Appl Phys A (2011) 104:47–53DOI 10.1007/s00339-011-6448-3

The inverted solar cells with the polymer:fullerene blend filmpossessing a stratified composition profile

Wei Quan · Cuiran Cheng · Jinsuo Liu · Jidong Zhang ·Donghang Yan · Dashan Qin

Received: 24 January 2011 / Accepted: 20 April 2011 / Published online: 19 May 2011© Springer-Verlag 2011

Abstract The inverted polymer:fullerene solar cells withstructure of ITO/TiO2/P3HT:PCBM/MoO3/Al have beenfabricated, where P3HT and PCBM stand for poly (3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methylester, respectively. It is discovered that the P3HT:PCBMblend film manipulated into the improved stratificationstructure, characterized as P3HT crystallite-rich zone closeto the top surface and PCBM constituent-rich zone adjacentto the bottom surface, can offer nearly the same power con-version efficiency of solar cell, compared to the one growninto the bulk heterojunction structure, characterized as thebicontinuous interpenetrating network of P3HT and PCBM.We provide an alternative insight to the morphology controlof inverted polymer:fullerene solar cells.

1 Introduction

Polymer solar cells have been extensively studied in thefields of science and technology as a potential cost-effectivealternative to silicon based solar cells [1–3]. The commoncharacter possessed by the state-of-the-art polymer solarcells is the involvement of photoactive bulk heterojunction

W. Quan · C. Cheng · J. Liu · D. Qin (�)Institute of Polymer Science and Engineering, Schoolof Chemical Engineering, Hebei University of Technology,Tianjin 300130, People’s Republic of Chinae-mail: qindashan@yahoo.com.cn

J. Zhang · D. YanState Key Laboratory of Polymer Physics and Chemistry,Changchun Institute of Applied Chemistry, Chinese Academyof Sciences, Changchun 130022, Jilin, People’s Republic of China

J. Zhange-mail: jdzhang@ciac.jl.cn

thin film self-assembled via the spontaneous phase sepa-ration between polymer and fullerene [4–8]. The under-lying advantage for polymer:fullerene bulk heterojunctionblend film is to enable the nearly 100% excitons dissoci-ated at the large donor/acceptor interface prior to decay-ing, due to the average sizes for the separated polymerand fullerene nanoscale domains comparable to organic ex-citon diffusion lengths [3] and, therefore, allow for usingthick polymer:fullerene blend film to increase the light ab-sorption of the solar cell. In addition, the phase-separatedmorphology of polymer:fullerene bulk heterojunction blendfilm, represented as a bicontinuous interpenetrating net-work of polymer and fullerene phases, offers efficient pho-tocharges transport toward the due electrodes via the inter-connected conducting pathways. The morphology control ofpolymer:fullerene bulk heterojunction blend film has beenwidely thought crucial to the performance of polymeric so-lar cells [9, 10].

Recently, the inverted solar cell comprised of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acidmethyl ester (PCBM) has drawn much attention [11–22],due to its two major advantages over the regular structure.Firstly, in the regular solar cell, the adoption of the low-workfunction metals and the corrosion of the acidic conductingfilm (PEDOT:PSS) to the anode of indium tin oxide (ITO)lead to the degraded device stability [18], whereas, the in-verted structure is free from these drawbacks. Secondly, be-cause of the vertical composition profile of P3HT-enrichedzone close to the top blend and PCBM-enrich zone close tothe bottom blend in as-cast P3HT:PCBM blend film [23],which is similar to the hybrid planar-mixed molecular het-erojunction delivering a power conversion efficiency (PCE)of (5.0 ± 0.3)% [24, 25], the spin-coated blend film is moresuitably integrated into the inverted solar cell than into theregular one. Generally in the inverted solar cell, the transi-

48 W. Quan et al.

tion oxides, such as MoO3, WO3, V2O5, etc., are used as in-terfacial layers to enhance the photo-hole collection into thetop anode, and the cesium carbonate, zinc oxide, titaniumoxide (TiO2) are utilized to modify the bottom cathode. Liaoet al. reported an inverted P3HT:PCBM solar cell with aPCE of 4.2%, slightly lower than that of their best regularstructure (4.4%) [17]. However, more efforts are still neededto deeply probe the relationship between the blend film mor-phology and the photovoltaic performance of an invertedsolar cell. In particular, it remains unclear how the strati-fied P3HT:PCBM blend film, featuring P3HT-enriched zoneclose to the top and PCBM-enrich zone close to the bottom,affects the performance of an inverted solar cell. Recently,the roll-to-roll processing of inverted polymer:fullerene so-lar cells has been demonstrated [19–22].

