Hot-casting Technique Research Articles

Enhanced efficiency and stability of Dion–Jacobson quasi-two-dimensional perovskite solar cells by additive

Dion–Jacobson (DJ) quasi-two-dimensional (quasi-2D) perovskite solar cells (PSCs) have attracted increasing attention owing to their potential stability compared with the Ruddlesden-Popper (RP) phase. It is known that the low efficiency and poor thermal stability retard the application of quasi-2D PSCs. Here, DJ quasi-2D perovskite films based on propane-1.3-diammonium (PDA) spacer cations were prepared by a hot casting technique. Ionic liquid additive of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) was introduced into the quasi-2D perovskite precursor solution to improve the crystal quality and morphology of the films. The quasi-2D PSCs with BMIMBF4 achieved an optimal power conversion efficiency (PCE) of 14.07%, which is higher than the 12.45% of the control devices. The impressive 1.17 V high open-circuit voltage and 14.07% efficiency are the highest values for the reported PDA-based n–i–p type devices. Moreover, the thermal stability of the solar cells was significantly enhanced by the addition of BMIMBF4. The PCE of the devices with BMIMBF4 maintains 72% of the initial value after annealing at 85 °C for 120 h, while the PCE of control devices drops to 19% of the initial value. These results prove that the introduction of ionic liquid BMIMBF4 is a pragmatic method to elevate the efficiency and thermal stability of quasi-2D PSCs.

Temperature-induced strain management in MAPbI3-xClx hybrid perovskite films

Strain management in organometal halide perovskite materials is one of the important strategies to improve charge transport, ion migration and crystallinity of these materials. This work reveals the internal relationship between the perovskite processing temperature and the optoelectronics properties of MAPbI3-xClx perovskite films. The MAPbI3-xClx films have fabricated on various temperature ranges from 80 °C to 180 °C using hot casting technique and explored their structural, surface morphological, optical, and electrical properties. Lattice strain has observed and found to be reduced on increasing the processing temperature and at a particular processing temperature, perovskite films exhibit free of strain. Urbach energy and trap levels have significantly reduced at higher temperatures which confirm the superior optoelectronic feature of perovskite semiconductors. Further, Schottky diodes with device configuration FTO/MAPbI3-xClx/Ag were fabricated and ideality factor, barrier height and reverse saturation current density have been determined in order to explore the charge transport properties of the films.

α-CsPbI3 Bilayers via One-Step Deposition for Efficient and Stable All-Inorganic Perovskite Solar Cells.

The emerging inorganic CsPbI3 perovskites are promising wide-bandgap materials for application in tandem solar cells, but they tend to transit from a black α phase to a yellow δ phase in ambient conditions. Herein, a gradient grain-sized (GGS) CsPbI3 bilayer is developed to stabilize the α phase via a single-step film deposition process. The spontaneously upward migration of (adamantan-1-yl)methanammonium (ADMA) based on the hot-casting technique causes self-assembly of the hierarchical morphology for the perovskite layers. Due to the strong steric effect of the surficial ADMA cation, a self-assembly tiny grain-sized CsPbI3 layer is in situ formed at the surface site, which exhibits notably enhanced phase stability by its high surface energy. Meanwhile, a large grain-sized CsPbI3 layer is obtained at the bottom site with high charge mobility and low trap density of states, which benefits from the regulated growth rates by the interaction between ADMA and perovskites. The perovskite solar cell (PSC) based on the GGS CsPbI3 bilayer shows an efficiency of 15.5% and operates stably for 1000 h under ambient conditions. This work confirms that redistributing the surface energy of perovskite films is a facile strategy to stabilize metastable PSCs without the cost of efficiency loss.

