Design of Lead-Free Inorganic Halide Perovskites for Solar Cells via Cation-Transmutation.

Lead-free tin perovskite solar cells

With the rise of industry and technology development based on the utilization of fossil fuels as a main energy source as well as environmental and air pollution greenhouse gases have drastically increased with the alert and demand to alter the sources of energy. One way to do so is to reduce the consumption of fossil fuels and replace it by other sources such as renewable energies. Most interesting and promising is solar energy. After the discovery of the photovoltaic effect and applying it for making solar cells, an effective way to convert solar energy into electric energy has become achievable. Throughout history three different solar cell generations can be observed mostly based on the evolution of solar cells by the decrease of the solar cell thickness and the lowering of the active layer thickness from µm to nm. Beyond the decrease in thickness of the solar cells, the price of solar cells has dropped over the years. Still, the silicon-based technology is complex and cost-intensive. The third generation of solar cells are thin films with their most interesting and well-researched sub-group: perovskite solar cells. This new way of cheap and easy-to-fabricate solar cell has interested both, scientists and industry. A decade of lead-based perovskite solar cells in the scientific community has shown an increase in power conversion efficiency from 3.8 to 26 % and enormous interest of world scientists investigating the excellent photovoltaic properties of these lead-based perovskites (e.g. CH3NH3PbI3) but also the possibility of large commercialization. Organic–inorganic lead halide perovskite absorbers have excellent photovoltaic properties, such as suitable bandgap, high optical absorption, and long carrier lifetime. Unfortunately, underlying issues are the presence of toxic lead and the cell instability under ambient atmosphere (e.g. O2 and H2O). Bismuth-based lead-free double perovskites (e.g. Cs2AgBiBr6) have been considered as alternatives to the lead-based perovskites for solar cell applications. Trivalent cations, such as Bi3+ along with monovalent cations, Ag+, have been concurrently introduced to the B-sites of halide perovskites, leading to B cation double perovskites with the general chemical formula of A2B’B’’X6. These Pb-free double perovskites have been reported to have promising photovoltaic properties, including long carrier recombination lifetime, good stability against air and moisture, and low carrier effective masses. Thus, they are a potential alternative to the toxic lead halide perovskites. Nevertheless, device development is still in its infancy, and its performance is affected by severe hysteresis. In this work the realization of the synthesis and deposition of the double perovskite is presented via different deposition routes such as vacuum vapor and solution deposition. The double perovskite thin films have been optimized and characterized using different surface and material characterization methods. Afterwards, hysteresis-free planar and mesoporous double perovskite solar cells with no s-shape in the device characteristics and increased device open circuit voltage have been realized for the first time. This has been achieved by fine-tuning the material deposition parameters and layer optimization using several modification routes such as different temperature annealings, the thicknesses of mesoporous and perovskite layer, ozone and TiCl4 treatments leading to better infiltration of the double perovskite solution into mesoporous TiO2–ending with a significant improvement in solar cell performance. Except of device and interface engineering, to improve the material properties, compositional engineering has been conducted using mixing elements such as organic cation methylammonium, inorganic cation antimony and halide anion iodine. Finally, dimensional engineering has been achieved by adding large organic cations to the double perovskite crystal structure resulting in the new white emissive 2D and quasi 2D lead-free double perovskite materials.

The close-to-optimal band gap, large absorption coefficient, low manufacturing cost and rapid increase in power conversion efficiency make the organic–inorganic hybrid halide (CH3NH3PbI3) and related perovskite solar cells very promising for commercialization. The properties of point defects in the absorber layer semiconductors have important influence on the photovoltaic performance of solar cells, so the investigation on the defect properties in the perovskite semiconductors is necessary for the optimization of their photovoltaic performance. In this work, we give a brief review to the first-principles calculation studies on the defect properties in a series of perovskite semiconductors, including the organic–inorganic hybrid perovskites and inorganic halide perovskites. Experimental identification of these point defects and characterization of their properties are called for.

