Polariton-hybridization of light-matter oscillations can emerge from various quasiparticles, such as phonon, plasmon, exciton and magnon. Particularly, exciton polaritons are bosonic quasiparticles with half-light, halfmatter nature, which are originated from strong coupling between excitons and microcavity photons. The half-light nature results in extremely small effective mass, making it feasible to achieve high temperature even room-temperature Bose-Einstein condensation (BEC). Meanwhile, the half-matter nature leads to strong nonlinear interaction, which is missing between photons and can promote the polaritons relaxation to ground state and give rise to low threshold polariton lasing, compared to photonic lasing. The exciton polaritons are of great importance in applications of quantum simulation, topological quantum optics, ultrafast optical switch and low threshold lasers.

Halide perovskites have been extensively studied in last decade partially due to the unprecedentedly rapid increase of power conversion efficiency of perovskite based solar cells. In addition to the solar cells, perovskite based optoelectronic devices such as photodetectors and light emitting devices have also been demonstrated with impressive performance, benefited from the large absorption coefficient, tunable band gap, defect tolerance and long carrier diffusion length. Although significant progress has been made in these fields, a few challenges including long-term stability and toxicity of lead greatly limit their commercialization. Great effort has been put into those fields to address the long-last issues from aspects of fundamental understanding of their photophysics, material engineering and performance optimization.This special issue on “Halide perovskites: from materials to optoelectronic devices” including one comment, four reviews, and five original research articles covers all topics mentioned. In this special issue, Xiong et al. [1] from Nanyang Technological University (Singapore) give an in-depth comment on the current development and future research direction of Bose-Einstein condensation of exciton polariton based on perovskites. Koleilat et al. [2] give a thorough summary on how the dimensionality engineering including morphological engineering and molecular engineering could affect their band gap, binding energy and carrier mobility and subsequently the performance of photodetectors and solar cells. Li et al. [3] review the research process of self-trapped excitons in two-dimensional perovskite including the origins of self-trapped excitons, how to detect and control the self-trapped excitons and how the presence of self-trapped excitons influences the performance of perovskite based optoelectronic devices. Tang et al. [4] collect the performance matrix including external quantum efficiency, luminance, and stability status of perovskite based light emitting diodes, presenting a brief yet comprehensive overview of this field to readers. Chen et al. [5] summarize the possible top cells for next-generation Si-based tandem solar cells and further propose the promising candidate top cells. Mei et al. [6] explore how the precursor concentrations affects the performance of printable hole-conductor-free mesoscopic perovskite solar cells via a simple one-step drop-coating method while You et al. [7] investigate the performance and thermal stability of inorganic perovskite solar cells by using dopant-free polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT), as the holetransport layer. Zhong et al. [8] fabricate a wide-bandgap formamidinium lead bromide film using a doctor-bladecoating method and study the effect of the species of surfactants on the performance of the solar cells based on the as-fabricated film. Wei et al. [9] demonstrate how to fabricate efficient perovskite based light emitting diodes via composite engineering. Mu et al. [10] propose a corona modulation device structure to achieve random lasing in perovskite quantum dots under electron beam excitation.These ten articles appeared in this special issue only cover a rather small portion of the recent advances in this rapid growing perovskite community. We hope this special issue will provide useful references for halide perovskite community and motivate more investigations in these research fields.

Over the last decade, the power conversion efficiency of hybrid organic–inorganic perovskite solar cells (PSCs) has increased dramatically from 3.8% to 25.2%. This rapid progress has been possible due to the accurate control of the morphology and crystallinity of solution-processed perovskites, which are significantly affected by the concentration of the precursor used. This study explores the influence of precursor concentrations on the performance of printable hole-conductor-free mesoscopic PSCs via a simple one-step drop-coating method. The results reveal that lower concentrations lead to larger grains with inferior pore filling, while higher concentrations result in smaller grains with improved pore filling. Among concentrations ranging from 0.24–1.20 M1), devices based on a moderate strength of 0.70 M were confirmed to exhibit the best efficiency at 16.32%.

