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Application of a CdS nanostructured layer in inverted solar cells

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2013 J. Phys. D: Appl. Phys. 46 495114

(http://iopscience.iop.org/0022-3727/46/49/495114)

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 46 (2013) 495114 (8pp) doi:10.1088/0022-3727/46/49/495114

Application of a CdS nanostructuredlayer in inverted solar cellsD O Grynko1, O M Fedoryak1, P S Smertenko1, N A Ogurtsov2, A A Pud2,Yu V Noskov2 and O P Dimitriev1

1 V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, Kyiv, Ukraine2 Institute of Bioorganic Chemistry and Petrochemistry, NAS of Ukraine, Kyiv, Ukraine

E-mail: dimitr@isp.kiev.ua

Received 15 May 2013, in final form 11 October 2013Published 20 November 2013Online at stacks.iop.org/JPhysD/46/495114

AbstractAn inverted solar cell has been constructed by the growth of a CdS layer on an indium tinoxide (ITO)/glass substrate followed by spin-coating of organic poly(3-hexylthiophene) :[6,6]-phenyl C61-butyric acid methyl ester (P3HT : PCBM) or a zinc phthalocyanine derivative(ZnPc-4R): PCBM composite layer and using a free-standing PEDOT : PSS composite film asthe top electrode. It has been found that CdS plays the role of an electron-selective(hole-blocking) layer to rectify the electron flow from the P3HT : PCBM or ZnPc-4R : PCBMbulk heterojunction to the ITO electrode. By changing the morphology of the CdS layer from atextured continuous film to a nanowire array we demonstrate an improved collection efficiencyof charge carriers due to the increased organic–inorganic interface area. Thus, a bifunctionalrole of the CdS nanostructured layer as a bottom electrode with better electron affinity thanITO and as a means to increase the interface for better electron collection from the organicactive layer has been demonstrated.

(Some figures may appear in colour only in the online journal)

1. Introduction

Versatile 1D nanostructures made of nanowires (NWs),nanorods or nanotubes of semiconducting metal oxides, II–VIcompounds, silicon, etc, have attracted great interest inphotovoltaic (PV) applications due to their better extraction ofcharge carriers from the bulk heterojunction (BHJ) structureas compared to flat and even nanostructured heterojunctions[1]. Recently, semiconductor NWs have been explored forthe construction of inverted solar cells as well [2, 3]. Thechallenge to reversing the layer sequence of a conventionalorganic solar cell, which normally comprises of a bottomanode, active organic layer and top metal cathode, is to preparea selective contact to the bottom cathode and to achieve asuitable layer morphology for better charge carrier collection inthe inverted structure [4]. In an inverted solar cell, the bottomelectrode is typically made of an n-type semiconductor or ametal with a low work function in order to collect the electronsfrom dissociated excitons which come from the active layerdeposited above the bottom electrode and consisting normallyof an organic donor–acceptor BHJ, whereas the top electrodein this structure is made of material with a high work functionto extract holes from the active PV layer [5–7].

Inverted solar cells have some advantages as comparedwith the conventional geometry of organic solar cells, sincethey provide higher stability in air and their fabricationdoes not require high vacuum deposition of the top metalelectrode, which is not suitable for roll-to-roll processing[8, 9]. Moreover, the inverted solar cells demonstrate a suitableperformance as compared to the conventional organic solarcells [10], and the record power conversion efficiency (PCE)of 9.2% has been reported recently [11].

