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.     

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.     

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.     

Low‐Temperature‐Processed CdS as the Electron Selective Layer in an Organometal Halide Perovskite Photovoltaic Device

Organic–inorganic halide perovskites have recently been crowned as the leading next‐generation photovoltaic material due to their high efficiency and simple fabrication process. Herein, a low‐temperature‐processed CdS thin film (commonly used as a buffer layer in commercial CdTe or CIGSe solar cells) is reported as an electron selective layer in perovskite devices based on the following reasons: First, the photoelectric property of CdS thin film is investigated, illustrating the possibility of CdS as the electron selective layer in the application of methylammonium lead (II) iodide perovskite devices. More specifically, CdS semiconductor film presents a higher mobility compared with traditional TiO₂ thin film, which benefits the electron extraction and transmission; second, it is found that the perovskite thin film spun‐coating on the CdS substrates grows with an obvious tendency along the direction toward the thickness of thin film, which reduces the chance of recombination of electrons and hole, beneficial to their separation. It is also revealed that the perovskite‐device‐based CdS electron selective layer has a higher stability compared with that of TiO₂ due to the difference of substrates wetting.     

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.     

Electron transport layer engineering with rubidium chloride alkali halide to boost the performance of perovskite absorber layer

The mesoscopic perovskite solar cells (PSCs) based on titanium oxide (TiO₂) with a certified 25.2% efficiency are the forefront devices in the PSCs field. Hence, it can conclude the mesoporous titanium oxide (mp-TiO₂) is one of the most promising candidates to use as an electron transport layer (ETL) in PSCs. Improving the conductivity of mp-TiO₂ can consider as a simple route to motivate the electron extraction ability of this layer and increase the efficiency of PSCs. Herein, rubidium chloride (RbCl) was introduced as an additive source to boost the optoelectronic properties of mp-TiO₂ ETL. It was observed through ETL modification with RbCl, the optical transmittance of mp-TiO₂ remains constant but increases its electro-conductivity. Results showed that the morphology and crystalline properties of the perovskite layer with a modified ETL substrate is improved. It indicates a perovskite layer with enlarger grains and lower lead iodide (PbI₂) surplus. Altogether, ETL modification brings a champion efficiency of 11.10% for hole transport layer (HTL)-free PSCs higher than that of 8.65% for the HTL-free PSCs based on pristine ETL. Besides, Modified PSCs compared to pristine PSCs showed higher stability response as a result of lower grain boundaries in the modified perovskite layer.     

Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

Methylammonium lead iodide (CH₃NH₃PbI₃) based perovskite having low degrees of the disorder is of great interest for optoelectronic and photovoltaic applications. In this work, a layer of CH₃NH₃PbI₃ was successfully prepared using an ultrasonically sprayed-nebulous method. Changes in structural and optical properties alongside with photo-induced charge separation and transportation behavior were systematically studied. The surface photovoltage spectra reveal a significant reduction of the density of deep defect states as the organic content was increased. It was observed that the measured values of Urbach energies decrease from 33.36 to 28.24 meV as the amount of organic content was increased to an optimum value. The best perovskite solar cells obtained using the sprayed-on approach exhibited a J_sc of 16.54 mA/cm², a V_oc of 0.99 V, and a FF of 62.4, resulting in an overall PCE of 10.09%.     

Fabrication of mixed perovskite organic cation thin films via controllable cation exchange

Here in this paper, we demonstrate a facile technique for creating the mixed formamidinium(HN = CHNH_3~+, FA~+)and methylammonium(CH_3NH_3~+, MA~+) cations in the lead iodide perovskite. This technique entails a facile drop-casting of formamidinium iodide(FAI) solutions on as-prepared MAPbI_3 perovskite thin films under the controlled conditions,which leads to controllable displacement of the MA~+ cations by FA~+ cations in the perovskite structure at room temperature. Uniform and controllable mixed organic cation perovskite thin films without a "bi-layered" or graded structure are achieved. By applying this approach to photovoltaic devices, we are able to improve the performances of devices through extending their optical-absorption onset further into the infrared region to enhance solar-light harvesting. Additionally,this work provides a simple and efficient technique to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials for high-performance perovskite solar cells.     

Humidity controlled crystallization of thin CH3NH3PbI3 films for high performance perovskite solar cell

Control of crystallization of a solution‐processed perovskite layer is of prime importance for high performance solar cells. In spite of the negative effect of water on perovskite solar energy conversion in several previous works, we observed that humidity plays a critical role to develop a thin uniform, dense perovskite film with preferred crystals, in particular, in a device with architecture of ITO/PEDOT:PSS/CH₃NH₃PbI₃/ PC₇₁BM/LiF/Al fabricated by two‐step sequential spin‐coating process. Humidity controlled spin‐coating of CH₃NH₃I on the pre‐formed PbI₂ layer was the most influential process and systematic structural investigation as a function of humidity revealed that grains of CH₃NH₃PbI₃ perovskite crystals increase in size with their preferred orientation while film surface becomes roughened as the humidity increases. The performance of a device was closely related to the humidity dependent film morphology and in 40% relative humidity, the device exhibited the maximum power conversion efficiency of approximately 12% more than 10 times greater than that of a device fabricated at 20% humidity. The results suggest that our process with controlled humidity can be another efficient route for high performance and reliable perovskite solar cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The influence of localized states on the optical absorption and carrier transport properties of acylamino hybrid perovskites with tunable electronic structures

