Lead
halide perovskites have over the past few years attracted considerable
interest as photo absorbers in PV applications with record efficiencies
now reaching 22%. It has recently been found that not only the composition
but also the precise stoichiometry is important for the device performance.
Recent reports have, for example, demonstrated small amount of PbI2 in the perovskite films to be beneficial for the overall
performance of both the standard perovskite, CH3NH3PbI3, as well as for the mixed perovskites (CH3NH3)
x
(CH(NH2)2)(1–x)PbBr
y
I(3–y). In this
work a broad range of characterization techniques including X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM), photo electron spectroscopy (PES), transient
absorption spectroscopy (TAS), UV–vis, electroluminescence
(EL), photoluminescence (PL), and confocal PL mapping have been used
to further understand the importance of remnant PbI2 in
perovskite solar cells. Our best devices were over 18% efficient,
and had in line with previous results a small amount of excess PbI2. For the PbI2-deficient samples, the photocurrent
dropped, which could be attributed to accumulation of organic species
at the grain boundaries, low charge carrier mobility, and decreased
electron injection into the TiO2. The PbI2-deficient
compositions did, however, also have advantages. The record V
oc
was as high as 1.20 V and
was found in PbI2-deficient samples. This was correlated
with high crystal quality, longer charge carrier lifetimes, and high
PL yields and was rationalized as a consequence of the dynamics of
the perovskite formation. We further found the ion migration to be
obstructed in the PbI2-deficient samples, which decreased
the JV hysteresis and increased the photostability.
PbI2-deficient synthesis conditions can thus be used to
deposit perovskites with excellent crystal quality but with the downside
of grain boundaries enriched in organic species, which act as a barrier
toward current transport. Exploring ways to tune the synthesis conditions
to give the high crystal quality obtained under PbI2-poor
condition while maintaining the favorable grain boundary characteristics
obtained under PbI2-rich conditions would thus be a strategy
toward more efficiency devices.
Chemical doping of
inorganic–organic hybrid perovskites
is an effective way of improving the performance and operational stability
of perovskite solar cells (PSCs). Here we use 5-ammonium valeric acid
iodide (AVAI) to chemically stabilize the structure of α-FAPbI3. Using solid-state MAS NMR, we demonstrate the atomic-level
interaction between the molecular modulator and the perovskite lattice
and propose a structural model of the stabilized three-dimensional
structure, further aided by density functional theory (DFT) calculations.
We find that one-step deposition of the perovskite in the presence
of AVAI produces highly crystalline films with large, micrometer-sized
grains and enhanced charge-carrier lifetimes, as probed by transient
absorption spectroscopy. As a result, we achieve greatly enhanced
solar cell performance for the optimized AVA-based devices with a
maximum power conversion efficiency (PCE) of 18.94%. The devices retain
90% of the initial efficiency after 300 h under continuous white light
illumination and maximum-power point-tracking measurement.
Combining halide perovskites with tailored dimensionality into two/three-dimensional (2D/3D) systems has revealed a powerful strategy to boost the performances of perovskite photovoltaics (PVs). Despite recent advances, a clear understanding of the intimate link between interface structure and physics is still missing, leading so far to a blind optimization of the 2D/3D PVs. Here, we reveal the impact of 2D/3D crystal alignment in driving interface chargerecombination dynamics. The 2D crystal growth and orientation are manipulated by specific fluorination of phenethylammonium (PEA), used here as the organic cation backbone of the 2D component. By means of time-resolved optoelectronic analysis from the femto-to microsecond regions, we demonstrate a static function of the 2D layer as an electron barrier and homogeneous surface passivant, together with a dynamic role in retarding back charge recombination. Our results reveal a crucial dependence of such beneficial effects with the 2D layer, leading to an enhanced open-circuit voltage (V oc), mostly attributed to the 2D phase which orients parallel on the 3D layer. Such findings provide a deep understanding and delineate precise guidelines for the smart design of multidimensional perovskite interfaces for advanced PVs and beyond.
Evidence for an ultrafast light-induced cascade of energy and charge transfer between aggregated quantum-confined nanoplatelets and nanoparticles of CH3NH3PbBr3 perovskite.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.