Organic-inorganic Halide Perovskite Materials Research Articles

Improve the Charge Carrier Transporting in Two-Dimensional Ruddlesden-Popper Perovskite Solar Cells.

Conventional 3D organic-inorganic halide perovskite materials have shown substantial potential in the field of optoelectronics, enabling the power conversation efficiency of solar cells beyond 26%. A key challenge limiting the further commercial application of 3D perovskite solar cells is their inherent instability over outer oxygen, humidity, light, and heat. By contrast, 2D Ruddlesden-Popper (2DRP) perovskites with bulky organic cations can effectively stabilize the inorganic slabs, yielding excellent environmental stability. However, the efficiencies of 2DRP perovskite solar cells are much lower than those of the 3D counterparts due to poor charge carrier transporting property of insulating bulky organic cations. Their inner structural, dielectric, optical, and excitonic properties remain to be primarily studied. In this review, the main reasons for the low efficiency of 2DRP perovskite solar cells are first analyzed. Next, a detailed description of various strategies for improving the charge carrier transporting of 2DRP perovskites is provided, such as bandgap regulation, perovskite crystal phase orientation and distribution, energy level matching, interfacial modification, etc. Finally, a summary is given, and the possible future research directions and methods to achieve high-efficiency and stable 2DRP perovskite solar cells are rationalized.

In-plane uniaxial crystal growth in CH3NH3PbI3 bar-coated thin film by evaporation-site-controlled annealing

In the thin film fabrication of a typical organic–inorganic halide perovskite material, CH3NH3PbI3 (MAPbI3), by bar-coating, thermal conversion from its precursor to MAPbI3 was focused on, and an evaporation-site-controlled annealing method was proposed. The crystal grain size of MAPbI3 changed depending on the sweep speed of a cover plate set above the precursor liquid films. The crystallinity of the MAPbI3 thin film was clarified by analyzing electron backscatter diffraction. The typical inverted-type solar cells with the MAPbI3 thin films prepared by evaporation-site-controlled annealing were fabricated, and the improved performance was demonstrated in comparison with the conventional uniform annealing method.

Synthesis, crystal structure and optical properties of the quasi-0D lead-free organic-inorganic hybrid crystal (C6H14N)3Bi2I9·H2O

The organic-inorganic metal halide perovskite materials have remarkable semiconductor and light absorption characteristics. It shows excellent application prospects in the fields of optoelectronics and temperature sensors. In this work, zero-dimensional lead-free (C6H14N)3Bi2I9·H2O single crystals were synthesized by conventional solvothermal reaction. The crystal structure was analyzed using single crystal X-ray diffraction (SXRD). (C6H14N)3Bi2I9·H2O was crystallized in the orthorhombic crystal system with the space group Pmn21. The material is stable for long-term storage under ambient conditions and shows a reversible thermochromic phenomenon. Through ultraviolet–visible–near-infrared (UV–vis–NIR) absorption analysis, the band gap of (C6H14N)3Bi2I9·H2O at room temperature is found to be 2.12 ​eV and it gradually decreases as the temperature increases. This indicates that the thermochromism is probably caused by charge transfer. The response of material to optical signals was also studied and the maximum value of the responsivity (R) and external quantum efficiency (EQE) at 275 ​nm could reach 11 ​mA/W and 2.4%, respectively. It suggests that the material has a potential for applications in photovoltaic devices.

Phase segregation affects electron–phonon coupling in perovskite solar cell materials

Phase segregation has been described as a significant factor that limits solar cell efficiency and long-term stability in mixed organic–inorganic halide perovskite materials.

