Multifunctional Role of Nanostructured CdS Interfacial Layers in Hybrid Solar Cells

Copyright © 2014 American Scientific PublishersAll rights reservedPrinted in the United States of America

ArticleJournal of

Nanoscience and NanotechnologyVol. 14, 1–7, 2014www.aspbs.com/jnn

Multifunctional Role of Nanostructured CdSInterfacial Layers in Hybrid Solar Cells

D. O. Grynko1, O. M. Fedoryak1, P. S. Smertenko1, N. A. Ogurtsov2,A. A. Pud2, Yu. V. Noskov2, and O. P. Dimitriev1�∗

1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, Kyiv, Ukraine2Institute of Bioorganic Chemistry and Petrochemistry, NAS of Ukraine, Kyiv, Ukraine

We demonstrate here a multifunctional application of CdS layers with nanotextured and nanowiremorphology in four types of hybrid solar cells, i.e., (i) nanocrystal-polymer cell, (ii) nanocrystal-organic donor–acceptor bulk heterojunction (BHJ) inverted cell, (iii) nanocrystal-dye sensitized solidstate cell and (iv) nanocrystal-dye sensitized electrochemical cell. The role of CdS layer in each typeof the above cells has been elucidated and the photovoltaic (PV) performance of the PV cells hasbeen compared. It is shown that CdS acts as acceptor in the cells of types (i) and (iii), while it playsthe role of an electron-selective (hole-blocking) layer to direct electrons from the organic counterpartto anode in the cases (ii) and (iv). Morphology of the CdS layer makes a noticeable effect on the PVperformance. In particular, the nanowire array demonstrated an improved efficiency of collection ofcharge carriers as compared with the continuous textured surface due to the increased organic-CdSinterface area in PV cells of practically all types. It is demonstrated that the same nanocrystal-dyestructure can operate either as PV cell of type (iii) or PV cell of type (iv).

Keywords: CdS, Hybrid Solar Cell, Photovoltaics, Morphology, Nanowire.

1. INTRODUCTIONRecently, hybrid organic–inorganic solar cells based oninorganic semiconductor crystals and organic moleculeshave attracted great attention due to potential of use ofmutual advantages of the both materials in the samedevice.1 As to inorganic semiconductors, different com-pounds, such as II–VI semiconductors and metal oxideshave been applied in hybrid solar cells. CdS has attractedmuch attention due to its relative cheapness and proces-sibility in the form of thin films and nanocrystals, aswell as due to its promising applications in various fields,such as photonics,2,3 photoconducting elements,4 logicgates,5 field effect transistors,6,31 solar cells,8 etc. Overthe past few years, tremendous efforts have been made todecrease the size and to control the shape of CdS crys-tals, and a number of new methods have been reportedfor the synthesis of CdS nanocrystals integrated into one-dimensional nano- or microstructures.9,10 Use of semicon-ductor nanowires (NWs) instead of planar films, texturedsurfaces, or even semiconductor nanoparticles is one of

∗Author to whom correspondence should be addressed.

the perspective directions to achieve higher performanceof hybrid solar cells.11 Application of semiconductor NWarrays in solar cells has potential advantages, such as bet-ter light absorption due to the reduced reflection and lighttrapping (shadow effect) and better charge collection fromthe active layer, since charge carriers move straight tothe respective electrode through a NW crystal.12 Hybridsolar cells based on CdS crystals of different morphology,such as single crystals,13 polycrystalline films,14,15 quan-tum dots,16,17 and NW crystals18–22 have been investigatedin the last decade. However, the promising potential of CdSNW morphology in solar cells has not been unambiguouslydemonstrated yet. So far, NW-based solar cells showedlower efficiencies as compared to planar cells made fromthe same materials.22–24 For example, inorganic core–shellstructures based on CdS NW core and CdTe shell25 showeda modest power conversion efficiency (PCE) of 6.0% versus16.5% for the best planar cells of the same composition.26

Recently, comparable PV performance of NW and planarsolar cells based on CdS has been demonstrated for CdS–Cu2S

27 and CdS–Cu(In,Ga)Se2 (CIGS)28 heterostructures.In the latter case, for example, NW-based cell showed PCE

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of 6.18% which is 28.7% higher than that of the equiva-lent CIGS solar cells containing chemically deposited CdSthin film. The benefit of the NW structure for solar cells,therefore, is not evident from the obtained data and furtherstudies are needed in order to clarify the potential of theNW based solar cells.In this work, we provide systematic studies of different

types of solar cells, i.e., nanocrystal-polymer cells, dye-senstized solar cells (DSSCs) and inverted donor–acceptorsolar cells based on the template of CdS with the planar(textured) and NW morphologies. We show that PV per-formance of the cells of the same structural composition isalways better for the NW morphology. Thus, the potentialof the NW morphology in hybrid solar cells is unambigu-ously proved in this work.

