CdTe cells

CdTe cells

Thin Solid Films 361±362 (2000) 321±326 www.elsevier.com/locate/tsf Microstructure of electrodeposited CdS/CdTe cells Daniel R. Johnson BP Solarex Te...

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Thin Solid Films 361±362 (2000) 321±326 www.elsevier.com/locate/tsf

Microstructure of electrodeposited CdS/CdTe cells Daniel R. Johnson BP Solarex Technology Centre, 12 Brooklands Close. Windmill Road, Sunbury-on-Thames, Middlesex TW16 7DX, UK

Abstract The microstructures of chemical bath deposited CdS and electrodeposited CdTe polycrystalline thin ®lms, in n-CdS/p-CdTe solar cells, were studied. In both CdS and CdTe, the effect of grain size, re-crystallisation and interdiffusion are linked to the tin oxide/glass substrate and deposition control. Optical characterisation and device measurements are used to qualify these changes in the context of solar cell fabrication. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: CdS; CdTe; Photovoltaic; Tin oxide; Chemical bath; Electrodeposition; Microscopy

1. Introduction

o E ˆ ETe 1 RT=4Fln…aHTeO12 =aTe † 1 3RT=4FlnC1 H

n-CdS/p-CdTe thin ®lm heterojunctions provide an opportunity for low cost photovoltaic modules on conducting tin oxide coated glass (TCO). BP Solar's `Apollo' solar cell technology utilises aqueous solution chemical deposition of CdS and electrodeposition of CdTe. In chemical deposition of CdS, numerous TCO substrates are batch coated in a one shot decomposition reaction at ~758C

…8†

Cd‰COMPLEXŠ21 ! Cd21 1 COMPLEX

…1†

…NH2 †2 CS 1 OH2 ! HS21 CH2 N2 1 H2 O

…2†

However, the deposition potential of the Cd is shifted by an amount 2DG/2F, which provides the free energy of reaction to produce CdTe [1]. For an observed voltage plateau between 2250 and 2650 mV vs. Ag/AgCl, CdTe of nominal stoichiometry can be produced at an applied potential, VA, of 2400 mV. [2]. In practice, deposition onto largearea substrates leads to non-uniformity in VA, arising from the electrical resistivity of the TCO. Electrochemical engineering has been used to some extent to reduce these effects. The technology has produced devices on 30 £ 30 cm TCO with conversion ef®ciencies of 10% and pilot production batches with an average of 8% [2].

HS2 1 OH2 ! S22 1 H2 O

…3†

2. Experimental details

Cd21 1 S22 ! CdS

…4†

CdTe was electrodeposited onto the coated side of 30£30 cm sheets of CdS/SnO2/Glass substrates, after ®rst making electrical contacts to their edges and then immersing as cathodes into a 0.5 M CdSO4 plating solution at 708C and pH 1.5, with HTeO21. The deposition was carried out under potentiostatic control and a quasi-rest potential [1], QRP, of 2400 mV, was used to determine a suitable VA, of 2600 mV vs. Ag/AgCl to deposit CdTe. Post-deposition, small coupons were cut from the sheets and used to assess the uniformity of optical, device and microstructural properties associated with the TCO and VA. In microstructural analysis, SEM was performed using a JEOL JSM-6300 on 1-cm 2 sections. For TEM work, planar sections of CdS and CdTe ®lms were prepared using a combination of chemical etching in a solution of Br2/methanol and de-lamination of the ®lms from the substrate onto an

Controlled hydrolysis of thiourea and surface adsorption ensures heterogeneous CdS ®lm deposition. Improved engineering and bath chemistry provides uniform ®lms of ,100 nm, and high utilisation of reagents. In the electrochemical deposition of CdTe an aqueous electrolyte of Cd 21 and lower concentration of HTeO1 2 are co-deposited on the CdS/TCO by Cd21 1 2e2 ! Cd

