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The role of transparent conducting oxides in metal organic chemical vapourdeposition of CdTe/CdS Photovoltaic solar cells

S.J.C. Irvine ⁎, D.A. Lamb, V. Barrioz, A.J. Clayton, W.S.M. Brooks, S. Rugen-Hankey, G. KartopuCentre for Solar Energy Research (CSER), Glyndŵr University, OpTIC Glyndŵr, St Asaph Business Park, LL17 0JD, UK

a b s t r a c ta r t i c l e i n f o

Available online 4 May 2011

Keywords:Transparent conducting oxidesIndium tin oxideCadmium oxideCadmium tellurideCadmium zinc sulphideFluorine doped tin oxideSolar cells

A systematic study is made between the relationship of Cd0.9Zn0.1S/CdTe photovoltaic (PV) device propertiesfor three different commercial transparent conducting oxide (TCO) materials and some experimental CdO todetermine the role of the TCO in device performance. The resistance contribution from the TCO was measuredafter depositing the gold contact architectures directly onto the TCOs. These were compared with theCd0.9Zn0.1S/CdTe device properties using the same contact arrangements. Series resistance for the commercialTCOs correlated with their sheet resistance and gave good agreement with the PV device series resistance forthe indium tin oxide (ITO) and fluorine doped tin oxide (FTO) 15 Ω/Sq. superstrates. The devices on thethicker FTO 7 Ω/sq superstrates were dominated by a low shunt resistance, which was attributed to the roughsurface morphology causing micro-shorts. The device layers on the CdO substrate delaminated but deviceswere successfully made for ultra-thin CdTe (0.8 μm thick) and compared favourably with the comparabledevice on ITO. From the measurements on these TCOs it was possible to deduce the back contact resistanceand gave an average value of 2 Ω.cm2. The correlation of fill factor with series resistance has been comparedwith the predictions of a 1-D device model and shows excellent agreement. For high efficiency devices thecombined series resistance from the TCO and back contact need to be less than 1 Ω.cm2.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conducting oxides (TCO) play a vital part in theoverall efficiency of thin film photovoltaic (PV) solar cells. However,the precise contribution to the overall performance of the PV cell isoften difficult to quantify. The TCO provides the so called “frontcontact” which has to allow the solar radiation to be transmitted tothe underlying cell and to conduct the generated electricity to theexternal circuit. Thus it is clear that for efficient cells the TCOtransmission of solar radiation must be highly coupled with very lowelectrical resistance; however other properties of the TCO can alsoinfluence the cell performance in more subtle ways. A comprehensivereview of the TCOs used for CdTe PV cells is given in the review byBosio et al. [1]. The commonly used TCOs are fluorine doped SnO2

(FTO) and indium tin oxide (ITO) with an increasing interest intrivalent doped ZnO. The need for improved conductivity, achieved byimproving TCO mobility rather than the carrier concentration, isrecognised as a factor in improving PV cell and module performance[2]. High carrier concentrations in the region of 1020 to 1021 cm−3 canreduce transmission, particularly in the infrared part of the solarspectrum [3]. The world record CdTe cell performance has remainedon the value set by Wu et al. in 2001 and this was achieved using a

TCO combination of zinc stannate and cadmium stannate for the TCO[4]. Performance of CdTe PV cells using commercially available TCOs ismore in the 14–15% range and for modules, where the limitations ofthe TCO conduction tend to be more apparent, the best is around 11%[5]. The issues in transferring from idealised laboratory PV cells to PVmodules have been highlighted in the recent paper by Jingquan et al.[6] where it was shown that the laser groove width in the TCO, andmetallisation width, can significantly affect the module efficiency andeven under the best conditions the fill factor (FF) did not exceed 58%.This indicates the difficulty in translating performance from idealisedlaboratory PV cells tomodule device architectures. For laboratory cellsthe FF is expected to be over 70% and the decrease in modules isindicative of higher series resistance.

