Simultaneous Examination of Grain-Boundary Potential, Recombination, and Photocurrent in CdTe Solar Cells Using Diverse Nanometer-Scale Imaging

Abstract

We report on the simultaneous measurements of Kelvin probe force microscopy (KPFM), cathodoluminescence (CL), and electron-beam induced current (EBIC) for hundreds of grain boundaries (GBs) after different processing steps. The GB potential steadily increases from tens of mV for as-deposited samples, 80 mV after the Cl treatment, to about 120 mV after Cu diffusion (Cl+Cu). The latter is consistent with the emergence of deep Cu defects observed in CL GB spectra. At the same time, EBIC indicates that GB photocurrent is enhanced by 0% to 50% after Cu is introduced. However, this increase is very modest relative to expectations based on computational modeling and the measured GB potential. The EBIC enhancement coincides with increased GB recombination observed by CL. A plausible explanation is that GB defects introduce GB potentials and increase GB recombination; this can reconcile the CL, EBIC, and KPFM data.