Prospects for Tin-Containing Halide Perovskite Photovoltaics

Mechanistic Exploration of Determinants for the Fullerene@FASnI3 Interface Stability: Surface Termination and Monovalent Cation Rotation

The investigation into the interfacial properties between fullerene compounds and Sn-based perovskites (Sn-PVSK) holds extraordinary significance for advancing efficient and stable Pb-free perovskite solar cells. This study is the first theoretical exploration to examine their interfacial properties using Ab initio molecular dynamics (AIMD) simulations and trajectory analysis methods with C60@FASnI3 as a representative system. The impact of surface termination and FA+ rotation on interface stability has been assessed. Within the 10 ps AIMD simulations, the C60@FAI interface demonstrates greater stability compared to the C60@SnI interface due to the robustness of the single-bonded I on the FAI termination and weaker C60-FAI interactions. The C60@SnI interface has poor stability, but it can be enhanced by controlling the FA+ rotation, achieving optimal stability at a 45° rotation along the C-H bond axis. This is attributed to minimal hydrogen bond interactions and a reduced steric hindrance. This work not only substantiates the pivotal role of surface termination in maintaining interface stability but, most importantly, also reveals how FA+ rotational dynamics regulate the C60@SnI interface stability, providing valuable insights for further improving the efficiency of Sn-PVSK solar cells.

Synchronized crystallization in tin-lead perovskite solar cells

Tin-lead halide perovskites with a bandgap near 1.2 electron-volt hold great promise for thin-film photovoltaics. However, the film quality of solution-processed Sn-Pb perovskites is compromised by the asynchronous crystallization behavior between Sn and Pb components, where the crystallization of Sn-based perovskites tends to occur faster than that of Pb. Here we show that the rapid crystallization of Sn is rooted in its stereochemically active lone pair, which impedes coordination between the metal ion and Lewis base ligands in the perovskite precursor. From this perspective, we introduce a noncovalent binding agent targeting the open metal site of coordinatively unsaturated Sn(II) solvates, thereby synchronizing crystallization kinetics and homogenizing Sn-Pb alloying. The resultant single-junction Sn-Pb perovskite solar cells achieve a certified power conversion efficiency of 24.13 per cent. The encapsulated device retains 90 per cent of the initial efficiency after 795 h of maximum power point operation under simulated one-sun illumination.

Tuning the dimensionality in chiral and racemic organic/tin hybrids with halides

Chiral 1D tin iodides EBASnI3 were synthesized while incorporating enantiomerically pure and racemic ethylbenzylammonium (EBA) cations between the 1D shared inorganic corners. The dimensionality was reduced to 0D when replacing iodine with bromine. In all the cases, the presence of hydrogen bonds was observed between the organic part and the inorganic part, while transfer of chirality was evidenced for the EBASnI3 enantiomerically pure compounds.

Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin-lead (Sn-Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn-Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn-Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn-Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.

An open-cage bis[60]fulleroid as an electron transport material for tin halide perovskite solar cells

An open-cage bis[60]fulleroid (OC) was applied as an electron transport material (ETM) in tin (Sn) halide perovskite solar cells (PSCs). Due to the reduced offset between the energy levels of Sn-based perovskites and ETMs, the power conversion efficiency (PCE) of Sn-based PSCs with OC reached 9.6% with an open-circuit voltage (VOC) of 0.72 V. Additionally, OC exhibited superior thermal stability and provided 75% of the material without decomposition after vacuum deposition. The PSC using vacuum-deposited OC as the ETM could afford a PCE of 7.6%, which is a big leap forward compared with previous results using vacuum-deposited fullerene derivatives as ETMs.

Efficient and Stable Tin-Lead Mixed Perovskite Solar Cells Using Post-Treatment Additive with Synergistic Effects

Inhibiting the oxidation of Sn2+ during the crystallization process of Sn-Pb mixed perovskite film is found to be as important as the oxidation resistance of precursor solution to achieve high efficiency, but less investigated. Considering the excellent reduction feature of hydroquinone and the hydrophobicity of tert-butyl group, an antioxidant 2,5-di-tert-butylhydroquinone (DBHQ) was introduced into Sn-Pb mixed perovskite films using an anti-solvent approach to solve this problem. Interestingly, we find that DBHQ can act as function alterable additive during its utilization. On the one hand, DBHQ can restrict the oxidation of Sn2+ during the crystallization process, facilitating the fabrication of high-quality perovskite film; on the other hand, the generated oxidation product 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) can functionalize as defect passivator to inhibit the charge recombination. As a result, this synergetic effect renders the Sn-Pb mixed PSC a power conversion efficiency (PCE) up to 23.0 %, which is significantly higher than the reference device (19.6 %). Furthermore, the unencapsulated DBQH-modified PSCs exhibited excellent long-term stability and thermal stability, with the devices maintaining 84.2 % and 78.9 % of the initial PCEs after aging at 25 °C and 60 °C for 800 h and 120 h under N2 atmosphere, respectively. Therefore, the functional alterable strategy provides a novel cornerstone for high-performance Sn-Pb mixed PSCs.