Abstract
The association of the electron acceptor 4,4'-amino-bipyridinium (AmV2+) dication and BiI3 in an acidic solution affords three organic-inorganic hybrid materials, (AmV)3(BiI6)2 (1), (AmV)2(Bi4I16) (2), and (AmV)BiI5 (3), whose structures are based on isolated BiI63- and Bi4I164- anion clusters in 1 and 2, respectively, and on a one-dimensional (1D) chain of trans-connected corner-sharing octahedra in 3. In contrast with known methylviologen-based hybrids, these compounds are more soluble in polar solvents, allowing thin film formation by spin-coating. (AmV)BiI5 exhibits a broad absorption band in the visible region leading to an optical bandgap of 1.54 eV and shows a PV effect as demonstrated by a significant open-circuit voltage close to 500 mV. The electronic structure of the three compounds has been investigated using first-principles calculations based on density functional theory (DFT). Unexpectedly, despite the trans-connected corner-shared octahedra, for (AmV)BiI5, the valence state shows no coupling along the wire direction, leading to a high effective mass for holes, while in contrast, the strong coupling between Bi 6px orbitals in the same direction at the conduction band minimum suggests excellent electron transport properties. This contributes to the low current output leading to the low efficiency of perovskite solar cells based on (AmV)BiI5. Further insight is provided for trans- and cis-MI5 1D model structures (M = Bi or Pb) based on DFT investigations.
Highlights
Perovskite solar cells (PSCs) represent a new generation of photovoltaics, combining the advantages of lowtemperature thin film processing[1,2] and a high power conversion efficiency (PCE), currently reaching 25.5%.3 the more efficient solar cells contain a high percentage (>30%) of the toxic Pb element, which could prevent its industrial development
Depending on the experimental conditions, three compounds have been obtained as pure phases in the AmV2+/Bi3+/I− system: (AmV)3(BiI6)[2] (1) and (AmV)2(Bi4I16) (2) and (AmV)BiI5 (3)
The crystal structures are based on isolated BiI63− and Bi4I164− anion clusters in 1 and 2, respectively, and on 1D transconnected corner-sharing octahedral chains in 3. (AmV)BiI5 exhibits a broad absorption band in the whole visible region, and upon incorporation into a solar cell device, the presence of a PV effect is demonstrated with a significant open-circuit voltage close to 500 mV
Summary
Perovskite solar cells (PSCs) represent a new generation of photovoltaics, combining the advantages of lowtemperature thin film processing (the process costing less than half of the price of c-Si solar cell technology)[1,2] and a high power conversion efficiency (PCE), currently reaching 25.5%.3 the more efficient solar cells contain a high percentage (>30%) of the toxic Pb element, which could prevent its industrial development. Size, and charge of the organic cations, many different types of such anions, resulting from corner-, edge-, or face-sharing MI6 (M = Bi or Sb) octahedra, are known.[12] most of these anions are of the cluster type,[13−15] which lead to low structural and electronic dimensionalities for the corresponding materials. They lead to modest power conversion efficiencies; for instance, the past record efficiency of lead-free perovskites was reached with Cs3Bi2I9 clusters, with an efficiency of slightly >1% (0.6 FF, 0.85 V, and 2.15 mA cm−2).[16] While some one-dimensional (1D) and two-dimensional (2D) non-perovskite networks have been reported, such as the well-known 1D BiI4 chain of edge-sharing octahedra,17 1D and 2D perovskite networks (having octahedra in corner-sharing mode only) are rarer. Our strategy was to replace the methylviologen cation MV2+
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