CH3NH3PbI3 Films Research Articles

Influence of the reagents’ ratio on photoelectric and optical properties of perovskite films for photovoltaics

The properties of the synthesized films of organic-inorganic perovskites CH3NH3PbI3 obtained at various ratios of starting reagents (PbI2 and CH3NH3I) have been studied. As a solvent, we used chemically pure dried dimethylformamide (DMF). Organic-inorganic perovskites are promising for photovoltaic applications. It has been shown that regardless of the ratio of the starting reagents, single-phase perovskites are formed, at the same time the microstructure of the films changes significantly. It has been reported photoelectric and optical properties of synthesized films, namely: experimental and theoretical spectral dependences of the low-signal surface photovoltage and transmission. The band gap and the Urbach parameter dependence on the ratio of precursors were determined. It has been found that the materials’ band gap depends on the ratio of precursors and equals to 1.59, 1.62 and 1.57 eV, while the characteristic Urbach energy equals to 18, 19 and 22 meV for the PbI2:CH3NH3I films with PbI2 ratio of 1:1, 1:2 and 1:3, respectively. It has been ascertained that the spectral dependences of the low-signal surface photovoltage are much more sensitive to the material microstructure and its electronic structure close to the absorption edge, while the optical transmission spectra are not so sensitive. The limiting value of the short-circuit current density for the films with different PbI2 and CH3NH3I ratios has been determined.

Localized quenching sites in MAPbI3 investigated by fluorescence and photothermal microscopy.

In this work, we developed a fluorescence and photothermal microscope with extremely large scanning range and high spatial resolution. We demonstrated the capability of this instrument by simultaneously measuring the photoluminescence and photothermal signals of the CH3NH3PbI3 (MAPbI3) film. After scanning the MAPbI3 film on the scale of centimeters, we can obtain information of both emissive and nonemissive processes with a resolution of 200nm at any location of the large area. We can clearly see the localized photothermal signal while the photoluminescence signal is uniform. These results directly prove that the emissive recombination happens all over the materials, but the nonemissive recombination happens only at certain localized quenching sites. The fluorescence and photothermal microscope with both large scanning range and high spatial resolution can provide information of all the relaxation channels of the excitons, showing potential applications for investigation of photophysical mechanisms in photoelectric materials.

Silica as a Scaffold Layer via Electrospray for MAPbI3 Perovskite-based Solar Cell

We synthesized a novel hybridized perovskite-silica (SiO2) solar cell using an electrospray process under ambient conditions. The Silica scaffold not only provided optical transmission from visible to infrared regions but also blocked the back flow transport of the holes and electrons to reach the electrodes, leading to reduced charge recombination. Moreover, the CH3NH3PbI3 (MAPbI3) film quality can be improved by using the combination of vacuum annealing enhanced crystal growth and the methylamine (CH3NH2, MA) vapor-assisted diffusion at the grain boundaries, due to reduced surface roughness and the re-crystallization of the MAPbI3 film. Photovoltaic performances were evaluated using the glass/ITO/poly (3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS)/SiO2/MAPbI3/C60/BCP/Al device structure, with the champion MAPbI3 based perovskite-silica solar cell exhibited a short-circuit current density of 19.89 mA/cm2, an open-circuit voltage of 0.89 V, a fill factor of 0.50, and a power conversion efficiency of 8.85%.

Rapid microwave annealing of CH3NH3PbI3 with controllable crystallization for enhancing the resistive-switching performance

High switching fluctuation induced by the random formation and rupture of conductive filaments (CFs) is a critical issue for the development of organic-inorganic perovskite-based resistive switching (RS) memory devices. In this study, a rapid microwave-annealing method is developed to reduce grain boundaries (GBs) of CH3NH3PbI3 (MAPbI3) film and improve the switching reliability of the Au/MAPbI3/FTO device. The crystal grain size of MAPbI3 is increased and the corresponding GBs number is reduced with microwave irradiation time ranging from 40 to 120 s. In the optimized case, the dispersion of the RS parameters of the Au/MAPbI3/FTO cell is greatly reduced. Specifically, the fluctuations of the high/low resistance states and SET/RESET voltages dropped from 70.4%/69.5% and 40.5%/40.3% to 20.1%/26.7% and 16.0%/11.9%, respectively. The device can be operated with reduced compliance current. These results demonstrate that reducing the number of GBs is a feasible strategy to reduce the randomness of the CFs, and thus achieve more stable RS performance in MAPbI3-based memory devices.

