. McGehee , MRS Bull. 2005 , 30 , 37 .

[ 13 ] K. M. Coakley , Y. X. Liu , M. D. McGehee , K. L. Frindell , G. D. Stucky , Adv. Funct. Mater. 2003 , 13 , 301 .

[ 14 ] D. C. Olson , S. E. Shaheen , R. T. Collins , D. S. Ginley , J. Phys. Chem. C 2007 , 111 , 16670 .

[ 15 ] D. C. Olson , Y.-J. Lee , M. S. White , N. Kopidakis , S. E. Shaheen , D. S. Ginley , J. A. Voigt , J. W. P. Hsu , J. Phys. Chem. C 2007 , 111 , 16640 .

[ 16 ] A. P. Piechowski , G. R. Bird , D. L. Morel , E. L. Stogryn , J. Phys. Chem. 1984 , 88 , 923 .

[ 17 ] C. W. Tang , Appl. Phys. Lett. 1986 , 48 , 183 . [ 18 ] P. Peumans , A. Yakimov , S. J. Forrest , Appl. Phys. 2003 , 93 ,

3693 . [ 19 ] P. Peumans , S. Uchida , S. R. Forrest , Nature 2003 , 425 , 158 . [ 20 ] J. J. M. Halls , C. A. Walsh , N. C. Greenham , E. A. Marseglia ,

R. H. Friend , S. C. Moratti , A. B. Holmes , Nature 1995 , 376 , 498 . [ 21 ] C. R. McNeill , N. C. Greenham , Adv. Mater. 2009 , 21 , 3840 . [ 22 ] A. B. Tamayo , B. Walker , T.-Q. Nguyen , J. Phys. Chem. C 2008 , 112 ,

11545 . [ 23 ] B. Walker , B. Tamayo , X.-D. Dang , P. Zalar , J. H. Seo , A. Garcia ,

M. Tantiwiwat , T.-Q. Nguyen , Adv. Funct. Mater. 2009 , 19 , 3063 . [ 24 ] http://www.pv-tech.org/news/(last accessed December 2009 ). [ 25 ] C. J. Brabec , N. S. Sariciftci , J. C. Hummelen , Adv. Funct. Mater.

2001 , 11 , 15 . [ 26 ] C. J. Brabec , J. A. Hauch , P. Schilinsky , C. Waldauf , MRS Bull. 2005 ,

30 , 50 . [ 27 ] C. J. Brabec , J. R. Durrant , MRS Bull. 2008 , 33 , 670 . [ 28 ] G. Dennler , M. C. Scharber , C. J. Brabec , Adv. Mater. 2009 , 21 ,

1323 . [ 29 ] K. M. Coakley , M. D. McGehee , Chem. Mater. 2004 , 16 , 4533 . [ 30 ] K. M. Coakley , Y. Liu , C. Goh , M. D. McGehee , MRS Bull. 2005 , 30 ,

37 . [ 31 ] A. C. Mayer , S. R. Scully , B. E. Hardin , M. W. Rowell ,

M. D. McGehee , Mater. Today 2007 , 10 , 28 . [ 32 ] H. Spanggaard , F. C. Krebs , Sol. Energy Mater. Sol. Cells 2004 , 83 ,

125 . [ 33 ] E. Bundgaard , F. C. Krebs , Sol. Energy Mater. Sol. Cells 2007 , 91 .

954 . [ 34 ] M. Jørgensen , K. Norrman , F. C. Krebs , Sol. Energy Mater. Sol. Cells

2008 , 92 , 686 . [ 35 ] F. C. Krebs , Sol. Energy Mater. Sol. Cells 2009 , 93 , 394 . [ 36 ] H. Hoppe , N. S. Sariciftci , J. Mater. Res. 2004 , 19 , 1924 . [ 37 ] L.-M. Chen , Z. Hong , G. Li , Y. Yang , Adv. Mater. 2009 , 21 , 1434 . [ 38 ] R. A. J. Janssen , J. C. Hummelen , N. S. Sariciftci , MRS Bull. 2005 ,

30 , 33 . [ 39 ] S. Guenes , H. Neugebauer , N. S. Sariciftci , Chem. Rev. 2007 , 107 ,

1324 . [ 40 ] M. T. Lloyd , J. E. Anthony , G. G. Malliaras , Mater. Today 2007 , 10 ,

34 . [ 41 ] B. P. Rand , J. Genoe , P. Heremans , J. Poortmans , Prog. Photovolt.

