Perovskite Optoelectronics Research Articles

Intrinsic Carrier Diffusion in Perovskite Thin Films Uncovered by Transient Reflectance Spectroscopy.

Carrier diffusion and surface recombination are key processes influencing the performance of conventional semiconductor devices. However, the interplay of photon recycling together with these processes in halide perovskites obfuscates our understanding. Herein, we discern these inherent processes in a thin FAPbBr3 perovskite single crystal (PSC) utilizing a unique transient reflectance technique that allows accurate diffusion modeling with clear boundary conditions. Temperature-dependent measurements reveal the coexistence of shallow and deep traps at the surface. The inverse quadratic dependence of temperature on carrier mobility μ suggests an underlying scattering mechanism arising from the anharmonic deformation of the PbBr6 cage. Our findings ascertain the fundamental limits of the intrinsic surface recombination velocity (S) and carrier diffusion coefficient (D) in PSC samples. Importantly, these insights will help resolve the ongoing debate and clarify the ambiguity surrounding the contributions of photon recycling and carrier diffusion in perovskite optoelectronics.

Photoinduced Small Hole Polarons Formation and Recombination in All-Inorganic Perovskite from Quantum Dynamics Simulation.

We conducted ab initio molecular dynamics (AIMD) and nonadiabatic MD to simulate polaron formation and recombination in all-inorganic Cs3Bi2Br9 perovskite. The meticulously designed AIMD simulations show that two types of small hole polaron, including localized and semidelocalized small hole polaron on either an intralayer or an interlayer Br dimer, are adiabatically formed within 1.71 ps. The localized small hole polaron reduces nonadiabatic coupling and decoherence time and, thus, delays charge recombination to 213 ns. In contrast, the dominant semidelocalized polaron increases nonadiabatic coupling by enhancing electron-hole overlap and restores the energy gap and decoherence time to the pristine system, accelerating recombination to 4.7 ns compared to a 10 ns charge carrier lifetime in the pristine system. All the obtained time scales agree well with experiments. The study offers a fundamental understanding of the excited-state dynamics of small hole polaron in Cs3Bi2Br9 and helps to design high-performance perovskite optoelectronics and photovoltaics.

Near-UV Phototransistors Based on an All-Inorganic Lead-Free Cs3Cu2I5/CuTCNQ Hierarchical Heterostructure

With the rapid advances in metal halide perovskite optoelectronics, eliminating toxic lead from perovskites has been an urgent demand. However, state-of-the-art lead-free perovskite photodetectors are still challenged with issues of low photoresponse, poor stability, etc. Here, all-inorganic lead-free perovskite (Cs3Cu2I5) single crystals that possess good stability under air exposure are synthesized via a facile solid reaction method. Meanwhile, a higher photoluminescence quantum yield of 95.2% and a prolonged carrier lifetime of 1.127 μs are obtained by further optimizing the synthesis. Benefiting from the polyporous surface and hollow structure of Cu-7,7,8,8-tetracyanoquinodimethane (CuTCNQ) microtubes, more Cs3Cu2I5 nanocrystals can adhere on the innershell and outershell of CuTCNQ-15 microtubes. This unique structure contributes to the improved efficiency of utilizing incident light and promotes charge carrier generation and transportation. As a result, the hierarchical CuTCNQ/Cs3Cu2I5 (hollow microtube/nanocrystal) heterostructure phototransistor exhibits a high responsivity of 88.36 A W-1 and a large detectivity of 1.66 × 1012 Jones. The proposed lead-free perovskites and mixed-dimensional heterojunctions are promising for sensitive light detection.

Highly Fluorescent Collagen-Based Quantum Dots as an Efficient Interlinkage in the 2D Perovskite Bulk for Improved Solar Cells.