In order to combine high electron affinity and transparencyof the bottom electrode in an inverted solar cell, indium tinoxide (ITO), which is a p-type semiconductor, is typically usedto support the growth of a subsequent layer of a wide gap n-typesemiconductor, such as ZnO or TiO2. ZnO (work function∼4.3 eV, energy level of the conducting band ECB ∼ 4.1 eV)and TiOx (work function ∼4.3 eV, ECB ∼ 4.4 eV) areparticularly suitable as electron-transporting layers or hole-blocking layers due to their large band gaps (Eg > 3 eV) andgood energy level matching to rectify the electron flow to theITO side [12]. In this context, various types of inverted poly(3-hexylthiophene) : [6,6]-phenyl C61-butyric acid methyl ester(P3HT : PCBM) solar cells with nanostructured ZnO or TiOx

underlayers have been developed and evaluated. In these cells,

0022-3727/13/495114+08$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 46 (2013) 495114 D O Grynko et al

a stepwise electron transfer from the P3HT excited state toPCBM, followed by electron injection from the reduced PCBMto a conduction band of the nanocrystalline ZnO or TiOx

electrode, takes place [13]; therefore the double heterojunctionstructures of P3HT–PCBM and P3HT–ZnO (TiOx) have beenclaimed to be responsible for efficient charge separation [14].

One of the recent trends in the development of invertedsolar cells is the replacement of crystalline titanium oxide orzinc oxide layers with solution-processable layers of titanium(IV) chelates [15–17] or ZnO nanocrystals [18]. However, theapplication of other semiconductors with narrower band gaps(Eg < 3 eV) as an alternative to ZnO or TiO2 have not beenstudied extensively. The reason may be due to the fact thatsuch semiconductors absorb light in the visible spectrum andtherefore they are not completely transparent and can cease orrestrict light penetration through them to the active layer of thePV structure. However, some semiconductors, such as CdS,have a modest band gap which allows light to be absorbed onlyin the restricted part of the visible spectrum. As was shownpreviously, the spectral region of light absorption of CdS iscomplementary to that of many organic compounds [19] andtherefore this semiconductor can work not only as an electron-selective electrode, but also as an active element collectinglight from the blue part of the spectrum in such structures.Particularly, the active role of CdS has been well documentedin various hybrid solar cells [20, 21]. Moreover, CdS materialis well processable and can be prepared in the form of thin filmsof different morphology, including a high area NW structure.These advantages encouraged us to explore this material asa possible alternative to wide gap semiconductors in invertedsolar cells based on organic donor–acceptor BHJs. Recently, ithas been reported that CdS films deposited by using the spraydeposition technique can indeed serve as an electron-selectivelayer in inverted solar cells [22]. However, the effect of theCdS–organic interface morphology in this type of cells hasnot been studied yet. Here we concentrate on the effect ofthe morphology of an electron-selective CdS layer on the PVperformance of inverted solar cells.

2. Experimental

2.1. Sample preparation

CdS NW crystals have been synthesized by using a close-spacesublimation technique in a vessel with hot walls. CdS powderwas sublimated with decomposition at temperatures from670–790 ◦C in a fused quartz crucible. To compensate for thesulfur shortage in the final samples, because this component ismore volatile and condensates poorly, an additional source ofsulfur at temperatures of 45–160 ◦C was used. Substrates ofMo glass or fused quartz with a predeposited layer of ITOwere placed on the graphite thermostabilized holder in thequasi-closed space at temperatures of 450–670 ◦C. The reactorvessel was pumped by a nitrogen crio-sorption pump to reachthe residual pressure of 10−6 mm Hg. After the vessel had beenheated, the vapour oversaturation was created only within thetime interval of crystal growth by an additional increase of theprecursors’ temperature by 30–50 ◦C. Synthesis duration was3–10 min.

PEDOT:PSS

P3HT:PCBM

CdS

ITO

In

Figure 1. Layer sequence in the prepared cell.

ITO substrates with two different zones were prepared assubstrates for growth of CdS crystals. The first zone was abare ITO surface to grow CdS crystals by a vapour–solid (VS)mechanism. The second zone was an ITO surface coveredby gold nucleation seeds to grow CdS crystals by a vapour–liquid–solid (VLS) mechanism. The gold nucleation seedswere prepared by deposition of Au film of about 2 nm thicknesson the substrate surface by thermal evaporation followed byannealing in vacuum. In order to compare crystal growthby VLS and VS, both substrate zones were exposed to CdSvapours under identical conditions.

