Perovskite solar cells based on methylammonium lead triiodide witnessed unprecedented progress after the seminal work of Miyasaka and co-workers in 2009, where they employed perovskite nanocrystals as sensitizers in a dye-sensitized solar-cell configuration. After key breakthroughs with solid-state perovskite photovoltaics in 2012, research efforts have grown exponentially, and several groups have demonstrated that the perovskite concomitantly acts as a light absorber and an electron and hole transporter in both mesoscopic networks and solid polycrystalline layers, where the perovskite layer can be deposited using a broad range of techniques. The methylammonium lead triiodide perovskite bandgap has been tuned by substituting various cations and anions. By optimizing the crystalline quality of the perovskite absorber and film formation by solvent engineering, a remarkable power-conversion efficiency of over 20% has been demonstrated, highlighting the exceptional photovoltaic properties of perovskite materials. The high efficiencies are due to a combination of long carrier lifetimes, substantial charge-carrier mobilities, and remarkably benign electronic defects. This issue highlights various deposition methods of the perovskite absorber, such as single-step, sequential, dual-source sublimation, and solution and sublimation processes, as well as hole-transporting-free and tandem perovskite solar cells.

Crystalline silicon dominates the solar panel industry today but remains relatively expensive to manufacture. If devices could be fabricated from inexpensive materials by a simple solution process without the need for high-temperature annealing, their cost could be considerably reduced. Recently, inorganic–organic (I/O) hybrid systems based on inorganic nanoparticles (including quantum dots) and perovskite materials as light harvesters with organic hole-conducting materials have shown great potential for efficient solar cells due to the combination of superior optical properties and solution-based processes. In this review, we describe the relevant morphological factors and the performance of perovskite solar cells with tuned heterojunctions. In particular, we describe the mediator retarding the rapid crystallization of perovskite layers for a bilayer configuration. Appropriate processes and chemical engineering induced the formation of well-crystallized perovskite materials with extremely uniform and dense perovskite layers and remarkably improved the performance of the cells with a National Renewable Energy Laboratory (NREL)-certified record efficiency of 20.1%.

Perovskite solar cells based on organolead halide perovskite light absorbers have been considered a promising photovoltaic technology due to their superb power-conversion efficiency along with cheap material cost. Since the first work on long-term durable solid-state perovskite solar cells, a tremendous volume of research on perovskite solar cells has been carried out. A high photovoltaic performance is mainly attributed to the high-quality CH3NH3PbI3 (MAPbI3) material that is strongly dependent on the fabrication method used. MAPbI3 can be prepared by either a single-step procedure or a sequential two-step deposition technique. The two-step method was found, in general, to show better coverage, morphology, and infiltration into a mesoporous oxide layer, which led to high-quality perovskites with desirable optoelectronic properties and thereby high-efficiency perovskite solar cells.

Organic–inorganic perovskites have emerged as one of the most promising materials for future optoelectronics applications, most notably photovoltaics. The achievement of high-efficiency solar cells has been possible mainly through the understanding of the perovskite formation during the solution deposition of thin films. Vacuum deposition methods have also been developed and have intrinsic advantages over solution-based processing, including control over the film thickness and composition, low-temperature processing, and the possibility of preparing multilayer structures. This article summarizes the latest advances in the vacuum deposition of hybrid perovskites, with an emphasis on the application to photovoltaics. Methods for the deposition of perovskite thin films and the performances of the correspondent solar cells are reviewed.

Hybrid organic–inorganic perovskites (e.g., CH3NH3PbX3, X represents a halide) have been highlighted for various applications, especially as light absorbers in third-generation photovoltaics. In the pursuit of low-cost and efficient perovskite solar technology, it is crucial to develop a facile method to fabricate conformal, compact perovskite films in an inexpensive and reproducible manner. Here, we report high-quality perovskite films controllably deposited via a facile low-temperature (<150°C) vapor-assisted solution process (VASP). Key steps include deposition of the inorganic framework by solution first, followed by a subsequent in situ reaction between the inorganic species and the desired organic vapor. The VASP approach differs from other conventional solution processing techniques because it retards nucleation and enables vigorous reorganization for film growth, with an absence of solvation, hydration, and undesirable structural transitions. Facilitated by excellent film quality, perovskite materials enable a power-conversion efficiency of ∼16.8% in the planar configuration of a solar cell. This method provides a simple approach to perovskite film preparation and paves the way toward high reproducibility and mass production of high-quality absorber films for solar devices.

Recent discoveries have revealed a breakthrough in the photovoltaics (PVs) field using organometallic perovskites as light harvesters in the solar cell. The organometal perovskite arrangement is self-assembled as alternate layers via a simple low-cost procedure. These organometal perovskites promise several benefits not provided by the separate constituents. This overview concentrates on implementing perovskites in PV cells such that the perovskite layers are used as the light harvester as well as the hole-conducting component. Eliminating hole-transport material (HTM) in this solar-cell structure avoids oxidation, reduces costs, and provides better stability and consistent results. Aspects of HTM-free perovskite solar cells discussed in this article include (1) depletion regions, (2) high voltages, (3) panchromatic responses, (4) chemical modifications, and (5) contacts in HTM-free perovskite solar cells. Elimination of HTM could expand possibilities to explore new interfaces in these solar cells, while over the long term, these uniquely structured HTM-free solar cells could offer valuable benefits for future PV and optoelectronics applications.

A method to cost-effectively upgrade the performance of an established small-bandgap solar technology is to deposit a large-bandgap polycrystalline semiconductor on top to make a tandem solar cell. Metal-halide perovskites have recently been demonstrated as large-bandgap semiconductors that perform well even as a defective and polycrystalline material. We review the initial experimental and modeling work performed on these tandems. We also discuss in-depth the challenges of perovskite-based tandems and the innovations needed from the solar research community to propel perovskite-based tandems into the high-efficiency (>25%) regime and reach commercial competitiveness.

With more than two-thirds of utilized energy being lost as waste heat, there is compelling motivation for high-performance thermoelectric materials that can directly convert heat to electrical energy. However, over the decades, practical realization of thermoelectric materials has been limited by the hitherto low figure of merit, ZT, which governs the Carnot efficiency. This article describes our long-standing efforts to advance ZT to record levels starting from exploratory synthesis and evolving into the nanostructuring and panoscopic paradigm, which has helped to usher in a new era of investigation for thermoelectrics. The term panoscopic is meant as an attempt to integrate all length scales and multiple physical concepts into a single material. As in any other energy-conversion technology involving materials, thermoelectrics research is a challenging exercise in taming “contra-indicated” properties. Critical properties such as high electrical conductivity, thermoelectric power, low thermal conductivity, and mechanical strength do not tend to favor coexistence in a single material. How these can be achieved in certain systems leading to record values of ZT is also described. Endotaxial nanostructures and mesoscale engineering in thermoelectrics enable effective phonon scattering with negligible electron scattering. By combining all relevant length scales hierarchically, we can achieve large enhancements in thermoelectric performance. The field, however, continues to produce surprises.