A design-inexpensive, effective, and easy-to-prepare additive in the large-scale preparation of perovskite solar cells (PSCs) is urgently desired to alleviate the future energy crisis. Carbon-based quantum dots have demonstrated novel nanomaterials with excellent chemical stability and high electrical conductivity, which exhibit great potential as additives for perovskite optoelectronics. Herein, we designed novel highly fluorescent collagen-based quantum dots (Col-QDs) and thoroughly studied the micromorphological characteristics, photoluminescence properties, and the states of surface-functionalized groups on the Col-QDs. It is found that the introduction of Col-QDs in the two-dimensional (2D) perovskite precursor can be further confirmed as an efficient interlinkage via Col-Pb bands in the pure 2D perovskite heterojunction, which significantly improves the crystallinity, orientation, and interlayer coupling of perovskite crystal plates, as observed by grazing incidence X-ray diffraction (GIWAXS) and X-ray photoelectron spectroscopy (XPS). Finally, the champion Col-QD additive can efficiently modulate the photovoltaic performance of pure 2D PSCs with a significant increase of photoelectric conversion efficiency (PCE) from 8.18% up to 10.45%, which ranks among the best efficiencies of highly pure 2D PSCs. These results provide a facile and feasible approach to modulate the interlayer interaction of pure 2D perovskites and further improve their output of PSCs, which would further facilitate the burgeoning applications of the Col-QDs in various perovskite-based optical-related fields.

A biomineralization-inspired strategy of self-encapsulation for perovskite solar cells

Solution-processed perovskite films are promising for the cost-effective fabrication of optoelectronic devices. However, the poor environmental stability of perovskite materials hampers their commercial development, and a challenge remains in the universal technology for self-encapsulating perovskite films through controlling the crystallization. Herein, we designed a strategy of crystal growth assisted by the polymer bottom-up dynamic diffusion and created the self-encapsulated perovskite films with preferred crystalline orientation by mimicking the typical process of biomineralization. Thin polymer distributed at the surface, buried interface, and grain boundaries facilitated the efficient defect passivation and self-encapsulation of the entire film, and also, the coordination interaction between polymer and perovskite aligned the interface energy band. These collaborative advantages endowed perovskite solar cells (PSCs) with power conversion efficiency (PCE) of 22.90 %. In addition, the self-encapsulated perovskite films exhibited negligible decomposition and PCE remained 90 % after being stored in ambient air (30–50 % humidity) for 90 days. This universal strategy of biomineralization-inspired polymer self-encapsulating opens up a new avenue for efficient and stable perovskite optoelectronics.

Micro‐Nano Structure Functionalized Perovskite Optoelectronics: From Structure Functionalities to Device Applications

AbstractMetal halide perovskite, an emerging photosensitive semiconductor, has been widely employed in solar cells, light‐emitting diodes, photodetectors, and lasers owing to its excellent photophysical properties and simple solution preparation processing. However, as a photoactive layer, the higher refractive index and thinner thickness of perovskite film can cause reflection and transmission at the interface, and confine the emitted light within devices, resulting in the poor incident photon absorption and emitted photon extraction. In addition, the intrinsic brittleness of perovskite material restricts its potential applications in flexible optoelectronics. Therefore, great effort has been put into micro‐nano structured perovskite optoelectronics, and the reported reviews mainly focus on the fabrication process of micro‐nano patterned perovskite. Herein, the functionalities of micro‐nano structures in optoelectronics, including improving the light trapping, light extraction, light modulation, carrier dynamics, mechanical robustness, and other novel functionalities, are comprehensively reviewed. The specific applications of these functionalities in perovskite‐based optoelectronic devices are then discussed in detail to provide a better understanding of the photophysical properties of micro‐nano structure functionalized optoelectronics. Finally, promising strategies to promote the multifunctional commercial applications of micro‐nano structured perovskite optoelectronics are provided.

Vacuum‐Assisted Thermal Annealing of CsPbI3 for Highly Stable and Efficient Inorganic Perovskite Solar Cells

AbstractInorganic cesium lead iodide perovskite CsPbI3 is attracting great attention as a light absorber for single or multi‐junction photovoltaics due to its outstanding thermal stability and proper band gap. However, the device performance of CsPbI3‐based perovskite solar cells (PSCs) is limited by the unsatisfactory crystal quality and thus severe non‐radiative recombination. Here, vacuum‐assisted thermal annealing (VATA) is demonstrated as an effective approach for controlling the morphology and crystallinity of the CsPbI3 perovskite films formed from the precursors of PbI2, CsI, and dimethylammonium iodide (DMAI). By this method, a large‐area and high‐quality CsPbI3 film is obtained, exhibiting a much reduced trap‐state density with prolonged charge lifetime. Consequently, the solar cell efficiency is raised from 17.26 to 20.06 %, along with enhanced stability. The VATA would be an effective approach for fabricating high‐performance thin‐film CsPbI3 perovskite optoelectronics.

