Hole-mobility in nanocrystalline TiO2/P3HT composites is increased by over an order of magnitude when the TiO2 surface is treated with a 3-hexylthiophene oligomer bearing a cyanoacrylic acid group (4T).
The tunable bandgap energy (E g ) of metal halide perovskites has led to one of the most interesting developments in this context, namely the all-perovskite tandem solar cells (APTSCs), which is formed by combining two perovskite-based sub-cells with different bandgaps into a monolithically stacked structure. This tandem structure allows the solar cell to absorb different components of sunlight within the two absorber layers, extracting on average more useable energy per photon absorbed. Consequently, tandem solar cells can achieve a power conversion efficiency (PCE) above that of a single-junction solar cell, the limiting efficiency of which is marked by the single-junction radiative limit. [2] To most effectively implement this concept, the monolithic APTSCs usually consist of a narrow (≈1.20 eV) and a wide (≈1.80 eV) E g perovskite for the bottom and top subcells, [3] respectively. APTSCs have been reported with PCEs of 28.0%, i.e., well beyond 25.7% PCE of the current record single-junction perovskite solar cell (PSC). [4,5] This study concerns the bottom cell of APTSCs, which uses a narrow bandgap perovskite film as the light absorber. Mixing tin (Sn) and lead (Pb) as the B-site cation is required to achieve a sufficiently small E g . [6] Sn-Pb perovskites reach a minimum E g of ≈1.20 eV when the ratio of Sn to Pb is about Outstanding optoelectronic properties of mixed tin-lead perovskites are the cornerstone for the development of high-efficiency all-perovskite tandems. However, recombination losses in Sn-Pb perovskites still limit the performance of these perovskites, necessitating more fundamental research. Here, rubidium iodide is employed as an additive for methylammonium-free Sn-Pb perovskites. It is first investigated the effect of the RbI additive on the perovskite composition, crystal structure, and element distribution. Quasi-Fermi level splitting and transient photoluminescence measurements reveal that the RbI additive reduces recombination losses and increases carrier lifetime of the perovskite films. This finding is attributed to an approximately ten-fold reduction in the defect density following RbI treatment, as probed using constant final state yield photoelectron spectroscopy. Additionally, the concentration of Sn vacancies is also reduced, and the perovskite film becomes less p-type both in the bulk and at the interface towards the electron contact. Thus, the conductivity for electrons increases, improving carrier extraction. As a result, the open-circuit voltage of RbI-containing solar cells improves by 61 mV on average, with the best efficiency >20%. This comprehensive study demonstrates that RbI is effective at reducing recombination losses and carrier trapping, paving way for a new approach to Sn-Pb perovskite solar cell research.
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