The Critical Role of Anode Work Function in Non-Fullerene Organic Solar Cells Unveiled by Counterion-Size-Controlled Self-Doping Conjugated Polymers

In this work, we demonstrated an effective method to modulate the p-type self-doping effect as well as the molecular energy level of an anionic conjugated polymer, namely PCP-x (x = H, Li, Na, K, Cs) by changing the counterions, and hence developed a series of anode interlayer materials for nonfullerene organic solar cells (NF-OSCs). With the decreasing cation radius, self-doping effect of PCP-x can be enhanced and hence the work function (WF) of the PCP-x-modified anodes can be tuned from 4.78 to 5.11 eV. The photovoltaic performance of NF-OSCs based on J52–2F:IT-4F active layer was significantly affected by the WF of the anodes, and a PCE of 12.8% could be achieved by using the PCP-H-modified ITO anodes. Furthermore, the PCP-x-modified anodes were used to fabricate NF-OSCs with different active layers, including PBDTTT-E-T:IEICO and PBDB-T-2F:IT-4F. It was found that the energetic offsets between WF of the anodes and ionization potential (IP) of the donors should be controlled below 0.1 eV to minimize the loss in photovoltaic performance. This work provides not only an effective method for synthesizing a series of p-type interlayer materials with tunable WF but also an insight on the critical importance of developing anode interlayer materials with high WF.