Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Several organic photovoltaic blends can now operate with internal quantum efficiencies approaching unity, meaning that they can convert almost every absorbed photon into an electron-hole pair that does work in an external circuit. The underlying mechanism is still ill-understood. Here, we combine theoretical and experimental methods to obtain quantitative and general insight in the transient processes in operational organic solar cells. The obtained insight allows us to identify an efficient and universal mechanism for generation of free charges. The physical picture that arises is (a) that initial charge motion is highly diffusive and boosted by energetic relaxation in the disordered density of states, (b) that mobile charge carriers dissociate from and re-associate into Coulombically-associated pairs faster than they recombine, and (c) that the remaining Coulomb interaction between the nearly-free charges can be overcome by small electric fields to give rise to efficient generation of completely free charges.
In more detail, we present a rich multidimensional set of transient absorption observations which provide detailed information (on the ps to µs timescale) about the behavior of mobile, separating charge carriers in the prototypical high quantum efficiency PCDTBT:fullerene derivative blend. The measurements concurrently observe the relaxation of charge carriers within the disordered density of states (DOS), and the recombination of charge carriers at a sequence of different initial charge carrier concentrations. Understood through the lens of kinetic Monte Carlo calculations, these observations reveal how mobile charge carriers hop and relax, dissociating and re–associating potentially multiple times before recombination or extraction by contacts.
We find that a minimalistic kinetic Monte Carlo model, incorporating only exciton and charge motion in a disordered but otherwise homogeneous medium, gives an accurate description of the experimental data. This proves the relevance of the extended Gaussian disorder model, that was developed to describe charge transport in near-equilibrium situations, for gaining insight in the very-far-from-equilibrium processes governing charge generation in organic solar cells. A simple analytical calculation confirms the picture deduced from the MC simulations and hints at sub-Langevin recombination as the cause for quantitative deviations between the MC calculations and the measured concentration dependence of the charge recombination.
Figure | Spectral relaxation of the ground-state bleaching (upper panel) and corresponding transient charge concentration normalized to the initial value (lower panel) in PCDTBT:PCBM blend films after photoexcitation. Symbols denote experimental data, lines are calculations with the kinetic MC model. The color coding indicates the excitation fluences corresponding to initial electron and hole concentrations of 1.1, 2.1, 4.2, and 8.4% respectively in the simulations.
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