Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss

A new wide bandgap polymer donor, PNDT-ST, based on naphtho[2,3-b:6,7-b ']dithiophene (NDT) and 1,3-bis(thiophen-2-yl)-5,7-bis(2- ethylhexyl)benzo[1,2-c:4,5-c ']dithiophene-4,8-dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6-T is also developed. The extended pi-conjugation and less twist of PNDT-ST provides it with higher crystallinity and stronger aggregation than the PBDT-ST counterpart. The higher lowest occupied molecular orbital level of Y6-T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (V-oc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT-ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT-ST. To mediate the crystallinity and optimize the morphology, PNDT-ST and PBDT-ST are blended together with Y6-T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (E-loss) of 0.521 eV. Such small E-loss is the best record for polymer solar cells with PCEs over 16% to date.