Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss

Abstract

Donor–acceptor organic solar cells often show low open-circuit voltages (V OC) relative to their optical energy gap (E g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E g and that of intermolecular charge transfer (CT) states at the donor–acceptor interface. Here we study charge generation occurring in PIPCP:PC61BM, a system with a very low driving energy for initial charge separation (E g−E CT ~ 50 meV) and a high internal quantum efficiency (η IQE ~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC61BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state.

Spontaneous Exciton Dissociation at Organic Semiconductor Interfaces Facilitated by the Orientation of the Delocalized Electron–Hole Wavefunction

“…These interfaces usually have a relative large interfacial energy offset such that the direct charge generation from hot CT excitons is an isoenergetic process. Interestingly, recent works on non‐fullerene acceptors (NFAs) show that CT excitons can convert efficiently into free carriers despite the much smaller energy offset at the donor–acceptor interface 30–33. The small energy offset implies that CS is likely to be an energy uphill process because the singlet exciton has an energy lower than that of the free electron–hole pair.…”

Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

“…This is consistent with the previously reported empirical lower limit of a 0.3 eV driving force required for efficient CT in typical polymer:fullerene blends 12,13 . Understanding the impact of driving force on the CT dynamics is particularly important in organic solar cells, since the CT state energy also determines the open circuit voltage (V OC ), so that a small driving force is desirable, but might lead to a tradeoff in current generation if the recombination of excitons competes with their slow dissociation [13][14][15][16][17] .…”

Charge Pair Separation Dynamics in Organic Bulk‐Heterojunction Solar Cells

“…[21,22] While there are systems in which almost perfect CP separation takes place, [23] large losses by recombination can be observed in others. [21,22] While there are systems in which almost perfect CP separation takes place, [23] large losses by recombination can be observed in others.…”

“…It has proven to be particularly useful to model exciton and charge behavior in spatially and energetically disordered OPVs. [21,22] Typical values for the energetic disorder range from 70 meV in blends formed from poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend, down to 30 meV. OSC device models have emerged to simulate current-voltage characteristics, [37,38] the effect of blend morphology, [39,40] charge mobility extractions, [41,42] as well as CP separation, [30,[43][44][45][46][47][48] and its competing process, geminate recombination.…”

“…[36] A specific advantage of the kMC method is that it allows to include explicit particle-particle interactions and that it gives direct access to the charge trajectories which can be used to assess the CP displacement from the heterojunction. [21] Recombination in organic BHJs takes place at the donor/acceptor interface. [49] A pathway for highly efficient OPVs was given by Koster et al, [50] who suggested to increase the permittivity of the organic materials used within OSCs in order decrease the mutual attraction of CPs and enhance their separation.…”