Mahui

The photochemistry of halide‐related defects affects the optoelectronic properties of lead–halide perovskite semiconductors and their reactivity to external stimuli such as light and environmental molecules.

Abstract

The presence of various types of chemical interactions in metal‐halide perovskite semiconductors gives them a characteristic “soft” fluctuating structure, prone to a wide set of defects. Understanding of the nature of defects and their photochemistry is summarized, which leverages the cooperative action of density functional theory investigations and accurate experimental design. This knowledge is used to describe how defect activity determines the macroscopic properties of the material and related devices. Finally, a discussion of the open questions provides a path towards achieving an educated prediction of device operation, necessary to engineer reliable devices.

The factors that limit the luminescence efficiency (LE) of metal halide perovskite (MHP) light‐emitting diodes (PeLEDs) are reviewed by categorizing them into i) photophysical properties of MHPs, ii) morphological factors, and iii) problems caused by device architectures. Various strategies to overcome those LE‐limiting factors in MHPs and PeLEDs, and research directions to further increase the LE of MHPs are discussed.

Abstract

Metal‐halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light‐emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence‐limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next‐generation displays and solid‐state lightings are suggested.

The latest breakthroughs in interface and defect engineering as applied to metal halide perovskite solar cells and light‐emitting diodes (LEDs) are reviewed in order to shed light on their necessity and importance in tuning the optoelectronic properties of devices in an attempt to realize the best‐performing solar cells and LEDs.

Abstract

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Although high power conversion efficiency of up to 23.3% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Nonradiative recombination and charge back transfer at interfaces are mainly responsible for conversion loss. Interface engineering is the most important approach toward the theoretical efficiency in PSCs.

Abstract

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

The properties of excitons and photogenerated charge carriers in metal halide perovskites (MHPs) are explored. The properties of excitons including the exciton binding energy, exciton dynamics, and exciton–photon and exciton–phonon coupling, are discussed. The properties of photogenerated free charge carriers in MHPs such as diffusion length, mobility, and recombination are described. A brief review of recent applications is also demonstrated.

Abstract

Metal halide perovskites (MHPs) have recently attracted great attention from the scientific community due to their excellent photovoltaic performance as well as their tremendous potential for other optoelectronic applications such as light‐emitting diodes, lasers, and photodetectors. Despite the rapid progress in device applications, a solid understanding of the photophysical properties behind the device performance is highly desirable for MHPs. Here, the properties of excitons and photogenerated charge carriers in MHPs are explored. The unique dielectric constant properties, crystal–liquid duality, and fundamental optical processes of MHPs are first discussed. The properties of excitons and related phenomena in MHPs are then detailed, including the exciton binding energy determined by various methods and their influence factors, exciton dynamics, exciton–photon coupling and related applications, and exciton–phonon coupling in MHPs. The properties of photogenerated free charge carriers in MHPs such as the carrier diffusion length, mobility, and recombination are described. Recent progress in various applications is also demonstrated. Finally, a conclusion and perspectives of future studies for MHPs are presented.

Inorganic and layered perovskites have broadened research paradigm for a range of optoelectronic devices. Their unique electronic and photophysical properties show that they are an excellent material, leading forefronts of solar cells, light‐emitting diodes, photodetectors, lasers, and beyond. An overview of key research activities for these devices is provided and challenges for their future research are identified.

