Efficient charge generation by relaxed charge-transfer states at organic interfaces

Abstract

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…81 They employed the time-delayed-collection-field (TDCF) technique to investigate the field dependence of carrier generation. 82 TDCF is analogous to a pump-probe experiment with optical pumping and an electrical probe.…”

“…They reconstructed the absorption spectrum in this low-energy regime by measuring radiative decay from the thermally-relaxed CT manifold. 81 The measurement of the absorption and the EQE in the low-energy regime enabled the authors to determine the IQE of relaxed CT excitation. They found a constant IQE irrespective of the excitation energy for different materials combinations they investigated ( Figure 7).…”

Exciton Diffusion Length and Charge Extraction Yield in Organic Bilayer Solar Cells

“…Organic photovoltaics and photodetectors with a blended photoactive layer [1] benefit from a long ℓd: As the absorber phase separation can be increased, charge recombination is reduced, resulting in an increased operating voltage. [2] Meanwhile, photo-sensitive devices with a planar photoactive structure such as multilayer cascades [3] benefit from a long ℓd resulting in enhanced short-circuit currents as excitons generated further away from the charge dissociating heterojunction are harvested. For OLEDs on the other hand, a short ℓd for singlet excitons is required when containing both (efficient) phosphorescent and (stable) fluorescent emitter layers to confine singlet excitons to the latter.…”

“…For this purpose, ηc is assumed to be independent of λ, as wavelength independent internal quantum efficiencies have been experimentally demonstrated for both polymer and small molecule systems. [2,17] We exclude thin ZnPc layers up to a threshold thickness of xt = 11nm from the fitting routine due to layer roughness [18] and DA inter-diffusion [8,19] (details in SI.6b). For thicker absorber layers, we model the charge extraction to be spatially uniform, as no significant concentration of recombination centers is present in the bulk of the neat absorber.…”

Description of the Charge Transfer States at the Pentacene/C60 Interface: Combining Range-Separated Hybrid Functionals with the Polarizable Continuum Model

“…[1][2][3][4] Both processes are key for the performance of the photovoltaic cells and have therefore received significant attention from both experimental and theoretical sides.…”