The synthesized nanostructured CdS layer was then spin-coated with a 2 wt% mixture of P3HT and PCBM (1 : 1weight ratio) or zinc 2,9,16,23-tetra-tert-butyl-29H, 31H-phthalocyanine (ZnPc-4R) : PCBM (1 : 1 weight ratio) inchlorobenzene at 2000 rpm for 45 s followed by annealing at120 ◦C for 30 min under Ar. A 100 µm film of the compositeof poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT : PSS) and poly(ethylene oxide) (PEO) (3 : 2 weightratio) was used as the top electrode. To make the electricalcontact with the inverted heterostructure, the compositePEDOT : PSS-PEO film was simply pressed to the activeP3HT–PCBM layer in accord with [23], as shown in figure 1.In addition, structures with the cast PEDOT : PSS electrodewere also measured.

2.2. Measurements

Quasi-dc measurements of current–voltage (IV) characteristicswere performed by using a standard automated tester 14TKS-100. Voltage sweep was applied to the heterostructurestep by step with a 160 ms duration of each step and 90 msmeasurement time, respectively. A halogen lamp with variedpower irradiation from 3 to 100 mW cm−2 was used to obtainthe current–voltage characteristics under the light conditions.The morphology of the samples was studied by scanningelectron microscopy (SEM) using JEOL devices JSM35 andJXA8200.

3. Results and discussion

3.1. Characterization of the CdS layer

The morphology of the CdS layer was controlled by thepresence or absence of Au nucleation seeds on the substratesurface, by the vapour pressure of the reacting components,

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J. Phys. D: Appl. Phys. 46 (2013) 495114 D O Grynko et al

Figure 2. Morphology of CdS layers grown during the same process on (a) a bare ITO surface and (b) an ITO surface with Au nucleationseeds. The upper row corresponds to samples prepared at CdS powder temperature TCdS = 670 ◦C and substrate temperatureTsubst = 590 ◦C, whereas the lower row corresponds to samples prepared at TCdS = 750 ◦C and Tsubst = 630 ◦C, respectively; the synthesisduration was 10 min in both cases.

which is roughly proportional to the temperature of heating ofthe CdS powder, and by the duration of the deposition process.As expected, the use of the bare ITO substrate resulted inthe formation of a continuous polycrystalline CdS film with arough surface which grew through condensation of the vapouron the overall surface. Increased vapour pressure or synthesisduration leads to the formation of nanorod crystals on the topof this continuous layer (figure 2(a)). On the other hand,CdS NW crystals have been grown directly on the surfacewith Au nucleation seeds (figure 2(b)). The thicknesses ofthe CdS layers of different morphologies deposited in thesame technological run were different, whereas the NW layergrown on the Au seeds was virtually several times thickeras compared with the continuous layer obtained on the baresurface. That means that the Au catalyst accelerated the growthof the NW crystals through the VLS mechanism, as comparedto the continuous layer whose growth was controlled by the VSmechanism [24]. It should be noted that the increased durationof the deposition process or increased temperature of heatingof the CdS powder which gives a higher pressure of CdSvapour can result in a subsequent appearance of NW crystalswithout gold seeds as well. These are formed due to self-catalytic growth on some seed crystals of the polycrystallinelayer serving as nucleation sites (figure 2(a), bottom).

It has been found that the electrical, optical andphotoelectric properties of the CdS layer are largely dependent

on the technological parameters of the synthesis, such as thetemperature in the reaction zone, the temperature of heating ofthe CdS powder and the presence or absence of an additionalsource of sulfur.