Two-dimensional cyclohexane methylamine based perovskites as stable light absorbers for solar cells

Two-dimensional Ruddlesden-Popper perovskites (2D-RPPs) have attracted attention due to their superior moisture stability. In this study, we prepared and characterized a new group of 2D perovskites with cyclohexane methylamine (CMA+) as a spacer cation. The compounds (CMA)2(MA)n−1PbnI3n+1 (n = 1, 2, 3, and 4) showed high self-assembly (where MA is CH3NH3), which allowed for high-quality films to be deposited through the one-step spin-coating method in ambient air. Hot-casting technique, additives, and glove box were not required in the fabrication process. The bandgaps of this group of compounds can be varied from 2.36 eV for (CMA)2PbI4 (n = 1) to 1.52 eV for MAPbI3 (n = ∞) with increasing n value. Moreover, we found that the thin films of (CMA)2(MA)n−1PbnI3n+1 showed considerable stability in moisture and thermal tests. We successfully inserted these new 2D-RPPs in solar cells, and obtained the best power conversion efficiency of 10.66%, with an open-circuit voltage (Voc) of 1.00 V, and a short-circuit current density (Jsc) of 16.99 mA/cm2 for our first-generation device containing (CMA)2(MA)3Pb4I13 (n = 4). The CMA-based devices exhibited superior long-term stability, the unencapsulated (CMA)2(MA)3Pb4I13 solar cell retained nearly 95% of its initial efficiency when exposed to ambient conditions with a 60% relative humidity for 1500 h.

Perovskite solar cells based on a perovskite film with improved film coverage

Organic-inorganic halide perovskite solar cells have attracted considerable attention because of its high efficiency, low-cost fabrication, and flexibility. The film morphology has to be well controlled since the presence of pinholes in the perovskite films deteriorates the performance of the devices. Therefore, a variety of methods have been developed to realize high quality perovskite films and excellent device performance. In this work, hot-casting technique and additive engineering were investigated for obtaining a better film morphology. Optical microscopy, electron scanning microscopy and X-ray diffraction were used to monitor the morphology and the crystallinity of the perovskite films. We found that pinhole-free perovskite films cannot be obtained by the hot-casting technique alone. By using the hot-casting technique and incorporating the additive in the perovskite precursor, perovskite films with minimum pinholes were obtained when optimum hot-casting temperature and the amount of the additive were used. A reproducible average power conversion efficiency of 9.24 % was observed. The findings showed that the perovskite films depend not only on the usage of hot-casting technique but also through the incorporation of additives which can be used for other perovskite materials utilizing as solar cells.

Regulating active layer thickness and morphology for high performance hot-casted polymer solar cells

Fabrication of high performance polymer solar cells through the hot-casting technique, which modulates the thickness and roughness of the active layer and also the carrier mobility of the solar cell devices.

CH3NH3PbI3 thin films prepared by hot-casting technique in the air: growth mechanism, trap states and relating solar cells

Tetragonal CH3NH3PbI3 perovskite thin films with large crystallite sizes were successfully fabricated under atmospheric air using a one-step hot-casting technique. The casting temperature governed structural and optical properties of the prepared films. The energy gaps of the hot-casted films changed with changing casting temperature due to the variation of Urbach energy. The hot-casted perovskite thin films had superior structural stability to that of the two-step method films. However, the hot-casted perovskite films contained trap states as suggested by additional emissions other than bimolecular recombination in photoluminescence spectra. The origins of these trap states were believed to be attributed to the presence of iodine vacancies (VI), iodine interstitial sites (Ii) and methylammonium ion vacancies (VMA) in the prepared films. The fabricated perovskite solar cells showed that at low casting temperatures the power conversion efficiencies were relatively higher than the higher ones, this was attributed to their lower non-radiative recombination activities.

Crystallization and grain growth regulation through Lewis acid-base adduct formation in hot cast perovskite-based solar cells