Vacuum-Assisted Thermal Annealing of CsPbI3 for Highly Stable and Efficient Inorganic Perovskite Solar Cells.

Inorganic cesium lead iodide perovskite CsPbI3 is attracting great attention as a light absorber for single or multi-junction photovoltaics due to its outstanding thermal stability and proper band gap. However, the device performance of CsPbI3 -based perovskite solar cells (PSCs) is limited by the unsatisfactory crystal quality and thus severe non-radiative recombination. Here, vacuum-assisted thermal annealing (VATA) is demonstrated as an effective approach for controlling the morphology and crystallinity of the CsPbI3 perovskite films formed from the precursors of PbI2 , CsI, and dimethylammonium iodide (DMAI). By this method, a large-area and high-quality CsPbI3 film is obtained, exhibiting a much reduced trap-state density with prolonged charge lifetime. Consequently, the solar cell efficiency is raised from 17.26 to 20.06 %, along with enhanced stability. The VATA would be an effective approach for fabricating high-performance thin-film CsPbI3 perovskite optoelectronics.

4‐Hydrazinobenzoic‐Acid Antioxidant for High‐Efficiency Sn–Pb Alloyed Perovskite Solar Cells

Partial tin replacement of lead is expected to be an effective means to fine‐tune the bandgap of Sn–Pb alloyed lead‐halide perovskite to harvest near‐infrared light and thus further increase the efficiency of solar cells based on it, in particular for use as a bottom component cell in the tandem cell design to break through the theoretical Shockley–Queisser (S–Q) limit of the single‐junction solar cells. However, the efficiency of Sn–Pb alloyed perovskite solar cells (PSCs) is still lower than expected owing to the easy oxidation of Sn2+. Herein, a reducing agent 4‐hydrazinobenzoic acid is developed as an additive to suppress the oxidation of Sn2+ and, meanwhile, passivate surface defects. It is found that the optoelectronic performance of Sn–Pb alloyed perovskites is improved as evidenced by the mitigated trap state density and suppressed nonradiative recombination. As a result, the Sn–Pb alloyed PSC efficiency is increased to 21.09%, one of the highest for cells with this bandgap. It is expected that this method is applicable for general Sn–Pb‐based perovskite optoelectronics.

Enhancing the Intrinsic and Extrinsic Stability of Halide Perovskite Nanocrystals for Efficient and Durable Optoelectronics

Over the past few years, metal halide perovskite nanocrystals have been at the forefront of colloidal semiconductor nanomaterial research because of their fascinating properties and potential applications. However, their intrinsic phase instability and chemical degradation under external exposures (high temperature, water, oxygen, and light) are currently limiting the real-world applications of perovskite optoelectronics. To overcome these stability issues, researchers have reported various strategies such as doping and encapsulation. The doping improves the optical and photoactive phase stability, whereas the encapsulation protects the perovskite NCs from external exposures. This perspective discusses the rationale of various strategies to enhance the stability of perovskite NCs and suggests possible future directions for the fabrication of optoelectronic devices with long-term stability while maintaining high efficiency.

Manipulating Ion Migration and Interfacial Carrier Dynamics via Amino Acid Treatment in Planar Perovskite Solar Cells.

Instability caused by the migrating ions is one of the major obstacles toward the large-scale application of metal halide perovskite optoelectronics. Inactivating mobile ions/defects via chemical passivation, e.g., amino acid treatment, is a widely accepted approach to solve that problem. To investigate the detailed interplay, L-phenylalanine (PAA), a typical amino acid, is used to modify the SnO2/MAPbI3 interface. The champion device with PAA treatment maintains 80% of its initial power conversion efficiency (PCE) when stored after 528 h in an ambient condition with the relative humidity exceeding 70%. By employing a wide-field photoluminescence imaging microscope to visualize the ion movement and calculate ionic mobility quantitatively, we propose a model for enhanced stability in perspective of suppressed ion migration. Besides, we reveal that the PAA dipole layer facilitates charge transfer at the interface, enhancing the PCE of devices. Our work may provide an in-depth understanding toward high-efficiency and stable perovskite optoelectronic devices.