Res. Appl. 2007 , 15 , 659 . [ 42 ] B. C. Thompson , J. M. J. Frechet , Angew. Chem. Int. Ed. 2008 , 47 , 58 . [ 43 ] R. Kroon , M. Lenes , J. C. Hummelen , P. W. M. Blom , B. de Boer ,

Polym. Rev. 2008 , 48 , 531 .


[ 44 ] Semiconducting Polymers , 2nd ed. (Eds: G. Hadziioannou , G. G. Malliaras ), Wiley-VCH , Weinheim, Germany 2006 .

[ 45 ] Semiconducting Polymers—Chemistry, Physics and Engineering ; (Eds. G. Hadziioannou , P. F. van Hutten ) Wiley-VCH , Weinheim, Germany , 2000 .

[ 46 ] Special Issue on Organic-Based Photovoltaics, (Eds: S. E. Shaheen , D. S. Ginley , G. E. Jabbour ), MRS Bull 2005 , 30, pp. 10 – 57 .

[ 47 ] Special Issue on Organic Solar Cells, (Ed: J. Poortmans ), Prog. Photovolt, Res. Appl. 2007 , 15, pp. 657 – 754 .

[ 48 ] Special Issue on The Development of Organic and Polymer Photo-volatics, (Ed: F. C. Krebs ), The development of organic and polymer photo-voltaics, Sol. Energy Mater, Sol. Cells 2004 , 83, pp. 125 – 321 .

[ 49 ] Special Issue on Low Band Gap Polymer Materials for Organic Photovoltaics, (Ed: F. C. Krebs ), Sol. Energy Mater. Sol. Cells 2007 , 91 , pp. 953 – 1036 .

[ 50 ] Special Issue on Degradation and Stability of Polymer and Organic Solar Cells, (Ed: F. C. Krebs ), Sol. Energy Mater. Sol. Cells 2008 , 92 , pp. 685 – 820 .

[ 51 ] Special Issue on Processing and Preoaration of Polymer and Organic Solar Cells, (Ed: F. C. Krebs ), Sol. Energy Mater. Sol. Cells 2009 , 93 , pp. 393 – 412 .

[ 52 ] Organic Photovoltaics: Concepts and Realization (Eds: C. J. Brabec , V. Dyakonov , J. Parisi , N. S. Sariciftci ), Springer , New York , 2003 .

[ 53 ] Organic Photovoltaics: Materials, Device Physics and Manufacturing Technologies , (Eds: C. J. Brabec , V. Dyakonov , U. Scherf ) , Wiley-VCH , Weinheim, Germany , 2008 .

[ 54 ] Organic Photovoltaics: Mechanisms, Materials and Devices (Eds: S. S. Sun , N. S. Sariciftci ), CRC Press, Taylor & Francis Group , Boca Raton , Fl, USA , 2005 .

[ 55 ] Nanostructured Materials for Solar Energy Conversion (Ed: T. Soga ), Elsevier , UK , 2006 .

[ 56 ] Polymer Photovoltaics: A Practical Approach (Ed: F. C. Krebs ), SPIE Press , Bellingham , 2008 .

[ 57 ] M. A. Green , K. Emery , Y. Hishikawa , W. Warta , Prog. Photovoltaics 2009 , 17 , 85 .

