A Combinatorial Approach to the Optimisation of Cd(1−x)ZnxS Layersfor CdTe Solar Cells

Robert E. Treharne1, Andrew Clayton2, Laurie J. Phillips1, Jonathan D. Major1, Stuart J. C. Irvine2, Ken Durose1

1Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool, L69 7ZF, Merseyside, UK2Centre for Solar Energy Research, University of Glyndwr, St. Asaph, LL17 0JD, Denbighshire, UK

Abstract—A combinatorial methodology has been adopted todetermine the optimum composition of a Cd(1−x)ZnxS windowlayer for CdTe solar cells. The methodology generated a large, selfconsistent dataset which permitted an unambiguous relationshipbetween x, conversion efficiency and related cell parameters tobe determined. An optimum composition of x = 0.57 was shownto maximise cell efficiency. Analysis of J − V curves, measuredover 72 separate cells show that both short circuit current, JSC ,and fill factor, FF , values increase with respect to x over therange 0.1−0.57. EQE measurements show that further increasesin JSC value are limited by the band gap of the highly resistivetransparent (HRT) ZnO layer. The methodology demonstrates arapid route, compared to conventional experiments, to the furtheroptimisation of CdTe solar cells.

Index Terms—CdTe solar cells, CdZnS, magnetron sputtering,MOCVD, combinatorial analysis.

I. INTRODUCTION

A key route to improving the performance of CdTe basedsolar cells is via the thinning of the n-type CdS window layerto below 100 nm. This generates an improvement in the ‘blueresponse’ of the device at wavelengths below the CdS bandgap (i.e. 300 − 500 nm) and gives rise to an increase in theshort circuit current, JSC . However, this approach increasesthe probability of forming shunt paths through discontinuousCdS films with the result that any gain in JSC is generallynegated by a reduction in fill factor, FF , or open circuitvoltage, VOC . An alternative approach is to replace the windowlayer with a material with a higher optical band gap, preferably> 3 eV. This has been successfully demonstrated using filmsof CdS:O [1] - [2] and CdZnS [3] - [4]. It is most likely thatsuch alternatives to CdS are now being used commercially toproduce record module efficiencies [5].

In this work the effect of varying the composition of aCd(1−x)ZnxS window layer on the performance of CdTe solarcells is investigated. The key aim is to determine the optimumvalue of x that maximises conversion efficiency and determinethe source of efficiency gains, i.e. increases to JSC , VOC orFF . A novel combinatorial methodology is employed thatgreatly increases the rate of investigation through the use of au-tomated characterisation techniques. Furthermore, in contrastto conventional approaches involving multi-sample sets, thismethodology may be performed using a single combinatorialsample. This results in highly consistent data sets from whichunambiguous relationships between layer composition and cellperformance can be determined with high precision.

II. EXPERIMENTAL

A. Deposition and Characterisation of Cd(1−x)ZnxS Films

Deposition of Cd(1−x)ZnxS films was achieved by co-sputtering from separate CdS and ZnS targets (3” dia. Pi-Kem Ltd, UK) using RF powers of 100 W and 200 Wrespectively and an Ar pressure of 5 mTorr. The compo-sition profile was achieved by switching off the substraterotation. The deposition equipment used was a dual chamberOrion Phase-IIJ system (AJA Int.) The thickness profilesfrom pure CdS and ZnS films, deposited on bare soda-limeglass (OptiWhite, Pilkington NSG), were determined usingscanning spectrophotometric methods described fully in recentwork on similarly sputtered films [6]. The resultant profileof the Zn/(Cd+Zn) ratio, x, for a co-sputtered film was thenestimated by combining both profiles. The direct relationshipbetween band gap and x was determined using variable anglespectroscopic ellipsometry.

