Thin Solid Films 403–404(2002) 522–525

0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž01.01547-4

Microstructural changes of CdTe during the annealing process

A.J. Chapman *, D.W. Lane , K.D. Rogers , M.E. Ozsana, a a b¨

Department of Materials and Medical Sciences, Cranfield University, RMCS, Shrivenham, Swindon, SN6 8LA, UKa

Teksolar Ltd, 41 Queen Elizabeth way, Woking, UKb

Abstract

Complete thin film solar cells have been fabricated by physical vapour deposition of CdTe onto chemical-bath-deposited CdSsupported on a commercial SnOyglass substrate. CellI–V characteristics were measured under AM 1.5(G) illumination. Energy2

dispersive X-ray analysis was used to measure the average stoichiometry. Nuclear Reaction Analysis was used to measure thedegree of S diffusion within the CdTe thin films. X-ray diffraction was used to determine the lattice parameter of the CdTe.Results showed the CdTe films deposited at low deposition rates(-2 nm s ) exhibited greater S diffusion after a partialy1

recrystallisation during annealing. Higher CdTe deposition rates correlated to increased Te concentration for the as-deposited films,significantly reduced during annealing.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Sulfur; Diffusion; Cadmium telluride; Cadmium sulfide

1. Introduction

A full understanding of the microstructural changesin thin film CdSyCdTe heterostructures is a necessarycondition to improved solar cell efficiencies. For exam-ple, interdiffusion between the CdTe absorber and theCdS window layer is believed to accommodate thelattice mismatch between layers and hence improve thequality of the junction, whilst also reducing the opticalbandgap of the window layerw1,2x. Our previous workw3x has examined the time dependence of S diffusionusing synchroton X-ray diffraction data, which occursboth during and following recrystallization of the CdTelayer.

2. Experimental details

CdSySnO yglass substrates were fabricated by the2

process previously described by Ozsan et al.w4x. The¨

fabrication of the completed heterostructures has beenoutlined previously by Chapman et al.w5x and involved

* Corresponding author. Tel:q44-1793-785-531; fax:q44-1793-785-772.

E-mail address: a.j.chapman@rmcs.cranfield.ac.uk(A.J. Chapman).

a CdS anneal in air at 4008C for 20 min followed byan acetic acid etch. A 2-mm-thick layer of CdTe wasthen deposited by physical vapour deposition(PVD) atrates varying from 1 to 6 nm s . CdCl was theny1

2

deposited by PVD to a thickness of 50–100 nm on halfof the samples before all were annealed in air at 4508C.Anneal times of 5, 10 and 20 min were examined, afterwhich the samples were cooled in air at room tempera-ture. Completed solar cells were fabricated by theapplication CuI solution, made by dissolving 0.73 g ofCuI into 25 cm of acetonitrile(99.7% purity), on to3

the CdTe, followed by drying in air and spray applicationof a carbon back contact.

X-Ray diffraction(XRD) data were collected using aSiemens D500 powder diffractometer and Cu Ka radi-ation with a diffracted beam monochromator. Data werecollected over a 2u range of 22–788 in order to examinethe cubic CdTe(111), (220) and (333) peaks bothbefore and after annealing. Profile fitting of the(111)diffraction peak using an asymmetric Voigt functiongave the full width at half maximum(FWHM), integralbreath, integral intensity and peak position. The latticeparameters were calculated from the d-spacing of the(111) peak.

523A.J. Chapman et al. / Thin Solid Films 403 –404 (2002) 522–525

Fig. 1. Lattice parameter plotted against CdTe deposition rate.Fig. 2. CdTe(333) diffraction peak for films deposited at 1.5 nms .y1

Fig. 3. CdTe(111) diffraction peaks for CdTe deposited at 1.5 nms .y1

A JEOL JSM-840A scanning electron microscopewas used to examine film morphology and to providethe average composition of each CdTe layer. A 25-keVelectron beam at 1 nA was used, with energy dispersiveX-ray analysis with respect to a reference standard ofsingle crystal CdTe.

The S distribution within the annealed heterostructureswas examined using Nuclear Reaction Analysis NRAemploying the S(d, p ) S reactionw6x at an incident32 33

0

deuteron energy of 2.5 MeV. Three samples were exam-ined; thin film CdS, and two CdTeyCdS heterostructures(deposited at 1.5 and 6 nm s).y1

The electrical properties of the completed cells wereassessed by measuring theirI–V characteristics andquantum efficiency. The I–V characteristics wereobtained using an AM 1.5(G) solar simulator that wascalibrated using a gallium arsenide solar cell of knownefficiency.