[ 58 ] G. A. Chamberlain , Solar Cells 1983 , 8 , 47 . [ 59 ] J. Simon , J.-J. Andre , Molecular Semiconductors , Springer-Verlag ,

Berlin 1985 . [ 60 ] Handbook of Conducting Polymers , Vol. 1 (Ed: J. Kanicki ,

T. A. Skotheim ), Marcel Dekker , New York 1985 . [ 61 ] S. Karg , W. Riess , V. Dyakonov , M. Schwörer , Synth Metals 1993 ,

54 , 427 . [ 62 ] N. S. Sariciftci , L. Smilowitz , A. J. Heeger , F. Wudl , Science 1992 ,

258 , 1474 . [ 63 ] A. J Heeger , private communication. [ 64 ] G. Yu , J. Gao , J. C. Hummelen , F. Wudl , A. J. Heeger , Science 1995 ,

270 , 1789 . [ 65 ] J. J. M. Halls , C. A. Walsh , N. C. Greenham , E. A. Marseglia ,

R. H. Friend , , A. B. Holmes , Nature 1995 , 376 , 498 . [ 66 ] G. Schwartz , B. Maennig , C. Uhrich , W. Gnehr , S. Sonntag ,

O. Erfurth , E. Wollrab , K. Walzer , M. Pfeiffer , Proc SPIE (San Diego) 2009 , in press, ISBN: 9780819477064

[ 67 ] J. Hou , H.-Y. Chen , S. Zhang , G. Li , Y. Yang , J. Am. Chem. Soc. 2008 , 130 , 16144 .

[ 68 ] S. H. Park , A. Roy , S. Beaupre , S. Cho , N. Coates , J. S. Moon , D. Moses , M. Leclerc , K. Lee , A. J. Heeger , Nat. Photonics 2009 , 3 , 297 .

[ 69 ] J. Peet , J. Y. Kim , N. E. Coates , W. L. Ma , D. Moses , A. J. Heeger , G. C. Bazan , Nat. Mater. 2007 , 6 , 497 .

[ 70 ] J. Y. Kim , K. Lee , N. E. Coates , D. Moses , T.-Q. Nguyen , M. Dante , A. J. Heeger , Science 2007 , 317 , 222 .

[ 71 ] R. Gaudiana , C. J. Brabec , Nat. Photonics 2008 , 2 , 287 . [ 72 ] Y. Liang , D. Feng , Y. Wu , S. Tsai , G. Li , C. Ray , L. Yu , J. Am. Chem.

Soc. 2009 , 131 , 7792 .



REV

IEW

[ 73 ] J. C. Hummelen , B. W. Knight , F. Lepec , F. Wudl , J. Org. Chem. 1995 , 60 , 532 .

[ 74 ] C. J. Brabec , A. Cravino , D. Meissner , N. S. Sariciftci , T. Fromherz , M. T. Rispens , L. Sanchez , J. C. Hummelen , Adv. Funct. Mater. 2001 , 11 , 374 .

[ 75 ] G. Yu , A. J. Heeger , J. Appl. Phys. 1995 , 78 , 4510 . [76] C. W. Tang , US Patent 4,164,431 , August 14, 1979 . [ 77 ] S. Shaheen , C. J. Brabec , N. S. Sariciftci , F. Padinger , T. Fromherz ,

J. C. Hummelen , Appl. Phys. Lett. 2001 , 78 , 841 . [ 78 ] V. D. Mihailetchi , L. J. A. Koster , P. W. M. Blom , C. Melzer ,

B. de Boer , J. K. J. van Duren , R. A. J. Janssen , Adv. Funct. Mater. 2005 , 15 , 795 .

[ 79 ] R. Pacios , D. D. C. Bradley , J. Nelson , C. J. Brabec , Synth. Met. 2003 , 137 , 1469 .

[ 80 ] R. Pacios , J. Nelson , D. D. C. Bradley , C. J. Brabec , Appl. Phys. Lett. 2003 , 83 , 4764 .

[ 81 ] P. W. M. Blom , V. D. Mihailetchi , L. J. A. Koster , D. E. Markov , Adv. Mat. 2007 , 19 , 1551 .

