Solar Cells Based on All-Inorganic Halide Perovskites:Progress and Prospects

The emergence of perovskite solar cells(PSCs)based on all-inorganic metal halide(IMH)has generated enormous interest in the photovoltaic research community,and the power conversion efficiency(PCE)has exceeded13%.Despite its outstanding performance in thermal stability,PSCs based on IMH still face problems such as the lack of a suitable band gap and the inability to generate large areas.In this review,we will summarize the latest progress of PSCs based on IMH.

The structural, electronic, and optical properties of organic–inorganic mixed halide perovskites CH_3NH_3Pb(I_(1-y)X_y)_3(X = Cl, Br)

Methylammmonium lead iodide perovskites（CH3NH3PbI3） have received wide attention due to their superior optoelectronic properties. We performed first-principles calculations to investigate the structural, electronic, and optical properties of mixed halide perovskites CH3NH3Pb（I（1-y）Xy）3（X = Cl, Br; y = 0, 0.33, 0.67）. Our results reveal the reduction of the lattice constants and dielectric constants and enhancement of band gaps with increasing doping concentration of Cl-/Br-at I-. Electronic structure calculations indicate that the valance band maximum（VBM） is mainly governed by the halide p orbitals and Pb 6 s orbitals, Pb 6 p orbitals contribute the conduction band minimum（CBM） and doping does not change the direct semiconductor material. The organic cation [CH3NH3]~＋does not take part in the formation of the band and only one electron donates to the considered materials. The increasing trends of the band gap with Cl content from y = 0（0.793 eV） to y = 0.33（0.953 eV） then to y = 0.67（1.126 eV）. The optical absorption of the considered structures in the visible spectrum range is decreased but after doping the stability of the material is improving.

Synergistic Optimization of Buried Interface by Multifunctional Organic-Inorganic Complexes for Highly Efficient Planar Perovskite Solar Cells

For the further improvement of the power conversion efficiency(PCE)and stability of perovskite solar cells(PSCs),the buried interface between the perovskite and the electron transport layer is crucial.However,it is challenging to effectively optimize this interface as it is buried beneath the perovskite film.Herein,we have designed and synthesized a series of multifunctional organic-inorganic(OI)complexes as buried interfacial material to promote electron extraction,as well as the crystal growth of the perovskite.The OI complex with BF4−group not only eliminates oxygen vacancies on the SnO_(2) surface but also balances energy level alignment between SnO_(2) and perovskite,providing a favorable environment for charge carrier extraction.Moreover,OI complex with amine(−NH_(2))functional group can regulate the crystallization of the perovskite film via interaction with PbI2,resulting in highly crystallized perovskite film with large grains and low defect density.Consequently,with rational molecular design,the PSCs with optimal OI complex buried interface layer which contains both BF4−and−NH_(2) functional groups yield a champion device efficiency of 23.69%.More importantly,the resulting unencapsulated device performs excellent ambient stability,maintaining over 90%of its initial efficiency after 2000 h storage,and excellent light stability of 91.5%remaining PCE in the maximum power point tracking measurement(under continuous 100 mW cm−2 light illumination in N2 atmosphere)after 500 h.

4‑Terminal Inorganic Perovskite/Organic Tandem Solar Cells Offer 22%Efficiency

After fast developing of single-junction perovskite solar cells and organic solar cells in the past 10 years,it is becoming harder and harder to improve their power conversion efficiencies.Tandem solar cells are receiving more and more attention because they have much higher theoretical efficiency than single-junction solar cells.Good device performance has been achieved for perovskite/silicon and perovskite/perovskite tandem solar cells,including 2-terminal and 4-terminal structures.However,very few studies have been done about 4-terminal inorganic perovskite/organic tandem solar cells.In this work,semi-transparent inorganic perovskite solar cells and organic solar cells are used to fabricate 4-terminal inorganic perovskite/organic tandem solar cells,achieving a power conversion efficiency of 21.25%for the tandem cells with spin-coated perovskite layer.By using drop-coating instead of spin-coating to make the inorganic perovskite films,4-terminal tandem cells with an efficiency of 22.34%are made.The efficiency is higher than the reported 2-terminal and 4-terminal inorganic perovskite/organic tandem solar cells.In addition,equivalent 2-terminal tandem solar cells were fabricated by connecting the sub-cells in series.The stability of organic solar cells under continuous illumination is improved by using semi-transparent perovskite solar cells as filter.