Abstract

Organic–inorganic halide perovskites are making breakthroughs in a range of optoelectronic devices. Reports of >23% certified power conversion efficiency in photovoltaic devices, external quantum efficiency >21% in light‐emitting diodes (LEDs), continuous‐wave lasing and ultralow lasing thresholds in optically pumped lasers, and detectivity in photodetectors on a par with commercial GaAs rivals are being witnessed, making them the fastest ever emerging material technology. Still, questions on their toxicity and long‐term stability raise concerns toward their market entry. The intrinsic instability in these materials arises due to the organic cation, typically the volatile methylamine (MA), which contributes to hysteresis in the current–voltage characteristics and ion migration. Alternative inorganic substitutes to MA, such as cesium, and large organic cations that lead to a layered structure, enhance structural as well as device operational stability. These perovskites also provide a high exciton binding energy that is a prerequisite to enhance radiative emission yield in LEDs. The incorporation of inorganic and layered perovskites, in the form of polycrystalline films or as single‐crystalline nanostructure morphologies, is now leading to the demonstration of stable devices with excellent performance parameters. Herein, key developments made in various optoelectronic devices using these perovskites are summarized and an outlook toward stable yet efficient devices is presented.

Rapid advancement of perovskite solar cells confronts the challenges of reliable measurement, which is important for data analysis and results reproduction. Major measurement methods and the key factors affecting evaluation are summarized. A measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Abstract

Perovskite solar cells (PSCs) have undergone an incredibly fast development and attracted intense attention worldwide owing to their high efficiency and low‐cost fabrication. However, it is challenging to make a reliable measurement of PSCs, which creates great difficulty for researchers to compare and reproduce published results. Herein, the major measurement methods and key factors affecting evaluation of PSCs are summarized, such as hysteresis in current–voltage measurement, calibration of solar simulators for less mismatch in spectra and light intensity, and the area for the calculation of current density and power conversion efficiency. PSCs are also compared with n–i–p or p–i–n structures that exhibit different feedback under the same measurement methods. Finally, a measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Low‐toxicity tin‐based perovskites have excellent optical and electrical properties, and are a good alternative for lead‐based perovskites. However, the performance and stability of tin‐based perovskites are not comparable. The properties of tin‐based perovskite films and the performance of tin‐based perovskite solar cells are reviewed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

Abstract

The tremendous interest focused on organic–inorganic halide perovskites since 2012 derives from their unique optical and electrical properties, which make them excellent photovoltaic materials. Pb‐based halide perovskite solar cells, in particular, currently stand at a record efficiency of ≈23%, fulfilling their potential toward commercialization. However, because of the toxicity concerns of Pb‐based perovskite solar cells, their market prospects are hindered. In principle, Pb can be replaced with other less‐toxic, environmentally benign metals. Sn‐based perovskites are thus the far most promising alternative due to their very similar and perhaps even superior semiconductor characteristics. After years of effort invested in Sn‐based halide perovskites, sufficient breakthroughs have finally been achieved that make them the next runners up to the Pb halide perovskites. To help the reader better understand the nature of Sn‐based halide perovskites, their optical and electrical properties are systematically discussed. Recent progress in Sn‐based perovskite solar cells, focusing mainly on film fabrication methods and different device architectures, and highlighting roadblocks to progress and opportunities for future work are reviewed. Finally, a brief overview of mixed Sn/Pb‐based systems with their anomalous yet beneficial optical trends are discussed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

Lead‐halide perovskites have demo‐nstrated rapid rises in optoelectronic device performance, which directly links to their efficient luminescence properties. The current understanding of the physics of light emission in perovskites is discussed, along with current outstanding challenges and opportunities to push device performances beyond existing technologies.

Abstract

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light‐emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state‐of‐the‐art metal‐halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

The ultraslow cooling of hot carriers in a hybrid lead halide perovskite is intimately related to its dielectric function, which is responsible for the order‐of‐magnitude decrease in the Coulomb potential on the sub‐picosecond timescale. This dynamic screening reduces hot‐carrier scattering with longitudinal optical phonons, leading to partial retention of excess electronic energy on longer timescales.