[ 82 ] H. Hoppe , N. S. Sariciftci , J. Mater. Chem. 2006 , 16 , 45 . [ 83 ] P. W. Blom , V. D. Mihailetchi , L. J. A. Koster , D. E. Markov ,

Adv. Mater. 2007 , 19 , 1551 . [ 84 ] T. Munters , T. Martens , L. Goris , V. Vrindts , J. Manca , L. Lutsen ,

W. De Ceuninck , D. Vanderzande , L. De Schepper , J. Gelan , N. S. Sariciftci , C. J. Brabec , Thin Solid Films 2002 , 403–404 , 247 .

[ 85 ] C. J. Brabec , S. E. Shaheen , C. Winder , N. S. Sariciftci , P. Denk , Appl. Phys. Lett. 2002 , 80 , 1288 .

[ 86 ] M. M. Wienk , J. M. Kroon , W. J. H. Verhees , J. Knol , J. C. Hummelen , P. A. van Hal , R. A. J. Janssen , Angew. Chem. Int. Ed. 2003 , 42 , 3371 .

[ 87 ] K. J. Ihn , J. Moulton , P. Smith , J. Polym. Sci., Part B: Polym. Phys. 1993 , 31 , 735 .

[ 88 ] S. Morita , A. A. Zakhidov , K. Yoshino , Solid State Commun. 1992 , 82 , 249 .

[ 89 ] P. Schilinsky , C. Waldauf , C. J. Brabec , Appl. Phys. Lett. 2002 , 81 , 3885 .

[ 90 ] F. Padinger , R. S. Rittberger , N. S. Sariciftci , Adv. Funct. Mater. 2003 , 13 , 1 .

[ 91 ] V. D. Mihailetchi , H. Xie , B. de Boer , L. J. A. Koster , P. W. M. Blom , Adv. Funct. Mater. 2005 , 15 , 1260 .

[ 92 ] A. Pivrikas , G. Juska , A. J. Mozer , M. Scharber , K. Arlauskas , N. S. Sariciftci , H. Stubb , R. Österbacka , Phys. Rev. Lett. 2005 , 94 , 176806 .

[ 93 ] T. Erb , U. Zhokhavets , G. Gobsch , S. Raleva , B. Stühn , P. Schilinsky , C. Waldauf , C. J. Brabec , Adv. Funct. Mat. 2005 , 15 , 1193 .

[ 94 ] X. Yang , J. Loos , S. C. Veenstra , W. J. H. Verhees , M. M. Wienk , J. M. Kroon , M. A. J. Michels , R. A. J. Janssen , Nano Lett. 2005 , 5 , 579 .

[ 95 ] Y. Kim , S. Cook , S. M. Tuladhar , S. A. Choulis , J. Nelson , J. R. Durrant , D. D. C. Bradley , M. Giles , I. McCulloch , C.-S. Ha , M. Ree , Nat. Mater. 2006 , 5 , 197 .

[ 96 ] W. Ma , C. Yang ., X. Gong , K. Lee , A. J. Heeger , Adv. Funct. Mater. 2005 , 15 , 1617 .

[ 97 ] P. Schilinsky , U. Asawapirom , U. Scherf , M. Biele , C. J. Brabec , Chem. Mater. 2005 , 17 , 2175 .

[ 98 ] J. Kline , M. D. McGehee , E. N. Kadnikova . J. Liu , J. M. J. Fréchet , Adv. Funct. Mater. 2003 , 15 , 1519 .

[ 99 ] A. Zen , J. Pfl aum , S. Hirschmann . W. Zhuang , F. Jaiser , U. Asawapirom , J. P. Rabe , U. Scherf , D. Neher , Adv. Funct. Mater. 2004 , 14 , 757 .

[ 100 ] J. Kline , M. D. McGehee , E. N. Kadnikova . J. Liu , J. M. J. Fréchet , M. F. Toney , Macromolecules 2005 , 38 , 3312 .

[ 101 ] W. Ma , J. Y. Kim , K. Lee , A. J. Heeger , Macromol . Rapid. Commun. 2007 , 28 , 1776 .

[ 102 ] G. Li , V. Shrotriya , J. Huang , T. Mariarty , K. Emery , Y. Yang , Nat. Mater. 2005 , 4 , 864 .