2. EXPERIMENTAL DETAILSCdS NW crystals have been synthesized by close spacesublimation technique in the vessel with hot walls. CdSpowder was sublimated with decomposition at tempera-tures 670–790 �C from the fused quartz crucible. For com-pensation of sulfur shortage in the final samples, since thiscomponent is more volatile and condensates more slowly,an additional source of sulfur at temperatures of 45–160 �Cwas used. Substrates of Mo glass or fused quartz withpredeposited layer of ITO were placed on the graphitethermostabilized holder in the quasi-closed space at tem-peratures of 450–670 �C. The reactor vessel was pumpedby nitrogen to reach a residual pressure of 10−6 mm Hg.After the vessel has been heated, the vapor oversaturationwas created by additional raise of the precursor tempera-ture by 30–50 �C within the time interval of crystal growth.Synthesis duration was 3–10 minutes.ITO substrates with two different zones have been pre-

pared as substrates for growth of CdS crystals. The firstzone was a bare ITO surface to grow CdS crystals byvapor–solid (VS) mechanism. The second zone was anITO surface covered by gold nucleation seeds to growCdS crystals by vapor–liquid–solid (VLS) mechanism.The gold nucleation seeds were prepared by depositionof a thin Au film of about 2 nm thickness on the sub-strate surface by thermal evaporation followed by anneal-ing in vacuum. Film thickness was controlled via quartzmicrobalance during deposition and calculated through themass uptake value.In order to compare crystal growth by VLS and VS

the both substrate zones were exposed to CdS vaporsunder identical temperature conditions. As expected, theVS growth resulted in formation of nanotextured surfaceof CdS layer, while VLS growth in formation of NW crys-tal arrays (Fig. 1).An organic overlayer has been deposited on the CdS

layer using different organic materials, such as poly(3-hexylthiophene) (P3HT), zinc 2,9,16,23-tetra-tert-butyl-29H, 31H-phthalocyanine (ZnPc-4R), or blend of P3HT

Figure 1. Typical morphology of CdS layers obtained on ITO surfacesby different growth techniques. (a) VS growth on the bare substrate sur-face with the increasing synthesis duration (from the top to the bottom);an early stage of nucleation of NW crystals due to self-catalytic processcan be seen in the middle and bottom images. (b) VLS growth startingfrom Au nucleation seeds (the top image) with the increasing synthesisduration (middle to bottom images). Note that the images are tilted by450 in respect to the figure plane to show the layer thickness on the leftpart of the images.

and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)(1:1 weight ratio) to prepare nanocrystal-polymer,nanocrystal-dye and nanocrystal-organic donor–acceptorbulk heterojunction (BHJ) cells, respectively. Organic solu-tions were prepared in chlorobenzene with concentra-tion of 2 wt% and spin-coated on the CdS surface at2000 rpm for 45 s followed by annealing at 120 �Cfor 30 min under Ar. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) top electrode hasbeen prepared by drop-casting of PEDOT:PSS aque-ous solution (1.3 wt%) mixed with dimethyl sulfoxide(6 vol%) followed by annealing of the structure at 120 �Cfor 5 min.Two types of solid state DSSCs have been prepared, i.e.,

by (i) using hole-transporting layer (HTL) and (ii) with-out HTL. Dye molecules were deposited onto the CdSsurface via adsorption/spin-coating from chlorobenzenesolutions, using soluble ZnPc-4R or vanadyl 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine (NpPcVOR) or byclose space sublimation of unsubstituted ZnPc molecules.In the latter case the temperature of PcZn sublimationwas about 440 �C, whereas the CdS substrate was keptat 410–430 �C. For the first type of cells the CdS/dyeassembly was spin-coated with P3HT as a HTL. Then, the