Ferroelectric Properties and Applications of Hybrid Organic-Inorganic Perovskites

Hybrid organic-inorganic perovskites （e.g. CH;NH;PbI;） have attracted tremendous attention due to their promise for achieving next-generation cost-effective and high performance optoelectronic devices.These hybrid organic-inorganic perovskites possess excellent optical and electronic properties, including strong light absorption, high carrier abilities, optimized charge diffusion lengths, and reduced charge recombination etc., leading to their widespread applications in advanced solar energy technologies （e.g.high efficiency perovskite solar cells）. However, there is still a lack of investigations regarding fundamental properties such as ferroelectricity in these perovskites.As conventional ferroelectric ceramics are prepared at high temperature and have no mechanically flexibility,low-temperature proceed and flexible perovskite ferroelectrics have become promising candidates and should be exploited for future flexible ferroelectric applications. Here, ferroelectric properties in hybrid organic-inorganic perovskites and several state-of-the-art perovskite ferroelectrics are reviewed. Novel ferroelectric applications of hybrid organic-inorganic perovskites are discussed as well, providing guideline for realizing future high performance and flexible ferroelectric devices.

Recent advances in atomic imaging of organic-inorganic hybrid perovskites

Three-dimensional organic-inorganic hybrid perovskites(OHPs)hold a great prospect for photovoltaic applications due to their outstanding electronic and optical properties.These fascinating properties of OHPs in combination with their scalable and low-cost production make OHPs promising candidates for next-generation optoelectronic devices.The ability to obtain atomistic insights into physicochemical properties of this class of materials is crucial for the future development of this field.Recent advances in various scanning probe microscopy techniques have demonstrated their extraordinary capability in real-space imaging and spectroscopic measurements of the structural and electronic properties of OHPs with atomic-precision.Moreover,these techniques can be combined with light illumination to probe the structural and optoelectronic properties of OHPs close to the real device operation conditions.The primary focus of this review is to summarize the recent progress in atomic-scale studies of OHPs towards a deep understanding of the phenomena discovered in OHPs and OHP-based optoelectronic devices.

Characterization of structural transitions and lattice dynamics of hybrid organic–inorganic perovskite CH3NH3PbI3

By combining temperature-dependent x-ray diffraction(XRD) with temperature-dependent Raman scattering, we have characterized the structural transitions and lattice dynamics of the hybrid organic–inorganic perovskite CH3NH3PbI3.The XRD measurements cover distinct phases between 15 K and 370 K and demonstrate a general positive thermal expansion.Clear anomalies are found around the transition temperatures.The temperature evolution of the lattice constants reveals that the transition at 160 K/330 K is of the first-/second-order type.Raman measurements uncover three strong lowfrequency modes, which can be ascribed to the vibration of the Pb/I atoms.The temperature evolution of the modes clearly catches these transitions at 160 K and 330 K, and confirms the transition types, which are exactly consistent with the XRD results.The present study may set an experimental basis to understand the high conversion efficiency in methylammonium lead iodide.

Aromatic bromination with hydrogen production on organic‐inorganic hybrid perovskite‐based photocatalysts under visible light irradiation

Aromatic bromides are important chemicals in nature and chemical industries.However,their tra‐ditional synthesis routes suffer from low atomic economy and pollutant formation.Herein,we show that organic-inorganic hybrid perovskite methylammonium lead bromide(MAPbBr_(3))nanocrystals stabilized in aqueous HBr solution can achieve simultaneous aromatic bromination and hydrogen evolution using HBr as the bromine source under visible light irradiation.By hybridizing MAPbBr_(3) with Pt/Ta_(2)O_(5) and poly(3,4‐ethylenedioxythiophene)polystyrene sulfonate as electron‐and hole‐transporting motifs,aromatic bromides were achieved from aromatic compounds with high yield(up to 99%)and selectivity(up to 99%)with the addition of N,N‐dimethylformamide or its analogs.The mechanistic studies revealed that the bromination proceeds via an electrophilic attack pathway and that HOBr may be the key intermediate in the bromination reaction.

