Charge carrier transport and nanomorphology control for efficient non-fullerene organic solar cells. (June 2019) - Hu, Hanlin
Abstract: 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 cm 2 /V (D:A = 1:1 by weight) to 1.02 × 10 −5 cm 2 /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 E g /q-VOC of only 0.55 V. Graphical abstract: A combination of non-fullerene acceptor – EH-IDTBR and PTB7-Th yields remarkably high open-circuit voltage up to 1.03 V, along with PCE up to 10.79% in single-junction OPV devices free of post-treatment and additive. The enhanced aggregation of EH-IDTBR molecules leads to the increase of electron mobility by ∼one order due to the improved connectivity of electron pathways in the BHJ layer when the D:A ratio changed to 1:2. Further improvement of the electron mobility by increase EH-IDTBR component content in the BHJ layer did not benefit the performance of OSC devices as expected due to the over-sized phase segregation.Image 1 Highlights: 10.79% OPV devices based on non-fullerene acceptor (NFA) was achieved in a manufacture friendly way free from additive and post-treatment. Correlations are established among nanoscale phase segregation, carrier transport and solar cell performance. A high VOC of 1.03 V is achieved due to suppressed radiative recombination loss (0.32 eV) and non-radiative recombination loss (0.23 eV).