Two-step Sequential Deposition Method Research Articles

Synchronous Elimination of Excess Photoinstable PbI2 and Interfacial Band Mismatch for Efficient and Stable Perovskite Solar Cells.

Eliminating the undesired photoinstability of excess lead iodide (PbI2 ) in the perovskite film and reducing the energy mismatch between the perovskite layer and heterogeneous interfaces are urgent issues to be addressed in the preparation of perovskite solar cells (PVSCs) by two-step sequential deposition method. Here, the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4 ) is employed to convert superfluous PbI2 to more robust 1D EMIMPbI3 which can withstand lattice strain, while forming an interfacial dipole layer at the SnO2 /perovskite interface to reconfigure the interfacial energy band structure and accelerate the charge extraction. Consequently, the unencapsulated PVSCs device attains a champion efficiency of 24.28 % with one of the highest open-circuit voltage (1.19 V). Moreover, the unencapsulated devices showcase significantly improved thermal stability, enhanced environmental stability and remarkable operational stability accompanied by 85 % of primitive efficiency retained over 1500 h at maximum power point tracking under continuous illumination.

Enhanced Performance of Perovskite Solar Cell via Controlling Layer Thickness

In this study, Perovskite material methylammonium lead iodide (CH3NH3PbI3) was successfully fabricated using a two-step sequential solution deposition method. The precursor solution of lead iodide was first spin-coated, followed by immersing the film into a solution of CH3NH3I to form the Perovskite layer. The Perovskite material was then successfully applied as a light-harvesting material in Perovskite solar cells. The power conversion efficiency of the devices was optimized by varying TiO2 blocking layer, TiO2 mesoporous layer, and perovskite capping layer thickness. The best Perovskite solar cell exhibits a power conversion efficiency of 7.72% with a short-circuit photocurrent density JSC of 17.26 mA/cm2, an open-circuit photovoltage VOC of 0.94 V, and a fill factor of 47.6%; with the thicknesses of TiO2 blocking layer, TiO2 mesoporous layer, and perovskite capping layer of 100 nm, 200 nm, and 400 nm, respectively. The results show that Perovskite material has potential for solar cell application.

Formamidinium Lead Iodide Perovskite Thin Films Formed by Two-Step Sequential Method: Solvent-Morphology Relationship.

Thin films made of formamidinium lead iodide (FAPbI3) perovskites prepared by a two-step sequential deposition method using various solvents for formamidinium iodide (FAI) - isopropanol, n-butanol and tert-butanol, were studied with the aim of finding a correlation between morphology and solvent properties to improve film quality. They were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and their photophysical properties were studied by means of absorption and photoluminescence (PL) spectroscopies. XRD patterns, absorption and PL spectra proved α-phase formation for all selected solvents. An excessive amount of PbI2 found in perovskite films prepared with n-butanol indicates incomplete conversion. Thin film morphology, such as grain and crystallite size, depended on the solvent. Using tert-butanol, thin films with a very large grain size of up to several micrometers and with preferred crystallite orientation were fabricated. The grain size increased as follows: 0.2-0.5, 0.2-1 and 2-5 µm for isopropanol, n-butanol and tert-butanol, respectively. A correlation between the grain size and viscosity, electric permittivity and polarizability of the solvent could be considered. Our results, including fabrication of perovskite films with large grains and fewer grain boundaries, are important and of interest for many optoelectronic applications.

Efficient Inverted Perovskite Solar Cells via Improved Sequential Deposition.

Inverted-structure metal halide perovskite solar cells (PSCs) have attractive advantages like low-temperature processability and outstanding device stability. The two-step sequential deposition method shows the benefits of easy fabrication and decent performance repeatability. Nevertheless, it is still challenging to achieve high-performance inverted PSCs with similar or equal power conversion efficiencies (PCEs) compared to the regular-structure counterparts via this deposition method. Here, an improved two-step sequential deposition technique is demonstrated via treating the bottom organic hole-selective layer with the binary modulation system composed of a polyelectrolyte and an ammonium salt. Such improved sequential deposition method leads to the spontaneous refinement of up and buried interfaces for the perovskite films, contributing to high film quality with significantly reduced defect density and better charge transportation. As a result, the optimized PSCs show a large enhancement in the open-circuit voltage by 100mV and a dramatic lift in the PCE from 18.1% to 23.4%, delivering the current state-of-the-art performances for inverted PSCs. Moreover, good operational and thermal stability is achieved upon the improved inverted PSCs. This innovative strategy helps gain a deeper insight into the perovskite crystal growth and defect modulation in the inverted PSCs based on the two-step sequential deposition method.