[ 103 ] J. Peet , C. Soci , R. C. Coffi n , T. Q. Nguyen , A. Mihailovsky , D. Moses , G. C. Bazan , Appl. Phys. Lett. 2006 , 89 , 252105 .

[ 104 ] W. Wang , H. Wu , C. Yang , C. Luo , Y. Zhang , J. Chen , Y. Cao , Appl. Phys. Lett. 2007 , 90 , 183512 .

[ 105 ] A. J. Moule , K. Meerholz , Adv. Mater. 2008 , 20 , 240 . [ 106 ] M. D. Irwin , D. B. Buchholz , A. W. Hains , R. P. H. Chang ,

T. J. Marks , PNAS 2008 , 105 , 2783 . [ 107 ] S. Berson , R. De Bettignies , S. Bailly , S. Guillerez , Adv. Funct. Mater.

2007 , 17 , 1377 . [ 108 ] S. Malik , A. K. Nanti , J. Polym. Sci., Part B: Polym. Phys. 2002 , 40 ,

2073 . [ 109 ] E. Mena-Osteritz , A. Meyer , B. M. W. Langeveld-Voss , R. A. J. Janssen ,

E. W. Meijer , P. Bäuerle , Angew. Chem. Int. Ed. 2000 , 39 , 2679 ; Angew. Chem. 2000 , 112 , 2792 .

[ 110 ] Y. Kim , S. A. Choulis , J. Nelson , D. D. C: Bradley , S. Cook , J. R. Durrant , Appl. Phys. Lett. 2005 , 86 , 063502 .

[ 111 ] G. Li , V. Shrotriya , Y. Yao , Y. Yang , J. Appl. Phys. 2005 , 94 , 043704 . [ 112 ] M. Reyes-Reyes , K. Kim , D. L. Carroll , Appl. Phys. Lett. 2005 , 87 ,

083506 . [ 113 ] J. Y. Kim , S. H. Kim , H.-H. Lee , K. Lee , W. Ma , X. Gong ,

A. J. Heeger , Adv. Mater. 2006 , 18 , 572 . [ 114 ] M. S. White , D. C. Olson , S. E. Shaheen , N. Kopidakis , D. S. Ginley ,

Appl. Phys. Lett. 2006 , 89 , 143517 . [ 115 ] M. D. Irwin , D. B. Buchholz , A. W. Hains , R. P. H. Chang ,

T. J. Marks , PNAS 2008 , 105 , 2783 . [ 116 ] M .Green , K. Emery , Y. Hishikawa , W. Warta , Prog. Photovolt.: Res.

Appl., Solar conversion Effi ciency Table 28 , 2007 . [ 117 ] C. J. Brabec , P. Denk , M. Scharber , J. Kroon , K. Emery , W. Warta ,

Y. Hishikawa , in preparation. [ 118 ] G. Dennler , M. Scharber , C. J. Brabec , Adv. Mat 2009 , 21 1323 . [ 119 ] a) M. Svensson , F. Zhang , S. C. Veenstra , W. J. H. Verhees ,

J. C. Hummelen , J. M. Kroon , O. Inganäs , M. R. Andersson , Adv. Mater. 2003 , 15 , 988 ; b) X. Wang , E. Perzon , J. L. Delgado , P. de la Cruz , F. Zhang , F. Langa , M. R. Andersson , O. Inganäs , Appl. Phys. Lett. 2004 , 85 , 5081 ; c) A. Gadisa , X. Wang , S. Admassie , E. Perzon , F. Oswald , F. Langa , M. R. Andersson , O. Inganäs , Org. Electronics 2006 , 7 , 195 ; d) F. Zhang , K. G. Jespersen , C. Björström , M. Svensson , M. R. Andersson , V. Sundström , K. Magnusson , E. Moons , A. Yartsev , O. Inganäs , Adv. Funct. Mater. 2006 , 16 , 667 .

