  Perovskite solar cells are one of the most attracting cells because of
remarkably improved power conversion efficiency (PCE) recently. Toward their
practical application, it is important not only to increase the PCE but also to
elucidate the deterioration mechanism. Here, we present operando direct
observation of spin states in the cells using electron spin resonance (ESR)
spectroscopy in order to investigate the operation and deterioration mechanisms
from a microscopic viewpoint. By simultaneous measurements of solar-cell and
ESR characteristics of the same cell, the spin states in the hole-transport
material (HTM) spiro-OMeTAD are demonstrated to be changed at the molecular
level, which varies the device performance under device operation. These
variations are ascribed to the change of hole transport by charge-carrier
scatterings and filling of deep trapping levels in the HTM, and to interfacial
electric dipole layers formed at the HTM interfaces. In addition, reverse
electron transfer from TiO2 layer to the HTM layer is directly demonstrated at
the molecular level under ultraviolet light irradiation, which causes the
decrease in the HTM doping effect. Thus, conducting such operando microscopic
investigation on the internal states in the cells would be useful to obtain a
new further guideline for improving the device performance and durability.
