ZnO–SnO2 composite anodes in extremely thin absorber layer (ETA) solar cells

Ruvini Dharmadasa, K.G. Upul Wijayantha *, Asif Ali TahirDepartment of Chemistry, Loughborough University, Loughborough, LE11 3TU, UK

a r t i c l e i n f o

Article history:

Received 25 November 2009Received in revised form 23 April 2010Accepted 3 May 2010Available online 17 May 2010

Keywords:

ETA solar cellAerosol assisted chemical vapour depositionZnOASnO2

Indium sulphideLead sulphideSurface coverage

a b s t r a c t

ZnO–SnO2 composite electrodes have been deposited on fluorine-doped tin oxide (FTO) substrates by aer-osol assisted chemical vapour deposition (AACVD) from a single source precursor solution. The electrodeswere characterised using X-ray diffraction (XRD), atomic force microscopy (AFM), field emission gunscanning electron microscopy (FEGSEM) and energy dispersive X-ray analysis (EDX). The composite elec-trodes were used to construct ETA solar cells with the following structure; FTO/ZnO–SnO2/In2S3/PbS/PED-OT:PSS/Cgraphite/FTO. Performance of the cells were characterised by measuring the current–voltage (I–V)and incident photon to electron conversion efficiencies (IPCE). The effect of Zn:Sn ratio in the precursorand effect of post deposition annealing temperature on the morphology of the composite layers, in rela-tion to the performance of the fabricated cells were investigated. The highest performing cells were fab-ricated using the composite anode deposited from 50:50 mol% Zn:Sn in the precursor with postdeposition annealing at 400 �C. I–V characterisation under AM 1.5 solar simulated light reveals thatthe cell had an open circuit voltage (Voc) � 0.32 V, short circuit current density (Jsc) � 8.2 mA cm�2, a fillfactor (FF) � 0.26, an overall efficiency (g) � 0.68% and a maximum IPCE � 30%. The experimental IPCEagrees well with theoretically estimated IPCE when the PbS surface coverage is about 0.1–0.2.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Extremely thin absorber layer (ETA) solar cells are of greatinterest in the area of excitonic solar cells, due to their potentialto produce highly efficient but low cost devices. The projected costreduction is thought to be mainly associated with reduced materialusage and low temperature manufacturing routes. The ETA cellanalogues to the solid-state version of dye-sensitised solar cells(DSC) has a typical configuration of a porous n-type semiconduct-ing metal oxide anode to facilitate electron transport, a conformallight harvesting layer (widely known as the ETA layer), and a porefilling p-type semiconducting layer for hole transport. Charge car-rier generation occurs within the ETA layer, and has a typical thick-ness of a few hundred nanometers [1]. Consequently, this layer canbe made from rarer materials while still keeping production costsdown. In2S3 [2], CuInS2 [3], CdTe [4], Sb2S3 [5], and CdSe [6] are afew of the ETA materials currently under investigation.

Studies on n-type semiconducting materials for ETA solar cellshave so far only been limited to a few materials such as TiO2

[4,5] and ZnO [2,6]. Electrodes of these materials have been depos-ited using a range of methods including doctor-blading colloidalpastes [5], electrodeposition [7] and spray-pyrolysis [8], followedby a sintering step to provide a highly porous but still intercon-nected particle network. The use of other anodic materials such

as Nb2O5 [9], and composite electrodes, i.e. ZnO–TiO2 [10], SnO2–MgO [11], TiO2–Nb2O3 [9], SnO2–TiO2 [12] and SnO2–ZnO [13–15] have been widely studied to enhance the DSC performance.Hence, the use of composite anodic electrodes in ETA cells is ofconsiderable interest. We intended to employ the composite metaloxide electrodes as photoelectrodes with the objective of optimis-ing the key ETA cell properties such as open circuit photovoltage(Voc), short circuit photocurrent density (Jsc), and fill factor (FF).To study the performance of the composite electrodes, the ZnO–SnO2 films were employed in the following ETA cell configuration;FTO/ZnO–SnO2/In2S3/PbS/PEDOT:PSS/Cgraphite/FTO (see Fig. 1). TheETA cell performance was optimised by investigating the postdeposition annealing temperature of the composite layer and bycontrolling the ratio of Zn:Sn in the precursor.

