Tuning the molecular packing and energy levels of fullerene acceptors for polymer solar cells†
Fullerenes are considered to be promising acceptor materials for the 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 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 the favorable nanophase separation of the P3HT/MBTCBA based blending film, resulting in a higher electron mobility. Therefore, PSCs incorporating P3HT as a donor and MBTCBA as an 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 those of BTCBA (3.54%) and PCBM (3.76%) based devices. Our work proves that the PSC performance can be readily improved by modification of fullerene acceptors using an electron donating group, tuning the LUMO energy level and the molecular packing behavior.