Surface passivation of the methylammonium lead iodide (CH3NH3PbI3) by oxygen plasma to improve solar cell efficiency

The photoelectronic materials of hybrid organic-inorganic halide perovskite (HHPs) materials are an important research field. However, the defects on the surface of perovskite films need to be solved. For organic-inorganic halide perovskite materials, oxygen passivation has been regarded as an effective and convenient method to inhibit non-radiative recombination. In this paper, the unannealed CH3NH3PbI3 precursor films were passivated by oxygen plasma for different irradiation times. The prepared perovskite films have large grain sizes, lower trap density, improved crystallinity, and strong absorption. The J-V curve shows that the HHPs solar cells with oxygen treatment increase the solar cell's overall photoelectric conversion efficiency (PCE) from 10.24% to 14.23%. Experimental results and analysis demonstrate that oxygen plasma successfully passivates the surface defects of perovskite, minimizes non-radiation recombination, and enhances device stability.

Topological feature engineering for machine learning based halide perovskite materials design

Accelerated materials development with machine learning (ML) assisted screening and high throughput experimentation for new photovoltaic materials holds the key to addressing our grand energy challenges. Data-driven ML is envisaged as a decisive enabler for new perovskite materials discovery. However, its full potential can be severely curtailed by poorly represented molecular descriptors (or fingerprints). Optimal descriptors are essential for establishing effective mathematical representations of quantitative structure-property relationships. Here we reveal that our persistent functions (PFs) based learning models offer significant accuracy advantages over traditional descriptor based models in organic-inorganic halide perovskite (OIHP) materials design and have similar performance as deep learning models. Our multiscale simplicial complex approach not only provides a more precise representation for OIHP structures and underlying interactions, but also has better transferability to ML models. Our results demonstrate that advanced geometrical and topological invariants are highly efficient feature engineering approaches that can markedly improve the performance of learning models for molecular data analysis. Further, new structure-property relationships can be established between our invariants and bandgaps. We anticipate that our molecular representations and featurization models will transcend the limitations of conventional approaches and lead to breakthroughs in perovskite materials design and discovery.

CuInSe2 quantum dots doped MAPbI3 films with reduced trap density for perovskite solar cells

The organic-inorganic halide perovskite (OIHP) materials are becoming promising, but there is still inherent defect-mediated-carrier recombination in hybrid perovskite materials, thus, leading to inefficient power conversion efficiency (PCE). The defect states, which act as the recombination centers, are usually formed during crystallization process. Here, by a hot injection method, Na2S ligand exchange is applied to synthesize copper indium selenium quantum dots (CuInSe2 QDs), which are well dispersed in N,N-dimethylformamide (DMF) and then added into perovskite precursor solution. The resulted QD-in-perovskite structure is found to be beneficial for enlarging grain size, and thus exhibits an improved crystallinity and absorption due to the passivation of trap states. Hence, the perovskite solar cell (PSC) based on the MAPbI3-CuInSe2 QD hybrid film show an improved carrier lifetime from 58.17 ns to 243.23 ns and an enhanced PCE from 16.3% to 18.4% due to higher open-circuit voltage, compared to the cell device based on the pure perovskite film. Additionally, the optimized cell device presents better stability, which can still keep 75% of the initial PCE after 20-day storage. Such a simple Na2S ligand exchange to introduce QD into perovskite provides strong potential for preparing other hybrid materials.

Thickness control and photovoltaic properties of CH3NH3PbI3 bar-coated thin film

Thin films of a typical organic–inorganic halide perovskite material, CH3NH3PbI3 (MAPbI3), were fabricated by a bar-coating method, which is one of the candidate techniques for large-scale production. The film thickness of MAPbI3 markedly changed depending on the sweep speed of the coating bar, that is, it decreased in evaporation regime under the low-speed condition, and increased in the Landau–Levich regime under the high-speed condition. The typical inverted-type p-i-n planar solar cells with the MAPbI3 thin films demonstrated the photoelectric conversion efficiency of 14.0%, and their photovoltaic properties depending on the sweep speed were discussed by taking the surface morphology and crystallinity into consideration.

