Current Solar Cell Technologies and the Application of Nanomaterials

in Photovoltaics

Jennifer Cook

February 11, 2016

Abstract

Worldwide energy usage has increased by approximately 49% over the last 30 years, and the current, total,global power requirement stands above 1.7 TW.1 At present, the main source of energy conversion, are fossilfuels, with a larger amount of energy sourced from coal, rather than oil or gas. Technological advances innanoscience has opened many doors in the field of renewable energy conversion, particularly for photovoltaic(PV) devices. Constant progress is being made to improve band gap utilisation, spectral utilisation andto reduce toxicity. Nanowires and 1D nanoarrays can help to reduce parasitic losses via enhanced lightscattering and increase the amount of incident radiation absorbed into the device. Intermediate bandstructures may offer a wider utilisation of the spectrum, with quantum dots offering further efficiencyenhancements. With the ability to tune many properties of nano architectures, as well as control theirsize and morphology, it is not only third generation photovoltaics that will benefit from nano-technologicaladvancements. Current successful applications include the combination of nanoparticles with dyes for usein dye-sensitised cells, as well as the development of hybrid cells. Nanomaterials hold exciting prospects forthe optimisation of solar power and the future of global energy production, provided the limitations theypose, can be sufficiently supressed, for them to serve as a more efficient and economically viable option overcurrent technologies.

1 Introduction

Energy conversion for electricity using renewables inthe UK, has undergone an increase of 550% from theyear 2000 to 2014 as seen in figure 1.2 The graph dis-plays the main renewable energy sources and the cor-responding amount of energy produced in each yearin the UK. Increasing interest into photovoltaics as aviable solution to the energy crisis has led to the pub-lication of numerous research papers in this field.

Solar cells can be categorised by three generations (Fig-ure 2).3 Dotted lines are shown with a positive, linearcorrelation; in general, the larger the gradient of theline, the better the solar cell technology, in terms of itsefficiency to cost ratio.

First generation solar cells were based on single crys-tal or multi-crystalline wafer technologies, and mainlyon single-junctions, therefore possessing a theoreticalconversion efficiency limit, known as the ‘Shockley-Queisser limit’,4 which stands at approximately 32%for single junction cells.

Second generation thin-film PV technologies combinethe efforts to improve conversion efficiencies, whilstmaintaining the economic viability of solar cells (Fig-ure 2). By incorporating nanomaterials into currentPV technologies, efficiency improvements can be made,even exceeding the Shockley-Queisser limit.

2 P-n Junctions and the Photo-voltaic cell

Traditional solar cells are based on p-n junction semi-conductors. As with all p-n junctions, the drift currentis controlled by the density of minority charge carriers.When light is applied, it acts as a bias voltage to thejunction and the density of the minority charge carri-ers in each region increases which increases the drift.When the p/n regions are connected with a conductivematerial, a current is produced in the closed circuit.The electrons flow from the n-region through the cir-cuit and recombine with a hole in the p-region. If thecircuit is opened, an ‘open circuit voltage’ is produced.This is known as a photovoltaic cell.5

2.1 Performance of a solar cell

The band structure for a typical semiconductor is shownin figure 6.3 The band gap energy is directly propor-tional to the open circuit voltage, and can be increasedby increasing the lifetime τ of the minority charge car-riers where τ is determined by the recombination rate.

Recombination happens via three main mechanisms:Shockley Read Hall (SRH),6 Auger7 and Radiative.8

The radiative method is dominant within direct bandgap materials such as GaAs, as additional momentumdoes not need to be supplied for recombination. Re-combination also affects the diffusion length Lp, and asa rule, the thickness of the solar cell must not exceedLp.9

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Figure 1

The graph displays the main renewable energy sources

and the corresponding amount of energy produced in each

year. The largest increase in percentage FiT capacity

from 2013-2014 was from solar photovoltaics (PV), with

an increase of 565MW.

Figure 2

(a) Energy diagram to show the spectral utilisation of

multiple junctions.(b) Spectral splitting mechanism in the

solar cell.

Figure 54 represents excitation within a single junctioncell, where much of the incident light is useless in theproduction of charge carriers. The external quantumefficiency (EQE), measures the ratio of charge carriersproduced from incident light.10 One way of increasingthe EQE, would be to use a multi-junction cell (Figure2),4 which utilises a wider range of spectral light. Inaddition, changing the absorption coefficient α of thesolar cell material can change its absorbency efficiency,which in turn has an impact on how thick the cell canbe.

2.2 Parasitic Losses

Optical loss mechanisms such as shading and reflectionof light from the cell surface reduce absorption. In ad-dition, non-active PV layers in anti-reflection coatingswill absorb the light without contributing to chargecarrier generation (parasitic absorption). Another pos-sible way to reduce reflective losses is to texture thefilm (Figure 3).11 Radiation may also be transmittedthrough the length of the cell without absorption if thefilm is too thin.12

Figure 3

Textured surface of a solar cell, used to increase

absorption via light scattering.