This paper investigates the role that different TCOs play in theperformance of CdTe thin film PV devices. The deposition method forthe window and absorber layer in this study is metal organic chemicalvapour deposition (MOCVD) which enables excellent control over thefilm thickness and physical properties of the deposited layers [7]. Forexample, the conventionally used window layer CdS is alloyed withZn in the MOCVD process to allow more of the blue light to passthrough to the absorber layer than with CdS alone. Further, the CdTelayer is doped, in situ, with arsenic (As) to increase the p-type carrierconcentration and this doping is increased towards the back contactwhich enables non-rectifying contacts to be formed without any etchtreatment [7–9]. These features were considered to be important inthe present study where the purpose is to investigate the role of the

Thin Solid Films 520 (2011) 1167–1173

⁎ Corresponding author.E-mail address: Stuart.Irvine@optictechnium.com (S.J.C. Irvine).

0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2011.04.194

Contents lists available at ScienceDirect

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TCO in the final performance of the device. A study of amorphoussilicon thin film PV on fluorine doped tin oxide (FTO) has shown theimportance of morphology of the TCO in addition to the usualparameters of high transmission and low resistance [10]. Fourdifferent TCOs are investigated in this paper: two commercial FTOmaterials (NSG TEC C15 and NSG TEC A7), commercial indium tinoxide (ITO), and some experimental cadmium oxide (CdO). Thedevice layers of Cd0.9Zn0.1S and CdTe are deposited onto thesedifferent TCO superstrates in order to investigate the relationshipbetween the properties of TCO and PV device performance onlaboratory cells.

2. Experimental details

The commercial NSG TEC C15 and NSG TEC A7 superstrates onsoda lime glass were supplied by Pilkington Technology Centre. TheITO on boro-alumino silicate glass was supplied by Delta Technologiesand the CdO was grown in our own laboratory onto Corning 1737boro-alumino silicate glass substrates using MOCVD [12]. Theprecursors for the CdO deposition were dimethylcadmium (DMCd)and tertiarybutanol (tBuOH). The 500 nm thick films were grown at291 °C which had been shown previously to give low resistance filmsfor the undoped CdO [12].

The Cd0.9Zn0.1S/CdTe device structure was grown using the MOCVDprocess described previously [7]. This entailed in situ doping of the 2 μmthick CdTe layerwithAs from trisdimethlyaminoarsenic (tDMAAs)witha terminationofhigh concentration of As (N1×1019 cm−3) to give a lowback contact resistance. A CdCl2 layer was deposited and annealed insitu in the MOCVD reactor, as described by Barrioz et al. [12], and noetching of the surface was required prior to evaporating the gold backcontacts.

The electrical properties of the TCO layers were characterisedusing a Jandel RM3-AR 4-point probe for sheet resistance and anEcopia HMS 3000 Hall measurement system to determine carrierconcentration and carrier mobility. The Hall measurements weremade, at room temperature, on cleaved 1 cm2 samples with In-Sneutectic contacts soldered onto the extreme corners of the sample.The optical transmittance was measured over the spectral range from300 to 2000 nm using a Varian Cary 5000 spectrometer.

Laboratory cell device architectures were tested on 5×5 cm2 TCOsuperstrates by evaporating gold contact configurations directly ontothe TCO. This enabled measurement of lateral conduction through thedifferent TCOs between the front contact bus-bars and the individualdevices. These are shown in Fig. 1 where the devices are defined bythe 0.5×0.5 cm2 gold contacts and the front contacts are defined by

the bus-bars around the edge of the sample. For the PV devicefabrication the CdTe layer was removed from the areas shown by thebus-bars prior to evaporation of the gold. Thus the front and backcontacts were made in a single evaporation step where the revealedareas of TCO provided the front contact. The difference between thetwo device architectures was in the network of front contact bus-bars.The simple configuration in Fig. 1(a) relies on larger distances forlateral conduction in the TCO compared to the improved configura-tion shown in Fig. 1(b) where the bus-bar extends further around theperiphery of the sample and there is an addition of gold fingersextending between the rows of devices. The typical evaporated goldthickness was 50 to 60 nm, as measured using a dektak150 fromVeeco. For the PV devices it was possible to test both configurations onthe same sample to remove potential effect of small variance in theCdTe and Cd0.9Zn0.1S between theMOCVD growth runs. The first stagewas to deposit the contact architecture in Fig. 1(a) and following theI–V device measurements using an Abet Technologies AM1.5calibrated solar simulator, the contact architecture in Fig. 1(b) wasoverlaid and again I–V measurements were repeated.

