Organic-inorganic Lead Halide Research Articles

Efficient and Stable Perovskite Solar Cells Based on Inorganic Hole Transport Materials.

Although power conversion efficiencies of organic-inorganic lead halide perovskite solar cells (PSCs) are approaching those of single-crystal silicon solar cells, the working device stability due to internal and external factors, such as light, temperature, and moisture, is still a key issue to address. The current world-record efficiency of PSCs is based on organic hole transport materials, which are usually susceptible to degradation from heat and diffusion of dopants. A simple solution would be to replace the generally used organic hole transport layers (HTLs) with a more stable inorganic material. This review article summarizes recent contributions of inorganic hole transport materials to PSC development, focusing on aspects of device performance and long-term stability. Future research directions of inorganic HTLs in the progress of PSC research and challenges still remaining will also be discussed.

One-dimensional corner-sharing perovskites: Syntheses, structural evolutions and tunable photoluminescence properties

In recent years, organic-inorganic hybrid lead halide perovskites have emerged as a highly promising class of light emitting diodes (LEDs) due to their diverse structures and promising properties. Herein, by choosing various organic molecules as cations, we successfully constructed a series of 1D perovskites of [PMDETA]PbBr5, [PMDETA]PbCl5, [DMPZ][MA]PbBr5, [EEDA][MA]PbCl5, [EPZ][MA]PbCl5 and [EPZ]3Pb2Br10. Under the different structural directing effects of organic cations, these 1D perovskites feature three different kinds of linear or zigzag 1D [PbX5]3− chains based on purely corner-shared octahedrons. As expected, these 1D perovskites exhibit tunable broadband light emissions from blue to yellow spectral range stemming from the self-trapped excitons based on systematical characterizations including time-resolved, temperature-dependent PL emissions and theoretical calculations. This work provides crystal structural engineering for low-dimensional perovskites and paves a new way to regulate the luminescent properties.

Construction of nanostructured CH3NH3PbI3 layer for high-performance perovskite solar cells by Ar plasma etching

Organic-inorganic lead halide perovskite materials have attracted wide attention owing to their great optoelectronic properties and low-temperature solution processability. Mesostructured perovskite solar cells (PSCs) could help to further improve the photovoltaic property. However, the existing approaches by inserting a layer of metal-oxide are generally need special treatment and consume much energy. In this work, an Ar plasma etching method is employed to modify composition of the perovskite absorb material with its morphology remain unchanged, which can effectively avoid unfavorable defects and enhance the charge separation efficiency. Based on controllable pore structure and porosity, the idiosyncratic mesostructured perovskite solar cells yield an 18.7% improvement in power conversion efficiency as compared with the common planar-type counterparts. Therefore, this method offers a room temperature scheme for high-performance mesostructured perovskite solar cells, making the flexible PSC possible.

Resonant Degenerate Four-Wave Mixing at the Defect Energy Levels of 2D Organic-Inorganic Hybrid Perovskite Crystals.

Two-dimensional organic-inorganic lead halide perovskites are generating great interest due to their optoelectronic characteristics such as high solar energy conversion efficiency and a tunable direct band gap in the visible regime. However, the presence of defect states within the two-dimensional crystal structure can affect these properties, resulting in changes to their band gap emission as well as the emergence of nonlinear optical phenomena. Here, we have investigated the effects of the presence of defect states on the nonlinear optical phenomena of the 2D hybrid perovskite (BA)2(MA)2Pb3Br10. When two pulses, one narrowband pump pulse centered at 800 nm and one supercontinuum pulse with bandwidth from 800-1100 nm, are incident on a perovskite flake, degenerate four-wave mixing (FWM) occurs, with peaks corresponding to the energy levels of the defect states present within the crystal. The longer carrier lifetime of the defect state, in comparison to that of virtual transitions that take place in nonresonant FWM processes, allows for a larger population of electrons to be excited by the second pump photon, resulting in increased FWM signal at the defect energy levels. The quenching of the two-photon luminescence as flake thickness increases is also observed and attributed to the increased presence of defects within the flake at larger thicknesses. This technique shows the potential of detecting defect energy levels in crystals using FWM for a variety of optoelectronic applications.

A One-Dimensional Broadband Emissive Hybrid Lead Iodide with Face-Sharing PbI6 Octahedral Chains.

