Due to their enhanced performance and simplicity in manufacturing, scalability, and versatility, lead-halide perovskite-based solar cells (HPSCs) have received much attention in the domains of energy. Lead is present in nature as a poisonous substance that causes various issues to climate and human health and prevents its further industrialization. Over the past few years, there has been a noticeable interest in exploring some alternative lead-free perovskites. However, owing to some intrinsic losses, the performance that may be achieved from these photovoltaics is not up to standards. Thus, for the purpose of efficiency improvement, a comprehensive simulation is required to comprehend the cause of these losses. In the current research, an investigation into how to employ the promisingly efficient lead-free, all-inorganic cesium tin–germanium iodide (CsSnGeI3) perovskites as the photoactive layer in HPSCs was performed. Results exhibited a high efficiency of 12.95% with a CsSn0.5Ge0.5I3 perovskite thickness of 0.6 μm and a band gap of 1.5 eV at room temperature. High efficiency may be achieved using phenyl-C61-butyric acid methyl ester (PCBM) as an electron transport material because of its favorable energy-level alignment with the perovskite material. The research further tested the perovskite layer thickness and defect density in depth. The results showed that the carrier diffusion lengths have a big effect on how well the HPSC works. 

Comparative Study of the Correlation between Diffusion Length of Charge Carriers and the Performance of CsSnGeI3 Perovskite Solar Cells

Tin halide lead‐free perovskite solar cells (TPSCs) have received tremendous research interest recently due to their nearly ideal bandgap, broad light absorption, non‐toxicity, and environmental friendliness. However, the uncontrollable crystallization process and the facile oxidation of Sn2+ limit the further increase of power conversion efficiency (PCE). To solve these problems, a series of acetates are introduced into the perovskite precursor solution to regulate the crystallization process. It is revealed that formamidine acetate (FAAc) has strong COSn coordination with Sn2+ compared with acetic acid (HAc) and methylammonium acetate (MAAc), which can stabilize the lattice structure, minimize defect states and suppress the oxidation of Sn2+. Meanwhile, benefiting from this coordination ability, it not only leads to large‐size colloidal clusters in precursor but also slows down the crystallization process and improves the crystallinity of tin halide perovskite films. The device with FAAc achieved an increased PCE from initially 9.84% to 12.43%, and it could maintain 94% of its initial value for 2000 h in N2 atmosphere. This work provides a feasible strategy for depositing high‐quality tin perovskite films with low defect density and lattice distortion, which will be crucial for related photovoltaics and other optoelectronic devices.

Stabilization of Perovskite Lattice and Suppression of Sn2+/Sn4+ Oxidation via Formamidine Acetate for High Efficiency Tin Perovskite Solar Cells

Monolithic perovskite/organic tandem solar cells (POTSCs) have significant advantages in next‐generation flexible photovoltaics, owing to their capability to overcome the Shockley–Queisser limit and facile device integration. However, the compromised sub‐cells performance challenges the fabrication of high‐efficiency POTSCs. Especially for all‐inorganic wide‐bandgap perovskite front sub‐cells (AIWPSCs) based n‐i‐p structured POTSCs (AIPOTSCs), for which the power conversion efficiency (PCE) is much lower than organic–inorganic mixed‐halide wide‐bandgap perovskite based POTSCs. Herein, an ionic liquid, methylammonium formate (MAFm), based dual‐interface engineering approach is developed to modify the bottom and top interfaces of wide‐bandgap CsPbI2Br films. In particular, the Fm− group of MAFm can effectively passivate the interface defects, and the top interface modification can facilitate the formation of uniform perovskite films with enlarged grain size, thereby mitigating the defects and perovskite grain boundaries induced carrier recombination. As a result, CsPbI2Br‐based AIWPSCs with a high PCE of 17.0% and open‐circuit voltage (VOC) of 1.347 V are achieved. By integrating these dual‐interface engineered CsPbI2Br‐based front sub‐cells with the narrow‐bandgap PM6:CH1007‐based rear sub‐cells, a record PCE of 23.21% is obtained for AIPOTSCs, illustrating the potential of AIPOTSCs for achieving high‐efficiency tandem solar cells.

All‐Inorganic Perovskite‐Based Monolithic Perovskite/Organic Tandem Solar Cells with 23.21% Efficiency by Dual‐Interface Engineering

Low bandgap tin-lead perovskite solar cells (Sn-Pb PSCs) are expected to achieve higher efficiencies than Pb-PSCs and regarded as key components of tandem PSCs. However, the realization of high efficiency is challenged by the instability of Sn2+ and the imperfections at the charge transfer interfaces. Here, we demonstrate an efficient ideal bandgap formamidinium (FA)-based Sn-Pb (FAPb0.5Sn0.5I3) PSC, by manipulating the buried NiOx/perovskite interface with 4-hydroxyphenethyl ammonium halide (OH-PEAX, X = Cl-, Br-, or I-) interlayer, which exhibits fascinating functions of reducing the surface defects of the NiOx hole transport layer (HTL), enhancing the perovskite film quality, and improving both the energy level matching and physical contact at the interface. The effects of different halide anions have been elaborated and a 20.53% efficiency is obtained with OH-PEABr, which is the highest one for FA-based Sn-Pb PSCs using NiOx HTLs. Moreover, the device stability is also boosted.