3. Results

3.1. Properties of TCOs

The results of the Hall measurement and 4-point probe measure-ments for the 4 types of TCO used in this study are shown in Table 1.The thicknesses of the TCOs vary such that the thinnest sample is theDelta Tech ITO but still achieves a low sheet resistance of 11 Ω/sqthrough the high carrier concentration of 1.26×1021 cm−3. The twoNSG TEC glass samples differ in their thickness with values of 340 nmand 630 nm for TEC C15 and TEC A7 respectively. This leads to a lowersheet resistance for TEC A7 and the higher carrier mobility of36.9 cm2/Vs compared with 27.8 cm2/Vs also contributes to this lowsheet resistance. The CdO sample had the highest mobility and thelowest carrier concentration with values of 44.9 cm2/Vs and2.20×1020 cm−3 respectively. This led to a modest sheet resistance of19.0 Ω/sq. This set of TCO samples gives a significant range of electricalproperties and film thickness. The latter also relates to surfacemorphology and tomicrostructurewhereOswald ripeningmechanismswill lead to larger grain size for thicker layers and is reflected in the RMSsurface roughness measured by A Veeco Dimension 2400 Atomic ForceMicroscope (AFM) shown in Table 1. All these properties are relevantwhen considering thenucleation,morphology and grain structure of theMOCVD grown Cd0.9Zn0.1S/CdTe structure.

The specular transmittance spectra are shown in Fig. 2 for each ofthe four types of TCO. The lower optical transmission on the NSG TECsamples is not indicative of higher absorption in these films but isrelated to the rougher surfaces on these compared to the ITO and CdOsamples. The decrease in maximum transmission follows the trend ofincreasing RMS roughness in Table 1. The short wavelength cut-offson these TCOs are in line with their optical bandgaps with the longestwavelength cut-off for CdO around 500 nm corresponding to thebandgap of 2.5 eV. This will cause some loss of photocurrent when

Fig. 1. A schematic of the two variations of evaporated gold contacts used in this study,showing (a) the standard “simple” layout and (b) an improved layout to reduce theeffect of lateral resistance in the TCO layers. All samples measured with air as referenceNSG TEC are on 3 mm glass, ITO and CdO are on 0.7 mm glass.

Table 1Measured properties of the TCO films using Hall effect and 4-point probe. Allmeasurements were made at 294 K.

Property CdO ITO NSG TECC15

NSG TECA7

Thickness (nm)±20 501 140 340 630Carrier concentration (cm−3) 2.20×1020 1.26×1021 5.56×1020 4.06×1020

Carrier mobility (cm2/Vs) 44.9 33.7 27.8 36.9Resistivity (Ω cm) 6.33×10−4 1.47×10−4 4.04×10−4 4.17×10−4

Sheet resistance (Ω/sq) 19.0 11 15 6RMS roughness fromAFM(nm) 7.1 3.3 13.8 28.3

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using the wider bandgap window layer Cd0.9Zn0.1S but not with CdSwhich has a similar bandgap. The most striking feature of CdO is thehigh transmission to longer wavelengths where free carrier absorp-tion starts to become significant for wavelengths longer than 1200 nmin the ITO and FTO superstrates. The excellent long wavelengthtransmission for CdO has been discussed in a previous publication[11]. For the purposes of a single junction CdTe device this does notoffer any advantage but would be attractive for multi-junction thinfilm devices.