One-dimensional (1D) organic-inorganic hybrid lead halides with unique core-shell quantum wire structures and splendid photoluminescence properties have been considered one of the most promising high-efficiency broadband emitters. However, studies on the broadband emissions in 1D purely face-shared lead iodide hybrids are still rare so far. Herein, we report on a new 1D lead iodide hybrid, (2cepyH)PbI3 (2cepy = 1-(2-chloroethyl)pyrrolidine), characterized with face-sharing PbI6 octahedral chains. Upon UV photoexcitation, this material shows broadband yellow emissions originating from the self-trapped excitons associated with distorted Pb-I lattices on account of the strong exciton-phonon coupling, as proved by variable-temperature emission spectra. Moreover, experimental and calculated results reveal that (2cepyH)PbI3 is an indirect bandgap semiconductor, the band structures of which are governed by inorganic parts. Our work represents the first broadband emitter based on a 1D face-shared lead iodide hybrid and opens a new way to obtain the novel broadband emission materials.

First-principles calculation on electronic properties of Bismuth-halide inorganic perovskites for solar cell

Solar energy is a commonly used alternate source of energy and it can be utilized based on the principle of the photovoltaic effect. The photovoltaic effect converts sun energy into electrical energy using photovoltaic devices (solar cells). A solar cell device should have high efficiency and a long lifetime to be commercially beneficial. Presently, silicon and thin-film solar cells are widely employed. The crystalline solar cells are more efficient but they are also expensive. Thin-film solar cells are formed by placing one or more thin layers of photovoltaic materials on different substrates. Although these cells have a lower cost, they are also less efficient compared to Si-based solar cells. Organic-inorganic hybrid lead halide perovskite solar cells are one of the most promising low-cost power conversion efficiency technologies that could exceed the 26% threshold. However, the lack of environmental stability and of high lead toxicity are the main bottlenecks that impede the future industrialization and commercialization hybrid lead halide perovskite. Hence It is important to achieve high power conversion efficiency while also maintaining stability and non-toxicity in the development of new lead-free perovskite materials.

Limitations and solutions for achieving high-performance perovskite tandem photovoltaics

The large light absorption coefficient, long carrier diffusion length, and high defect tolerance enable organic-inorganic lead halide perovskites for excellent photovoltaic performance. The highest certified power conversion efficiency of single-junction perovskite solar cells (PSCs) has reported 25.5%. Besides, the bandgap of perovskites can be tuned by engineering their composition. These merits have made perovskites promising candidates for tandem photovoltaics, which can cross over the Shockley-Queisser limit of single-junction PSCs with economic costs. However, there are yet some hurdles in the wide-bandgap and narrow-bandgap subcells as well as interconnected layers (ICLs), which limit the commercial applications of perovskite tandem solar cells (PTSCs). In this review, we summarize the major scientific and technical limitations of PTSCs. We firstly demonstrate the configurations and working principles of PTSCs. Then, the developments of front subcells and rear subcells are discussed. Their main drawbacks, implemented technologies, and underlying mechanisms are analyzed in detail. Subsequently, the progress of ICLs which are responsible for guaranteeing continuous current in 2 T PTSCs are discussed. In addition, the stability of PTSCs is also summarized. The purpose of this review is to map and thrive the future development of PTSCs.

Reversible photochromic and photoluminescence in iodide perovskites

Color imaging experiment demonstrates a photochromic behavior of mixed-halide perovskites upon dark and light excitation, while the color emitted by the crystals changes from red to yellow and vice versa with the intensity of excitation light source. Unlike the photochromic property commonly observed in solution-state polymer materials, solid-state perovskite materials with photochromic behavior might open wide applications in consumer products and electronic devices. In the next part, we report on a reversible photoluminescence (PL) peak in iodide-based organic-inorganic lead halide perovskite materials under a 2-photon absorption process, while tuning the excitation wavelength. Intriguingly, two shorter wavelength peaks are visible and become prominent when the excitation photon energy is being tuned in the high energy spectrum, while laser power is remained constant. The same phenomenon of reversible PL peak is also observed in various iodine-based organic-inorganic halides as well as all-inorganic perovskite single crystals and polycrystals. We attribute to the reversible PL peak phenomenon to the photoinduced structural deformation and the associated change in the optical bandgap of iodide perovskites under the femtosecond laser excitation. Our findings will introduce a degree of freedom in future research as well as adding functionalities to optoelectronic applications in these emerging perovskite materials-based devices.

Robust Ultralong Lead Halide Perovskite Microwire Lasers.

