Photovoltage Mapping in Bulk- and Planar-Heterojunction Polymer Solar Cells by Scanning Photo-Illuminated Kelvin Probe Force Microscopy

This study elucidated the quantitative relationship between the experimental surface potential (SP) data obtained by scanning Kelvin probe force microscopy (KPFM) and the open-circuit voltage ( V OC ) of bulk heterojunction (BHJ) and planar heterojunction (PHJ) polymer solar cells prepared using four types of polymers as the donor material and fullerene as the acceptor material. The shift in SP value under illumination relative to that in the dark (ΔSP) recorded on the top metal electrodes of the devices exhibited excellent agreement with V OC , regardless of the heterojunction structure and donor type. In the PHJs, the average ΔSP on the active layer was slightly higher than V OC , indicating that ΔSP tracks electron–hole quasi-Fermi-level splitting across the donor–acceptor heterojunction. The ΔSP mapping in the PHJ devices showed a uniform V OC with spatial variations of ∼20 mV. By contrast, the ΔSP on the active layer in the BHJs did not represent the full value of V OC ; instead, it was lower than V OC at all locations in the images because of convolution effects owing to the proximate accumulation of holes and electrons in the nanoscale phase-segregated donor and acceptor domains below the tip. The findings clarify the applicability and limitations of photoilluminated KPFM in the quantitative analysis and mapping of the local V OC in polymer solar cells.