Mixed halide hybrid perovskite compounds
such as MAPbI1.5Br1.5 are among highly acclaimed
absorber materials for
solar cells due to their easy tunability of band gap. These compounds
are intrinsically unstable and often demix into separate phases upon
white light illumination, and this undesired process commonly known
as photoinduced phase segregation presently impedes the development
of mixed halide perovskite (MHP)-formulated solar devices. Understanding
the pathways that lead to such phase segregation and controlling them
are the key to solve their device instability problems. The miscibility
gap that I/Br possesses at room temperature easily allows the separation
of the mixed phase of MAPbI1.5Br1.5 into two
phases as MAPbBr3 and MaPbI3 on irradiation,
which leads to a continuous decrease in V
oc in these devices. The low-lying valence band (VB) of the segregated
iodine-rich phase facilitates trapping of holes, leading to migration
of I– ions toward grain boundaries. Here, we investigate
the effects of TiO2-intercalated graphene nanoribbons (GNRs)
to hinder the migration and accumulation of I– at
grain boundaries. It is found that the remarkably low charge resistance
and low electron impedance of the modified electron transport layer
(ETL), along with a reduced polaron-induced strain gradient in MHP
due to GNRs facilitate against phase segregation, hole accumulation,
and carrier recombination. This enables the device to retain its V
oc unlike in devices without GNR wherein V
oc decreases to zero in less than 60 min of
illumination. Using conductive atomic force microscopy (AFM) studies,
we reveal the electrical conductivity of TiO2 and TiO2-GNR films. We conclude by positing that TiO2-GNR
with improved charge transport properties reduces electric field-induced
halide ion migration that may give rise to a decrease in V
oc upon continuous light illumination.