Electronic and optical properties of inorganic lead-free perovskite Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>

Organic-inorganic halide perovskite materials are widely used in solar cells because of their excellent photoelectric properties. However, the stability and lead toxicity problems associated with materials and devices have restrict their production and development. Compared with the hybrid perovskite, the inorganic lead-free perovskite Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> has attracted wide attention because of its stronger stability and environmental friendliness. The Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> has three crystal types: monoclinic type, trigonal type, and hexagonal type. At present, the researches of Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> focus mainly on the hexagonal phase. In this paper, based on the first principles of density functional theory, the electronic properties, carrier effective mass values, stabilities, and optical properties of Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> monoclinic, trigonal, and hexagonal phases are studied theoretically. It is suggested that the stabilities of the three crystal phases are similar, and the direct band gap (1.21 eV) of the trigonal phase would be noticeable. For the three phases, their effective mass values show that their properties are the same along both the <i>a</i> direction and the <i>b</i> direction, but different along the <i>c</i> direction. The effective mass of electron of the trigonal phase is significantly smaller along the <i>a</i>-direction than along the <i>c</i>-direction. Corresponding to the red shift phenomenon of optical properties, the trigonal phase shows the better optical absorption performance than other phases. In addition, the optical properties also show that the properties are the same along the <i>a</i> direction and the <i>b</i> direction, but different along the <i>c</i> direction, and the optical absorption performance is better along the <i>a</i>-direction than along the <i>c</i>-direction.

Manipulated Crystallization and Passivated Defects for Efficient Perovskite Solar Cells via Addition of Ammonium Iodide

Organic-inorganic metal halide perovskite materials have been widely studied as the light absorber for efficient photovoltaics. However, perovskite layers with defective nature are typically prepared with an uncontrollable crystallization process, intrinsically limiting further advance in device performance, and thus require delicate manipulation of crystallization processes and defect density. Here, we demonstrate an ammonium-assisted crystallization of perovskite absorbers during a two-step deposition to fabricate efficient solar cells. Addition of ammonium iodide (NH4I) is devised to manipulate the nucleation and crystal growth of perovskite, wherein the formation and transition of intermediate x[NH4+]•[PbI3]x- enables high-quality perovskite layers with an enlarged grain and reduced defect density. As a result, the perovskite solar cells (PSCs) achieve an average efficiency of 21.36% with a champion efficiency of 22.15% and improved environmental stability over 30 days in ambient conditions with varied relative humidity. These results with addition of NH4I provide an available and ingenious way to construct high-quality perovskite layers for efficient solar cells and will advance the commercial application of perovskite-based photovoltaics.

Performance optimization of lead free-MASnI3 based solar cell with 27% efficiency by numerical simulation

Abstract The photovoltaic devices based on organic-inorganic halide perovskite materials have witnessed a rapid increase in performance and are marching towards commercialization. Recently, lead-free perovskites have drawn huge attention as one of the major research topics over toxic lead-based materials. Here, we report the optimized device performance of methylammonium tin iodide (MASnI3) based perovskite solar cell by using numerical simulation. The influence of different key parameters, such as different hole transport layers (HTLs), doping density, thickness of different layers including defect density on the device performances is thoroughly analysed through numerical simulation. The optimized device architecture with copper (I) oxide (Cu2O) as the hole transport layer and TiO2 as electron transport layer shows the highest power conversion efficiency of 27.43%, short circuit current density of 25.97 mA/cm2, open-circuit voltage of 1.203 V, and fill factor of 87.79%. This indicates that by optimizing the device parameters, it is possible to achieve high performance lead-free perovskite solar cells experimentally in future research.

