Navigating Organo-Lead Halide Perovskite Phase Space via Nucleation Kinetics toward a Deeper Understanding of Perovskite Phase Transformations and Structure-Property Relationships

How varying surface wettability of different PEDOT:PSS formulations and their mixtures affects perovskite crystallization and the efficiency of inverted perovskite solar cells

The physico-chemical interaction, surface, and electrical properties of hole transport layers (HTLs) significantly affect the wettability and film crystallization of the deposited perovskite and the corresponding performance of inverted perovskite solar cells (PSCs). One of the most frequently used HTLs for inverted PSCs is poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). Various commercial PEDOT:PSS formulations have already been tested as HTLs. Until now mixtures of these remained rather unexplored. In this study, three commercially available PEDOT:PSS formulations (PH, PH1000, and AI4083), as well as three mixtures (PH:PH1000, PH:AI4083, and PH:PH1000:AI4083; in a 1 : 1 and 1 : 1 : 1 ratios) were used as HTLs to investigate the crystallization of perovskite films and the performance of associated PSCs. Of the three formulations, PEDOT:PSS PH showed better perovskite crystallization, resulting in better solar cell performance followed by both PH:AI4083 and PH:PH1000:AI4083 layers. The pioneering work on mixing PEDOT:PSS resulted in new combinations of PEDOT:PSS, with new properties (work function, surface wettability, and roughness) which are very important parameters for perovskite crystallization and corresponding device efficiencies and stabilities. All PSCs that use the mixed PEDOT:PSS as HTLs revealed higher fill factor and open-circuit voltage values above 900 mV, which is not the same except for PEDOT:PSS PH. As a result, we believe that such a mixing strategy could aid in the creation of various PEDOT:PSS combinations with tuneable properties for certain applications.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

Perovskites in the Energy Grid and CO2 Conversion: Current Context and Future Directions

CO2 emissions from the consumption of fossil fuels are continuously increasing, thus impacting Earth’s climate. In this context, intensive research efforts are being dedicated to develop materials that can effectively reduce CO2 levels in the atmosphere and convert CO2 into value-added chemicals and fuels, thus contributing to sustainable energy and meeting the increase in energy demand. The development of clean energy by conversion technologies is of high priority to circumvent these challenges. Among the various methods that include photoelectrochemical, high-temperature conversion, electrocatalytic, biocatalytic, and organocatalytic reactions, photocatalytic CO2 reduction has received great attention because of its potential to efficiently reduce the level of CO2 in the atmosphere by converting it into fuels and value-added chemicals. Among the reported CO2 conversion catalysts, perovskite oxides catalyze redox reactions and exhibit high catalytic activity, stability, long charge diffusion lengths, compositional flexibility, and tunable band gap and band edge. This review focuses on recent advances and future prospects in the design and performance of perovskites for CO2 conversion, particularly emphasizing on the structure of the catalysts, defect engineering and interface tuning at the nanoscale, and conversion technologies and rational approaches for enhancing CO2 transformation to value-added chemicals and chemical feedstocks.

Effect of intermediate phases on the optical properties of PbI2-rich CH3NH3PbI3 organic-inorganic hybrid perovskite

Methylammonium lead halide perovskite (CH3NH3PbI3) films, with high PbI2 concentration, were grown by the two-step spin coating method. The influence of the precursor concentration and annealing time on the optical and structural properties of the perovskite films was analyzed by optical absorption, photoluminescence, X-ray diffraction and scanning electron microscopy. The results showed that, in addition to the CH3NH3PbI3 and PbI2 phases, intermediate phases, such as (MA)2(DMF)2Pb3I8, were formed in the films, depending on the time and temperature of annealing, which can tune the optical absorption in the visible spectra. This intermediate phase induced the formation of perovskite nanowires, identified by SEM images, and their growth may be associated with the presence of the DMF solvent remaining in the PbI2 film.

