Suppression of geminate charge recombination in organic photovoltaic devices with a cascaded energy heterojunction

How High Local Charge Carrier Mobility and an Energy Cascade in a Three‐Phase Bulk Heterojunction Enable >90% Quantum Efficiency

“…[ 5,6 ] Work by many groups has shown that the presence and composition of the mixed region can have a dramatic impact on device performance. [7][8][9] This impact could be due to the fact that the mixed region has been reported to have energy levels that are shifted with respect to the aggregated phases, producing an energy cascade that assists in free charge generation. [ 8,[10][11][12] For example, PCBM has been shown to have a 100-200 meV shift in electron affi nity upon aggregation and P3HT, the prototypical OPV donor, displays a 300 meV change in optical bandgap between amorphous and crystalline regions.…”

“…[7][8][9] This impact could be due to the fact that the mixed region has been reported to have energy levels that are shifted with respect to the aggregated phases, producing an energy cascade that assists in free charge generation. [ 8,[10][11][12] For example, PCBM has been shown to have a 100-200 meV shift in electron affi nity upon aggregation and P3HT, the prototypical OPV donor, displays a 300 meV change in optical bandgap between amorphous and crystalline regions. [ 6,10,13 ] In this report, we studied the role of the mixed region in assisting geminate splitting using Kinetic Monte Carlo (KMC) simulations of idealized trilayer (pure donor/ mixed region/pure acceptor) morphologies.…”

Computationally connecting organic photovoltaic performance to atomistic arrangements and bulk morphology

“…While both contributions are material dependent, density functional theory and alternative theoretical treatments predict that λ ext is independent of chromophore length, whereas λ int decreases monotonically for longer chromophores [117][118][119]. Some investigations have had success from approximating the reorganization energy to be twice the charge carrier polaron energy, which can be tuned to match the simulated material [100,120,121], rather than explicitly calculating λ ij through QCCs or molecular dynamics [122,123].…”

Simulating charge transport in organic semiconductors and devices: a review

“…QCMC simulations of charge separation in the 5nm around a P3HT:PC61BM heterojunction suggested that these factors alone could explain GR as low as 50% for strongly-bound charge transfer states, in agreement with experiment. Other device-scale KMC simulations have shown similar reductions in GR from cascade regions [401,402]. It should also be noted that the energy levels of aggregated PCBM are lower than those in the polymer:PC61BM mixed phase [342], hence aggregated fullerene within the OPV morphology may encourage HCT states and cascade regions simultaneously.…”

“…Before leaving the discussion on GR, we note that cascade heterojunctions can be realised by multi-layer structures as well as by self-organisation. KMC simulations suggest a thin (<3nm) 'cascade' layer between donor and acceptor, and  = 150meV is sufficient to reduce GR from over 90% to levels reported in P3HT:PC61BM OPVs [402]. Izawa et al [403], Heidel et al [404], and Tan et al [405] all report experiments in which a thin layer (<2nm) of 'cascade' material with  = 120meV, 300meV and 400meV respectively was interposed between the donor and acceptor.…”