…5†

1 2 HTeO1 2 1 3H 1 4e ! Te 1 2H2 O

…6†

The deposition potential, E, for each is given by o 1 RT=2Fln…a21 E ˆ ECd Cd =aCd †

…7†

0040-6090/00/$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00779-8

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adhesive. Back-etching of the de-laminated ®lms at this stage enabled different slices to be taken throughout the ®lm thickness. Flakes of de-laminated ®lm were dispersed in solvent and dropped onto a 3-mm 2 TEM grid containing lacy carbon. Cross-sectional TEM samples were prepared by ion beam thinning. TEM analysis was conducted using diffraction contrast to provide bright-®eld images of grains using a JEOL 200CX and lattice images were provided using this instrument and a JEOL 4000EX. Optical studies were performed on CdS using a `Monolite' spectrophotometer. Devices were produced by making electrical contacts to the CdTe, using 4-mm gold dots by vacuum evaporation, after an initial treatment to remove surface oxides. 3. Results Experiments were carried out using commercially available TCO from Libbey Owens Ford to determine (1) the in¯uence of tin oxide morphology and (2) deposition control on devices from BP Solar's standard thin ®lm `Apollo' technology. First, a microstructural and optical survey was performed to determine normal ®lm properties followed by an investigation of departures from this behaviour. 3.1. Normal ®lm properties 3.1.1. Tin oxide To discover the dependence of CdS/CdTe on tin oxide morphology, two types were chosen, type I being `tec10' with a sheet resistance of ~10 V and 5% haze and type II, Kglass, with ~15 V and 1% haze. Fig. 1 shows SEM images of the two, where Type I has large grains (~0.3 mm) and rougher morphology (~100 nm RA) than type II with small grains (~0.1 mm) and smooth morphology (~30 nm RA) (measured using a Dektak II surface pro®lometer). 3.1.2. Cadmium sulphide Films from chemical bath deposition have been produced on both tin oxide types. The SEM image in Fig. 2a shows the nodular structure of CdS with features similar in size to the morphology of the underlying type I tin oxide. The ®lm viewed after air annealing at 4008C is in fact a composite made up from small grains ~10 nm in size and randomly orientated (Fig. 2b). Additional image contrast arising from thickness variations is caused by the deposition of CdS into the underlying type I tin oxide morphology. Electron diffraction rings indicate a spacing typical of the cubic form F-43m. Fig. 2c shows a grain containing stacking faults, consistent with polytypism between cubic and hexagonal phases [3]. CdS ®lms coated with CdCl2 and heat treated at 4008C produce signi®cant inter-grain fusion and grain growth (Fig. 3a). Similarly, following deposition and annealing to recrystallise and type convert the CdTe layer, the underlying CdS ®lm undergoes the same re-crystallisation and grain

Fig. 1. Type I and Type II Tin oxide TCO.

growth. In both cases CdS becomes predominantly the hexagonal phase and is highly oriented, with the 112Å0 axis normal to the substrate. Defects are replaced by larger twins boundaries from [112Å0] to [0001] orientation, with a Ê along [101Å0]. lattice spacing of 3.59 A The in¯uence on the optical band gap, Eg, of CdS ®lms from typical deposition and re-crystallisation processes is summarised in Table 1. The data were derived from studies on the optical absorption, a , and calculated from plots of a n vs. photon energy (eV). In the case of CdS underlying annealed CdTe, Eg is 2.25 eV compared to an Eg of ~2.4 eV for CdCl2 re-crystallised CdS. The difference is associated with Te diffusion into CdS from CdTe during recrystallisation, reducing the optical transmission and magnitude of Jsc in the resulting device [6]. 3.1.3. Cadmium telluride Electrodeposited CdTe grows as columnar grains ~0.1 mm in cross-section, with a strong [111] preferred orientation normal to the substrate. Fig. 4a shows a TEM image of a planar section from an as-deposited CdTe ®lm and Fig. 4b a

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Table 1 Variation in the optical properties of CdS CdS status

Grain speci®cation

Optical band gap (eV)

