Tunability of electronic and optical properties of the Ba–Zr–S system via dimensional reduction

Transition metal sulfide perovskites offer lower band gaps and greater tunability than oxides, along with other desirable properties for applications. Here, we explore dimensional reduction as a tuning strategy using the Ruddlesden–Popper phases in the Ba–Zr–S system as a model. The three-dimensional perovskite BaZrS₃ is a direct gap semiconductor, with a band gap of 1.5 eV suitable for solar photovoltaic application. However, the three known members of the Ruddlesden–Popper series, are all indirect gap materials, and additionally have lower fundamental band gaps. This is accompanied in the case of Ba₂ZrS₄ by a band structure that is more favorable for carrier transport for oriented samples. The layered Ruddlesden–Popper compounds show significantly anisotropic optical properties, as may be expected. The optical spectra show tails at low energy, which may complicate experimental characterization of these materials.     

Enhance the performance and stability of methylammonium lead iodide perovskite solar cells with guanidinium thiocyanate additive

Employing additive to regulate the morphology of perovskite film is an effective method to enhance both the power conversion efficiency and long term stability of organic-inorganic hybrid perovskite solar cells. Here, we demonstrate that guanidinium thiocyanate (GuSCN) is a suitable additive for methylammonium lead iodide (MAPbI₃) perovskite materials. Addition of GuSCN into MAPbI₃ can simultaneously enhance the crystallinity, enlarge the crystal size, and reduce the trap density of the perovskite films. As a result, the MAPbI₃ perovskite with 10% GuSCN exhibits superior power conversion efficiency of 16.70% compared to the pristine MAPbI₃ perovskite solar cell (15.57%). At the same time, the MAPbI₃ perovskite solar cell with GuSCN additive shows better stability, power conversion efficiency retains ～90% of its initial value compared to only ～60% for pristine MAPbI₃ perovskite solar cells after being stored for 15 days without encapsulation.     

DFT and k · p modelling of the phase transitions of lead and tin halide perovskites for photovoltaic cells

3D hybrid organic perovskites, CH₃NH₃PbX₃ (X = halogen), have recently been used to strongly improve the efficiency of dye sensitized solar cells (DSSC) leading to a new class of low‐cost mesoscopic solar cells. CsSnI₃ perovskite can also be used for hole conduction in DSSC. Density functional theory and GW corrections are used to compare lead and tin hybrid and all‐inorganic perovskites. The room temperature optical absorption is associated to electronic transitions between the spin–orbit split‐off band in the conduction band and the valence band. Spin–orbit coupling is about three times smaller for tin. Moreover, the effective mass of relevant band edge hole states is small (0.17). The high temperature phase sequence of CsSnI₃ leading to the room temperature orthorhombic phase and the recently reported phases of CH₃NH₃MI₃ (where M = Pb, Sn) close to the room temperature, are also studied. Tetragonal distortions from the ideal cubic phase are analysed by a k · p perturbation, including spin–orbit effect. In addition, the non‐centrosymmetric phases of CH₃NH₃MI₃ exhibit a splitting of the electronic bands away from the critical point. The present work shows that their physical properties are more similar to conventional semiconductors than to the absorbers used in DSSC. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modulation of strain,electric field and organic cation rotation on the band gap and electronic structures of organic-inorganic hybrid perovskite CH3NH3PbI3

Band gap modulation engineering is an important step in the application of optoelectronic materials. In this paper, the first-principles calculations were carried out to study the influence of strain, external electric field, spatial orientation of organic cation on the band gaps and electronic structures of organic-inorganic hybrid halide perovskites CH₃NH₃PbI₃. The results show that both the uniform strain and the tetragonal deformation can modulate the band gap obviously. The electric field of 0.2 V/Å is the critical point of the band gap modulation. The band gap increases when an electric field is applied from 0 to 0.2 V/Å. The electric field above 0.2 V/Å will cause the band gap to decrease. The spatial orientation of the organic cation also has modulation influence on the band gap of CH₃NH₃PbI₃, but has no effect on the direct semiconductor characteristics. The above results will be helpful to study the band gap modulation of other organic-inorganic hybrid halide perovskites.     

