Incorporation of Cl into sequentially deposited lead halide perovskite films for highly efficient mesoporous solar cells

Lead-Free Perovskite Materials for Solar Cells

The toxicity issue of lead hinders large-scale commercial production and photovoltaic field application of lead halide perovskites. Some novel non- or low-toxic perovskite materials have been explored for development of environmentally friendly lead-free perovskite solar cells (PSCs). This review studies the substitution of equivalent/heterovalent metals for Pb based on first-principles calculation, summarizes the theoretical basis of lead-free perovskites, and screens out some promising lead-free candidates with suitable bandgap, optical, and electrical properties. Then, it reports notable achievements for the experimental studies of lead-free perovskites to date, including the crystal structure and material bandgap for all of lead-free materials and photovoltaic performance and stability for corresponding devices. The review finally discusses challenges facing the successful development and commercialization of lead-free PSCs and predicts the prospect of lead-free PSCs in the future.

Causes and Solutions of Recombination in Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Tailoring the Band Gap in 3D Hybrid Perovskites by Substitution of the Organic Cations: (CH3 NH3 )1-2y (NH3 (CH2 )2 NH3 )2y Pb1-y I3 (0≤y≤0.25)

Tuning the optical properties of MAPbI₃ (MA=methylammonium) is a key requirement to increase the efficiency of perovskite solar cells (PSCs). Simple precipitation from solution allows the partial substitution of MA in MAPbI₃ by H₃ NCH₂ CH₂ NH₃ (H₂ en). Surprisingly, there is 1:1 exchange of the monovalent cation MA by the dication H₂ en. The charge compensation results from a deficit of Pb²⁺ , leading to a series MA_1-2y (H₂ en)_2y Pb_1-y I₃ with 0≤y≤0.25. This model has been supported by single-crystal measurements and NMR investigations. The substitution results in a continuous shift of the band gap from 1.51 to 2.1 eV and a color change from black to orange-red. The H₂ en content stabilizes the cubic high-temperature (HT) form of MAPbI₃ . There is a linear correlation between band gap and unit cell volume. The substitution enables controlled band gap tuning because the extent of substitution is closely related to the applied MA:H₂ en ratio in solution.

A PbI2-xClx seed layer for obtaining efficient planar-heterojunction perovskite solar cells via an interdiffusion process

Despite the previous reports on the fabrication of CH₃NH₃PbI_3-xCl_x films via sequential deposition, the positioning and formation of PbI₂ in MAPbI_3-xCl_x perovskite films made from the seed layer containing PbI₂ and PbCl₂ in different ratios have not yet been addressed. In this study, the PbI₂ content in a perovskite absorber layer is controlled by changing the PbCl₂ ratio in a PbI_2-xCl_x seed layer. The addition of PbCl₂ in the seed layer facilitates PbI₂ generation and affects the morphology of the perovskite film. By integrating a perovskite absorber via the PbI_2-xCl_x seed-layer into a solar cell, we investigated the effects of the correlation between the chlorine and PbI₂ contents on the device performance through intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy. Elemental depth profiling analyses confirm that not only was the formed PbI₂ preferentially located near the metal-oxide layer, but residual chlorine was adsorbed at the TiO₂ layer. Our findings demonstrate that the geometric features of the formed PbI₂ affected the perovskite solar cells according to the chlorine content, likely because of the elemental gradient induced by annealing. The PbI_2-xCl_x-derived planar-heterojunction perovskite solar cells exhibited maximum power-conversion efficiencies of 17.56% at reverse scan and 17.21% at forward scan, suppressed current density-voltage hysteresis, and good performance distributions.

Achieving Large-Area Planar Perovskite Solar Cells by Introducing an Interfacial Compatibilizer

Despite the recent unprecedented increase in the power conversion efficiencies (PCEs) of small-area devices (≤0.1 cm² ), the PCEs deteriorate drastically for PSCs of larger areas because of the incomplete film coverage caused by the dewetting of the hydrophilic perovskite precursor solutions on the hydrophobic organic charge-transport layers (CTLs). Here, an innovative method of fabricating scalable PSCs on all types of organic CTLs is reported. By introducing an amphiphilic conjugated polyelectrolyte as an interfacial compatibilizer, fabricating uniform perovskite films on large-area substrates (18.4 cm² ) and PSCs with the total active area of 6 cm² (1 cm² × 6 unit cells) via a single-turn solution process is successfully demonstrated. All of the unit cells exhibit highly uniform PCEs of 16.1 ± 0.9% (best PCE of 17%), which is the highest value for printable PSCs with a total active area larger than 1 cm² .