Realize larger grain size of CH3NH3PbI3 film with reduced non-radiative recombination for high performance perovskite solar cells via precursor colloidal size engineering

In CH3NH3PbI3 perovskite solar cells, enhancement grain size of CH3NH3PbI3 to reduce the non-radiative at grain boundaries is an important way to reach high performance perovskite solar cell. However, it is still a challenge to enhance the grain size of CH3NH3PbI3 through a simple and low cost way. In this work, a larger precursor colloidal size is realized through tuning morphology of precursor CH3NH3I using a polar solvent of ethanol during purification, yielding a larger grain size of CH3NH3PbI3 film, and the as-prepared perovskite solar cells are shown to be dramatically increased to 17.49% with an increase in short circuit density, fill factor and open circuit voltage, as compared to that (14.28%) in the control device with CH3NH3I purified by non-polar solvent of diethyl ether. The investigation result showed the increased efficiency of perovskite solar cells prepared by ethanol purification is ascribed to a faster charge transfer at CH3NH3PbI3/TiO2 interface resulting from the reduced grain boundary defects. Our work provides a route for improving the CH3NH3PbI3 device efficiency through a simple yet effective approach.

The study of the morphology and structural, optical, and J-V characterizations of (CH3NH3PbI3) perovskite photovoltaic cells in ambient atmosphere

In this paper, the synthesis processes of perovskite active films of CH3NH3PbI3 (MAPbI3) have been studied for perovskite hybrid solar cells by spin and dip coating in ambient atmosphere, and without glove box to reduce production cost for technological applications. The morphology and structural, electrical and optical properties of perovskite films have been investigated by X-ray Diffraction (XRD), Field-Emission Scanning Electron Microscopes (FE-SEM), Atomic Force Microscopes (AFM) and Ultraviolet–visible spectroscopy (UV-Vis). A precursor solution of lead Iodide (PbI2) with Dimethylformamide (DMF) solvent by spin coating at two stages (a) R=4500 rpm and (b) R=6000 rpm, and methyl ammonium iodide (CH3NH3I) with isopropanol solvent by dip coating, have been prepared for active perovskite layer (CH3NH3PbI3).

Defect passivation in CH3NH3PbI3 films using alkali metal fluoride additives for highly efficient perovskite solar cells

Solution-processed organic/inorganic hybrid halide perovskites are a class of very promising photovoltaic materials because of their extraordinary optoelectronic properties. However, numerous intrinsic defects within perovskite films limit the performance enhancement of perovskite solar cells (PSCs). Herein, we introduce three different alkali metal fluoride additives into the perovskite precursor to modulate the defect behaviors. The addition of alkali metal fluorides, especially potassium fluoride (KF), significantly passivates grain boundaries and point defects within bulk perovskite films, contributing to increased carrier lifetime and reduced defect state density. By optimizing the concentration of additives, the PSC device with 1% KF added has realized a significantly enhanced power conversion efficiency of 20.11%, and ignorable hysteresis.