Abstract

Among the exceptional properties of lead halide perovskites (LHPs) is the ultraslow cooling of hot carriers. Carrier densities below the Mott density for large polarons (≤ ≈10¹⁸ cm⁻³) are focused on here. As in other semiconductors, a nascent hot electron distribution initially cools down via emission of longitudinal optical (LO) phonons on the 10⁻¹⁴–10⁻¹³ s timescale. What distinguishes LHPs from conventional semiconductors is the exceptionally efficient screening in the former. The dielectric screening in LHPs on the 10⁻¹³ s timescale results in an order‐of‐magnitude reduction in the Coulomb potential upon the formation of a large polaron, likely with ferroelectric‐like local ordering. Further LO‐phonon emission is inhibited, and this leads to partial retention of hot electron energy on the 10⁻¹² s timescale, more so in hybrid LHPs than in their all‐inorganic counterparts. Further cooling of hot polarons occurs on the 10⁻¹⁰ s timescale, and this can be attributed to the slow diffusion of heat out of the large polaron volume due to the low thermal conductivity of LHPs. Like other carrier properties, slow hot carrier cooling in LHPs can be intimately related to efficient screening in a soft, anharmonic, and dynamically disordered lattice.

Perovskites are versatile ABX₃ crystals, hosting many intriguing physical properties. While most are inorganic compounds with cationic A‐ and B‐, and anionic X‐sites, recently, the introduction of organic ions (hybrid perovskites) and structures with inverted ionic charges (inverse (hybrid) perovskites) have been explored. Thus, the combinatorial space for design with optimized properties has new dimensions.

Abstract

Materials science evolves to a state where the composition and structure of a crystal can be controlled almost at will. Given that a composition meets basic requirements of stoichiometry, steric demands, and charge neutrality, researchers are now able to investigate a wide range of compounds theoretically and, under various experimental conditions, select the constituting fragments of a crystal. One intriguing playground for such materials design is the perovskite structure. While a game of mixing and matching ions has been played successfully for about 150 years within the limits of inorganic compounds, the recent advances in organic–inorganic hybrid perovskite photovoltaics have triggered the inclusion of organic ions. Organic ions can be incorporated on all sites of the perovskite structure, leading to hybrid (double, triple, etc.) perovskites and inverse (hybrid) perovskites. Examples for each of these cases are known, even with all three sites occupied by organic molecules. While this change from monatomic ions to molecular species is accompanied with increased complexity, it shows that concepts from traditional inorganic perovskites are transferable to the novel hybrid materials. The increased compositional space holds promising new possibilities and applications for the universe of perovskite materials.

Halide perovskites exhibit extraordinary properties of slow hot‐carrier cooling, long‐range hot‐carrier transport, and efficient hot‐carrier extraction, and are capable of unlocking disruptive high‐efficiency hot‐carrier photovoltaics which will overcome the Shockley–Queisser limit. The intricate photophysical mechanisms behind the novel phenomena are distilled, an engineering and developmental toolkit is assembled, and the challenges and opportunities in this fledging area are examined.

Abstract

Rapid hot‐carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot‐carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot‐carrier properties: slow hot‐carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, long‐range hot‐carrier transport (up to ≈600 nm), and highly efficient hot‐carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot‐carrier cooling properties in halide perovskites and prospects for perovskite hot‐carrier solar cells are also discussed.

Metal‐halide perovskites have attracted much attention in recent years due to their use as light‐harvesting and light‐emitting semiconductors. Their superior optical and electronic properties, low processing cost, and the ease with which their bandgaps can be tuned suggest that they will be useful in various optoelectronic applications. This special issue aims to address the fundamentals of perovskite materials and devices, to explore recent progress in this exciting field, and to overcome the hurdles that remain to commercialization of optoelectronic perovskite devices.

Passivating nonradiative recombination centers is of crucial importance to guide experimentalists to further enhance perovskite solar cell efficiency approaching the Shockley–Queisser limit. Comprehensive first‐principles defect studies reveal the passivation mechanism of Br as a detrimental DX center in CH₃NH₃PbI₃. As a result, the carrier lifetime can be enhanced from 3.2 ns in defective CH₃NH₃PbI₃ to 19 ns in CH₃NH₃Pb(I_0.96Br_0.04)₃.