Recent progress on all-inorganic metal halide perovskite solar cells

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

ConspectusThree-dimensional (3D) halide perovskite (HP) solar cells have been thriving as promising postsilicon photovoltaic systems. However, despite the decency of efficiency, they suffer from poor stability. Partial dimensionality reduction from 3D to 2D was found to significantly meliorate the instability, thus mixed-dimensional 2D/3D HP solar cells have been expected to combine favorable durability and high efficiency. Nevertheless, their power conversion efficiency (PCE) does not live up to the expectation, hardly exceeding 19%, in sharp contrast with the ∼26% benchmark for pure 3D HP solar cells. The low PCE primarily arises from the restricted charge transport of the mixed-phasic 2D/3D HP layer. Understanding its photophysical dynamics, including its nanoscopic phase distribution and interphase carrier transfer kinetics, is essential for fathoming the underlying restriction mechanism. This Account outlines the three historical photophysical models of the mixed-phasic 2D/3D HP layer (denoted as models I, II, and III hereafter). Model I opines (i) a gradual dimensionality transition in the axial direction and (ii) a type II band alignment between 2D and 3D HP phases, hence favorably driving global carrier separation. Model II takes the view that (i) 2D HP fragments are interspersed in the 3D HP matrix with a macroscopic concentration variation in the axial direction and (ii) 2D and 3D HP phases instead form a type I band alignment. Photoexcitations would rapidly transfer from wide-band-gap 2D HPs to narrow-band-gap 3D HPs, which then serve as the charge transport network. Model II is currently the most widely accepted. We are one of the earliest groups to unveil the ultrafast interphase energy-transfer process. Recently, we further amended the photophysical model to consider also (i) an interspersing pattern of phase distribution but (ii) the 2D/3D HP heterojunction to be a p-n heterojunction with built-in potential. Anomalously, the built-in potential of the 2D/3D HP heterojunction increases upon photoexcitation. Therefore, local 3D/2D/3D misalignments would severely impede charge transport due to carrier blocking or trapping. Contrary to models I and II which hold 2D HP fragments as the culprit, model III rather suspects the 2D/3D HP interface for blunting the charge transport. This insight also rationalizes the distinct photovoltaic performances of the mixed-dimensional 2D/3D configuration and the 2D-on-3D bilayer configuration. To extinguish the detrimental 2D/3D HP interface, our group also developed an approach to alloy the multiphasic 2D/3D HP assembly into phase-pure intermediates. The accompanying challenges that are coming are also discussed.

Organic–inorganic lead halide perovskite solar cells are regarded as one of the most promising technologies for the next generation of photovoltaics due to their high power conversion efficiency (PCE) and simple solution manufacturing. Among the different compositions, the formamidinium lead iodide (FAPbI3) photoactive phase has a bandgap of 1.4 eV, which enables the corresponding higher PCEs according to the Shockley–Queisser limit. However, the photoactive crystal phase of FAPbI3 is not stable at room temperature. The most high‐performing compositions to date have reduced this problem by incorporating the methylammonium (MA) cation into the FAPbI3 composition, although MA has poor stability at high temperatures and in humid environments, which can limit the lifetime of FAxMA1−xPbI3 films. CsxFA1−xPbI3 perovskites are also explored, but despite better stability they still lag in performance. Herein, the additive engineering of MA‐free organic−inorganic lead halide perovskites using divalent cations Sr2+ and Ca2+to enhance the performances of CsxFA1−xPbI3 perovskite compositions is explored. It is revealed that the addition of up to 0.5% of Sr2+ and Ca2+ leads to improvements in morphology and reduction in microstrain. The structural improvements observed correlate with improved solar cell performances at low additive concentrations.

Large grain size with reduced non-radiative recombination in potassium incorporated methylammonium-free perovskite solar cells

Inorganic lead-based halide perovskites: From fundamental properties to photovoltaic applications

The research on solution processed metal halide perovskite solar cells (PSCs) as a new type of solar cells has experienced explosive growth since the first report in 2009. It is impressive that solar energy conversion efficiency has increased to over 23%. Outstanding optoelectronic properties including high absorption coefficient, high mobility, and long diffusion length of charge carriers have been revealed in the family of hybrid organic inorganic halide perovskite materials that are considered the heart of solar cells. A long-anticipated feature for solar cells that the diffusion lengths of charge carriers outstrip the active layer thickness of a device has been demonstrated in PSCs so that the efficiency of extracting photocarriers, particularly electrons at the interfaces becomes a key parameter controlling global device performance. The n-type semiconductor TiO2 with the merits of thermal and chemical stability, low cost, and suitable band edge positions has been regarded an ideal electron transporting layer (ETL) material in PSCs performing the function of selectively extracting photoelectrons and subsequently delivering them toward a current collector. Besides the highly concerning energy conversion efficiency of PSCs, the challenge of the current–voltage hysteresis phenomenon and instability of PSCs are also revealed to be closely related with TiO2 ETLs. In this review, the recent progress on strategies for modifying TiO2 ETLs by controlling morphology, surface modification, doping, and constructing composites to improve global performance of PSCs is reviewed. Moreover, the perspective on future development of TiO2 based ETLs for high performance PSCs is proposed on the basis of the comprehensive and deep understanding of TiO2 from the area of photocatalysis. It is anticipated that finely tailoring the features and properties of TiO2 ETLs will further release large room for exciting enhancements in the global performance of PSCs.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Progress and challenges in the fabrication of lead-free all-inorganic perovskites solar cells using solvent and compositional engineering Techniques-A review