2. Experimental procedure

The ZnO–SnO2 composite layer was deposited by aerosol as-sisted chemical vapour deposition (AACVD). In the AACVD processa single precursor solution was used to deposit multi-componentmetal oxide electrodes in a single step. For the fabrication of theZnO–SnO2 composite electrodes, a single precursor solution con-taining 50:50 mol% Zn:Sn was made by dissolving 2.97 g(10 mmol) of Zn(NO3)2�6H2O (Fisher) in 200 ml of methanol usinga flask placed in an ice bath. This was followed by the addition of3.56 g (40 mmol) of N,N-dimethylaminoethanol (dmaeH, 99%,ACROS Organics) and 2.62 g (10 mmol) of SnCl4 (99%, Aldrich)

1572-6657/$ - see front matter � 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jelechem.2010.05.003

* Corresponding author. Tel.: +44 (0) 1509 222574; fax: +44 (0) 1509 223925.E-mail address: U.Wijayantha@lboro.ac.uk (K.G. Upul Wijayantha).

Journal of Electroanalytical Chemistry 646 (2010) 124–132

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and stirred for 30 min. To fully characterise the precursor by singlecrystal X-ray analysis, attempts were made to crystallise the pre-cursor from the methanol solution. The resulting product was atransparent viscous liquid, therefore, the structure of the precursorwas proposed on the basis of elemental analysis. The CHN analysiswas in a good agreement with the calculations for the proposed[ZnSn(dmae)(dmaeH)2(NO3)Cl4]�2H2O structure. Analysis. Calc.: C,20.93%; H, 5.09%; N, 8.14%; Found: C, 20.06%; H, 5.02%; N, 8.14%.

The ratio of Zn:Sn in the composite films were investigated bychanging the Zn mol% in the precursor solution in the range of0 �mol% � 100. The resulting solution with known Zn:Sn mol%was then transferred to a round bottom flask and an aerosol wasgenerated using a piezoelectric transducer. This aerosol was thendirected to the deposition chamber using air (500 ml min�1) as acarrier gas [16]. Three fluorine-doped tin oxide (FTO) glass sub-strates (TEC 8, 8X/h, Pilkington) were placed side by side in depo-sition chamber and aligned so that aerosol falls directly onto themiddle substrate. Decomposition of the precursor occurred in thedeposition chamber resulting in a thin film of ZnO–SnO2 compositeon the FTO. The substrate (FTO) temperature was maintained at�380 �C and the deposition was carried out for 30 min to obtainZnO–SnO2 composite electrodes with similar thickness. The ZnO–SnO2 composite electrodes were then annealed in air at tempera-tures ranging from 200 to 500 �C for 1 h.

The composite films were then used to fabricate ETA solar cellswith the following configuration; FTO/ZnO–SnO2/In2S3/PbS/PED-OT:PSS/Cgraphite/FTO. A thin layer of In2S3, was deposited by chem-ical bath deposition (CBD) on the ZnO–SnO2 composite film using aslightly modified method reported by Bayon et al. [8]. In2S3 isknown to act as a buffer layer to reduce charge carrier recombina-tion at semiconductor interfaces [17]. In a typical CBD experiment,the In2S3 layer was deposited for 30 min in a bath containing 25 mlof 0.025 M InCl3, 25 ml 0.1 M C2H5NS and 2 ml of 1 M HCl. Thedeposition process was repeated three times in order to obtainan appropriate film thickness and was then annealed in argon at300 �C for 30 min [8]. Similarly, the ETA layer of PbS was alsodeposited using CBD, and the deposition bath contained 1 MPb(CH3COO)�H2O, 1 M NaOH, 1 M CS(NH2)2, and 1 M C6H15NO3 asreported earlier [8,18]. The thickness of the PbS layer was con-

trolled by changing the deposition time and best ETA cell perfor-mance was obtained for 10 min deposition. The hole collector,PEDOT:PSS was deposited by spin coating. Fabrication of the ETAcell was completed by a Cgraphite back contact (by sandwichingthe cell with a graphite coated FTO counter electrode). The typicalarea of a completed cell was approximately 0.3 cm2.

