Zwitterion Dual-Modification Strategy for High-Quality NiOx and Perovskite Films for Solar Cells.

Nickel oxide (NiOx) has been limited in use as a hole transport layer for its low conduction, surface defects, and redox reactions with the perovskite layer. To address these issues, the incorporation of zwitterion L-tryptophan (Trp) is proposed at the NiOx/Trp interface. The carboxyl group of Trp effectively passivates the surface positive defects of NiOx, thereby improving its optical and electrical properties. The ammonium group of Trp not only passivates negative defects but modulates the growth of the perovskite layer, resulting in an improved perovskite film quality. Furthermore, the Trp layer acts as a buffer layer, suppressing adverse interfacial reactions between the perovskite and NiOx. Consequently, perovskite solar cells with 1.56 and 1.68 eV absorbers achieve the champion power conversion efficiency (PCE) of 23.79% and 20.41%, respectively. Moreover, the unencapsulated devices demonstrate excellent long-term stability, retaining above 80% of the initial PCE value after 1600 h of storage in the air with a humidity of 50-60%.

Perovskite Solar Cells Based on Polymerized Chlorophyll Films as Environmentally Friendly Hole-Transporting Layers.

Hole-transporting layers (HTLs) play a crucial role in the performance of inverted, p-i-n perovskite solar cells (PSCs). Chlorophylls (Chls) are naturally abundant organic photoconductors on earth, with good charge carrier mobility and appropriate Fermi energy levels that make them promising candidates for use in photovoltaic devices. However, Chls films prepared using the solution method exhibit lower carrier mobility compared to other organic polymer films, which limits their application in PSCs. To address this issue, Chls molecules are chemically linked to reduce the charge transfer barrier, thus the transfer of charges between molecules is transformed to intramolecular charge transfer. This study synthesizes and characterizes two polymerized Chl films, PolyCuChl and PolyNiChl, as HTLs of CH3 NH3 PbI3 -based PSCs. PSCs based on the electrochemical polymerization of PolyChl HTLs demonstrate an enhanced power conversion efficiency (PCE) of up to 19.0%, which is the highest efficiency among devices based on Chl materials. Furthermore, these devices demonstrated exceptional long-term stability. These results highlight the potential of polymerized Chl films as a viable alternative to conventional HTLs in PSCs. The approach utilizes abundant, environmentally friendly, and versatile Chl derivatives, and can be extended to develop next-generation HTL materials for improved PSC performance.

Organic Dye/Cs2AgBiBr6 Double Perovskite Heterojunction Solar Cells.

The photovoltaic performance of Cs2AgBiBr6 perovskite is limited by its light-harvesting ability owing to its broad bandgap. Here, we introduced three indoline dyes, D102, D131, and D149, to sensitize the TiO2 electron transport layer that was employed in the Cs2AgBiBr6 perovskite solar cells (PSCs). The perovskite-indoline dye hybrid cells worked with higher power conversion efficiencies (PCEs) than the corresponding dye-sensitized solar cells and the PSC. Extended absorption resulted in a higher short-circuit current density, up to 8.24 mA cm-2, and a maximum PCE of 4.23% in the case of D149, for instance. The double perovskite worked as a p-type interlayer between the dyes and spiro-OMeTAD to convey the holes from the former to the latter, resulting in enhancement in the overall performance.

Chlorophyll Derivative-Sensitized TiO2 Electron Transport Layer for Record Efficiency of Cs2AgBiBr6 Double Perovskite Solar Cells.

The power conversion efficiency (PCE) of Cs2AgBiBr6-based perovskite solar cells (PSCs) is still low owing to the inherent defects of Cs2AgBiBr6 films. Herein, we demonstrate a carboxy-chlorophyll derivative (C-Chl)-sensitized mesoporous TiO2 (m-TiO2) film as an electron transport layer (ETL) to enhance and extend the absorption spectrum of Cs2AgBiBr6-based PSCs. The C-Chl-based device achieves a significantly improved PCE, exceeding 3% for the first time, with an increase of 27% in short-circuit current density. Optoelectronic investigations confirm that the introduction of C-Chl reduces the defects, accelerates the electron extraction, and suppresses charge recombination at the interface of ETL/perovskite. Moreover, the unencapsulated PSCs display restrained hysteresis and great stability under ambient conditions.