Effectively Inhibit Phase Separation to Improve Efficiency and Stability of All-Inorganic Planar CsPbIBr2 Perovskite Solar Cells

The advancement in a power conversion efficiency(PCE)to reach 25%,the inorganic perovskites are being explored intensively as promising optoelectronic materials due to their excellent photovoltaic performance,i.e.,thermal stability and efficiency.Lately,the inorganic cesium lead halide perovskite is studied to show enhanced light absorption,however,it suffers from the phase separate into I-rich and Br-rich phase which leads to poor film quality due to difference of electronegativity.Herein,we propose a unique solution of controlling the rate of solvent volatilization followed by gel method to inhibit phase separation effectively to obtain the homogenous and pinhole-free CsPbIBr2 films with high crystalline quality.In this study,an inverted planar device based on a light absorber of CsPbIBr2 is prepared to achieve a power conversion efficiency of 8.8%(maintain a stabilized value of 8%in ambient air conditions).Surprisingly,the optimized cell without encapsulation shows excellent long-term stability,as it maintained 90%initial efficiency over 500 h and controlled storage at around 45%relative humidity and 25℃.

New insight into the ultra-long lifetime of excitons in organic-inorganic perovskite： Reverse intersystem crossing

Recently, an effective exciton diffusion length L exceeding 100μm has been reported for organic- inorganic halide perovskites owing to both the high mobility and ultra-long lifetime of the excitons; however, the origin of ultra-long L is still unclear in nature. In some photoelectric materials, reverse intersystem crossing （RISC） from the triplet to the singlet state can enhance the quantum yield of pho- toluminescence greatly. In this study, our theoretical investigation indicated that the energy difference △E_st between the singlet state and the triplet state of CH_3NH_3Pbl_3 was less than 0.1 eV, which represents one crucial prerequisite for the occurrence of RISC. Meanwhile, the experimental results showed that the photoluminescence lifetime increased with the increasing temperature, a typical feature of RISC. Based on this study, we put forward the hypothesis that the ultra-long lifetime of excitons in organic-inorganic halide perovskite might be caused by the RISC process. This may provide a new insight into the important photophysical properties of such novel photovoltaic materials.

Synthesis and Characterization of Layered Perovskite-type Organic-inorganic Hybrids(CnH2n+1NH3)2(CH3NH3)m-1PbmI3m+1(n=5-10,m=1,2)

Layered organic-inorganic hybrids（CnH2n＋1NH3）2（CH3NH3）m-1PbmI3m＋1 containing monolayer（m=1） and bilayer（m=2） perovsikte were synthesized by reactions in solution.The influences of the reactant ratio,solvent,reaction temperature,and reaction time on the structures of the products were investigated.The structures and the properties of the hybrids were characterized using X-ray diffraction（XRD）,scanning electron microscopy（SEM）,and ultraviolet and visible（UV） absorption spectroscopy.The XRD patterns and the SEM images demonstrate that the pure bilayer perovskite hybrids are obtained.The UV-vis spectra indicate that the number of the inorganic perovskite layer（m） has greater impact on the band gap than the number of the carbon atoms（n）.The band gap of bilayer hybrids（around 1.9 eV） is significantly less than that of monolayer hybrids（around 2.2 eV）.

Synthesis and Characterization of Layered Perovskite-type Organic-inorganic Hybrids (R-NH_3)_2(CH_3NH_3)Pb_2I_7

Layered organic-inorganic hybrids containing bilayer perovsikte （R-NH3）2（CH3NH3）Pb2I7 （where R=C12H25,C6H5C2H4） were synthesized by reactions in solution. The influences of the solvents and the reactant ratio on the structures of the products were investigated. The structures and the properties of the hybrids were characterized using X-ray diffraction （XRD） and ultraviolet and visible （UV） adsorption spectra. For comparing with the bilayer perovskite hybrids in structure and band gap magnitude, the hybrids containing monolayer perovskite （R-NH3）2PbI4 were also synthesized and characterized. The results demonstrate that the thickness of inorganic layer has obvious effect on the tunneling magnitude of the band gap but the organic part can be micro actuator of band gap.

