Local charge neutgrality condition, Fermi level, and carrier compensation of CdTe polycrystalline thin film in CdS/CdTe solar cells

In this paper we discuss the classification of localized intrinsic/impurity defect states in the band gap of semiconductors according to the charging and transition energy levels of the state being single or multiple, and according to the atomic configuration and formation of energy of the state being single or multiple. For semiconductors that have multi-level intrinsic/impurity defect states (such as Cd vacancies V_Cd^(o/-) and V_Cd^(-/2-) in CdTe thin film), the general formulation of charge neutrality condition is given to determine the Fermi level and majority carrier density. For semiconductors that have multi-configuration intrinsic/impurity defect states (such as acceptor Cu_Cd and donor Cu_i in CdTe), the concept of transformation of state and self-compensation is introduced and discussed. The effect of state transformation and self-compensation on charge neutrality condition, Fermi level, and majority carrier density is explored. Numerical examples are given for CdTe to illustrate the relevance and importance of multi-level and multi-configuration intrinsic/impurity defect states for understanding the performance of CdTe thin film photovoltaics (PV).