Phase and crystallinity of the electrodes were characterisedusing a Bruker D8 X-ray diffractometer (XRD) operating withmonochromatic intensity Cu Ka (k = 1.54 Å) radiation using a PSDdetector. Surface morphology and the film thickness were studiedusing a Leo 1530 VP field emission gun scanning electron micro-scope (FEGSEM) at an accelerating voltage of 5 kV and a workingdistance of 6 mm. Elemental analysis of the films were also studiedusing a EDX detector on the FEGSEM. Surface topography of thecomposite films was also studied using the Veeco di Caliber atomicforce microscope (AFM) in contact mode, and surface roughnessanalysis was carried out using the SPM Lab Analysis V7.00 soft-ware. Current–voltage (I–V) measurements of the cells were car-ried out using a potentiostat (Eco Chemie micro-Autolab type III)while the cells were illuminated using an AM 1.5 Class A solar sim-ulator (Solar Light 16S – 300 solar simulator), at 100 mW cm�2

light intensity, calibrated by a solar pyranometer (Solar Light Co.,PMA2144 Class II). The light was manually chopped at regularintervals to record dark and photocurrent simultaneously.

Incident photon to electron conversion efficiency (IPCE) mea-surements were conducted using a 75W Xenon lamp connectedto a monochromator (Bentham, TMc300) and the system was cal-ibrated using a silicon diode (Bentham). The IPCE spectra of ETA so-lar cells were recorded at zero bias and were illuminated throughthe ZnO–SnO2 composite electrode over the 300–1100 nm range,using a chopping frequency of 11 Hz.

3. Results and discussion

3.1. Structural characterisation

The composition of the ZnO–SnO2 electrodes was studied byXRD analysis. Fig. 2a(ii) shows the diffractogram for a compositefilm deposited on FTO from a precursor solution containing100 mol% Sn. All the reflections have a good match with the crys-talline SnO2 [ICDD 01-088-0287] and also with the FTO substrate(Fig. 2a(i)). The high intensity reflections at 38.0� and 65.5� indicatethat the SnO2 follows the orientation of the FTO substrate.Fig. 2a(iii) shows the XRD pattern of the composite layer depositedon FTO from a precursor solution containing 50:50 mol% Zn:Sn.Comparing this diffractogram to that of the film made from100 mol% Sn, the intensity of the SnO2 reflections were reducedand a strong reflection appears at 58.7� which corresponds to crys-talline ZnO [ICDD 00-021-1486]. The reflections at 26.8�, 34.0� and55.0� belong to both SnO2 and ZnO. The reflection at 58.7� is onlypresent for composite films fabricated with Zn in the precursorsolution. The XRD analysis proved the formation of a compositeelectrode which was supported by EDX analysis. Further confirma-tion for the formation of ZnO–SnO2 composite was obtained bydepositing films from 50:50 mol% Zn:Sn on plain glass. The crystal-lite size was calculated by applying the Scherrer equation to thereflections at 58.7� and 51.6�, and was found to be 16 nm forZnO and 33 nm for SnO2, respectively.

Fig. 2b(i) shows the XRD of the In2S3 buffer layer deposited byCBD and annealed at 300 �C in argon. The reflections correspondingto the FTO substrate are labelled, while the remaining reflectionscorrespond to In2S3 [ICDD 00-032-0456]. The CBD deposition of in-dium sulphide is known to be affected by the pH of the depositionbath. It was reported that films deposited at high pH, have a higherhydroxide content forming In(OH)xSy [8,19]. However at low pH,

Fig. 1. Schematic of FTO/ZnO and SnO2/In2S3/PbS/PEDOT:PSS/Cgraphite/FTO, ETAsolar cell.