Figure 4

Graphical representation of solar cell efficiency (%) vs cell

cost (US$/ m2). The oval shaped regions demonstrate how

the three generations of solar cell technology, compare.

3 Thin-film PV technologies

The most promising thin film PV technologies haveshown to be copper indium gallium deselenide (CIGS)and cadmium telluride (CdTe).13 However, much re-search is being undertaken to eliminate the amount ofcadmium used in photovoltaics due to its bio-toxicity.14

3.1 CIGS

Copper indium gallium selenide (CIGS) solar cells, haveconversion efficiencies around 20% and are being pro-duced at the fastest rate among thin film solar cells.They have high absorption coefficients as they possess

Figure 5

Single junction cell band gap. Not all incident light has

sufficient energy to breach the band gap (qVoc).

2

Figure 6

Band structure detailing various loss mechanisms: (1)

Thermal loss, (2) Junction/contact voltage losses,

(4)Recombination loss.

Figure 7

Cross section of a CIGS solar cell, showing the materials

used in each layer and their approximate thicknesses.

a direct band gap structure. Figure 7 shows the crosssection of a typical CIGS cell.15

CIGS may be considered unsustainable, because in-dium is not an abundant element, presenting a po-tential limit on their up-scaling. CZTS may offer areplacement to CIGS in solving this issue, as they arenon-toxic and currently more abundant in availabil-ity.16

Deb, et al.17 and Kolodinski, et al.18, have describedan increase in cell efficiency through the creation ofmultiple electron-hole pairs upon a single incident pho-ton.

This is supported by similar work (Figure 8) publishedby Semonin, et al.,19 confirming that EQEs above 100%can be achieved via MEG(Figure 9).20 The intention isto reduce energy loss via heat, by utilising carriers ex-cited to higher levels in the conduction band, in orderto generate more electron-hole pairs.

4 Mechanisms

4.1 Down Conversion

The general concept behind down conversion involvessplitting the energy received from energetic photonsinto multiple lower energy photons, prior to absorp-tion into the PV active material.

Nanoparticles prove to be extremely useful when utilis-

Figure 8

Graph of EQE (%) vs Photon Energy (eV), showing the

energies at which the EQE surpassed 100%.

Figure 9

Schematic diagram detailing multiple exciton generation.

An incident photon induces the creation of two e-/h+

pairs (b and c). Non-radiative relaxation follows via

phonon emission (d). The resulting biexciton decays into

single exciton. in this diagram, Franceschetti, et al. have

denoted dissipative processes as dashed arrows and

energy-conserved processes as the solid arrows.

ing this mechanism. In figure 10, van Wijngaarden, etal.21 have proposed a mechanism for the Pr+3 - Yb+3

couple. The quantised energy of an electron excitedinto the conduction band in such nanoparticle systems,will not be transferred as heat to the lattice. Instead,the energy is transferred to a neighbouring quantumdot, which can then excite an electron into the con-duction band of this neighbouring nanocrystal. Twodifferent excitons have essentially been produced fromthe irradiation of a single, high energy photon.

Now, lower energy photons, produced from the radia-tive recombination of the QDs, may be absorbed bythe PV material, provided other recombination meth-ods can be supressed.

Yang, et al.22, proved that down conversion using quan-tum dots, can be enhanced even further, by embeddingthe QDs into fabricated photonic crystals, which im-proves the conversion efficiency in contrast to the loneQDs.

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Figure 10

(a) energy level scheme depicting the simultaneous

transfer of the Y b+3 ions. (b) Down conversion - the 1G4

state is used as a bridging level.

Figure 11

Triple-band intermediate cell.

4.2 Intermediate Band Cell

Figure 113 is a schematic representation of a solar cellwith one intermediate band, which helps to utilise awider range of incident spectral light. Luque, et al.,23

attempted to increase cell efficiency upon addition of athird impurity band, and proved that the efficiency ex-ceeded the Shockley-Quiesser limit both for ideal anddouble-tandem cells.

However, the addition of an intermediate level may in-crease the rate of recombination. This effect is inves-tigated by Ichimura, et al.24, where it was duly con-cluded, that implanting silicon wafers with hydrogendid not produce significant efficiency improvements.This suggests that the choice of material when consid-ering intermediate level addition is also an importantfactor.

4.3 Hot Carrier Solar Cell

Energy losses in the form of phonons may be avoidedthrough the use of a ’hot carrier cell’, appropriatelynamed for its ability to help eliminate heat losses. Fig-ure 123 shows how the energy that would traditionallybe lost in a conventional cell, may be stored by a ‘hotcarrier’. In addition, figure 13 shows how the use ofquantum dots in hot carrier cells may also prove ben-eficial.3

Figure 12

(a) Carriers have reached thermal equilibrium withinthe lattice. (b) Hot carrier distribution - carriers can

be held in higher energy states (storing excessenergy).

Figure 13

Simple schematic of the use of quantum dots in hot carrier

solar cells.