Defect control based on interfacial passivation via post-treatment of 1-ethylpyridine hydrobromide for achieving efficient and stable perovskite solar cells

The carrier recombination caused by a series of crystal defects in perovskite materials is still the main obstacle to further improve the photoelectric conversion efficiency. Herein, as an agent for interfacial engineering, 1-ethylpyridine hydrobromide (EPB) was employed to treat the FA1-xMAxPbI3-yBry perovskite film using a two-step sequential deposition method. The influence and mechanism of the post-treatment with EPB on the film properties and photovoltaic performance of the relevant perovskite solar cells (PSCs) were studied. The champion EPB-treated perovskite solar cell achieved a remarkable power conversion efficiency (PCE) of 20.71 % with an enhanced short current density (Jsc) of 24.14 mA cm−2, which is higher than that (PCE = 18.85 %, Jsc = 23.40 mA cm−2) of the pristine device without EPB. The nitrogen (N) atom in EPB exhibits a strong coordination with lead (Pb) from the perovskite, which reduces defect density and suppresses non-radiative recombination for achieving high-performance PSCs. The UV–vis and UPS results reveal that the effective passivation effect of EPB reduces the drop of the valence band between perovskite and SpiroOMeTAD, which effectively promoting the interfacial transport of carriers. As a result, The PCE of the EPB-treated device remains more than 90 % of the initial efficiency after 400 h of unpackaged and about 40 % ambient humidity.

Methylammonium thiocyanate seeds assisted heterogeneous nucleation for achieving high-performance perovskite solar cells

Thiocyanate (SCN-) has been reported to manage crystallization of perovskite films and improve their optoelectronic properties in solar cells. However, the mechanism of SCN- in preparation of high-quality (MAFA)Pb(IBr)3 based light-absorbing layers has not been well investigated yet. In this work, we investigate the effects of SCN- on phase transition of (MAFA)Pb(IBr)3 perovskite films and the corresponding device performance by introducing methylammonium thiocyanate (MASCN) in a PbI2 precursor solution which is used to make perovskite by a two-step sequential deposition method. Characterization of the perovskite films has shown that a proper amount (1 mg/mL) of MASCN in PbI2 can produce PbI2-MASCN seeds which act as a nucleation center for assisting heterogeneous nucleation of the (MAFA)Pb(IBr)3. Meanwhile, MASCN helps to effectively control crystallization of the perovskite film by obtaining preferential crystal growth orientation. Furthermore, MASCN is beneficial to eliminate residual PbI2 and prepare perovskite films with low trap-state density. Consequently, the power conversion efficiency (PCE) of the perovskite solar cells increases from 18.36% to 20.4%. An enhanced device stability against moisture is also observed in the MASCN-treated device which exhibits 90% retention of the initial efficiency after exposed to the ambient air for 1780 h.

Additive effect of bromides and chlorides on the performance of perovskite solar cells fabricated via sequential deposition

A two-step sequential deposition method has been applied to prepare the solar cells with two types of perovskites Cs0.15FA0.85Pb(I0.95Cl0.05)3 and Cs0.15FA0.85Pb(I0.95Br0.05)3. In order to obtain the perovskite layers, the different sources of bromine and chlorine atoms were used for synthesis. The performance and time stability of chloride-based photocells are worse in comparison to the bromide-based devices. It can be explained by the effect of an accumulation of Cl atoms at the interfaces between the chloride-based perovskites and the layer of PCBM. Such a process causes an increasing of interface recombination. Also, the bulk density of states and, consequently, the bulk recombination of charge carriers seem to be higher for the perovskite layers obtained with chlorine atoms. The two-step technique applied to create the bromide perovskites less influences the photocells performance as in the case of one-step deposition. We can explain this observation by an existence of nucleation sites in the inorganic layer which improve the growth of a perovskite material.

