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Perovskite solar cell device performance is affected by optical and electronic losses. To minimize these losses in solar cells, it is important to identify their sources. Here, we report the optical and electronic losses arising from physically mixed interfacial layers between the adjacent component materials in highly efficient two terminal (2T) all-perovskite tandem, single-junction wide-bandgap, and single-junction narrow-bandgap perovskite-based solar cells. Physically mixed interfacial layers as the sources of optical and electronic losses are identified from spectroscopic ellipsometry measurements and data analysis followed by comparisons of simulated and measured external quantum efficiency spectra. Parasitic absorbance in the physically mixed regions between silver metal electrical contacts and electron transport layers (ETLs) near the back contact and a physical mixture of commercial indium tin oxide and hole transport layers (HTL) near the front electrical contact lead to substantial optical loss. A lower-density void + perovskite nucleation layer formed during perovskite deposition at the interface between the perovskite absorber layer and the HTL causes electronic losses because of incomplete collection of photogenerated carriers likely originating from poor coverage and passivation of the initially nucleating grains.

High-performance perovskite/perovskite tandem solar cells require high-efficiency and stable low-bandgap perovskite subcells. State-of-the-art low-bandgap mixed tin–lead iodide perovskite solar cells exhibit either a high power-conversion efficiency or improved stability, but not both. Here we report a two-step bilayer interdiffusion growth process to simultaneously meet both requirements for formamidinium-based low-bandgap mixed tin–lead iodide perovskite solar cells. The bilayer interdiffusion growth process allows for the formation of high-quality and large-grained perovskite films with only 10 mol% volatile methylammonium. Additionally, one-dimensional pyrrolidinium perovskite was applied to passivate the perovskite film and improve the junction quality, which resulted in a carrier lifetime of 1.1 μs and an open circuit voltage of 0.865 V for our perovskite film and device with a bandgap of 1.28 eV. Our strategies enabled a power-conversion efficiency of 20.4% for low-bandgap perovskite solar cells under AM 1.5G illumination. More importantly, an encapsulated device can retain 92% of its initial efficiency after 450 h of continuous 1 sun illumination. Low-bandgap tin–lead perovskites are key to all-perovskite tandem solar cells but simultaneous improvement in efficiency and stability has proven challenging. Now, Li et al. fabricate tin–lead perovskite cells with reduced methylammonium content that are 20.4% efficient and stable under illumination for 450 h.

We have studied the growth of epitaxial films of stannate pyrochlores with a general formula A 2 Sn 2 O 7 (A = La and Y) and find that itis possible to incorporate ∼ 25% excess of the A-site constituent; in contrast, any tin excess is expelled. We unravel the defect chemistry,allowing for the incorporation of excess A-site species and the mechanism behind the tin expulsion. An A-site surplus is manifested by ashift in the film diffraction peaks, and the expulsion of tin is apparent from the surface morphology of the film. In an attempt to increaseLa 2 Sn 2 O 7 conductivity through n-type doping, substantial quantities of tin have been substituted by antimony while maintaining good filmquality. The sample remained insulating as explained by first-principles computations, showing that both the oxygen vacancy and antimony-on-tin substitutional defects are deep. Similar conclusions are drawn on Y 2 Sn 2 O 7 . An alternative n-type dopant, fluorine on oxygen, is shallowaccording to computations and more likely to lead to electrical conductivity. The bandgaps of stoichiometric La 2 Sn 2 O 7 and Y 2 Sn 2 O 7 filmswere determined by spectroscopic ellipsometry to be 4.2 eV and 4.48 eV, respectively.

Optical characterization and simulations have been applied to study formamidinium + cesium lead triiodide (FA1-xCsxPbI3) perovskite based thin films and solar cells. Near infrared to ultraviolet complex dielectric function (e = e1 + ie2) spectra of varying cesium-to-formamidinium ratios in solution processed perovskite thin films have been extracted. Analysis of these e spectra track changes in the positions of critical point (CP) energies, including band gaps and above band gap transitions, with increasing cesium contents. Band gap values are identified as the lowest energy CP, with additional sub-gap features attributed to the presence of defects. Absorption onset values, for which absorption coefficient (α) = 4000 cm−1, are extracted, with x = 0.2 showing the sharpest absorption edge and the least contribution to absorption from defects below the band gap. External quantum efficiency (EQE) simulations of solar cells using e spectra as input are compared to experimental results and corroborate that the observed sub-gap absorption is due to defects as it does not contribute to EQE.

Multi-junction all-perovskite tandem solar cells are a promising choice for next-generation solar cells with high efficiency and low fabrication cost. However, the lack of high-quality low-bandgap perovskite absorber layers seriously hampers the development of efficient and stable two-terminal monolithic all-perovskite tandem solar cells. Here, we report a bulk-passivation strategy via incorporation of chlorine, to enlarge grains and reduce electronic disorder in mixed tin–lead low-bandgap (~1.25 eV) perovskite absorber layers. This enables the fabrication of efficient low-bandgap perovskite solar cells using thick absorber layers (~750 nm), which is a requisite for efficient tandem solar cells. Such improvement enables the fabrication of two-terminal all-perovskite tandem solar cells with a champion power conversion efficiency of 21% and steady-state efficiency of 20.7%. The efficiency is retained to 85% of its initial performance after 80 h of operation under continuous illumination. Two-terminal monolithic all-perovskite tandem solar cells are attractive due to their flexible nature and low-cost fabrication. Here the authors develop a process to obtain high-quality Sn–Pb perovskite thin films by incorporating chlorine. Such layers are employed to fabricate 20.7%-efficient tandem cells with 80 h operational stability.

In situ real-time spectroscopic ellipsometry (RTSE) measurements have been conducted on MAPbI3, MA0.7FA0.3PbI3, and (FAPbI3)0.95(MAPbBr3)0.05 perovskite thin films when exposed to different levels of relative humidity at given temperatures over time. Analysis of RTSE measurements track changes in the complex dielectric function spectra and structure, which indicate variations in stability influenced by the underlying material, preparation method, and perovskite composition. MAPbI3 and MA0.7FA0.3PbI3 films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited on soda lime glass directly. (FAPbI3)0.95(MAPbBr3)0.05 films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition.

The phenomenon of spontaneously growing metal whiskers (MW) raises significant reliability concerns due to its related arcing and shorting in electric equipment. The growth kinetics of MW remains poorly predictable. Here we present a theory describing the earlier observed intermittent growth of MW as caused by local energy barriers related to variations in the random electric fields generated by surface imperfections. We find the probabilistic distribution of MW stopping times, during which MW growth halts, which is important for reliability projections., 4 pages, 3 figures