Charge carrier transport and nanomorphology control for efficient non-fullerene organic solar cells

Single junction organic photovoltaic devices (OPVs) have exceeded 15% power conversion efficiency (PCE) with the help of fused ring based low-bandgap non-fullerene acceptors (NFAs). As a major type of NFA, the indacenodithiophene derivative NFA (IDTBR) has been shown to have superior OPV stability with outstanding VOC, but the efficiency is relatively lower compared to the reported OPV champion devices. Further improvements towards high efficiencies in this OPV system remains challenging due to the relatively poor charge carrier transport properties in the bulk heterojunction film, particularly the electron transport in small molecule non-fullerene acceptor network. Here we conducted detailed study on the dependence of carrier transport on BHJ donor–acceptor (D–A) composition. Our results show that the nano-morphology or phase aggregation of non-fullerene acceptor (NFA) molecules can be tuned via D–A composition in bulk heterojunction layer, and the improvement of electron mobility was shown to be enhanced by almost one order – from 1.23 × 10−6 cm2/V (D:A = 1:1 by weight) to 1.02 × 10−5 cm2/V (D:A = 1:2) – due to the improved connectivity of electron transport pathways. Further increase of NFA component content, however, has led to over-sized phase segregation, deteriorating the photovoltaic performance of organic soar cells. The optimized BHJ cell shows more balanced charge carrier transport and phase segregation, which yields a PCE of 10.79%. Furthermore, it shows a VOC as high as 1.03 V, which is ascribed to the significantly suppressed radiative and non-radiative recombination losses with bandgap-VOC offset Eg/q-VOC of only 0.55 V.