Crystallization Of Perovskite Films Research Articles

Improved crystallization of perovskite films using PbTiO3-decorated mesoporous scaffold layers for high stable carbon-counter-electrode solar cells

Abstract The quality of perovskite film has great influence on photovoltaic performance of mesoscopic organic-inorganic halide perovskite solar cells, which is related to the mesoporous scaffold layers. In this paper, we present a modified two-step method to fabricate PbTiO3-decorated mesoporous scaffold layers for growing high-quality perovskite film, in which the mesoporous layers are treated by different concentration of Pb2+ solution. The introduction of moderate concentration of PbTiO3 is seen to promote the perovskite film growing and crystallizing, leading to reducing carrier recombination loss and defect density. The perovskite film based on treating concentration of 0.05 M exhibits larger size of crystal blocks, better quality of films, and higher surface coverage. Such a film applies in hole-counter-free perovskite solar cells with carbon-counter-electrode to obtain a power conversion efficiency of 10.55% along with a high open circuit voltage of 1.00 V. Moreover, these devices exhibit excellent long-term stability in an ambient atmosphere, which is related to the presence of the carbon electrode and PbTiO3.

Efficient Perovskite Solar Cells Prepared by Hot Air Blowing to Ultrasonic Spraying in Ambient Air

Substrate heating is the most common method for controlling crystallization during spray coating. However, due to poor controllability during substrate heating, the sprayed films have variable thicknesses and rich pores, which limit the efficiency of the device. Here, hot air blowing was applied to spray coating to promote the crystallization of perovskite films under ambient conditions. Upon employing a hot air blowing method that stimulated uniformly distributed nuclei growth, the pinhole-free and thickness-controllable perovskite film was prepared. This enabled more reproducible high-quality perovskite films to achieve a power conversion efficiency of 13.5% and obtain a stabilized power output of >12% in ambient conditions.

An Air Knife–Assisted Recrystallization Method for Ambient‐Process Planar Perovskite Solar Cells and Its Dim‐Light Harvesting

The photovoltaic performance of perovskite solar cells is highly dependent on the control of morphology and crystallization of perovskite film, which usually requires a controlled atmosphere. Therefore, fully ambient fabrication is a desired technology for the development of perovskite solar cells toward real production. Here, an air-knife assisted recrystallization method is reported, based on a simple bath-immersion to prepare high-quality perovskite absorbers. The resulted film shows a strong crystallinity with pure domains and low trap-state density, which contribute to the device performance and stability. The proposed method can operate in a wide process window, such as variable relative humidity and bath-immersion conditions, demonstrating a power conversion efficiency over 19% and 27% under 1 sun and 500-2000 lux dim-light illumination respectively, which is among the highest performance of ambient-process perovskite solar cells.

A Review of the Role of Solvents in Formation of High-Quality Solution-Processed Perovskite Films.

Recently, perovskite solar cells have attracted great attention because of their outstanding photovoltaic performance and ease of fabrication. High-quality perovskite films hold a key in getting highly efficient perovskite solar cells. Solution-processed fabrication technique is the most widely adopted for preparing perovskite films because of its low cost. In the solution-proceed perovskite films, solvents not only play the role of dissolving the solute but also participate in the crystallization of perovskite. In the one-step method, solvents play key roles in controlling morphology, widening process window, and achieving room-temperature crystallization of perovskite films. In addition, the solvents play important roles in controlling the nuclei/growth, suppressing volume expansion during the two-step method. Especially, the solvent can induce grain coarsening during the annealing process. A deep understanding of the multiplicity of roles during the formation of perovskite films will help understand the formation mechanism of perovskite films. Here, a systematic review on the progress in fabrication of high-quality perovskite films by making use of solvent to control the crystallization is presented. Meanwhile, we elucidate the key roles of solvent in the fabrication of high-quality perovskite films.

Control of Crystal Growth toward Scalable Fabrication of Perovskite Solar Cells

AbstractWith the impressive record power conversion efficiency (PCE) of perovskite solar cells exceeding 23%, research focus now shifts onto issues closely related to commercialization. One of the critical hurdles is to minimize the cell‐to‐module PCE loss while the device is being developed on a large scale. Since a solution‐based spin‐coating process is limited to scalability, establishment of a scalable deposition process of perovskite layers is a prerequisite for large‐area perovskite solar modules. Herein, this paper reports on the recent progress of large‐area perovskite solar cells. A deeper understanding of the crystallization of perovskite films is indeed essential for large‐area perovskite film formation. Various large‐area coating methods are proposed including blade, slot‐die, evaporation, and post‐treatment, where blade‐coating and gas post‐treatment have so far demonstrated better PCEs for an area larger than 10 cm2. However, PCE loss rate is estimated to be 1.4 × 10−2% cm−2, which is 82 and 3.5 times higher than crystalline Si (1.7 × 10−4% cm−2) and thin film technologies (≈4 × 10−3% cm−2) respectively. Therefore, minimizing PCE loss upon scaling‐up is expected to lead to PCE over 20% in case of cell efficiency of >23%.

