Role of Energetic Disorder in Energy Loss of Bulk Heterojunction Organic Solar Cells

The power conversion efficiency (PCE) of organic solar cells (OSCs) has recently progressed significantly with a rapid increase from 10% to 19% due to state-of-the-art research on non-fullerene acceptor molecules and various device processing strategies. However, OSCs still exhibit significant open circuit energy loss (~0.6 eV) due to disorder and excitonic nature of the organic semiconductors. In this work, we explored the role of energetic disorder on the total energy loss of bulk heterojunction OSCs. For this purpose, different donor-acceptor combinations were used to fabricate the OSCs, and various electrical characterizations were carried out in detail. The approximation of the energetic disorder was measured in term of Urbach energy (EU) from the band tail of spectra. The correlation between open circuit energy loss due to different recombination processes and Urbach energy were observed. As EU was decreased, the energy loss decreased due to more ordered molecular packing and lower energetic disorder, resulting in better device performance. These finding show the need of reducing the energetic disorder, hence lower energy losses for boosting the PCE of the OSCs.