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects.

The photovoltaics of organic-inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regarding compositional engineering of the hybrid perovskite structure are discussed, including potentials for developing all-inorganic and lead-free perovskite materials. Looking at the latest cutting-edge research, the prospects for perovskite-based photovoltaic and optoelectronic devices, including non-photovoltaic applications such as X-ray detectors and image sensing devices in industrialization, are described. In addition to the aforementioned major topics, we also review, as a background, our encounter with perovskite materials for the first solar cell application, which should inspire young researchers in chemistry and physics to identify and work on challenging interdisciplinary research problems through exchanges between academia and industry.

Synthesis, optoelectronic properties and applications of halide perovskites.

Halide perovskites have emerged as a class of most promising and cost-effective semiconductor materials for next generation photoluminescent, electroluminescent and photovoltaic devices. These perovskites have high optical absorption coefficients and exhibit narrow-band bright photoluminescence, in addition to their halide-dependent tuneable bandgaps, low exciton binding energies, and long-range carrier diffusion. These properties make these perovskites superior to classical semiconductors such as silicon. Most importantly, the simple synthesis of perovskites in the form of high quality films, single crystals, nanocrystals and quantum dots has attracted newcomers to develop novel perovskites with unique optoelectronic properties for optical and photovoltaic applications. Here, we comprehensively review recent advances in the synthesis and optoelectronic properties of films, microcrystals, nanocrystals and quantum dots of lead halide and lead-free halide perovskites. Followed by the classification of synthesis, we address the ensemble and single particle properties of perovskites from the viewpoints of the confinement and transport of charge carriers or excitons. Further, we correlate the charge carrier properties of perovskite films, microcrystals, nanocrystals and quantum dots with the crystal structure and size, halide composition, temperature, and pressure. Finally, we illustrate the emerging applications of perovskites to solar cells, LEDs, and lasers, and discuss the ongoing challenges in the field.

VOC Over 1.4 V for Amorphous Tin-Oxide-Based Dopant-Free CsPbI2Br Perovskite Solar Cells.

CsPbI2Br perovskite solar cells have attracted much attention because of the rapid development in their efficiency and their great potential as a top cell of tandem solar cells. However, the VOC outputs observed so far in most cases are far from that desired for a top cell. Up to now, with various kinds of treatments, the reported champion VOC is only 1.32 V, with a VOC deficit of 0.60 V. In this work, we found that aging of the SnCl2 precursor solution for the electron-transporting layer can promote the VOC of CsPbI2Br solar cells by employing a dopant-free-polymer hole transport material (HTM) over 1.40 V and efficiency over 15.5% with high reproducibility. With the champion VOC of 1.43 V, the VOC deficit was reduced to <0.50 V, which is achieved for the first time. This simple technique of SnCl2 solution aging forms a uniform and smooth amorphous SnOx film with pure Sn4+, elevates the conduction band of SnOx, and reduces the interfacial gaps and the trap state density of the device, resulting in enhancement in average VOC from ∼1.2 V in the nonaged case to ∼1.4 V in the aged case. Furthermore, the device using an aged SnCl2 solution also exhibits a much better long-term stability than that made of the fresh solution. These achievements in dopant/additive-free CsPbI2Br solar cells can be useful for future research on CsPbI2Br and tandem solar cells.

Quantum cutting-induced near-infrared luminescence of Yb3+ and Er3+ in a layer structured perovskite film.

Quantum cutting is an attractive optical phenomenon where one high-energy photon is converted into two low-energy photons, resulting in photoluminescence quantum yields (PLQYs) above 100%. In this report, we demonstrate a novel approach to enhance the quantum cutting energy transfer from an all-inorganic perovskite (CsPbCl3) to ytterbium (Yb3+) and erbium (Er3+) ions as near-infrared (NIR) emitters by using the highly orientated crystalline film. Yb3+ ions are fixed in the neighborhood of the CsPbCl3 lattice by preparing a one-to-one layer arrangement consisting of quasi-2D CsPbCl3 perovskite and Yb3+ layers. The successful preparation of layer arrangements resulted in the highly sensitized luminescence of Yb3+ by CsPbCl3 with NIR PLQYs exceeding 130%, which is attributed to quantum cutting. In addition, Er3+ luminescence at 1540 nm is acquired by the co-existence of Er3+ with Yb3+ in a layer, which is a result of the intralayer metal-to-metal energy transfer from Yb3+ activated by CsPbCl3 via the interlayer quantum cutting process. The PLQY of Er3+ luminescence reaches to 12.6%, which is the highest value ever observed for Er3+ compounds, resulting from the efficient interlayer quantum cutting process over 100% and the following intralayer resonance metal to metal energy transfer with the efficiency over 80%.

