In recent years, the record efficiency of perovskite solar cells (PSCs) has been updated from 9.7% to 20.1%. However, there has been very little study of the issue of stability, which restricts the outdoor application of PSCs. The issues of the degradation of perovskite and the stability of PSC devices should be urgently addressed to achieve good reproducibility and long lifetimes for PSCs with high conversion efficiency. Without studies on stability, exciting achievements cannot be transferred from the laboratory to industry and outdoor applications. In order to improve their stability, a basic understanding of the degradation process of PSCs in different conditions should be acquired via thorough study. This review summarizes recent studies of the relationship of the chemical stability of PSCs with their environment (oxygen and moisture, UV light, solution process, temperature) and corresponding possible solutions.

Review of recent progress in chemical stability of perovskite solar cells

Organic and inorganic hybrid perovskites (e.g., CH(3)NH(3)PbI(3)), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic-inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.

Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications

There are only few semiconducting materials that have been shaping the progress of third generation photovoltaic cells as much as perovskites. Although they are deceivingly simple in structure, the archetypal AMX3-type perovskites have built-in potential for complex and surprising discoveries. Since 2009, a small and somewhat exotic class of perovskites, which are quite different from the common rock-solid oxide perovskite, have turned over a new leaf in solar cell research. Highlighted as one of the major scientific breakthroughs of the year 2013, the power conversion efficiency of the title compound hybrid organic–inorganic perovskite has now exceeded 18%, making it competitive with thin-film PV technology. In this minireview, a brief history of perovskite materials for photovoltaic applications is reported, the current state-of-the-art is distilled and the basic working mechanisms have been discussed. By analyzing the attainable photocurrent and photovoltage, realizing perovskite solar cells with 20% efficiency for a single junction, and 30% for a tandem configuration on a c-Si solar cell would be realistic.

Organohalide lead perovskites for photovoltaic applications

Halide perovskite quantum dots (QDs), primarily regarded as optoelectronic materials for LED and photovoltaic devices, have not been applied for photochemical conversion (e.g., water splitting or CO2 reduction) applications because of their insufficient stability in the presence of moisture or polar solvents. Herein, we report the use of CsPbBr3 QDs as novel photocatalysts to convert CO2 into solar fuels in nonaqueous media. Under AM 1.5G simulated illumination, the CsPbBr3 QDs steadily generated and injected electrons into CO2, catalyzing CO2 reduction at a rate of 23.7 μmol/g h with a selectivity over 99.3%. Additionally, through the construction of a CsPbBr3 QD/graphene oxide (CsPbBr3 QD/GO) composite, the rate of electron consumption increased 25.5% because of improved electron extraction and transport. This study is anticipated to provide new opportunities to utilize halide perovskite QD materials in photocatalytic applications.

A CsPbBr3 Perovskite Quantum Dot/Graphene Oxide Composite for Photocatalytic CO2 Reduction

In recent years, heterogeneous photocatalysis has received much research interest because of its powerful potential applications in tackling many important energy and environmental challenges at a global level in an economically sustainable manner. Due to their unique optical, electrical, and physicochemical properties, various 2D graphene nanosheets-supported semiconductor composite photocatalysts have been widely constructed and applied in different photocatalytic fields. In this review, fundamental mechanisms of heterogeneous photocatalysis, including thermodynamic and kinetics requirements, are first systematically summarized. Then, the photocatalysis-related properties of graphene and its derivatives, and design rules and synthesis methods of graphene-based composites are highlighted. Importantly, different design strategies, including doping and sensitization of semiconductors by graphene, improving electrical conductivity of graphene, increasing eloectrocatalytic active sites on graphene, strengthening interface coupling between semiconductors and graphene, fabricating micro/nano architectures, constructing multi-junction nanocomposites, enhancing photostability of semiconductors, and utilizing the synergistic effect of various modification strategies, are thoroughly summarized. The important applications including photocatalytic pollutant degradation, H2 production, and CO2 reduction are also addressed. Through reviewing the significant advances on this topic, it may provide new opportunities for designing highly efficient 2D graphene-based photocatalysts for various applications in photocatalysis and other fields, such as solar cells, thermal catalysis, separation, and purification.

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Graphene in Photocatalysis: A Review

Perovskite solar cells have garnered great attention in recent years as promising high performance next-generation solar cells with long-term stability at low cost. Since the seminal work of Miyasaka and others in 2009, the power conversion efficiency (PCE) of perovskite-based dye-sensitized solar cells (DSSCs) has rapidly increased from 3.8% to 15% over the past four years, exceeding the highest efficiency of conventional organic dye-sensitized DSSCs. Recently, the perovskite has been demonstrated to act successfully as an active layer in simple planar-heterojunction solar cells with no need of complex nanostructured DSSC architectures, leading to an attractively high PCE of 15.4% at a competitive low manufacturing cost. In this Feature Article, we aim to review the recent impressive development in perovskite solar cells, and discuss the prognosis for future progress in exploiting perovskite materials for high efficiency solar cells.

High efficiency perovskite solar cells: from complex nanostructure to planar heterojunction

National Natural Science Foundation of China [51072170, 21021002]; National Basic Research Program of China [2012CB932900]; Georgia Institute of Technology

Hierarchically Structured Nanotubes for Highly Efficient Dye-Sensitized Solar Cells

Highly ordered rutile TiO2 nanorod arrays (NRAs) are promising architectures in dye-sensitized solar cells (DSCs). However, the efficiency of DSCs based on such photoanodes is still relatively low, largely due to the limited internal surface area. Herein, we report that highly oriented rutile TiO2 NRAs with film thickness up to ∼30 μm was developed by a facile hydrothermal method. More importantly, an optimized porous rutile TiO2 NRAs with a large internal surface area was fabricated on the FTO (fluorine-doped tin oxide) substrate via a secondary hydrothermal treatment and when applied as the photoanodes in DSCs, a record efficiency of 7.91% was achieved.

Optimized porous rutile TiO2 nanorod arrays for enhancing the efficiency of dye-sensitized solar cells

One-dimensional (1-D) TiO2 nanorod arrays (NRAs) with large inner surface area are desired in dye-sensitized solar cells (DSSCs). So far, good performance of DSSCs based on 1-D rutile TiO2 NRAs remains a challenge mainly owing to their low dye-loading ability resulting from the insufficient specific surface area of 1-D TiO2 nanostructures. In this paper, densely aligned TiO2 NRAs with tunable thickness were grown directly on transparent conductive fluorine-doped tin oxide (FTO) substrates by hydrothermal method, followed by a facile chemical etching route to further increase the specific surface area of the TiO2 NRAs. The etching treatment leads to the split of TiO2 nanorods into secondary nanorods with a reduced diameter, which markedly enlarges the inner surface area of the TiO2 NRAs. The formation of 1-D rutile TiO2 nanotube arrays (NTAs) is observed as well in the etched TiO2 films. Finally, a DSSC efficiency of 5.94% was achieved by utilizing an etched TiO2 NRA as the photoanode, which is so far the best DSSC efficiency that has been reported for the 1-D rutile TiO2 NRA films.

Dajiang Zheng

Papers

High efficiency perovskite solar cells: from complex nanostructure to planar heterojunction

Hierarchically Structured Nanotubes for Highly Efficient Dye-Sensitized Solar Cells

Optimized porous rutile TiO2 nanorod arrays for enhancing the efficiency of dye-sensitized solar cells

Densely aligned rutile TiO2 nanorod arrays with high surface area for efficient dye-sensitized solar cells.

CdSe/CdS quantum dots co-sensitized TiO2 nanotube array photoelectrode for highly efficient solar cells