Understanding steric-charge-dependence of conjugated passivators on π-Pb2+ bond strength for efficient all-inorganic perovskite solar cells

Intramolecular structure of interfacial passivator highly determines the bond strength with detrimental defects located at perovskite surfaces and grain boundaries. Herein, we investigate the steric-charge-dependence of various π-conjugated pyridine derivatives on electron-donating ability to passivate inorganic CsPbIBr2 perovskite film. The primary results demonstrate that the photovoltaic performances tightly correlate with the multidentate coordination sites and structure-steric hindrance because of their synergistic regulation on intramolecular electron-cloudy density and charge extraction. Comparing to pyridine and terpyridine treated devices, the bipyridine passivated device delivers an increased efficiency of 11.04% with a high open-circuit voltage of 1.301 V and improved stability after persistent irradiation over 20 h and storage for 60 days, which is one of the highest efficiencies for carbon-based all-inorganic CsPbIBr2 solar cells. This work provides a direction to choose ideal passivator for high-efficiency perovskite optoelectronics.

2-CF3-PEAI to eliminate Pb0 traps and form a 2D perovskite layer to enhance the performance and stability of perovskite solar cells

As a deep level trap in perovskite, metallic lead (Pb0) greatly deteriorates the performances of perovskite optoelectronics such as perovskite solar cells. Here, we designed 2-trifluoromethyl-phenylethylamine hydroiodide (2-CF3-PEAI) to serve as a multifunctional trap deactivating reagent which can react with metallic lead to eliminate deep level Pb0 traps, forming a thin layer of 2D perovskite (2-CF3-PEA)2PbI4 atop the 3D perovskite without impediment of charge carrier transport. Moreover, functional groups in 2-CF3-PEAI can deactivate a variety of traps in perovskite by multiple interactions, i.e. coordination between I- and Pb2+, hydrogen bond between CF3 and formamidine, ionic interaction between NH3+ group and lead halide anion etc. Thus, non-radiative recombination was dramatically inhibited. The surface coverage of hydrophobic thin 2D perovskite (2-CF3-PEA)2PbI4 layer impedes the diffusion of metal (from anode) and humidity (from the environment) to 3D perovskite, leading to improved device stabilities. As a result, a 23.17% power conversion efficiency was achieved and an enhanced T80 (~850 h) was obtained under continuous light-soaking without encapsulation, 121 times longer than that of the untreated devices.

Substitutional alkaline earth metals delay nonradiative charge recombination in CH3NH3PbI3 perovskite: A time-domain study.

Experiments reported that alkaline earth metal dopants greatly prolong carrier lifetime and improve the performance of perovskite solar cells. Using state-of-the-art ab initio time-domain nonadiabatic molecular dynamics (NAMD), we demonstrate that incorporation of alkaline earth metals, such as Sr and Ba, into MAPbI3 (MA = CH3NH3 +) lattice at the lead site is energetically favorable due to the largely negative formation energies about -7 eV. The replacement widens the bandgap and increases the open-circuit voltage by creating no trap states. More importantly, the substitution reduces the mixing of electron and hole wave functions by pushing the hole charge density away from the dopant together with no contribution of Sr and Ba to the conduction band edge state, thus decreasing the NA coupling. The high frequency phonons generated by enhanced atomic motions and symmetry breaking accelerate phonon-induced loss of coherence. The synergy of the three factors reduces the nonradiative recombination time by a factor of about 2 in the Sr- and Ba-doped systems with respect to pristine MAPbI3, which occurs over 1 ns and agrees well with the experiment. The study highlights the importance of various factors affecting charge carrier lifetime, establishes the mechanism of reduction of nonradiative electron-hole recombination in perovskites upon alkaline earth metal doping, and provides meaningful insights into the design of high performance of perovskite solar cells and optoelectronics.

Ice Assisted Electron-Beam Lithography for Halide Perovskite Optoelectronic Nanodevices

Ice Assisted Electron-Beam Lithography for Halide Perovskite Optoelectronic Nanodevices

Columnar liquid crystals as stability enhancing hole transport layers for Ruddlesden–Popper perovskite optoelectronics

The stability of Ruddlesden–Popper perovskite (RPP) films is studied. A triphenylene discotic liquid crystal is compared with spiro-OMeTAD as an alternative hole transport layer. Results show improvement in the stability of the RPPs.

Impacts of the Electron Transport Layer Surface Reconstruction on the Buried Interface in Perovskite Optoelectronic Devices.