Photoinduced Migration of Ions in Optically Resonant Perovskite Nanoparticles

Organic–inorganic perovskites with a mixed anion composition are widely used in solar cells, light-emitting diodes, and nanophotonic structures. Light nanosources based on resonant perovskite nanoparticles are of particular interest. However, perovskites with such a composition demonstrate the light-induced segregation of anions, which leads to a reversible dynamic rearrangement of the optical properties of a material and photoluminescence spectra. In this work, the photoinduced process of change in optical properties in resonant hybrid perovskite nanoparticles with a mixed anion composition (MAPbBr_1.5I_1.5, where MA = NH₃CH ₃ ⁺ ) has been studied. Comparison with a similar process in a perovskite thin film with a similar composition has shown that the photoinduced migration of halogen ions in a nanoparticle occurs cyclically. This is due to the competition of two processes: the concentration of ions near the boundaries of the particle and migration caused by the gradient of the density of light-generated electron–hole pairs. This effect in resonant nanoparticles makes it possible to obtain optically tunable nanoantennas.     

First-principles study on the electronic and optical properties of cubic ABX3 halide perovskites

The electronic properties of ABX₃ type compounds in the cubic phase are systematically studied using the first-principles calculations. The chemical trend of their properties as A or B or X varies is fully investigated. The optical properties of the ABX₃ compounds are also investigated. Our calculations show that taking into account the spin–orbit coupling effect is crucial for predicting the accurate band gap of these halide perovskites. We predict that CH₃NH₃SnBr₃ is a promising material for solar cells absorber with a perfect band gap and good optical absorption.     

MAPbI3-xBrx钙钛矿薄膜中溴梯度对界面电荷转移的促进作用

混合卤素钙钛矿由于具有优异的光物理性质成为了光电子领域应用中的明星材料. 因此,钙钛矿材料中光生载流子动力学的探究和调控对于进一步提升材料的性能具有重要意义. 本文通过表面离子交换法制备了具有溴梯度的MAPbI_3-xBr_x钙钛矿薄膜,并对其内部载流子传输及界面电荷转移动力学过程进行了系统的研究. 在MAPbI_3-xBr_x薄膜中,溴离子梯度分布所导致的能带梯度能有效促进光生空穴在薄膜内部的传输过程及在界面的提取过程. 同时,由于卤素离子交换的后处理方法对薄膜表面起到了修饰作用,薄膜界面处的本征电子转移速率也得到了显著的提升. 研究表明,在通过表面后处理方法制备的混合卤素钙钛矿薄膜中,有可能同时实现界面电子和空穴转移速率的提升,这对于进一步提升钙钛矿太阳能电池的能量转换效率具有一定的启发作用.     

钙钛矿/银电极界面降解和离子迁移的原位研究

本文对CH_3NH_3PbI_3钙钛矿层与Ag电极之间的界面降解和离子迁移过程进行了全面地研究.利用原位光电子能谱检测于段,发现了Ag电极会诱导钙钛矿层的降解,导致PbI_2和AgI物种的形成以及Pb~(2+)离子在界面处还原成金属Pb物种.I 3d谱峰强度的反常增强为碘离子从CH_3NH_3PbI_3下表面迁移到Ag电极提供了直接的实验证据.此外,Ag电极和钙钛矿层接触会在CH_3NH_3PbI_3/Ag界面处诱导0.3 eV的界面偶极,这可能进一步促进碘离子扩散迁移,导致钙钛矿层的分解和Ag电极的侵蚀.     