[ 120 ] D. J. D. Moet , L. H. Slooff , J. M. Kroon , S. S. Chevtchenko , J. Loos , M. M. Koetse , J. Sweelssen , S. C. Veenstra , Mater. Res. Soc. Symp. Proc. 2007 , 974 , 0974-CC03-09 .

[ 121 ] E. Wang , L. Wang , L. Lan , C. Luo , W. Zhuang , J. Peng , Y. Cao , Appl. Phys. Lett. 2008 , 92 , 033307 .

[ 122 ] M. Chen , J. Hou , Z. Hong , G. Yang , S. Sista , L. Chen , Y. Yang , Adv. Mat. 2009 , 21 , 1 .

[ 123 ] N. Blouin , A. Michaud , M. Leclerc , Adv. Mat. 2007 , 19 , 2295 . [ 124 ] N. Blouin , A. Michaud , D. Gendron , S. Wakim , E. Blair ,

R. N. Plesu , M. Bellette , G. Durocher , Y. Tao , M. Leclerc , J. Am. Chem. Soc. 2008 , 130 , 732 .

[ 125 ] S. H. Park , A. Roy , S. Beaupre’ , S. Cho , N. Coates , J. S. Moon , D. Moses , M. Leclerc , K. Lee , A. J. Heeger , Nat. Photonics 2009 , 3 , 297 .

[ 126 ] M. C. Scharber , D. Mühlbacher , M. Koppe , P. Denk , C. Waldauf , A. J. Heeger , C. J. Brabec , Adv. Mat. 2006 , 18 , 789 .

[ 127 ] R. Tipnis , D. Laird , High Effi ciency Solar Cells , SPIE News-room Website, December 2008, http://spie.org/x31682.xml?ArticleID = x31682, (last accessed December 2008).

[ 128 ] Z. Zhu , D. Waller , R. Gaudiana , M. Morana , D. Mühlbacher , M. Scharber , C. Brabec , Macromolecules 2007 , 40 , 1981 .

[ 129 ] P. Coppo , M. L. Turner . J. Mat. Chem 15 2005 , 15 , 1123 . [ 130 ] A. Tsami , T. W. Bunnagel , T. Farrell , M. Scharber , S. Choulis ,

C. J. Brabec , U. Scherf , J. Mat. Chem. 17 , 2007 , 1353 . [ 131 ] M. Zhang , H. Tsao , W. Pisula , C. Yang , A. K. Mishra , K. Muellen ,

J. Am. Chem. Soc. 2007 , 129 , 3472 .


3856


REV

IEW
[ 132 ] M. C. Scharber , M. Koppe , Jia Gao , F. Cordella , M. A. Loi ,
P. Denk , M. Morana , H.-J. Egelhaaf , K. Forberich , G. Dennler , R. Gaudiana , D. Waller , Z. Zhu , X. Shi , C. J. Brabec , Adv. Mater. 2010 , 22 , 367 .

[ 133 ] Y. Liang , Y. Wu , D. Feng , S.-T. Tsai , H.-J. Son , G. Li , L. Yu , J. Am. Chem. Soc. 2009 , 131 , 56 .

[ 134 ] Y. Wu , presented at LOPE-C, Messe Frankfurt, Germany, June 2009 .

[ 135 ] a) J. C. Hummelen , D. Kronholm , Organic Photovoltaics: Materials, evice Physics and Manufacturing Technologies , (Eds: C. J. Brabec , V. Dyakonov , U. Scherf ), Wiley-VCH , Weinheim , 2008 ; [ 135 ] ) http://www.solennebv.com/(last accessed June 2010).

[ 136 ] F. B. Kooistra , J. Knol , F. Kastenberg , L. M. Popescu , W. J. H. Verhees , J. M. Kroon , J. C. Hummelen , Org. Lett. 2007 , 9 , 551 .

[ 137 ] C. J. Brabec , A. Cravino , D. Meissner , N. S. Sariciftci , T. Fromherz , M. T. Rispens , L. Sanchez , J. C. Hummelen , Adv. Funct. Mater. 2001 , 11 , 374 .