B. Device Fabrication and Characterisation

The device structure: ZnO/Cd(1−x)ZnxS/CdTe/CdTe:As(p+)was fabricated on a 45×90 mm2 substrate of SnO2:F (TEC7,Pilkington NSG). A highly resistive transparent (HRT) ZnOlayer (50nm) was sputtered from a ceramic ZnO target andthe Cd(1−x)ZnxS film was deposited according to the methoddescribed above. The CdTe (2 µm) and doped CdTe:As (0.5µm) layers were deposited using MOCVD and an in-situchloride activation treatment was also performed according tothe procedures described in ref. [4]. The device was contacteddirectly with a grid of 72 5 × 5 mm2 gold contacts. Thecollection of current-voltage, J − V , data, under AM1.5illumination, from each of these ‘cells’ was automated usingan in-house constructed scanning stage. External quantumefficiency, EQE, measurements were performed on a sub-setof the cells.

III. RESULTS AND DISCUSSION

Fig. 1 shows the resultant x profile for the Cd(1−x)ZnxSfilm, as determined by the automated optical transmit-tance measurement and fitting procedure. The profile wariessmoothly over the range x = 0.1 − 0.8. The film thick-ness varies in the narrow range of 180 − 200 nm over themajority of the layer (increasing to ∼ 230 nm) in the topright corner). This helped ensure that the effect of thickness

Fig. 1: Profile of Zn/(Cd+Zn) ration x, across co-sputteredCd(1−x)ZnxS film. x varies almost linearly in the range0.1 − 0.8 from top to bottom. The black contour lines showthe variation in thickness profile was determined using spec-troscopic ellipsometry.

variation on the resultant cell performance was minimised dur-ing the subsequent analysis of cell performance with respectto Cd(1−x)ZnxS film composition. Fig. 2 shows a contourmap of the extracted cell conversion efficiency, η, for thecompleted device structure with incorporated Cd(1−x)ZnxSfilm. The profile varied very smoothly indicating a high degreeof consistency between cells. Cell efficiency was recordedin the range 2 − 11%, with an average cell efficiency of10.0 ± 2.1% over the 72 contacts. Fig. 3 shows the directrelationship between x, the cell conversion efficiency, η, andthe corresponding performance parameters: JSC , VOC andFF . Each distribution was calculated by combining the datafrom contour plots such as those shown in Fig. 1 and Fig.2. The distributions are generally continuous, with varyingdegrees of scatter, and show a clear relationship with respectto x. The main source of scatter within the distributionsis attributed to a non-uniformity of the chloride annealingtreatment, with cells at the very edge of the sample beingover-treated, and hence having lower performances, relativeto those in the central portion of the sample. All ‘edge cells’are identified separately in the sub plots of Fig. 3. Apart

Fig. 2: Efficiency map of completed device having aCd(1−x)ZnxS window layer. A direct comparison with Fig. 1indicates that the efficiency is maximised for values of x in therange 0.5− 0.6. Fully black areas of the profile represent cellefficiencies below 8%. The dotted line indicates the locationsof J − V and EQE measurements shown in Fig. 4 and Fig. 5respectively.

from scatter, there are some systematic variations in the data,with there being cusps in the curves at x ∼ 0.25, 0.32, 0.4,0.5, 0.62 and 0.7 (see e.g. the JSC curve). This is causedby the systematic variation of the series resistance in themeasurements as the plate is probed. There is an especiallystrong cusp at x = 0.75, which looks different: it may be dueto a particularly resistive composition of Cd(1−x)ZnxS - thisshould be investigated further.

Efficiency is shown to increase from ∼ 9% to a maximumof 11.8% (a 24% relative increase) over the range x = 0.1−0.57. This maximum in the cell efficiency corresponded tothe following parameters: JSC = 25.1 mAcm−2, VOC = 700mV, FF = 67.4%. It is clear that the distribution of efficiencywith respect to x is dominated by changes in FF . In the rangex = 0.1−0.57, FF values show a relative increase of ∼ 20%before decreasing sharply as x increased further.