Hybrid organic-inorganic lead halide perovskite microwires are potential building blocks for realizing on-chip integrated optoelectronic devices. However, the length-controllable synthesis of one-dimensional hybrid perovskite microwires has been rarely reported, especially the ones with lengths in the millimeter scale. Herein, methylammonium lead bromide (MAPbBr3) and formamidinium lead bromide (FAPbBr3) micro and milliwires are demonstrated using single-crystal PbBr2 microwires synthesized via template-free solution-phase growth as the lattice framework. Following the PbBr2 template, the as-converted perovskite microwires possess controllable lengths ranging from tens to thousands of micrometers. In addition, Fabry-Perot (FP) lasing was realized in both MAPbBr3 and FAPbBr3 microwires, attesting to their excellent crystal quality and the efficient optical confinement of the natural cavity. These unique properties result in the first demonstration of FP perovskite microwire lasers with submillimeter lengths. More interestingly, the microwire lasers show excellent photostability under repetitive pulsed laser excitation for over 8 × 106 cycles. Such findings demonstrate that the solution-converted hybrid lead bromide microwires have excellent optoelectronic performances promising for practical applications, and the size controllability indicates that this novel fabrication process may be feasible for large-scale industrial production.

Single-atom Pt-I3 sites on all-inorganic Cs2SnI6 perovskite for efficient photocatalytic hydrogen production

Organic-inorganic lead halide perovskites are a new class of semiconductor materials with great potential in photocatalytic hydrogen production, however, their development is greatly plagued by their low photocatalytic activity, instability of organic component and lead toxicity in particular. Herein, we report an anti-dissolution environmentally friendly Cs2SnI6 perovskite anchored with a new class of atomically dispersed Pt-I3 species (PtSA/Cs2SnI6) for achieving the highly efficient photocatalytic hydrogen production in HI aqueous solution at room temperature. Particularly, we discover that Cs2SnI6 in PtSA/Cs2SnI6 has a greatly enhanced tolerance towards HI aqueous solution, which is very important for achieving excellent photocatalytic stability in perovskite-based HI splitting system. Remarkably, the PtSA/Cs2SnI6 catalyst shows a superb photocatalytic activity for hydrogen production with a record turnover frequency of 70.6 h−1per Pt, about 176.5 times greater than that of Pt nanoparticles supported Cs2SnI6 perovskite, along with superior cycling durability. Charge-carrier dynamics studies in combination with theory calculations reveal that the dramatically boosted photocatalytic performance on PtSA/Cs2SnI6 originates from both unique coordination structure and electronic property of Pt-I3 sites, and strong metal-support interaction effect that can not only greatly promote the charge separation and transfer, but also substantially reduce the energy barrier for hydrogen production. This work opens a new way for stimulating more research on perovskite composite materials for efficient hydrogen production.

Fighting Health Hazards in Lead Halide Perovskite Optoelectronic Devices with Transparent Phosphate Salts.

Organic-inorganic lead halide perovskite (CH3NH3PbI3) solar cells have surpassed 25% power conversion efficiency, being ready for industrial-scale production of cheap photovoltaic (PV) panels. In this action, the major hurdle is its lead content, which in case of device failure, could be washed into the soil, entering the food chain. Since there is a zero tolerance on lead in the human organism, this health hazard is a critical obstacle for commercialization. Here, we propose a solution to this problem by incorporating phosphate salts (e.g., (NH4)2HPO4) in PV and other perovskite-based optoelectronic devices in various architectures. Phosphate salts do not react with CH3NH3PbI3 and do not alter its advantageous optoelectronic properties, but in a wet environment, they react immediately with lead, forming a highly insoluble compound, precluding this way the spread of lead into the environment. It is expected that this study will stimulate research, enabling lead halide perovskite solar cells to reach a similar environmental risk category as the commercially available, nonwater-soluble heavy metal-containing CdTe and gallium diselenide technologies.

Unraveling the degradation process of 2D passivated and Cs stabilized FAPbI3 by optical pump THz probe spectroscopy

The carrier dynamics and time-dependent photo-physical properties in the degradation period of hybrid organic-inorganic lead halide perovskites (HOIPs) are still unclear. Here a non-contact optical pump THz probe measurement was performed to observe the long-term degradation process of 2D passivated and cesium stabilized formamidinium lead iodide perovskite (FAPbI3) films. We find that photoconductivity and carrier mobility of the FA0.9Cs0.1PbI3 film decay steadily, while these of the 2D phenylethylammonium lead iodide (PEA2PbI4) passivated FAPbI3 film show an accelerated decay rate. Moreover, it’s also revealed that the localization of charge carriers increases with the transformation process from the fitted results by the Drude-Smith model. These results indicate that the stabilized effect of Cs incorporation is continuous, while the passivated effect of PEA2PbI4 can be weakened by the newly emerged phase boundaries. This report proposes a new perspective and sheds light on the degradation process of HOIPs.

Simulations of Trions and Biexcitons in Layered Hybrid Organic-Inorganic Lead Halide Perovskites.