R. Dharmadasa et al. / Journal of Electroanalytical Chemistry 646 (2010) 124–132 125

deposition baths produced In2S3 [19,20]. The absorber layer, PbSwas also deposited by CBD and the XRD pattern of the as-depositedlayer on FTO is shown in Fig. 2b(ii). The labelled reflections corre-spond to the FTO substrate, while the remaining reflections matchwell to crystalline PbS [ICDD 01-077-0244].

The morphology of the ZnO–SnO2 layers deposited on FTO sub-strateswere then investigated using FEGSEM (Fig. 3). The compositefilms made using 50:50 mol% Zn:Sn in the precursor (Fig. 3a)showed needle-like structures with diameters ranging from 150to 200 nm. In the SEM image it is not possible to differentiate theSnO2 and ZnO crystallites. The image shows that the ZnO andSnO2 crystallites have combined together to form the rod like parti-cles. Therefore the diameters of the particles calculated from SEM

are not comparable to the crystallite size obtained from the broad-ening of the XRD reflections for the materials. We believe that thesmall needle-like structures grew up from the surface of thesubstrate at different angles, due to the uneven morphology of theFTO substrate. The growth of many of these structures in closeproximity results in an approximately 1 lm thick compact layer.The needle-like structures that have grown perpendicular to theFTO substrate surface continued further growth resulting in highsurface area composite electrode. The surface roughness of the elec-trodes was calculated by analysing 10 � 10 lm2 surface area fromAFM images of the composite layer (Fig. 2c and d). The estimatedroot mean square (RMS) surface roughness of the film was 95 nmwhile the surface area for this portion (10 � 10 lm2) of the compos-ite film was 146 lm2. The surface roughness and areas calculatedfrom the AFM images represent a lower limit for the samples asthese values correspond to the top �0.7 lm of the composite film.

As in the DSC, the ETA cell concept relies on the porous metaloxide film being transparent in the visible region of the solar spec-tra, as well as providing a large internal surface area. Therefore themorphology of the composite films made from 50:50 mol% Zn:Snin the precursor makes them suitable for constructing the ETAcells. Previous studies on ZnO nanorod and nanoparticle basedelectrodes in DSCs have shown that charge transport was ‘‘tensto hundreds of times faster” in the nanorod electrodes [21]. Inour composite electrodes the needle-like structures grew almostperpendicular to the substrate surface and in principle should facil-itate an easy passage for electron transport through the films.

3.2. Effect of annealing temperature

The effect of post deposition annealing temperature on thecomposite films was investigated. Films deposited using50:50 mol% Zn:Sn were used in this study as they showed a largeinternal surface area, as required by ETA solar cells. It is believedthat annealing of the composite layer improves inter-particle neck-ing, reduces defects within the metal oxide films, and improveselectron transport which is a key factor in the performance ofETA solar cells.

The cells were fabricated using composite electrodes annealedin air at temperatures ranging between 200 and 500 �C. As theannealing temperature of the electrode was increased up to400 �C, the cell performance was improved (Fig. 4). The improve-ment seen in the Jsc and g of the cells may have been a result of bet-ter electron transport routes in the composite electrode. Furtherincrease of annealing temperature diminishes the cell performance(i.e. Jsc, g). FEGSEM investigations of the surfaces of the ZnO–SnO2

electrodes reveal that annealing above 300 �C leads to an increasein particle size (Fig. 5). The films annealed above 400 �C displayedcomplete particle coalescence resulting in the reduction of internalsurface area. This significant change in morphology was reflectedby a drop in device efficiency. The lack of change in the Voc, sug-gests that the energetics of the ETA cell remain unchanged withthe variation of post deposition annealing temperature.

