Hybrid Organic-inorganic Halide Perovskites Research Articles

Perovskite films grown with green mixed anti-solvent for highly efficient solar cells with enhanced stability

Organic–inorganic hybrid halide perovskite solar cells have attracted considerable attention owing to their high power conversion efficiency. Up to now, anti-solvent assisted one-step spin-coating crystallization method is still a major process to achieve high quality perovskite film. However, the toxicity of commonly used anti-solvents and the nonrepeatability of corresponding solar cell still obstruct the potential of commercial applications. Here, high-quality perovskite film, fabricated with anti-solvent engineering, is successfully developed by using a green mixed anti-solvent of ethylacetate with isopropanol as additive. Compared with pure ethylacetate anti-solvent, the mixed anti-solvent can significantly improve the surface morphology and passivate the grainboundary/surface defects of perovskite film, distinctly enhancing the photovoltaic property and the repeatability of P-i-N planar devices. The best-performing device exhibits a remarkable efficiency of 18.98% with negligible hysteresis and praiseworthy stability over 30 days in dry conditions. Our findings provide an avenue for high efficiency and stable planar perovskite devices with low cost and environmental-friendly manufacturing.

A Water-Stable Organic-Inorganic Hybrid Perovskite for Solar Cells by Inorganic Passivation

Organic-inorganic hybrid halide perovskite solar cells (PSCs) have been a trending topic in recent years. Significant progress has been made to increase their power conversion efficiency (PCE) to more than 20%. However, the poor stability of PSCs in both working and non-working conditions results in rapid degradation through multiple environmental erosions such as water, heat, and UV light. Attempts have been made to resolve the rapid-degradation problems, including formula changes, transport layer improvements, and encapsulations, but none of these have effectively resolved the dilemma. This paper reports our findings on adding inorganic films as surface-passivation layers on top of the hybrid perovskite materials, which not only enhance stability by eliminating weak sites but also prevent water penetration by using a water-stable layer. The surface-passivated hybrid perovskite layer indicates a slight increase of bandgap energy (Eg = 1.76 eV), compared to the original methylammonium lead iodide (MAPbI3, Eg = 1.61 eV) layer, allowing for more stable perovskite layer with a small sacrifice in the photoluminescence property, which represents a lower charge diffusion rate and higher bandgap energy. Our finding offers an alternative approach to resolving the low stability issue for PSC fabrication.

First-Principles Modeling of Lead-Free Perovskites for Photovoltaic Applications

The material class of hybrid organic–inorganic halide perovskites has been rapidly progressed in the field of photovoltaic applications. However, this class of materials has limitations associated with its poor stability and toxicity of the lead element. Therefore, there is a strong desire to search for environmentally friendly perovskite materials without affecting the conversion efficiency. First-principles calculations have been employed to study thermodynamic stability, electronic, and optical properties of potential candidate lead-free perovskites. The results show that the ionization energy at the A-site can provide an explanation for the stabilities of hybrid perovskites. The bond lengths and bond angles are likely responsible for the discrepancy in the band gap. The results indicate that DAGeI3 has better stability and suitable band gap among Ge-based AGeI3 perovskites. It is observed that the Si-doped perovskites are energetically favorable. CsGe2/3Si1/3I3 can become the potential candidate for t...

DFT study of electronic structure and optical properties of layered two-dimensional CH3NH3PbX3 (X=Cl, Br, I)