As-deposited (standard formulation) Air-annealed on type I TCO Air-annealed on type II TCO Re-crystallised with Cl ¯ux Re-crystallised under annealed CdTe

Indistinct cubic

2.42

10 nm random cubic

2.15

6 nm random cubic

2.26

300 nm 112Å0 oriented hexagonal 100 nm 112Å0 oriented hexagonal

2.46 2.25

reduced and larger grains up to 1.5 mm across appear by lateral fusion of grains. X-ray and electron diffraction show an increase in the [220] orientation amongst the larger grains. In Fig. 5a the grains of fused [220] type are characterised by twinning and some intra-grain pores. Fig. 5b shows a lattice image of a typical {111} /{111} boundary. Here, transformational twinning and mismatch strain yields a non-stoichiometric surface with associated edge dislocations having b ~1/2k110l. Durose et al. describe similar twinning in the (112) plane and suggest their role in possible carrier recombination [4]. The in¯uence of grain boundaries on the performance of polycrystalline solar cells has also been investigated [5]. 3.2. Variations arising from TCO morphology and deposition control

Fig. 2. (a) SEM image of air annealed CdS. (b) TEM bright-®eld image of a CdS planar section removed from the tin oxide, showing 10 nm grain contrast. (c) Lattice image of a 10 nm grain with stacking faults.

corresponding cross-section. The material exhibits poor crystallinity in electron diffraction and is electron beam sensitive producing molten material after a short duration of exposure. Air-annealed CdTe samples are more stable in the microscope, which con®rms the propensity for the former to undergo re-crystallisation. In cross-section, the grains of as-deposited CdTe contain striations in the plane of the ®lm and lattice images show a high density of stacking faults possibly arising from rotational strain around the (111) plane from ®lm deposition. The defects terminate as edge and interstitial loop dislocations with burgers vector, b, ~1/2k111l (Fig. 4c). Air annealing at 4508C produces re-crystallisation in the CdTe ®lm and formation of a CdTe/CdS heterojunction. The preferred [111] orientation to the substrate is normally

3.2.1. Cadmium sulphide In Table 1, tin oxide appears to in¯uence both the grain size and optical band gap of air annealed, chemical bath deposited CdS. The effect of these changes on the device is small, however, compared to the extent of re-crystallisation and inter-diffusion expected from the chloride ¯ux used in the CdTe anneal. 3.2.2. Cadmium telluride Under normal conditions of annealing, type II TCO produced smaller CdTe grains, ~0.3 mm in cross-section (Fig. 5a), found typically within a bi-modal size distribution ~12 cm either side of the centre of substrate. In type I TCO, the grain size was uniformly larger (~1 mm) for the same centre portion of the substrate (Fig. 5a) and as in the type II substrate, wholly small grained in the vicinity of the plating contacts at the substrate edges. Photovoltaic devices formed from CdTe with small versus large grain size are illustrated in Fig. 6a,b. Poor devices are attributable to material with a high occupancy of small-grain CdTe and are signi®ed by a maximum current density, J (mA/cm 2), dependent on voltage biasing to over-

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for only a few hours. The lost component may be readily recovered after a short duration heat treatment at ~1508C. For both cited examples of small-grained CdTe, further use of CdCl2 and annealing produces signi®cant intergrain fusion and growth, which in turn re¯ects in improved Jsc, represented by the I±V curves typifying larger grain material in Fig. 6a,b.

4. Discussion and conclusions Substrate morphology and non-uniformity in VA have a

Fig. 3. TEM image of CdS on type I TCO with (a) The ®lm re-crystallised using a chloride containing ¯ux at 4008C; (b) CdS underlying annealed CdTe; (c) Lattice image of a grain from b showing the highly oriented [112Å0] hexagonal phase.

ride carrier recombination. In these devices, short-circuit current density, Jsc, exhibits a further degradation by ~30% after exposure to light of 100 W/cm 2 and AM1.5

Fig. 4. TEM images of as-deposited CdTe on type II TCO. (a) Planar section through the [111] oriented ®lm. (b) Cross-section of ®lm with the k111l direction indicated with respect to the TCO substrate, showing striations. (c) Lattice image of the same cross section, showing striations as stacking faults.

