Silver bismuth iodide (SBI) materials have recently gained attention as nontoxic alternatives to lead perovskites. Although most of the studies have been focusing on photovoltaic performance, the inherent ionic nature of SBI materials, their diffusive behavior, and influence on material/device stability is underexplored. Herein, AgBi2I7, Ag2BiI5, and Ag3BiI6 thin films are developed in controlled ambient humidity conditions with a decent efficiency up to 2.32%. While exploring the device stability, it is found that Ag3BiI6 exhibits a unique ion‐migration behavior where Ag+, Bi3+, and I− ions migrate and diffuse through the dopant‐free hole transport layer (HTL) leading to degradation. Interestingly, this ion‐migration behavior is relatively fast for the case of antisolvent‐processed Ag3BiI6 thin‐film‐based devices contrasting the case of without antisolvent and is not observed for other SBI material‐based devices. Theoretical calculations suggest that low decomposition enthalpy favors the decomposition of Ag3BiI6 to AgI and BiI3 causing migration of ions to the electrode which is protected by using a thick HTL . The new mechanism reported herein underlines the importance of SBI material composition and fundamental mechanism understanding on the stability of Ag3BiI6 material for better solar cell design and also in extending the applications of unique ion‐migration behavior in various optoelectronics.
Investigation
of the perovskite (MAPbI3; MA+ = CH3NH3
+) crystallization, location
of residual lead iodide (PbI2), and its influence on the
optoelectronic and photovoltaic properties is important to resolve
the low-efficiency issue in radio frequency (RF) sputtered nickel
oxide (sp-NiO
x
) based perovskite solar
cells (PSCs). Despite obtaining the perovskite layer from a solution
of the stoichiometric ratio of precursor materials using a one-step
method (OSM) for spin-coating, residual PbI2 grains are
observed (from the top surface scanning electron microscopy (SEM)
image) in the bulk of perovskite (deposited on sp-NiO
x
). To investigate the dependence of residual PbI2 on the preparation methods, we prepared the perovskite (on
sp-NiO
x
) by a two-step method (TSM) in
which methylammonium iodide (MAI) solution is spin-coated on top of
the deposited PbI2 layer. The cross-sectional SEM image
of the TSM-based perovskite device, confirms the presence of the residual
PbI2 layer (20–50 nm) at the interface of perovskite/NiO
x
. Although a higher X-ray diffraction (XRD)
peak intensity of residual PbI2 was observed, the higher
crystallinity of the perovskite film prepared by TSM was confirmed
based on the evaluation of the photoluminescence (PL) peaks and the
PL lifetime. In comparison with the devices prepared by OSM, the higher
photovoltaic performance of PSCs is observed for devices fabricated
using TSM. An investigation on the relation of the device performance
with the location of residual PbI2 is presented in this
work and can be insightful to further improve the performance of sp-NiO
x
-based PSCs.
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