Inorganic metal halide perovskite materials as sunlight absorbers for solar cells exhibit better thermal stability than organic-inorganic hybrid counterparts. Pure cesium lead triiodide (CsPbI3), with the most suitable band gap, suffers phase instability under an ambient environment. Nucleation and crystal growth are two crucial steps in fabricating a solution-processed perovskite film. A high-quality perovskite film with good morphology makes a significant impact on the efficiency and stability of perovskite solar cells. Dimethylformamide (DMF) is a commonly used aprotic solvent. However, it is difficult to obtain a high-quality inorganic perovskite film using DMF as a single solvent due to its slow evaporation and strong coordination with Pb2+. Here, we investigate dimethylacetamide (DMAc)/DMF as a cosolvent to prompt nucleation during the spin-coating process, leading to higher nucleation density and better surface coverage. In addition, we introduce CsBr in dimethylammonium lead triiodide (DMAPbI3)/CsI precursors to slow down the crystal growth process. CsBr does not increase the film band gap but leads to a pinhole-free film with better crystallinity. Through nucleation and crystal growth engineering, the power conversion efficiency of inorganic perovskite devices is improved to 17.67%, and ambient environment stability is significantly enhanced.

New photovoltaic materials have been extensively searched for in the last decade for clean and renewable solar energy conversion. Among them, the organic–inorganic hybrid lead halide perovskite solar cell is of current interest to the solar cell community because of its low processing cost, ease of fabrication and high power conversion efficiency. These perovskites show great potential to be photovoltaic absorbers with excellent optoelectronic properties, for example, high absorption coefficient, long charge carrier diffusion length, moderate ambipolar mobilities, tunable bandgap, etc. The unique physical and chemical properties of these materials are based on the crystal structure of the perovskites, named after the mineral form of CaTiO3. Most inorganic materials with a perovskite crystal structure typically show ferroelectric properties. Ferroelectricity in methylammonium lead tri-iodide (MAPbI3), an absorbing material in perovskite solar cells with a perovskite crystal structure, is now a topic of debate. Some theoretical and experimental investigations have shown the presence of a ferroelectric domain in these perovskites, although a lot of controversial results and conclusions also exist in the literature. In this review, notable signs of progress in the theoretical and experimental studies regarding ferroelectricity in organic-inorganic hybrid perovskites are summarized. In addition, future improvements of ferroelectric perovskites are proposed, paving the way towards prospective high-performance flexible and wearable ferroelectric solar cells and optoelectronic devices.

ConspectusOwing to the remarkable progress achieved over the past decade with research efforts from the perspectives of material synthesis, device configuration, and theoretical investigation, metal halide perovskites have emerged as a revolutionary class of light-absorbing semiconductors. The perovskite photovoltaic devices have demonstrated an impressive increase in power conversion efficiency. For single-junction perovskite solar cells, the value has risen from the initial one-digit maximum to the state-of-art record of 25.5%. Among various chemical and structural variations of perovskites, inorganic lead halides possess a more favorable operational stability and hold greater potential for perovskite/silicon tandem photovoltaics' top cells. At the initial stage of exploring all-inorganic perovskites for optoelectronic applications, many concepts, technical routes, and modification strategies were directly adopted from research on the more-developed field of organic-inorganic hybrid perovskite (OIHP). However, as understandings on inorganic perovskite deepen with research experience gained from both experimental and theoretical progression, it has been found that the difference between the asymmetric, volatile organic cations and the spherical, stable inorganic cations can lead to drastic changes on overall material properties and the subsequent device performances. In detail, the disparities reflect the crystalline and phase profiles of the material, the fabrication and passivation rationales of perovskite thin films, and the photophysics in the assembled optoelectronic devices. Therefore, the discussions of all-inorganic perovskites have their own exclusivity and are worthy of a specialized scrutinization.Here in this Account, the latest progress on the stabilization of inorganic lead halide perovskites for efficient photovoltaic applications is highlighted. A library of chemical compositions will be discussed with a focus on notable works about CsPbI3, which possesses a more favorable bandgap as a tandem to commercialized silicon solar cells. To underscore the influence of the crystal phase on the stability of inorganic perovskites, fundamental investigations regarding the chemical and physical properties, including experimental and theoretical studies, will be summarized as a means of phase control. The stability of inorganic lead halide perovskites can also be improved by the strategic introduction of external components to the light-absorbing layer, such as the incorporation of inorganic halides, organic cations, OIHPs with low dimension, etc. These strategies can synergistically improve the stability and efficiency of the fabricated devices from the perspectives of compositional tuning, dimensional engineering, surface termination, and low-dimension capping. On the basis of a careful examination and an analysis of works achieved in these categories from our group and others, we will then discuss some promising approaches toward achieving more stable and efficient photovoltaics using inorganic lead halide perovskites.