Cesium-based inorganic perovskite solar cells (PSCs) are paid more attention because of their potential thermal stability. However, prevalent salt-doped 2,2′,7,7′- tetrakis(N,N-dipmethoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD) as hole-transport materials (HTMs) for a high-efficiency inorganic device has an unfortunate defective thermal stability. In this study, we apply poly (3-hexylthiophene-2,5-diyl) (P3HT) as the HTM and design all-inorganic PSCs with an indium tin oxide (ITO)/SnO2/LiF/CsPbI3-xBrx/P3HT/Au structure. As a result, the CsPbI3-xBrx PSCs achieve an excellent performance of 15.84%. The P3HT HTM-based device exhibits good photo-stability, maintaining ~80% of their initial power conversion efficiency over 280 h under one Sun irradiation. In addition, they also show better thermal stability compared with the traditional HTM Spiro- OMeTAD.

Organic–inorganic hybrid perovskite solar cells have generated wide interest due to the rapid development of their photovoltaic conversion efficiencies. However, the majority of the reported devices have been fabricated via spin coating with a device area of 2. In this study, we fabricated a wide-bandgap formamidinium lead bromide (FAPbBr3) film using a cost-effective, high-yielding doctor-blade-coating process. The effects of different surfactants, such as l-α-phosphatidylcholine, polyoxyethylene sorbitan monooleate, sodium lauryl sulfonate, and hexadecyl trimethyl ammonium bromide, were studied during the printing process. Accompanying the optimization of the blading temperature, crystal sizes of over 10 mm and large-area perovskite films of 5 cm × 5 cm were obtained using this method. The printed FAPbBr3 solar cells exhibited a short-circuit current density of 8.22 mA/cm2, an open-circuit voltage of 1.175 V, and an efficiency of 7.29%. Subsequently, we replaced the gold with silver nanowires as the top electrode to prepare a semitransparent perovskite solar cell with an average transmittance (400–800 nm) of 25.42%, achieving a highpower efficiency of 5.11%. This study demonstrates efficient doctor-blading printing for preparing large-area FAPbBr3 films that possess high potential for applications in building integrated photovoltaics.

Metal halide perovskites have received considerable attention in the field of electroluminescence, and the external quantum efficiency of perovskite lightemitting diodes has exceeded 20%. CH3NH3PbBr3 has been intensely investigated as an emitting layer in perovskite light-emitting diodes. However, perovskite films comprising CH3NH3PbBr3 often exhibit low surface coverage and poor crystallinity, leading to high current leakage, severe nonradiative recombination, and limited device performance. Herein, we demonstrate a rationale for composition engineering to obtain high-quality perovskite films. We first reduce pinholes by adding excess CH3NH3Br to the actual CH3NH3PbBr3 films, and we then add CsBr to improve the crystalline quality and to passivate nonradiative defects. As a result, the (CH3NH3)1-xCsxPbBr3 based perovskite light-emitting diodes exhibit significantly improved external quantum and power efficiencies of 6.97% and 25.18 lm/W, respectively, representing an improvement in performance dozens of times greater than that of pristine CH3NH3PbBr3-based perovskite light-emitting diodes. Our study demonstrates that composition engineering is an effective strategy for enhancing the device performance of perovskite light-emitting diodes.

Although laser pumping using electron beam (EB) has high transient power output and easy modulation based on perovskite quantum dot (PQD) film, its lasing emitting direction is the same as the pumped EB’s direction. Thus, realizing the conventional direct device structure through the film lasing mechanism is extremely difficult. Therefore, using the random lasing principle, herein, we proposed a corona modulation device structure based on PQDs random laser pumped using an EB. We discussed and stimulated the optimized designed method of the device in terms of parameters of the electronic optical device and the utilization ratio of output power and its modulation extinction ratio, respectively. According to the simulation results, this type of device structure can effectively satisfy the new random lasing mechanism in terms of high-speed and high-power modulation.