Lead-Free Organic-Inorganic Hybrid Perovskites for Photovoltaic Applications: Recent Advances and Perspectives

Organic-inorganic hybrid halide perovskites (e.g., MAPbI₃ ) have recently emerged as novel active materials for photovoltaic applications with power conversion efficiency over 22%. Conventional perovskite solar cells (PSCs); however, suffer the issue that lead is toxic to the environment and organisms for a long time and is hard to excrete from the body. Therefore, it is imperative to find environmentally-friendly metal ions to replace lead for the further development of PSCs. Previous work has demonstrated that Sn, Ge, Cu, Bi, and Sb ions could be used as alternative ions in perovskite configurations to form a new environmentally-friendly lead-free perovskite structure. Here, we review recent progress on lead-free PSCs in terms of the theoretical insight and experimental explorations of the crystal structure of lead-free perovskite, thin film deposition, and device performance. We also discuss the importance of obtaining further understanding of the fundamental properties of lead-free hybrid perovskites, especially those related to photophysics.

Transformation of PbI2, PbBr2 and PbCl2 salts into MAPbBr3 perovskite by halide exchange as an effective method for recombination reduction

Properties of MAPbBr₃ layers and devices prepared using halide exchange strongly depend on the precursor PbX₂ halide salts.

Stability of Perovskite Solar Cells: A Prospective on the Substitution of the A Cation and X Anion

In recent years, organometal trihalide perovskites have emerged as promising materials for low-cost, flexible, and highly efficient solar cells. Despite their processing advantages, before the technology can be commercialized the poor stability of the organic-inorganic hybrid perovskite materials with regard to humidity, heat, light, and oxygen has be to overcome. Herein, we distill the current state-of-the-art and highlight recent advances in improving the chemical stability of perovskite materials by substitution of the A-cation and X-anion. Our hope is to pave the way for the rational design of perovskite materials to realize perovskite solar cells with unprecedented improvement in stability.

Nanostructuring Mixed-Dimensional Perovskites: A Route Toward Tunable, Efficient Photovoltaics

2D perovskites is one of the proposed strategies to enhance the moisture resistance, since the larger organic cations can act as a natural barrier. Nevertheless, 2D perovskites hinder the charge transport in certain directions, reducing the solar cell power conversion efficiency. A nanostructured mixed-dimensionality approach is presented to overcome the charge transport limitation, obtaining power conversion efficiencies over 9%.

Lead-Free MA2CuCl(x)Br(4-x) Hybrid Perovskites

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)2CuCl(x)Br(4-x) was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu(+) ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material's properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.

Enhancing the carrier thermalization time in organometallic perovskites by halide mixing

Non-adiabatic molecular dynamics simulations of non-radiative relaxation dynamics of charge carriers in hybrid perovskites show that the carrier relaxation time can be considerably increased by halide mixing.

Material Innovation in Advancing Organometal Halide Perovskite Functionality

Organometal halide perovskites (OMHPs) have garnered much attention recently for their unprecedented rate of increasing power conversion efficiency (PCE), positioning them as a promising basis for the next-generation photovoltaic devices. However, the gap between the rapid increasing PCE and the incomplete understanding of the structure-property-performance relationship prevents the realization of the true potential of OMHPs. This Perspective aims to provide a concise overview of the current status of OMHP research, highlighting the unique properties of OMHPs that are critical for solar applications but still not adequately explained. Stability and performance challenges of OMHP solar cells are discussed, calling upon combined experimental and theoretical efforts to address these challenges for pioneering commercialization of OMHP solar cells. Various material innovation strategies for improving the performance and stability of OMHPs are surveyed, showing that the OMHP architecture can serve as a promising and robust platform for the design and optimization of materials with desired functionalities.

Impact of Capacitive Effect and Ion Migration on the Hysteretic Behavior of Perovskite Solar Cells

In the past five years, perovskite solar cells (PSCs) based on organometal halide perovskite have exhibited extraordinary photovoltaic (PV) performance. However, the PV measurements of PSCs have been widely recognized to depend on voltage scanning condition (hysteretic current density-voltage [J-V] behavior), as well as on voltage treatment history. In this study, we find that varied PSC responses are attributable to two causes. First, capacitive effect associated with electrode polarization provides a slow transient non-steady-state photocurrent that modifies the J-V response. Second, modification of interfacial barriers induced by ion migration can modulate charge-collection efficiency so that it causes a pseudo-steady-state photocurrent, which changes according to previous voltage conditioning. Both phenomena are strongly influenced by ions accumulating at outer interfaces, but their electrical and PV effects are different. The time scale for decay of capacitive current is on the order of seconds, whereas the slow redistribution of mobile ions requires several minutes.

Control and Study of the Stoichiometry in Evaporated Perovskite Solar Cells

Herein, we present the precise stoichiometric control of methlyammonium lead iodide perovskite thin-films using high vacuum dual-source vapor-phase deposition. We found that UV/Vis absorption and emission spectra were inadequate for assessing precisely the perovskite composition. Alternatively, inductively coupled plasma mass spectrometry (ICP-MS) is used to give precise, reproducible, quantitative measurements of the I/Pb ratio without systematic errors that often result from varying device thicknesses and morphologies. This controlled deposition method enables better understanding of the evaporation and deposition processes; methylammonium iodide evaporation appears omnidirectional, controlled using the chamber pressure and incorporated in the film through interaction with the unidirectionally evaporated PbI2. Furthermore, these thin-films were incorporated into solar cell device architectures with the best photovoltaic performance and reproducibility obtained for devices made with stoichiometric perovskite active layers. Additionally, and particularly pertinent to the field of perovskite photovoltaics, we found that the I-V hysteresis was unaffected by varying the film stoichiometry.