Abstract

After a period of rapid, unprecedented development, the growth in the efficiency of perovskite solar cells has recently slowed. Further improvement of cell efficiency will rely on the in‐depth understanding and delicate control of defect passivation. Here, the formation mechanism of iodine vacancies (V_I), a typical deep defect in CH₃NH₃PbI₃ (MAPbI₃), is elucidated. The structural and electronic behaviors of V_I are like those of a DX center, a kind of detrimental defect formed by large atomic displacement. Aided by the passivation mechanism of DX centers in tetrahedral semiconductors, it is found that the introduction of Br strengthens chemical bonds and prevents large atomic displacements during defect charging. It therefore reduces the defect states and diminishes electron–phonon coupling. Using time‐domain density functional theory (DFT) combined with nonadiabatic molecular dynamics, it is found that the carrier lifetime can be enhanced from 3.2 ns in defective MAPbI₃ to 19 ns in CH₃NH₃Pb(I_0.96Br_0.04)₃. This work advances our understanding of how a small amount of Br doping improves the carrier dynamics and cell performance of MAPbI₃. It may also provide a route to enhance the carrier lifetimes and efficiencies of perovskite solar cells by defect passivation.

A crosslinkable organic small molecule, thioctic acid (TA), is introduced into perovskite solar cells as a new bifacial passivation agent. This TA can simultaneously be chemically anchored to the surface of TiO₂ and methylammonium lead iodide through coordination effects and then in situ crosslinked to form a robust continuous polymer (Poly(TA)) network after thermal treatment.

Abstract

Defects, inevitably produced within bulk and at perovskite‐transport layer interfaces (PTLIs), are detrimental to power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). It is demonstrated that a crosslinkable organic small molecule thioctic acid (TA), which can simultaneously be chemically anchored to the surface of TiO₂ and methylammonium lead iodide (MAPbI₃) through coordination effects and then in situ crosslinked to form a robust continuous polymer (Poly(TA)) network after thermal treatment, can be introduced into PSCs as a new bifacial passivation agent for greatly passivating the defects. It is also discovered that Poly(TA) can additionally enhance the charge extraction efficiency and the water‐resisting and light‐resisting abilities of perovskite film. These newly discovered features of Poly(TA) make PSCs herein achieve among the best PCE of 20.4% ever reported for MAPbI₃ with negligible hysteresis, along with much enhanced ultraviolet, air, and operational stabilities. Density functional theory calculations reveal that the passivation of MAPbI₃ bulk and PTLIs by Poly(TA) occurs through the interaction of functional groups (COOH, CS) in Poly(TA) with under‐coordinated Pb²⁺ in MAPbI₃ and Ti⁴⁺ in TiO₂, which is supported by X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

The discovery of a hydrogen doping effect in correlated nickelates mediated by incoherent interfaces is reported. It increases electronic conductance by tuning the Fermi‐level in the Mott–Hubbard band. With this concept, a reconfigurable Mottronic device through defect engineering is demonstrated.

Abstract

The discovery of hydrogen‐induced electron localization and highly insulating states in d‐band electron correlated perovskites has opened a new paradigm for exploring novel electronic phases of condensed matters and applications in emerging field‐controlled electronic devices (e.g., Mottronics). Although a significant understanding of doping‐tuned transport properties of single crystalline correlated materials exists, it has remained unclear how doping‐controlled transport properties behave in the presence of planar defects. The discovery of an unexpected high‐concentration doping effect in defective regions is reported for correlated nickelates. It enables electronic conductance by tuning the Fermi‐level in Mott–Hubbard band and shaping the lower Hubbard band state into a partially filled configuration. Interface engineering and grain boundary designs are performed for H _x SmNiO₃/SrRuO₃ heterostructures, and a Mottronic device is achieved. The interfacial aggregation of hydrogen is controlled and quantified to establish its correlation with the electrical transport properties. The chemical bonding between the incorporated hydrogen with defective SmNiO₃ is further analyzed by the positron annihilation spectroscopy. The present work unveils new materials physics in correlated materials and suggests novel doping strategies for developing Mottronic and iontronic devices via hydrogen‐doping‐controlled orbital occupancy in perovskite heterostructures.