3.2. Device architecture

The two different contact layouts shown in Fig. 1 enable acomparison to be made of the effect of lateral conduction in the TCOon series resistance in a PV device. By testing the contact architectureson the TCOs, without the PV layers, it will help to interpret the seriesresistance and other properties of the Cd0.9Zn0.1S/CdTe devices grownonto these TCO superstrates. In addition to the lateral conduction ofthe TCO, the contact resistance of the gold to the TCO and probecontact resistance must also be considered. The front contact bus-barswere connected with crocodile clips (both sides) to a 2400 Keithleysource meter, with the other terminal being connected via goldprobes to each of the back contact pads in turn. The contact resistanceof the crocodile clips was checked by contacting the same bus bardifferent distances apart and extrapolating to zero separation. Thisgave a contact resistance for the crocodile clips to the gold bus bar ofless than 0.25 Ω.cm2. The resistance was determined from the slope ofthe I–V plot where in all cases they were Ohmic. Tests were carriedout with and without the AM1.5 solar simulator and no significantdifference was observed. In order to make a direct comparison ofresistance with the device series resistance, the resistance values hereare multiplied by the back contact area (0.25 cm2) but this must notbe confused with resistivity of the TCO as this would only be correctfor vertical conduction through the back contact pad and not lateralconduction being considered here. An example of the resistance mapsis shown in Fig. 3 for the NSG TEC C15 superstrate, where acomparison is made between the two contact architectures. Thesimple contact architecture was used for the resistance map in Fig. 3(a) and shows a systematic variation across the sample from averageof 2.5 Ω.cm2 in column 1 to 1.8 Ω.cm2 in column 4. The absence of anyrandom variation in resistance across the area indicates that the probecontact resistance to each pad is very small but the systematicvariation does not reflect the 2-fold symmetry of the contact pattern.The thickness of the gold contacts was measured using a stylusprofilometer and showed an increase in thickness of the gold goingfrom column 1 to column 4, mirroring the reduction in resistance. Theaverage thickness in row 1 was 50 nm and in column 4 was 64 nm. Itis also noted that the column 3 and column 4 resistance values aresimilar and correlates with similar thicknesses for columns 3 and 4.

The pattern in Fig. 3(b) is different with the lowest resistance valuesin columns 2 and 3, increasing to each end. This pattern was moreusual than that shown in Fig. 3(a) across most of the samplesmeasured and is consistent with the 2-fold symmetry of the contactlayout. However, this “valley” pattern of resistance indicates that thelateral conduction in the gold bus-bars is adding significant seriesresistance to the measurements for columns 1 and 4. From the centreto edge measurement it is possible to estimate the maximumcontribution of the gold bus bar resistance to the series resistanceand this is 0.42 Ω.cm2. The average “valley” resistance of 1.64 Ω.cm2 isattributed to the lateral conduction in the NSG TEC C15 for theimproved contact architecture. Carrying out a similar calculation forthe Delta-Tech ITO (which displayed similar “valley” behaviour) gavea bus-bar series resistance of 0.27 Ω.cm2 and an average “valley”resistance of 1.45 Ω.cm2. Some variance is expected on the seriesresistance depending on the variance in gold thickness. For the Delta-Tech ITO sample the average column 1 thickness was 59 nm andaverage column 4 thickness was 47 nm. For the NSG TEC A7 samplewith the same, improved, contact architecture, the estimated bus-barresistance was 0.20 Ω.cm2 and the “valley” resistance was 1.23 Ω cm2.

It was not possible to obtain sufficiently uniform CdO over thesame 5×5 cm2 area so a 5×2.5 cm2 sample was used instead.Although this is not a direct comparison with the 5×5 cm2 samplesit provided an average series resistance to compare with the PV deviceresults that were on similar small format architecture. The averageresistance was 3.1±0.3 Ω.cm2, which is significantly higher than forthe other TCOs but in agreement with the higher sheet resistance.

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Fig. 2. Spectrophotometer measurements for the four TCO samples over thewavelength range from 200 to 3300 nm.

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Fig. 3. Maps of resistance across NSG TEC C15 superstrate using (a) the simple contactarchitecture in Fig. 1(a), (b) the improved contact architecture from Fig. 1(b).

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In order to establish the primary contribution to the seriesresistance a correlation graph was plotted from the series resistancefor each superstrate, taken from the “valley” region of contacts, withthe corresponding sheet resistance values (Fig. 4). Comparison ismade for both contact architectures and in both cases they show goodagreement between series resistance and TCO sheet resistance,confirming that these series resistance measurements are attributableto the TCO. It is clear that the additional gold bus-bars reduce theseries resistance, giving a shallower slope for the improved contactarchitecture, which was designed to be less dependent on the lateralconductivity of the TCO.

