Photovoltaic solar cells based on metal halide perovskites have gained considerable attention over the past decade because of their potentially low production cost, earth-abundant raw materials, ease of fabrication and ever-increasing power conversion efficiencies of up to 25.2%. This type of solar cells offers the promise of generating electricity at a more competitive unit price than traditional fossil fuels by 2035. Nevertheless, the best research cell efficiencies are still below the theoretical limit defined by the Shockley-Queissier theory owing to the presence of non-radiative recombination losses. In this Review, we analyse the predominant pathways that contribute to non-radiative recombination losses in perovskite solar cells, and evaluate their impact on device performance. We then discuss how non-radiative recombination losses can be estimated through reliable characterization techniques, and highlight some notable advances in mitigating these losses, which hint at pathways towards defect-free perovskite solar cells. Finally, we outline directions for future work that will push the efficiency of perovskite solar cells towards the radiative limit.

Minimizing non-radiative recombination losses in perovskite solar cells

Organic-inorganic hybrid perovskite (OIHP) solar cells have achieved comparable efficiencies to those of commercial solar cells, although their instability hinders their commercialization. Although encapsulation techniques have been developed to protect OIHP solar cells from external stimuli such as moisture, oxygen, and ultraviolet light, understanding of the origin of the intrinsic instability of perovskite films is needed to improve their stability. We show that the OIHP films fabricated by existing methods are strained and that strain is caused by mismatched thermal expansion of perovskite films and substrates during the thermal annealing process. The polycrystalline films have compressive strain in the out-of-plane direction and in-plane tensile strain. The strain accelerates degradation of perovskite films under illumination, which can be explained by increased ion migration in strained OIHP films. This study points out an avenue to enhance the intrinsic stability of perovskite films and solar cells by reducing residual strain in perovskite films.

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Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells.

Novel photovoltaic technologies such as perovskites hold the promise of a reduced levelized cost of electricity, but the low-cost potential depends on the ability to scale-up solution-based deposition. So far, complex fluid dynamics have limited the solution deposition of uniform pinhole-free organic–inorganic perovskite thin films over large areas. Here, we show that very small amounts (tens of parts per million) of surfactants (for example, l-α-Phosphatidylcholine) dramatically alter the fluid drying dynamics and increase the adhesion of the perovskite ink to the underlying non-wetting charge transport layer. The additives enable blading of smooth perovskite films at a coating rate of 180 m h–1 with root-mean-square roughness of 14.5 nm over 1 cm. The surfactants also passivate charge traps, resulting in efficiencies over 20% for small-area solar cells. Fast blading in air of perovskite films results in stabilized module efficiencies of 15.3% and 14.6% measured at aperture areas of 33.0 cm2 and 57.2 cm2, respectively. Scaling up perovskite film deposition necessitates controlling the film formation dynamics. Here, Deng et al. use amphoteric choline surfactants to blade-coat well-passivated films, reaching module efficiencies of ~15% for aperture areas up to 57 cm2.

Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic modules

Since the report in 2012 of a solid-state perovskite solar cell (PSC) with a power-conversion efficiency (PCE) of 9.7% and a stability of 500 h, intensive efforts have been made to increase the certified PCE, reaching 25.2% in 2019. The PCE of PSCs now exceeds that of conventional thin-film solar-cell technologies, and the rate at which this increase has been achieved is unprecedented in the history of photovoltaics. Moreover, the development of moisture-stable and heat-stable materials has increased the stability of PSCs. Small-area devices ( 100 cm2) substrates required for commercialization. Thus, materials and methods need to be developed for coating large-area PSCs. In this Review, we discuss solution-based and vapour-phase coating methods for the fabrication of large-area perovskite films, examine the progress in performance and the parameters affecting the properties of large-area coatings, and provide an overview of the methodologies for achieving high-efficiency perovskite solar modules. The scalable fabrication of perovskite solar cells and solar modules requires the development of new materials and coating methods. In this Review, we discuss solution-based and vapour-phase coating methods for large-area perovskite films and examine the progress in performance and the parameters affecting large-area coatings.