Chlorine Incorporation for Enhanced Performance of Planar Perovskite Solar Cell Based on Lead Acetate Precursor

We show the effects of chlorine incorporation in the crystallization process of perovskite film based on a lead acetate precursor. We demonstrate a fabrication process for fast grain growth with highly preferred {110} orientation upon only 5 min of annealing at 100 °C. By studying the correlation between precursor composition and morphology, the growth dynamic of perovskite film in the current system is discussed. In particular, we found that both lead acetate precursor and Cl incorporation are beneficial to perovskite growth. While lead acetate allows fast crystallization process, Cl improves perovskite crystallinity. Planar perovskite solar cells with optimized parameters deliver a best power conversion efficiency of 15.0% and average efficiency of 14.0% with remarkable reproducibility and good stability.

Energetics and dynamics in organic-inorganic halide perovskite photovoltaics and light emitters

The rapid transcendence of organic-inorganic metal halide perovskite solar cells to above the 20% efficiency mark has captivated the broad photovoltaic community. As the efficiency race continues unabated, it is essential that fundamental studies keep pace with these developments. Further gains in device efficiencies are expected to be increasingly arduous and harder to come by. The key to driving the perovskite solar cell efficiencies towards their Shockley-Queisser limit is through a clear understanding of the interfacial energetics and dynamics between perovskites and other functional materials in nanostructured- and heterojunction-type devices. In this review, we focus on the current progress in basic characterization studies to elucidate the interfacial energetics (energy-level alignment and band bending) and dynamical processes (from the ultrafast to the ultraslow) in organic-inorganic metal halide perovskite photovoltaics and light emitters. Major findings from these studies will be distilled. Open questions and scientific challenges will also be highlighted.

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

Organo-lead halide perovskite photovoltaics have developed faster than our understanding of the material itself. Using the vast body of work on perovskite processing created in just the past few years, it is possible to create a better picture of this material's complex phase-transformation behavior. This concept paper summarizes and correlates the current understanding of structural intermediates, kinetic controls, and structure-property relationships of organo-lead iodide perovskites. To this end, a new way of graphically relating information is developed, allowing the simultaneous mapping of schematic kinetic relationships between all currently prevailing perovskite deposition and growth techniques.

Efficient inorganic solid solar cells composed of perovskite and PbS quantum dots

Lead halide perovskite solar cells have attracted great interest due to their high efficiency and simple fabrication process. However, the high efficiency heavily relies on expensive organic hole-transporting materials (OHTMs) such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD), it is preferable to replace these expensive OHTMs by inorganic and low cost materials. Here, we report colloidal PbS quantum dots synthesized by a facile method and used as the inorganic hole-transporting material in a hybrid perovskite solar cell. By controlling the crystalline morphology of the perovskite capping layer, the recombination process is significantly retarded. Furthermore, a pure inorganic solar cell prepared by a two-step process demonstrated a nearly 8% power conversion efficiency due to efficient charge separation by a cascade of junctions and retarding charge recombination by a void-free capping layer. The stability of the inorganic solar cell was also tested with a little decay observed within ca. 100 h.

Perovskite Solar Cells: Beyond Methylammonium Lead Iodide

Organic-inorganic lead halide based perovskites solar cells are by far the highest efficiency solution-processed solar cells, threatening to challenge thin film and polycrystalline silicon ones. Despite the intense research in this area, concerns surrounding the long-term stability as well as the toxicity of lead in the archetypal perovskite, CH3NH3PbI3, have the potential to derail commercialization. Although the search for Pb-free perovskites have naturally shifted to other transition metal cations and formulations that replace the organic moiety, efficiencies with these substitutions are still substantially lower than those of the Pb-perovskite. The perovskite family offers rich multitudes of crystal structures and substituents with the potential to uncover new and exciting photophysical phenomena that hold the promise of higher solar cell efficiencies. In addressing materials beyond CH3NH3PbI3, this Perspective will discuss a broad palette of elemental substitutions, solid solutions, and multidimensional families that will provide the next fillip toward market viability of the perovskite solar cells.

Morphology control of the perovskite films for efficient solar cells

In the past two years, the power conversion efficiency (PCE) of organic–inorganic hybrid perovskite solar cells has significantly increased up to 20.1%.

DMSO-based PbI2 precursor with PbCl2 additive for highly efficient perovskite solar cells fabricated at low temperature

The advantages of both doping chloride and using dimethylsulfoxide (DMSO) as solvent are combined, by which the performance of the as-prepared perovskites solar cells is significantly improved.

Efficient and reproducible CH3NH3PbI3−x(SCN)xperovskite based planar solar cells

CH₃NH₃PbI_3−x(SCN)_xperovskite based planar solar cells exhibit a maximum power conversion efficiency of 11.07% with remarkably high reproducibility and good stability.