Suitability of atmospheric-pressure MOCVD CdTe solar cells for inline production scale

CdTe, with a direct band gap of 1.45 eV is well suited to the terrestrial AM1.5 solar irradiance and currently makes up half of the thin film (TF) photovoltaic (PV) market. There are 4 main factors that determine final cost of PV modules: the conversion efficiency, materials amount per unit area of module, production yield, and the economy of scale. It is therefore valuable to investigate alternative and/or innovative deposition techniques and processes which have the potential to impact on these factors. Metal organic chemical vapour deposition (MOCVD) is a powerful technique offering increased process repeatability, achieving a high level of control of materials characteristics. Recent improvements in CdTe devices using atmospheric-pressure (AP) MOCVD have led to efficiencies of 13.3 % using 2 μm absorber layers and 11 % with a 1 μm absorber layer [11, 12]. These results were achieved: by extending the optical band gap of the window layer, using a ternary alloy (Cd _0.9Zn_0.1S), with intentional p-type doping of the CdTe layers with As , and the use of an in situ (dry) deposited CdCl₂ layer and anneal. All layers, except the front and back contact, are grown by a sequential MOCVD process. Furthermore this is a dry process without the need for any etch treatment. The inherent design of the horizontal MOCVD laboratory chambers do not lend themselves well to large scale production. However, the CSER group has designed and built an experimental inline reaction chamber to evaluate AP-MOCVD as an inline production process. Discussion is made based on kinetically limited growth and molar supply models to assess the suitability of the MOCVD process to deposit fast enough for an inline process. The AP-MOCVD inline reactor uses a showerhead to deliver the precursors onto a moving 5 x 7.5 cm² substrate and preliminary results for deposited layers are given. From these preliminary results it has been extrapolated that a 1 μm thick CdTe layer can be deposited on substrates moving at 60 cm/min.