Grain size control in sublimation-grown CdTe solar cells
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Grain size control in sublimation-grown CdTe solar cells J. D. Major, Y. Y. Proskuryakov and K. Durose Department of Physics, Durham University, South Road, Durham, DH1 3BH, UK ABSTRACT The early stage formation mechanisms operating during the sublimation growth of CdTe films on CdS has been evaluated using a growth interrupt methodology for deposition under 100 Torr of N2. Key stages of the growth were identified and are discussed in terms of the processes of island nucleation, island growth/coalescence, channel formation and secondary nucleation that have been reported for other materials systems. It was demonstrated that the grain size could be manipulated by means of controlling the gas pressure in the range 2 – 200 Torr, with the grain diameter increasing with pressure linearly as D (µm) = 0.027(± 0.011) x P (Torr) + 0.90(± 0.31). For a series of solar cells made using such material, the performance parameters were seen to increase with grain size up to a plateau corresponding to grains of ~4 µm in this case. Equivalent circuit parameters for resistive components arising from grain boundaries, and the contact to the CdTe, were measured. It is considered that grain boundary barriers in CdTe are harmful to PV performance, and that the plateau in performance occurs when the grain size is increased to the level where the contact resistance is greater than that due to grain boundaries. INTRODUCTION Grain boundaries in thin film solar cells are widely considered to be deleterious to efficient photovoltaic operation. In this work, study of the basic processes operating during growth by sublimation of CdTe films has been investigated. The results suggested a mechanism of controlling and increasing the grain size which is reported here. Cells made with a series of layers with increasing grain size were fabricated, and the efficiency limiting mechanisms are commented upon. Prior to describing the experiments, key papers on the action of grain boundaries in thin film CdTe are reviewed, and an outline of the grain structure properties of thin films presented. An account of the generic processes observed during the formation of thin metallic polycrystalline films is also provided, since it gives a framework for the interpretation of the growth processes observed in this work. Influence of grain boundaries on thin film solar cells Grain boundary and granular effects are considered to affect the performance and stability of thin film solar cells in the following ways: a) grain boundaries are conduits for chemical diffusion.1 Hence they may be generally implicated in mechanisms that affect the long term stability of polycrystalline devices. b) grain boundaries are associated with band bending2, and they may present electrical barriers to the transport of minority carriers. c) grain boundaries are generally considered to be associated with a sheet of electrical charge3 arising from either impurity decoration or ‘dangling bonds’. Whatever its origin, this charge is considered to be associated with deep levels, and t
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