A data fusion approach to optimize compositional stability of halide perovskites

Summary Search for resource-efficient materials in vast compositional spaces is an outstanding challenge in creating environmentally stable perovskite semiconductors. We demonstrate a physics-constrained sequential learning framework to subsequently identify the most stable alloyed organic-inorganic perovskites. We fuse data from high-throughput degradation tests and first-principle calculations of phase thermodynamics into an end-to-end Bayesian optimization algorithm using probabilistic constraints. By sampling just 1.8% of the discretized CsxMAyFA1−x−yPbI3 (MA, methylammonium; FA, formamidinium) compositional space, perovskites centered at Cs0.17MA0.03FA0.80PbI3 show minimal optical change under increased temperature, moisture, and illumination with >17-fold stability improvement over MAPbI3. The thin films have 3-fold improved stability compared with state-of-the-art multi-halide Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3, translating into enhanced solar cell stability without compromising conversion efficiency. Synchrotron-based X-ray scattering validates the suppression of chemical decomposition and minority phase formation achieved using fewer elements and a maximum of 8% MA. We anticipate that this data fusion approach can be extended to guide materials discovery for a wide range of multinary systems.

Calculation of electronic and optical properties of methylammonium lead iodide perovskite for application in solar cell.

Organic-inorganic metal halide perovskite materials, i.e., ABX3 (A = methylammonium, B = Pb, X = Cl, Br, I) have been proved to be outstanding for solar energy conversion. They provide a solution to renewable energy problems with good efficiency and cost-effective technology. Here, we report the initial calculations done by solving Kohn-Sham equations by the use of density function theory. The electronic structural and band gap of CH3NH3PbI3 material are obtained by using different exchange-correlation potential (PBE, PBE-sol, GGA). Further, solar cell devices with CH3NH3PbI3 as absorption layer and CdS/TiO2/ZnTe as buffer layer have been modeled; device physics is discussed and performance of solar cell structure is analyzed in terms of short circuit current density, open circuit voltage, efficiency, fill factor, and quantum efficiency. The maximum efficiency of CH3NH3PbI3 solar cell is found to be 19.6% with TiO2 buffer layer, whereas efficiency with ZnTe buffer layer is also comparable which is 19.5%. Further the effect of layer thickness and temperature are analyzed for maximum efficiency.

Role of BCP layer on nonlinear properties of perovskite solar cell

Organic-inorganic hybrid halide perovskite materials have been most attractive for wide range of optoelectronic applications. Investigating the photo-response in these devices will be very useful to understanding cell performance in different conditions and applications. For understanding non-linearity photo-response we use the Bathocuproine (BCP) as blocking layer in two interfaces of inverted perovskite solar cell (FTO/NiO/BCP/Perovskite/PCBM/Au and FTO/NiO/Perovskite/PCBM/BCP/Au). Employing thick BCP layer (≈15 nm) caused s-shaped JV curve due to charge accumulation at interfaces. The nonlinearity effect was also explored for devices with s-shaped JV and established that device parameters respect to light intensity were nonlinear. Short circuit current had a nonlinear response respect to light intensity while the voltage behavior in terms of light intensity indicated that monomolecular recombination is dominant mechanism. The open circuit voltage is almost 100% nonlinear from 1 to 0.1 sun in device A, while in second device shown from 1 to 0.5 sun almost 50% nonlinearity. The results suggest that our simple strategy not only can be useful for understanding how cell performance is but also can be used in some optical setup such as Focus induced photo-response (FIP).

Synergetic Effects of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) with Nanomaterials for Efficient Hole Extracted Perovskite Photovoltaics