ABSTRACTTwo-dimensional (2D) organic–inorganic hybrid halide perovskites (OIHPs) have been considered by researchers in the field of solar cells due to their high-temperature stability. In this paper, the electronic and optical properties of single-layer (SL) and multilayer (ML) structures of MAPbX3 (X = Cl, Br, I and MA = CH3NH3) have been studied by density functional theory (DFT) in order to predict its photovoltaic capabilities. The results have shown that SL- and ML-MAPbX3 have a direct band gap in the range of 1.76–2.70 eV. The calculation of dielectric constants has depicted that the static dielectric constants (SDCs) of SL-MAPbX3 are smaller than SDCs of ML-MAPbX3. However, as we expected, the reaction of the structures to in-plane (║) and out-of-plane (┴) polarizations was different; therefore, the SL- and ML-MAPbX3 (X = Cl, Br, I) were optically anisotropic. In addition, the intensity of the optical absorption spectrum for ML-MAPbX3 structures is approximately three times higher than that of SL-MAPbX3 structures. By increasing the radius of halogens (RCl<RBr<RI), surface area under the absorption curve increases and absorbs more. Furthermore, our results have shown that the electronic and optical behavior of 2D-MAPbX3 is suitable for photovoltaic applications and makes them useful for OIHP solar cells.

Significance of hydrogen bonding and other noncovalent interactions in determining octahedral tilting in the CH3NH3PbI3 hybrid organic-inorganic halide perovskite solar cell semiconductor

The CH3NH3PbI3 (methylammonium lead triiodide) perovskite semiconductor system has been viewed as a blockbuster research material during the last five years. Because of its complicated architecture, several of its technological, physical and geometrical issues have been examined many times. Yet this has not assisted in overcoming a number of problems in the field nor in enabling the material to be marketed. For instance, these studies have not clarified the nature and type of hydrogen bonding and other noncovalent interactions involved; the origin of hysteresis; the actual role of the methylammonium cation; the nature of polarity associated with the tetragonal geometry; the unusual origin of various frontier orbital contributions to the conduction band minimum; the underlying phenomena of spin-orbit coupling that causes significant bandgap reduction; and the nature of direct-to-indirect bandgap transition features. Arising from many recent reports, it is now a common belief that the I···H–N interaction formed between the inorganic framework and the ammonium group of CH3NH3+ is the only hydrogen bonded interaction responsible for all temperature-dependent geometrical polymorphs of the system, including the most stable one that persists at low-temperatures, and the significance of all other noncovalent interactions has been overlooked. This study focussed only on the low temperature orthorhombic polymorph of CH3NH3PbI3 and CD3ND3PbI3, where D refers deuterium. Together with QTAIM, DORI and RDG based charge density analyses, the results of density functional theory calculations with PBE with and without van der Waals corrections demonstrate that the prevailing view of hydrogen bonding in CH3NH3PbI3 is misleading as it does not alone determine the a−b+a− tilting pattern of the PbI64− octahedra. This study suggests that it is not only the I···H/D–N, but also the I···H/D–C hydrogen/deuterium bonding and other noncovalent interactions (viz. tetrel-, pnictogen- and lump-hole bonding interactions) that are ubiquitous in the orthorhombic CH3NH3PbI3/CD3ND3PbI3 perovskite geometry. Their interplay determines the overall geometry of the polymorph, and are therefore responsible in part for the emergence of the functional optical properties of this material. This study also suggests that these interactions should not be regarded as the sole determinants of octahedral tilting since lattice dynamics is known to play a critical role as well, a common feature in many inorganic perovskites both in the presence and the absence of the encaged cation, as in CsPbI3/WO3 perovskites, for example.

Predicted photovoltaic performance of lead-based hybrid perovskites under the influence of a mixed-cation approach: theoretical insights

Hybrid organic–inorganic halide perovskite solar cells have recently attracted much attention because of their highly efficient photovoltaic performance.

Orthogonal Lithography for Halide Perovskite Optoelectronic Nanodevices.

3D organic-inorganic hybrid halide perovskites have attracted great interest due to their impressive optoelectronic properties. Recently, the emergence of 2D layered hybrid perovskites, with their excellent and tunable optoelectronic behavior, has encouraged researchers to develop the next generation of optoelectronics based on these 2D materials. However, device fabrication methods of scalable patterning on both types of hybrid perovskites are still lacking as these materials are readily damaged by the organic solvents in standard lithographic processes. We conceived the orthogonal processing and patterning method: Chlorobenzene and hexane, which are orthogonal to hybrid perovskites, are utilized in modified electron beam lithography (EBL) processes to fabricate perovskite-based devices without compromising their electronic or optical characteristics. As a proof-of-concept, we used the orthogonal EBL technique to fabricate a 2D layered single-crystal (C6H5C2H4NH3)2PbI4 photodetector featuring nanoscale patterned electrodes and superior photodetection ability with responsivity of 5.4 mA/W and detectivity of 1.07 × 1013 cm Hz1/2/W. Such orthogonal processing and patterning methods are believed to fully enable the high-resolution, high-throughput fabrication of complex perovskite-based electronics in the near future.

