Nanoscale mapping of chemical composition in organic inorganic hybrid perovskite films

Lead-based organic-inorganic hybrid perovskite (OIHP) solar cells can attain efficiencies over 20%. However, the impact of ion mobility and/or organic depletion, structural changes, and segregation under operating conditions urge for decisive and more accurate investigations. Hence, the development of analytical tools for accessing the grain-to-grain OIHP chemistry is of great relevance. Here, we used synchrotron infrared nanospectroscopy (nano-FTIR) to map individual nanograins in OIHP films. Our results reveal a spatial heterogeneity of the vibrational activity associated to the nanoscale chemical diversity of isolated grains. It was possible to map the chemistry of individual grains in CsFAMA [Cs(0.05)FA(0.79)MA(0.16)Pb(I0.83Br0.17)(3)] and FAMA [FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3)] films, with information on their local composition. Nanograins with stronger nano-FTIR activity in CsFAMA and FAMA films can be assigned to PbI2 and hexagonal polytype phases, respectively. The analysis herein can be extended to any OIHP films where organic cation depletion/accumulation can be used as a chemical label to study composition

Managing phase purities and crystal orientation for high-performance and photostable cesium lead halide perovskite solar cells

Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6% and the longest operational lifetime, T S80, of ≈300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 − x Brx perovskite solar cells

Hybrid Perovskite Degradation from an Optical Perspective A Spectroscopic Ellipsometry Study from the Deep Ultraviolet to the Middle Infrared

A quantitative analysis of the thermally induced degradation of various device relevant multi cation hybrid perovskite films is performed using spectroscopic ellipsometry, for temperatures between 80 and 120 C. The studied compositions are a triple cation perovskite Cs0.05 MA0.17FA0.83 0.95Pb Br0.17I0.83 3, a Rb containing variant Rb0.05Cs0.05 MA0.17FA0.83 0.90Pb Br0.17I0.83 3, and a methylammonium free Rb0.05Cs0.10FA0.85PbI3 composition. A very wide combined spectral range of 200 nm to 25 amp; 956;m is covered by combining the data from two separate instruments. The relative changes in organic cation concentrations are quantified from the middle infrared molecular absorption bands, leveraging the use of point by point fitting for increased sensitivity. Additionally, the formation of PbI2 and non perovskite amp; 948; CsPbI3 phases is evidenced from Bruggemann effective medium fits to the visible and ultraviolet complex refractive indices. Methylammonium is almost completely depleted from the relevant compositions within 100 to 285 min of thermal annealing. The MA free perovskite degrades faster at intermediate temperatures, which is attributed to phase instability due to the formation of amp; 948; CsPbI3 in addition to PbI

Large Conduction Band Energy Offset Is Critical for High Fill Factors in Inorganic Perovskite Solar Cells

Wang Q, Zu F, Caprioglio P, et al. Large Conduction Band Energy Offset Is Critical for High Fill Factors in Inorganic Perovskite Solar Cells. ACS Energy Letters. 2020;5(7):2343-2348.Although SnO2 has been reported to give high efficiencies of over 20% for organic–inorganic perovskite solar cells and has been frequently used in perovskite tandem solar cells, very few contributions have explored its feasibility in inorganic perovskite solar cells (IPSCs). Inorganic perovskites with a wide bandgap tunable from 1.7 to 2.0 eV are promising candidates for top cells in tandem structures; development of IPSCs based on SnO2 will greatly benefit their integration into tandem solar cells. We examined SnO2 in comparison to the prevalent TiO2. We found that although SnO2 had a good energy alignment with the inorganic perovskite and exhibited slower nonradiative recombination, the relatively low conduction band minimum energy offset restricted efficient charge extraction. In contrast, TiO2 that had a large energy offset of ∼400 meV led to a high fill factor of 78.7% and a state-of-the-art efficiency of 14.2% for IPSCs with a bandgap of 1.93 eV

Solvents for Processing Stable Tin Halide Perovskites

Tin is one of the most promising alternatives to lead to make lead free halide perovskites for optoelectronics. However, the stability of tin based perovskites is hindered by the oxidation of Sn II to Sn IV . Recent works established that dimethyl sulfoxide, which is one of the best performing solvents for processing perovskite, is the primary source of tin oxidation. The quest for a stable solvent could be a game changer in the stability of tin based perovskites. Starting from a database of over 2000 solvents, we identified a series of 12 new solvents suitable for the processing of formamidinium tin iodide perovskite FASnI3 by investigating 1 the solubility of the precursor chemicals FAI and SnI2, 2 the thermal stability of the precursor solution, and 3 the possibility of forming perovskite. Finally, we demonstrate a new solvent system to produce solar cells outperforming those based on DMSO. Our work provides guidelines for further identification of new solvents or solvent mixtures for preparing stable tin based perovskite

Large grain size with reduced non radiative recombination in potassium incorporated methylammonium free perovskite solar cells

Organic inorganic hybrid halide perovskites are widely considered to be one the most promising material in photovoltaic technology, the use of this semiconductor as absorbent layer in solar cells has attracted considerable interest due to their excellent properties. It has been reported that the incorporation of potassium ion is a powerful strategy to tune the perovskites properties, notwithstanding there has been some disagreement regarding the role of this monovalent alkali metal within the perovskite structure. Here, we investigated the impact of K on the film properties and photovoltaic performance in double cation perovskite solar cells Cs0.1FA0.9PbI3. Our results show that K intervenes in the crystallization process inducing the extraction of non reactive PbI2 from the bulk, resulting in a notable enhancement in morphology and reduced non radiative recombination. The solar cells fabricated with 3 of K content achieve a PCE of 19.3 , showing a significative improvement in Jsc, Voc and stability values compared with control device

Fluoride Chemistry in Tin Halide Perovskites

Tin is the frontrunner for substituting toxic lead in perovskite solar cells. However, tin suffers the detrimental oxidation of SnII to SnIV. Most of reported strategies employ SnF2 in the perovskite precursor solution to prevent SnIV formation. Nevertheless, the working mechanism of this additive remains debated. To further elucidate it, we investigate the fluoride chemistry in tin halide perovskites by complementary analytical tools. NMR analysis of the precursor solution discloses a strong preferential affinity of fluoride anions for SnIV over SnII, selectively complexing it as SnF4. Hard X ray photoelectron spectroscopy on films shows the lower tendency of SnF4 than SnI4 to get included in the perovskite structure, hence preventing the inclusion of SnIV in the film. Finally, small angle X ray scattering reveals the strong influence of fluoride on the colloidal chemistry of precursor dispersions, directly affecting perovskite crystallizatio