On the Durability of Tin-Containing Perovskite Solar Cells

Tin (Sn)-containing perovskite solar cells (PSCs) have gained significant attention in the field of perovskite optoelectronics due to lower toxicity than their lead-based counterparts and their potential for tandem applications. However, the lack of stability is a major concern that hampers their development. To achieve the long-term stability of Sn-containing PSCs, it is crucial to have a clear and comprehensive understanding of the degradation mechanisms of Sn-containing perovskites and develop mitigation strategies. This review provides a compendious overview of degradation pathways observed in Sn-containing perovskites, attributing to intrinsic factors related to the materials themselves and environmental factors such as light, heat, moisture, oxygen, and their combined effects. The impact of interface and electrode materials on the stability of Sn-containing PSCs is also discussed. Additionally, various strategies to mitigate the instability issue of Sn-containing PSCs are summarized. Lastly, the challenges and prospects for achieving durable Sn-containing PSCs are presented.

Reductant Engineering in Stable and High-Quality Tin Perovskite Single Crystal Growth for Heterojunction X-Ray Detectors

Tin perovskites have emerged as a promising alternative material to address the toxicity of lead perovskites and the low bandgap of around 1.1 eV is also compatible with tandem solar cell applications. Nevertheless, the optoelectronic performance of solution-processed tin perovskite single-crystal counterparts still lags behind because of the tin instability under ambient conditions during crystal growth and limited reductants to protect the Sn2+ ions from oxidation. Here, the reductant engineering to grow high-quality tin perovskite single crystals under ambient conditions is studied. Oxalic acid (H2 C2 O4 ) serves as an excellent reductant and sacrificial agent to protect Sn2+ ions in methanol due to its suitable redox potential of -0.49 V, and the CO2 as the oxidation product in the gas state can be easily separated from the solution. The FPEA2 SnI4 single crystal grown by this strategy exhibits low trap density perovskite surface by constructing an FPEA2 PbI4 -FPEA2 SnI4 (FPI-FSI) single crystal heterojunction for X-ray detection. An improved X-ray sensitivity of 1.7 × 105 µC Gy-1 cm-2 is realized in the heterojunction device, outperforming the control FPEA2 PbI4 counterpart.

Highly Efficient HTM-Free Tin Perovskite Solar Cells with Outstanding Stability Exceeding 10000 h

The bottleneck in the rapid development of tin-based perovskite solar cells (TPSCs) is the inherent chemical instability. Although this is being addressed continuously, the device performance has not improved further due to the use of PEDOT:PSS as the hole-transport material (HTM), which has poor long-term stability. Herein we have applied commercial ITO nanoparticles over ITO glass substrates and altered the surface chemistry of the ITO electrode via a simple two-step thermal annealing, followed by a UV-ozone treatment. These surface-modified ITO electrodes display promising interfacial characteristics, such as a suitable band alignment owing to significantly reduced surface carbon contamination, increased In-O bonding, and reduced oxygen vacancies, that enabled fabrication of an HTM-free TPSC device according to a two-step method. The fabricated device possessed an outstanding power conversion efficiency (PCE) of 9.7%, along with a superior long-term stability by retaining over 90% of the initial PCE upon shelf storage in a glovebox for a period of over 10000 h. The application of ITO nanoparticles led to effective interfacial passivation, whose impacts on the long-term durability were assessed using electrochemical impedance spectroscopy, time-resolved photoluminescence decay profiles, and femtosecond transient absorption spectroscopy techniques.

Sn-Based Perovskite Solar Cells towards High Stability and Performance

Recent years have witnessed rapid development in the field of tin-based perovskite solar cells (TPSCs) due to their environmental friendliness and tremendous potential in the photovoltaic field. Most of the high-performance PSCs are based on lead as the light-absorber material. However, the toxicity of lead and the commercialization raise concerns about potential health and environmental hazards. TPSCs can maintain all the optoelectronic properties of lead PSCs, as well as feature a favorable smaller bandgap. However, TPSCs tend to undergo rapid oxidation, crystallization, and charge recombination, which make it difficult to unlock the full potential of such perovskites. Here, we shed light on the most critical features and mechanisms affecting the growth, oxidation, crystallization, morphology, energy levels, stability, and performance of TPSCs. We also investigate the recent strategies, such as interfaces and bulk additives, built-in electric field, and alternative charge transport materials that are used to enhance the performance of the TPSCs. More importantly, we have summarized most of the recent best-performing lead-free and lead-mixed TPSCs. This review aims to help future research in TPSCs to produce highly stable and efficient solar cells.

Lasing in Two-Dimensional Tin Perovskites

Two-dimensional (2D) perovskites have been proposed as materials capable of improving the stability and surpassing the radiative recombination efficiency of three-dimensional perovskites. However, their luminescent properties have often fallen short of what has been expected. In fact, despite attracting considerable attention for photonic applications during the last two decades, lasing in 2D perovskites remains unclear and under debate. Here, we were able to improve the optical gain properties of 2D perovskite and achieve optically pumped lasing. We show that the choice of the spacer cation affects the defectivity and photostability of the perovskite, which in turn influences its optical gain. Based on our synthetic strategy, we obtain PEA₂SnI₄ films with high crystallinity and favorable optical properties, resulting in amplified spontaneous emission (ASE) with a low threshold (30 μJ/cm²), a high optical gain above 4000 cm^-1 at 77 K, and ASE operation up to room temperature.