The remaining question for the lateral conduction in the ITO andFTO superstrates is whether the inner rows of contacts (away from thebus-bars) in the simple contact arrangement (Fig. 1(a)) have a higherseries resistance than the rows adjacent to the bus-bars. For thiscomparison an average was taken for rows B and C and comparedwith the average for A and D. These results are shown in Table 2. It isclear that there is no significant difference between the inner andouter sets of contacts. This might seem surprising considering thedependence of the series resistance on the TCO sheet resistance.However, the integrated conduction paths with these contactarchitectures are complex and contacting from bus-bars on bothsides of the 5×5 cm2 sample will provide overlap of spreadingresistance and an averaging between the bus-bars. This wasconfirmed by contacting from just one bus bar on the ITO superstratewith the simple contact architecture. The average series resistance ofthe row adjacent to the contacted bus-bar was 1.73 Ω.cm2 andincreased for each row to 3.13 Ω.cm2 for the row furthest away. Theseresults also provide further insight to the interplay between TCO sheetresistance and the device contact architecture.

3.3. Results on CdZnTe/CdTe devices on different TCOs

Having established the contributions to series resistance from thechosen contact architecture and from the sheet resistance of the TCO,the next objective was to see if this will translate into the PV devices.The full contact array was made onto the Cd0.9Zn0.1S/CdTe deposited(and CdCl2 treated) layers, except for the CdO, which had to be madeon a smaller superstrate size. A map of I–V characteristics underAM1.5 illumination could be created by probing each of the contacts.The full set of maps is shown in Fig. 5 for the PV structure depositedonto the Delta-Tech ITO superstrate, using the improved contactarchitecture. A summary of average values, for all the commercialsubstrate material, is given in Table 3. The flow direction in theMOCVD reactor causes some non-uniformity due to small tempera-ture variations and depletion of precursors from the gas stream.

However, the overall uniformity is good on the Delta-Tech ITO andprovides a suitable template for testing the properties of this TCO. TheAM1.5 conversion efficiency (Fig. 5(a)) ranged from 9.5 to 11.9%. Theseries resistance was measured from the forward bias section of theI–V curve. These were all Ohmic with no roll-over that would beindicative of a rectifying contact. The series resistance values rangedfrom 2.7 Ω.cm2 to 4.4 Ω.cm2, with an average of 3.4 Ω.cm2 (Fig. 5(e)),with no particular pattern across the surface. The fill factor (FF) shownin Fig. 5(d) is a parameter that depends on a low series resistance aswell as a high shunt resistance. The shunt resistance is reliant on goodsurface coverage of the window layer and CdTe absorber layers andfor this device were mostly high (N1000 Ω) but with more variancethan for the other parameters. Similarly, the FF is variable but with allvalues over 60% and six contacts over 70%.

The same device sample was also processed with the simplecontact pattern (Fig. 1(a)) with average values given in Table 3, andshowed similar uniformity of device parameters with efficiencyranging from 9.5 to 11.6%. The series resistance was in the range 2.3to 3.5 Ω.cm2, slightly lower than for the results shown in Fig. 5(e) butconsidering the variance in this measurement no conclusions can bedrawn from this. The difference between the two device architecturesis very small and in line with expectation from the relatively smalldifferences in series resistance from Section 3.2. The more significantdifference is with the device series resistance and the TCO seriesresistance measurements from Section 3.2. Average series resistanceover the same area of the device, as for the TCO “valley”measurements gives a value of 3.4±0.5 Ω.cm2, using the improvedcontact architecture. When compared with the series resistance forthe ITO using the same (improved) contact architecture, which was1.45 Ω.cm2, there is an additional series resistance of approximately2 Ω.cm2 arising from the PV devices that can't be attributed to the TCOresistance. If we use the lower value for the simple architecture,considering the variance in series resistance, this non-TCO resistancecould be as low as 1 Ω.cm2.