Theoretical study on photoelectric properties of lead-free mixed inorganic perovskite RbGe1-xSnxI3

The recent discoveries of lead-free halide perovskites have come into notice as promising photovoltaic materials due to their high solar-to-electrical efficiency conversion. However, these perovskites suffer from large effective masses, wide band gap and affected photovoltaic performance. It is well known that it is an effective means to overcome the above shortcomings by changing the metallic ion concentration and position for the inorganic perovskite. Herein, we study the geometrical, electronic, and optical properties of RbGe_1-xSn_xI₃ with various compositions of metal atoms by performing the Density Functional Theory (DFT). Besides, we systematically investigate how the doping positions of stannum (Sn) atoms affect the electronic structure by taking mixed metal RbGe_0.50Sn_0.50I₃ compound as an example. The results show that RbGe_1-xSn_xI₃ exhibits the semiconducting property with the tunable direct band gaps by changing its proportions. Compared to other two doping positions in the perovskite RbGe_0.50Sn_0.50I₃, the configuration with Sn atom at equator plane has better mobility of electron and optical absorption properties. Our works demonstrate that the modification of metal concentration and position will modulate the optoelectronic performance and photovoltaic properties of mixed metal perovskites.     

无机钙钛矿太阳能电池Cs2SnI6的电子结构和光学性质的第一性原理研究

近年来,Cs2SnI6作为一种无毒性、稳定性好的新型钙钛矿材料应用于太阳能电池中,其电池的光电转换效率由最初不到1%增长到现在的8.5%,使之成为有可能替代铅基钙钛矿太阳能电池的新型太阳能电池。本文采用基于广义密度泛函和杂化密度泛函的第一性原理方法研究了Cs2SnI6的电子结构、光学特性和钙钛矿太阳能电池的光电性能参数。研究结果表明,导带底和价带顶位于同一高对称点Γ而属于直接跃迁型半导体,且电子态主要来自于I-5p轨道和Sn-5s轨道。在近红外和可见光波长范围内有较高的吸收系数,当Cs2SnI6钙钛矿厚度达到10μm时,吸收率在311~989 nm之间接近100%,不考虑潜在损失的情况下,理论上太阳能电池可获得短路电流为32.86 mA/cm2、开路电压0.91 V、填充因子87.4%、光电转换效率26.1%。为实验上制备高效Cs2SnI6钙钛矿太阳能电池提供了参考。     

掺杂质子化石墨相氮化碳增强碳基太阳能电池中钙钛矿的结晶

钙钛矿层的品质极大影响钙钛矿太阳能电池性能. 然而,在溶液法生成多晶钙钛矿膜过程中会不可避免地形成缺陷和陷阱位. 通过在钙钛矿层中嵌入添加物改善钙钛矿晶化,用于减少和钝化缺陷是非常重要的. 本文合成一种环境友好的二维纳米材料质子化石墨相氮化碳(p-g-C₃N₄),并掺杂于碳基钙太阳能电池的钙钛矿层中. 实验证明,在钙钛矿前驱体溶液中添加p-g-C₃N₄不仅能调解碘铅甲胺(MAPbI₃)结晶的成核和生长速率,获得大晶粒尺寸的平滑表面,还能减少钙钛矿层的本征缺陷. 质子化过程在氮化碳表面引入活性基团-NH₂/-NH₃,它们和钙钛矿晶体表面N-H键发生强化学作用,有效地钝化电子陷阱,提高钙钛矿结晶质量. 结果表明,与不掺杂的对照电池(效率为4.48%)和掺杂石墨相氮化碳(g-C₃N₄)电池(效率为5.93%)相比,掺杂质子化石墨相氮化碳(p-g-C₃N₄)的电池获得了6.61%的较高效率. 本工作展示了一种通过掺杂改性添加物改善钙钛矿膜的简单方法,为碳基钙钛矿太阳能电池的低成本制备提供了建议.     

