Contemporary
thin-film photovoltaic (PV) materials contain elements
that are scarce (CIGS) or regulated (CdTe and lead-based perovskites),
a fact that may limit the widespread impact of these emerging PV technologies.
Tin halide perovskites utilize materials less stringently regulated
than the lead (Pb) employed in mainstream perovskite solar cells;
however, even today’s best tin-halide perovskite thin films
suffer from limited carrier diffusion length and poor film morphology.
We devised a synthetic route to enable in situ reaction between metallic
Sn and I2 in dimethyl sulfoxide (DMSO), a reaction that
generates a highly coordinated SnI2·(DMSO)
x
adduct that is well-dispersed in the precursor solution.
The adduct directs out-of-plane crystal orientation and achieves a
more homogeneous structure in polycrystalline perovskite thin films.
This approach improves the electron diffusion length of tin-halide
perovskite to 290 ± 20 nm compared to 210 ± 20 nm in reference
films. We fabricate tin-halide perovskite solar cells with a power
conversion efficiency of 14.6% as certified in an independent lab.
This represents a ∼20% increase compared to the previous best-performing
certified tin-halide perovskite solar cells. The cells outperform
prior earth-abundant and heavy-metal-free inorganic-active-layer-based
thin-film solar cells such as those based on amorphous silicon, Cu2ZnSn(S/Se)4 , and Sb2(S/Se)3.
Tin
oxide (SnO2) has recently emerged as a promising
electron transport layer for perovskite solar cells (PSCs) in light
of the material’s optical and electronic properties and its
low-temperature processing. However, SnO2 films are prone
to surface defect formation, which results in energy loss in PSCs.
We report that surface treatment using ammonium fluoride (NH4F) leads to reduced surface defects and that it also induces chemical
doping of the SnO2 substrate simultaneously. The effects
of NH4F treatment on SnO2 properties are revealed
by surface chemical analysis, computational studies, and energy level
investigations, and PSCs with the treatment achieve photovoltaic performance
of 23.2% in light of higher voltage than in relevant controls.
Many of the best-performing perovskite photovoltaic devices make use of 2D/3D interfaces, which improve efficiency and stability – but it remains unclear how the conversion of 3D-to-2D perovskite occurs and how these interfaces are assembled. Here, we use in situ Grazing-Incidence Wide-Angle X-Ray Scattering to resolve 2D/3D interface formation during spin-coating. We observe progressive dimensional reduction from 3D to n = 3 → 2 → 1 when we expose (MAPbBr3)0.05(FAPbI3)0.95 perovskites to vinylbenzylammonium ligand cations. Density functional theory simulations suggest ligands incorporate sequentially into the 3D lattice, driven by phenyl ring stacking, progressively bisecting the 3D perovskite into lower-dimensional fragments to form stable interfaces. Slowing the 2D/3D transformation with higher concentrations of antisolvent yields thinner 2D layers formed conformally onto 3D grains, improving carrier extraction and device efficiency (20% 3D-only, 22% 2D/3D). Controlling this progressive dimensional reduction has potential to further improve the performance of 2D/3D perovskite photovoltaics.
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