A Rapid Screening Method for Investigating the Effect of Processing Parameters on CdTe/CdS Solar Cell Performance
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A Rapid Screening Method for Investigating the Effect of Processing Parameters on CdTe/CdS Solar Cell Performance Mohammed Al Turkestani1*, Ken Durose1, Ben Wakeling2, David Lane2, Stuart Irvine3 and Vincent Barrioz3. 1 Department of Physics, Durham University, The Science Laboratories, South Rd, Durham, DH1 3LE, UK 2 Centre for Materials Science and Engineering, Dept of Materials and Medical Sciences, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK 3 Centre of Solar Energy Research, Glyndŵr University/ OpTIC Technium, St Asaph Business Park, St Asaph, LL17 OJD, UK ABSTRACT A rapid screening method is reported in which material processing parameters are investigated as a function of the CdTe absorber thickness in CdTe/CdS solar cells. The technique has been used to investigate i) the optimum absorber thickness for CdCl2 processing at 380°C for 10 mins, and ii) the effect on device performance of post-growth annealing of CdS layer with H2, N2, and O2. It was found that the optimum thickness of CdTe compatible with the processing conditions was ~3µm. The device results were independent of the post-growth treatment of the CdS for the conditions investigated here. The bevel method allowed for ~30 data points to be obtained from each sample, giving a significant advantage over conventional experimental methods. INTRODUCTION Contrary to the way in which silicon based microelectronic devices and many optoelectronic devices are designed and fabricated, the development of thin film polycrystalline devices has been largely conducted by process optimisation. In the case of the CdTe/CdS/TCO/glass solar cell, the contacting, CdCl2 processing, interdiffusion, grain boundaries, impurities (including oxides) are all thought to interact. The number of experiments required for complete optimisation is therefore very large. As a result there is a need for rapid screening methodologies with the aim of reducing the number of samples required in order to fully explore a region of processing parameter space. Such methods may find application both in existing technologies, and for new and emerging thin film systems. One particular embodiment of rapid screening is the use of a wedge-shaped sample in order to simultaneously explore a range of film thicknesses in a single processing run. In this work the use of a bevelling method to produce CdTe/CdS samples having a range of CdTe thicknesses has been explored. The CdTe thickness is therefore selected to be the primary variable. Two variations of this methodology have been explored: screening process A, in which the sample was bevelled prior to CdCl2 processing, and screening process B, in which the bevelling followed the CdCl2 treatment step. Both methodologies were applied to each of three kinds of cells in which the CdS had itself been processed differently i.e. treated with N2, H2 and O2 (as described in the Experimental section).
In a preliminary study using this same method, fully clear results were not obtained and that was attributed to non-uniformity in the plate
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