Here, the performance variation of the invertedP3HT:PCBM solar cell was studied via tuning the solventevaporating time of wet P3HT:PCBM blend film. It wasfound that the blend film possessing a graded compositionprofile with a lower degree of P3HT crystallinity offerednearly the same PCE, compared to the conventional bulkheterojunction blend film with a higher degree of P3HTcrystallinity. The current research is believed useful for op-timizing the morphology and performance of inverted poly-mer:fullerene solar cell.

2 Methods

2.1 Materials and preparation

The inverted structure of ITO/TiO2/P3HT:PCBM/MoO3/Alwas adopted. The ITO coated glass substrates were commer-cially bought with a sheet resistance of 10 � per square. TheP3HT with a number-average molecular weight of 16,000and a polydispersity index of 1.4 and PCBM were obtainedfrom Synwit Technology Company and Luoyang Micro-light Company, respectively. The MoO3 was purchased fromthe Sigma–Aldrich Company. The carefully cleaned ITOglass substrates were treated in UV-ozone for 15 min priorto the usage.

The TiO2 thin films were spin-coated onto ITO glassfrom a TiO2 sol (prepared in the molar ratio of Ti(OC4H9)4:NH(C2H4OH)2:C2H5OH:H2O being 5:5:133:6) at a speedof 2000 rpm for 30 sec, and then calcinated at 450°C for halfan hour [26]. The resulting structure of TiO2 was confirmedanatase via its Raman spectrum conducted on a JY HR800spectrometer. The role of TiO2 was to conduct photoelec-trons into the ITO cathode. The blend films of P3HT:PCBMwere all spin-coated onto TiO2 film for 30 sec at a speedof 1000 rpm from a mixed 15 mg/ml P3HT and 12 mg/mlPCBM solution using 1,2-dichlorobenzene (DCB). Theirthicknesses were measured ca. 70 nm using a Dektek 6M

profiler. An as-cast P3HT:PCBM film looked orange and be-came deep purple when the residual DCB was evaporatedcompletely. The time scale for the color of the blend filmchanging from orange to deep purple was defined as thesolvent evaporation time (SET). Li et al. reported to con-trol the SET of the blend film via adjusting the spin-coatingtime [27], however, because both the film thickness and ini-tial morphology are subject to the spin-coating time, it isnot manageable to properly control the blend morphologyvia this method. In an effort to achieve various SETs forthe same fabricated wet blends, two simple setups were pro-vided as shown in Scheme 1. The SETs more than 4 minwere obtained via Scheme 1(a) and those less than 4 minwere achieved via Scheme 1(b). The solvent evaporatingprocesses were done under the ambient condition with aroom temperature of about 16°C. Finally, 10 nm MoO3 at arate of 0.2 nm/s, followed by 100 nm Al at a rate of 2.5 nm/s,was thermally evaporated onto P3HT:PCBM blend film un-der a base vacuum pressure of 3 × 10−4 Pa. Five solar cellsmanipulated via the solvent annealing were prepared as fol-lows: devices 1, 2, 3, 4, and 5 corresponded to the SETs of39, 70, 112, 269, and 346 s, respectively. The active areas ofsolar cells were controlled to be 0.08 cm2.

Scheme 1 (a) The schematic diagram for the setup used to achieve theSET of more than 4 min for wet P3HT:PCBM blend film. D representsthe distance from the top part to the bottom part of the Peri dish. Thedashed arrows signify the solvent evaporation out of the Peri dish. TheSET can be varied via adjusting the value of D. (b) The setup used toachieve the SET of less than 4 min for wet P3HT:PCBM blend film.The horizontal arrows signify blowing wind over the blend film by ahair drier. The SET can be varied via adjusting the distance of the hairdrier to the blend film

The inverted solar cells with the polymer:fullerene blend film possessing a stratified composition profile 49

2.2 Characterizations

The TiO2 thin film was firstly activated via illuminating theinverted solar cell at a bias of 1 V. Then the current-voltage(I–V ) characteristics of the inverted solar cell under the1 sun simulated AM 1.5 G illumination (CXE-400 arc Xelamp) were recorded by a programmable Keithley 2400 DCsourcemeter. The morphologies of the blend films were ex-amined using an Amray 1910FE scanning electron micro-scope (SEM). The UV-Vis absorption and X-ray diffraction(XRD) measurements were carried out using a Cary 300spectrophotometer and a Bruker D8 Discover X-ray Reflec-tor, respectively. The device characterization was carried outin the air.

