Development of efficient and stable back contacts on CdTe/CdS solar cells

To make CdTe/CdS solar cells highly efficient, a Cu containing back contact (BC) is generally used. These cells degrade due to Cu diffusion to the front contact which causes shunting; this is shown with secondary ion mass spectroscopy (SIMS) depth profiling. To get a stable but still highly efficient cell, different BC materials and etching treatments were investigated. Chemical etching creates a back surface field (BSF) due to a p+-doped Te-rich CdTe surface. To overcome the naturally existing Schottky barrier between p-CdTe and any metal, a thin buffer layer was evaporated prior to the metallization. Amongst the many investigated BC materials, the most suitable are Sb or Sb2Te3 as a buffer and Mo for metallization. These cells showed high stability under accelerated tests corresponding to 70 years.