Development of Hybrid Pseudohalide Tin Perovskites for Highly Stable Carbon-Electrode Solar Cells

Suppression of deep-level traps via semicarbazide hydrochloride additives for high-performance tin-based perovskite solar cells

Tin perovskites with exemplary optoelectronic properties offer potential application in lead-free perovskite solar cells. However, Sn vacancies and undercoordinated Sn ions on the tin perovskite surfaces can create deep-level traps, leading to non-radiative recombination and absorption of nucleophilic O2 molecules, impeding further device efficiency and stability. Here, in this study, a new additive of semicarbazide hydrochloride (SEM-HCl) with a N-C=O functional group was introduced into the perovskite precursor to fabricate high-quality films with a low concentration of deep-level trap densities. This, in turn, serves to prevent undesirable interaction between photogenerated carriers and adsorbed oxygen molecules in the device's operational environment, ultimately reducing the proliferation of superoxide entities. As the result, the SEM-HCl-derived devices show a peak efficiency of 10.9% with improved device stability. These unencapsulated devices maintain almost 100% of their initial efficiencies after working for 100 h under continuous AM1.5 illumination conditions.

Are formation and adsorption energies enough to evaluate the stability of surface-passivated tin-based halide perovskites?

Surface passivation is one of the effective and widely-used strategies to enhance the stability of halide perovskites with reduced surface defects and suppressed hysteresis. Among all existing reports, the formation and adsorption energies are popularly used as the decisive descriptors for screening passivators. Here, we propose that the often-ignored local surface structure should be another critically important factor governing the stability of tin-based perovskites after surface passivation, but has no detrimental effect on the stability of lead-based perovskites. It is verified that poor surface structure stability and deformation of the chemical bonding framework of Sn-I caused by surface passivation are ascribed to the weakened Sn-I bond strength and facilitated formation of surface iodine vacancy (V_I). Therefore, the surface structure stability represented by the formation energy of V_I and Sn-I bond strength should be used to accurately screen preferred surface passivators of tin-based perovskites.

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.

Quasi-2D Bilayer Surface Passivation for High Efficiency Narrow Bandgap Perovskite Solar Cells

The combination of comprehensive surface passivation and effective interface carriers transfer plays a critical role in high-performance perovskite solar cells. A 2D structure is an important approach for surface passivation of perovskite film, however, its large band gap could compromise carrier transfer. Herein, we synthesize a new molecule 2-thiopheneethylamine thiocyanate (TEASCN) for the construction of bilayer quasi-2D structure precisely on a tin-lead mixed perovskite surface. This bilayer structure can passivate the perovskite surface and ensure effective carriers transfer simultaneously. As a result, the open-circuit voltage (V_oc ) of the device is increased without sacrificing short-circuit current density (J_sc ), giving rise to a high certified efficiency from a credible third-party certification of narrow band gap perovskite solar cells. Furthermore, theoretical simulation indicates that the inclusion of TEASCN makes the bilayer structure thermodynamically more stable, which provides a strategy to tailor the number of layers of quasi-2D perovskite structures.

Pseudohalide Anions to Suppress Oxidative Degradation for Efficient Formamidinium-Based Sn-Pb Halide Perovskite Solar Cells

Although binary Sn-Pb perovskites possess optimal band gap approaching to the Shockley-Queisser limit efficiency, the enhancement on power conversion efficiency (PCE) of Sn-Pb perovskite solar cells (PSCs) is impeded by the detrimental oxidation of Sn²⁺. Herein, a novel and effective strategy is developed to introduce pseudohalide anion thiocyanate (SCN^-) with similar ionic radius to iodide to occupy the X-site of the perovskite lattice, thus restraining the rapid oxidation of Sn²⁺ to Sn⁴⁺. The incorporation of SCN^- into perovskite stabilizes the perovskite crystal structure thermodynamically and increases the adsorption-energy-barrier of oxygen molecules. The coordination between Sn²⁺ and SCN^- can reduce the defect density by healing the undercoordinated Sn²⁺ and suppressing the Sn and I vacancies. With the incorporation of SCN^-, the ion migration behavior and lattice strain associated with the defects are remarkably relaxed. The study on carrier dynamics based on steady-state and time-resolved photoluminescence suggests that the carrier lifetime and non-radiative recombination rate of SCN^- PSCs can be remarkably prolonged and depressed, respectively. As a result, FASn_0.5Pb_0.5I₃-based PSCs achieve a 14.5% increase in PCE, reaching 13.74% under AM 1.5G illumination. This strategy takes a noteworthy step toward high efficiency and high stability FA-based Sn-Pb PSCs.

Low-Dimensional Inorganic Tin Perovskite Solar Cells Prepared by Templated Growth

The manipulation of the dimensionality and nanostructures based on the precise control of the crystal growth kinetics boosts the flourishing development of perovskite optoelectronic materials and devices. Herein, a low-dimensional inorganic tin halide perovskite, CsSnBrI_2-x (SCN)_x , with a mixed 2D and 3D structure is fabricated. A kinetic study indicates that Sn(SCN)₂ and phenylethylamine hydroiodate can form a 2D perovskite structure that acts as a template for the growth of the 3D perovskite CsSnBrI_2-x (SCN)_x . The film shows an out-of-plane orientation and a large grain size, giving rise to reduced defect density, superior thermostability, and oxidation resistance. A solar cell based on this low-dimensional film reaches a power conversion efficiency of 5.01 %, which is the highest value for CsSnBr_x I_3-x perovskite solar cells. Furthermore, the device shows enhanced stability in ambient air.

Low-Toxicity Perovskite Applications in Carbon Electrode Perovskite Solar Cells—A Review

Perovskite solar cells (PSCs) with earth-abundant carbon as an effective replacer for unstable hole-transporting materials and expensive electrodes is a recently proposed structure promising better air and moisture stability. In this review paper, we report on the latest advances and state of the art of Pb-free and low-Pb-content perovskites, used as absorbers in carbon-based perovskite solar cells. The focus is on the implementation of these, environmentally friendly and non-toxic, structures in PSCs with a carbon electrode as a replacement of the noble metal electrode typically used (C-PSCs). The motivation for this study has been the great potential that C-PSCs have shown for the leap towards the commercialization of PSCs. Some of their outstanding properties include low cost, high-stability, ambient processability and compatibility with most up-scaling methods (e.g., printing). By surpassing the key obstacle of toxicity, caused by the Pb content of the highest-performing perovskites, and by combining the advantages of C-PSCs with the Pb-free perovskites low toxicity, this technology will move one step further; this review summarizes the most promising routes that have been reported so far towards that direction.