Fig. 6 shows the I–V characteristics of an ETA solar cell preparedby using a composite electrode annealed at 400 �C. The light anddark current was recorded by manually chopping the AM 1.5 ClassA solar simulated light beam at regular intervals. The cell showedgood rectification with a very low dark current. In order to find theelectrical properties such as series resistance, RS and shunt resis-tance, RSh, the I–V characteristics were fitted with the diode equa-tion for a thin film photovoltaic cell [22].

JlightðVÞ ¼ Jsc � JdarkðVÞ � JR

¼ Jsc � Jo eqðVþJARS Þ

kBT

� �

� 1

!

�V þ JARS

RShð1Þ

Fig. 2. (a) XRD of: (i) FTO substrate, (ii) composite film deposited from 100 mol% Snprecursor on FTO, and (iii) ZnO–SnO2 composite film deposited from 50:50 mol%Zn:Sn precursor on FTO; and (b) XRD of: (i) In2S3 deposited on FTO by CBD and (ii)PbS deposited on FTO by CBD. The labelled reflections in (b) correspond to the FTOsubstrate.

Fig. 3. FEGSEM (a and b) and AFM (c and d) images of the surface and cross sectionof the ZnO–SnO2 electrodes deposited from 50:50 mol% Zn:Sn precursor on FTO.

126 R. Dharmadasa et al. / Journal of Electroanalytical Chemistry 646 (2010) 124–132

where q is the charge on an electron, kB is the Boltzmann constant, Tis the temperature, J is the photocurrent density, JR is the currentlost to resistance, and Jo is the reverse saturation current density.For the I–V curve shown in Fig. 6, the RS of the cell was estimated

to be 23X cm2 and RSh of 179X cm2. The relatively low Voc

(0.32 V) and Jsc (8.2 mA cm�2) measured for the cell reflect thelow RSh and high RS. As a result of these parasitic resistances, theFF (0.26), and g (0.68%) of the cell remains low. The results obtained

Fig. 4. The effect of post deposition annealing of composite electrode deposited from 50:50 mol% Zn:Sn on the ETA cells performance. (a) Jsc, (b) Voc, (c) FF and (d) g vs.annealing temperature.

Fig. 5. FEGSEM images of the surface of ZnO–SnO2 electrodes deposited from 50:50 mol% Zn:Sn for (a) as-deposited films, and films annealed at (b) 300 �C, (c) 400 �C and (d)500 �C.

R. Dharmadasa et al. / Journal of Electroanalytical Chemistry 646 (2010) 124–132 127

in the present study are close to the values reported for the TiO2/In(OH)xSy/PbS/PEDOT:PSS ETA solar cells [8].

3.3. Effect of Zn:Sn ratio

The ratio of Zn:Sn in the films deposited onto FTO glass sub-strates was varied by changing the amount of zinc nitrate in theprecursor solution and were annealed in air at 400 �C for 1 h. FEG-SEM studies of the surface of the composite films shows that thefilm deposited from 100 mol% of Sn in the precursor solutionformed highly compact triangular-based pyramidal particles(Fig. 7a). Addition of Zn to the precursor solution resulted in thegrowth of square-based pyramidal particles (Fig. 7b). Further in-crease of the Zn mol% resulted in the growth of rectangular cuboidshaped rods from the substrate, leading to a highly structured film(Fig. 7c). As the mol% of Zn in the precursor solution exceeded theSn mol% (e.g. 67:33 mol% Zn:Sn), films with large particles wereobtained (Fig. 7e–f). These results show that the morphology ofthe composite layers is strongly dependant on the mol% of Zn inthe precursor solution. The EDX analysis conducted on compositeelectrodes with different Zn mol% shows that the Zn content inthe composite layer is different to the amount of Zn present inthe precursor solution (Fig. 8). In fact it was found that films depos-ited from 50:50 mol% Zn:Sn in the precursor solution has only26 mol% Zn and 74 mol% Sn in the film. The trend was followedfor all composite electrodes deposited from precursors with Zn

Fig. 6. Current–voltage characteristics of ETA cell under light and dark conditionscorresponding to ZnO–SnO2 electrodes deposited from 50:50 mol% Zn:Sn andannealed at 400 �C. Illumination source is AM 1.5 simulated light, intensity�100 mW cm�2. Inset shows the circuit diagram for the photodiode with parasiticresistances [23].

