Improved
Understanding of the Electronic and Energetic
Landscapes of Perovskite Solar Cells: High Local Charge Carrier Mobility,
Reduced Recombination, and Extremely Shallow Traps
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Abstract
The
intriguing photoactive features of organic–inorganic
hybrid perovskites have enabled the preparation of a new class of
highly efficient solar cells. However, the fundamental properties,
upon which the performance of these devices is based, are currently
under-explored, making their elucidation a vital issue. Herein, we
have investigated the local mobility, recombination, and energetic
landscape of charge carriers in a prototype CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite (PVK) using a laser-flash time-resolved
microwave conductivity (TRMC) technique. PVK was prepared on mesoporous
TiO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub> by one or two-step
sequential deposition. PVK on mesoporous TiO<sub>2</sub> exhibited
a charge carrier mobility of 20 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, which was predominantly attributed to holes.
PVK on mesoporous Al<sub>2</sub>O<sub>3</sub>, on the other hand,
exhibited a 50% lower mobility, which was resolved into balanced contributions
from both holes and electrons. A general correlation between crystal
size and mobility was revealed irrespective of the fabrication process
and underlying layer. Modulating the microwave frequency from 9 toward
23 GHz allowed us to determine the intrinsic mobilities of each PVK
sample (60–75 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>), which were mostly independent of the mesoporous scaffold. Kinetic
and frequency analysis of the transient complex conductivity strongly
support the superiority of the perovskite, based on a significant
suppression of charge recombination, an extremely shallow trap depth
(10 meV), and a low concentration of these trapped states (less than
10%). The transport mechanism was further investigated by examining
the temperature dependence of the TRMC maxima. Our study provides
a basis for understanding perovskite solar cell operation, while highlighting
the importance of the mesoporous layer and the perovskite fabrication
process