The electronic absorption of the CdS samples had a sharpedge near 520 nm (figure 3), which is due to the band–bandelectronic excitation. It should be noted that, depending onthe synthesis conditions, some samples demonstrated a longtail behind the band absorption edge, extending to the red partof the spectrum, which can be assigned to the presence of anamorphous component (Urbach tail) or a scattering effect fromthe nanostructured surface. The band gap of the synthesizedsamples was calculated from the optical absorption spectraand was found to be markedly dependent on the temperatureof heating of the precursors, i.e., CdS and S powders, whileit was less dependent on the substrate temperature (table 1).For example, heating the sulfur powder to temperatures above120 ◦C resulted in an abnormal reduction of the band gap value.This result can be explained by oversaturation of the samplecomposition with the sulfur component and deviation from thenormal CdS stoichiometry, respectively.

On the other hand, an increased content of sulfur in thesample caused higher photoconductivity of the CdS layer. Ifthe sample stoichiometry had a sulfur shortage (S<Cd by 3–5%in samples grown without an additional sulfur source), the

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J. Phys. D: Appl. Phys. 46 (2013) 495114 D O Grynko et al

400 500 600 7000,1

0,2

0,3

0,4

0,5

0,6

0,7

Eg=2.4 eV

optic

al d

ensi

ty

wavelength, nm

Figure 3. Electronic absorption spectrum of a continuous CdS layerwith thickness of 0.25 µm, prepared via a VS growth mechanismunder conditions of TCdS = 710 ◦C, Tsubst = 532 ◦C.

Table 1. Dependence of the band gap Tg of CdS samples on thetemperature of heating of sulfur (TS), CdS (TCdS) powders andsubstrate (Tsubst). The samples have been prepared via VLS growthmechanism.

TS (◦C) 60 60 120 120 165 200Tsubst (◦C) 600 635 637 637 636 608TCdS (◦C) 750 750 720 690 747 750Tg (eV) 2.37 2.38 2.38 2.44 2.34 2.24

conductivity under light conditions (3 mW cm−2) increased byone order of magnitude over dark conductivity, whereas thesample oversaturated with sulfur (S>Cd by 2–3%) showedphotoconductivity up to three orders of magnitude higher thanthe dark conductivity. So, the deficit of sulfur leads to anoticeable decrease in photoconductivity.

The morphology of the CdS layer was found to alsoinfluence the photoconductivity of the sample. Studies ofas-prepared CdS layers showed that the photocurrent of theCdS NW arrays was higher as compared with the continuousCdS film. For example, the ratio of photocurrent to darkcurrent under applied bias of 1 V for the NW sample was127, while for the continuous film this value was only 4.Asymmetry between direct and reverse currents in the darkfor the structure ITO/CdS/In was also more significant for theNW sample, which showed a current ratio of 38 at 1 V, whilefor the continuous film this ratio was only 9 (figure 4).

3.2. Inverted PV cells with a P3HT : PCBM organic layer

Deposition of an organic film onto the CdS nanostructuredlayer results in the formation of a hybrid organic–inorganicheterojunction which is capable of exciton dissociation atthe interface normally due to the different electron affinitiesof the CdS and organic material and also due to theband bending at the surface of CdS [25]. For example,deposition of P3HT only on the CdS layer and formationof an ITO/CdS/P3HT/PEDOT : PSS structure facilitated a PVresponse under illumination. However, the performance

of this cell was too low, so the effect of the CdS/P3HTheterojunction can be neglected further. Instead, weconcentrate here on the P3HT : PCBM BHJ which producesdissociated electron–hole pairs within the organic layerquite efficiently. As is known, a P3HT : PCBM BHJsandwiched between the respective anode and cathode, forexample, an ITO/PEDOT : PSS/P3HT : PCBM/Al structure,can demonstrate a PCE approaching 5% [26, 27]. In ourcase, the P3HT : PCBM BHJ has been taken to verifythe role of the CdS layer in the inverted structure ofITO/CdS/P3HT : PCBM/PEDOT : PSS.

