Donor dilution in D18:L8-BO organic solar cells: visualization of morphology and effects on device characteristics

The increased interest in tandem solar cells and semitransparent building-integrated photovoltaics raises the need for organic solar cells with enhanced visible light transmittance. Reducing the donor content successfully enhances semitransparency in the visible wavelength range, however, it can introduce issues in charge carrier separation and transport. We address this key issue and concentrate on a detailed investigation of the nanomorphology of D18:L8-BO bulk heterojunctions with reduced donor content to provide new insights into the morphological changes and their impact on device performance. Scanning transmission electron microscopy combined with electron energy loss spectroscopy based elemental ratio mapping provides good contrast between the donor and acceptor domains and we observed that the donor phase forms a well interconnected network, which surprisingly persists even at low donor contents down to 2%. Investigations of the solar cell characteristics align with these findings and reveal that despite a significant reduction of the donor, the fundamental device physics remains largely unaffected. Rather than changes in charge carrier mobility or exciton dissociation, we identify charge carrier collection and a significant reduction in shunt resistance as critical loss factors that have been previously underestimated. A thorough understanding of these changes will contribute to optimizing donor-diluted organic solar cells for semitransparent applications.