Organic-inorganic hybrid perovskite solar cells have excellent optoelectronic properties, but their low thermal and chemical stabilities limit their commercial applications. In this paper, a new type of organic-inorganic hybrid perovskite is proposed. Malondiamide (MA,CH₂(CONH₂)₂) and propionamide (PA, CH₃CH₂CONH₂) were used as organic layers, with Pb-I octahedral inorganic layers to form quasi three-dimensional (3D) perovskites. The crystal structure, stability, electronic structure, and optical properties of MAPbI₄ and PAPbI₄ perovskites were investigated, and the results showed that there were localized states that corresponded to the number of acyl groups in the two perovskites. Energy band calculations showed that the localized states of the two perovskites rose above the bottom of the conduction band. This can be used to regulate the band gap of the two perovskites, which affects the electronic properties and optical absorption characteristics of the two perovskites. Compared with PAPbI₄, MAPbI₄ has a lower formation energy, lower band gap, lower effective mass of electrons and holes, wider energy range, and larger absorption coefficient, which indicates that MAPbI₄ is more suitable for use in solar cells. This study provides guidance for obtaining efficient and stable photovoltaic materials.     

Study on the stability of organic–inorganic perovskite solar cell materials based on first principle

ABSTRACT

In order to better understand and elucidate the structural stability of perovskite materials, the lattice parameters and tolerance factors of three crystal structures of perovskite materials are calculated based on the first principle of density functional theory. We find that the perovskite crystal structures are relatively stable and is consistent with the experimental facts as the tolerance factor 0.81?<?T?<?1.11. The elastic modulus of three crystal structures of MAPbI₃, FAPbI₃ and the elastic modulus of FA_0.75Cs_0.25Sn_0.5PB_0.5I₃ are studied. By Voigt-Reuss-Hill approximation, the elastic properties such as bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are obtained. From the elastic modulus C_ij, we can find that the other six kinds of crystal structures are relatively stable except for the orthogonal structure of MAPbI₃ (c). The ductility and brittle toughness of the material are also discussed by B/G and Poisson’s ratio. It is found that MAPbI₃ (a) is the hardest and FAPBI₃ (a) the weakest. Form the three-dimensional surface view of Young's modulus it is found that their dependence in three-dimensional direction is spherical for an isotropic system. The degree of deviation of the Young's modulus sphere reflects the anisotropy of crystal structures. The degree of elastic anisotropy of organic–inorganic perovskite materials follows the order of FAPbI₃(c)?>?MAPbI₃(a)?>?FA_0.75?Cs_0.25?Sn_0.5Pb_0.5I₃?>?FAPbI₃(a)?>?MAPbI₃(b)?>?MAPbI₃(c)?>FAPbI₃(b). Furthermore, by the adsorption energies and density of states (DOS) of these seven crystals for water molecules, the reasons why perovskite materials are easily denatured in high humidity environment were explored. The results show that perovskite materials are easy to denaturate in high humidity environment.     

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.     

Effect of thickness on the structural,optical and electrical properties of thermally evaporated PbI2 thin films

This work describes the physical properties of lead iodide (PbI₂) thin films with different thicknesses that were deposited on ultrasonically cleaned glass substrates using a thermal evaporation technique at 5×10^-6 torr. The initial material was purified by the zone refining technique under an atmosphere of argon gas. XRD analysis of the material demonstrates that the thin films were preferably oriented along the (001) direction. The size of the crystallites was calculated from the Scherer relation and found to be in the range of ～5–10 nm, with higher values being observed for increasing film thicknesses. The optical energy band gaps were evaluated and determined to belong to direct transitions. Because the band gap increased with decreasing film thickness, a systematic blue shift was observed. The surface morphologies of PbI₂ films exhibited a clear increase in grain size with increasing film thickness. The photoluminescence and dc conductivity of the thin films are also discussed.     

Large grained and high charge carrier lifetime CH3NH3PbI3 thin-films: implications for perovskite solar cells

Spin coated perovskite thin films are known to have an issue of pinholes & poor morphology control which lead to poor device-to-device repeatability, that is an impediment to scale-up. In this work, Methylamine vapor annealing process is demonstrated which consistently leads to high-quality perovskite thin-films with an average grain-size of 10–15 μm. The improvement in film morphology enables improvement in effective carrier recombination lifetime, from 21 μs in as-deposited films to 54 μs in vapor-annealed films. The annealed films with large-grains are also more stable in ambient conditions. Devices made on annealed perovskite films are very consistent, with a standard deviation of only 0.7%. Methylamine vapor annealing process is a promising method of depositing large-grain CH₃NH₃PbI₃ films with high recombination lifetime and the devices with improved performance.     