Bionic photodetectors with microcavities mimicking the compound eyes of butterflies are developed to realize selective light response in the NIR‐I biological window. Inorganic halide perovskite with comprehensive composition and film engineering is adopted as the photoactive layer. The overall device affords superior performance for practical applications.

Abstract

Fluorescence imaging with photodetectors (PDs) toward near‐infrared I (NIR‐I) photons (700–900 nm), the so‐called “optical window” in organisms, has provided an important path for tracing biological processes in vivo. With both excitation photons and fluorescence photons in this narrow range, a stringent requirement arises that the fluorescence signal should be efficiently differentiated for effective sensing, which cannot be fulfilled by common PDs with a broadband response such as Si‐based PDs. In this work, delicate optical microcavities are designed to develop a series of bionic PDs with selective response to NIR‐I photons, the merits of a narrowband response with a full width at half maximum (FWHM) of <50 nm, and tunability to cover the NIR‐I range are highlighted. Inorganic halide perovskite CsPb_0.5Sn_0.5I₃ is chosen as the photoactive layer with comprehensive bandgap and film engineering. As a result, these bionic PDs offer a signal/noise ratio of ≈10⁶, a large bandwidth of 543 kHz and an ultralow detection limit of 0.33 nW. Meanwhile, the peak responsivity (R) and detectivity (D*) reach up to 270 mA W⁻¹ and 5.4 × 10¹⁴ Jones, respectively. Finally, proof‐of‐concept NIR‐I imaging using the PDs is demonstrated to show great promise in real‐life application.

The latest breakthroughs in interface and defect engineering as applied to metal halide perovskite solar cells and light‐emitting diodes (LEDs) are reviewed in order to shed light on their necessity and importance in tuning the optoelectronic properties of devices in an attempt to realize the best‐performing solar cells and LEDs.

Abstract

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Although high power conversion efficiency of up to 23.3% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Nonradiative recombination and charge back transfer at interfaces are mainly responsible for conversion loss. Interface engineering is the most important approach toward the theoretical efficiency in PSCs.

Abstract

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Rapid advancement of perovskite solar cells confronts the challenges of reliable measurement, which is important for data analysis and results reproduction. Major measurement methods and the key factors affecting evaluation are summarized. A measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Abstract

Perovskite solar cells (PSCs) have undergone an incredibly fast development and attracted intense attention worldwide owing to their high efficiency and low‐cost fabrication. However, it is challenging to make a reliable measurement of PSCs, which creates great difficulty for researchers to compare and reproduce published results. Herein, the major measurement methods and key factors affecting evaluation of PSCs are summarized, such as hysteresis in current–voltage measurement, calibration of solar simulators for less mismatch in spectra and light intensity, and the area for the calculation of current density and power conversion efficiency. PSCs are also compared with n–i–p or p–i–n structures that exhibit different feedback under the same measurement methods. Finally, a measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Low‐toxicity tin‐based perovskites have excellent optical and electrical properties, and are a good alternative for lead‐based perovskites. However, the performance and stability of tin‐based perovskites are not comparable. The properties of tin‐based perovskite films and the performance of tin‐based perovskite solar cells are reviewed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

