Characterization of CdS/CdTe thin-film solar cells by admittance spectroscopy and deep-level transient spectroscopy

Abstract

Admittance spectroscopy, deep-level transient spectroscopy, and deep-level optical spectroscopy were used to characterize CdS/CdTe thin-film solar cells prepared by close-spaced sublimation. These devices are n+/p-type hetero- (or shallow homo-) junctions, and deep levels in the p-CdTe layer are observed. The energy levels within the band gap can be divided into three groups. The first group consists of the shallow (<0.1 eV from the band edge) acceptors and donors, observed in electrical characterization only as the net carrier concentration. A band of midgap levels with activation energies of 0.6–0.8 eV in deep-level transient spectroscopy is observed. The emission rates and activation energies for these levels agree with deep donor levels found in n-type CdTe. A third band of levels is found with activation energies of 0.28–0.45 eV in admittance spectroscopy. These levels are seen in deep-level optical spectroscopy and also in deep-level transient spectroscopy, provided that a large forward-bias pulse is used to fill the level. The emission rates in the third band agree with donor levels observed in n-CdTe. Estimates of concentrations suggest that the concentration of the group two defects (midgap donors) is ∼0.8 of the net shallow acceptor concentration.