Fig. 7. FEGSEM images of the surface of the ZnO–SnO2 electrodes deposited from different Zn:Sn mol% in the precursor solution: (a) 0:100, (b) 20:80, (c) 33:67, (d) 50:50, (e)67:33 and (f) 100:0.

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content 650 mol% (Fig. 8). As the Zn mol% in the precursor ex-ceeded this value, the films showed a higher Zn mol% than wasoriginally in the precursor. Therefore the phase dominance of thecomposite films depend on the ratio of Zn:Sn in the precursor.

The key characteristics of the ETA solar cell related to the Znmol% in the precursor of the composite films is shown in Fig. 9.The Jsc of the cell gradually increased with the increase of the Znmol% in the precursor solution up to 50 mol%, and after that it de-creased rapidly. This effect may be due to the variation of internalsurface area and morphology of the composite layers as shown inFig. 7. The different Zn content of the precursor solution had littleeffect on the Voc and FF. The highest performing cell has been fab-ricated from composite films deposited using 50:50 mol% Zn:Sn inthe precursor. The Voc, FF and Jsc of this cell remain in the range of0.28–0.32 V, 0.28–0.30 and 7.0–8.2 mA cm�2, respectively indicat-ing the reproducibility of the ETA cells constructed in this study.

3.4. Incident photon to electron conversion efficiency (IPCE) study

To study the ETA solar cell performance, the IPCE spectra weremeasured by illuminating the cell through the composite layer

(Fig. 10). The ZnO–SnO2 layer provides the route for electron trans-port and in principle will not contribute to light harvesting in thecell. The relatively low IPCE in the UV region (300–400 nm) maybe partly due to the strong light absorption of the FTO layer.

The IPCE of the ETA cell shows appreciable photon to electronconversion efficiencies in the visible region, with a maximum effi-ciency of �30% in the 500–600 nm range. The maximum peak cor-responds to the estimated bandgap of In2S3, 2.3 eV which wasconfirmed by optical measurements. Therefore it appears thatthe In2S3 also acts as a light harvesting layer up to 540 nm whileacting as a buffer layer.

PbS has been widely used as a light harvesting material in ETAcells and quantum dot solar cells due to its high absorption coeffi-cient (10–105 cm�1 [28]) and strong size quantization effect. As theparticle size is reduced below 15 nm [27], the bandgap widens dueto the quantum confinement effect. The bandgap widening allowedphotogenerated charge injection to the composite layer. This

Fig. 8. EDX analysis of the Zn mol% in the composite ZnO–SnO2 films vs. Zn mol% inthe precursor solution (the Zn mol% is based on the total mol% of metal ions in thefilm).

Fig. 9. The effect of the Zn mol% in the precursor solution on the ETA cells performance. (a) Jsc, (b) Voc, (c) FF and (d) g vs. Zn mol%.

Fig. 10. IPCE spectra of ETA cell measured at chopping frequency of 11 Hz. Aschematic representation is superimposed on the IPCE plot to show the effect of PbSparticle size on the IPCE spectra.

R. Dharmadasa et al. / Journal of Electroanalytical Chemistry 646 (2010) 124–132 129

phenomenon has been clearly seen in the IPCE spectra. The cell isconsiderably photoactive in the region (540–1100 nm) suggestingthat the PbS layer consists of a size distribution of PbS nanoparti-cles. This has been schematically represented in the onset of theIPCE spectra (Fig. 10).

