Precontact surface chemistry effects on CdS/CdTe solar cell performance and stability

Carbon-based dag pastes containing mixtures of various Cu-, Sb-, Hg-, and Bi-telluride compounds offer convenient backcontact materials for CdS/CdTe superstrate solar cells. Open-circuit stress tests at 100°C and 1.5 to 2.0 Suns illumination show that the performance of devices utilizing Cu-based pastes improves during the first 1-10 hours of stress. Of these, the best stability is exhibited by pastes containing no HgTe. Carbon-based dag pastes can be viewed as sacrificial dopant sources on properly treated CdTe surfaces. CdTe surfaces consisting of thick Te layers, resulting from nitric-phosphoric (NP) acid etches, show better incorporation of active Cu-based dopants than thinner Te layers. Deep-level transient spectroscopy (DLTS) confirms the presence of acceptor-like defects at ~E_v+0.3 eV believed to be associated with Cu_Cd substitutional defects in NP-treated devices. The presence of Te and its ability to getter Cu is paralleled by significant improvements in CdTe device stability