A novel method for mapping open-circuit voltage in solar cells with nanoscale resolution (Presentation Recording)

The electrical characteristics of thin-film compound semiconductor solar cells have been successfully probed by scanning probe microscopy. Nevertheless, a direct relationship between the measured signals and the figures of merit that define the device performance is still missing. Here we present a novel method to image and spatially resolve the Voc of solar cells with truly nanoscale resolution (<100 nm), based on a variant of illuminated Kelvin probe force microscopy (KPFM) [1]. We map the Voc by measuring the difference between the contact potential difference under illumination and in the dark, which is equal to the photo-generated voltage of the device (and is proportional to the Fermi level splitting). We complement our new metrology by applying scanning photocurrent microscopy using near-field scanning microscopy (NSOM) probes as a local source of excitation to image local variations in Jsc within the material, also with nanoscale resolution. Further, we spatially and spectrally resolve the external quantum efficiency (EQE) within the devices, also with nanoscale resolution, while mimicking the power density operation conditions of real devices [2]. Combined, these new tools provide a complete picture of the local optoelectric characteristics of PV devices, including an indirect measurement of the centers for non-radiative recombination, and a direct mapping of the local collection properties of the material, respectively. We apply our novel metrology to polycrystalline solar cells, where we find Voc local variations of >200 mV. [1] E.M. Tennyson et al., Nature Commun., in review; [2] M.S. Leite et al., ACS Nano. 11, 11883 (2014).