5 Dye-sensitised Solar Cells

Dye-sensitised solar cells (DSSC), are based on a photo-electrochemical system as opposed to a p-n junction.25

They are promising for solar energy conversion, due totheir low production costs and energy-efficient manu-facturing process, usually favoured when aesthetics areimportant.

DSSC’s possess a similar theoretical conversion effi-ciency of the Si solar cell,26 and are currently the mostefficient27 of the excitonic cells.

Developments have been made regarding the efficiencyand economic viability of DSSCs in recent years. ‘Liq-uid electrolyte-based dye sensitised solar cells’ havemade advancements through the introduction of TiO2

nanoparticle films used to capture more incident pho-tons, promoting dye absorption.27 Efficiencies for thistype of DSSC have been improved, by using varyingnanoparticle morphologies as opposed to spherical par-ticulates, in order to reduce energy losses.28 29

Smooth 1D nano arrays have been seen to possess su-perior properties over nano-particulates,30 however, donot have sufficient roughness for dye attachment.31 In-vestigations have been undertaken, to raise the rough-ness factor of the nanoarrays. Varghese, et al.32 de-posited titanium films onto fluorine-doped glass sub-

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Figure 14

Results from investigation by Standridge, et al., showing

the IPCE (%) vs irradiation wavelength (nm).

strates and noted that the fill factor for DSSCs wasapproximatelt 25% lower than the nanoparticle counterparts. The group suggested that this occurred due toan increase in width at the TNO-FTO interface andthat by increasing the nanotube roughness factor, thisproblem could be resolved and greater efficiencies couldbe achieved. Liao, et al.33 confirmed this hypothe-sis, by proving that an increase in surface roughnessof TiO2 nano architectures, will create surfaces moreadapt for sunlight harvesting and dye absorption.

Standridge, et al.34 showed that the combination of sil-ver nanoparticles and dye, promoted the production ofmore electrons than in TiO2 or the dye alone. Figure14 displays part of the group’s results, which clearlyshow that at almost all wavelengths, the incident pho-ton conversion efficiency (IPCE) is higher with the NP-Dye combination.

Controlling the plasmonic frequency of nanoparticlesfor efficiency optimisation, has proven to be a difficulttask. However, groups have discovered that variationin nano architecture morphology, does shift the plas-mon frequency and boost the IPCE in DSSCs.35 36

There is no doubt on the success in the enhancementof DSSCs through the use of nanostructured materi-als, however currently, their economic viability is notpromising, due to the complex methods involved intheir syntheses.3 37

6 Polymer-hybrid Cells

Polymer-hybrid cells offer easier manufacture, in com-bination with high absorption coefficients, typical ofpolymers (105cm−1).38 They offer large scale solar en-ergy conversion with thin cells at lower costs.

The disadvantage, however, being that PV devices con-taining polymers tend to possess poor charge trans-port,39 suggesting that if maximum efficiencies are re-

Figure 15

Scanning electron microscopy (SEM) images of different

nanowire arrays; (a) Si, (b) ZnO, (c) InGaN.

quired, nanowire arrays may be ideal.40

7 Semiconductor Nanowires andNano architectures

Nanowires can be identified as long flexible rods with adiameter in the range 1 – 100 nm as seen in figure 15.41

In contrast to the bulk, the nanoscale materials havepromising prospects for higher efficiency photovoltaicdevices.

Conventional silicon wafers often have to be thick toabsorb enough light, adding to the recombination rate.Nanowires may offer a solution to this problem by re-ducing the diffusion length of the minority charge car-riers. Kato, et al.42 investigated surface passivationtechniques on nanowires and discovered that atomiclayer deposition (ALD) did increase the lifetime of thecharge carriers. Nanowire diameters also affect resis-tance43 suggesting a change in the occupied volume byconduction carriers.

Studies based on Si-nanowire PV applications, revealthat longer nanowires enhance the absorption and ge-ometry based on nanowire arrays provide more efficientlight scattering, in comparison to thin films.44 Fang, etal. discovered that although ’slantingly-aligned’ nanowirearrays are superior to the original vertical alignment,they are still limited by a high rates of surface recom-bination.45

Numerous groups have undertaken research on the P3HT- ZnO nanowire arrays. Greene et.al46 significantlyincreased efficiencies through the addition of polycrys-talline TiO2 with confirmation of this result from Plank

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et.al47 through the incorporation of an MgO shell.

8 Conclusion

Many aspects of solar technologies currently on themarket have been discussed, as well as the importanceof developing innovative new materials, to keep up withthe ever increasing demand for energy and the ongoingquest to reduce environmental impacts.

It is evident that there are countless applications forthe incorporation of nanomaterials into PV devices,ranging from technologies modelled purely around nano-materials to small additions such as quantum dots ornanoparticle-dye mixtures. If researchers can sufficientlysuppress the current limitations, that some advancedconcepts pose, nanomaterials really could be the keyto efficient, sustainable energy. There is no doubt thatthe future is bright for solar energy.

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