Behaving like atomically precise two-dimensional quantum wells with non-negligible dielectric contrast, the layered hybrid organic-inorganic lead halide perovskites (HOIPs) have strong electronic interactions leading to tightly bound excitons with binding energies on the order of 500meV. These strong interactions suggest the possibility of larger excitonic complexes like trions and biexcitons, which are hard to study numerically due to the complexity of the layered HOIPs. Here, we propose and parametrize a model Hamiltonian for excitonic complexes in layered HOIPs and we study the correlated eigenfunctions of trions and biexcitons using a combination of diffusion MonteCarlo and very large variational calculations with explicitly correlated Gaussian basis functions. Binding energies and spatial structures of these complexes are presented as a function of the layer thickness. The trion and biexciton of the thinnest layered HOIP have binding energies of 35 and 44meV, respectively, whereas a single exfoliated layer is predicted to have trions and biexcitons with equal binding energies of 48meV. We compare our findings to available experimental data and to that of other quasi-two-dimensional materials.

Dialkylamines Driven Two-Step Recovery of NiOx/ITO Substrates for High-Reproducibility Recycling of Perovskite Solar Cells.

Because of the toxicity of water-soluble lead, the recycling of organic-inorganic lead-halides perovskite solar cells (PSCs) has attracted increasing attention. Here, we report a highly reliable two-step process to recycle cost-dominated indium-tin-oxide (ITO) substrates coated with NiOx and regenerate their based PSCs by function of dialkylamines. The champion recycled PSC can achieve 20% in conversion-efficiency, higher than 17.92% of the fresh one. Strikingly, the regenerated devices can remain superior to the fresh ones in the first 7 of 10 recycles. The comprehensive X-ray photoelectronic spectroscopy analysis reveals that dipropylamine has a suitable interaction with NiOx surfaces by Ni-N coordination, enabling its effective interfacial passivation and template effect of high-quality growth of perovskites. That leads to the suppressed nonradiative recombination of both interfacial and bulk, and finally improves the device performances. The dialkylamines driven two-step recycling process offers a promising and highly reproducible strategy to recycle PSCs, especially the cost-dominated NiOx/ITO substrates.

Efficient Dual-Band White-Light Emission with High Color Rendering from Zero-Dimensional Organic Copper Iodide.

Broad-band white-light emissions from organic-inorganic lead halide hybrids have attracted considerable attention in energy-saving solid-state lighting (SSL) applications. However, the toxicity of lead in these hybrids hinders their commercial prospects, and the low photoluminescence quantum yields (PLQYs) cannot meet the requirements for efficient lighting. Here, we report a highly efficient dual-band white-light emission from organic copper iodide, (C16H36N)CuI2, which exhibits a high PLQY of 54.3% and excellent air stability. The single-crystalline (C16H36N)CuI2 possesses a unique zero-dimensional (0D) structure, in which the isolated [Cu2I4]2- dimers are periodically embedded in the wide band gap organic framework of C16H36N+. This perfect 0D structure can cause significant quantum confinement and strong electron-phonon coupling, which contributes to efficient emissions from self-trapped excitons (STEs). Photophysical studies revealed the presence of two self-trapped emitting states in [Cu2I4]2- dimers, whose populations are highly sensitive to the temperature that governs the molecular environment for [Cu2I4]2- dimers and the thermal activation energy of STEs. An ultraviolet (UV) excited white light-emitting diode fabricated using this single-phase white-light emitter exhibits a high color rendering index (CRI) of 78. The new material provides a promising emitter, having a high PLQY and a high CRI simultaneously, for SSL and display applications.

A polymer controlled nucleation route towards the generalized growth of organic-inorganic perovskite single crystals

Recently, there are significant progresses in the growth of organic-inorganic lead halide perovskite single crystals, however, due to their susceptible nucleation and growth mechanisms and solvent requirements, the efficient and generalized growth for these single crystals is still challenging. Here we report the work towards this target with a polymer-controlled nucleation process for the highly efficient growth of large-size high-quality simple ternary, mixed-cations and mixed-halide perovskite single crystals. Among them, the carrier lifetime of FAPbBr3 single crystals is largely improved to 10199 ns. Mixed MA/FAPbBr3 single crystals are synthesized. The crucial point in this process is suggested to be an appropriate coordinative interaction between polymer oxygen groups and Pb2+, greatly decreasing the nuclei concentrations by as much as 4 orders of magnitudes. This polymer-controlled route would help optimizing the solution-based OIHPs crystal growth and promoting applications of perovskite single crystals.

