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One of the organic components in the perovskite photo-absorber, the methylammonium cation, has been suggested to be a roadblock to the long-term operation of organic–inorganic hybrid perovskite-based solar cells. In this work we systematically explore the crystallographic and optical properties of the compositional space of mixed cation and mixed halide lead perovskites, where formamidinium (FA + ) is gradually replaced by cesium (Cs + ), and iodide (I − ) is substituted by bromide (Br − ), i.e. , Cs y FA 1− y Pb(Br x I 1− x ) 3 . Higher tolerance factors lead to more cubic structures, whereas lower tolerance factors lead to more orthorhombic structures. We find that while some correlation exists between the tolerance factor and structure, the tolerance factor does not provide a holistic understanding of whether or not a perovskite structure will fully form. By screening 26 solar cells with different compositions, our results show that Cs 1/6 FA 5/6 PbI 3 delivers the highest efficiency and long-term stability among the I-rich compositions. This work sheds light on the fundamental structure–property relationships in the Cs y FA 1− y Pb(Br x I 1− x ) 3 compositional space, providing vital insight to the design of durable perovskite materials. Our approach provides a library of structural and optoelectronic information for this compositional space.
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Impact of cesium on the phase and device stability of triple cation Pb–Sn double halide perovskite films and solar cells
Triple cation Cs/methylammonium (MA)/formamidinium (FA) and double halide Br/I lead perovskites improved the stability and efficiency of perovskite solar cells (PVSCs). However, their effects on alloyed Pb–Sn perovskites are unexplored. In this work, perovskite thin films with the composition Cs x (MA 0.17 FA 0.83 ) 1−x Pb 1−y Sn y (I 0.83 Br 0.17 ) 3 are synthesized utilizing a one-step solution process plus an anti-solvent wash technique and deployed in PVSCs with an inverted architecture. All films show a cubic crystal structure, demonstrating that compositional tuning of both the tolerance factor and crystallization rate allows for dense, single phase formation. The band gaps, affected by both lattice constriction and octahedral tilting, show opposite trends in Pb-rich or Sn-rich perovskites with the increase of Cs for fixed Sn compositions. The Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb 0.25 Sn 0.75 (I 0.83 Br 0.17 ) 3 PVSCs achieve a power conversion efficiency (PCE) of 11.05%, a record for any PVSC containing 75% Sn perovskites, and the Cs 0.10 (MA 0.17 FA 0.83 ) 0.90 Pb 0.75 Sn 0.25 (I 0.83 Br 0.17 ) 3 PVSCs reach a record PCE of 15.78%. Moreover, the triple cation and double halide alloyed Pb–Sn perovskites exhibit improved device stability under inert and ambient conditions. This study, which illustrates the impact of cation and halide tuning on alloyed Pb–Sn perovskites, can be used to further eliminate Pb and improve device performance of high Sn PVSCs and other optoelectronic devices.
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- PAR ID:
- 10101029
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 6
- Issue:
- 36
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 17426 to 17436
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Metal halide perovskites (MHP) can be made more stable through the addition of small amounts of cesium. Despite the improvement, these multication absorbers still display strong environmental sensitivity to any combination of factors, including water, oxygen, bias, temperature, and light. Here, the relationship is elucidated between light absorption, charge carrier radiative recombination, and relative humidity (rH) for the Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3composition, revealing partially reversible reductions in the extinction coefficient and fully reversible 25× enhancements in absolute light emission registered across the same humidity cycles up to 70% rH. With in situ excitation wavelength‐dependent measurements, irreversible changes are identified in the perovskite after a single cycle of humidity‐dependent photoluminescence (PL) performed with 450 nm excitation. The in situ measurement platform can be extended to test the effect of other stressors on thin films’ optical behavior.more » « less
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null (Ed.)Achieving efficient and stable tin-based perovskite solar cells remains challenging. In this work, we incorporate the ethylenediammonium diiodide (EDAI 2 ) additive into a cesium–guanidinium doped formamidinium tin triiodide perovskite with the composition of (CsGA) x FA 1−2x SnI 3 + y % EDAI 2 . This new perovskite utilizes the strong hydrogen bonding of the guanidinium cation and the lattice strain relaxation of the small cesium cation as well as the hollowing and passivation effects of the EDAI 2 additive. The EDAI 2 additive not only yields pinhole-free cubic phase perovskite films but also decreases both shallow and deep trap states in the perovskite films. These effects are pronounced with the increase of substitution of the pair of GA + and Cs + . The new perovskites are deployed in inverted planar solar cells. A maximum power conversion efficiency (PCE) of 5.01% is achieved with the (CsGA) 0.15 FA 0.70 SnI 3 + 0% EDAI 2 device but the device degrades after storage in a nitrogen-filled glove box for 30 days. Both performance and stability are improved with the addition of EDAI 2 . A maximum PCE of 5.72% is achieved with the (CsGA) 0.15 FA 0.70 SnI 3 + 1.0% EDAI 2 device. The (CsGA) 0.15 FA 0.70 SnI 3 + 1.5% EDAI 2 devices exhibit a maximum PCE of 5.69% and the performance is further increased to 6.39% after storage in a nitrogen-filled glove box for 4 days; 70% of the initial PCE is retained after 45 days. This study demonstrates the benefit of tuning cation sizes and introducing divalent cations to integrate stabilizing factors into pure Sn perovskites, creating new routes for efficient and stable lead-free perovskite solar cells.more » « less
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/c8ta06391e