3 Results and discussion

3.1 The solar cell characterization

Figure 1 shows the I–V characteristics of devices 1–5 un-der the photocondition, and Fig. 2 summarizes the depen-dence of the device parameters on the SET. The value ofopen-circuit voltage (Voc) of device slightly decreased from0.69 V to 0.63 V, when the SET increasing from 39 sto 346 s. The short-circuit current (Isc) of device variedmarkedly with the SET. It was 2.02 mA/cm2 at SET = 39 s,increased to 2.70 mA/cm2 at SET = 70 s and further to3.31 mA/cm2 at SET = 112 s, and then became saturatedat ∼3.74 mA/cm2 when SET ≥ 269 s. The variation trendof the PCE of device with the SET is as follows. It increasedfrom 0.56% at SET = 39 s to 1.03% at SET = 112 s, andthen appeared saturated at 1.03% when the SET ≥ 112 s.The fill factor (FF) of device showed a reverse V-shaped de-pendence on the SET. The maximum FF of device reached

Fig. 1 The I–V characteristics of devices 1–5 under the photo condi-tion. D represents device

47.9% at SET = 112 s. The FFs of device for SETs = 39and 70 s were smaller than those for SETs = 269 and 346 s.The SET dependence of the performance of P3HT:PCBMinverted solar cell can be attributed to the morphology evo-lution of the blend film with the SET.

3.2 The optical, morphological, and electrical properties ofblend films

The P3HT:PCBM blend films for devices 1, 3, 4, and 5were selected to characterize. As shown in Fig. 3(a), due

Fig. 2 The Plots of the device parameters, Voc (a), Isc and PCE (b),and FF (c), versus the SET

50 W. Quan et al.

Fig. 3 The UV-Vis absorption (a) and grazing XRD spectra (b) ofdevices 1, 3, 4, and 5. D represents device

to the short SET, the fast-grown blend film for device 1exhibited three weak vibronic absorption peaks locating at517, 549, and 599 nm and also some residual absorption ofP3HT:PCBM solution, indicating the poor π–π stack be-tween P3HT chains in this blend film; whereas, the blendfilms for the other devices appeared with increased π–π

stacks between P3HT chains, as a consequence of the ex-tended SETs. Moreover, the intensity of the shoulder at599 nm, assigned to highly interchain delocalized excita-tion [15], increased in the order of device 5 > device 4 >

device 3. Thus, it can be concluded that the degrees of P3HTcrystallinity for the four blend films varied in the sequenceof device 5 > device 4 > device 3 > device 1, coincidentwith the observation that the Voc of the inverted solar cellpresented some decreasing trend with increasing the SETas seen in Fig. 2(a). It is understood as follows. The maxi-mum Voc of polymer solar cell is determined by the energyof the charge transfer state residing at the donor/acceptor in-terface, which can be reduced by increasing the crystallinefraction of donor component [28]. The increased visible-light absorptions of devices 4 and 5 than devices 1, 2, and 3partly accounted for the greater Isc of devices 4 and 5 than

devices 1, 2, and 3. Figure 3(b) compares the grazing XRDpatterns of these four blend films. There was a prominentdiffraction peak at 2θ = 5.6◦ from (100) stacking for eachblend film, indicating in the P3HT crystallites the P3HTchains were approximately parallel to the substrate withthe plane of thiophene ring oriented vertical to the sub-strate [29].

The surface morphologies of these four blend films werealso investigated. As seen in Figs. 4(a) and (b), the blendfilms for devices 1, 3 exhibited nearly the same rough andfeatureless surfaces. However, Fig. 4(c) shows that the sur-face of the blend film for device 4 severely cracked, signi-fying the occurrence of the lateral phase separation betweenP3HT and PCBM. Figure 4(d) indicates there existed manycrystalline grains of P3HT on the surface of the blend filmfor device 5, demonstrating that the adequate lateral phaseseparation between P3HT and PCBM took place at the SETof 346 s and thereby that the blend film for device 5 hadbeen well organized into the conventional bulk heterojunc-tion structure, represented by the bicontinuous interpenetrat-ing network of donor and acceptor.

In order to further ascertain the morphology differencebetween devices 3 and 5, the electron-only devices withstructure of ITO (anode)/TiO2/P3HT:PCBM/LiF 1 nm/Al(cathode) were fabricated at SETs = 118, 331, and 489 s.As shown in Fig. 5, at a given driving voltage, the currentfor the electron-only device with SET = 489 s was slightlyhigher than that with SET = 331 s, and both were remark-ably larger than that with SET = 118 s. Thus, it is deducedthat the electron transporting property through the blend filmwith SET = 346 s in device 5 must be much superior to thatthrough the blend film with SET = 112 s in device 3, in clearcontrast to the observation that device 3 presented nearly thesame PCE as device 5. As a result, it can be suggested thatthe blend film in device 3 possessed a graded compositionprofile with a lower degree of P3HT crystallinity, different tothe conventional bulk heterojunction structure with a higherdegree of P3HT crystallinity owned by the blend film in de-vice 5.

