  Fundamental working mechanisms of perovskite solar cells remain an elusive
topic of research. Impedance Spectroscopy (IS) application to perovskite-based
devices generates uncommon features and misleading outputs, mainly due to the
lack of a stablished model for the interpretation of the results. In this work
we control the perovskite precursor concentration to fabricate a series of
perovskite-based solar cells with different amounts of perovskite absorber. Low
concentration devices present the well-known dye sensitized solar cell (DSSCs)
impedance pattern. As the amount of perovskite is increased, the characteristic
impedance spectra of thin-film perovskite solar cells (PSCs) arises. This
transition is characterized by a change in the working principles, determined
by an evolution of the dominant capacitance: from the intermediate frequency
chemical capacitance of TiO2 in devices with isolated perovskite domains, to a
large low-frequency capacitance signal which divides the spectra in two
sections, yet with no direct influence in final device performance. This study
allows to link experimentally, in terms of impedance behavior, PSCs with the
rest of solar cell devices via DSSCs. We observe that it is not possible to
assign a single physical origin to the different resistances determined in the
impedance spectra except for the series resistance. In contrast, resistive
element present contributions from different physical processes, observing a
transport-recombination coupling. Based on this analysis we provide an
equivalent circuit model to evaluate the impedance pattern of PSCs in terms of
the processes directly affecting the final performance (i.e. considering
transport-related and recombination-related losses), a crucial tool for further
development of perovskite photovoltaics.