Multistep spin–spray deposition of large-grain-size CH3NH3PbI3 with bilayer structure for conductive-carbon-based perovskite solar cells

Large-grain-size and void-free CH₃NH₃PbI₃ films with bilayer structure are fabricated by spin-coating a PbI₂ layer onto a mesoporous TiO₂ layer and sequentially spraying CH₃NH₃I (methylammonium iodide, MAI) multilayers. The sprayer is controlled by a homemade three-axis computer numerical control machine; thus, the substrates are coated by successive parallel passes achieved by moving the nozzle. Spray deposition at the optimal spray rate and substrate temperature produces a large-grain-size and void-free methylammonium lead iodide (MAPbI₃) bilayer structure. The mesoporous TiO₂ layer plays an important role in electron transport by preventing the return of electrons to the perovskite layer and decreasing the contact resistance at the perovskite/compact TiO₂/fluorine tin oxide interface. When the films are incorporated into a solar cell device with a conductive carbon counter electrode, a maximum power conversion efficiency of 10.58% is realised.     

Inverted structure perovskite solar cells: A theoretical study

We analysed perovskite CH₃NH₃PbI_3-xCl_x inverted planer structure solar cell with nickel oxide (NiO) and spiro-MeOTAD as hole conductors. This structure is free from electron transport layer. The thickness is optimized for NiO and spiro-MeOTAD hole conducting materials and the devices do not exhibit any significant variation for both hole transport materials. The back metal contact work function is varied for NiO hole conductor and observed that Ni and Co metals may be suitable back contacts for efficient carrier dynamics. The solar photovoltaic response showed a linear decrease in efficiency with increasing temperature. The electron affinity and band gap of transparent conducting oxide and NiO layers are varied to understand their impact on conduction and valence band offsets. A range of suitable band gap and electron affinity values are found essential for efficient device performance.     

KTa0.4Nb0.6O3 nanoparticles synthesized through solvothermal method

Potassium tantalate niobate (KTa_0.4Nb_0.6O₃, KTN) nanoparticles of perovskite structure were successsfully synthesized by a solvothermal method. The KTN nanoparticles synthesized at 250 °C for 8 h with 1 to 4 M KOH concentration using isopropyl alcohol [(CH₃)₂ CHOH] as the solvent was composed of a single phase of cubic perovskite structure. Futhermore, the KTN powers synthesized at the same conditions besides of using (CH₃)₂CHOH/H₂O as a solvent compose of a single phase of tetragonal perovskite structure. The nanoparticles exhibit a mixture of cubic and prism-like shapes with lengths of 100 nm to 500 nm and average cross sections of 200×200 nm². The solvent dependence of the powder formation is discussed. X-ray diffraction and electron diffraction results show that the powders have the needed tetragonal perovskite structure. The band gap of KTN nanoparticles is determined to be 3.26 eV from the optical absorption spectra.      

CH3NH3PbI3/poly‐3‐hexylthiophen perovskite mesoscopic solar cells: Performance enhancement by Li‐assisted hole conduction

To address if the non‐triphenylamine derivative hole transporting materials such as P3HT (poly‐3‐hexylthiophene) could also exhibit high device efficiency in mesoscopic MAPbI₃ perovskite solar cells, we examined the effect of Li‐TFSI (Li‐bis(trifluoromethanesulfonyl) imide) and t‐BP (4‐tert‐butylpyridine) additives added in P3HT on device performance. Unlike the triphenylamine HTMs, the P3HT thiophene HTM without amine moiety was not doped by the additives but its conductivity was significantly improved by the Li‐TFSI/t‐BP mediated additional hole conduction. By inclusion of Li‐TFSI/t‐BP additive, we could fabricate more efficient mesoscopic MAPbI₃ perovskite solar cells with smaller hysteresis with respect to scan direction due to Li mediated additional hole conduction. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