First-principles study on optic-electronic properties of doped formamidinium lead iodide perovskite**Project supported by the National Natural Science Foundation of China (Grant No. 11164004), the Industrial Research Project of Guizhou Province, China (Grant No. GY[2012]3060), the Project of Education Department of Guizhou Province, China (Grant No. [2016]215), and the Special Laboratory Fund of Education Department of Guizhou Province, China (Grant No.

We have discussed the materials of solar cell based on hybrid organic–inorganic halide perovskites with formamidinium ( or FA) lead iodide. Firstly, we build the structure of formamidinium lead iodide (FAPbI3) by using the material studio. By using the first-principles calculations, the energy band structure, density of states (DOS), and partial DOS (PDOS) of the hydrazine-iodide lead halide are obtained. Then, we theoretically analyze a design scheme for perovskite solar cell materials, published in [Science 354, 861 (2016)], with the photoelectric conversion efficiency that can reach 20.3%. Also, we use non-toxic elements to replace lead in FAPbI3 without affecting its photoelectric conversion efficiency. Here in this work, we explore the energy band structure, lattice constant, light absorption efficiency, etc. After the Ca, Zn, Ge Sr, Sn, and Ta atoms replacing lead (Pb) and through comparing the spectral distributions of the solar spectrum, it can be found that FAGeI3, FASnI3, and FAZnI3 have better absorbance characteristics in the solar spectrum range. If the band gap structure is taken into account, FAGeI3 will become an ideal material to replace FAPbI3, although its performance is slightly lower than that of FAPbI3. The toxicity of Pb is taken into account, and the Ge element can be used as a substitute element for Pb. Furthermore, we explore one of the perovskite materials, i.e., FA0.75Cs0.25Sn0.25Ge0.75I3 whose photovoltaic properties are close to those of FA0.75Cs0.25Sn0.5Pb0.5I3, but the former does not contain toxic atoms. Our results pave the way for further investigating the applications of these materials in relevant technologies.

Unique ion diffusion properties in lead-free halide double perovskites: A first-principles study

Pb-free halide double perovskites (HDPs) are proposed as potential candidates for various optoelectronic applications to replace the mainstream hybrid organic-inorganic halide perovskites, e.g., CH3NH3PbI3. While it is known that ion diffusion is a critical problem to affect the structural and electronic stability of CH3NH3PbI3, the mechanism of ion diffusions in HDPs is still unclear and highly desired to be revealed. In this study, taking Cs2AgInX6 (X = Cl, Br) HDPs as prototypes, for the first time we suggest that the fast ion diffusion of the dominant defects may play an important role in the performance stability of HDPs. Importantly, we find that the Agi+ diffusion in a multi-ion concerted fashion has a much faster diffusion rate, compared to the VAg− and VX+ diffusion in a single-ion fashion. It is revealed that HDPs exhibit quite different diffusion properties from CH3NH3PbI3. Furthermore, we demonstrate that the diffusion rate of Agi+ in HDPs can be effectively suppressed by applying an epitaxial strain, which opens a promising way to enhance the performance stability of perovskite materials for various device applications.