The precise control of perovskite crystallization process that could provide smooth films with large grains is a prerequisite for solar cell fabrication to achieve higher efficiencies. To regulate this crystallization process and to achieve it multiple times remains a humongous challenge. This process has been reproducibly demonstrated here through efficient molecular exchange via a Lewis acid-base adduct formation. In this work, a Lewis acid-base adduct approach in concurrence with hot casting technique was employed to efficiently control the perovskite crystallization by judiciously adding dimethyl sulfoxide (DMSO) to a precursor solution of lead iodide (PbI 2 ) and methyl ammonium chloride (MACl) in dimethylformamide (DMF). High quality perovskite films were fabricated by precisely controlling the volume of DMSO which resulted in low recombination rate and which had better light harvesting ability. Uniform films of MAPbCl x I 3-x having large grain size were formed reproducibly on addition of 1.5 equivalents (eq.) DMSO to the precursor solution. Perovskite solar cells fabricated using this solution resulted in maximum power conversion efficiency (PCE) of 14.11% with enhanced stability as compared to the reduced PCE values obtained with other DMSO ratios and pure DMF solution cast films. • Hot casting using Lewis acid-base approach resulted in large grain perovskite. • 1.5 equivalents of Lewis base (DMSO) gave largest grain size and smooth film. • A power conversion efficiency >14% achieved in inverted perovskite solar cell. • This method also improved the overall stability of the solar cells.

Effect of hot-casted NiO hole transport layer on the performance of perovskite solar cells

NiO is extensively studied as a hole transport layer in perovskite solar cells but syntheses of NiO precursor involves toxic chemicals and time-consuming processes. Moreover, the synthesized NiO contains surface defects acting as trapping sites at the NiO/perovskite interfaces, resulting in poor charge extraction, hysteresis and light soaking. In this manuscript, we developed a non-toxic methodology for NiO precursor solution by using a simple mixture of NiO powder and HCl in an air environment. In addition, a new hot-casting technique was developed to successfully fabricate densely-packed, less defective NiO films. Interestingly, the hot-casting temperature was found to significantly affect morphology, film coverage and surface defects of NiO films. When a hot-casting temperature was below 100 °C, non-uniform NiO films were sparsely formed on the FTO surface and were characterized by defects in the form of hydroxyl groups and water on the surface. Such defective NiO films resulted in severe hysteresis and light soaking effect due to the trapped charges at the defective NiO/perovskite interface of perovskite solar cells. In contrast, when the hot-casting temperature was 120 °C, the NiO film formed densely-packed morphologies, covering the FTO surface. Furthermore, this film exhibited an ordered chemistry with strong Ni-O octahedral bonding and facilitated charge extraction at NiO/perovskite interface, resulting in a negligible hysteresis and light soaking. Finally, this non-toxic and simple method of fabricating NiO film will assist further development of perovskite solar cells.

Optical properties of the perovskite films deposited on meso-porous TiO2 by one step and hot casting techniques

In this work, CH3NH3PbI3 is coated on meso-porous TiO2 films via one step method (OSM) and hot casting technique (HCT). The optical properties of the solution-processed organic inorganic halide perovskite layers deposited on meso-structured TiO2 scaffolds are investigated using UV–Vis reflectance and transmittance spectroscopies at the wavelength range of 400–900 nm. Optical parameters including refractive index, extinction coefficient, real and imaginary parts of the dielectric constants, optical conductivity and direct and indirect optical band gaps are calculated and compared for the perovskite films created by above mentioned methods. Optical band gaps are determined from the absorption coefficient values using Tauc's power law. The obtained values of direct band gap for two perovskite films are close to each other (1.56 and 1.58 eV for perovskite layer deposited by OSM and HCT respectively). For both prepared films, the difference between direct and indirect band gaps is 60 meV which is in very good agreement with that reported in literature. In addition to the difference between the perovskite morphology, the optical absorption of perovskite layers created by the HCT is less than that of layers prepared by OSM and therefore the amount of generated photo current is less. This is the main reason that causes the efficiency of perovskite solar cells prepared by HCT (3.0%) is ~38% less than that of cells created by OSM (4.15%).

Superior Performance of Silver Bismuth Iodide Photovoltaics Fabricated via Dynamic Hot‐Casting Method under Ambient Conditions

AbstractBismuth‐based ternary halides have recently gained a lot of attention as lead‐free perovskite materials. However, photovoltaic performances of these devices remain poor, mostly due to their low‐dimensional crystal structure and large bandgap. Here, a dynamic hot casting technique to fabricate silver bismuth iodide‐based perovskite solar cells under an ambient atmosphere with power conversion efficiencies above 2.5% is demonstrated. Silver bismuth iodides are 3D analogs of complex ternary bismuth halides with a suitable bandgap for a single junction solar cell. As far as it is known, these results represent the highest efficiency for solution processed air‐stable lead‐free perovskite solar cells. The enhanced solar cell performance via this dynamic hot casting technique can be attributed to long carrier lifetimes, micrometer‐sized crystalline grains, and pinhole free thin‐film formation with uniform morphology. This work provides a new direction for fabrication of solution‐processed lead‐free perovskite solar cells with a rapid fabrication strategy irrespective of the processing environment.

