Investigating the Trade-Off between Device Performance and Energy Loss in Nonfullerene Organic Solar Cells

The large energy loss (E_loss) in organic solar cells (OSCs) relative to those of silicon or inorganic/organic hybrid perovskite solar cells is one of the major factors limiting the power conversion efficiency (PCE) of OSCs. Recently, OSCs based on nonfullerene acceptors (NFAs) have achieved high PCEs at decreased E_loss values. Therefore, the present study investigates the relationship between E_loss and the device performance of NFA-based OSCs. Here, we select two polymer donors (PBDB-T and its fluorinated derivative PBDB-TF) and blend each polymer donor with each of three NFAs (indaceno[1,2-b:5,6-b']dithiophene and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IEIC) and its respective fluorinated and chlorinated derivatives IE-4F and IE-4Cl), which provide varied energy-level alignments. The six blends exhibit similar morphologies and charge transport properties but varied E_loss values in OSCs. The results indicate that the charge generation and PCE of the OSCs increase with the increasing E_loss. Accordingly, the PBDB-T:IE-4Cl-based device yields the highest PCE of 11.1% with an E_loss of 0.64 eV, while the PBDB-TF:IEIC-based device provides a significantly decreased PCE of 3.8% with a diminished E_loss of 0.52 eV. These results demonstrate the great importance of finely tuning the energy-level alignments of these types of donor/acceptor systems to achieve the best device performance.