Efficient lead (Pb)-free inverted planar formamidinium tin triiodide (FASnI ) perovskite solar cells (PVSCs) are demonstrated. Our FASnI PVSCs achieved average power conversion efficiencies (PCEs) of 5.41% ± 0.46% and a maximum PCE of 6.22% under forward voltage scan. The PVSCs exhibit small photocurrent-voltage hysteresis and high reproducibility. The champion cell shows a steady-state efficiency of ≈6.00% for over 100 s.

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Mixed tin (Sn)-lead (Pb) perovskites with high Sn content exhibit low bandgaps suitable for fabricating the bottom cell of perovskite-based tandem solar cells. In this work, we report on the fabrication of efficient mixed Sn-Pb perovskite solar cells using precursors combining formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3). The best-performing cell fabricated using a (FASnI3)0.6(MAPbI3)0.4 absorber with an absorption edge of ∼1.2 eV achieved a power conversion efficiency (PCE) of 15.08 (15.00)% with an open-circuit voltage of 0.795 (0.799) V, a short-circuit current density of 26.86(26.82) mA/cm(2), and a fill factor of 70.6(70.0)% when measured under forward (reverse) voltage scan. The average PCE of 50 cells we have fabricated is 14.39 ± 0.33%, indicating good reproducibility.

PEALD deposition was used to reduce the effective deposition temperature of SnO2 electron selective layers without compromising the performance of perovskite solar cells.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10. 1002/aenm.201700414. used mesoporous TiO 2 ESLs, but the complex synthesis procedure and requirement of sintering at temperatures above 450 °C make the process undesirable for large-scale roll-to-roll manufacturing. [3] Therefore, significant efforts have been made on developing efficient low-temperature planar PVSCs. [23][24][25][26][27] For PVSCs with a regular configuration, the more popular low-temperature processed ESL materials are transition metal oxides such as TiO 2 , [26,28] ZnO, [29,30] SnO 2 , [31,32] and Zn 2 SnO 4 . [23,24] Due to its relatively simple synthesis process, chemical stability, [33][34][35] and high electron mobility, [33,36,37] SnO 2 ESL has shown great promise in making efficient planar PVSCs. For example, Ke et al. first fabricated PVSCs with PCE over 16% using a low-temperature solution processed SnO 2 ESL. [31] Since then, many other groups have also reported efficient PVSCs using low-temperature processed ESLs. [23,33,37] Recently, Gratzel and co-workers reported planar PVSCs with efficiencies approaching 21% using SnO 2 ESLs synthesized by low-temperature chemical bath deposition. [32] Low-temperature SnO 2 ESLs have been deposited by various methods including spin-coating, [31] dual-fuel combustion, [38] chemical-bath deposition, [32] and atomic-layer deposition (ALD). [34,39] ALD produces the most compact thin films compared to the other methods mentioned. However, PVSCs using ALD SnO 2 ESLs often show current-density-voltage (J-V) Adv. Energy

Formamidinium lead triiodide (FAPbI ) is considered as an alternative to methylammonium lead triiodide (MAPbI ) because of its lower band gap and better thermal stability. However, owing to the large size of FA cations, it is difficult to synthesize high-quality FAPbI thin films without the formation of an undesirable yellow phase. Smaller sized cations, such as MA and Cs, have been successfully used to suppress the formation of the yellow phase. Whereas FA and MA lead triiodide perovskite solar cells (PVSCs) have achieved power conversion efficiencies (PCEs) higher than 20 %, the PCEs of formamidinium and cesium lead triiodide (FA Cs PbI ) PVSCs have been only approximately 16.5 %. Herein, we report our examination of the main factors limiting the PCEs of (FA Cs PbI ) PVSCs. We find that one of the main limiting factors could be the small grain sizes (≈120 nm), which leads to relatively short carrier lifetimes. We further find that adding a small amount of lead thiocyanate [Pb(SCN) ] to the precursors can enlarge the grain size of (FA Cs PbI ) perovskite thin films and significantly increase carrier lifetimes. As a result, we are able to fabricate (FA Cs PbI ) PVSCs with significantly improved open-circuit voltages and fill factors and, therefore, enhanced PCEs. With an optimal 0.5 mol % Pb(SCN) additive, the average PCE is increased from 16.18±0.50 (13.45±0.78) % to 18.16±0.54 (16.86±0.63) % for planar FA Cs PbI PVSCs if measured under reverse (forward) voltage scans. The champion cell registers a PCE of 19.57 (18.12) % if measured under a reverse (forward) voltage scan, which is comparable to that of the best-performing MA-containing planar FA-based lead halide PVSCs.

We show that the cooperation of lead thiocyanate additive and a solvent annealing process can effectively increase the grain size of mixedcation lead mixed-halide perovskite thin films while avoiding excess lead iodide formation. As a result, the average grain size of the wide-bandgap mixed-cation lead perovskite thin films increases from 66 ± 24 to 1036 ± 111 nm, and the mean carrier lifetime shows a more than 3-fold increase, from 330 ns to over 1000 ns. Consequently, the average open-circuit voltage of wide-bandgap perovskite solar cells increases by 80 (70) mV, and the average power conversion efficiency (PCE) increases from 13.44 ± 0.48 (11.75 ± 0.34) to 17.68 ± 0.36 (15.58 ± 0.55)% when measured under reverse (forward) voltage scans. The best-performing wide-bandgap perovskite solar cell, with a bandgap of 1.75 eV, achieves a stabilized PCE of 17.18%.