Effect of crystallization strategies on CH3NH3PbI3 perovskite layer deposited by spin coating method: Dependence of photovoltaic performance on morphology evolution

In this paper, the effects of humidity, annealing temperature and deposition method including one step method, sequential deposition rout and hot casting technique are investigated on the morphology of perovskite layer. The optimal conditions for deposition of perovskite layers are introduced. Formation and growth of CH3NH3PbI3 perovskite crystals by these procedures leads to completely different crystal shapes and sizes and as a result different cell efficiency and environmental stability. Humidity and annealing temperature have great effect on the morphology of perovskite layer in one step method. It was found that in one step method by increasing the humidity from <10% to ~40% (during spin coating and annealing the CH3NH3PbI3 perovskite) morphology of the spin coated layer evolves from mostly ribbon shape with a large number of voids to layers comprising hollow fibers. However increase of the annealing temperature from 100 to 140 °C results in decreasing the diameter of the formed hollow fibers. In hot casting technique, by controlling the casting temperature, a very compact flat layer including lotus leaf-like morph with fewer pinholes can be created. Cells fabricated using CH3NH3PbI3 cuboid crystals grown by sequential deposition rout showed the highest photocurrent and power conversion efficiency (4.8%). However deposited layers via sequential deposition rout and then the cells fabricated using them are not chemically as stable as those prepared by one step method for long time performance due to humidity induced degradation of large crystal of CH3NH3PbI3 to smaller size.

Efficient and stable planar heterojunction perovskite solar cells fabricated under ambient conditions with high humidity

Abstract The quality of perovskite film plays an important role on the performance and stability of perovskite solar cells (PSCs). Herein, dense and uniform perovskite (CH3NH3PbI3) films are fabricated by combining hot-casting technique (HCT) and methylamine (CH3NH2) gas treatment (MGT) under ambient conditions with a relative humidity of about 60%. These high quality CH3NH3PbI3 films are further used to fabricate planar heterojunction (PHJ) PSCs with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Ag, showing a power conversion efficiency (PCE) up to 14.2% with negligible hysteresis. More importantly, these PHJ-PSCs show excellent stability, and over 80% of their original efficiencies could be maintained even stored in air (humidity ∼45%) for 20 days without encapsulation. Meanwhile, the PCE up to 13.6% could be achieved for PHJ-PSCs fully fabricated in air including electron transport layer PCBM. The researches provide an ease-of-fabrication method to fully fabricate highly efficient and stable PHJ-PSCs under ambient conditions with high humidity, which would potentially accelerate the commercialization of PHJ-PSCs.

Investigation of Dynamics of Perovskites using Chemically, Spatially and Temporally Resolved 2D Mapping

Currently, there is an ongoing strong discussion regarding the nature of organometallic CH3NH3PbI3-xClx perovskites for solar cells. In this work, perovskites have been investigated using chemically, spatially, and temporally resolved measurements. To investigate the carrier dynamics of perovskites, perovskite (CH3NH3PbI3-xClx) films were prepared using hot casting technique in which perovskite (CH3NH3PbI3-xClx) solution was prepared by dissolving equimolar ratios of lead iodide (PbI2, Sigma-Aldrich, 99%) and methylamine hydrochloride (MACl, Sigma-Aldrich) in N, N-dimethylformamide (DMF, Sigma-Aldrich, anhydrous, 99.8%) in 11% concentration. A lower hot-casting temperature (100 °C) resulted in smaller grain size, with typical grain sizes of 1~2µm. Hot-casting temperatures higher than 100 °C produced perovskite samples with larger grains of ~ more than tens of micrometers, which was confirmed by high-resolution scanning electron microscopy (SEM). The PL intensity and peak-position mapping image of submicron-scale were performed using confocal micro-spectroscopy. In-plane spatial resolution of ~380nm was indicated by an objective lens with numerical aperture of 0.7 and a solid-state laser with 405nm wavelength for the excitation. The signal collected by the objective lens was detected by a thermoelectrically cooled CCD detector. For the analysis of time resolved PL (TRPL), multifunctional confocal microscopy including a time-correlated single photon counting (TCSPC) system was employed. In addition, 2-D energy-dispersive X-ray (EDS) measurement was performed to elucidate the distribution of chemistries for the surface of perovskites, which were correlated to spatially and temporally resolved PL. Particularly, the carrier dynamics between grain interiors and grain boundaries were systematically investigated.