Turning now to the NSG TEC C15 superstrate, where averagedevice measurements are given in Table 3, the range of AM1.5efficiency for the simple contact architecture was somewhat lowerthan that for the devices on ITO with a range of 8.3–10.1% over anarray of 2×3 contacts within the larger matrix. The FF was also poorerover this contact array with an average value of 56%. However, theshunt resistance was over 1000 Ω cm2 for all these contacts but theseries resistance had significantly increased, compared with the ITO,with an average of 6.7±2.2 Ω.cm2. For the improved contactarchitecture on this sample the AM1.5 efficiency is similar to theresults from the simple architecture, ranging from 8.2 to 10.4%, withan average FF of 63% and is consistent with a lower average seriesresistance of 3.9±0.3 Ω.cm2 compared with the simple architecture.Comparing this valuewith the estimated TCO series resistance for NSGTEC C15 superstrates of 1.64 Ω gives a difference of 2.3 Ω cm2, similarto the difference for the results using the ITO superstrate. The error onthe series resistance on the simple contact architecture was too largefor any meaningful calculation of the residual series resistance. Thesignificant decrease in series resistance, when using the improvedcontact architecture, was expected for this relatively high impedancesuperstrate where the difference in contact architecture can make alarger difference in the device series resistance (as seen in Fig. 4).

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1.6

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stan

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Fig. 4. Correlation graph of series resistance (averaged from columns 2 and 3 in bothcontact architectures shown in Fig. 1 where “simple” just has the straight bus bar frontcontacts and “improved” has the additional gold bus bars dividing the rows of goldcontacts) versus the measured sheet resistance values for each superstrate.

Table 2Correlation of series resistance between the inner and outer rows of contacts for each ofthe TCO superstrates.

Average of rows B and C(Ω.cm2)

Average of rows A and D(Ω.cm2)

Delta-Tech ITO 1.73±0.13 1.64±0.20NSG TEC A7 1.40±0.11 1.36±0.15NSG TEC C15 2.07±0.26 2.02±0.20

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The device results for the lowest resistance TCO in this series, theNSG TEC A7, were disappointingly low and with larger variancebetween the devices. The AM1.5 efficiency was in the range of 6.9% to9.3% for the simple contact architecture and 5.2% to 8.1% for theimproved architecture. The overlaying of the additional contacts hasclearly caused some deterioration but the main cause of the largevariance and poor performance can be seen in the shunt resistancewith average values shown in Table 3 of 270 Ω.cm2 and 327 Ω.cm2 for

the simple and improved contacts respectively. This is the main causeof the poor FF with average values of 61% and 54% respectively. Theseries resistance for the devices on the NSG TEC A7 was much lower(as expected) than for the NSG TEC C15 with average values of 2.9±0.3 Ω.cm2 and 3.4±0.5 Ω.cm2 for simple and improved contactsrespectively. The difference between these series resistance values isnot significant considering the observed variance and would bedifficult to distinguish for a low sheet resistance TCO. The difference in

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Fig. 5.Maps of PV device characteristics under AM1.5 illumination for Cd0.9Zn0.1S/CdTe on Delta-Tech ITO superstrate, showing (a) the cell efficiency, (b) the Jsc, (c) the Voc, (d) the fillfactor (FF) and (e) the series resistance Rs. The contact architecture was the improved architecture in Fig. 1(b). The flow direction of the gases in the horizontal reactor is markedwiththe arrow.

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series resistance between the devices and the TCO series resistance inFig. 4 is 1.7 and 2.1 Ω cm2 respectively for the simple and improvedarchitectures. These values are in agreement with the non-TCO seriesresistance derived from the other superstrate devices. The explana-tion for the poor FF and low shunt resistance lies with the incompletecoverage of the Cd0.9Zn0.1S window layer creating micro-shorts. Aseparate growth of the Cd0.9Zn0.1S under identical conditions to thedevice layer showed a patchy nucleation onto the NSG TEC A7 surface.This was confirmed in the devices by taking a blue laser beam inducedcurrent (LBIC) image of the device as shown in Fig. 6. The laserwavelength of 405 nm is highly absorbing in the Cd0.9Zn0.1S, leadingto a very low photo-current over most of the contact area. The spikesare regions of very thin Cd0.9Zn0.1S where the blue laser lightpenetrates into the CdTe absorber layer. The optimal red laserimage doesn't show these spikes, where penetration through thewindow layer is good. This confirms the non-uniformity seen in theCd0.9Zn0.1S coatings and helps to illustrate where micro-shorts arelikely to occur where the CdTe layer might contact the underlyingTCO. For the rougher morphology on the NSG TEC A7 superstrate theRMS roughness was 28 nm from Table 1 and peak to valley height ofover 100 nmwas observed in the AFM image. For the very thin regionsof the window layer it is likely that TCO spikes uncoated with thewindow layer would punch through into the CdTe layer, causingmicro-shorts. The reason for the regions of poor Cd0.9Zn0.1S nucleationis not clear but uniform nucleation by the MOCVD process doesdepend on a high surface cleanliness and this was more difficult toachieve on the rougher surfaces. The open circuit voltage Voc for thedevices on NSG TEC A7 superstrates is also significantly lower than for

