Ionic Liquid-Induced Ostwald Ripening Effect for Efficient and Stable Tin-Based Perovskite Solar Cells

All-perovskite tandem solar cells: from fundamentals to technological progress

Organic-inorganic perovskite materials have gradually progressed from single-junction solar cells to tandem (double) or even multi-junction (triple-junction) solar cells as all-perovskite tandem solar cells (APTSCs). Perovskites have numerous advantages: (1) tunable optical bandgaps, (2) low-cost, e.g. via solution-processing, inexpensive precursors, and compatibility with many thin-film processing technologies, (3) scalability and lightweight, and (4) eco-friendliness related to low CO2 emission. However, APTSCs face challenges regarding stability caused by Sn2+ oxidation in narrow bandgap perovskites, low performance due to V oc deficit in the wide bandgap range, non-standardisation of charge recombination layers, and challenging thin-film deposition as each layer must be nearly perfectly homogenous. Here, we discuss the fundamentals of APTSCs and technological progress in constructing each layer of the all-perovskite stacks. Furthermore, the theoretical power conversion efficiency (PCE) limitation of APTSCs is discussed using simulations.

Retrograde Solubility of Methylammonium Lead Iodide in γ-Butyrolactone Does Not Enhance the Uniformity of Continuously Coated Films

Halide perovskite thin films can be the centerpiece of high-performance solar cells, light-emitting diodes, and other optoelectronic devices if the films are of high uniformity and relatively free of pinholes and other defects. A common strategy to form dense films from solution has been to generate a high density of nuclei by rapidly increasing supersaturation, for example, by timely application of an antisolvent or forced convection. In this work, we examine the role of retrograde solubility, wherein solubility decreases with increasing temperature, as a means of increasing the nucleation density and film coverage of slot-die-coated methylammonium lead iodide (MAPbI3) from γ-butyrolactone (GBL) solution. Coverage was investigated as a function of the substrate temperature and the presence and temperature of an air knife. Results were considered within the framework of the dimensionless modified Biot number, which quantifies the interplay between evaporation and horizontal diffusion. Moderate temperatures and a heated air knife improved film coverage and morphology by enhanced nucleation up to ∼80 °C. However, despite the dense nucleation enabled by retrograde solubility, slow evaporation as a result of the low vapor pressure of GBL, combined with Ostwald ripening at high temperatures, prevented the deposition of void-free, device-quality films. This work has provided a more detailed understanding of the interplay between perovskite processing, solvent parameters, and film morphology and ultimately indicates the obstacles to forming dense, uniform films from solvents with high boiling points even in the presence of rapid nucleation.

Significantly Improved Efficiency and Stability of Pure Tin-Based Perovskite Solar Cells with Bifunctional Molecules

Tin-based perovskite solar cells (TPSCs) have become one of the most prospective photovoltaic materials due to their remarkable optoelectronic properties and relatively low toxicity. Nevertheless, the rapid crystallization of perovskites and the easy oxidization of Sn²⁺ to Sn⁴⁺ make it challenging to fabricate efficient TPSCs. In this work, a piperazine iodide (PI) material with -NH- and -NH₂⁺- bifunctional groups is synthesized and introduced into the PEA_0.1FA_0.9SnI₃-based precursor solution to tune the microstructure, charge transport, and stability of TPSCs. Compared with piperazine (PZ) containing only the -NH- group, the PI additive displays better effects on regulating the microstructure and crystallization, inhibiting Sn²⁺ oxidation and reducing trap states, resulting in an optimal efficiency of 10.33%. This is substantially better than that of the reference device (6.42%). Benefiting from the fact that PI containing -NH- and -NH₂⁺- groups can passivate both positively charged defects and negatively charged halogen defects, unencapsulated TPSCs modified with the PI material can maintain about 90% of their original efficiency after being kept in a N₂ atmosphere for 1000 h, much higher than the value of 47% in reference TPSCs without additives. This work provides a practical method to prepare efficient and stable pure TPSCs.

Ligand Engineering in Tin-Based Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted aggressive attention in the photovoltaic field in light of the rapid increasing power conversion efficiency. However, their large-scale application and commercialization are limited by the toxicity issue of lead (Pb). Among all the lead-free perovskites, tin (Sn)-based perovskites have shown potential due to their low toxicity, ideal bandgap structure, high carrier mobility, and long hot carrier lifetime. Great progress of Sn-based PSCs has been realized in recent years, and the certified efficiency has now reached over 14%. Nevertheless, this record still falls far behind the theoretical calculations. This is likely due to the uncontrolled nucleation states and pronounced Sn (IV) vacancies. With insights into the methodologies resolving both issues, ligand engineering-assisted perovskite film fabrication dictates the state-of-the-art Sn-based PSCs. Herein, we summarize the role of ligand engineering during each state of film fabrication, ranging from the starting precursors to the ending fabricated bulks. The incorporation of ligands to suppress Sn2+ oxidation, passivate bulk defects, optimize crystal orientation, and improve stability is discussed, respectively. Finally, the remained challenges and perspectives toward advancing the performance of Sn-based PSCs are presented. We expect this review can draw a clear roadmap to facilitate Sn-based PSCs via ligand engineering.