Emission properties of sequentially deposited ultrathin CH3NH3PbI3/MoS2 heterostructures

Hybrid organic-inorganic perovskite materials have obtained considerable attention due to their exotic optoelectronic properties and extraordinarily high performance in photovoltaic devices. Herein, we successively converted the ultrathin PbI₂/MoS₂ into the CH₃NH₃PbI₃/MoS₂ heterostructures via CH₃NH₃I vapor processing. Atomic force microscopy (AFM)、Scanning electron microscopy (SEM) and X-ray photoemission spectroscopy (XPS) measurements prove the high-quality of the converted CH₃NH₃PbI₃/MoS₂. Both MoS₂ and CH₃NH₃PbI₃ related photoluminescence (PL) intensity quenching in CH₃NH₃PbI₃/MoS₂ implies a Type-II energy level alignment at the interface. Temperature-dependent PL measurements show that the emission peak position shifting trend of CH₃NH₃PbI₃ is opposite to that of MoS₂ (traditional semiconductors) due to the thermal expansion and electron-phonon coupling effects. The CH₃NH₃PbI₃/TMDC heterostructures are useful in fabricating innovative devices for wider optoelectronic applications.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Crystallization of lead zirconate titanate films by laser annealing

The structure of lead zirconate titanate Pb(Zr _x Ti_{1 ? x} )O₃ thin films grown by chemical solution deposition on Si-SiO₂-Ti-Pt substrates has been studied using transmission electron microscopy and energy-dispersive analysis. Films crystallization has been performed using laser annealing. It has been found that, in contrast to isothermal annealing where nucleation on the platinum layer dominates, crystallization and growth of spherical perovskite crystals occur in the film bulk. The perovskite phase crystal size increases from 10 to 120 nm with increasing laser beam energy.     

Thermodynamic Study of Formamidinium Lead Iodide (CH5N2PbI3) from 5 to 357 K

In the present study, the molar heat capacity of solid formamidinium lead iodide (CH₅N₂PbI₃) was measured over the temperature range from 5 to 357 K using a precise automated adiabatic calorimeter. In the above temperature interval, three distinct phase transitions were found in ranges from 49 to 56 K, from 110 to 178 K, and from 264 to 277 K. The standard thermodynamic functions of the studied perovskite, namely the heat capacity C°_p(T), enthalpy [H⁰(T) − H⁰(0)], entropy S⁰(T), and [G°(T) − H°(0)]/T, were calculated for the temperature range from 0 to 345 K based on the experimental data. Herein, the results are discussed and compared with those available in the literature as measured by nonclassical methods.     

Optical and structural properties of PbI<Subscript>2</Subscript> thin film produced via chemical dipping method

PbI₂ thin films were deposited on glass substrates via chemical bath deposition. The characteristics of PbI₂ thin films were examined through their structural and optical properties. X-ray diffraction spectra showed the presence of rhombohedral structure and atom planes were subject to change with the pH of the bath. Scanning electron microscope indicated uniform distribution of grains. Optical properties were examined via UV–VIS; optical spectrum of the thin films was measured at the range of 200–1100 nm wavelength. Optimum pH levels for producing thin films were found to be pH 4–5. It has been observed that transmission and optical band gap (E _g) increased with the pH of the bath, which varied between 66–95 and 2.24–2.50 %, respectively; on the other hand film thickness of PbI₂ thin films was decreased with the pH of the bath. Energy-dispersive X-ray spectroscopy analysis were in accordance with theoretical value of PbI₂ at pH = 4 and 5. Refractive index was negatively correlated with pH of the chemical bath; it has been calculated as 1.97, 1.40, 1.29 and 1.24 for the films produced at pH 2, 3, 4 and 5. The results of the study were compared with similar studies in the literature.     

Hole‐extraction layer dependence of defect formation and operation of planar CH3NH3PbI3 perovskite solar cells

Three planar CH₃NH₃PbI₃ (MAPbI₃) solar cells having the same structure except a hole‐extraction layer (HEL) showed distinctive difference in operation characteristics. Analysis of frequency‐dependent capacitance and dielectric‐loss spectra of the three MAPbI₃ devices showed two types of recombination‐loss channels with different time constants that we attributed respectively to interface and bulk defects. Discrepancy in defect formation among the three devices with a HEL of PEDOT:PSS, NiO_x, or Cu‐doped NiO_x was not surprising because grain‐size distribution and crystalline quality of MAPbI₃ can be affected by surface energy and morphology of underlying HELs. We were able to quantify interface and bulk defects in these MAPbI₃solar cells based on systematic and simultaneous simulations of capacitance and dielectric‐loss spectra, and current–voltage characteristics by using the device simulator SCAPS.