Recent Advances in Synthesis and Properties of Hybrid Halide Perovskites for Photovoltaics

The progress made by the scientific community in emerging photovoltaic technologies over the past two decades has been outstanding. Numerous methods have been developed for the preparation of hybrid organic–inorganic perovskite solar cells. The power conversion efficiency has been up to 14% by a one-step vacuum deposition technique. A serious concern is the toxicity of the materials. In this review, several methods aimed at resolving these problems to some extent have been compiled, including eco-friendly synthesis. Further efficiency enhancements are expected following optimization, and a better fundamental understanding of the internal electron charge transfer, electron–hole diffusion to the corresponding layers, flexibility, and stability-dependent bandgaps is reported. This paper explores the green synthesis of organic–inorganic perovskites for industrialization. Concerning the above facts, a simple low-cost model called‘‘dispersed photovoltaic cells' ' is presented.

A New Bismuth(Ⅲ) Iodide Inorganic/organic Hybrid Solid Containing Large Heterocyclic Conjugated Organic Ligand: Synthesis, Properties and Calculations

A new bismuth（III） iodide inorganic/organic hybrid containing larger heterocyclic conjugated organic ligand, [（dppz）2Bi2I6]·2DMF （1） （dppz = dipyrido[3,2-a：2＇,3＇-c]phenazine）, has been synthesized by solution process and structurally determined by X-ray diffraction method. 1 crystallizes in triclinic, space group Pī with Mr= 1844.08, α = 9.215（5）, b = 11.488（6）, c = 12.424（9） ？, α = 106.66（2）, β = 90.84（2）, γ = 92.34（2）°, V = 1258.5（13） ？3, Z = 1, Dc = 2.433 g/cm3, F（000） = 830, μ（MoKα） = 10.702 mm–1, the final R = 0.0834 and wR = 0.1947 for 2749 observed reflections with I 〉 2σ（I）. 1 consists of a dimeric structure [（dppz）2Bi2I6], in which two [（dppz）BiI3] fragments are bridged by a pair of iodine atoms. Hydrogen bonds and π···π stacking interactions contribute to the structural extension and stabilization. Experimental band gap of about 2.13 eV indicates its semiconductor nature. The optical absorption spectrum and electronic structure were also discussed.

Recent progress on advanced transmission electron microscopy characterization for halide perovskite semiconductors

Halide perovskites are strategically important in the field of energy materials. Along with the rapid development of the materials and related devices, there is an urgent need to understand the structure–property relationship from nanoscale to atomic scale. Much effort has been made in the past few years to overcome the difficulty of imaging limited by electron dose,and to further extend the investigation towards operando conditions. This review is dedicated to recent studies of advanced transmission electron microscopy(TEM) characterizations for halide perovskites. The irradiation damage caused by the interaction of electron beams and perovskites under conventional imaging conditions are first summarized and discussed. Low-dose TEM is then discussed, including electron diffraction and emerging techniques for high-resolution TEM(HRTEM) imaging. Atomic-resolution imaging, defects identification and chemical mapping on halide perovskites are reviewed. Cryo-TEM for halide perovskites is discussed, since it can readily suppress irradiation damage and has been rapidly developed in the past few years. Finally, the applications of in-situ TEM in the degradation study of perovskites under environmental conditions such as heating,biasing, light illumination and humidity are reviewed. More applications of emerging TEM characterizations are foreseen in the coming future, unveiling the structural origin of halide perovskite’s unique properties and degradation mechanism under operando conditions, so to assist the design of a more efficient and robust energy material.

A New Inorganic-organic Hybrid Based on Biisoquinoline and Hexachloridostannate:Structure, Photoluminescence, Electrochemical Behavior and Theoretical Study

A new inorganic-organic hybrid constructed from biisoquinoline dication and tin halide, [（BIQBT）（Sn Cl6）]n（1, BIQBT = 1,4-bis（isoquinoline） butane）, has been synthesized and structurally determined by X-ray diffraction method. 1 crystallizes in the monoclinic system, space group Cc with Mr = 644.82, a = 16.589（3）, b = 18.388（4）, c = 8.5532（17）A, β = 108.75（3）°, V = 2470.6（9） A3, Z = 4, Dc = 1.736 g/cm^3, F（000） = 1281, μ（Mo Kα） = 1.697 mm^–1, the final R = 0.0197 and wR = 0.0493 for 4614 observed reflections with I 〉 2（I）. 1 consists of BIQBT^2＋dications and mononuclear hexachloridostannate Sn Cl62- anion, and hydrogen bonds among them contribute to the formation of a 1-D chain. Strong fluorescence can be detected in 1, which was explained by theoretical calculation. Its electrochemical behavior was investigated, and the theoretical calculations reveal that the π···π stacking interaction is dominated for their structural stabilization.