Sn-Based Perovskite Halides for Electronic Devices

Because of its less toxicity and electronic structure analogous to that of lead, tin halide perovskite (THP) is currently one of the most favorable candidates as an active layer for optoelectronic and electric devices such as solar cells, photodiodes, and field-effect transistors (FETs). Promising photovoltaics and FETs performances have been recently demonstrated because of their desirable electrical and optical properties. Nevertheless, THP's easy oxidation from Sn²⁺ to Sn⁴⁺ , easy formation of tin vacancy, uncontrollable film morphology and crystallinity, and interface instability severely impede its widespread application. This review paper aims to provide a basic understanding of THP as a semiconductor by highlighting the physical structure, energy band structure, electrical properties, and doping mechanisms. Additionally, the key chemical instability issues of THPs are discussed, which are identified as the potential bottleneck for further device development. Based on the understanding of the THPs properties, the key recent progress of THP-based solar cells and FETs is briefly discussed. To conclude, current challenges and perspective opportunities are highlighted.

The Growth Dynamics of Organic-Inorganic Metal Halide Perovskite Films

Organic-inorganic metal halide perovskite films have emerged as potential candidate materials for photoelectric devices because of their superior optoelectronic properties. The performance of these devices depends on the quality of perovskite films defined by their grain size, crystallinity, and absence of pinholes. While solution-based processing is the most cost-effective and scalable approach to producing these films, the impact of the process parameters on the film quality and nanoscale details of these processes are unknown. Specifically, it is unclear how perovskites grow from a liquid precursor to form solid-phase nanocrystals and how these nanocrystals arrange to form a uniform film. Here, using liquid-phase transmission electron microscopy (TEM), we show how perovskite nanocrystals nucleate from a precursor solution and then grow and coalesce to form a polycrystalline film. Furthermore, we show how additives, such as urea, can improve the film crystallinity by increasing perovskite solubility, which induces the dissolution and subsequent redeposition of smaller crystals onto larger grains. Our approach to studying the growth of perovskite films provides an important insight into improving the synthesis of perovskites and other technologically relevant crystalline films.

Mitigating Voc Loss in Tin Perovskite Solar Cells via Simultaneous Suppression of Bulk and Interface Nonradiative Recombination

Tin-based perovskite solar cells (PSCs) have recently attracted extensive attention as a promising alternative to lead-based counterparts due to their low toxicity and narrow band gap. However, the severe open-circuit voltage (V_oc) loss remains one of the most significant obstacles to further improving photovoltaic performance. Herein, we report an effective approach to reducing the V_oc loss of tin-based PSCs. We find that introducing ethylammonium bromide (EABr) as an additive into the tin perovskite film can effectively reduce defect density both in the tin perovskite film and at the surface as well as optimize the energy level alignment between the perovskite layer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) transport material, thereby suppressing nonradiative recombination both in the bulk film and at the interface. Furthermore, it is demonstrated that the V_oc loss is gradually mitigated along with increasing storage duration due to the slow passivation effect. As a result, a remarkable V_oc of 0.83 V is achieved in the devices optimized with the EABr additive, which shows a significantly improved power conversion efficiency (PCE) of 10.80% and good stability.

Ionic Liquid-Assisted Crystallization and Defect Passivation for Efficient Perovskite Solar Cells with Enhanced Open-Circuit Voltage

Perovskite materials have demonstrated many excellent properties in next-generation photovoltaic devices, but the intrinsic defects and the quality of perovskite film still limit the performance and stability of PSCs. Here, 1,3-dimethylimidazolium iodide (DMII) ionic liquid was employed as an additive to passivate the various defects and produce the high-quality perovskite film with enlarged grain sizes. DMII could act as an "ionic stabilizer" for passivating the point defects including the vacancies defects of organic cations and halogen anions of perovskite. At the same time, the extra problematic PbI₂ on surfaces and at grain boundaries of the perovskite film could also be reacted by DMII, leading to the reduction of recombination centers and trap states. On the other hand, the DMII ionic liquid with a "Ostwald ripening effect" could retard the crystallization process of perovskite crystals and yield better film quality with higher crystallinity, smoother morphology and larger grains. As a result, the optimal device achieved a champion power conversion efficiency (PCE) of 20.4 %. Particularly, the modified devices demonstrated a significant elevation in open-circuit voltage from 1.03 to 1.10 V. The hydrophobicity of perovskite films modified by DMII was enhanced and the un-encapsulated DMII devices retained 91 % of their initial PCE after aging 60 days under 15±5 % relative humidity.