It is known that molecular sensitisers such as organic dyes withappropriate linkers form self assemble monolayers on metal oxidesurfaces (i.e. adsorption of dyes in a monolayer form on TiO2 nano-particle surface in dye-sensitised solar cells). As the light harvest-ing efficiency is determined by the surface coverage of sensitisermaterial, obtaining a good coverage on the porous metal oxide sur-face is paramount in the development of quantum dot sensitisedsolar cells and ETA solar cells. However, obtaining such a conformalcoverage of low band gap semiconductor sensitiser materials (i.e.metal chalcogenides) on porous metal oxide surface is not easy.Considering the physical and surface properties of high band gapmetal oxide porous matrix, anchoring the pre-prepared nanoparti-cles through a suitable molecular linker by using a bifunctionalmolecule is generally preferred over other wet chemical routes[23]. Even this approach may not provide a coverage as good asthat of molecular sensitisers. For example, the literature suggests

that the coverage could be as low as 14% for CdSe nanoparticleswith linkers on nanoporous TiO2 [23]. The use of molecular linkersto adsorb photosensitising semiconductor nanoparticles is furthercomplicated by inadequate material loading to harness most ofthe incoming photons and light losses through scattering. Wetchemical methods such as CBD have their own merits and draw-backs. For example, in the CBD method there is no control overthe surface coverage, particle size and the deposited materialscould be far from the stoichiometric composition. On the otherhand it is well known that CBD allows a far better control overthe amount of sensitiser material loading hence relatively high ab-sorbed photons to electrons conversion efficiency. Liquid basedphotoelectrochemical cells made with CBD light absorber have alsoshown relatively high charge recombination resistance which isbeneficial in device operation [24].

In this study we used the CBD method to obtain the light absor-ber PbS layer. The amount of PbS coating was controlled by adjust-ing the growth time (i.e. 2, 4, 6, 8 and 10 min). Although theamount of material loading in the CBD method can be quantita-tively determined, relating that information to a meaningfulsurface coverage of absorber material is always challenging.

Fig. 11. (a) IPCE spectra of ETA cells with PbS deposition time (b) IPCE at k = 600 nm vs. PbS deposition time. Estimated IPCE spectra based on the effect of PbS particle radiusfor surface coverage, h; (c) h = 0.79, (d) h = 0.4, (e) h = 0.1 respectively, and (f) for average PbS surface coverage from h = 0.1–0.79.

130 R. Dharmadasa et al. / Journal of Electroanalytical Chemistry 646 (2010) 124–132

Although an analysis can be performed on the basis of formation ofa closed packed nanoparticle monolayer, it may not provide infor-mation on the true coverage. Therefore, we systematically variedthe deposition time of PbS and constructed ETA cells in each case.Due to the nature of PbS nucleation and subsequent growth in theCBD process, it is possible to form multi-layers of PbS.

It was also assumed that the PbS nanoparticles directly attachedto the In2S3 are mainly responsible for charge injection. The ab-sorbed photons to electrons conversion efficiency data recordedagainst the PbS deposition time clearly indicates that the IPCEstarts to level off after �6 min deposition time (see Fig. 11a andb). This suggests that after �6 min further growth of PbS may betaking place on PbS particles already formed along with newIn2S3 sites. An alternative explanation for this behaviour is the pos-sible blocking of the necks in the ZnO–SnO2 matrix after 6 mingrowth time hence arresting further increase of PbS surface cover-age. The experimental IPCE data related to this apparently opti-mum surface coverage was compared with the theoreticallyestimated IPCE values in order to deduce an idea of the true surfacecoverage.

The IPCE was estimated theoretically by considering the en-hanced internal surface area over a projected area by taking intoaccount the electrode thickness (obtained from cross-sectionalFEGSEM images of composite electrode in Fig. 3b). Here the spec-tral region beyond the In2S3 optical band gap where PbS is effectivein harnessing the light (kP 540 nm), was considered. The surfacecoverage, h and PbS particle radius were considered to be the keyparameters in the model. IPCE is a function of light harvesting,charge injection and collection efficiencies. As the film thicknessof the porous region of composite electrode is only about 1 lm(Fig. 3b), it is fair to assume that the electron collection efficiencyreaches unity (it should be mentioned here that the charge lossduring the transport could be substantial if the porous electrodethickness is relatively high. Indeed this is the case for dye-sensi-tised solar cells where on average a 10 lm thick porous electrodewas employed [25]). As size quantised PbS nanoparticles are capa-ble of injecting electrons into composite metal oxide at a faster rate[26], the IPCE (0 6 IPCE 6 1) was calculated according to the fol-lowing expression.

