Recent Research Progress of Photovoltaic Materials for Nonfullerene Polymer Solar Cells

Nonfullerene polymer solar cells (PSCs) based on p-type conjugated polymer as donor and n-type organic semiconductor (n-OS) as acceptor have attracted great attention in recent years, due to the advantages of the n-OS acceptors, such as strong and broad absorption in visible-NIR region, easy tuning of absorption and electronic energy levels, and good morphology stability in comparison with the traditional fullerene derivative acceptors. This article reviews the recent research progress of the n-OS acceptors (including n-type conjugated polymers and n-OS solution-processable organic small molecules) and the conjugated polymer donor photovoltaic materials for the application in nonfullerene PSCs. The n-type conjugated polymer acceptor materials include the perylene diimide (PDI)- and naphthalene diimide (NDI)-based D-A copolymers, as well as the D-A copolymers based on the new B <- N bonded acceptor unit. The highest power conversion efficiency (PCE) reached 8.24% for the all polymer PSCs with a medium bandgap p-type conjugated polymer J51 as donor and a narrow bandgap n-type NDI-based D-A copolymer N2200 as acceptor. The n-OS small molecules acceptors include the PDI- or NDI-based molecules, the narrow bandgap A-D-A structured small molecules with a fused ring central donor unit and two electron-withdrawing end groups (such as ITIC), and other n-OS small molecules. The p-type conjugated polymer donor materials, matching with the nonfullerene acceptors, include the narrow bandgap oligothiophene-or banzodithiophene (BDT)-based copolymers, and the medium bandgap two-dimension-conjugated D-A copolymers based on BDT with thiophene conjugated side chains. The best nonfellerene PSCs, with a medium bandgap polymer as donor and a narrow bandgap A-D-A structured n-OS as acceptor, have been recently demonstrated to have high PCE of over 12%. The complementary absorption in the visible-NIR region, matching electronic energy levels of the donor and acceptor materials, is very important for the high performance nonfullerene PSCs. In the end of the article, we give some comments and point out the key issues for the next-step studies and future development of the nonfullerene PSCs.