Organic-inorganic halide perovskite materials have been touted for their excellent light-absorbing properties, implying great potential for low-cost fabrication of high-performance solar cells. Nevertheless, numerous studies revealed that choice of materials and optimization of device architecture is crucial for realizing high efficiency photovoltaics consisting perovskite photoactive materials. PEDOT:PSS, poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate), has been widely selected as a hole transport material, for its high transparency, which allows enough light to the photoactive layer. However, perovskite solar cells with PEDOT:PSS hole transport layer often suffers low stability, due to the high acidity compared to the other devices.In this research, we introduce various effective solutions for controlling electrical and chemical properties of PEDOT:PSS, for realization of high efficiency and stability of perovskite solar cells. Introducing inorganic copper-based chalcogenide, (such as copper sulfide, Cu2-xS) nanoparticles [1] and organic nanosheets [2] (covalent organic nanosheets, CONs) to PEDOT:PSS hole transport layer resulted in improved charge dynamics factors and device performance. The incorporation of the CuS nanoparticles induced interaction between PEDOT molecules and the particles, which resulted in structural transition of PEDOT:PSS and its higher hole mobility. Interestingly, UV-selective absorbing properties of CuS particles reduced photo-induced damage on perovskite materials, by filtering UV light from the sunlight which enhanced the devices’ stability significantly. On the other hand, the additional organic interlayer between PEDOT:PSS and hole-selective electrode facilitates formation of even film morphology, which contributed to the higher crystallinity of perovskite layers and enhanced charge separation, which resulted in higher performance compared to the pristine device. Suppressing acidity of PEDOT:PSS layer has been another target for enhancing stability of perovskite solar cells. Acidity modification by incorporation of imidazole [3] not only prevented unintentional degradation of device structure, and the PEDOT:PSS with modified pH brought about higher performance which originated from tuned surface properties and molecular rearrangement, which resulted in better crystallization of perovskite coated onto the PEDOT:PSS layers, and efficient charge transport. Also, tailoring ratio of amount of PEDOT to PSS molecules by inducing solution reaction between neutral and acid PEDOT:PSS [4] revealed encouraging electric properties and stability of PEDOT:PSS hole transport layer. This work provides solutions for effective tailoring of electrical and chemical properties of PEDOT:PSS, which will shed light on the low-cost and reliable fabrication of perovskite photovoltaics.[1] Kim, B. G., Jang, W., Choa, Y. H., and Wang, D. H., ACS Sustain. Chem. Eng., 2020, 8, 20, 7617–7627.[2] Park, S., Kim, M. S., Jang, W., Park, J. K., and Wang, D. H. Nanoscale, 2018, 10, 4708-4717.[3] Yi, M., Jang, W., and Wang, D. H. (2019). ACS Sustain. Chem. Eng., 2019, 7, 8245–8254.[4] Kim, M., Yi, M., Jang, W., Kim, J. K., and Wang, D. H. Polymers, 2020, 12, 129.

Natural passivation of the perovskite layer by oxygen in ambient air to improve the efficiency and stability of perovskite solar cells simultaneously

Abstract For the hybrid organic-inorganic halide perovskite (HHPs) material, oxygen passivation could enhance the photoluminescence intensity, while it also deteriorates the stability of HHPs film or device. It is a challenge how to use oxygen passivation to improve the efficiency and stability of the HHPs solar cells simultaneously. Here we reported a novel and simple method for natural passivation of the perovskite layer with oxygen in ambient air by exclusion of light illumination. The HHPs solar cells were fabricated by deposition of hole transport layer after oxygen passivation. By optimizing the passivation conditions, the power conversion efficiency of the solar cells increases to 20.1% from 17.3% for those without passivation. Meanwhile, the stability of the HHPs solar cells with passivation was improved. This work is the first report applying oxygen passivation to improve the efficiency and stability of HHPs solar cells simultaneously.

Improvement of Characteristics of Metal Doped TiO₂ Thin Film and Application to Perovskite Solar Cell.

Over the past three decades, the development of renewable energy technologies has attracted significant attention to overcome both environmental pollution and global warming. Recently, a new type of solar cell based on an organic-inorganic halide perovskite material has been developed. Perovskite solar cells (PSC) were first reported in 2009; their efficiencies increased rapidly from 3.8% to 22%. PSCs have many advantages owing to their use of simple processing technology and stable materials. Perovskite materials have a general formula of ABX, where A is generally methyl ammonium CH₃NH3+ (MA), B is a metal ion, such as Pb or Sn, and × represents a halogen ion. A distinct advantage of lead-based perovskites (i.e., MAPbX₃) is that their band gaps can be easily tuned, from 1.2 to 2.3 eV, by varying their compositions and anions. Titanium dioxide is as often used as an electron transport layer due to its high chemical and optical stability, non-toxicity, low cost, and resistance to corrosion. TiO₂ films can be characterized by the defects in their preparation, such as density fluctuations, pinholes, and cracks; these defects can reduce electrical conductivity and cause recombination. In this study, we have demonstrated that the electrical conductivity of TiO₂ thin films is improved by its doping with Al3+. When applied to a PSC, the doped thin film improves the charge transfer of the solar cell and increases its efficiency. Our results suggest that Al3+ nanoparticles in the TiO₂ layer may contribute to the improvement of the PSC.