Highly Efficient Sn/Pb Binary Perovskite Solar Cell via Precursor Engineering: A Two‐Step Fabrication Process

AbstractRegulation of the crystallization of perovskite films and avoiding the oxidation of Sn2+ during the deposition process are very important for achieving Sn/Pb binary perovskite solar cells (PVSCs) with high power conversion efficiency (PCE) and producibility. In this work, a high‐quality HC(NH2)2Pb0.7Sn0.3I3 (FAPb0.7Sn0.3I3) film deposited from the two‐step solution process by introducing methylammonium thiocyanate (MASCN) as a bifunctional additive into the precursor solution containing PbI2 and SnI2 is reported. MASCN can not only tune the morphology of the perovskite film but also stabilize the precursor solution via retarding the oxidation of Sn2+ through a strong coordination between SCN− and Sn2+. The Sn/Pb binary inverted PVSCs based on FAPb0.7Sn0.3I3 present a high fill factor of 0.79 and the best PCE of 16.26% in the case of 0.25 MASCN addition. The device fabrication producibility is also greatly improved due to the stabilized precursor solution with the aid of MASCN. The PCE of the device is almost independent of the storage time of the precursor solution within 124 d in the N2‐filled glove box. These results indicate that the precursor engineering with multifunctionality additive is an effective approach toward highly efficient and producible PVSCs for future commercialization.

Mixed-steam annealing treatment for perovskite films to improve solar cells performance

To improve the photovoltaic performance of the perovskite solar cells, it is necessary to reduce the density of surface defect for perovskite film with smooth surface. In this paper, we demonstrate a DMSO/CB mixed vapor annealing process to fabricate high-performance planar perovskite solar cells. The mixed vapor annealing treatment can significantly enhance the crystallization of perovskite film, leading to less crystal surface defects, effective charge-separation and the electron transport rate at the perovskite interface. Finally, the efficiency and reproducibility of the cells has been greatly improved. The power conversion efficiency (PCE) improved from 16.6% to 18.4% under AM 1.5G 100 mW·cm−2 irradiation. We anticipate that the mixed vapor annealing treatment will become a promising crystallization method for the fabrication of high performance PSCs in the future commercialization.

Critical role of chloride in organic ammonium spacer on the performance of Low-dimensional Ruddlesden-Popper perovskite solar cells

Low-dimensional Ruddlesden-Popper (LDRP) perovskites attracted remarkable attention due to their technologically relevant intrinsic photo- and chemical-stability, suppressed ion migration, and ultralow self-doping effect over their 3D counterpart. The power conversion efficiency over 14% was recently achieved since the initial demonstration of LDRP perovskite solar cells (PSCs) in 2014. However, further improvements require a fundamental understanding on the components functionality in LDRP perovskites, e.g., bulky organic ammonium spacer and halogen ions, which are critical for designing efficient LDRP PSCs. Here, we report the critical role of the chloride that are derived from halogenated organic ammonium salts on the LDRP perovskite film crystallization, growth, opto-electric properties, and device performance. We found that the expected improvements in perovskite morphology with increased grain size, enhanced crystallinity, and uniform and smooth surface were revealed no matter which introduced chloride either by bulky organic ammonium or methyl ammonium salts. We also unambiguously demonstrated that photocurrent and photovoltage of LDRP PSCs are highly related to the position of chloride on organic ammonium salts. Moreover, the films and devices maintain excellent stability by the introduction of chloride due to the excellent film quality. The resulting LDRP PSCs exhibited best efficiency of 12.78%, which is two times enhancement compared to all iodide-contained device (6.52%) commonly used in previous reports. These findings demonstrated that chloride plays a significant role in LDRP perovskite and detected a key parameter for the development of future LDRP perovskite absorbers and relevant optoelectronic devices.

High performance and stable all-inorganic perovskite light emitting diodes by reducing luminescence quenching at PEDOT:PSS/Perovskites interface

Abstract Compared to organic-inorganic halide perovskites (e.g., MAPbBr3 and FAPbBr3), all-inorganic CsPbBr3 perovskite has shown to be a more promising candidate for application in light-emitting diodes (LEDs) due to higher photoluminescence quantum efficiency and thermal stability. Yet, the external quantum efficiency (EQE) of polycrystalline (3D) CsPbBr3 based PeLEDs needs further improvement. The low efficiency was attributed to severe luminescence quenching at PEDOT:PSS (Poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate))/CsPbBr3 interface arising from (i) interfacial energy barrier between PEDOT:PSS and CsPbBr3 layer, (ii) poor crystallization and large pinholes in perovskite film, and (iii) indium tin oxide (ITO) anode etching due to PEDOT:PSS acidity. Therefore, it is necessary to address all these problems simultaneously to reduce the luminescence quenching and to improve device efficiency. Here, we introduce a feasible approach to modify the PEDOT:PSS layer in order to reduce the luminescence quenching at PEDOT:PSS/CsPbBr3 interface. In this modification, PEDOT:PSS is mixed with a MoO3 ammonia solution at desired volume ratios. Our studies demonstrate that upon this treatment, not only hole injection and crystallization of perovskite film are improved but also ITO is protected from etching by the acidic nature of PEDOT:PSS. This modification result in a high performance all inorganic CsPbBr3 LED with a maximum luminance of ∼34420 cd/m2 and a current efficiency of ∼7.30 cd/A. In addition, our state-of-the-art perovskite LEDs (PeLEDs) exhibit no degradation after 2 h of continuous operation, representing a stable PeLEDs based on one-step solution processed polycrystalline CsPbBr3.