Scalable fabrication and coating methods for perovskite solar cells and solar modules

Over the last several years, inorganic–organic hybrid perovskites have shown dramatic achievements in photovoltaic performance and device stability. Despite the significant progress in photovoltaic application, an in-depth understanding of the fundamentals of precursor solution chemistry is still lacking. In this review, the fundamental background knowledge of nucleation and crystal growth processes in solution including the LaMer model and Ostwald ripening process is described. This review article also highlights the recent progress in precursor-coordinating molecule interaction in solution along with the role of anti-solvent in the solvent engineering process to control nucleation and crystal growth. Moreover, chemical pathways from precursor solution to perovskite film formation are given. This represents identification of the intermediate phase induced by precursor-coordinating molecule interaction and responsible intermediate species for uniform and dense perovskite film formation. Further to the description of chemical phenomena in solution, the contemporary progress in chemical precursor composition is also provided to comprehend the current research approaches to further enhance photovoltaic performance and device stability. On the basis of the critical and comprehensive review, we provide some perspectives to further achieve high-performance perovskite solar cells with long-term device stability through precisely controlled nucleation and crystal growth in precursor solution.

Perovskite precursor solution chemistry: from fundamentals to photovoltaic applications.

Hybrid lead halide perovskites have emerged as high-performance photovoltaic materials with their extraordinary optoelectronic properties. In particular, the remarkable device efficiency is strongly influenced by the perovskite crystallinity and the film morphology. Here, we investigate the perovskites crystallisation kinetics and growth mechanism in real time from liquid precursor continually to the final uniform film. We utilize some advanced in situ characterisation techniques including synchrotron-based grazing incident X-ray diffraction to observe crystal structure and chemical transition of perovskites. The nano-assemble model from perovskite intermediated [PbI6]4- cage nanoparticles to bulk polycrystals is proposed to understand perovskites formation at a molecular- or nano-level. A crystallisation-depletion mechanism is developed to elucidate the periodic crystallisation and the kinetically trapped morphology at a mesoscopic level. Based on these in situ dynamics studies, the whole process of the perovskites formation and transformation from the molecular to the microstructure over relevant temperature and time scales is successfully demonstrated.

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In situ dynamic observations of perovskite crystallisation and microstructure evolution intermediated from [PbI 6 ] 4− cage nanoparticles

Volatile organic compounds (VOCs) are atmospheric pollutants that have been of concern for researchers in recent years because they are toxic, difficult to remove, and widely sourced and easily cause damage to the environment and human body. Most scholars use low-temperature plasma biological treatment, catalytic oxidation, adsorption, condensation, and recovery techniques to treat then effectively. Among them, catalytic oxidation technology has the advantages of a high catalytic efficiency, low energy consumption, high safety factor, high treatment efficiency, and less secondary pollution; it is currently widely used for VOC degradation technology. In this paper, the catalytic oxidation technology for the degradation of multiple types of VOCs as well as the development of a single metal oxide catalyst have been briefly introduced. We also focus on the research progress of composite metal oxide catalysts for the removal of VOCs by comparing and analyzing the metal component ratio, preparation method, and types of precursors and the catalysts' influence on the catalytic performance. In addition, the reason for catalyst deactivation and a correlation between the chemical state of the catalyst and the electron distribution are discussed. Development of a composite metal oxide catalyst for the catalytic oxidation of VOCs has been proposed.

Research Progress of a Composite Metal Oxide Catalyst for VOC Degradation.