Stability of solution-processed MAPbI3 and FAPbI3 layers.

We provide a semi-empirical model based on in situ degradation measurements to predict the durability of hybrid perovskite materials under simulated thermal operation conditions. In the model, the degradation path of MAPbI3 layers is proved to follow an Arrhenius-type law. The predictive role is played by the activation energy combined with its pre-exponential factor. Our comparative study under moisture conditions with respect to vacuum and nitrogen treatments has assessed the occurrence of an intrinsic dynamic exchange of protons between the organic cations and the inorganic cage with a direct impact on the lattice stability, for which the presence of water molecules is not mandatory. This mutual interaction produces defects inside the material and volatile species, such as HI, CH3NH2 or MAI, with an associated experimental activation energy of 1.54 eV measured under vacuum conditions in dark. This value is comparable to that calculated by the density functional theory for defect generation in MAPbI3. In air, the action of water molecules reduces the activation energy for proton exchanges in dark to 0.96 eV. As an alternative solution to increase the material stability, we demonstrate that the substitution of methylammonium (MA(+)) with the formamidinium (FA(+)) cations inside the inorganic cage gives greater robustness to the overall lattice and extends the material durability due to a different interaction between the organic molecules and the inorganic cage. This definitely supports the use of FAPbI3 in applications, provided its structure can be stabilized in the dark phase at room temperature.

Similar Structural Dynamics for the Degradation of CH3 NH3 PbI3 in Air and in Vacuum.

We investigate the degradation path of MAPbI3 (MA=methylammonium) films over flat TiO2 substrates at room temperature by means of X-ray diffraction, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI2 . We describe how this degradation product is structurally coupled with the original MAPbI3 lattice through the orientation of its constituent PbI6 octahedra. Our results suggest a slight octahedral rearrangement after volatilization of HI+CH3 NH2 or MAI, with a relatively low energy cost. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material.

A metallocene molecular complex as visible-light absorber for high-voltage organic-inorganic hybrid photovoltaic cells.

A thin solid-state dye-sensitized photovoltaic cell is fabricated by composing organic and inorganic heterojunctions in which the visible-light sensitizers are cyclopentadiene derivatives (Cp*) coordinated to a metal oxide, typically TiO2. The coordination bonds of the metallocene molecular complex (Ti-Cp*) create a new LMCT (ligand-to-metal charge transfer) absorption band and induce a rectified charge transfer from the organic ligands to TiO2, leading to photocurrent generation. Photovoltaic junctions are completed by coating crystalline organic molecules (perylene) as a hole-transport layer on the Cp*-coordinated TiO2 surface by using the vapor deposition method. The molecular plane of Cp* on the TiO2 surfaces seems to help the hole-transport layer to form ordered structures, which effectively improve carrier conductivities and minimize interfacial resistance. The organic-inorganic hybrid thin-film photocell with metallocene molecular complexes is capable of generating high open-circuit voltages exceeding 1.2 V.

Quantum conversion enhancement with TiO(x) compact layers for ITO-plastic-film-based low-temperature-processed dye-sensitized photoelectrodes.

A plastic substrate-based dye-sensitized photoelectrode was fabricated with a thin mesoporous titania film prepared on a TiO(x)-coated indium tin oxide-polyethylene naphthalate (ITO-PEN) substrate by a low-temperature process. The 3.5 µm-thick titania film sensitized with an indoline dye (D205) yielded a power conversion efficiency of 5.2%, with the internal quantum efficiency reaching 100%.

Investigation of the Temperature Coefficients of Perovskite Solar Cells for Application in High-Temperature Environments.

