Do Diffusion Lengths Really Matter for Perovskite Solar Cells?
The diffusion length is frequently used to explain poor charge collection in halide perovskite solar cells. However, in the presence of only moderately conductive charge transport layers, non-linear series resistances can modulate the carrier density and thereby the recombination losses even if transport in the perovskite itself is nearly perfect.
The diffusion length of halide perovskite absorbers for photovoltaics is a desirable property that is frequently mentioned as a synonym for good electronic properties. It is also assumed to be a necessary and possibly sufficient condition for efficient charge collection, originating from the logic of p-n junction solar cells and the assumption of perfect collection at the edge of the space-charge region. However, this assumption is not always realistic in halide perovskites due to the presence of various organic contact layers with lower electron and hole mobilities than the perovskite. The physics of charge-carrier collection in perovskite solar cells is therefore different from that of other photovoltaic technologies. A sufficiently long diffusion length remains a necessary condition for efficient charge collection, but it is not a sufficient criterion. Instead, the conductivity of the transport layers is key to understanding efficient collection in perovskite solar cells and is currently not being discussed in the literature. While the contact layers can cause ohmic losses and reduced fill factors, lowly doped or intrinsic contact layers can also impact the short-circuit current without necessarily affecting the fill factor. The ability of the charge transport layers to modulate the carrier density at short circuit or low forward bias inside the absorber layer is due to their ability to lower the carrier density and increase the recombination rate at a given charge-carrier lifetime. The higher the mobility inside the ETL or HTL, the lower the carrier density, and the lower the recombination rate. The value of the diffusion length inside the absorber depends on the charge-carrier density inside the absorber, and this latter quantity depends on the conductivity of undoped contact layers. This effect is present to some degree in any photovoltaic technology, but it is particularly strong in halide perovskites, as they have thin absorber layers with decent mobilities and long lifetimes that are then interfaced with low-mobility transport layers. Furthermore, we also illustrate the importance of voltage-dependent photoluminescence measurements. These measurements can identify and quantify recombination losses that are caused by inefficient charge collection and inefficient transport through the ETL or HTL. They allow identifying situations where the loss is mainly at short circuit and the shape of the current-voltage curve does not indicate a significant fill-factor loss.
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