Unveiling defect-mediated charge-carrier recombination at the nanometer scale in polycrystalline solar cells
Yohan Yoon, Wei-Chang D. Yang, Dongheon Ha, Paul M. Haney, Daniel Hirsch, Heayoung Yoon, Renu Sharma, Nikolai B. Zhitenev
In this work, we employ two techniques, near-field scanning photocurrent microscopy (NSPM) and transmission electron microscope based cathodoluminescence spectroscopy (TEM-CL), to analyze the nanoscale electrical and optical properties of CdTe solar cells. It is found that the high- efficiency cells, where CdTe thin-films are deposited at high substrate temperature (500 ºC) and treated by chlorine, display a reversed signal contrast at grain boundaries (GBs) between the two techniques. NSPM reveals that photocurrent is enhanced at GBs, while in TEM-CL, GBs exhibit reduced CL intensity and a red shift of the CL spectra peak. We attribute the distinct electrical and optical properties of GBs from the adjacent grains to the GB passivation by CdCl2 treatment and chlorine segregation at GBs. We discuss the advantages of the TEM geometry of the CL measurement, and we present numerical simulations and analytical models to extract the ratio of GB recombination velocity to minority carrier diffusivity.