Using density functional theory combined with ab initio molecular dynamics, we comprehensively investigated the performance enhancement mechanism of the device after surface reconstruction by passivating different halogen groups (i.e., F or Cl) at the ETL/perovskite interface. We demonstrated that the halogen group at the ETL layer could stabilize the geometric structure of the perovskite surface by balancing the interfacial interaction, ionic migration, and lead iodide framework. Even though halogen passivation decreased and increased the interface charge transfer at the O- and SnO-terminated MAPbI3/SnO2 interfaces, respectively, halogen passivation optimized surface reconstruction and could theoretically relieve the interface carrier recombination according to the changes in conduction band offsets generated by halogen passivation. Furthermore, the interfacial carrier recombination of the MAPbI3/SnO2 interface was also connected to the interfacial gap states, which were smaller for O-terminated MAPbI3/SnO2 interfaces with halogen passivation-induced surface reconstruction but larger for the SnO-terminated cases. Hence, our findings have implications for the design of buried interface optimization in perovskite optoelectronic devices.

Micro–Nanostructure‐Assisted Luminescence in Perovskite Devices

Micro–Nanostructure-Assisted Luminescence The micro-nanostructure engineering provides an effective strategy for improving the luminescent characteristics of perovskite materials and optoelectronics. In article number 2100084, Yan-Qing Li, Jian-Xin Tang, and co-workers summarize the recent developments of micro–nanostructure-assisted optical manipulation approaches to enhance photo/electroluminescence of perovskite emitters, light-outcoupling in light-emitting diodes, and stimulated light emission for perovskite lasers.

Trifluoromethyl‐Group Bearing, Hydrophobic Bulky Cations as Defect Passivators for Highly Efficient, Stable Perovskite Solar Cells

Solution‐processed perovskite films are rich in surface defects and grain boundaries, which limits their performance and stability in photovoltaic application. Surface passivation using bulky organic cations can effectively reduce the surface defects of a perovskite film without affecting its fundamental properties. Herein, the use of hydrophobic bulky aromatic molecules, namely 4‐trifluoromethyl‐benzylammonium iodide/bromide (CF3BZA‐I/Br), as defect‐passivators to heal the surface defects and grain boundaries of perovskite films is introduced. Owing to the presence of the trifluoromethyl (CF3) moieties, CF3BZA‐I/Br‐passivated perovskite films exhibit a hydrophobic surface with significantly fewer grain boundaries. By suppressing the surface and interfacial imperfections, CF3BZA‐Br‐treated perovskite solar cells achieve an outstanding power conversion efficiency (PCE) of 20.75%. The PCE improvement originates mainly from the reduction of trap states and nonradiative carrier recombination. The ultrathin hydrophobic barrier layer formed after passivation also shields the perovskite film surface from moisture ingress and environmental degradation, leading to improved stability of the devices. By optimizing the passivation conditions, the bulky CF3BZA‐I/Br molecules could be the ideal defect passivators, with versatile applications in a wide variety of perovskite optoelectronics.

Machine learning for high-throughput experimental exploration of metal halide perovskites

<h2>Summary</h2> Metal halide perovskites (MHPs) have catapulted to the forefront of energy research due to the unique combination of high device performance, low materials cost, and facile solution processability. A remarkable merit of these materials is their compositional flexibility allowing for multiple substitutions at all crystallographic sites, and hence thousands of possible pure compounds and virtually a near-infinite number of multicomponent solid solutions. Harnessing the full potential of MHPs necessitates rapid exploration of multidimensional chemical space toward desired functionalities. Recent advances in laboratory automation, ranging from bespoke fully automated robotic labs to microfluidic systems and to pipetting robots, have enabled high-throughput experimental workflows for synthesizing MHPs. Here, we provide an overview of the state of the art in the automated MHP synthesis and existing methods for navigating multicomponent compositional space. We highlight the limitations and pitfalls of the existing strategies and formulate the requirements for necessary machine learning tools including causal and Bayesian methods, as well as strategies based on co-navigation of theoritical and experimental spaces. We argue that ultimately the goal of automated experiments is to simultaneously optimize the materials synthesis and refine the theoretical models that underpin target functionalities. Furthermore, the near-term development of automated experimentation will not lead to the full exclusion of human operator but rather automatization of repetitive operations, deferring human role to high-level slow decisions. We also discuss the emerging opportunities leveraging machine learning-guided automated synthesis to the development of high-performance perovskite optoelectronics.