Engineering Stable Lead-Free Tin Halide Perovskite Solar Cells: Lessons from Materials Chemistry

Substituting toxic lead with tin (Sn) in perovskite solar cells (PSCs) is the most promising route toward the development of high-efficiency lead-free devices. Despite the encouraging efficiencies of Sn-PSCs, they are still yet to surpass 15% and suffer detrimental oxidation of Sn(II) to Sn(IV). Since their first application in 2014, investigations into the properties of Sn-PSCs have contributed to a growing understanding of the mechanisms, both detrimental and complementary to their stability. This review summarizes the evolution of Sn-PSCs, including early developments to the latest state-of-the-art approaches benefitting the stability of devices. The degradation pathways associated with Sn-PSCs are first outlined, followed by describing how composition engineering (A, B site modifications), additive engineering (oxidation prevention), and interface engineering (passivation strategies) can be employed as different avenues to improve the stability of devices. The knowledge about these properties is also not limited to PSCs and also applicable to other types of devices now employing Sn-based perovskite absorber layers. A detailed analysis of the properties and materials chemistry reveals a clear set of design rules for the development of stable Sn-PSCs. Applying the design strategies highlighted in this review will be essential to further improve both the efficiency and stability of Sn-PSCs.

Vapor-Assisted In Situ Recrystallization for Efficient Tin-Based Perovskite Light-Emitting Diodes

Tin-based perovskites are a promising candidates to replace their toxic lead-based counterparts in optoelectronic applications, such as light-emitting diodes (LEDs). However, the development of tin perovskite LEDs is slow due to the challenge of obtaining high-quality tin perovskite films. Here, a vapor-assisted spin-coating method is developed to achieve high-quality tin perovskites and high-efficiency LEDs. It is revealed that solvent vapor can lead to in situ recrystallization of tin perovskites during the film-formation process, thus significantly improving the crystalline quality with reduced defects. An antioxidant additive is further introduced to suppress the oxidation of Sn²⁺ and increase the photoluminescence quantum efficiency up to ≈30%, which is an approximately fourfold enhancement in comparison with that of the control method. As a result, efficient tin perovskite LEDs are achieved with a peak external quantum efficiency of 5.3%, which is among the highest efficiency of lead-free perovskite LEDs.

Fast-Response Oxygen Optical Fiber Sensor based on PEA2 SnI4 Perovskite with Extremely Low Limit of Detection

Oxygen sensor is an important technique in various applications including industrial process control, medical equipment, biological fabrication, etc. The reported optical fiber-based configurations so far, using gas-sensitive coating do not meet the stringent performance targets, such as fast response time and low limit of detection (LOD). Tin-based halide perovskites are sensitive to oxygen with potential use for sensor applications. Here, the halide perovskite-based oxygen optical fiber sensor by combining phenylethylammonium tin iodide (PEA₂ SnI₄ ) and tilted fiber Bragg grating (TFBG) is demonstrated. The PEA₂ SnI₄ -based oxygen optical fiber sensor is reversible at room temperature with a response time of about 10 s, and the experimental LOD approaches to an extremely low oxygen concentration of about 50 ppm. The as-fabricated oxygen sensor shows a relative response change of 0.6 dB for an oxygen concentration increase from 50 ppm to 5% with good gas selection against NO₂ , CO, CO₂ , H₂ . This work extends the sensor applications of halide perovskites, providing a novel technique for rapid and repeatable oxygen gas detection at a low level.

Superior Stability and Emission Quantum Yield (23% ± 3%) of Single-Layer 2D Tin Perovskite TEA2 SnI4 via Thiocyanate Passivation

Tin-based perovskite, which exhibits narrower bandgap and comparable photophysical properties to its lead analogs, is one of the most forward-looking lead-free semiconductor materials. However, the poor oxidative stability of tin perovskite hinders the development toward practical application. In this work, the effect of pseudohalide anions on the stability and emission properties of single-layer 2D tin perovskite nanoplates with chemical formula TEA₂ SnI₄ (TEA = 2-thiophene-ethylammonium) is reported. The results reveal that ammonium thiocyanate (NH₄ SCN) is the most effective additive in enhancing the stability and photoluminescence quantum yield of 2D TEA₂ SnI₄ (23 ± 3%). X-Ray photoelectron spectroscopic investigations on the thiocyanate passivated TEA₂ SnI₄ nanoplate show less than a 1% increase of Sn⁴⁺ signal upon 30 min exposure to air under ambient conditions (298 K, humidity ≈70%). Furthermore, no noticeable decrease in emission intensity of the nanoplate is observed after 20 h in air. The SCN^- passivation during the growth stage of TEA₂ SnI₄ is proposed to play a crucial role in preventing the oxidation of Sn²⁺ and hence boosts both stability and photoluminescence yield of tin perovskite nanoplates.