Thermal-Induced Volmer–Weber Growth Behavior for Planar Heterojunction Perovskites Solar Cells

Recently, a hot casting technique was developed to prepare pinhole free perovskite thin films with millimeter-scale grains. However, its intrinsic formation mechanism has not been studied in the literature up to now. Here, we demonstrate a Volmer–Weber growth mechanism during the hot casting, a process that typically involves the formation of island shaped grains and the following integration into dense perovskite films. It was found that such crystal growth was determined by the multiple effect of thermal energy and force centrifugal field. Particularly, the thermal energy can facilitate the formation of CH3NH3PbI3 and overcome the energy barriers of the precursor solutions on the substrates. The detailed morphologies of CH3NH3PbI3 films can be optimized by regulating deposition parameters including casting temperature and rotate speed. Solar cells constructed with these thin films achieve an average power conversion efficiency of 12.6 ± 0.3% under standard AM 1.5 G conditions.

Solar cells. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals.

The fundamental properties and ultimate performance limits of organolead trihalide MAPbX3 (MA = CH3NH3(+); X = Br(-) or I(-)) perovskites remain obscured by extensive disorder in polycrystalline MAPbX3 films. We report an antisolvent vapor-assisted crystallization approach that enables us to create sizable crack-free MAPbX3 single crystals with volumes exceeding 100 cubic millimeters. These large single crystals enabled a detailed characterization of their optical and charge transport characteristics. We observed exceptionally low trap-state densities on the order of 10(9) to 10(10) per cubic centimeter in MAPbX3 single crystals (comparable to the best photovoltaic-quality silicon) and charge carrier diffusion lengths exceeding 10 micrometers. These results were validated with density functional theory calculations.

Large-crystal perovskite films

Solar Cells The performance of organic-inorganic hybrid perovskite planar solar cells has steadily improved. One outstanding issue is that grain boundaries and defects in polycrystalline films degrade their output. Now, two studies report the growth of millimeter-scale single crystals. Nie et al. grew continuous, pinhole-free, thin iodochloride films with a hot-casting technique and report device efficiencies of 18%. Shi et al. used antisolvent vapor-assisted crystallization to grow millimeter-scale bromide and iodide cubic crystals with charge-carrier diffusion lengths exceeding 10 mm. Science , this issue p. [522][1], p. [519][2] [1]: /lookup/doi/10.1126/science.aaa0472 [2]: /lookup/doi/10.1126/science.aaa2725

Solar cells. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains.

State-of-the-art photovoltaics use high-purity, large-area, wafer-scale single-crystalline semiconductors grown by sophisticated, high-temperature crystal growth processes. We demonstrate a solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites with millimeter-scale crystalline grains. We fabricated planar solar cells with efficiencies approaching 18%, with little cell-to-cell variability. The devices show hysteresis-free photovoltaic response, which had been a fundamental bottleneck for the stable operation of perovskite devices. Characterization and modeling attribute the improved performance to reduced bulk defects and improved charge carrier mobility in large-grain devices. We anticipate that this technique will lead the field toward synthesis of wafer-scale crystalline perovskites, necessary for the fabrication of high-efficiency solar cells, and will be applicable to several other material systems plagued by polydispersity, defects, and grain boundary recombination in solution-processed thin films.