Efficient Planar Heterojunction FA1- xCs xPbI3 Perovskite Solar Cells with Suppressed Carrier Recombination and Enhanced Open Circuit Voltage via Anion-Exchange Process

Introduction of Cs into FAPbI₃ displayed great potential to stabilize the black perovskite phase by forming FA_{1- x}Cs _xPbI₃, which has been investigated widely based on solution process. During solution processing, the over-rapid intercalating reaction rate between PbI₂ and A cations (FA⁺ and Cs⁺) can bring some undesirable structural transitions. However, in vapor-assisted solution process (VASP), the over-rapid intercalating reaction rate can be reduced effectively. In addition, the formation process can be regulated significantly by the intermediate perovskite phase. In this study, FACl was employed together with FAI to improve the FA_0.9Cs_0.1PbI₃ films by VASP. In the vapor deposition process, the FACl and FAI vapor coreacted with the PbI₂ solid films, preferentially forming the intermediate perovskite phase FA_0.9Cs_0.1PbI _xCl _y. The intermediate perovskite phase FA_0.9Cs_0.1PbI _xCl _y supplied a plenty of seeds for rapid nucleation of perovskite, which prolonged the crystallization time of FA_0.9Cs_0.1PbI₃, and thus, a smooth FA_0.9Cs_0.1PbI₃ film with suppressed nonradiative recombination, prolonged carrier lifetime and decreased trap state density was acquired. Corresponding planar heterojunction perovskite solar cells achieved a champion power conversion efficiency (PCE) of 16.39% with a V_oc of 0.99 V, J_sc of 22.87 mA/cm², and fill factor of 74.82% under reverse scanning. Meanwhile, a hysteresis index of the FACl-10 device was decreased to 0.024 compared with 0.075 of the control device. Moreover, under the condition of nitrogen atmosphere, the normalized PCE of FACl-10 device diminished only 4.9% which was more stable comparing with 31.88% diminishing of the control device after 30 days.

Transformation from crystalline precursor to perovskite in PbCl2-derived MAPbI3

Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI₃) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl₂ + 3MAI) (MA = CH₃NH₃). The as-spun film consists of crystalline MA₂PbI₃Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.

ZnO-Assisted Growth of CH3NH3PbI3- xCl x Film and Efficient Planar Perovskite Solar Cells with a TiO2/ZnO/C60 Electron Transport Trilayer

Appropriate electron transport layers (ETL) are essential in perovskite solar cells (PSCs) with high power conversion efficiency (PCE). Herein, a TiO₂/ZnO/C₆₀ trilayer fabricated on a transparent fluorine-doped tin oxide (FTO) glass substrate is used as a compound ETL in planar PSCs. The trilayer shows positive effects on both perovskite synthesis and device performance. The ZnO layer assists growth of CH₃NH₃PbI_{3- x}Cl _x ( x ≈ 0) annealed at a lower temperature and with a shorter time, which is due to a more rapid and easier decomposition of the intermediate CH₃NH₃PbCl₃ phase in the growth of CH₃NH₃PbI_{3- x}Cl _x. All three materials in the trilayer are important for obtaining PSCs with a high PCE. ZnO is critical for enhancing the open circuit voltage by ensuring proper energy alignment with the TiO₂ and C₆₀ layers. C₆₀ enhances carrier extraction from the CH₃NH₃PbI_{3- x}Cl _x layer. TiO₂ eliminates charge recombination at the FTO surface and ensures efficient electron collection. The best-performing PSC based on the TiO₂/ZnO/C₆₀ electron transport trilayer features a PCE of 18.63% with a fill factor of 79.12%. These findings help develop an understanding of the effects of ZnO-containing ETLs on perovskite film synthesis and show promise for the future development of high-performance PSCs with compound ETLs.

Perovskite Nanowire Extrusion

The defect tolerance of halide perovskite materials has led to efficient optoelectronic devices based on thin-film geometries with unprecedented speed. Moreover, it has motivated research on perovskite nanowires because surface recombination continues to be a major obstacle in realizing efficient nanowire devices. Recently, ordered vertical arrays of perovskite nanowires have been realized, which can benefit from nanophotonic design strategies allowing precise control over light propagation, absorption, and emission. An anodized aluminum oxide template is used to confine the crystallization process, either in the solution or in the vapor phase. This approach, however, results in an unavoidable drawback: only nanowires embedded inside the AAO are obtainable, since the AAO cannot be etched selectively. The requirement for a support matrix originates from the intrinsic difficulty of controlling precise placement, sizes, and shapes of free-standing nanostructures during crystallization, especially in solution. Here we introduce a method to fabricate free-standing solution-based vertical nanowires with arbitrary dimensions. Our scheme also utilizes AAO; however, in contrast to embedding the perovskite inside the matrix, we apply a pressure gradient to extrude the solution from the free-standing templates. The exit profile of the template is subsequently translated into the final semiconductor geometry. The free-standing nanowires are single crystalline and show a PLQY up to ∼29%. In principle, this rapid method is not limited to nanowires but can be extended to uniform and ordered high PLQY single crystalline perovskite nanostructures of different shapes and sizes by fabricating additional masking layers or using specifically shaped nanopore endings.

