Understanding the Charge Transfer State and Energy Loss Trade-offs in Non-fullerene-Based Organic Solar Cells

Abstract

Energy losses significantly reduce the open-circuit voltage among current state-of-the-art organic solar cells (OSCs), which limits the further enhancement of their power conversion efficiencies (PCEs). In this study, the bulk heterojunction blends of PM6 donor and halogenated nonfullerene acceptors (NFAs) display a trade-off between radiative energy losses, i.e., charge transfer state (CTS) radiative energy loss (ΔE rad ) and the loss associated with CTS formation from acceptor singlet excitons (ΔE CT EL ). Similarly, a trade-off between ΔE rad and the nonradiative energy loss (ΔE nr ) is found, reflecting a competition in radiative and nonradiative charge recombination pathways. Further, the energy levels of relaxed CTS (E CT EL ) are shown to exhibit dependence on morphologically induced energetic traps, suggesting that it should not be associated merely to blend constituents. Interestingly, these correlations extend even to thermally degraded devices considered herein. Accordingly, this work provides further understandings of energy losses relevant to overcome the current limitations concerning NFA-based OSC developments.

N-doping of nonfullerene bulk-heterojunction organic solar cells strengthens photogeneration and exciton dissociation

“…In these two relations, the constants of α and ω describe the short-circuit bimolecular recombination and the open-circuit trap-assisted recombination, respectively. 45 As shown in Fig. 5e, the optimized device with 0.005 wt% N -DMBI dopant possesses both moderate α (0.95) and ω (1.04) values, while the undoped and highly doped devices obtain the α (0.98) and ω (1.03) that are closest to unity, respectively.…”

“N‐π‐N” Type Oligomeric Acceptor Achieves an OPV Efficiency of 18.19% with Low Energy Loss and Excellent Stability

“…This is probably because their small energy offset between E g and energy of charge transfer (CT) states estimated from normalized fourier‐transform photocurrent spectroscopy external quantum efficiency (FTPS‐EQE) curves and electroluminescence (EL) spectra of devices (as shown in Figure S8 and Table S5 , Supporting Information) creates an equilibrium state between the CT states and acceptor singlet, which is believed to enhance the efficiency of radiative decay, while suppressing its non‐radiative decay. [ 30 ] Hence, the previous analysis indicates there is negligible difference in the Δ E for the three systems, and their disparity is due to their slightly different energy levels of themselves and complex energy alignments due to the morphology optimization. Further applying a more rigid molecular backbone or monomer, and proper HOMO energy level of the donor would be a potential approach to decrease the Δ E of N‐ π ‐N oligemic type OSCs.…”

Effects of Vertical Molecular Stratifications and Microstructures on the Properties of Fullerene‐Free Organic Solar Cells

“…These are consistent with the observations from other systems, such as PM6:BTP-4Cl, as reported recently. [20] Intuitively, such discrepancies can generally be expected in this class of materials, and energy losses from former reports could potentially be over/underestimated. Time-resolved PL (TRPL) was also performed to describe the corresponding dynamics of excited states, more precisely for hole transfer, as IT-4Cl has a much lower E g than PM6.…”

“…These are consistent with the observations from other systems, such as PM6:BTP‐4Cl, as reported recently. [ 20 ] Intuitively, such discrepancies can generally be expected in this class of materials, and energy losses from former reports could potentially be over/underestimated.…”