Charge Transport and Recombination in Perovskite (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> Sensitized TiO<sub>2</sub> Solar
Cells
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Abstract
We report on the effect of TiO<sub>2</sub> film thickness on the
charge transport, recombination, and device characteristics of perovskite
(CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> sensitized solar cells
using iodide-based electrolytes. (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> is relatively stable in a nonpolar solvent (e.g., ethyl acetate)
with a low iodide concentration (e.g., 80 mM). Frequency-resolved
modulated photocurrent/photovoltage spectroscopies show that increasing
TiO<sub>2</sub> film thickness from 1.8 to 8.3 μm has little
effect on transport but increases recombination by more than 10-fold,
reducing the electron diffusion length from 16.9 to 5.5 μm,
which can be explained by the higher degree of iodide depletion within
the TiO<sub>2</sub> pores for thicker films. The changes of the charge-collection
and light-absorption properties of (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> sensitized cells with varying TiO<sub>2</sub> film thickness
strongly affect the photocurrent density, photovoltage, fill factor,
and solar conversion efficiency. Developing alternative, compatible
redox electrolytes is important for (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> or similar perovskites to be used for potential photoelectrochemical
applications