Metal halide perovskites are a class of materials that are ideal for photodetectors and solar cells due to their excellent optoelectronic properties. Their lowcost and low temperature synthesis have made them attractive for extensive research aimed at revolutionizing the semiconductor industry. The rich chemistry of metal halide perovskites allows compositional engineering resulting in facile tuning of the desired optoelectronic properties. Moreover, using different experimental synthesis and deposition techniques such as solution processing, chemical vapor deposition and hot-injection methods, the dimensionality of the perovskites can be altered from 3D to 0D, each structure opening a new realm of applications due to their unique properties. Dimensionality engineering includes both morphological engineering–reducing the thickness of 3D perovskite into atomically thin films–and molecular engineering–incorporating long-chain organic cations into the perovskite mixture and changing the composition at the molecular level. The optoelectronic properties of the perovskite structure including its band gap, binding energy and carrier mobility depend on both its composition and dimensionality. The plethora of different photodetectors and solar cells that have been made with different compositions and dimensions of perovskite will be reviewed here. We will conclude our review by discussing the kinetics and dynamics of different dimensionalities, their inherent stability and toxicity issues, and how reaching similar performance to 3D in lower dimensionalities and their large-scale deployment can be achieved.

With strong electron–phonon coupling, the self-trapped excitons are usually formed in materials, which leads to the local lattice distortion and localized excitons. The self-trapping strongly depends on the dimensionality of the materials. In the three-dimensional case, there is a potential barrier for self-trapping, whereas no such barrier is present for quasi-one-dimensional systems. Two-dimensional (2D) systems are marginal cases with a much lower potential barrier or nonexistent potential barrier for the self-trapping, leading to the easier formation of self-trapped states. Self-trapped excitons emission exhibits a broadband emission with a large Stokes shift below the bandgap. 2D perovskites are a class of layered structure material with unique optical properties and would find potential promising optoelectronic. In particular, self-trapped excitons are present in 2D perovskites and can significantly influence the optical and electrical properties of 2D perovskites due to the soft characteristic and strong electron–phonon interaction. Here, we summarized the luminescence characteristics, origins, and characterizations of self-trapped excitons in 2D perovskites and finally gave an introduction to their applications in optoelectronics.

Perovskite-based optoelectronic devices, especially perovskite light-emitting diodes (PeLEDs) and perovskite solar cells, have recently attracted considerable attention. The National Renewable Energy Laboratory (NREL) chart inspires us to develop a counterpart for PeLEDs. In this study, we collect the record performance of PeLEDs including several new entries to address their latest external quantum efficiency (EQE), highest luminance, and stability status.We hope that these performance tables and future updated versions will show the frontiers of PeLEDs, assist researchers in capturing the overview of this field, identify the remaining challenges, and predict the promising research directions.

Si-based solar cells, which have the advantages of high efficiency, low manufacturing costs, and outstanding stability, are dominant in the photovoltaic market. Currently, state-of-the-art Si-based solar cells are approaching the practical limit of efficiency. Constructing Si-based tandem solar cells is one available pathway to break the theoretical efficiency limit of single-junction silicon solar cells. Various top cells have been explored recently in the construction of Si-based tandem devices. Nevertheless, many challenges still stand in the way of extensive commercial application of Si-based tandem solar cells. Herein, we summarize the recent progress of representative Si-based tandem solar cells with different top cells, such as III-V solar cells, wide-bandgap perovskite solar cells, cadmium telluride (CdTe)-related solar cells, Cu(In,Ga)(Se,S)2 (CIGS)-related solar cells, and amorphous silicon (a-Si) solar cells, and we analyze the main bottlenecks for their next steps of development. Subsequently, we suggest several potential candidate top cells for Si-based tandem devices, such as Sb2S3, Se, CdSe, and Cu2O. These materials have great potential for the development of high-performance and low-cost Si-based tandem solar cells in the future.