Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time-of-flight, time-resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs-incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open-circuit voltage and fill factor of Cs-containing devices. By comparison, Rb-addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current-voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs-Rb-FA-MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple-cation perovskite solar cells, which are essential for the design of high-performance devices.

State-of-the-art perovskite-based solar cells employ expensive, organic hole transporting materials (HTMs) such as Spiro-OMeTAD that, in turn, limits the commercialization of this promising technology. Herein an HTM (EDOT-Amide-TPA) is reported in which a functional amide-based backbone is introduced, which allows this material to be synthesized in a simple condensation reaction with an estimated cost of <$5 g⁻¹. When employed in perovskite solar cells, EDOT-Amide-TPA demonstrates stabilized power conversion efficiencies up to 20.0% and reproducibly outperforms Spiro-OMeTAD in direct comparisons. Time resolved microwave conductivity measurements indicate that the observed improvement originates from a faster hole injection rate from the perovskite to EDOT-Amide-TPA. Additionally, the devices exhibit an improved lifetime, which is assigned to the coordination of the amide bond to the Li-additive, offering a novel strategy to hamper the migration of additives. It is shown that, despite the lack of a conjugated backbone, the amide-based HTM can outperform state-of-the-art HTMs at a fraction of the cost, thereby providing a novel set of design strategies to develop new, low-cost HTMs.

Perovskite-based photovoltaics have been rapidly developed, with record power conversion efficiencies now exceeding 22%. In order to rationally design efficient and stable perovskite solar cells, it is important to understand not only charge trapping and recombination events, but also processes occurring at the perovskite/transport material (TM) interface, such as charge transfer and interfacial recombination. In this work, time-resolved microwave conductivity measurements are performed to investigate these interfacial processes for methylammonium lead iodide and various state-of-the-art organic TMs. A global kinetic model is developed, which accurately describes both the dynamics of excess charges in the perovskite layer and transfer to charge-specific TMs. The authors conclude that for state-of-the-art materials, such as Spiro-OMeTAD and PCBM, the charge extraction efficiency is not significantly affected by intra-band gap traps for trap densities under 10¹⁵ cm^–3. Finally, the transfer rates to C60, PCBM, EDOT-OMeTPA, and Spiro-OMeTAD are sufficient to outcompete second order recombination under excitation densities representative for illumination by AM1.5.