Abstract

The tremendous interest focused on organic–inorganic halide perovskites since 2012 derives from their unique optical and electrical properties, which make them excellent photovoltaic materials. Pb‐based halide perovskite solar cells, in particular, currently stand at a record efficiency of ≈23%, fulfilling their potential toward commercialization. However, because of the toxicity concerns of Pb‐based perovskite solar cells, their market prospects are hindered. In principle, Pb can be replaced with other less‐toxic, environmentally benign metals. Sn‐based perovskites are thus the far most promising alternative due to their very similar and perhaps even superior semiconductor characteristics. After years of effort invested in Sn‐based halide perovskites, sufficient breakthroughs have finally been achieved that make them the next runners up to the Pb halide perovskites. To help the reader better understand the nature of Sn‐based halide perovskites, their optical and electrical properties are systematically discussed. Recent progress in Sn‐based perovskite solar cells, focusing mainly on film fabrication methods and different device architectures, and highlighting roadblocks to progress and opportunities for future work are reviewed. Finally, a brief overview of mixed Sn/Pb‐based systems with their anomalous yet beneficial optical trends are discussed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

Metal‐halide perovskites have attracted much attention in recent years due to their use as light‐harvesting and light‐emitting semiconductors. Their superior optical and electronic properties, low processing cost, and the ease with which their bandgaps can be tuned suggest that they will be useful in various optoelectronic applications. This special issue aims to address the fundamentals of perovskite materials and devices, to explore recent progress in this exciting field, and to overcome the hurdles that remain to commercialization of optoelectronic perovskite devices.

In article number https://doi.org/10.1002/adma.2019044941904494, Yueh‐Lin Loo and co‐workers demonstrate a near‐infrared‐harvesting perovskite solar cell with enhanced power‐conversion efficiency as high as 21.6% and improved stability, with an operational half‐life of 1900 h, by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layers. This work provides a promising approach to prepare highly efficient and stable perovskites solar cells and opens a new application field for the rational design of narrow‐bandgap organic semiconductors.

The power conversion efficiency of inorganic perovskite solar cells (PSCs) is still low compared with hybrid PSCs. The use of lithium fluoride on SnO₂ and PbCl₂ additive in perovskite is reported for reducing the charge recombination; 18.64% efficiency of CsPbI_3–x Br _x solar cells is demonstrated; and the devices show over than 1000 h light soaking stability.

Abstract

Cesium‐based inorganic perovskite solar cells (PSCs) are promising due to their potential for improving device stability. However, the power conversion efficiency of the inorganic PSCs is still low compared with the hybrid PSCs due to the large open‐circuit voltage (V _OC) loss possibly caused by charge recombination. The use of an insulated shunt‐blocking layer lithium fluoride on electron transport layer SnO₂ for better energy level alignment with the conduction band minimum of the CsPbI_{3‐ x} Br _x and also for interface defect passivation is reported. In addition, by incorporating lead chloride in CsPbI_{3‐ x} Br _x precursor, the perovskite film crystallinity is significantly enhanced and the charge recombination in perovksite is suppressed. As a result, optimized CsPbI_{3‐ x} Br _x PSCs with a band gap of 1.77 eV exhibit excellent performance with the best V _OC as high as 1.25 V and an efficiency of 18.64%. Meanwhile, a high photostability with a less than 6% efficiency drop is achieved for CsPbI_{3‐ x} Br _x PSCs under continuous 1 sun equivalent illumination over 1000 h.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.

The power conversion efficiency of inorganic perovskite solar cells (PSCs) is still low compared with hybrid PSCs. The use of lithium fluoride on SnO₂ and PbCl₂ additive in perovskite is reported for reducing the charge recombination; 18.64% efficiency of CsPbI_3–x Br _x solar cells is demonstrated; and the devices show over than 1000 h light soaking stability.

Abstract

Cesium‐based inorganic perovskite solar cells (PSCs) are promising due to their potential for improving device stability. However, the power conversion efficiency of the inorganic PSCs is still low compared with the hybrid PSCs due to the large open‐circuit voltage (V _OC) loss possibly caused by charge recombination. The use of an insulated shunt‐blocking layer lithium fluoride on electron transport layer SnO₂ for better energy level alignment with the conduction band minimum of the CsPbI_{3‐ x} Br _x and also for interface defect passivation is reported. In addition, by incorporating lead chloride in CsPbI_{3‐ x} Br _x precursor, the perovskite film crystallinity is significantly enhanced and the charge recombination in perovksite is suppressed. As a result, optimized CsPbI_{3‐ x} Br _x PSCs with a band gap of 1.77 eV exhibit excellent performance with the best V _OC as high as 1.25 V and an efficiency of 18.64%. Meanwhile, a high photostability with a less than 6% efficiency drop is achieved for CsPbI_{3‐ x} Br _x PSCs under continuous 1 sun equivalent illumination over 1000 h.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.

