Replacement of lead in the hybrid organic–inorganic perovskite solar cells invokes the need for non-toxic materials such as Sn. Although solution processed CsSnI3 has been demonstrated as a lead-free halide perovskite which can function as a light absorber with high photocurrent densities, the power conversion efficiencies were bottlenecked by low open circuit voltages. In this work, the open circuit voltages are modulated by chemical doping of CsSnI3 with Br leading to formation of CsSnI3‑x Br x (0 ≤ x ≤ 3) perovskites. The beneficial effect of Br incorporation for V oc improvement is evident for CsSnI3 system even without the addition of SnF2. There is an evolution of the crystal structure of CsSnI3 from orthorhombic to cubic for CsSnBr3 accompanied by changes in its optical properties with a blue shift of the absorption and IPCE onset, as the Br– doping is increased. The V oc enhancement is attributed to the decrease in Sn vacancies which is reflected by the lower charge carrier densities of 1015 cm–3 and a high resistance to charge recombination in case of Br rich CsSnI3‑x Br x perovskite. By the addition of SnF2 to CsSnI3‑x Br x perovskite, the current densities are improved significantly.

Even though perovskite solar cells have reached 22% efficiency within a very short span, the presence of lead is a major bottleneck to its commercial application. Tin and Germanium based perovskites failed to be viable replacements due to the instability of their +2 oxidation states. Antimony could be a possible replacement, forming perovskites with structure A3M2X9. However, solution processing of Cs, organic ammonium based Sb perovskites result in the formation of the dimer phase with poor charge transport properties. Here we demonstrate that Rb can template the formation of the desired layered phase irrespective of processing methodologies, enabling the demonstration of efficient lead-free perovskite solar cells.

Perovskite material with a bandgap of 1.7-1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation.

Lead (Pb) halide perovskites have attracted tremendous attention in recent years because of their rich optoelectronic properties, which have resulted in more than 22% power conversion efficient photovoltaics (PVs). Nevertheless, Pb-metal toxicity remains a huge hurdle for extensive applications of these compounds. Thus, alternative compounds with similar optoelectronic properties need to be developed. Bismuth possesses electronic structure similar to that of lead with the presence of ns electrons that exhibit rich structural variety as well as interesting optical and electronic properties. Herein, we critically assess CsBiI as a candidate for thin-film solar cell absorber. Despite a reasonable optical band gap (∼2 eV) and absorption coefficient, the power conversion efficiency of the CsBiI mesoscopic solar cells was found to be severely lacking, limited by the poor photocurrent density. The efficiency of the CsBiI solar cell can be slightly improved by changing the stoichiometry of the precursor solutions, which is most probably due to the reduction in nonradiative defects as evident from our single-crystal photoluminescence spectroscopy. However, detailed investigations on pristine CsBiI reveal that zero-dimensional molecular crystal structure remains one of the main bottlenecks in achieving high performance. On the basis of our comprehensive studies, we have proposed that a continuous network of three-dimensional crystal structure should be another major criterion in addition to proper band gap and suitable optical properties of the future PV compounds.

In addition to a good perovskite light absorbing layer, the hole and electron transport layers play a crucial role in achieving high-efficiency perovskite solar cells. Here, a simple, one-step, solution-based method is introduced for fabricating high quality indium-doped titanium oxide electron transport layers. It is shown that indium-doping improves both the conductivity of the transport layer and the band alignment at the ETL/perovskite interface compared to pure TiO 2 , boosting the fill-factor and voltage of perovskite cells. Using the optimized transport layers, a high steady-state efficiency of 17.9% for CH 3 NH 3 PbI 3 -based cells and 19.3% for Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 3based cells is demonstrated, corresponding to absolute efficiency gains of 4.4% and 1.2% respectively compared to TiO 2 -based control cells. In addition, a steady-state efficiency of 16.6% for a semi-transparent cell is reported and it is used to achieve a four-terminal perovskite-silicon tandem cell with a steady-state efficiency of 24.5%.