The method of obtaining quantum efficiencies hasbeen described previouslyw7x and the resulting datawere analysed by assuming a simple two layer hetero-junction with no intermixingw8x. Analysis involved astatistical Pearson correlation coefficient calculationbetween the degree of recrystallisation(220y111 peakarea ratio) and the quantum efficiency, assuming acontinuous linear response. Grouping the cells accordingto the degree of recrystallisation and performing aStudent’s t-test on their quantum efficiency thenassessed the significance of recrystallisation.

3. Results and discussion

The CdTe lattice parameter calculated from the posi-tion of the (111) peak is plotted as a function of itsdeposition rate in Fig. 1. This suggests increased Sdiffusion at the lower deposition rates for the Cl-treatedsamples, as indicated by a decrease in the lattice param-eter, with less S diffusion at the higher deposition rates.This assumes that the decrease in lattice parameter issolely caused by the presence of S within the CdTelattice. Fig. 2 details the CdTe(333) diffraction peakfor a 2-mm film deposited at 1.5 nm s . The asymmetryy1

in the peak confirms S diffusion and is consistent tosimilar work by McCandless et al.w9x.

Fig. 3 shows the CdTe(111) diffraction peaks forspecimens deposited at 1.5 nm s , and annealed for 5y1

and 10 min. This suggests that S diffusion occurs afterrecrystallisation as indicated by the difference in peakposition. Both annealing times demonstrated somedegree of recrystallisation, as indicated by the ratio ofthe 111y220 peak areas.

A similar analysis was done for CdTe films grown atthe higher deposition rate. The(111) peak for thesespecimens is shown in Fig. 4, for a 5- and 10-minanneal. The smaller shift in peak position demonstratesthat there is less S diffusion at the higher rates ofdeposition.

The NRA results for CdS thin film and the annealedCdSyCdTe heterostructure are shown in Fig. 5. Boththe high and low CdTe deposition rates demonstratedsimilar spectra. The resolvable shift in the front edgeposition indicates that the S remains at or near theinterface after diffusion for both CdTe deposition rates,with no evidence for S diffusion through to the exposedface of the CdTe film.

Fig. 6 shows the CdTe films variation in the com-y

position(y) with deposition rate. The electron micropro-be indicated that the as-deposited CdTe films are Te-rich,with an increase in the Te content,(i.e. y) with CdTe

524 A.J. Chapman et al. / Thin Solid Films 403 –404 (2002) 522–525

Fig. 4. CdTe(111) diffraction peaks for CdTe deposited at 6 nms .y1

Fig. 5. NRA results for thin film CdS and annealed CdSyCdTe.

Fig. 7. Quantum efficiency as a function of the degree ofrecrystallisation.

Fig. 6. Variation of the composition(y) of the CdTe films as a function of deposition rate.y

deposition rate. Significant changes in stoichiometryoccur after annealing, withy decreasing toyf1.02,suggesting evaporation of excess Te. Fig. 6 also showsthat specimens annealed in the presence of Cl becomeTe-deficient,(i.e. Cd rich). This suggests that Cl acts asa flux for Te evaporation, possibly as a result of theformation of more volatile TeCl .4

The results for the quantum efficiency measurementsare shown in Fig. 7. The Pearson correlation coefficientwas found to bers0.64, and thet-test indicated thatthe difference in the two groups shown was significant(P- 0.05). The quantum efficiency for the group thatexhibits the greater degree of recrystallisation was foundto be just below 70%, indicating that recrystallisationmay result in higher quality CdTe with a larger grainsize and fewer traps. The mean quantum efficiency forall samples was 53%, suggesting reasonable carriergeneration. When this is compared with theI–V curvein Fig. 8 the reasonable open circuit voltage and thepoor short circuit current(I ) suggests that the problemsc

with the completed heterostructure was with the backcontact.

4. Conclusion

In summary, this work has found that CdTe filmsdeposited at deposition rates-2 nm s exhibited Sy1

diffusion after a partial recrystallisation during anneal-ing. It should be noted that rates-2 nm s arey1

commonly used to deposit CdTe films for solar cell

525A.J. Chapman et al. / Thin Solid Films 403 –404 (2002) 522–525

Fig. 8. I–V curve for completed CdSyCdTe cell.

applications. We have also shown that higher depositionrates correlate to increases in the Te concentration ofas-deposited CdTe films. This is significantly reducedduring the type conversion anneal, giving a Cd-rich filmin the case of specimens annealed in the presence ofCdCl . The poorer quantum efficiencies resulting from2

the corresponding lower deposition rates suggest that

the lower degree of recrystallisation may allow manyfilm defects to remain.

Acknowledgements

This work is supported by EPSRC under awardnumber 98000728, and was sponsored by BP Solar.

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