Tuning the molecular packing and energy level of fullerene acceptors for polymer solar cells

摘要： Fullerenes are considered to be promising acceptor materials for fabrication of bulk-heterojunction polymer solar cells (PSCs) due to their excellent physical and chemical properties. Herein, two novel fullerene derivatives with different specific functional groups, 2-benzylthiophene-C60 bis-adduct (BTCBA) and 2-(4-methoxybenzyl)thiophene-C60 bis-adduct (MBTCBA), were synthesized and utilized as the acceptors for PSCs with the purpose of investigating the influence of the fullerene derivative substituents on the device performance. Compared with BTCBA, MBTCBA exhibits a higher lowest unoccupied molecular orbital (LUMO) energy level due to the electron donating properties of the 2-(4-methoxybenzyl)thiophene functional group. Meanwhile, the appropriate intermolecular interaction of MBTCBA molecules promotes favorable nanophase separation of the P3HT/MBTCBA based blending film, resulting in higher electron mobility. Therefore, the PSCs incorporating P3HT as donor and MBTCBA as acceptor yield a power conversion efficiency (PCE) of 5.29% with an open-circuit voltage (VOC) of 0.81 V, which is obviously higher than that of BTCBA (3.54%) and PCBM (3.76%) based devices. Our work proves that the PSCs performance can be readily improved by modification of fullerene acceptors using electron donating group, tuning the LUMO energy level and the molecular packing behavior.