No abstract

Lead-free double perovskites have recently attracted growing attention as possible alternatives to lead-based halide perovskites in photovoltaics and other optoelectronic applications. The most prominent compound Cs2AgBiBr6, however, presents issues such as a rather large and indirect band gap, high exciton binding energies, and poor charge carrier transport, especially in thin films. In order to address some of these challenges, we systematically modified the stoichiometry of the precursors used for the synthesis of thin films toward a BiBr3-deficient system. In combination with a stoichiometric excess of AgBr, we obtained highly oriented double perovskite thin films. These modifications directly boost the lifetime of the charge carriers up to 500 ns as observed by time-resolved photoluminescence spectroscopy. Moreover, time-resolved microwave conductivity studies revealed an increase of the charge carrier mobility from 3.5 to around ∼5 cm2/(V s). Solar cells comprising the modified films as planar active layers reached power conversion efficiency (PCE) values up to 1.11%, exceeding the stoichiometric reference film (∼0.97%), both on average and with champion cells. The results in this work underline the importance of controlling the nanomorphology of the bulk film. We anticipate that control of precursor stoichiometry will also offer a promising approach for enhancing the efficiency of other perovskite photovoltaic absorber materials and thin films.

Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current‐voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter‐laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin‐film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance‐free JV‐curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra‐grain charge transport is characterized by terahertz sum mobilities of ≈32 cm2 V−1 s−1. Drift‐diffusion simulations indicate that these intra‐grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best‐realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented.

By slowly evaporating organic solvents, we succeeded in growing lead-free double perovskite Cs2AgBiBr6 crystals. Solubility curves prove the advantage of this growth technique and allow comparison with the controlled cooling...

Heteroatom alloying of lead-free perovskite derivatives is a highly promising route to tailor their optoelectronic properties and stability for multiple applications. Here, we demonstrate the facile solution-based synthesis of Sn-alloyed layered MA 3 Sb 2 I 9 thin films by precursor engineering, combining acetate and halide salts. An increasing concentration of tin halides in different oxidation states leads to a strong boost in absorption over the whole visible spectrum. We demonstrate phase-pure synthesis and elucidate the heterovalent incorporation of Sn into the MA 3 Sb 2 I 9 lattice, proving the formation of additional electronic states in the bandgap by theoretical calculations. On this basis, we dissect the strong absorption increase into three components that we attribute to intervalence and heteroatom-induced interband absorption. Finally, we show the charge-stabilizing effect of the system through robustness toward precursors in mixed oxidation states and trace the improved ambient stability of this material back to this feature.