Modulation of PEDOT:PSS pH for Efficient Inverted Perovskite Solar Cells with Reduced Potential Loss and Enhanced Stability

Inverted p-i-n perovskite solar cells (PVSCs) using PEDOT:PSS as the hole-transporting layer (HTL) is one of the most widely adopted device structures thus far due to its facile processability and good compatibility for high throughput manufacturing processes. However, most of the PEDOT:PSS-based CH₃NH₃PbI₃ PVSCs reported to date suffered an inferior open-circuit voltage (V_OC) (0.88-0.95 V) compared to that (1.05-1.12 V) obtained for common CH₃NH₃PbI₃ PVSCs, revealing a severe potential loss issue. Herein, we describe a simple method to alleviate this problem by tuning the pH value of PEDOT:PSS with a mild base, imidazole. Accompanied by the pH modulation, the blended imidazole concurrently tailors the surface texture and electronic properties of PEDOT:PSS to promote the quality and crystallization of the perovskite film deposited on top of it and enable better energy-level alignment at this corresponding interface. Consequently, the PVSC using this modified PEDOT:PSS HTL yields an enhanced power conversion efficiency (PCE) of 15.7% with an enlarged V_OC of 1.06 V and improved long-term stability. These outperform the pristine device showing a PCE of 12.7% with a much smaller V_OC of 0.88 V and unsatisfactory environmental stability.

Formation Mechanism and Control of Perovskite Films from Solution to Crystalline Phase Studied by in Situ Synchrotron Scattering

Controlling the crystallization and morphology of perovskite films is crucial for the fabrication of high-efficiency perovskite solar cells. For the first time, we investigate the formation mechanism of the drop-cast perovskite film from its precursor solution, PbCl₂ and CH₃NH₃I in N,N-dimethylformamide, to a crystalline CH₃NH₃PbI_3-xCl_x film at different substrate temperatures from 70 to 180 °C in ambient air and humidity. We employed an in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique for this study. When the substrate temperature is at or below 100 °C, the perovskite film is formed in three stages: the initial solution stage, transition-to-solid film stage, and transformation stage from intermediates into a crystalline perovskite film. In each stage, the multiple routes for phase transformations are preceded concurrently. However, when the substrate temperature is increased from 100 to 180 °C, the formation mechanism of the perovskite film is changed from the "multistage formation mechanism" to the "direct formation mechanism". The proposed mechanism has been applied to understand the formation of a perovskite film containing an additive. The result of this study provides a fundamental understanding of the functions of the solvent and additive in the solution and transition states to the crystalline film. It provides useful knowledge to design and fabricate crystalline perovskite films for high-efficiency solar cells.

Enhanced Ambient Stability of Efficient Perovskite Solar Cells by Employing a Modified Fullerene Cathode Interlayer

A novel fullerene cathode interlayer is employed to facilitate the fabrication of stable and efficient perovskite solar cells. This modified fullerene surfactant significantly increases air stability of the derived devices due to its hydrophobic characteristics to enable 80% of the initial PCE to be retained after being exposed in ambient condition with 20% relative humidity for 14 days.

Current Challenges and Prospective Research for Upscaling Hybrid Perovskite Photovoltaics

Organic-inorganic hybrid perovskite photovoltaics (PSCs) are poised to push toward technology translation, but significant challenges complicating commercialization remain. Though J-V hysteresis and ecotoxicity are uniquely imposing issues at scale, CH3NH3PbI3 degradation is by far the sharpest limitation to the technology's potential market contribution. Herein, we offer a perspective on the practical market potential of PSCs, the nature of fundamental PSC challenges at scale, and an outline of prospective solutions for achieving module scale PSC production tailored to intrinsic advantages of CH3NH3PbI3. Although integrating PSCs into the energy grid is complicated by CH3NH3PbI3 degradation, the ability of PSCs to contribute to consumer electronics and other niche markets like those organic photovoltaics have sought footing in rests primarily upon the technology's price point. Thus, slot die, roll-to-roll processing has the greatest potential to enable PSC scale-up, and herein, we present a perspective on the research necessary to realize fully printable PSCs at scale.

