Metal halide perovskites are mixed electronic–ionic semiconductors with an extraordinarily rich optoelectronic behavior and the capability to function very efficiently as active layers in solar cells, with a record efficiency surpassing 23% nowadays.
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Interpreting the impedance response of perovskite solar cells is significantly more challenging than for most other photovoltaics. Here we provide a way to obtain useful information from the spectrum using insights from drift-diffusion simulation.
Frequency
domain techniques are useful tools to characterize processes
occurring on different time scales in solar cells and solar fuel devices.
Intensity-modulated photocurrent spectroscopy (IMPS) is one such technique
that links the electrical and optical responses of the device. In
this review, a summary of the fundamental application of IMPS to semiconductor
photoelectrodes and nanostructured solar cells is presented, with
a final goal of understanding the IMPS response of the perovskite
solar cell (PSC) to shed light on its complex physical mechanisms
of operation. The historical application of IMPS that connects its
transfer function to the charge transfer efficiency of the semiconductor
electrode and subsequently the considerations of diffusive transport
for the dye-sensitized solar cell is summarized. These models prioritize
the association of spectral features with time constants, which has
led to a neglect of other absolute aspects of the spectra by the photovoltaic
community. We clarify these aspects by developing the fundamental
connection between the absolute value of the IMPS transfer function
and the external quantum efficiency (EQEPV) of a photovoltaic
cell. Basic models for the solar cell are developed using kinetic
equations and equivalent circuits (EC), stressing their equivalence
and the advantage of the EC representation to adequately account for
different capacitances in the system. A critique of the current interpretations
of the PSC IMPS spectra is performed, where time constants and their
evolution are associated with characteristic transport processes of
either electronic or ionic carriers within the PSC. These are clarified
using the EC representation to identify that the generated characteristic
processes are only related to coupling between different elements
of the EC and are not reflective of transport phenomena in general.
Furthermore, a general model is developed that identifies charge accumulation
at the interfaces as a general feature for both low- and high-efficiency
PSCs, whose charging/discharging resistances are the main factor in
controlling the electrical response of the device. This model shows
a separation of the photovoltage within the PSC that causes a reduction
in its EQEPV at low frequencies. Further development of
the PSC will involve gaining control over the low-frequency charge
kinetics in the device to overcome these limitations.
Perovskite solar cells (PSCs) have reached impressively high efficiencies in a short period of time; however, the optoelectronic properties of halide perovskites are surprisingly complex owing to the coupled ionic...
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