Gap states present a new approach to develop multi‐photon upconversion light emission in quasi‐2D perovskite films under continuous‐wave infrared excitation. Magneto‐photoluminescence (PL) and polarization‐dependent PL reveal that the gap states are essentially spatially extended states involved in orbit–orbit interaction toward generating multi‐photon excitation in quasi‐2D perovskite films.

Abstract

A new approach to generate a two‐photon up‐conversion photoluminescence (PL) by directly exciting the gap states with continuous‐wave (CW) infrared photoexcitation in solution‐processing quasi‐2D perovskite films [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two‐photon up‐conversion PL occurring in quasi‐2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two‐photon up‐conversion PL signal. This confirms that the gap states are indeed responsible for generating the two‐photon up‐conversion PL in quasi‐2D perovskites. Furthermore, mechanical scratching indicates that the different‐n‐value nanoplates are essentially uniformly formed in the quasi‐2D perovskite films toward generating multi‐photon up‐conversion light emission. More importantly, the two‐photon up‐conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi‐photon excitation. Polarization‐dependent up‐conversion PL studies reveal that the gap states experience the orbit–orbit interaction through Coulomb polarization to form spatially extended states toward developing multi‐photon up‐conversion light emission in quasi‐2D perovskites.

An effective composite electron transport layer (ETL) is fabricated using carboxylic‐acid‐ and hydroxyl‐rich red‐carbon quantum dots to dope low‐temperature solution‐processed SnO₂. The electron mobility of SnO₂ is dramatically increased by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. A planar perovskite solar cell based on this novel SnO₂ ETL demonstrates an outstanding improvement in efficiency up to 22.77%.

Abstract

An efficient electron transport layer (ETL) plays a key role in promoting carrier separation and electron extraction in planar perovskite solar cells (PSCs). An effective composite ETL is fabricated using carboxylic‐acid‐ and hydroxyl‐rich red‐carbon quantum dots (RCQs) to dope low‐temperature solution‐processed SnO₂, which dramatically increases its electron mobility by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. The mobility achieved is one of the highest reported electron mobilities for modified SnO₂. Fabricated planar PSCs based on this novel SnO₂ ETL demonstrate an outstanding improvement in efficiency from 19.15% for PSCs without RCQs up to 22.77% and have enhanced long‐term stability against humidity, preserving over 95% of the initial efficiency after 1000 h under 40–60% humidity at 25 °C. These significant achievements are solely attributed to the excellent electron mobility of the novel ETL, which is also proven to help the passivation of traps/defects at the ETL/perovskite interface and to promote the formation of highly crystallized perovskite, with an enhanced phase purity and uniformity over a large area. These results demonstrate that inexpensive RCQs are simple but excellent additives for producing efficient ETLs in stable high‐performance PSCs as well as other perovskite‐based optoelectronics.

High‐efficiency organic solar cells are achieved through the use of a new electron acceptor AQx‐2 with a quinoxaline‐containing fused core. The increase in performance is attributed to the optimized phase separation morphology that significantly boosts hole transfer and suppresses geminate recombination. The power conversion efficiency of these devices, 16.4%, is the highest certified value for binary organic solar cells.

Abstract

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene‐electron‐acceptor‐based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx‐2 with quinoxaline‐containing fused core and PBDB‐TF as donor. The significant increase in photovoltaic performance of AQx‐2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx‐1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure–morphology–property relationship is established. The stronger π–π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx‐2‐based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next‐generation high‐performance OSCs.