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top PEDOT:PSS electrode was drop-cast onto the HTLas descrbed above, whereas for the cells without HTL a100 �m free-standing composite film of PEDOT:PSS andpoly(ethylene oxide) (PEO) (3:2 weight ratio) was usedas the top electrode in order to escape shortcut to theCdS layer. To make the electrical contact this compos-ite PEDOT:PSS-PEO film was simply pressed to the dye-sensitized layer as described in Ref. [29].Electrochemical DSSCs were assembled on the

CdS/ITO electrode (photoanode) clipped together withglassy carbon counter electrode. A small gap (∼ 100 �m)between the electrodes was made by a Parafilm tape placedat the periphery of the CdS layer. This gap was filledby injecting a polysulfide aqueous electrolyte containing1 M Na2S and 1 M S. I–V measurements were performedby using a potentiostat PI-50-1 (Belarus) interfaced to acomputer or by HP 4140B device. The active area of themeasurements was several square millimeters. White lightillumination of the samples was provided by a 50 W halo-gen lamp. The results below are referred to light intensityof 100 mW/cm2, otherwise, the other light intensity is indi-cated in some specific cases.

3. RESULTS AND DISCUSSION3.1. CdS Nanocrystal-Polymer CellsDeposition of an organic film onto the CdS nanostructuredlayer resulted in formation of a hybrid organic–inorganicheterojunction which is capable of exciton dissociation atthe hybrid interface due to different electron affinities ofCdS and organic material and also due to the band bend-ing at the surface of CdS.30 PV cells based on P3HT/CdSheterojunction have been studied recently elsewhere, withconcentration on the effects of CdS surface modificationwith different chemical agents15,31 or CdS layer modifica-tion itself (thickness, CdCl2 content32). However, no pro-duction and comparison of samples possessing differentinterface morphology and the same composition have beendemonstrated.Studies of the as-prepared bare CdS layers showed that

photosensitivity of NW arrays was higher as comparedwith the textured CdS films. For example, the ratio of pho-tocurrent to dark current under applied bias of 1 Volt forthe NW sample was 127, while for the textured film thisvalue was only 4. Asymmetry between direct and reversecurrents in the dark for the structure of ITO/CdS/In wasalso more significant for the NW samples.In the hybrid PV cell, both CdS and P3HT in the

hybrid PV cell can generate excitons which dissociateat the organic–inorganic interface with holes moving tothe P3HT and electrons moving to CdS. P3HT plays therole of donor in this assembly, since its LUMO level(ELUMO = 3�0 eV33) is above the conducting band of CdS(Ec = 4�4 eV), therefore P3HT can donate a photo-excitedelectron to CdS (Fig. 2). The performance of the PVcells was found to be of the same order of magnitude

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rgy,

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Figure 2. Energy level diagram showing relative positions of energylevels of the components of the nanocrystal-polymer PV cell. Surfaceband bending of CdS is not shown.

as similar cells with unmodified CdS surfaces reported inRefs. [15, 31]. However, here we revealed a substantial dif-ference in PV performance depending on the CdS layermorphology (Fig. 3). PCE of the NW based cell was higheras compared to the cell based on the textured interfacemorphology by ca. 450%, mainly due to improved pho-tocurrent (Table I). At the same time, fill factor of the NWcell was low, pointing out that incidental recombination ofcharge carriers at the NW interface takes place.

3.2. CdS Nanocrystal-Organic Inverted Hybrid CellsIn this section, we concentrate on inverted PV cellswith P3HT:PCBM BHJ layer which produces dissociatedelectron–hole pairs within this organic layer followed bycapture of electrons by electron-selective inorganic layernear cathode. Although CdS has recently been studied asan electron-selective layer in organic inverted solar cells,34

no effect of the interface morphology on PV performancewas shown.The structures of ITO/CdS/P3HT:PCBM/PEDOT:PSS

showed ability to separate negative charges at the ITOelectrode and positive charges at the PEDOT:PSS electrodeunder illumination.That means that CdS serves as an electron-selective

layer in this structure with a high affinity to electrons,so that electrons after dissociation of the electron–holepairs at the PCBM/P3HT interface go to CdS, while holesmove to the PEDOT:PSS electrode, respectively. Indeed,

Table I. PV performance of CdS-polymer solar cells with CdS layersprepared under the same temperature conditions.

