Chapter 5: Thin Film Cadmium Telluride Solar Cells

This chapter discusses a number of deposition techniques used to produce polycrystalline CdTe solar cells, including progress of photovoltaic (PV) performances in recent years. Focus is on the CdTe absorber and the effects from impurities, which are dependent on the process conditions used, but also due to self-compensating nature of the material itself influencing the acceptor levels in the layer. Impurities can introduce deep donor/acceptor levels that act as traps for both majority and minority carriers. This leads to greater recombination and reduced carrier lifetimes, causing a loss in the level of generated photocurrent and overall performance of the PV cell. Impurities are typically concentrated at the CdTe grain boundaries, making grain size an important parameter for defect density control. Post-growth treatment using CdCl₂ and annealing improves PV performances in several ways: grain re-crystallisation and growth; inter-diffusion at the CdS–CdTe interface removing defects related to the lattice mismatch between the two layers; and passivation of deep acceptor states through complex formation with the Cl_Te⁺ shallow donor. High p-type doping is necessary for the formation of a back contact with good ohmic properties without a Schottky barrier restricting conduction of majority carriers. Stable back contacts are also required, with the Sb₂Te₃–Mo system possibly offering the best solution. Finally MOCVD is presented as a prospective technique for large-scale industrial production of CdTe solar modules, with discussion of the beneficial impact in reducing CdTe absorber thickness and the processing challenges associated with it.