Enthalpy of formation of CsSn(Clx(Br,I)1-x)3 and CsPb(Clx(Br,I)1-x)3

Organic-inorganic hybrid halide perovskites, e.g., MAPbI3, have revolutionized the field of solution-processable photovoltaic applications. However, due to organic cation, they are found to be unstable under external environment such as moisture and high temperature. Moreover, the toxicity of lead (Pb) in their structure is harmful for human health and hurdle for commercialization. These inspire us to pursue for families of perovskites structure with the formula CsSn(Clx(Br,I)1-x)3 with x = 1/3, 2/3, 1, which are nontoxic and could have better structural stability under ambient conditions. We have investigated the enthalpy of formation of CsSn(Clx(Br,I)1-x )3 and in based on the density functional theory (DFT) with the generalized gradient approximation (GGA). The main objective is to seek for structural stability with respect to the composition x’s. We found that the structural stability increases linearly with composition x’s of halide atom from I to Br to Cl. We also calculate enthalpy of formation of CsPb(Cix(Br,I)1-x)3 perovskites for comparative purpose, and found that the structural stability of CsSn(Clx(Br,I)1-x)3 is slightly lower than its Pb-counterpart. Thus CsSn(Clx(Br,I)1-x)3 is still the promise candidate for photovoltaics. These studies could serve as a guidance to compromise the stability, by varying composition of halide atoms, with the optimal band gap or other solar-cell-desired properties.

Precursor solution temperature dependence of the optical constants, band gap and Urbach tail in organic–inorganic hybrid halide perovskite films

The band gap energy and optical constants are important parameters for solar cell design. Here, organic–inorganic hybrid halide perovskite CH3NH3PbI3 films (about 140 nm thick) were prepared by vapor deposition at precursor solution temperatures of 30 °C, 50 °C, 70 °C, and 90 °C, respectively. By fitting the ellipsometric spectra in the photon energy range of 0.496–4 eV, we can find that the optical constants, absorption coefficients, and dielectric function strongly depend on the precursor temperature. Due to the change of electronic band structures and spin–orbit coupling, the band gap energy can be modulated from 1.634 eV to 1.516 eV with different precursor temperatures. As band gap energy increases, the Urbach energy decreases with each precursor temperature. The film with a precursor temperature of 70 °C shows the lowest Urbach tail energy (107 meV). The Urbach tail effects can be ascribed to the cumulative effect of impurities, the degree of disorder, and electron–phonon interaction. This study is helpful for understanding the intrinsic optical properties of perovskite films fabricated under different precursor temperatures, so as to provide important information for understanding the device physics and fabrication of high performance perovskite solar cells.

Integral Method Analysis of Electroabsorption Spectra and Electrophotoluminescence Study of (C4H9NH3)2Pbl4 Organic–Inorganic Quantum Well

Electric-field-induced changes in absorption and photoluminescence (PL) spectra and in PL decay profile have been measured for two-dimensional hybrid organic–inorganic halide perovskite semiconductor, ((C4H9NH3)2Pbl4) (N1). Electroabsorption (E-A) spectra observed at room temperature and at a low temperature of 45 K were analyzed by assuming the Stark shift, and the magnitudes of the change in electric dipole moment and polarizability following photoexcitation were determined. The strong signal observed in the E-A spectra at 45 K was interpreted in terms of the weak absorption band which shows extremely large Stark shift resulting from the large change in polarizability following photoexcitation. Electrophotoluminescence spectra of this compound, that is, field-induced change in PL spectra, show that PL of N1 is quenched by the application of electric field. Field effects on PL decay profiles show that the quenching results both from the field-induced decrease of the population of the emitting state follo...

Significant Stability Enhancement of Perovskite Solar Cells by Facile Adhesive Encapsulation

The long-term stability of organic–inorganic hybrid halide perovskite solar cells (PSCs) is one of the major concerns for future commercial applications. This study demonstrated that adhesive encapsulation by employing commercial polyimide tape, which significantly and simultaneously improved the water resistance and thermal stability of MAPbI(3–x)Brx perovskite films and PSCs. The stable operation of encapsulated PSCs in water with a power conversion efficiency of 19.1% was achieved, which was slightly higher than that of the device in air (18.2%). The devices with a stable power output could retain 96.3% of their initial efficiency in water for 1620 s under continuous illumination. In addition, the encapsulated MAPbI(3–x)Brx films could retain their tetragonal perovskite crystal structure after being heated at 240 °C for 180 min in an ambient environment. This adhesive encapsulation offers an avenue to improve the stability of halide perovskite devices for future commercialization.