Short circuit current, JSC , values increased overall from ∼23 to 25 mAcm−2 (a 5% relative increase) over the range x =

Fig. 3: Measured device parameters plotted with respect tox. Cell efficiency increases with increasing x up to a value ofx = 0.57. Beyond this value, efficiency drops off rapidly. Thedotted line indicates the value of x at which the maximumcell efficiency was recorded.

Fig. 4: Series of J − V measurements made along linearcross-section of cell grid (see Fig. 2. The dominant gains inefficiency are shown to be due to improvements in FF valuesas x increases in the range 0.1 − 0.57. For all cells the VOC

remains relatively independent of x.

Fig. 5: Series of EQE measurements made on the cell gridat the points shown in Fig. 2. The effect of increasing x isshown to shift the left shoulder of EQE curves towards shortedwavelengths. Beyond the value of x = 0.4 the increase in blueresponse is limited by the band gap of the ZnO HRT layer(∼ 3.2 eV).

0.1−0.57. This increase is likely to be due to a shift in the bandgap of the Cd(1−x)ZnxS from ∼ 2.3 to 3.0 eV (determinedvia ellipsometry). Despite the expectation of further increasesin the JSC with x, values plateau beyond x = 0.4 beforedropping off at x > 0.6. The decrease in JSC and FF values,and hence in η, at high values of x is likely to be due to asharp increase in the resistivity of the Cd(1−x)ZnxS film as itbecomes zinc rich. The VOC is shown to be independent of xover its full range, maintaining a level of ∼ 0.7 V.

Fig. 4 shows a series of J − V curves for the cell sub-set

shown in Fig. 2 (dotted line), the colours of the J −V curvesalso correspond directly to those in the contour plot. The seriesconfirms that the gain in efficiency, as x increases towards theoptimal value of 0.57, is dominated by improvements to theFF with secondary gains generated from an increase in theJSC . The origin of the increase in FF values is likely to bedue to a decrease in the series resistance associated with animproved interface between CdTe and Cd(1−x)ZnxS layers.Further investigations are required to determine how the re-lated structural and electrical changes within the Cd(1−x)ZnxSfilm affect device performance. The corresponding EQE mea-surements for the series show that the increase JSC with x isdue to the shift of the left shoulder of the EQE towards towardsshorter wavelengths. This shift corresponds to an increase inthe band gap of the Cd(1−x)ZnxS. Beyond values of x ∼ 0.4,further increase in the blue-response (and hence the JSC) islimited by the band gap of the HRT ZnO layer (∼ 3.2 eV).The removal of this layer or its replacement with a higherband-gap material (e.g SnO2, EG ∼ 3.7 eV) will most likelyallow higher JSC values to be achieved.

IV. CONCLUSIONS

The combinatorial methodology described in this work wasdemonstrated to be highly effective for the optimisation ofCdTe solar cells. An unambiguous relationship was determinedbetween the composition of the Cd(1−x)ZnxS window layerand cell efficiency, and an optimal value of x = 0.57 wasidentified. EQE measurements who that further increases inJSC are limited by the band gap of a HRT ZnO layer. Sucha layer is widely used to ensure that device performanceis maintained when using then(< 100 nm) CdS windowlayers. The incorporated of a relatively thick (∼ 200 nm)Cd(1−0.43)Zn0.57S layer should eliminate the need for a HRTlayer. This would permit additional gains to the JSC , viafurther improvements of the blue-response, while permittingVOC and FF values to be maintained.

Conventionally, the determination of the relationship be-tween x and η would have required the fabrication of a multi-sample set over which the value of x is varied incrementally.Unavoidable drifts in the run-to-run deposition conditions oversuch a multi-sample set would result in a relationship with amuch higher degree of scatter in the data. The combinatorialapproach is extremely powerful as it eliminates this scatterthrough use of a single sample. Furthermore, the rate at whichinvestigations can be made is greatly increased.

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