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.

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.

Solution-processed lead halide perovskites have established themselves as one of the most important absorber materials in solar cells with power conversion efficiencies now exceeding 22%. [1] Unfortunately, the over reliance on highly toxic Pb 2+ remains a key issue for widespread commercial applications. The significant concentration of Pb 2+ in high performing halide perovskites and its water solubility make it highly hazardous compound to the environment. [2] Another apparent issue is the inherent instability of lead-based halide perovskites in ambient atmosphere. [3] Although the stability of Pb-based halide perovskites has improved impressively in recent times along with the simultaneous development of passivation techniques, the fabrication of lead-based halide perovskites still requires stringent environmental control. [4] To address these potential issues, there is an increased interest toward leadfree halide perovskites and their analogues in photovoltaics. Replacing Pb 2+ with Sn 2+ or Ge 2+ could minimize the toxicity associated with lead-based halide perovskite; however, the increased environmental instability of these compounds poses significant challenges in solar cell development. [5] Even after incorporating 2D/3D mixtures of perovskites, the efficiency of the highest performing Sn-based system reduced to nearly 50% of its original value within 3 d under 20% humidity. [6] Considering atmospheric stability, trivalent cations such as bismuth and antimony-based ternary halides were also investigated as potential absorber materials due to their inherent atmospheric stability and low toxicity. [7] The incorporation of protonated cations such as MA or Cs with Bi-I octahedra forms Bi-based ternary halides (structural formula A 3 Bi 2 I 9 : A = Cs, MA) exhibits high absorption coefficients and facile solution processability. Nevertheless, the photovoltaic performances of Bi-based ternary halides remained poor mostly due to high optical bandgap and low electronic dimensionality. [8] Replacement of the A-site protonated cations with transition metals such as Ag or Cu is a promising strategy to improve the dimensionality. These transition metals also take part in bonding with Bi-I octahedra, resulting in complex halide bismuthates. In comparison to Bismuth-based ternary halides have recently gained a lot of attention as lead-free perovskite materials. However, photovoltaic performances of these devices remain poor, mostly due to their low-dimensional crystal structure and large bandgap. Here, a dynamic hot casting technique to fabricate silver bismuth iodide-based perovskite solar cells under an ambient atmosphere with power conversion efficiencies above 2.5% is demonstrated. Silver bismuth iodides are 3D analogs of complex ternary bismuth halides with a suitable bandgap for a single junction solar cell. As far as it is known, these results represent the highest efficiency for solution processed air-stable lead-free perovskite solar cells. The enhanced solar cell performance via this dynamic hot casting ...

Bismuth-based halide perovskite derivatives have now attracted huge attention for photovoltaic (PV) applications after the unparalleled success of lead-based halide perovskites. However, the performances of PV devices based on these compounds are poor, despite theoretical predictions. In this Article, we have investigated the electronic structure and defect formation energies of Cs3Bi2I9 using density functional theory (DFT) calculations. The calculated electronic bandstructure indicates an indirect bandgap and high carrier effective masses. Our calculations reveal a large stability region for this compound; however, deep level defects are quite prominent. Even the varying chemical potentials from the stoichiometric region do not eliminate the presence of deep defects, ultimately limiting photovoltaic efficiencies.

A new broadband‐emitting 2 D hybrid organic–inorganic perovskite (CyBMA)PbBr4 based on highly flexible cis‐1,3‐bis(methylaminohydrobromide)cyclohexane (CyBMABr) core has been designed, synthesized, and investigated, highlighting the effects of stereoisomerism of the templating cation on the formation and properties of the resulting perovskite. The new 2 D material has high exciton binding energy of 340 meV and a broad emission spanning from 380 to 750 nm, incorporating a prominent excitonic band and a less intense broad peak at room temperature. Significant changes in the photoluminescence (PL) spectrum were observed at lower temperatures, showing remarkable enhancement in the intensity of the broadband at the cost of excitonic emission. Temperature‐dependent PL mapping indicates the effective role of only a narrow band of excitonic absorption in the generation of the active channel for emission. Based on the evidences obtained from the photophysical investigations, we attributed the evolution of the broad B‐band of (CyBMA)PbBr4 to excitonic self‐trapped states.

Double perovskite halides are a class of materials with diverse chemistries that are amenable to solution-based synthesis routes, and display a range of properties for diverse potential applications. Starting from a consideration of the octahedral and tolerance factors of 2000 candidate double-perovskite compounds, we compute structural, electronic and transport properties of 1000 compounds using first-principles 1 calculations. The computational results have been assembled in a database that is accessible through the Materials Project online. As one potential application, double perovskites are promising candidates in the search for lead-free halide photovoltaic absorbers. We present the application of our database to aid the discovery of new double perovskite halide photovoltaic materials. Forty compounds from five categories were identified as promising solar absorber candidates and the complex chemical trends for band gap within each category are analyzed, to provide guidelines for the use of subtitutional alloying as a means of tuning the electronic structure. Other possible applications of the database are also discussed briefly.

Lead-free halide perovskite semiconductors are necessary due to the atmospheric instability and lead toxicity associated with the 3D lead halide perovskites. However, a stable lead-free perovskite with an ideal band gap (1.2−1.4 eV) for photovoltaics is still missing. In this work, we synthesized organic− inorganic gold halide double perovskites ((CH 3 NH 3 ) 2 Au 2 X 6 , X = Br, I) through a solution-processed route that offers an ideal direct band gap for photovoltaic applications. Density functional theory calculations confirm the direct nature of the band gap with reasonable absorption coefficients in the visible range and excellent charge transport properties. In addition, the Au-halide perovskites show high chemical stability and photoresponse. These combined properties demonstrate that Au-based halide perovskites can be a promising group of compounds for optoelectronic applications.

Typical fabrication methods for laboratory‐scale (<1 cm2) perovskite solar cells (PSCs) are undeniably not scalable and the control of crystallization of large‐area perovskite layer for commercial sized modules is also particularly challenging. Here, a seed‐assisted crystallization approach is demonstrated through addition of alkali salts, CsPbBr3 and KPb2Br5, to the perovskite precursor ink for enabling homogeneous and highly crystalline large‐area Cs0.15FA0.85Pb(I0.83Br0.17)3 (CsFA) perovskite films via scalable slot‐die coating technique. X‐ray photoelectron spectroscopy analysis reveals the segregation of potassium ions at SnO2/perovskite interface which serve as nucleation sites for the crystallization of perovskite layer. The uniformly slot‐die coated CsFA films (100 cm2) from the additives containing precursor inks possess larger grains with enhanced optoelectronic properties and the corresponding devices display higher reproducibility and consistency. A champion device efficiency of 18.94% under 1 sun illumination for slot‐die coated PSCs in n‐type/intrinsic/p‐type structure is demonstrated with improved stability with 82% of its initial efficiency tested at 65 °C for 1150 h. The slot‐die coated methylammonium‐free perovskite module with an active area of 57.5 cm2 shows an efficiency of 16.22% and retains 82% of its initial efficiency after 4800 h under 30% relative humidity without encapsulation.