Structural Diversity in White-Light-Emitting Hybrid Lead Bromide Perovskites.

Hybrid organic-inorganic halide perovskites are under intense investigations because of their astounding physical properties and promises for optoelectronics. Lead bromide and chloride perovskites exhibit intrinsic white-light emission believed to arise from self-trapped excitons (STEs). Here, we report a series of new structurally diverse hybrid lead bromide perovskites that have broad-band emission at room temperature. They feature Pb/Br structures which vary from 1D face-sharing structures to 3D corner- and edge-sharing structures. Through single-crystal X-ray diffraction and low-frequency Raman spectroscopy, we have identified the local distortion level of the octahedral environments of Pb2+ within the structures. The band gaps of these compounds range from 2.92 to 3.50 eV, following the trend of "corner-sharing < edge-sharing < face-sharing". Density functional theory calculations suggest that the electronic structure is highly dependent on the connectivity mode of the PbBr6 octahedra, where the edge- and corner-sharing 1D structure of (2,6-dmpz)3Pb2Br10 exhibits more disperse bands and smaller band gap (2.49 eV) than the face-sharing 1D structure of (hep)PbBr3 (3.10 eV). Using photoemission spectroscopy, we measured the energies of the valence band of these compounds and found them to remain almost constant, while the energy of conduction bands varies. Temperature-dependent PL measurements reveal that the 2D and 3D compounds have narrower PL emission at low temperature (∼5 K), whereas the 1D compounds have both free exciton emission and STE emission. The 1D compound (2,6-dmpz)3Pb2Br10 has the highest photoluminescence quantum yield of 12%, owing to its unique structure that allows efficient charge carrier relaxation and light emission.

Hamiltonians and order parameters for crystals of orientable molecules

Many modern functional materials exhibit microscopic degrees of freedom that are difficult to describe mathematically or do not conform to conventional models in solid state physics. In this paper, we consider crystals containing orientable molecular species, which encompass the class of promising photovoltaic materials ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbX}}_{3}$. Abstracting these materials as crystals of orientable rigid rotors, we develop an effective Hamiltonian that expresses the crystal potential energy in terms of collective orientations of interacting rigid rotors. The approach is motivated by the cluster expansion framework from alloy theory and is appropriate for describing both chiral and achiral molecules. This framework utilizes a quaternion parametrization of molecular orientation and makes full use of the symmetry both of the rotors and of the crystal in order to constrain the functional form of the final Hamiltonian expression. The resulting Hamiltonian is compact, systematically improvable, and is suitable for Monte Carlo or molecular dynamics simulation. We apply this formalism to ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ and report symmetry-adapted basis functions that can be used to construct order parameters or Hamiltonians that describe orientational correlation of methylammonium ions in hybrid organic-inorganic halide perovskite materials.

Direct Measurements of Carrier Transport in Polycrystalline Methylammonium Lead Iodide Perovskite Films with Transient Grating Spectroscopy.

Hybrid organic-inorganic halide perovskites have been proposed in many optoelectronic applications, but critical to their increasing functionality and utility is understanding and controlling carrier transport. Here, we use light-induced transient grating spectroscopy to probe directly carrier transport in polycrystalline methylammonium lead iodide perovskite thin films using a weakly perturbative and noncontact method. The data reveal intrinsic diffusion characteristics of the charge carriers in the material and agree well with a simulated model of charge transport in which grain boundaries act as barriers to carrier movement.

