Processing options for CdTe thin film solar cells

Processing options for addressing critical issues associated with the fabrication of thin film CdTe solar cells are presented, including window and buffer layer processing, post-deposition treatment, and formation of stable low resistance contacts. The paper contains fundamental data, engineering relationships and device results. Chemical surface deposited CdS and Cd1-xZnxS films are employed as the n-type heteropartner window layers. Maintaining junction quality with ultra-thin window layers is facilitated by use of a high resistance oxide buffer layer, such as SnO2, In2O3 or Ga2O3, between the heteropartner and the transparent conductive oxide. Thermal annealing of the CdTe/CdS heterostructure in the presence of CdCl2 and O-2 shifts the chemical equilibrium on the surface of the absorber layer, which influences the bulk electrical properties. Aspects of back contacting CdTe/CdS devices, including etching, Cu application, contact annealing, back contact chemistry and secondary contacts, are discussed. Two commonly employed etches used to produce a Te-rich layer, nitric acid/phosphoric acid mixtures and Br-2/methanol are compared, including the nature and stability of the final treated CdTe surface. The diagnostic abilities of the surface sensitive VASE and GIXRD techniques are highlighted. Various methods of Cu delivery are discussed with consideration to; reaction with Te, processing simplicity, processing time and possible industrial scale-up. Some aspects of back contact stability are presented, including discussion of apparent robust back contacts, which contain a thick Te component. (C) 2004 Elsevier Ltd. All rights reserved.