Kelvin probe force microscopy: imaging open-circuit voltage in optoelectronic devices

Scanning probe microscopy has been successfully implemented to probe the electrical characteristics of optoelectronic devices. Currently, a method that directly correlates measured signals to device performance is missing. We implement illuminated Kelvin probe force microscopy (KPFM) to spatially resolve the open-circuit voltage of optoelectronics with nanoscale resolution, 5 orders of magnitude better than previous methods. In illuminated-KPFM, the surface photovoltage, is the difference between the contact potential difference under illumination and in the dark, and proportional to the Fermi level splitting. We apply our imaging method to a variety of solar cells and find that the open-circuit voltage in some materials varies locally by \textgreater 0.2 V, suggesting the spatial variation of non-radiative recombination strongly affects performance. A detailed examination of possible topography pick-up was excluded by measuring samples with modified surface morphology and considering the tip-sample separation dependence of the signal. This novel metrology enables new insights into the loss mechanisms that hinder solar cells and provides a new platform to image device performance with nanoscale resolution.