Perovskite solar cells are actively investigated for their potential as highly efficient and cost-effective photovoltaic devices. However, a significant challenge in their practical application is enhancing their durability. Particularly, these cells are expected to be subjected to heating by sunlight in real-world operating environments. Therefore, high-temperature durability and device operation under such conditions are critical. Our study aims to improve the durability of perovskite solar cells for practical applications by examining their temperature coefficients at elevated temperatures using MA-free compositions. We assessed these coefficients and investigated their correlation with the ideality factor, revealing that carrier recombination markedly affects the temperature behavior of these cells. Our methodology involves simple J-V measurements to evaluate device degradation at high temperatures, paving the way for further research to enhance device performance in such environments.

Ionic Liquid-Assisted MAPbI3 Nanoparticle-Seeded Growth for Efficient and Stable Perovskite Solar Cells.

With the rapid improvement of perovskite solar cells (PSCs), long-life operational stability has become a major requirement for their commercialization. In this work, we devised a pristine cesium-formamidinium-methylammonium (termed as CsFAMA) triple-cation-based perovskite precursor solution into the ionic liquid (IL)-assisted MAPbI3 nanoparticles (NPs) through a seeded growth approach in which the host IL-assisted MAPbI3 NPs remarkably promote high-quality perovskite films with large single-crystal domains, enhancing the device performance and stability. The power conversion efficiency (PCE) of the MAPbI3 NP-seeded growth of MAPbI3 NPs/CsFAMA-based PSCs is as high as 19.44%, which is superior to those of MAPbI3 NPs and pristine CsFAMA films as the photoactive layer (9.52 and 17.33%, respectively). The long-term light-soaking and moisture stability of IL-aided MAPbI3 NPs/CsFAMA-based devices (non-encapsulated) remain above 90 and 80%, respectively, of their initial output after 2 h of light illumination (1 sun) and 6000 h storage at ambient with a relative humidity range of 30-40%. The use of the IL-assisted MAPbI3 NP-seeded growth for PSCs is a significant step toward developing stable and reliable perovskite photovoltaic devices.

Sensitized Yb3+ Luminescence in CsPbCl3 Film for Highly Efficient Near-Infrared Light-Emitting Diodes.

Near-infrared (NIR) light emitting diodes (LEDs) with the emission wavelength over 900 nm are useful in a wide range of optical applications. Narrow bandgap NIR emitters have been widely investigated using organic compounds and colloidal quantum dots. However, intrinsically low charge mobility and luminescence efficiency of these materials limit improvement of the external quantum efficiency (EQE) of NIR LEDs, which is far from practical applications. Herein, a highly efficient NIR LED is demonstrated, which is based on an energy transfer from wide bandgap all inorganic perovskite (CsPbCl3) to ytterbium ions (Yb3+) as an NIR emitter doped in the perovskite crystalline film. High mobility of electrically excited carriers in the perovskite crystalline film provides a long carrier diffusion and enhances radiative recombination of an emission center due to minimized charge trapping losses, resulting in high EQE value in LEDs. The NIR emission of Yb3+ at around 1000 nm is found to be sensitized by CsPbCl3 thin film with a photoluminescence quantum yield over 60%. The LED based on Yb3+-doped CsPbCl3 film exhibits a high EQE of 5.9% with a peak wavelength of 984 nm, achieved by high carrier transporting ability and effective sensitized emission property in the solid-film structure.

Low-Temperature Synthesized Nb-Doped TiO2 Electron Transport Layer Enabling High-Efficiency Perovskite Solar Cells by Band Alignment Tuning.

An Nb-doped TiO2 (Nb-TiO2) film comprising a double structure stacked with a bottom compact layer and top mesoporous layers was synthesized by treating a Ti precursor-coated substrate using a one-step low-temperature steam-annealing (SA) method. The SA-based Nb-TiO2 films possess high crystallinity and conductivity, and that allows better control over the conduction band (CB) of TiO2 for the electron transport layer (ETL) of the perovskite solar cells by the Nb doping level. Optimization of power conversion efficiency (PCE) for the Nb-TiO2-based ETL was combined with the CB level tuning of the mixed-halide perovskite by changing the Br/I ratio. This band offset management enabled to establish the most suitable energy levels between the ETL and the perovskites. This method was applied to reduce the band gap of perovskites to enhance the photocurrent density while maintaining a high open-circuit voltage. As a result, the optimal combination of 5 mol % Nb-TiO2 ETL and 10 mol % Br in the mixed-halide perovskite exhibited high photovoltaic performance for low-temperature device fabrication, achieving a high-yield PCE of 21.3%.

