Abstract
Perovskite solar cells (PSCs) based on organic-inorganic hybrid perovskites containing a small fraction of substituted alkali-metal cations have shown remarkable performance and stability. However, the role of these cations is unclear. The thermal- and moisture-induced degradation of FA1-xCsxPbI3 and (FA1-xCsx)1-yRbyPbI3 (where FA represents formamidinium, x, y = 0.1, 0.05) is investigated using in situ photoelectron spectroscopy (PES). Both compositions exhibit superior moisture stability compared with methylammonium lead iodide under 9 mbar of water vapor. Ga Kα hard X-ray PES is used to investigate the composition of the perovskites at depths up to 45 nm into the surface. This allows more accurate quantification of the alkali-metal distribution than is possible using conventional X-ray PES. The addition of RbI results in a fairly homogeneous distribution of both Cs+ and Rb+ in the surface layers (in contrast to surface Cs depletion seen in its absence), together with a marked reduction in surface iodide vacancies. Overall, RbI is found to play a critical role in increasing the thermal stability of FA1-xCsxPbI3 by providing a source of I- that fills iodine vacancy sites in the perovskite lattice, while Rb+ is not substantially incorporated into the perovskite. We suggest that the concomitant increase in ion migration barriers in the surface layers is key to improved PSC performance and long-lasting stability.
Highlights
Organic−inorganic hybrid perovskites have emerged as important material systems in the field of photovoltaics and optoelectronics due to interesting optical and electrical properties
The XRD patterns of the FACs and FACsRb samples processed under ambient conditions are shown in Figure S1a in Supporting Information. Both samples exhibit a strong reflection at 13.9° corresponding to the (101) plane of αFAPbI3.49 Typically, the conventional perovskites MAPbI3 or FAPbI3 prepared under ambient conditions show a reflection at 2θ = 12.7° assigned to PbI2, indicating some degradation or incomplete conversion to perovskite under ambient conditions
By using a novel laboratory-based Ga Kα source for HAXPES, we are able to quantify the distribution of the dilute alkali-metal cations in the near-surface layers more accurately that has hitherto been possible
Summary
Organic−inorganic hybrid perovskites have emerged as important material systems in the field of photovoltaics and optoelectronics due to interesting optical and electrical properties These properties include tuneable band gaps (1.2−2.9 eV) and strong light absorption coefficients (104 to 105 cm−1) in the solar spectrum region, long charge carrier diffusion lengths (100 to 1 μm), low exciton-binding energies, and relatively low production costs.[1−4] In 2009, Kojima et al applied methylammonium lead iodide (MAPbI3) as the light absorber in a perovskite-sensitized solar cell which exhibited a power conversion efficiency (PCE) of 3.8% but poor stability.[5] Since several material compositions and preparation methods have been developed which have resulted in a PCE of 25.2%, which was achieved by using multiple cations and halides in the perovskite composition.[6] the intrinsic instability of MAPbI3 under atmospheric conditions such as moisture, light illumination, and thermal stress is preventing the widespread commercial application of these materials.[7,8] It has been previously reported that MAPbI3 undergoes a phase transition at low temperature (55 °C) and further decomposes into volatile compounds such as CH3NH2 and HI upon contact with moisture.[1,9].
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