Cosensitization-based halide perovskite in aqueous solution: A photoelectrochemical and first-principles investigation

The halide perovskite films suffer from poor photoelectrochemical performance in aqueous solution, inhibiting their further industrial deployment. Here, we propose a new concept of “cosensitized perovskites” that employ the cosensitization strategy to improve the photoelectrochemical properties of the prototypical CH3NH3PbI3 film in the aqueous solution via two commercial coumarin dyes. The resulting photocurrent of the cosensitization-modified perovskite film in the aqueous solution increases by 40 % than those of the single dye-based films, and is several orders of magnitude higher than that offered by the bare perovskite film under the same conditions. The first-principles calculations reveal the detailed atomic and electronic structures of the cosensitized systems, and suggest that the improved photoelectrochemical performance is attributed to the multiple types of surface coordination when the cosensitization is available and the conjugation in the dye molecules; these effectively reduce the surface charge traps. This study serves as the first example to highlight the cosensitization strategy for the perovskite materials, and expects the cosensitization method for a wide range of perovskite-based applications, including solar cells, photodetectors, light-emitting diodes, photocatalysts, and water-splitting systems.

Limits to Electrical Mobility in Lead-Halide Perovskite Semiconductors.

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH3NH3PbI3. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators.

Efficient and Stable Carbon-Based Perovskite Solar Cells via Passivation by a Multifunctional Hydrophobic Molecule with Bidentate Anchors.

Surface passivation has demonstrated to be an effective strategy to improve the power conversion efficiency (PCE) and long-term stability of perovskite solar cells (PSCs). Passivation treatment can effectively reduce the density of defect states at the surface and grain boundaries of perovskite films. Herein, a passivation agent of 2-amino-5-(trifluoromethyl)pyridine (5-TFMAP) with bidentate groups is applied to passivate perovskite CH3NH3PbI3 films for the first time. Two types of electron-rich nitrogen atoms from both the pyridine ring and the amino group provide strong interaction with the under-coordinated Pb2+. Additionally, the trifluoromethyl group offers a hydrophobic property and improves moisture stability of the as-fabricated PSCs. It is found that the 5-TFMAP passivation layer can effectively reduce the defect states, promote better carrier transport, and suppress non-radiation recombination of the perovskite films. The best PCE of carbon-based PSCs passivated by the 5-TFMAP agent achieves a high efficiency of 14.96% compared with that of 11.90% for the control PSCs. Moreover, the long-term stability of PSCs with the 5-TFMAP passivation treatment is greatly improved, and its PCE can maintain 80% of its original PCE after being stored for 1200 h with a relative humidity of around 35% at room temperature.

Small grains as recombination hot spots in perovskite solar cells

Summary Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells.

Slow halide exchange in CsPbIBr2 films for high-efficiency, carbon-based, all-inorganic perovskite solar cells

Halide exchange offers a versatile way to modify the properties of halide perovskites, but it is particularly challenging to slow the reaction rate to restrain defect growth in the products. Herein, we propose a slow halide exchange strategy to simultaneously fine-tune the optical and microstructural characteristics of CsPbIBr2 films by physically pairing CsPbIBr2 and CH3NH3PbI3 films. Once a proper heating treatment is applied, halide exchange of Br− and I− ions between the films is activated, and the reaction rate can be well controlled by the heating recipe, in which a high temperature can accelerate the exchange reaction, while a low temperature slows or stops it. By using an optimal halide exchange temperature (110°C) and time (2 h), the parent CsPbIBr2 film was transformed into high-quality CsPbI1+xBr2−x film, featuring an extended absorption onset from 590 to 625 nm, coarsened grains, improved crystallinity, reduced surface roughness, suppressed halide phase segregation, and identical stability to the pristine film. Accordingly, the efficiency of a carbon-based, all-inorganic perovskite solar cell (PSC) was boosted to 10.94%, which was much higher than that of the pristine CsPbIBr2 film (8.21%). The CsPbI1+xBr2−x PSC also possessed excellent tolerance against heat and moisture stresses.

Photostable and Uniform CH3NH3PbI3 Perovskite Film Prepared via Stoichiometric Modification and Solvent Engineering.