Cesium Acetate-Induced Interfacial Compositional Change and Graded Band Level in MAPbI3 Perovskite Solar Cells.

Compositional engineering and interfacial modifications have played pivotal roles in the accomplishment of high-efficiency perovskite solar cells (PSCs). Different interfaces in the PSCs influence the performance remarkably either by altering the crystallization of the active material or shifting the energy levels or improving the electrical contact. This work reports how a thin layer of cesium acetate on the TiO2 electron transport layer (ETL) induces generation of a PbI2-rich methylammonium lead iodide (MAPbI3) composition at the ETL/MAPbI3 interface, which downshifts the conduction band level of MAPbI3 to create an energy level gradient favorable for carrier collection, resulting in higher photocurrent, fill factor, and overall power conversion efficiency.

Full Efficiency Recovery in Hole-Transporting Layer-Free Perovskite Solar Cells With Free-Standing Dry-Carbon Top-Contacts.

Carbon-based top electrodes for hole-transporting-layer-free perovskite solar cells (PSCs) were made by hot press (HP) transfer of a free-standing carbon-aluminum foil at 100°C and at a pressure of 0.1 MPa on a methylammonium lead iodide (MAPbI3) layer. Under these conditions, the perovskite surface was preserved from interaction with the solvent. Over a timescale of 90 days, HP-PSCs were systematically compared to reference cells with carbon-based top electrodes deposited by doctor blading (DB). We found that all the photovoltaic parameters recorded in HP-PSCs during time under ambient conditions settled on values systematically higher than those measured in the reference DB-PSCs, with efficiency stabilized at around 6% within the first few measurements. On the other hand, in DB-PSCs, a long-lasting (~14 days) degrading transient of the performances was observed, with a loss of efficiency from an initial ~8% to ~3%. Moreover, in HP-PSCs, a systematic day-by-day recovery of the efficiency after operation was observed (Δ~2%) by leaving the cell under open circuit, a nitrogen environment, and dark conditions. Noteworthily, a full recovery of all the parameters was observed at the end of the experiment, while DB-PSCs showed only a partial recovery under the same conditions. Hence, the complete release of solvent from the carbon contact, before an interface is established with the perovskite layer, offers a definite advantage through the long period of operation in preventing irreversible degradation. Our findings indeed highlight the crucial role of the interfaces and their feasible preservation under nitrogen atmosphere.

Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.

Two organolead halide perovskite nanocrystals, CH(3)NH(3)PbBr(3) and CH(3)NH(3)PbI(3), were found to efficiently sensitize TiO(2) for visible-light conversion in photoelectrochemical cells. When self-assembled on mesoporous TiO(2) films, the nanocrystalline perovskites exhibit strong band-gap absorptions as semiconductors. The CH(3)NH(3)PbI(3)-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The CH(3)NH(3)PbBr(3)-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%.

Photomultiplying Visible Light Detection by Halide Perovskite Nanoparticles Hybridized with an Organo Eu Complex.

A molecular layer of an organo europium (Eu) complex was hybridized with the surface of halide perovskite (MAPbI3) nanocrystals to control charge transport between photoexcited perovskites and a metal electrode. A thin hybridized nanocrystalline film was found to cause an efficient photomultiplication reaction based on hole accumulation at photoexcited perovskite nanoparticles and charge tunneling across the Eu complex molecules under application of a low external bias (-0.5 V). A photodetecting device based on the Eu complex and perovskite absorber demonstrated photoinduced current generation at extremely high quantum conversion efficiency of 290 000% for weak monochromatic light (<1 mW cm-2), yielding a power responsivity value up to 1289 A W-1. High sensitivity of photodetection was enabled by using nanosized particles (<5 nm) of perovskite having a high surface area in contact with the Eu complex, while sensitivity was reduced by use of thick and large particle perovskite layers.