Abstract
In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice. In conventional materials, this assumption is experimentally justified as carrier thermalization completes before any significant carrier transport has occurred. Here, we demonstrate by unifying time-resolved optical and electrical experiments and Monte Carlo simulations over an exceptionally wide dynamic range that in the case of organic photovoltaic devices, this assumption is invalid. As the photo-generated carriers are transported to the electrodes, a substantial amount of their energy is lost by continuous thermalization in the disorder broadened density of states. Since thermalization occurs downward in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by direct experiments. We identify the time and distance scales relevant for carrier extraction and show that the photo-generated carriers are extracted from the operating device before reaching thermal equilibrium.
Highlights
In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice
High-performance inorganic solar cells, for organic photovoltaic (OPV) cells it is often tacitly assumed that following photon absorption and free charge carrier generation—but before any transport to the electrodes occurs—the free charge carrier populations have already fully thermalized in their respective density of states (DOS)
The free charge carriers are assumed to be transported at their corresponding equilibrium energy levels—lowest unoccupied molecular orbital (LUMO) for electrons and highest occupied molecular orbital (HOMO) for holes—in analogy to the conduction and valence bands of inorganic semiconductors
Summary
The free charge carriers are assumed to be transported at their corresponding equilibrium energy levels—lowest unoccupied molecular orbital (LUMO) for electrons and highest occupied molecular orbital (HOMO) for holes—in analogy to the conduction and valence bands of inorganic semiconductors. This assumption implies that no further charge carrier energy losses to thermalization can occur. The results that follow are explained by a coherent quantitative model In literature these techniques are mostly discussed in isolation, making it hard if not impossible to get the full picture, here all relevant time scales are investigated: a unification of the most commonly employed optical, electrical and numerical techniques to study OPV devices is presented. Device models based on the assumption of quasiequilibrium have to be reconsidered
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