P3HT : PCBM has been deposited onto CdS layers ofvarious morphology and a free-standing film of PEDOT : PSS-PEO composite served as the top electrode. The polymer filmwas clamped onto the sample surface instead of the usualspin-coated PEDOT : PSS layer for the following reasons.First, the free-standing film does not cause irreversible changesin the sample and allows one to further optimize the thicknessof the P3HT : PCBM active layer by subsequent depositions ofadditional layers, if necessary. Second, the free-standing filmprovides a fast method of measurements and does not requireadditional steps, such as drying and annealing, etc, of the topelectrode [18].

The structure of ITO/CdS/P3HT : PCBM/PEDOT : PSS-PEO showed an ability to separate negative charges at theITO electrode and positive charges at the PEDOT : PSS-PEOcomposite electrode under illumination. That means thatCdS serves as an electron-selective layer in this structurewith a high affinity to electrons, so that electrons afterdissociation of the electron–hole pairs at the PCBM/P3HTinterface go to CdS, while holes move to the PEDOT : PSS-PEO electrode, respectively. Indeed, the energy diagram(figure 5(a)) shows that P3HT plays the role of donor inthis assembly, since its lowest unoccupied molecular orbital(LUMO) level (ELUMO = 3.0 eV [28]) is above that ofany other component, therefore P3HT can donate a photo-excited electron to either CdS (Ec = 4.4 eV) or PCBM(ELUMO = 3.8 eV). Whereas the energy level offset betweenP3HT and CdS is high, the electron transfer rate from P3HTto PCBM is very fast and occurs within a picosecond ofphotoexcitation of the conjugated polymer, which guaranteesthat the quantum efficiency for charge transfer at this interfaceapproaches unity [29–31], with electrons on the PCBMnetwork and holes on the polymer network. On the otherhand, the response speed of CdS is of the order of hundredsof microseconds at best [32]. Therefore, the charge transferfrom P3HT to PCBM prevails that from P3HT to CdS. Thisconclusion is consistent with the much higher performance ofthe CdS/P3HT : PCBM composition (table 2) as compared tothe CdS/P3HT heterostructure; PCE of the latter under thesame conditions was of the order of 10−4%. In order toconfirm the role of CdS, an ITO/P3HT : PCBM/PEDOT : PSS-PEO structure without a CdS layer was studied separately,but it did not show a noticeable PV response, therefore thecontribution of the CdS layer to the PV performance of theinverted solar cell cannot be rejected. It should be notedthat in the ITO/CdS/P3HT : PCBM/PEDOT : PSS structure,PCBM is an acceptor in respect to P3HT, but it is a donor

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Figure 4. Current–voltage characteristics of an ITO/CdS/In assembly (a) in the dark and (b) under illumination with continuous textured(filled symbols) and NW (open symbols) CdS morphology. Square symbols correspond to positive potential and circles to negative potentialon ITO.

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

hole transfer

electron transfer

vacuum level

5.0 5.1

3.0

6.1

3.7

6.8

4.44.7

PEDOT:PSSP3HT

PCBM

CdSITO

Ene

rgy,

eV

(a) (b)

-7.0

-6.5

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-5.5

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-4.5

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hole transfer

electron transfer

vacuum level

5.0 5.3

3.3

6.1

3.7

6.8

4.44.7

PEDOT:PSSZnPc

PCBM

CdSITO

Ene

rgy,

eV

Figure 5. Energy level diagrams showing the relative positions of energy levels of the components of the inverted PV cells with a CdS layerand (a) a P3HT : PCBM BHJ, (b) a ZnPc-4R : PCBM BHJ.

Table 2. PV performance of PV structures with organic P3HT : PCBM layer and different morphologies of CdS layer under illumination of3 mW cm−2.

