Fullerene-Liquid-Crystal-Induced Micrometer-Scale Charge-Carrier Diffusion in Organic Bulk Heterojunction

The short charge-carrier diffusion length (L_D ) (100-300 nm) in organic bulk heterojunction (BHJ) impedes the further improvement in power conversion efficiency (PCE) of organic solar cells (OSCs), especially for thick-film (>400 nm) devices matching with industrial solution processing. Here a facile method is developed to efficiently increase L_D and then improve PCEs of OSCs via introducing a fullerene liquid crystal, F1, into the active layer. F1 combines the inherent high electron mobility of fullerene and strong self-assembly capacity of liquid crystal, providing a fast channel for charge-carrier transport and reducing energetic disorder and trap density in BHJ film via enhancing crystallization. Typically, in PM6:Y6:F1 BHJ, the enhanced charge-carrier mobility (>10^-2 cm^-2 V^-1 s^-1 ) and prolonged charge-carrier lifetime (55.3 µs) are acquired to realize the record L_D of 1.6 or 2.4 µm for electron or hole, respectively, which are much higher than those of the PM6:Y6 binary sample and comparable to or even better than those values reported for some inorganic/hybrid materials, such as CuIn_x Ga_{(1- x )} Se₂ (CIGS) and perovskite thin films. Benefitting from the micrometer-scale L_D , the PM6:Y6:F1 ternary OSCs sustain a remarkable PCE of 15.23% with the active layer thickness approaching 500 nm.