The effects of crystal structure on the photovoltaic performance of perovskite solar cells under ambient indoor illumination

Organic-inorganic hybrid lead halide perovskite materials with the general formula MAPbI3-nBrn (n = 0, 1, 2, and 3) exhibiting a range of crystal structures and a wide range of optical bandgaps are explored for their potential to harvest energy from ambient light sources. The replacement of I− with Br− is found to transform the perovskite crystal structure from the tetragonal for MAPbI3 to the pseudo cubic for MAPbI2Br and MAPbIBr2 and cubic for MAPbBr3 while increasing the optical bandgap from 1.59 eV to 2.31 eV. Photovoltaic cells (PVCs) are constructed using these perovskites and are found to exhibit power conversion efficiencies (PCEs) of 29.83 (MAPbI3), 23.75 (MAPbI2Br), 21.47 (MAPbIBr2), and 19.94% (MAPbBr3) under LED light illumination (1000 lux; ~0.371 mW cm−2), with a record high open-circuit voltage (VOC) of 1.15 V being observed for the MAPbBr3. The effect of crystal structure variation on the charge carrier mobility, trap states, recombination losses, photovoltaic properties, and operational stabilities has been investigated systematically under indoor illumination. Overall, this work provides a clear vision on the influence of perovskite crystal structures upon the photovoltaic properties and shows a pathway to achieve high VOC in perovskite PVCs under low-intensity ambient light sources.

Spray deposition of NiOx hole transport layer and perovskite photoabsorber in fabrication of photovoltaic mini-module

Organic-inorganic lead halide perovskite solar cells have received global attention, and their upscaling to perovskite solar modules under ambient conditions merits investigation. Herein, an upscaled mini-module consisting of five serially interconnected cells demonstrate a power conversion efficiency of 6.18% measured on an aperture area of 10.4 cm2. The perovskite photovoltaic mini-module with an inverted configuration is realized on a 5 × 5.5 cm2 ITO glass substrate using a technique combining ultrasonic spray-coating of both the NiOx hole transport layer (HTL) and perovskite photoabsorber layer with mechanical scribing for module patterning. The influence of the post-annealing duration (3, 5, 8, 10, and 15 min) of the spray-coated NiOx films under optimized fabrication conditions of the perovskite mini-module is investigated. The bulk properties of the NiOx thin films as functions of annealing time and their interfacial properties with methylammonium lead iodide (MAPbI3) are comprehensively discussed. The use of spray-coating to assemble the NiOx HTL/perovskite photoactive layer of the mini-module demonstrate the significant potential of this technique to facilitate scalable production of perovskite-based photovoltaic modules.

Close-loop recycling of perovskite solar cells through dissolution-recrystallization of perovskite by butylamine

Organic-inorganic lead halide perovskite solar cells (PSCs) have achieved impressive progress in both conversion efficiency and stability. However, the leakage risk of toxic lead in the fabrication and service of PSCs hinders their commercialization; hence, the management of lead is imperative. Here, we report a close-loop recycling strategy to collect the key materials involved in devices with butylamine (BA), even regenerating solar cells with recycled materials including perovskite absorbers and NiOx-coated ITO glass substrates. The regenerated devices show performance equivalent to that of fresh ones and nearly 18% PCE for the champions. The total recycling yield of lead is more than 98.9%. The recycling can be attributed to the unique interaction between lead halides and BA. Our strategy could move PSCs toward commercialization by effective close-loop recycling and refabrication of whole PSC devices.

Photophysics of 2D Organic-Inorganic Hybrid Lead Halide Perovskites: Progress, Debates, and Challenges.

2D organic–inorganic hybrid Ruddlesden–Popper perovskites (RPPs) have recently attracted increasing attention due to their excellent environmental stability, high degree of electronic tunability, and natural multiquantum‐well structures. Although there is a rapid development of photoelectronic applications in solar cells, photodetectors, light emitting diodes (LEDs), and lasers based on 2D RPPs, the state‐of‐the‐art performance is far inferior to that of the existing devices because of the limited understanding on fundamental physics, especially special photophysics in carrier dynamics, excitonic fine structures, excitonic quasiparticles, and spin‐related effect. Thus, there is still plenty of room to improve the performances of photoelectronic devices based on 2D RPPs by enhancing knowledge on fundamental photophysics. This review highlights the special photophysics of 2D RPPs that is fundamentally different from the conventional 3D congeners. It also provides the most recent progress, debates, challenges, prospects, and in‐depth understanding of photophysics in 2D perovskites, which is significant for not only boosting performance of solar cells, LEDs, photodetectors, but also future development of applications in lasers, spintronics, quantum information, and integrated photonic chips.