Amount of Morphology of Uoc Isc

PCBM : P3HT layers CdS layer (V) (µA cm−2) FF PCE (%)

One layer Textured film 0.45 2.2 0.33 9.6 × 10−3

Nanowire array 0.3 1.8 0.33 5.4 × 10−3

Two layers Textured film 0.45 1.0 0.30 4.1 × 10−3

Nanowire array 0.3 9.0 0.33 2.7 × 10−2

in respect to CdS since its LUMO level lies above the Ec ofCdS (figure 5(a)).

Advantages of a NW CdS layer as compared to the texturedcontinuous one in PV cells were demonstrated using a samplewith two regions. The first region of the sample was acontinuous textured CdS film with thickness of about 0.8 µm,which was grown through the VS mechanism, whereas thesecond region was a NW array with total thickness of about2.0 µm and NW diameter of the order of 100 nm, which wassynthesized via the VLS mechanism (figure 6). An equaloverlayer of P3HT : PCBM was spin-coated onto the both

regions, so that the effective thickness of this layer (∼100 nm)was the same in each region. The IV measurements with thetop PEDOT : PSS-PEO electrode showed that the continuoustextured film had a higher performance, with PCE almost twiceabove that of the NW PV cell (table 2). It should be noted thatthe above thickness of the organic layer is insufficient for athicker NW structure which resulted in poorer PV performanceof this cell. When an additional layer of P3HT : PCBM wasdeposited onto the structure, the performance of the NW cellimproved substantially, which showed an increase in bothphotocurrent and PCE by a factor of five (table 2). Thus, an

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J. Phys. D: Appl. Phys. 46 (2013) 495114 D O Grynko et al

(a) (b)

0,01 0,1 1 101E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0,01

3

4

2

1

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Voltage, V0,01 0,1 1 10

1E-10

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1E-5

1E-4

1E-3

0,01

0,1

1

3

4

2

1

Current, A/cm2

Voltage, V

Figure 6. Current–voltage characteristics of an ITO/CdS/(PCBM : P3HT)/PEDOT : PSS cell with (a) a CdS NW array and a doubledorganic layer; (b) textured continuous CdS and a single organic layer. Filled symbols correspond to dark measurements, open symbols tolight measurements; squares correspond to positive potential on ITO, circles to negative potential on ITO. Morphologies of the respectiveCdS layers are indicated.

Table 3. PV performance of inverted solar cells with one layer ofP3HT : PCBM and different morphologies of thin CdS layers(0.3–0.5 µm) under illumination of 100 mW cm−2.

Morphology of Uoc Isc, PCECdS layer (V) (µA cm−2) FF (%)

Textured film 0.27 89 0.21 5.0 × 10−3

Nanowire array 0.43 310 0.25 3.3 × 10−2

optimal thickness of the organic layer should be applied in sucha type of structure. This optimal thickness is dictated by therequirement that the entire inorganic layer should be soakedand also its surface should be covered to prevent a shortcutwith the top electrode.

The other way to adequately compare the inverted solarcells with a P3HT : PCBM layer of the same thickness is toreduce the thickness of the NW array itself and to make itcomparable with the thickness of the P3HT : PCBM layer.Inverted solar cells prepared with a single P3HT : PCBMlayer (∼100 nm) and thin CdS layers of textured and NWmorphology with the drop-cast PEDOT : PSS top electrodeagain showed a difference in PV performance, with the superiorperformance of cells based on the NW array (table 3).

The higher performance of the optimized NW cell ascompared to the cell based on the continuous textured CdSlayer can be explained by the increased interface area inthe NW structure, which allows it to collect more chargecarriers from the adjacent organic active layer and to get

a higher photocurrent, respectively, as compared to the flatheterojunction.