Morphology Uoc IscPV structure of CdS (V) (�A/cm2) FF PCE (%)

ITO/CdS/P3HT/ Textured film 0.230 24 0.41 2�2 ·10−3

PEDOT:PSS Nanowire array 0.347 100 0.30 1�0 ·10−2

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Multifunctional Role of Nanostructured CdS Interfacial Layers in Hybrid Solar Cells Grynko et al.

0.01 0.1 11E–8

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rent

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sity

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dark

light

Figure 3. I–V curves of the polymer ITO/CdS/P3HT/PEDOT:PSS cellsbased on textured (circles) and NR (squares) layers of CdS.

the energy diagram (Fig. 3) shows that P3HT plays therole of donor in this assembly, since its LUMO level at3.0 eV is above that of any other component, thereforeP3HT can donate a photo-excited electron to either CdS(Ec = 4�4 eV) or PCBM (ELUMO = 3�8 eV). Whereas theenergy level offset between P3HT and CdS is high, theelectron transfer rate from P3HT to PCBM is very fastand occurs within a picosecond of photoexcitation of theconjugated polymer, which guarantees that the quantumefficiency for charge transfer at this interface approachesunity,35–37 with electrons on the PCBM network and holeson the polymer network, respectively. On the other hand,the response speed of CdS is of the order of hundredsof microseconds at best. Therefore, the charge transferfrom P3HT to PCBM prevails that from P3HT to CdS.This conclusion is consistent with higher performance ofCdS/P3HT:PCBM structure (Table II) as compared to theCdS/P3HT heterostructure (Table I).In order to confirm the role of CdS, an ITO/P3HT:

PCBM/PEDOT:PSS structure without CdS layer was stud-ied separately, but it did not show noticeable PV response;therefore contribution of CdS layer to the PV performanceof the inverted solar cell cannot be neglected. It shouldbe noted that in the ITO/CdS/P3HT:PCBM/PEDOT:PSSstructure, PCBM is an acceptor in respect to P3HT, but it isa donor in respect to CdS since its LUMO level lies aboveEc of CdS (Fig. 4).Advantages of the NW CdS layer as compared to the

textured one in PV cells were demonstrated using thesamples of the different interface morphology, but a simi-lar overlayer of P3HT:PCBM. The NW cell demonstratedhigher performance, with PCE exceeding that of the PVcell based on the textured morphology by more than 600%(Table II). The NW cell showed a noticeable increase inboth photocurrent and open-circuit voltage (Fig. 5). Thehigher performance of the NW cell as compared to thecell based on the continuous textured CdS layer can be

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PCBM

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Figure 4. Energy level diagram showing relative positions of energylevels of the components of the inverted PV cell.

Table II. PV performance of inverted solar cells with different mor-phology of CdS layers.

Morphology of CdS Uoc (V) Isc (�A/cm2) FF PCE (%)

Textured film 0.27 89 0.21 5�0 ·10−3

Nanowire array 0.43 310 0.25 3�3 ·10−2

explained by the increased interface area in the NW struc-ture which allows one to collect more charge carriersfrom the adjacent organic active layer and to get a higherphotocurrent, respectively, as compared to more planarheterojunction.

3.3. CdS-Dye Sensitized Solid State CellsOne of the advantages of CdS-dye heterostructures inhybrid solar cells is light adsorption of CdS and dyemolecules in complementary parts of the visible range.13

While CdS absorbs mainly in the UV-blue part of the spec-trum, i.e., below 2.4 eV, many phthalocyanines, for exam-ple, absorb in the red part of the visible range (Fig. 6).

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Figure 5. I–V curves of the inverted ITO/CdS/PCBM:P3HT/PEDOT:PSS cells based on (1) textured and (2) NR array CdS layers.

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Figure 6. Electronic absorption spectrum of the CdS-NpPcVOR het-erostructure. Arrow shows the band gap edge of the CdS absorption.