Surface-Orientation Elimination of Vapor-Deposited PbI2 Flakes for Efficient Perovskite Synthesis on Curved Solar Cells.

Vapor deposition of perovskite solar cells (PSCs) has attracted considerable interest for its dry processing characteristics. However, a two-step sequential vapor deposition method suffers from ineffective conversion of PbI2 to perovskite with reasons still unclear. In this report, we carefully investigated the crystallization orientation of PbI2 films deposited by physical vapor deposition via synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) and observed an asymmetric scattering pattern with respect to the qz-axis. The observed oriented morphology and texture hinder the diffusion of MAI molecules in the PbI2 films synthesized by vapor deposition, resulting in over 15% PbI2 remaining at the buried interface after reaction with MAI vapor. As a result, the MAPbI3 synthesized in this way was also highly oriented, especially in the surface layers. Surface fumigation (SF) step was introduced to decrease the orientational anisotropy of PbI2, which successfully breaks the diffusion barriers of MAI molecules by forming a complex layer on the PbI2 surface with polar solvent vapors, like dimethyl sulfoxide or 1,3-dimethyl-2-imidazolidinone. We infer that the SF treatment changes the vapor-solid reaction mechanism from reaction-crystallization to dissolution-recrystallization, which largely promotes the conversion of PbI2 to perovskite. Defects were reduced in perovskite synthesized in this way, and a p-i-n device with 19.56% efficiency was fabricated, which is among the highest efficiencies reported for sequential-vapor-deposited PSCs. Notably, this method enables the fabrication of conformal perovskite layers on uneven substrates. An exemplary PSC showing efficiency of 8.93% was fabricated on a precurved substrate. We believe that the method is applicable to the fabrication of tandem or curved PSCs that are compatible with wearable or building/autocar-integrated photovoltaics in the future.

Highly efficient wide-band-gap perovskite solar cells fabricated by sequential deposition method

Perovskite-based double junction tandem solar cells, including perovskite/silicon, perovskite/perovskite, and perovskite/Cu(In, Ga)Se 2 , have received increasing attention in recent years. While most wide-band-gap perovskite solar cells so far were synthesized by the one-step deposition method which has a simple procedure but is hindered by its narrow antisolvent dripping time window, exploration of other method is in demand. In this study, a highly efficient wide-band-gap perovskite solar cell device with a bandgap of 1.63–1.65 eV has been successfully fabricated using the two-step sequential deposition method by optimizing the solution composition and the spinning speed of the second step. The best solar cell obtained has a power conversion efficiency of 20.35% and a fill factor (FF) of 81.53%, and is believed to be the first wide-band-gap solar cell device fabricated by the sequential deposition method with an efficiency over 20%. The obtained FF value is also pretty high among all reported perovskite solar cells fabricated by sequential deposition method. Our work indicates that sequential deposition method not only can be applied to the traditional narrow-band-gap perovskite solar cells but also has a potential for fabricating highly efficient wide-band-gap perovskite solar cells. A highly efficient wide-band-gap perovskite solar cell device with an energy conversion efficiency of 20.35% and a fill factor of 81.53% has been successfully fabricated using the two-step sequential deposition method by optimizing the solution composition and the spinning speed of the second step. • A highly efficient wide-band-gap perovskite solar cell device was fabricated using sequential deposition method. • The obtained FF value 81.53% is the maximum among all reported perovskite solar cells fabricated by sequential deposition method. • This work has shown the potential of sequential deposition method for fabricating highly efficient and reproducible wide-band-gap perovskite active layers.