Photoelectrochemical and first-principles investigation on halide perovskite/TiO2 film improved by dicyano dye

Abstract The severe degradation and limited optoelectronic performance in water is the Achilles' heel of hybrid organic-inorganic halide perovskite materials, which prevents their further industrial deployment in hostile conditions. In this manuscript, we propose a new molecular dye bearing a dicyano group to improve the water stability and optoelectronic properties of the halide perovskite/TiO2 film. The halide perovskite-based photoelectrochemical setup bearing the dicyano anchor-based dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran, witnesses a significant enhancement in the photocurrent generated by the halide perovskite CH3NH3PbI3-based film in aqueous solution under the visible light irradiation. Such enhanced optoelectronic performance is accompanied with a much improved stability of the halide perovskite film in water. The first-principles calculations suggest that the molecular dye bearing the dicynao anchor not only adsorbs on the TiO2 substrate, but also strongly interacts with the halide perovskite surfaces; therefore, the bifunctional dicyano anchor could stabilize the CH3NH3PbI3/TiO2 interfacial structure and improve the optoelectronic performance and stability of the halide perovskite electrode. This study benefits the development of the dye-sensitized perovskite materials for the halide-perovskite-based energy devices such as solar cells and water-splitting systems.

Surface polished of bulk methylammonium lead tribromide single crystal

Organic-inorganic halide perovskite materials have been demonstrated with wide applications in optoelectronics and ionization radiation detection. For bulk as-grown crystals, the existence of surface cracks and defects can significantly increase charges recombination and reduce the performance of the device. Herein, we polished the crystal surfaces with both mechanical and chemical mechanical methods at room temperature. After been chemical-mechanical polished, the crystal surface with root mean square roughness about 0.5 nm was obtained. Optical transmission and photoluminescence spectra indicate that chemical mechanical polishing technology can effectively reduce the density of crystal surface defects. The achieved low leakage current density on the surface and bulk crystal is 0.05 nA mm−2 and 0.07 nA mm−2, respectively. Furthermore, the current-voltage curve under visible photons and X-ray photons reveals that surface polishing treatment can suppress the charges recombination and increase the charges transportation.

Stable fluorescent NH3 sensor based on MAPbBr3 encapsulated by tetrabutylammonium cations

Recently, organic-inorganic halide perovskite materials were investigated on gas sensing due to their excellent optical properties and gas sensitivities. Here, we designed a stable fluorescent perovskite-based sensor for NH3 detection, in which the CH3NH3PbBr3 (MAPbBr3) film was deposited on the GeO2 substrate and also capped by tetrabutylammonium (TBA) ligand as a stabilizing agent. This as-fabricated MAPbBr3-TBA-based sensor exhibited the relatively strong and stable photoluminescence (PL) intensity, thereby expanding the fluorescence response range for NH3 sensing. Upon exposure to NH3 gas, the PL intensity quenched rapidly by 62.5% with short response time (61 s) and recovery time (65 s). The sensor also possessed a linear relationship between the PL intensity and concentration of NH3 in the range of 0–100 ppm, and presented excellent reversibility, high gas selectivity, and humidity stability. Furthermore, the NH3 sensing mechanism was investigated based on X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Differential thermal analysis-Thermogravimetry (DTA-TG) and fluorescence lifetime measurements, in which the NH3 molecules might permeate the capped TBA ligand and then induce structure transformation of inner MAPbBr3 crystal. This study indicated that the as-prepared MAPbBr3-TBA-based sensor might provide promising applications on NH3 gas detection.