With the widespread use of fossil energy, the environmental problems caused by CO2-based greenhouse gases are becoming more and more serious. The key to solving these problems lies in the reduction and reuse of CO2. Recent studies have shown that metal–organic framework (MOF) materials are widely used in CO2 catalytic reduction because of their porous structures, large specific surface areas, regular pore morphologies, and flexible tunability in terms of components and structure. This paper reviews recent research on several common MOF catalysts for the CO2 conversion process. The mechanism, research progress, and development prospects of MOFs for thermal catalytic reduction, photocatalytic reduction, and plasma catalytic reduction of CO2 in recent years are then discussed. Finally, the prospects and challenges of MOFs in CO2 conversion are proposed. The paper is instructive for the discovery of novel and efficient MOF catalysts. 

Research progress of Metal–Organic Frameworks (MOFs) for CO2 Conversion in CCUS

The SmMn2O5 catalyst is considered as an excellent substitute for noble-metal and transition-metal oxide catalysts due to its unique crystal structure. In order to further improve its catalytic activity for acetone degradation, SmMn2O5 nanocatalysts with four morphologies were synthesized by the hydrothermal method by adjusting the type of manganese source reagent, the content of mineralizing agent, and hydrothermal temperature. Among them, the SMO-NF catalyst prepared with Mn(NO3)2 as the manganese source, 10 mL of NaOH, and a hydrothermal temperature of 220 °C has a special flower morphology and can achieve 100% acetone conversion at 118 °C. Because of its thin and large “petal” morphology providing active sites and oxygen vacancies, the excellent catalytic performance of the SMO-NF catalyst is an important breakthrough in the field of acetone degradation. In addition, the highly stable crystalline phase, abundant surface reactive oxygen species, and excellent reducibility of the SMO-NF catalyst also lay the foundation for its high catalytic activity, water resistance, and stability. The SMO-NS catalyst with a spherical morphology can completely degrade acetone at 136 °C, and its activity is better than that of the SMO-CP catalyst prepared by the coprecipitation method and Pt/TiO2 catalysts. The relationship between the formation process of SmMn2O5 nanocatalysts with special morphology and its catalytic activity is discussed as well. The successful synthesis of the SMO-NF catalyst also opens up a way for the synthesis of the special morphology of acetone degradation nanocatalysts. 

Morphology-Controlled SmMn2O5 Nanocatalysts for Improved Acetone Degradation

The key of catalytic oxidation technology is to develop a stable catalyst with high activity. It is still a serious challenge to achieve high conversion efficiency of acetone with an integral catalyst at low temperature. In this study, the SmMn2O5 catalyst after acid etching was used as the support, and the manganese mullite composite catalyst was prepared by loading Ag and CeO2 nanoparticles on its surface. By means of SEM, TEM, XRD, N2-BET, XPS, EPR, H2-TPR, O2-TPD, NH3-TPD, DRIFT, and other characterization methods, the related factors and mechanism analysis of acetone degradation activity of the composite catalyst were discussed. Among them, the CeO2-SmMn2O5-H catalyst has the best catalytic activity at 123 and 185 °C for T50 and T100, respectively, and shows excellent water and thermal resistance and stability. In essence, the surface and lattice defects of highly exposed Mn sites were formed by acid etching, and the dispersibility of Ag and CeO2 nanoparticles was optimized. Highly dispersed Ag and CeO2 nanoparticles have a highly synergistic effect with the support SmMn2O5, and the reactive oxygen species provided by CeO2 and the electron transfer brought by Ag further promote the decomposition of acetone on the carrier SMO-H. In the field of catalytic degradation of acetone, a new catalyst modification method of high-quality active noble metals and transition metal oxides supported by acid-etched SmMn2O5 has been developed.

Ziyi Zhang

Papers

In situ dynamic observations of perovskite crystallisation and microstructure evolution intermediated from [PbI 6 ] 4− cage nanoparticles

Research Progress of a Composite Metal Oxide Catalyst for VOC Degradation.

Research progress of Metal–Organic Frameworks (MOFs) for CO2 Conversion in CCUS

Morphology-Controlled SmMn2O5 Nanocatalysts for Improved Acetone Degradation

Catalytic Oxidation of Acetone on SmMn2O5: Effect of Acid Etching and Loading Treatment.