Solution-processed organometal halide perovskites (OMHPs) have been widely used in optoelectronic devices, and have exhibited brilliant performance. One of their generally recognized advantages is their easy fabrication procedure. However, such a procedure also brings uncertainty about the opto-electric properties of the final samples and devices, including morphology, stability, coverage ratio, and defect concentration. Normally, one needs to find a balanced condition, because there is a competitive relation between these parameters. In this work, we fabricated CH3NH3PbI3 films by carefully changing the ratio of the PbI2 to CH3NH3I, and found that the stoichiometric and solvent engineering not only determined the photoluminescence efficiency and defects in the materials, but also affected the photostability, morphology, and coverage ratio. Combining solvent engineering and the substitution of PbI2 by Pb(Ac)2, we obtained an optimized fabrication condition, providing uniform CH3NH3PbI3 films with both high photoluminescence efficiency and high photostability under either I-rich or Pb-rich conditions. These results provide an optimized fabrication procedure for CH3NH3PbI3 and other OMHP films, which is crucial for the performance of perovskite-based solar cells and light emitting devices.

Observing and Understanding the Corrosion of Silver Nanowire Electrode by Precursor Reagents and MAPbI3 Film in Different Environmental Conditions

AbstractSince metal electrode is an important component in the perovskite solar cells, which can be used as back contact, transparent electrode, and the transparent interlayer of tandem devices, the corrosion of metal electrode by perovskite film, should be understood. Here, the corrosion of conventional reagents of organic–inorganic perovskite material, i.e., CH3NH3X (MAX) and PbX2, X = I, Br, and Cl, against silver nanowire (AgNW) based transparent electrode are investigated under four different environments, i.e., nitrogen protection, light irradiation, oxygen exposure, and moisture exposure, separately. The corrosion process is inspected by monitoring the time‐dependent resistance variation of the electrode coated by the film of precursor reagent. In addition, the perovskite film of CH3NH3PbI3 (MAPbI3) is also coated on the silver nanowire electrode without or with the hole transporting layer of poly(3,4‐ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) or nanostructured CuSCN for understanding the corrosion process under different environmental conditions. The experimental results presented in this paper would be helpful to the understanding of metal electrode stability in perovskite solar cells.

Perovskite with inhomogeneous composition: Presence of the Cl-rich layer improves the device performance

A perovskite compositional difference was engineered at the perovskite/electron transport layer (ETL) interface. The devices with inhomogeneous perovskite composition achieve the best power conversion efficiency of 16.1% under the optimal conditions, compared with 12.5% for the control, and the device stability is also improved. Further characterizations indicate that the performance enhancement is due to the larger perovskite grain size with less pinholes, the improved charge extraction at the perovskite/ETL interface, and the reduced charge recombination. This modification effect is induced by constructing an additional Cl-rich layer on bottom of the primary CH3NH3PbI3 film.

Enhancement of charge transfer efficiency from CH3NH3PbI3 perovskite film in layer of titanium dioxide in the presence of Ag/SiO2 nanostructures

The perovskite CH3NH3PbI3 films doped with Ag/SiO2 nanostructures on the surface of compact TiO2 layers have been synthesized. Ag/SiO2 nanostructures were embedded into the bulk of CH3NH3PbI3 perovskite by introducing them into the perovskite precursor’s solution. Ag/SiO2 nanostructures have insignificant effect on the perovskite films absorption spectra, but they significantly change the intensity and lifetime of CH3NH3PbI3 luminescence. The blue-shift of the luminescence intensity maximum in the CH3NH3PbI3 − Ag/SiO2 composite films, and time delay between the luminescence intensity maxima in perovskites without and with nanostructures indicate an increase of the charge transfer efficiency from perovskite film to titanium dioxide layer in the presence of Ag/SiO2 nanostructures.