[ 138 ] L. Zheng , Q. Zhou , X. Deng , M. Yuan , G. Yu , Y. Cao , J. Phys. Chem. B 2004 , 108 , 11921 .

[ 139 ] H. Hoppe , S. Sariciftci , Adv. Polym. Sci. 2007 , 12 , 121 . [ 140 ] M Lenes , G.-J. A. H. Wetzelaer , F. B. Kooistra , S. C. Veenstra ,

K. J. Hummelen , P. W. M. Blom , Adv. Mater. 2008 , 20 , 2116 . [ 141 ] M. Lenes , S. W. Shelton , A. B. Sieval , D. F. Kronholm ,

J. C. Hummelen , P. W. M. Blom , Adv. Funct. Mater. 2009 , 19 , 3002 . [ 142 ] R. B. Ross , C. M. Cardona , D. M. Guldi , S. G. Sankaranarayanan ,

M. O. Reese , N. Kopidakis , J. Peet , B. Walker , G. C. Bazan , E. Van Keuren , B. C. Holloway , M. Drees , Nat. Mater. 2009 , 8 , 208 .

[ 143 ] P. Wuerfel , Physics of Solar Cells , Wiley-VCH , Berlin 2004 . [ 144 ] M. Hiramoto , M. Suezaki , M. Yokoyama , Chem. Lett. 1990 , 327 . [ 145 ] J. Y. Kim , K. Lee , N. E. Coates , D. Moses , T.-Q. Nguyen , M. Dante ,

A. J. Heeger , Science 2007 , 317 , 222 . [ 146 ] S. Sista , M.-H. Park , Z. Hong , Y. Wu , J. Hou , W. L. Kwan , G. Li ,

Y. Yang , Adv. Mater. 2010 , in press. [ 147 ] J. Gilot , M. M. Wienk , R. A. J. Janssen , Appl. Phys. Lett. 2007 , 90 ,

143512 . [ 148 ] M. A. Green , K. Emery , Y. Hishikawa , W. Warta , Prog. Photovolt:

Res. Appl. 2009 , 17 , 320 . [ 149 ] http://www.wikispaces.com/opvlifetime (last accessed June

2010). [ 150 ] S. Schuller , P. Schilinsky , J. Hauch , C. J. Brabec , Appl. Phys. A 2004 ,

79 , 37 . [ 151 ] R. De. Bettignies , J. Leroy , M. Firon , C. Sentein , Synthetic Metals

2006 , 156 , 510 . [ 152 ] R. Tipins , J. Bernkof , S. Jia , J. Krieg , S. Li , M. Storch , D. Laird ,

Sol. Energy Mater. Sol. Cells 2009 , 93 , 442 . [ 153 ] E. A. Katz , S. Gevorgyan , M. S. Orynbayev , F. C. Krebs , Eur. Phys. J.

Appl. Phys. 2007 , 36 , 307 . [ 154 ] D. Berman , D. Faiman , Sol. Energy Mater. Sol. Cells 1997 , 45 ,

401 .


[ 155 ] J. Hauch , P. Schilinsky , S. A. Choulis , R. Childers , M. Biele , C. J. Brabec , Sol. Energy Mater. Sol. Cells 2008 , 92 , 727 .

[ 156 ] C. Lungenschmied , G. Dennler , H. Neugebaueeer , S. N. Sacrifi ti , M. Glatthaar , T. Meyer , A. Meyer , Sol. Energy Mater. Sol. Cells 2007 , 91 , 379 .

[ 157 ] J. Hauch , P. Schilinsky , S. A. Choulis , S. Rajoelson , C. J. Brabec , Appl. Phys. Lett 2008 , 93 , 103306 .

[ 158 ] J. Lewis , Mater. Today 2006 , 9 , 38 . [ 159 ] F. C. Krebs , K. Normann , Prog. Photovoltaics 2007 , 15 , 697 . [ 160 ] J. Brirgerson , M. Fahlman , P. Broms , W. R. Salaneck , Synth. Met.