钙钛矿晶体的电荷复合动力学研究

本文利用紫外吸收光谱和稳态荧光光谱技术结合理论模型,研究了钙钛矿材料CH₃NH₃PbI₃晶体在光激发过程中的电荷复合动力学行为,进而获得晶体的扩散长度. 电荷载体的扩散长度是判断光电材料的重要参数. 研究通过合成两种不同缺陷态浓度的CH₃NH₃PbI₃晶体,测量这两种晶体在0.019∽4.268 μJ/cm²的激光激发下的时间分辨荧光光谱,利用动力学模型对光谱进行拟合,可以获得每个晶体的掺杂浓度,空穴浓度以及电荷复合参数. 将这些参数结合已有公式,最终可获得每个晶体的电荷载体的扩散长度.     

Theoretical study of p-d hybridization in Co perovskite

The real-space recursion method and unrestricted Hartree-Fock approximation have been applied to calculate the density of states of various Co perovskite, CeCoO₃, SrCoO₃ and Sr_1−xCe_xCoO₃. We have studied the magnetically ordered states of these Co perovskites in an enlarged double cell, and find its various magnetic structures due to the occupancy of 3d band and its interaction with neighboring Co ions. In this study, we have studied the p-d hybridization of the three Co perovskites, we find t_2g electrons are localized and the flat e_g band is responsible for the itinerant behavior, and although the rare earth elements itself contribute little to the DOS at the Fermi energy, the DOS at Fermi energy and the magnetic moment changed consequently because of different valence of Co ions in these compounds and p-d hybridization effect is very important.     

Controlled growth of perovskite microplates arrays for functional optoelectronics

In this study, a novel and facile route was utilized to synthesize methylammonium lead iodide (MAPbI₃) perovskite microplate arrays with uniform morphology and predefined locations. Initially, large-area single-crystalline lead iodide (PbI₂) thin sheets were synthesized through a solvent evaporation crystallization method for synthesizing PbI₂ thin sheets. These PbI₂ sheets were then employed as a seed layer to grow uniaxially aligned arrays of PbI₂ microplates through lithographically defined microscale windows in a polymeric film. Thereafter, the PbI₂ microplates were further intercalated with methylammonium iodide (MAI) to produce perovskite crystals. Structural and optical characterizations showed that the synthesized materials have a distinct heterojunction structures consisting of MAPbI₃ perovskite microplates arrays and the underlying PbI₂ thin sheet layer. The two-step process reported herein, which involves the uniaxial growth of PbI₂ microplates and their conversion to MAPbI₃ perovskite microplates with little dimensional change offers a new pathway for the fabrication of materials for integrated electronic and optoelectronic systems.     

Optical properties of zinc peroxide and zinc oxide multilayer nanohybrid films

Zinc peroxide and zinc oxide nanoparticles were prepared and self-assembled hybrid nanolayers were built up using layer-by-layer (LbL) technique on the surface of glass substrate using the layer silicate hectorite and an anionic polyelectrolyte, sodium polystyrene sulfonate (PSS). Light absorption, interference and morphological properties of the hybrid films were studied to determine their thickness and refractive index. The influence of layer silicates and polymers on the self-organizing properties of ZnO₂ and ZnO nanoparticles was examined. X-ray diffraction revealed that ZnO₂ powders decomposed to ZnO (zincite phase) at relatively low temperatures (less than 200 °C). The optical thickness of the films ranged from 190 to 750 nm and increased linearly with the number of layers. Band gap energies of the ZnO₂/hectorite films were independent from the layer thickness and were larger than that of pure ZnO₂ nanodispersion. Decomposition of ZnO₂ to ZnO and O₂ at 400 °C resulted in the decrease of the band gap energy from 3.75 to 3.3 eV. Concomitantly, the refractive index increased in correlation with the formation of the zincite ZnO phase. In contrast, the band gap energies of the ZnO₂/PSS hybrid films decreased with the thickness of the nanohybrid layers. We ascribe this phenomenon to the steric stabilization of primary ZnO₂ particles present in the confined space between adjacent layers of hectorite sheets.