Development of Materials and Structures for p-type Contacts in Cadmium Telluride Solar Cells.

摘要

Solar cells based on CdTe absorbers are attractive due to the optimal direct band gap energy and large absorption coefficient of CdTe, however, their performance and commercialization is hindered by the lack of reliable p-type contacts. CdTe has a low carrier concentration and a large electron affinity, which results in a requirement of non-realistic work functions for metals to be used as back contacts in the solar cell. Even noble metals such as Ag present a significantly large potential barrier for holes, thereby reducing the hole current through the semiconductor/metal interface. Several attempts to resolve this challenge have been tried, however, many drawbacks have been encountered.;Two particular systems, namely Cu2Te thin films and CdTe/ZnTe strained-layer superlattices, are investigated for their potential use as ohmic contacts in CdTe solar cells. A detailed analysis of the optical, electrical, and structural properties of Cu2Te thin films deposited by magnetron sputtering is presented. It is shown that these films have an indirect band gap and highly degenerate semiconductor behavior. The large p-type carrier concentration of Cu2Te films is highly desirable for the application of Cu2Te as a p-type contact to CdTe.;In-depth studies of optical transitions and miniband transport in strained-layer CdTe/ZnTe superlattices are presented as well. The band offsets between CdTe and ZnTe were determined by comparison of measured and calculated optical transitions. Superlattice structures that offer best contact performance have been identified by use of tunneling probability simulations.;Characterization of CdTe solar cells with above mentioned contacts indicated that contacts based on CdTe/ZnTe superlattices are a viable Cu free option for stable and reliable p-type contacts in CdTe solar cell. The contact performance of Cu2Te thin films was comparable to that of CdTe/ZnTe superlattices and both demonstrated an advantage over contacts based on ZnTe:N thin films which were used a standard.