Manipulation of the Buried Interface for Robust Formamidinium-based Sn-Pb Perovskite Solar Cells with NiOx Hole-Transport Layers.

Abstract Tin halide perovskites (Sn HaPs) are the top lead‐free choice for perovskite optoelectronics, but the oxidation of perovskite Sn2+ to Sn4+ remains a key challenge. However, the role of inconspicuous chemical processes remains underexplored. Specifically, the halide component in Sn HaPs (typically iodide) has been shown to play a key role in dictating device performance and stability due to its high reactivity. Here we describe the impact of native halide chemistry on Sn HaPs. Specifically, molecular halogen formation in Sn HaPs and its influence on degradation is reviewed, emphasising the benefits of iodide substitution for improving stability. Next, the ecological impact of halide products of Sn HaP degradation and its mitigation are considered. The development of visible Sn HaP emitters via halide tuning is also summarised. Lastly, halide defect management and interfacial engineering for Sn HaP devices are discussed. These insights will inspire efficient and robust Sn HaP optoelectronics.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/anie.202213966

Halide Chemistry in Tin Perovskite Optoelectronics: Bottlenecks and Opportunities

Organic‐free and lead‐free CsSnI3 perovskite solar cells (PSCs) have recently gained growing attention as a promising template to mitigate the thermal instability and lead toxicity of hybrid lead‐based PSCs. However, the relatively low device efficiency due to the high content of Sn(II)‐related defects hinders its further development. Herein, highly performed CsSnI3−xBrx compositional perovskite‐based PSCs are achieved by using dimethyl ketoxime (C3H7NO, DMKO) as a multifunctional additive. As a commercially used deoxidant, DMKO can effectively neutralize the oxygen molecule and reduce Sn4+ back to Sn2+, enhancing the oxidation resistance of the film. Besides, the electron‐rich oxime group (NOH) in DMKO tends to interact with Sn2+ ions with extremely low adsorption energy less than −15 eV and inhibits defect formation, resulting in films with low defect density. The corresponding PSCs deliver a considerable open‐circuit voltage (Voc) of 0.75 V with a record efficiency as high as 11.2%, which represents the highest reported efficiency for lead‐free all‐inorganic PSCs thus far. More importantly, the grain surface distributed DMKO provides an in situ encapsulation of the perovskite, which results in greatly enhanced ambient stability of the un‐encapsulated devices.

Organic‐Free and Lead‐Free Perovskite Solar Cells with Efficiency over 11%

Sn-Ge mixed perovskites have been proposed as promising lead-free candidates in the photovoltaics (PV) field. In this work, we discovered a stable P1 phase Sn-Ge mixed structure (CsSn0.5Ge0.5I3) with an appropriate band gap value of 1.19 eV, which manifests its unique structural stability and physics properties. The thermodynamic stability of this mixed structure under different growth conditions and all possible native defects are depicted in detail. The formation energies and dominant native point defects indicate that P1 phase CsSn0.5Ge0.5I3 exhibits unipolar self-doping behavior (p-type conductivity) and good defect tolerance while the growth condition changes. In addition, the calculation of light absorption confirmed that the P1 phase has a higher light absorption coefficient than that of MAPbI3 in the visible light range, showing excellent light absorption. Our work not only provides theoretical guidance for unraveling the unusual structural stability of Sn-Ge mixed perovskites, but also offers a useful scheme to modulate the stability and optoelectronic properties of Ge-based perovskites through alloy engineering.

Remarkable Stability and Optoelectronic Properties of an All-Inorganic CsSn0.5Ge0.5I3 Perovskite Solar Cell.

The effect of spacer cations on optoelectronic properties of two-dimensional perovskite based on first-principles calculations

Vacancy defects are universally regarded to be the main defect that limits the photoelectric conversion efficiency of perovskite solar cells. In perovskite, iodine vacancy dominates the defect proportion due to its low formation energy. However, the defect property of iodine vacancy (VI) is still in dispute. Ideally, the VI defect is considered to be a shallow level defect near conduction band minimum, meaning that it does not act as a Shockley–Read–Hall (SRH) nonradiative recombination center. Herein, we find a direct correlation between compressive strain and VI defect behavior. The compressive strain along the lattice vector b/c direction will drive the VI defect from shallow level to deep level defect, which is related to the formation of Pb-dimer. In addition, the influence of extra electrons is also considered during the structural evolution of VI, which is often observed in the experiments. Therefore, we find that the elimination of compressive strain and extra electrons can be of great significance for improving the photoelectric performance of perovskite solar cells. Our work reveals the defect properties of VI from shallow level one to the SRH recombination center and the inherent physics mechanism of defect evolution under external factors, which provides a strategy to control device defects and eliminate recombination losses.

Yating Cai

Papers

Organic‐Free and Lead‐Free Perovskite Solar Cells with Efficiency over 11%

Remarkable Stability and Optoelectronic Properties of an All-Inorganic CsSn0.5Ge0.5I3 Perovskite Solar Cell.

All‐Inorganic Perovskite‐Based Monolithic Perovskite/Organic Tandem Solar Cells with 23.21% Efficiency by Dual‐Interface Engineering

The effect of spacer cations on optoelectronic properties of two-dimensional perovskite based on first-principles calculations

First-principles study of detrimental iodine vacancy in lead halide perovskite under strain and electron injection