devices on the other substrates and is consistent with this interpre-tation of the poor FF and shunt resistance.

An attempt to repeat the PV device structure growth onto the CdOsuperstrate resulted in delamination of the CdO film from thesubstrate. The adhesion of the as grown film was sufficient to passthe Scotch tape test so it appeared that interfacial strain caused by thethicker CdTe layer had caused the delamination. A further PV devicewas prepared where the MOCVD grown CdTe was only 800 nm thick.Previous reports on ultra-thin CdTe devices have shown that gooddevice quality can be obtained but with some loss of Jsc due to thereduction in solar absorption [13,14]. An 800 nm CdTe absorber layerwas grown onto 240 nm thick CdS onto CdO superstrate with thecharacteristics given in Table 1. This was compared with growth ontoan ITO substrate in the same MOCVD batch run. The adhesion of thisCdS/CdTe film was good on both substrates and the results are givenin Table 4 along with the series resistance. Although the TCO had alower series resistance, with corresponding improvement in the FFover the CdO superstrate device, the Voc is significantly higher for thelatter and indicates factors relating to the TCO that might favourimproved CdS nucleation and a reduction inmicro-shorts. This featureof the MOCVD growth where nucleation can be carefully controlled isnow being investigated in more detail. Comparing the seriesresistance from Table 4 for the CdO superstrate device with the CdOseries resistance from Section 3.2, gives a difference of 1.9 Ω.cm2

which is in agreement with the non-TCO series resistance for theother TCO superstrate devices.

In this results section a careful analysis of the series resistance of thePV device has been made and contributions associated with lateralconduction in the TCO, the contact architecture and the non-TCOcontribution attributed to the back contact. The observed range of deviceseries resistance for the5×5 cm2 format is 2.6 to 6.7 Ω.cm2 andhas beenattributed to theTCO sheet resistance. TheSCAPSmodel [15]was used tointerpret impact on PV device performance. This is a 1-D device modeltaking into account properties of the window layer, absorber layer, backcontact and series resistance. The cell parameters and materialsproperties used in this modelling were the same as those reported byJones et al. [13]. Fig. 7 shows the predicted correlation between FF andseries resistance from the SCAPS model and the from the device resultsfor the Delta Tech ITO and NSG TEC C15 superstrates. The NSG TEC A7wasnot included in this comparisonas the FFwasdominatedby thepoorshunt resistance and therefore not limited by series resistance. Theresults in Fig. 7 show excellent agreement between the experimentalresults and theoretical prediction, confirming that for these devices,where the shunt resistance isN1000Ω.cm2, theFF is limitedby the seriesresistance. The SCAPS model predicts that to achieve a FF of 75% theseries resistance will have to be less than 1Ω.cm2. This means that thetarget series resistance contributions from both the TCO and backcontact will both have to be significantly below 1Ω.cm2.

4. Conclusions

A detailed study has been made of the role of TCOs in theperformance of Cd0.9Zn0.1S/CdTe PV solar cells where the solar cellstructure has been deposited by MOCVD. It was found that the factorsinfluencing device performance were:

a) The device architecture where front contacts can mitigate againstpoor lateral conduction in the TCO.