1996 , 80 , 125 . [ 161 ] M. P. De. Long , L. J. van Ijzendoom , M. J. A.. de Voigt , Appl. Phys.

Lett. 2000 , 77 , 2255 . [ 162 ] G. Greczynski , Th. Kugler , M. Keil , W. Osikowicz , M. Hahlman ,

W. R. Salaneck , J. Electron Spectrosc. Relat. Phenom. 2001 , 121 , 1 . [ 163 ] Y. Sahin , S. Alem , R. De. Bettignies , J. Nunzi , Thin Solid Films 2005 ,

476 , 340 . [ 164 ] A. Watanabe , A. Kasuya , Thin Solid Films 2005 , 483 , 358 . [ 165 ] M. Glatthaar , M. Niggemann , B. Zimmermann , P. Lewer , M. Riede ,

A. Hinsch , J. Luther , Thin Solid Films 2005 , 491 , 298 . [ 166 ] C. Waldauf , M. Morana , P. Denk , P. Schilinsky , K. Coakley ,

S. A. Choulis , C. J. Brabce , Appl. Phys. Lett. 2006 , 89 , 233517 . [ 167 ] R. Steim , S. A. Choulis , P. Schilinsky , C. J. Brabec , 2008 , Appl. Phys.

Lett. 92 , 093303 . [ 168 ] S. K. Hau , H. Yip , N. S. Baek , J. Zou , K. O’Malley , A. K. Jen , Appl.

Phys. Lett. 2008 , 92 , 253301 . [ 169 ] B. Zimmermann , U. Wurfel , M. Niggemann , Sol. Energy Mater. Sol.

Cells 2009 , 93 , 491 . [ 170 ] N. Damn , R. D. Surlock , B. Wang , L. Ma , M. Sundahl , P. O. Ogilby ,

Chem. Mater. 1999 , 11 , 1302 . [ 171 ] H. Neugebauer , C. Brabec , J. C. Hummelen , N. S. Sacriciftci ,

Sol. Energy Mater. Sol. Cells 2000 , 61 , 35 . [ 172 ] H. Neugebauer , C. Brabec , J. C. Hummelen , R. A. J. Janssen ,

N. S. Sacriciftci , Synthetic Metals 1999 , 102 , 1002 . [ 173 ] R. Pacios , A. J. Chatten , K. Kawano , J.. R. Durrant , D. D. C. Bradley ,

J. Nelson , Adv. Funct. Mater. 2006 , 16 , 2117 . [ 174 ] L. Luer , H. J. Egelhaff , D. Oelkrug , G. Cerullo , G. Lanzani ,

B. H. Huisman , D. de Leeuw , Organic Electronics 2004 , 5 , 83 . [ 175 ] K. Normann , S. A. Gevorgyan , F. C.Krebs , ACS Appl. Mater. Inter-

faces 2009 , 1 , 102 . [ 176 ] M. Manceau , A. Rivaton , J. Gardette , S Guillerez , N. Lemaitre ,

Polymer Degradation and Stability 2009 , 94 , 898 . [ 177 ] M. Manceau , A. Rivaton , J.-L. Gardette , Macromol . Rapid Commun.

2008 , 29 , 1823 . [ 178 ] B. Paci , A. Generosi , V. R. Albertini , P. Perfetti , R. de. Bettignies ,

M. Firon , J. Leroy , C. Sentein , Appl. Phys. Lett. 2005 , 87 , 194110 . [ 179 ] R. Steim , S. A. Choulis , P. Schilinsky , U. Lemmer , C. J. Brabec ,

2009 , Appl. Phys. Lett. 94 , 043304 . [ 180 ] http://www.neubers.de/vmchk/Solar-Taschen/View-all-products.

html (last accessed August 2009).


Polymer–Fullerene Bulk-Heterojunction Solar Cellsfotonica/teaching... · 2011. 5. 19. · solar...

Documents

Transcript of Polymer–Fullerene Bulk-Heterojunction Solar Cellsfotonica/teaching... · 2011. 5. 19. · solar...