Excited state and charge-carrier dynamics in perovskite solar cell materials

Organo-metal halide perovskites (OMHPs) have attracted enormous interest in recent years as materials for application in optoelectronics and solar energy conversion. These hybrid semiconductors seem to have the potential to challenge traditional silicon technology. In this review we will give an account of the recent development in the understanding of the fundamental light-induced processes in OMHPs from charge-photo generation, migration of charge carries through the materials and finally their recombination. Our and other literature reports on time-resolved conductivity, transient absorption and photoluminescence properties are used to paint a picture of how we currently see the fundamental excited state and charge-carrier dynamics. We will also show that there is still no fully coherent picture of the processes in OMHPs and we will indicate the problems to be solved by future research.

Identification and characterization of the intermediate phase in hybrid organic–inorganic MAPbI3perovskite

The intermediate phase bridges a reversible cycle between PbI₂and high quality perovskite.

A Low-Temperature, Solution-Processable, Cu-Doped Nickel Oxide Hole-Transporting Layer via the Combustion Method for High-Performance Thin-Film Perovskite Solar Cells

Low-temperature, solution-processable Cu-doped NiOX (Cu:NiOx ), prepared via combustion chemistry, is demonstrated as an excellent hole-transporting layer (HTL) for thin-film perovskite solar cells (PVSCs). Its good crystallinity, conductivity, and hole-extraction properties enable the derived PVSC to have a high power conversion efficiency (PCE) of 17.74%. Its general applicability for various elecrode materials is also revealed.

Transition from the Tetragonal to Cubic Phase of Organohalide Perovskite: The Role of Chlorine in Crystal Formation of CH3NH3PbI3 on TiO2 Substrates

The role of chlorine in the superior electronic property and photovoltaic performance of CH3NH3PbI(3-x)Clx perovskite has attracted recent research attention. Here, we study the impact of chlorine in the perspective of the crystal structure of the perovskite layer, which can provide important understanding of its excellent charge mobility and extended lifetimes. In particular, we find that in the presence of chlorine (PbCl2 or CH3NH3Cl), when CH3NH3PbI3 films are deposited on a TiO2 mesoporous layer instead of a planar TiO2 substrate, a stable cubic phase rather than the commonly observed tetragonal phase is formed in CH3NH3PbI3 perovskite at room temperature. The relative peak intensity of two major facets of cubic CH3NH3PbI3 crystals, (100)C and (200)C facets, can also be easily tuned, depending on the film thickness. Furthermore, compared with pristine CH3NH3PbI3 perovskite films, in the presence of chlorine, CH3NH3PbI3 crystals grown on planar substrates exhibit strong preferred orientations on (110)T and (220)T facets.

Enhanced Organo-Metal Halide Perovskite Photoluminescence from Nanosized Defect-Free Crystallites and Emitting Sites

Photoluminescence (PL) of organo-metal halide perovskite semiconductors can be enhanced by several orders of magnitude by exposure to visible light. We applied PL microscopy and super-resolution optical imaging to investigate this phenomenon with spatial resolution better than 10 nm using films of CH3NH3PbI3 prepared by the equimolar solution-deposition method, resulting in crystals of different sizes. We found that PL of ∼100 nm crystals enhances much faster than that of larger, micrometer-sized ones. This crystal-size dependence of the photochemical light passivation of charge traps responsible for PL quenching allowed us to conclude that traps are present in the entire crystal volume rather than at the surface only. Because of this effect, "dark" micrometer-sized perovskite crystals can be converted into highly luminescent smaller ones just by mechanical grinding. Super-resolution optical imaging shows spatial inhomogeneity of the PL intensity within perovskite crystals and the existence of <100 nm-sized localized emitting sites. The possible origin of these sites is discussed.