Enhanced efficiency and stability of perovskite solar cell by adding polymer mixture in perovskite photoactive layer

The key to commercialization of organic‐inorganic lead halide perovskite solar cells (PSCs) is to achieve high photoelectric conversion efficiency (PCE) and long-term stability under ambient condition. In this paper, polymer mixture of polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) with appropriate mass ratio are incorporated into PbI2 precursor solution in the preparation process of perovskite film via two-step sequential deposition method in air. The PCE of polymer mixture modified PSC based on ZnO@TiO2 nanorod arrays is better than those of the PSC with single polymer（PVP or PEG）and the pristine one, and the PCE of 1.5 wt%PVP/0.5 wt%PEG modified device increased by 30% compared with that of the unmodified one, which is mainly a consequence of the better quality of perovskite film caused by the synergistic effect including the strong attraction between the C=O in PVP and Pb2+ in perovskite, and the intense hydrogen bond between O atoms from PEG and H atoms in CH3NH3+. Moreover, polymer mixture modified PSC also represents better air stability compared with the device with single polymer or the pristine one，which is largely attributed to the exceptional hygroscopicity of PVP/PEG mixture, the synergistic stabilization of perovskite structure and inhibition of ion migration by the bondings between polymer mixture and perovskite. Furthermore, the mechanism of the interaction between PVP/PEG mixture and CH3NH3PbI3 to increase the efficiency and stability of the PSC was also studied. Our work offers an effective additive to simultaneously improve the efficiency and stability of the PSCs in high humidity environment for further commercial application.

New insights into interaction of proteins in extracellular polymeric substances of activated sludge with ciprofloxacin using quartz crystal microbalance with dissipation

Proteins in extracellular polymeric substances play a vital role in adsorbing organic contaminants in biological wastewater treatment processes, but there is still lack of a fast and effective approach to monitor their interaction. Quartz crystal microbalance with dissipation (QCM-D) was used to investigate the binding and viscoelastic properties of ciprofloxacin (CIP) on extracellular proteins from activated sludge by a two-step sequential deposition method. A saturated viscoelastic monolayer of proteins was formed on the crystal by injecting 500 mg L−1 extracellular proteins. Binding of CIP with the extracellular proteins film followed the pseudo-first-order kinetic equation and Langmuir model, with the maximum binding capacity of 172.4 mg g−1. The binding mass, energy dissipation, and reaction rate constant increased with increasing CIP concentration. A strong binding was obtained at pH 5, suggesting electrostatic interactions as the dominating binding mechanism. Cations inhibited CIP binding with extracellular proteins, probably due to cations competition. Two binding periods were distinguished according to the viscoelastic properties of CIP layer: viscous binding in the initial period and elastic towards binding saturation. Results highlighted QCM-D as an effective and real-time technique to evaluate the role of extracellular proteins in contaminants removal.

Optimized active layer morphology toward efficient and polymer batch insensitive organic solar cells

Morphology control in laboratory and industry setting remains as a major challenge for organic solar cells (OSCs) due to the difference in film-drying kinetics between spin coating and the printing process. A two-step sequential deposition method is developed to control the active layer morphology. A conjugated polymer that self-assembles into a well-defined fibril structure is used as the first layer, and then a non-fullerene acceptor is introduced into the fibril mesh as the second layer to form an optimal morphology. A benefit of the combined fibril network morphology and non-fullerene acceptor properties was that a high efficiency of 16.5% (certified as 16.1%) was achieved. The preformed fibril network layer and the sequentially deposited non-fullerene acceptor form a robust morphology that is insensitive to the polymer batches, solving a notorious issue in OSCs. Such progress demonstrates that the utilization of polymer fibril networks in a sequential deposition process is a promising approach towards the fabrication of high-efficiency OSCs.

Precursor engineering for performance enhancement of hole-transport-layer-free carbon-based MAPbBr3 perovskite solar cells

An optimized two-step sequential deposition method to fabricate hole-transport-layer-free carbon-based methyl ammonium lead bromide (MAPbBr3) perovskite solar cells is reported. Small amounts of MABr are introduced into the PbBr2 precursor solution during the first step to prepare MAPbBr3 perovskite films (labeled as MAPB-xMABr), which promotes the conversion of PbBr2 into perovskite phase and results in denser perovskite films with increased crystallinity, lower trap density, and longer carrier lifetime. After optimization, a maximum power conversion efficiency (PCE) of 7.64% (VOC = 1.36 V) is obtained for MAPB-0.2MABr based solar cells. Significantly, the non-encapsulated devices exhibit excellent long-term stability in ambient air (25–30 °C and 20–30% relative humidity), showing no degradation after a year’s exposure. While, it also shows superior thermal stability with PCE retaining 95% of the initial efficiency after 120 h under thermal stress of 80 °C and 40–70% relative humidity.