3.3 The influence of the P3HT:PCBM blend filmmorphology evolution on the performance of invertedsolar cell

The vertical phase segregation of P3HT:PCBM blend filmhas been suggested [16, 23]. If a hydrophilic surface isused as the substrate, the vertical composition profile of as-cast blend film can be described as the concentration gra-dient varying from PCBM-rich near the hydrophilic sub-strate to P3HT-rich near the blend top surface [23], whichis hereafter named the composition-graded structure match-ing to the polarity of the inverted solar cell structure. Themorphology evolution of the P3HT:PCBM blend film with

The inverted solar cells with the polymer:fullerene blend film possessing a stratified composition profile 51

Fig. 4 The SEM observationson the P3HT:PCBM blend filmsfor devices 1 (a), 3 (b), 4 (c),and 5 (d)

Fig. 5 The dark I–V characteristics of the electron-only devices withstructure of ITO (cathode)/TiO2/P3HT:PCBM/LiF 1 nm/Al (anode)fabricated at SETs = 118, 331, and 489 s. Prior to the electric mea-surement, the TiO2 thin film was firstly activated via illuminating thedevice at a bias of 1 V

the SET contains not only the P3HT crystallization butalso the subsequent PCBM diffusion towards the blend

top surface. Based on the vertical composition distribu-tion of as-cast P3HT:PCBM blend film, Scheme 2 is pro-posed to explain the device performance variation with theSET.

The poorest photovoltaic performance of device 1 can beattributed to the lowest degree of P3HT crystallinity of de-vice 1 as shown in Fig. 3(a), which leads to the weakestphoto-hole transport through the P3HT-rich zone to the an-ode and thereby the smallest FF (40.2%). The blend film ofdevice 1 remains in the composition-graded structure as pro-posed in Scheme 2(i), due to the short SET offering the inad-equate PCBM diffusion toward the anode. When the SET isextended, as proposed in Scheme 2(ii), the degree of P3HTcrystallinity increases but the stratified composition profilestays almost unchanged. Such an intermediate phase state iscalled a composition-graded structure with an improved de-gree of P3HT crystallinity, functioning as the hybrid planar-mixed molecular heterojunction [9]: after the efficient ex-citon dissociation in the middle blend film, the photo-holesand -electrons can be nondispersively transported throughthe P3HT crystallite-rich zone and PCBM-rich zone into theanode and cathode, respectively. When the SET is furtherprolonged, as shown in Scheme 2(iii), the lateral phase sep-

52 W. Quan et al.

Scheme 2 The schematic diagrams for illustrating the nanoscalephase separation in the P3HT:PCBM blend films for devices 1 (i),3 (ii), and 5 (iii). The black spots represent PCBM and the short linesrepresent P3HT rigid chains. The close and parallel arrangement be-tween the neighboring short lines denotes the P3HT crystallites. (i),(ii), and (iii) signify the composition-graded structure with a low de-gree of P3HT crystallinity, the composition-graded structure with animproved degree of P3HT crystallinity, the bicontinuous interpenetrat-ing bulk heterojunction, respectively. Note that, (i) and (iii) have beenwell understood and characterized in the literatures [5, 10, 18, etc.],while (ii) is proposed as a transition structure from (i) to (iii)

aration emerges as a result of the growth of P3HT crystal-lites followed by the sufficient PCBM diffusion toward theblend surface [23]. After the evolution of the P3HT:PCBMblend film from a composition-graded structure into a bi-continuous interpenetrating network of donor and acceptor,the charge-separating interfacial area becomes larger, partlyanswering for the increased Isc in device 5 than device 3,nevertheless, the photo-charges conduction becomes perco-lated and dispersive, giving rise to the reduced FF in de-vice 5 than in device 3, because the nondispersive hole mo-bility of P3HT is reported to be about one order of magni-tude higher than the dispersive one [5]. Therefore, device 3showed nearly the same PCE as device 5.

4 Conclusion

The inverted solar cells were fabricated with theP3HT:PCBM blend films progressively tuned via control-ling the solvent evaporation time. It was demonstrated thatthe composition-graded structure with an improved degreeof P3HT crystallinity showed comparable photovoltaic per-

formance to the conventional bulk heterojunction structure.We provide a simple, helpful concept to optimize the mor-phology of the inverted P3HT:PCBM solar cells.

Acknowledgements The authors are grateful for the financial sup-port from the National Science Foundation of China (Grant No.50803014) and from Open Research Fund of State Key Laboratoryof Polymer Physics and Chemistry, Changchun Institute of AppliedChemistry, Chinese Academy of Sciences.

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