Effect of Diode Field on Time-Resolved Photoluminescence of CdTe-Based Solar Cells

Abstract

Time-resolved photoluminescence (TRPL) measurements with externally applied voltage bias were performed on a variety of thin-film CdTe-based solar cells. These measurements revealed a substantial variation in luminescence decays with the application of external voltage bias in many of the samples. Experimental results were replicated with a MATLAB-based modeling tool. Decay variations are attributed to the changing strength of internal p-n junction electric fields as bias is modulated; this exercise, therefore, illustrates whether or not field effects influence a particular TRPL measurement and how. It is found via modeling that the field influence on TRPL decays is dependent on the cell's bulk minority carrier lifetime, mobility, and carrier concentration, as well as the laser excitation power during a measurement. While nearly insignificant in poorer quality legacy cells, the field effects are substantial in state-of-the-art CdTe cells with high lifetime, mobility, and doping, complicating determination of bulk lifetime. It is found that forward bias is effective in eliminating junction fields. It is also shown that fields can be effectively suppressed by increasing excitation power, but doing so is imprecise and can also introduce additional errors unrelated to fields. It is therefore concluded that lifetime is most accurately measured in completed cells under forward bias at low excitation intensity.