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

Over the past few decades, polymer solar cells have gained attention as alternative sustainable energy resources because of their benefits over inorganic devices, such as their light weight, flexibility, wide range of material choices, etc. Despite the positive legacy, the power conversion efficiency (PCE), which is roughly 18~20%, is limited because of the ambiguity of the operational mechanisms. To surpass this limit, the relationship of the morphology-photovoltaic functions crucially needs to be unravelled. One of the effective approaches is Kelvin probe force microscopy (KPFM), where the surface potential of the active layer of organic solar cells is evaluated as the contact potential difference (CPD) between the sample surface and the AFM probe. Nevertheless, the interpretation of the obtained data and the relationship between the data and the open-circuit voltage (<i>V</i>_OC) of the device has not been well established. In this study, with the illumination below the sample, the photovoltage is evaluated by the CPD shift under the illumination from the dark condition (△CPD) for donor-acceptor planar heterojunction devices (PHJ) first and moves to the bulk heterojunction devices (BHJ). Finally, the spatial distribution of △CPD and its correspondence to the device<i> V</i>_OC were discussed.<br/><br/>Firstly, the surface topography and the corresponding △CPD image of PHJ devices reveal the homogeneous distribution of photovoltage, with FWHM smaller than 10 mV. Next, △CPD was characterized in different donor devices, and the values of△CPD are approximately the same as the device <i>V</i>_OCs. We conclude that, in PHJ, only holes are accumulating on the donor surface since electrons travel to the bottom electrode (cathode) through the acceptor layer, causing the shift in the surface potential to be as equal as the <i>V</i>_OC. As to the BHJ devices, △CPD observed on the active layer shows lower values than <i>V</i>_OC, even though △CPD on the top electrode (anode) matches the device <i>V</i>_OC. The proximity between donor and acceptor results in averaging the surface potential caused by hole and electron accumulation in the same space, leading to smaller values of △CPD.<br/><br/>In conclusion, the successful evaluation of <i>V</i>_OC by △CPD mapping, as demonstrated in PHJ devices, provides a promising approach to insightfully study the morphology-<i>V</i>_OC relationship in polymer solar cells.