3.3. Inverted PV cells with a ZnPc-4R : PCBM organic layer

The donor–acceptor pair of zinc phthalocyanine (ZnPc)–fullerene C60 is used in organic PV cells both in bilayer and BHJgeometry, where high absorption coefficients and reasonablehole and electron conductivities are provided [33–35]. Here,we demonstrate that soluble derivatives of the above donor andacceptor components, i.e., ZnPc-4R and PCBM, respectively,are able to form BHJ structures which successively functionin an inverted solar cell. Similar to the P3HT in theP3HT : PCBM BHJ cell, ZnPc-4R plays the role of donorin respect to both PCBM and CdS in the inverted structure(figure 5(b)). Under illumination, the PV response of the cellwas measured with a negative charge on the ITO electrodeand a positive charge on the PEDOT : PSS-PEO compositeelectrode. In order to confirm the role of CdS, an ITO/ZnPc-4R : PCBM/PEDOT : PSS-PEO structure without a CdS layerwas prepared and studied separately, however, no PV responsewas observed; therefore, the contribution of the CdS layerto the PV performance of the inverted solar cell is evident.CdS again plays the role of the electron-selective layer in thisstructure, similar to what was observed above for the cell basedon P3HT : PCBM BHJ. However, the PV performance of theITO/CdS/ZnPc-4R : PCBM/PEDOT : PSS-PEO cell was muchlower than that based on P3HT : PCBM, with Uoc = 0.15 V,

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J. Phys. D: Appl. Phys. 46 (2013) 495114 D O Grynko et al

Jsc = 2 × 10−8 A cm−2, FF = 0.25, PCE = 2.5 × 10−5%under illumination of 3 mW cm−2. The possible reason isthat ZnPc-4R molecules in this structure form much shorterassociates as compared with P3HT polymer chains, whichmeans that holes should overcome a higher amount of barriersin the ZnPc-4R network when moving to the anode.

4. Conclusions

Inverted solar cells with CdS films of different morphology asan electron-selective layer have been demonstrated. It shouldbe noted that the obtained performance of our inverted PVcells was low. There are a few ways in which to optimizethem further. First, the thickness of the CdS layer used inthe above cells (∼1 µm) is high and it also requires relativelythick organic layers. As is known, an optimized thicknessof the organic layer in PV cells should be of the order of100 nm, which allows one to completely absorb the visiblelight and to keep the length of charge carrier pathways short.The increased thickness of the organic layer means that onlyone part of this layer absorbs light and generates excitons,while the other part is passive and contributes to the seriesresistance only. In order to overcome this drawback, morethin NW layers should be used. On the other hand, a reducedlength of CdS NWs in more thin layers yields a decreasedaspect ratio (ratio of the NW length to its diameter), whichmay result in poorer carrier collection. Therefore, specialtechnological efforts should be undertaken in order to reducethe NW diameter, which is currently of the order of 100 nmfor conventional CdS synthesis routes. Second, the surface ofCdS NWs should be well controlled in order to provide goodcontact with the organic counterpart and to serve as a chargesink, respectively. Some phenomena, such as surface charging,band bending, etc., can affect surface quality and thereforerequire detailed studies as applied to CdS NWs. Third, theCdS layer can work not only as a selective electrode, but also asan active component of the PV cell, for example, participatingin the dissociation of excitons generated in the organic layer.In this case, the distance between the CdS NWs should bewell controlled and be equal to the exciton diffusion lengthwhich is of the order of 10 nm for the organic PV material [24].Taking into account that the CdS NW diameter is of the order of100 nm, there will be a great challenge for technology to reduceboth the diameter and the interwire distance in the respectivestructure. Fourth, we mainly used a clamped top contact ofPEDOT : PSS-PEO composite free-standing film in the abovestructure. The clamped contact was used only for purposeof demonstration of how the cell works and requires furtheroptimization. For example, a spin-coated film is supposed toprovide better physical contact with the underlying layer andthus a higher ability to extract photocurrent. Solution of theabove problems is believed to allow one to construct invertedPV cells with better performance.

Acknowledgments

This publication is based on work supported by AwardNo UKE2-7035-KV-11 of the US Civilian Research and

Development Foundation (CRDF). The work was alsosupported by the State Agency for Science, Innovation andInformation of Ukraine.

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