Effects of alkali and transition metal-doped TiO2 hole blocking layers on the perovskite solar cells obtained by a two-step sequential deposition method in air and under vacuum.

Planar perovskite solar cells (PPSCs) have received great attention in recent years due to their intriguing properties, which make them a good choice for photovoltaic applications. In this work, the effect of alkali and transition metal-doped TiO2 (cesium-doped TiO2 (Cs-TiO2) and yttrium-doped TiO2 (Y-TiO2)) compact layers on the optical, structural and the photovoltaic performance of the PPSCs have been investigated. The perovskite layer syntheses were carried out by depositing a lead iodide (PbI2) layer via spin-coating; converting PbI2 into methyl ammonium iodide (CH3NH3PbI3) by chemical vapor deposition (CVD) and spin-coating at 60 min and 60 s conversion times respectively. The as-deposited PPSCs were studied layer-by-layer using an X-ray diffractometer, scanning electron microscope, and UV-vis diffuse reflectance, transmittance and absorbance. The power conversion efficiency for stable processed perovskite solar cells were 3.61% and 12.89% for air and vacuum processed, respectively.

Sulfur-Vacancy Passivation in Solution-Processed Sb2S3 Thin Films: Influence on Photovoltaic Interfaces

We have presented a modified two-step sequential deposition method in forming Sb2S3 thin films. In contrast to conventional chemical bath deposition (CBD) route, this sequential deposition approach...

Effects of Bromide-mixing in Methylammonium Lead Iodide on Photovoltaic Properties of ZnO-based Perovskite Solar Cells

To improve efficiency of ZnO-based perovskite solar cells, a mixture iodide-bromide (I-Br) of FA0.4MA0.6Pbl3-2xBr2x perovskite structure was prepared by two-step sequential deposition method and used as photo absorber layer. Solar cells with structure of ITO/ZnO/PCBM/perovskite/P3HT/Au were fabricated and characterized. The perovskite films were prepared by mixing precursor PbI2 and PbBr2 at various molar ratio. It was found that the grain sizes and crystallite sizes of perovskite crystal were comparable for different Br doping. The existence of Br ions in perovskite layer was confirmed. In addition, the increase in band gap energy was observed. The highest average power conversion efficiency (PCE) of 2.17% was achieved from the device with FA0.4MA0.6PbI2.8Br0.2 layer. This PCE value is higher than the FA0.4MA0.6PbI3 device which exhibited 1.53%-efficiency. The enhancement in PCE could be explained by the development of light absorption and charges transport.

Tailoring perovskite conversion and grain growth by in situ solvent assisted crystallization and compositional variation for highly efficient perovskite solar cells

Abstract The performance of perovskite solar cell devices is strongly dependent on the morphology, crystallinity, and phase purity of perovskite films. However, the complete conversion of PbI2 into perovskite and tailoring perovskite grain growth as well as its surface morphology are challenging in the conventional two-step sequential deposition method. In this study, a facile and effective approach was developed to improve perovskite conversion rate and film quality via in situ solvent assisted crystallization (ISAC) and compositional adjustment. It was found that in the presence of in situ solvent molecules, CH3NH3I (MAI) molecules readily intercalated in the PbI2 lattice to form small-grain (∼210 nm) MAPbI3 perovskite. By partially substituting MAI with a small amount of CH(NH2)2I (FAI), flat FAxMA1–xPbI3 films composed of much larger grains (∼450 nm) could be obtained due to the in situ formation and decomposition of (MA)2Pb3I8(DMSO)2 intermediates. Photovoltaic devices based on ISAC-FAxMA1–xPbI3 films exhibited an enhanced efficiency of 17.31% under AM 1.5G solar illumination at 100 mW cm−2 when compared to the devices based on ISAC-MAPbI3 films (14.30%), which can be mainly ascribed to the higher optical absorption and reduced defect density of the former. The results highlight the distinct effects of in situ solvent molecules on perovskite crystallization dynamics in various material systems and the importance of in situ solvent molecules in perovskite conversion, film morphology, and even device performance.