Table 3Comparison of Cd0.9Zn0.1S/CdTe PV devices on three different commercial superstrates,Deltatec ITO, NSG TEC A7 and NSG TEC C15. The device parameters are AM1.5 efficiency(η), short circuit current density (Jsc), open circuit voltage (Voc), series resistance (Rs)and shunt resistance (Rsh). For each superstrate two different contact architectureswere used (shown in Fig. 1) where “simple” just has the straight bus bar front contactsand “improved” has the additional gold bus bars dividing the rows of gold contacts.

Contacts η(%)

Jsc(mA cm−2)

Voc

(mV)FF(%)

Rs

(Ω.cm2)Rsh

(Ω.cm2)

Delta-Tech ITO Simple 10.9 23.3 665 70 2.6 1573Improved 10.9 22.5 711 68 3.4 1500

NSG TEC A7 Simple 8.5 20.8 655 61 2.9 270Improved 7.2 20.1 654 54 3.4 327

NSG TEC C15 Simple 9.0 21.6 670 56 6.7 2105Improved 9.3 22.0 680 63 3.9 1894

Fig. 6. LBIC image over one of the device contacts on the NSG TEC A7 superstrate devicestaken at 405 nm showing higher response from areas of thin Cd0.9Zn0.1S window layer.

Table 4Comparison of ultra-thin CdTe devices on a Deltatec ITO superstrate with a CdOsuperstrate.

η (%) Jsc (mA cm−2) Voc (mV) FF (%) Rs (Ω.cm2)

Delta-Tech ITO 6.7 16.1 596 69 1.5MOCVD CdO 7.2 16.8 626 68 5.0

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Author's personal copy

b) The gold contact bus-bars can contribute to series resistance(estimated to be 0.4 Ω.cm2 for Au bus-bars less than 60 nmthickness)

c) Series resistance measured using the two different contactarchitectures, directly onto the different TCOs, correlated withthe measured sheet resistance and the improved contact archi-tecture falling on a line below the simple architecture.

d) The PV devices on the Delta-Tech. ITO and on the NSG TEC C15superstrates were limited by series resistance arising from lateralconduction in the TCO and a residual series resistance associatedwith the back contact that fell in the range from 1 to 2.3 Ω.cm2

with most of the results in the range 1.7 to 2.3 Ω.cm2. Therefore, areasonable estimate for the back contact resistance for thesedevices is 2 Ω.cm2.

e) Surface morphology of the TCO and consequent nucleation of theCd0.9Zn0.1S window layer was shown to be the limiting factor withthe rougher NSG TEC A7 superstrate causing shunt resistanceb1000 Ω.cm2 and poor FF.

f) A good correlation was obtained between the device seriesresistance and the FF which was also in good agreement withthe prediction of SCAPS modelling.

Although the sheet resistance of the TCO was shown to contributeto the device series resistance it is clear from this study that otherfactors are also important and no one factor dominates. Other factorsthat have been shown to be important are the back contact resistance,the surface morphology of the TCO and the nucleating surface. Thelatter may be particularly important for MOCVD where surfacechemistry is important. However, this provides an opportunity to

achieve greater control over the nucleation and hence improve thequality of the window layer. This appears to strongly influence the Voc

and further work is being carried out to see what effect differentnucleation treatments have on the Voc. It proved to be difficult tomake a direct comparison with CdO as we were only able to obtainadherent devices on ultra-thin CdO superstrates. Further work will becarried out on doping the CdO to reduce sheet resistance and workwith thinner CdO to see if this avoids delamination from thin filmstrain. A final comment is that the sheet resistance of the TCO is notnecessarily the limitation on device performance provided suitablecontact architecture is used to increase lateral conduction. Therefore,further work will be carried out using the NSG TEC C15 superstrateswith improved nucleation.

Acknowledgements

The authors would like to acknowledge EPSRC for their financialsupport through the PV SUPERGEN project (PV Materials for the 21stCentury). We would also like to thank Dr Paul Warren and Dr NeilMcSporran of the Pilkington Group Ltd. for providing the NSG TECglass. Technical support from Fraser Hogg inmeasuring large numbersof devices is also gratefully acknowledged.

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0 5 10 15

FF

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Fig. 7. A comparison of the theoretical fill factor (FF) versus the series resistance fromSCAPS model with the results for devices on the Delta-Tech and NSG TEC C15superstrates.

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