Abstract of Chapter 3

Ohmic contacts on p-type CdTe are difficult to achieve, since the CdTe electron affinity of around 4.3 eV combined with a band gap of 1.45 eV is quite high. Theoretically, only metals with work functions higher than 5.5 eV would allow Ohmic contacts. As a consequence, almost all metals would form a Schottky barrier. This Schottky barrier is then reverse biased to the main junction and limits the cell performance. In order to achieve a quasi Ohmic contact free of loss, a modification of the CdTe surface is needed.
The usual approach is a wet chemical etch of the CdTe which creates a Te layer on the surface by dissolving Cd and the subsequent deposition of a buffer material between the semiconductor and the metal contact. The Te layer on the surface acts as an effective p+-doping, which, together with the thin buffer layer, influences the fermi level position andband alignment. Layer intermixing and doping effects are finally achieved by moderate annealing. The conventional back contact containing Cu exhibits low barriers, but usually cell performance degrades severely in the long term. In order to get stable cells and “non-rectifying” contacts, the CdTe surface modification processes — i.e., chemical etching, buffer layer deposition, post-deposition annealing, and substitutional Te deposition by evaporation in order to avoid wet chemical processing — have been investigated with respect to performance and stability.