Hybrid Lead Halide Perovskite Films with Large Grain Size Via Spin‐Coating Free Fabrication

Herein, a spin‐coating‐free method to deposit CH3NH3PbI3 perovskite films with large grains is demonstrated. The effect of processing temperature on the property of hybrid lead halide perovskite films is investigated. It is found that the grain domain size is enlarged by increasing processing temperature. Dense and uniform CH3NH3PbI3 films with grain domain size close to 1 mm are obtained using this method. A higher processing temperature also promotes the conversion from precursors solution to perovskite, but excessive temperature induces a poor crystalline state. About 150 °C is proved to be close to the optimal processing temperature to form perovskite films with less defect density, lower charge recombination, and longer charge carrier lifetimes. Space‐charge‐limited current method is used to investigate the charge transport efficiency based on CH3NH3PbI3 perovskite films. The results show that both the electron and hole mobility improve remarkably as the processing temperature increases.

Raman spectroscopy real-time detection for the performance of perovskite solar cell

In this work, Raman signals are used to characterize the crystallinity of perovskite films and the photoelectric conversion efficiency (PCE) of perovskite solar cells. By controlling the ratios of EtOH and CHCl3 during the perovskite crystallization process, the CH3NH3PbI3 film with larger crystal grain size, higher crystallinity and no pinholes can be obtained. Good crystallinity of perovskite films promotes charge transfer (CT), and Raman spectroscopy is a practical method for the real time detection of CT in perovskite films. The results manifest that SERS intensities are conforming to X-ray diffraction (XRD) intensities and the PCE of perovskite solar cells. Meanwhile, the Raman signals are tested in the nitrogen environment, which can avoid the decomposition of perovskite films caused by H2O and O2. These results indicate that this method provides an early, real-time and harmless method to study the performance of perovskite photovoltaic devices.

Electron-Beam Irradiation Induced Regulation of Surface Defects in Lead Halide Perovskite Thin Films.

Organic-inorganic hybrid perovskites (OIHPs) have been intensively studied due to their fascinating optoelectronic performance. Electron microscopy and related characterization techniques are powerful to figure out their structure-property relationships at the nanoscale. However, electron beam irradiation usually causes damage to these beam-sensitive materials and thus deteriorates the associated devices. Taking a widely used CH3NH3PbI3 film as an example, here, we carry out a comprehensive study on how electron beam irradiation affects its properties. Interestingly, our results reveal that photoluminescence (PL) intensity of the film can be significantly improved along with blue-shift of emission peak at a specific electron beam dose interval. This improvement stems from the reduction of trap density at the CH3NH3PbI3 surface. The knock-on effect helps expose a fresh surface assisted by the surface defect-induced lowering of displacement threshold energy. Meanwhile, the radiolysis process consistently degrades the crystal structure and weaken the PL emission with the increase of electron beam dose. Consequently, the final PL emission comes from a balance between knock-on and radiolysis effects. Taking advantage of the defect regulation, we successfully demonstrate a patterned CH3NH3PbI3 film with controllable PL emission and a photodetector with enhanced photocurrent. This work will trigger the application of electron beam irradiation as a powerful tool for perovskite materials processing in micro-LEDs and other optoelectronic applications.

Morphological improvement of CH3NH3PbI3 films using blended solvents for perovskite solar cells

Perovskite solar cells with the structure of glass/florine-doped tin oxide (FTO)/electron transfer layer (ETL)/perovskite/hole transfer layer (HTL)/Ag were fabricated. The effects of blending solvents and thermal annealing on the surface morphology, structural, and optical properties of perovskite (CH3NH3PbI3) active layer were investigated. The active layer was optimized by adding 2-butanol (2-BTA) as an eco-friendly solvent into methyl ammonium iodide (MAI) solution used for spin coating on PbI2/(dimethylformamide (DMF)+dimethyl sulfoxide (DMSO)) film in the second step. The resulting morphology of CH3NH3PbI3 films was smooth and pinholes in the films were also reduced without change in the structure. The effects of thermal annealing on the surface morphology and structural properties of active layer were also studied. The fabricated device for the optimized condition showed the maximum power conversion efficiency (PCE) of ~8.6%.