Here we compare two types of solid state DSSCs, i.e.,the cells prepared by using HTL and without it. The cellsprepared without HTL showed very low photocurrents andPCE, respectively (Table III). This low performance canbe explained by the fact that there is bad interconnectionof the molecules in the dye layer spin-coated on the CdSsurface, so that the top PEDOT:PSS-PEO pressed elec-trode used in these cells can extract charge carriers onlyfrom dye molecules which are in contact with the elec-trode material directly or which locate within a very shortdistance from it. Therefore, only the top surface layer inboth textured and NW structures can contribute to the cellperformance. At the same time, an open-circuit voltageand fill factor in NW cells are noticeably lower as com-pared with the cells based on the textured interface. Thisresult cannot be explained by small shunt resistance Rsh inthe NW cell, because Rsh was evaluated to be three ordersof magnitude higher in the NW cell as compared with thetextured one. Probably, the low Uoc in NW cells can bedue to a high recombination rate of charge carriers at theNW interface.

Table III. PV performance of solid state DSSCs (white light illumination ∼ 12 mW/cm2).

CdS morphology PV structure Uoc (V) Isc (�A/cm2) FF PCE (%)

Textured layer ITO/CdS/NpPcVOR/PEDOT:PSS 0.350 0�07 0.30 6�1 ·10−5

ITO/CdS/ZnPc-4R/PEDOT:PSS 0.300 0�11 0.30 1�0 ·10−4

Nanowire array ITO/CdS/ZnPc-4R/PEDOT:PSS 0.100 0�01 0.23 1�9 ·10−6

ITO/CdS/ZnPc/PEDOT:PSS 0.049 1�0 0.23 9�4 ·10−5

Table IV. Effect of dye sensitization in CdS-polymer solar cells.

CdS morphology Cell type PV structure Uoc (V) Isc (mA/cm2) FF PCE (%)

Textured layer Polymer cell ITO/CdS/P3HT/PEDOT:PSS 0.331 20 0.42 2�8 ·10−3

DSSC ITO/CdS/ZnPc-4R/P3HT/PEDOT:PSS 0.450 14 0.26 1�7 ·10−3

Nanowire array Polymer cell ITO/CdS/P3HT/PEDOT:PSS 0.178 56 0.30 3�0 ·10−3

DSSC ITO/CdS/ZnPc-4R/P3HT/PEDOT:PSS 0.190 205 0.32 1�2 ·10−2

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Figure 7. I–V curves of solid state DSSCs of ITO/CdS/PcZn-4R/P3HT/PEDOT:PSS based on (1) textured and (2) NW array CdS layers.

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Figure 8. Current–voltage characteristics of electrochemical cells ofITO/CdS/PcZn-4R/polysulfide electrolyte/glassy carbon based on (1) tex-tured and (2) NW array CdS layers.

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Table V. PV performance of solid state and electrochemical DSSCs based on the same CdS/dye structures.

CdS morphology Cell type PV structure Uoc (V) Isc (mA/cm2) FF PCE (%)

Textured layer Solid-state cell ITO/CdS/ZnPc-4R/PEDOT:PSS 0.340 0.6 0.30 6�0 ·10−5

Electro-chemical cell ITO/CdS/ZnPc-4R/polysulfide electrolyte/glassy carbon 0.279 35 0.40 3�9 ·10−3

Nanowire array Solid-state cell ITO/CdS/ZnPc-4R/PEDOT:PSS 0.080 0.05 0.25 1�0 ·10−6

Electro-chemical cell ITO/CdS/ZnPc-4R/polysulfide electrolyte/glassy carbon 0.292 114 0.32 1�1 ·10−2

Introduction of a HTL increases performance of thecells substantially, since HTL material penetrates into thecell pores, extracts charge carries from the higher surfacearea and has better molecular interconnection to conductcharge carriers to anode. P3HT has been used as a con-ventional HTL in DSSCs by Zhang et al.38 and it wasapplied in respect to our cells as well. It is interesting toelucidate the effect of dye sensitization in cells of differentmorphology, i.e., to compare how introduction of a dyelayer to the CdS-polymer cell influences cell performance.Such a comparison was performed for the same CdS layerto escape fine differences in CdS layer itself, which canvary from sample to sample. CdS/P3HT heterostructurewas studied first, followed by washing out of the polymerlayer, successive deposition of dye and P3HT layers andmeasurement of PV characteristics again. It was found thatfor the textured CdS layer dye sensitization almost has noeffect in terms of PCE of the cell, although deposition of adye layer resulted in reducing shunt resistance and increas-ing series resistance, with corresponding increase in Uoc

and decrease in fill factor, respectively (Table IV). At thesame time, dye sensitization was very effective in the NWstructure yielding an increase in photocurrent by almostfour times and increase in PCE by almost one order. Thiseffect certainly is due to increased interface area in theNW cell as compared to the textured morphology. As aresult, the solid state DSSC based on the CdS NW arraydemonstrated markedly better performance as comparedwith the analogous cell based on the textured CdS layer(Fig. 7).