First-principles study on the material properties of the inorganic perovskite Rb1−xCsxPbI3 for solar cell applications

Recently, replacing or mixing organic molecules in the hybrid halide perovskites with the inorganic Cs or Rb cations has been reported to increase the material stability with the comparable solar cell performance. In this work, we systematically investigate the electronic and optical properties of all-inorganic alkali iodide perovskites Rb1-xCsxPbI3 using the first-principles virtual crystal approximation calculations. Our calculations show that as increasing the Cs content x, lattice constants, band gaps, exciton binding energies, and effective masses of charge carriers decrease following the quadratic (linear for effective masses) functions, while static dielectric constants increase following the quadratic function, indicating an enhancement of solar cell performance upon the Rb addition to CsPbI3. When including the many-body interaction within the GW approximation and incorporating the spin-orbit coupling (SOC), we obtain more reliable band gap compared with experiment for CsPbI3, highlighting the importance of using GW+SOC approach for the all-inorganic as well as organic-inorganic hybrid halide perovskite materials.

Solvent-Antisolvent Ambient Processed Large Grain Size Perovskite Thin Films for High-Performance Solar Cells

In recent years, hybrid organic-inorganic halide perovskites have been widely studied for the low-cost fabrication of a wide range of optoelectronic devices, including impressive perovskite-based solar cells. Amongst the key factors influencing the performance of these devices, recent efforts have focused on tailoring the granularity and microstructure of the perovskite films. Albeit, a cost-effective technique allowing to carefully control their microstructure in ambient environmental conditions has not been realized. We report on a solvent-antisolvent ambient processed CH3NH3PbI3−xClx based thin films using a simple and robust solvent engineering technique to achieve large grains (>5 µm) having excellent crystalline quality and surface coverage with very low pinhole density. Using optimized treatment (75% chlorobenzene and 25% ethanol), we achieve highly-compact perovskite films with 99.97% surface coverage to produce solar cells with power conversion efficiencies (PCEs) up-to 14.0%. In these planar solar cells, we find that the density and size of the pinholes are the dominant factors that affect their overall performances. This work provides a promising solvent treatment technique in ambient conditions and paves the way for further optimization of large area thin films and high performance perovskite solar cells.

Efficient planar CsPbBr3 perovskite solar cells by dual-source vacuum evaporation

Organic–inorganic hybrid halide perovskites materials have inspired enormous interest in the photovoltaic research community; however, they are currently plagued by insufficient environmental stability. To overcome this problem, all-inorganic halide perovskites have been developed and exhibit significantly improved stability. Here, vacuum thermal evaporation deposition of CsPbBr3 perovskite films is developed by dual-source coevaporation of CsBr and PbBr2 precursors. The evaporation rate ratio of CsBr to PbBr2 determines the stoichiometry of the deposited CsPbBr3 films, and the substrate temperature and post annealing temperature affect the crystallinity of the CsPbBr3 films. Through systematic optimization of the deposition parameters, high-quality CsPbBr3 films with good crystallinity and uniformity are obtained. Planar CsPbBr3 perovskite solar cells with high efficiency of 6.95% in small size (0.09 cm2) and 5.37% in large size (1 cm2) are fabricated. Moreover, the CsPbBr3 solar cells show good long-term stability under ambient conditions without encapsulation.

Controllable synthesis of organic-inorganic hybrid halide perovskite quantum dots for quasi-solid-state solar cells

Photovoltaics are regarded as the promising solution to resolve energy and environmental problems owing to their high power conversion efficiency and zero emissions. However, the narrow-spectra absorption range and serious recombination reactions at electrode/electrolyte interfaces are two major drawbacks for high-performance solar cells. Herein, a new way of synthesizing organic-inorganic hybrid CH3NH3PbX3 (X = Cl, Br or I) perovskite quantum dots (PQDs) is demonstrated by dropping perovskite precursor solution into anhydrous toluene. After a systematic study, the results demonstrate that the energy levels of PQDs can be tuned through optimizing the stoichiometric ratio of halide ions, showing good optical properties for photovoltaic applications. A quasi-solid-state sensitized solar cell with configuration of FTO/m-TiO2/PQDs/dye/long persistence phosphor/gel electrolyte/Pt/FTO is fabricated, yielding an impressive power conversion efficiency as high as 7.91%, which is higher than 7.26% for referenced device free of PQDs. The improved performances are mainly attributed to the increased light absorption as well as reduced recombination upon the incorporation of PQDs and long persistence phosphor into devices.