Silver bismuth iodides are non-toxic and comparatively cheap photovoltaic materials, but their wide bandgaps and downshifted valence band edges limit their This article is protected by copyright. All rights reserved. performance as light absorbers in solar cells. Herein, we introduce a strategy to tune the optoelectronic properties of silver bismuth iodides by partial anionic substitution with the sulfide dianion. A consistent narrowing of the bandgap by 0.1 eV and an upshift of the valence band edge by 0.1-0.3 eV upon modification with sulfide are demonstrated for AgBiI 4 , Ag 2 BiI 5 , Ag 3 BiI 6 and AgBi 2 I 7 compositions. Solar cells based on silver bismuth sulfoiodides embedded into a mesoporous TiO 2 electron transporting scaffold, and a poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] hole transporting layer significantly outperform devices based on sulfide-free materials, mainly due to enhancements in the photocurrent by up to 48 %. A power conversion efficiency of 5.44 ± 0.07 % (J sc = 14.6 ± 0.1 mA cm -2 ; V oc = 569 ± 3 mV; fill factor = 65.7 ± 0.3 %) under 1 sun irradiation and stability under ambient conditions for over a month are demonstrated. The results reported herein indicate that further improvements should be possible with this new class of photovoltaic materials upon advances in the synthesis procedures and an increase in the level of sulfide anionic substitution.
Remarkable power conversion efficiencies (PCE) of metal halide perovskite solar cells (PSCs) are overshadowed by concerns about their ultimate stability, which is arguably the prime obstacle to commercialisation of this promising technology. Herein, the problem is addressed by introducing ethane-1,2-diammonium ( + NH 3 (CH 2 ) 2 NH 3 + , EDA 2+ ) cations into the methyl ammonium (CH 3 NH 3 + , MA + ) lead iodide perovskite, which enables, inter alia, systematic tuning of the morphology, electronic structure, light absorption and photoluminescence properties of the perovskite films. Incorporation of <5 mol% EDA 2+ induces strain in the perovskite crystal structure with no new phase formed. With 0.8 mol% EDA 2+ , PCE of the MAPbI 3 -based PSCs (aperture of 0.16 cm 2 ) improves from 16.7 ±0.6 % to 17.9 ± 0.4% under 1 sun irradiation, and fabrication of larger area devices (aperture 1.04 cm 2 ) with a certified PCE of 15.2 ± 0.5% is demonstrated. Most importantly, EDA 2+ /MA + -based solar cells retain 75% of the initial performance after 72 hours of continuous operation at 50% relative humidity and 50 ºC under 1 sun illumination, whereas the MAPbI 3 devices degrade by approximately 90% within only 15 hours. This substantial improvement in stability is attributed to the steric and coulombic interactions of embedded EDA 2+ in the perovskite structure.
The search for lead‐free alternatives to lead‐halide perovskite photovoltaic materials resulted in the discovery of copper(I)‐silver(I)‐bismuth(III) halides exhibiting promising properties for optoelectronic applications. The present work demonstrates a solution‐based synthesis of uniform CuxAgBiI4+x thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI5 at x = 1, 2D Cu2AgBiI6 at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu2AgBiI6 has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, a valence band level at ‐5.25 eV, improved carrier lifetime, and higher recombination resistance as compared to CuAgBiI5. These differences are mirrored in the power conversion efficiencies of the CuAgBiI5 and Cu2AgBiI6 solar cells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The latter value is the highest reported for this class of materials owing to the favorable film morphology provided by the hot‐casting method. Future performance improvements might emerge from the optimization of the Cu2AgBiI6 layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated Cu2AgBiI6 solar cells display storage stability over 240 days.
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