Flexible Photodetector Arrays Based on Patterned CH3 NH3 PbI3- x Clx Perovskite Film for Real-Time Photosensing and Imaging.

The quest for novel deformable image sensors with outstanding optoelectronic properties and large-scale integration becomes a great impetus to exploit more advanced flexible photodetector (PD) arrays. Here, 10 × 10 flexible PD arrays with a resolution of 63.5 dpi are demonstrated based on as-prepared perovskite arrays for photosensing and imaging. Large-scale growth controllable CH3 NH3 PbI3- x Clx arrays are synthesized on a poly(ethylene terephthalate) substrate by using a two-step sequential deposition method with the developed Al2 O3 -assisted hydrophilic-hydrophobic surface treatment process. The flexible PD arrays with high detectivity (9.4 × 1011 Jones), large on/off current ratio (up to 1.2 × 103 ), and broad spectral response exhibit excellent electrical stability under large bending angle (θ = 150°) and superior folding endurance after hundreds of bending cycles. In addition, the device can execute the functions of capturing a real-time light trajectory and detecting a multipoint light distribution, indicating that it has widespread potential in photosensing and imaging for optical communication, digital display, and artificial electronic skin applications.

Effects of the concentration of PbI2 and CH3NH3I on the perovskite films and the performance of perovskite solar cells based on ZnO-TiO2 nanorod arrays

To date the research of the effects of the concentration of PbI2 and CH3NH3I in the two-step sequential deposition process on the perovskite films and the performance of the perovskite solar cells (PSCs) based on nanorod arrays is rarely reported. In this paper, the perovskite films and the PSCs based on ZnO-TiO2 nanorod arrays were prepared by two-step sequential deposition method in ambient atmosphere. The effects of the concentration of PbI2 and CH3NH3I solution on the morphology, structure, optical absorption and luminescence properties of perovskite films and cell performance were systematically investigated by using field emission scanning electron microscope, X-ray diffractometer, UV–vis spectrophotometer, fluorophotometer, and photocurrent density-voltage (J-V) curves. The best power conversion efficiency (PCE) of perovskite solar cell reaches 10.38% by using PbI2 with a concentration of 1.0 M and CH3NH3I with a concentration of 0.063 M in ambient atmosphere with a relative humidity of about 40%. The PCE of the device may be further improved by optimizing the density and length of nanorod arrays and using glove box. These results provide valuable information for preparing more efficient all-solid-state PSC based on nanorod arrays.

Synergic solventing-out crystallization with subsequent time-delay thermal annealing of PbI2 precursor in mesostructured perovskite solar cells

Although the two-step sequential deposition method provides an efficient route to fabricate high performance perovskite solar cells (PSSCs) with increasing reproducibility, the inefficient and incomplete conversion of PbI2 to perovskite is still quite a challenge. Following pioneering works, we found that the conversion process from PbI2 to perovskite mainly involves diffusion, infiltration, contact and reaction. In order to facilitate the conversion from PbI2 to perovskite, we demonstrate an effective method to regulate supersaturation level (the driving force to crystallization) of PbI2 by solventing-out crystallization combining with subsequent time-delay thermal annealing of PbI2 wet film. Enough voids and spaces in resulting porous PbI2 layer will be in favor of efficient diffusion, infiltration of CH3NH3I solution, and further enhance the contact and reaction between PbI2 and CH3NH3I in the whole film, leading to rapid, efficient and complete perovskite conversion with a conversion level of about 99.9%. Enhancement of light harvesting ranging from visible to near-IR region was achieved for the resultant high-quality perovskite. Upon this combined method, the fabricated mesostructured solar cells show tremendous power conversion efficiency (PCE) improvement from 3.2% to about 12.3% with less hysteresis owing to the simultaneous enhancement of short-circuit photocurrent density (Jsc), open-circuit voltage (Voc) and fill factor (FF).