Electrocaloric effect and pyroelectric properties of organic–inorganic hybrid （C2H5NH3）2CuCl4

The organic–inorganic hybrid(C2H5NH3)2 CuCl4(EA2CuCl4) single crystals are prepared by the solvothermal condition method. The x-ray diffraction, scanning electron microscopy, dielectric permittivity, pyroelectric current, and heat capacity are used to systematically investigate the electrocaloric performances of EA2CuCl4. The pyroelectric currents are measured under various voltages, and the electrocaloric effect(ECE) is calculated. Its ECE exhibits an isothermal entropy change of 0.0028 J/kg·K under an electric field of 30 kV/cm associated with a relatively broad temperature span. Further, the maximum pyroelectric coefficient(p) is 4× 10^-3 C/m^2·K and the coefficient β for generating ECE from electric displacement D is 1.068× 10^8 J·cm·K^-1·C^-2 at 240 K. Our results indicate that the ECE behavior of organic–inorganic hybrid EA2CuCl4 is in accordance with Jona and Shirane’s opinion in which the ECE should occur both below and above the Curie temperature Tc.

High performance solid-state thermoelectric energy conversion via inorganic metal halide perovskites under tailored mechanical deformation

Solid-state thermoelectric energy conversion devices attract broad research interests because of their great promises in waste heat recycling,space power generation,deep water power generation,and temperature control,but the search for essential thermoelectric materials with high performance still remains a great challenge.As an emerging low cost,solution-processed thermoelectric material,inorganic metal halide perovskites CsPb(I_(1–x)Br_(x))_(3) under mechanical deformation is systematically investigated using the first-principle calculations and the Boltzmann transport theory.It is demonstrated that halogen mixing and mechanical deformation are efficient methods to tailor electronic structures and charge transport properties in CsPb(I_(1–x)Br_(x))_(3) synergistically.Halogen mixing leads to band splitting and anisotropic charge transport due to symmetry-breakinginduced intrinsic strains.Such band splitting reconstructs the band edge and can decrease the charge carrier effective mass,leading to excellent charge transport properties.Mechanical deformation can further push the orbital energies apart from each other in a more controllable manner,surpassing the impact from intrinsic strains.Both anisotropic charge transport properties and ZT values are sensitive to the direction and magnitude of strain,showing a wide range of variation from 20%to 400%(with a ZT value of up to 1.85)compared with unstrained cases.The power generation efficiency of the thermoelectric device can reach as high as approximately 12%using mixed halide perovskites under tailored mechanical deformation when the heat-source is at 500 K and the cold side is maintained at 300 K,surpassing the performance of many existing bulk thermoelectric materials.

All-inorganic perovskite solar cells with efficiency>20%

The power conversion efficiency(PCE)of organic-inorganic hybrid metal halide perovskite solar cells(PSCs)has rocketed from around 3%to more than 25%in a decade,showing a miracle in the development history of photovoltaics^([1]).However,the hybrid perovskites still suffer from the issue of thermodynamic instability due to the volatile organic cations in perovskites.All-inorganic metal halide perovskites.

ITO-free silicon-integrated perovskite electrochemical cell for light-emission and light-detection

Halide perovskite light-emitting electrochemical cells are a novel type of the perovskite optoelectronic devices that differs from the perovskite light-emitting diodes by a simple monolayered architecture.Here,we develop a perovskite electrochemical cell both for light emission and detection,where the active layer consists of a composite material made of halide perovskite microcrystals,polymer support matrix,and added mobile ions.The perovskite electrochemical cell of CsPbBr3:PEO:LiTFSI composition,emitting light at the wavelength of 523 nm,yields the luminance more than 7000 cd/m2 and electroluminescence efficiency of 4.3 lm/W.The device fabricated on a silicon substrate with transparent single-walled carbon nanotube film as a top contact exhibits 40%lower Joule heating compared to the perovskite optoelectronic devices fabricated on conventional ITO/glass substrates.Moreover,the device operates as a photodetector with a sensitivity up to 0.75 A/W,specific detectivity of 8.56×1011 Jones,and linear dynamic range of 48 dB.The technological potential of such a device is proven by demonstration of 24-pixel indicator display as well as by successful device miniaturization by creation of electroluminescent images with the smallest features less than 50μm.