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nantly columnar [111] oriented grains fuse into larger [220] type. The transformation con®rms the role of grain boundaries as sites for recombination and has signi®cant implications for both device performance and stability under irradiation. Grain size and the ratio of [111]:[220] is in¯uenced by three factors: ² normal re-crystallisation of CdTe, where ‰220Š . ‰111Š under conditions of increasing time at temperature [6]; ² non-uniformity in VA, where ‰220Š . ‰111Š as cathodic potential reduces with distance from the plating contacts, owing to TCO resistivity; ² substrate morphology, where ‰220Š , ‰111Š for smallergrained TCO.

Fig. 5. TEM images of (a) diffraction contrast of bi-modal distribution of small and large CdTe grains, and (b) lattice image of the grain boundary, GB, between two small [111] oriented grains.

similar detrimental effect on grain growth of CdTe derived from electrodeposition and placed under normal conditions of anneal. The presence of sites for bias-dependent carrier recombination may be removed by employing CdCl2 ¯ux which further extends grain growth, where the predomi-

CdTe re-crystallisation is most strongly dependent on the presence and concentration of a chloride ¯ux, but in this study is shown to be sensitive to deposition and morphological effects. Separate studies have shown that reduced VA produces CdTe containing an excess of tellurium in the deposit. Defects accommodating this non-stoichiometry would seem to assist nucleation and grain growth in CdTe. The role of substrate is less understood, but again appears to limit lateral nucleation between grains. In conclusion, improving device performance of CdS/ CdTe solar cells from electrodeposition requires careful tailoring of the re-crystallisation process, primarily to maximise CdTe grain size. However, this process will need to account for inter-diffusion with the underlying CdS [7], which to excess has yielded greater opacity and consumption of the layer, affecting Jsc and device voltage, respectively. Future work on device improvement will concentrate on factors in the CdS layer, such as thickness, stoichiometry, defect density and grain size, and should also consider other possible window layers.

Fig. 6. Typical I±V characteristics for devices from ®lms with variable CdTe grain size: (a) comparison of type I to type II TCO; (b) positional variation in devices associated with VA in type I TCO.

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5. Terminology

R F T a Eo Eg CH1 VA RA J Jsc

gas constant Faraday constant temperature activity coef®cient standard reference potential in¯uence on optical band gap acid concentration applied potential in CdTe electrodeposition .v. Ag/ AgCl average roughness of a surface current density (mA/cm 2) short-circuit current density

Acknowledgements I am grateful for the TEM work, which was carried out by

Dr I. Horsfell (RMCS Cran®eld), Dr J.L Hutchison (Oxford University), Dr. D. White (BP/AMOCO) and also BP Solar's UK Shared Technology Team for their support.

References [1] F.A. Kroger, J. Electrochem. Soc. 125 (1978) 11. [2] J. Barker, S.P. Binns, D.R. Johnson, et al., Int. J. Sol. Energy 12 (1992) 79. [3] D. Lincot, B. Mokili, M. Froment, R. Cortes, M.C. Bernards, C. Witz, J. Lafait, J. Phys. Chem. B 101 (1997) 2174. [4] K. Durose, G.J. Russell, Microsc. Semicond. Mater. Inst. Conf. Ser. 87 (1987) 327. [5] L.M. Fraas, J. Appl. Phys. 49 (2) (1978) 87. È zsan, D. Johnson, S. Oktik, M.H. Patterson, D. Sivapathasun[6] M.E. O daram, J.M. Woodcock, Proc. 12th Eur. Photovoltaic Sol. Energy Conf. 2 (1994) 1604. È zsan, D. Johnson, D.W. Lane, K.D. Rogers, Proc. 12th Eur. [7] M.E. O Photovoltaic Sol. Energy Conf. 2 (1994) 1600.