3.4. CdS-Dye Sensitized Electrochemical CellsTo our knowledge, electrochemical DSSCs based on CdSinstead of metal oxides (TiO2, ZnO, etc.) as electron-accepting layers have not been discussed in the litera-ture yet and, therefore, we present here first results todemonstrate a possibility of the CdS-DSSCs operation asa basis for further improvement of this new compositionfor DSSCs.To compare an effect of the CdS surface morphology on

PV characteristics of these cells we used the CdS texturedfilm/ITO and CdS NW array/ITO as the cell anodes. It isinteresting to note that CdS-dye heterostructure can workboth in electrochemical and solid state cells. The sameITO/CdS/dye assembly was studied first as a solid stateDSSC with PEDOT:PSS-PEO counter electrode and thenas an electrochemical cell with polysulfide electrolyte andglassy carbon counter electrode.

The solid state cells showed a poor performance, withobvious dependence of Uoc and fill factor on the cell mor-phology (Table V). These results are in complete accor-dance with those discussed in the previous section (seeTable III). Electrochemical cells demonstrated much betterperformance as compared with their solid state analogs.The observed difference in performance of the solid stateand electrochemical cells based on the same CdS morphol-ogy can be explained by more effective electron transferfrom the dye to CdS crystal facilitated by electric field ofan electric double layer. However, PCE of electrochemi-cal cells based on CdS was still low as compared withDSSCs based on semiconductor metal oxide (TiO2, ZnO,etc.) structures.39 On the other hand, the effect of CdS mor-phology was different in respect to the solid state and elec-trochemical cells. While NW morphology showed evensome poorer performance as compared with the texturedmorphology in the solid state cells without HTL, the NWarray structure was superior in the electrochemical cells,demonstrating an increase in PCE by almost one orderof magnitude as compared with the textured analog. Theimproved performance is mainly due to increased pho-tocurrent in the NW structure, however, both Uoc and fillfactor become improved too in the electrochemical cell ascompared with the solid state cell (Table V). Thus, theNW morphology has been justified as a better choice forelectrochemical cells as well.

4. CONCLUSIONSDifferent types of hybrid solar cells based on CdS layers ofdifferent morphology and organic counterparts have beenstudied and compared. Particularly, it was shown for thefirst time operation of DSSCs based on the CdS insteadof metal oxides (TiO2, ZnO etc.) as electron-acceptinglayer and simultaneous application of the same CdS-dyestructure in both electrochemical and solid state cells.Application of CdS nanostructured layers in PV cells ofthe different type discloses multifunctional role of thismaterial in photovoltaics. Although the cells showed rela-tively low efficiency, a tendency of their increasing perfor-mance can be clearly traced. This can be taken as a basisto forecast or to model certain situations in photovoltaics.It has been clearly demonstrated that in PV cells of

different types, i.e., in organic inverted cells, hybrid CdS-polymer cells and DSSCs PCE can be enhanced up to oneorder of magnitude due to application of the NW morphol-ogy of the CdS layer instead of the planar or textured one.

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Such a dramatic increase is mainly due to the increase inphotocurrent, which evidences that charge carriers are col-lected from the higher interface area in the NW cells. Atthe same time, the NW cells showed certain drawbacks,such as a low open-circuit voltage and fill factor, whichwas suggested to occur due to high recombination of exci-tons at the NW interface. Therefore, further optimizationof PV cells based on the NW morphology is still needed.

Acknowledgment: This publication is based on worksupported by Award No. UKE2-7035-KV-11 of the U.S.Civilian Research and Development Foundation (CRDF).The work was also supported by the State Agency for Sci-ence, Innovation and Information of Ukraine.

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