IPCE ¼ 1� expð�AÞ ð2Þ

where A is the light absorbance, which is a function of the surfacecoverage and PbS particle radius. The maximum attainable surfacecoverage is considered to be 0.79 (corresponding to cubic close-packing scenario). In the model, the surface coverage varied in therange of 0.1 6 h 6 0.79. Only the PbS nanoparticles with the radiusof 1–5 nm were taken into account as the band gap widening of PbSis significant in this regime [27]. As it is possible to form PbS nano-particles with a range of sizes due to particle coagulation in the CBDprocess, the average particle size effect was also analysed in eachcoverage. The light absorption coefficient data of PbS was takenfrom the study conducted by Scanlon [28].

Fig. 11c–e shows the theoretical IPCE plots correspond to sur-face coverage 0.1, 0.4 and 0.79. Fig. 11f describes the average par-ticle size effect when the surface coverage varied in the range of0.1 6 h 6 0.79. The experimental IPCE data related to apparentlyoptimum surface coverage (Fig. 10) was compared with the theo-retically estimated IPCEs in Fig. 11c and d. The best fit was foundwhen the surface coverage is about 0.1–0.2. The surface coverageof PbS seems to be low in the present cell configuration and agreeswell with literature reported surface coverage for similar cells [23].Although our theoretical treatment depends on a number of sim-plifications, it gives an insight to the direction of future work.

The low coverage implies that, further increase of compositelayer thickness may not yield improved cell performance whichis in agreement with the observations made in this work. Finding

the ways of improving the surface coverage seems to be an areaof significant interest for further development of ETA cells as wellas quantum dot sensitised solar cells. The strong dependence ofIPCE on the PbS particle size is a direct result of the amount ofPbS loading on the surface. Even from an experimental view point,it could be possible to obtain better point contacts between In2S3and PbS when the particle size is smaller.

4. Conclusions

This study has proved that AACVD deposited ZnO–SnO2 com-posite electrodes can be successfully used to fabricate ETA solarcells. It has been found that the Zn:Sn mol% in the single sourceprecursor solution strongly affected the morphology of the com-posite electrode. The composite electrode with a surface roughnessof 95 nm having needle-like structures were fabricated from50:50 mol% Zn:Sn in the precursor solution. It was found that theZn content in the solution had a pronounced effect on the perfor-mance of the ETA cell. The post annealing temperature of theZnO–SnO2 film was also found to affect the morphology, and per-formance of the ETA cell. The best performing cells were obtainedwhen the ZnO–SnO2 anodes deposited from a solution with50:50 mol% Zn:Sn content and were annealed at 400 �C in air for1 h. I–V characterisation under AM 1.5 simulated light reveals thatthe best cells has an Voc � 0.32 V, Jsc � 8.2 mA cm�2, FF � 0.26 andan g � 0.68%.

IPCE studies of the ETA cells reveals maximum efficiency �30%in the visible region. A theoretical estimation of the IPCE spectrawas performed in the region of 540–1100 nm by considering theenhanced internal surface area over a projected area. The theoret-ical IPCE spectra matched well with the experimental IPCE datawhen PbS surface coverage is about 0.1–0.2. Further work is re-quired to increase the internal surface area of the composite layerand PbS coverage to improve the performance of the ETA cell.

Acknowledgements

KGUW has been very fortunate to engage scientific activitieswith Professor Laurie Peter over the last 12 years, first as a doctoralstudent and then as a colleague. This work is dedicated to ProfessorLaurie Peter for his service to the scientific community over manyyears. The authors would like to thank Sina Saremi-Yarahamdi, AsriMat-Teridi, Tom Smith, Henry Burch, and John Bates for their assis-tance during this project. This work was supported by the EPSRCaward EP/F057342/1.

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