Constructing efficient mixed-ion perovskite solar cells based on TiO2 nanorod array

Oriented TiO2 nanorod array (TiO2 NA) is very attractive in the fields of halide perovskite solar cells (PSCs) due to its fewer grain boundaries and high crystallinity for effective charge collection. The optimization of TiO2 nanostructures has been proved to be an effective approach for efficient PSCs. On the other hand, tuning the crystallization of perovskite films on top of the TiO2 NA is very important for efficient TiO2-NA based PSCs. Herein, scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) were used to study the crystallization of different mixed-ion Cs0.1(FA0.83MA0.17)0.9Pb(I0.83Br0.17)3 perovskite (in which MA = CH3NH3+, and FA = CH(NH2)2+) films, from different perovskite precursor concentrations, on the TiO2 nanorod arrays. A mechanism was proposed to reveal the inherent connection between the precursor concentration and the crystallite growth of the perovskite film prepared with anti-solvent quenching process. Meanwhile, both faster charge separation at perovskite/TiO2 NA interface and longer charge transport were observed on thicker perovskite film with larger grains, revealed by the time-resolved method. However, atomic force microscopy (AFM) results indicated that too thick perovskite film impaired the charge collection owing to the increased recombination. By balancing the charge collection and film thickness, highly efficient PSCs were prepared with a champion power conversion efficiency (PCE) of 19.33% with little hysteresis. The study highlights a great potential of incorporating oriented one-dimensional electron extraction materials in high-performance PSCs and other applications.

Phase Engineering of Perovskite Materials for High-Efficiency Solar Cells: Rapid Conversion of CH3NH3PbI3 to Phase-Pure CH3NH3PbCl3 via Hydrochloric Acid Vapor Annealing Post-Treatment.

Organometal halide CH3NH3PbI3 (MAPbI3) has been commonly used as the light absorber layer of perovskite solar cells (PSCs), and, especially, another halide element chlorine (Cl) has been often incorporated to assist the crystallization of perovskite film. However, in most cases, a predominant MAPbI3 phase with trace of Cl- is obtained ultimately and the role of Cl involvement remains unclear. Herein, we develop a low-cost and facile method, named hydrochloric acid vapor annealing (HAVA) post-treatment, and realize a rapid conversion of MAPbI3 to phase-pure MAPbCl3, demonstrating a new concept of phase engineering of perovskite materials toward efficiency enhancement of PSCs for the first time. The average grain size of perovskite film after HAVA post-treatment increases remarkably through an Ostwald ripening process, leading to a denser and smoother perovskite film with reduced trap states and enhanced crystallinity. More importantly, the generation of MAPbCl3 secondary phase via phase engineering is beneficial for improving the carrier mobility with a more balanced carrier transport rate and enlarging the band gap of perovskite film along with optimized energy level alignment. As a result, under the optimized HAVA post-treatment time (2 min), we achieved a significant enhancement of the power conversion efficiency (PCE) of the MAPbI3-based planar heterojunction-PSC device from 14.02 to 17.40% (the highest PCE reaches 18.45%) with greatly suppressed hysteresis of the current-voltage response.

Manipulation of the crystallization of perovskite films induced by a rotating magnetic field during blade coating in air

A rotating magnetic field (RMF) was employed to manipulate the crystallization of perovskite films during blade coating.

Graphene oxide as an additive to improve perovskite film crystallization and morphology forhigh-efficiency solar cells.

The quality of a perovskite film has a great impact on its light absorption and carrier transport, which is vital to improve high-efficiency perovskite solar cells (PSCs). Herein, it is demonstrated that graphene oxide (GO) can be used as an effective additive in the precursor solution for the preparation of high-quality solution-processed CH3NH3PbI3 (MAIPbI3) films. It is evidenced by scanning electron microscopy that the size of the grains inside these films not only increases but also becomes more uniform after the introduction of an optimized amount of 1 vol% GO. Moreover, 1 vol% GO also enhances the crystallization of perovskite film with intact preferential out-of-plane orientation as proven by 2-dimensional grazing-incidence X-ray diffraction. As a consequence of the improved film quality, enhanced charge extraction efficiency and optical absorption are demonstrated by photoluminescence (PL) spectroscopy and UV-visible absorption spectroscopy, respectively. Using 1 vol% GO, the fabricated champion heterojunction PSC with a structure of ITO/SnO2/perovskite/spiro-OMeTAD/Au shows a significant power conversion efficiency increase to 17.59% with reduced hysteresis from 16.10% for the champion device based on pristine perovskite. The present study thus proposes a simple approach to make use of GO as an effective and cheap addictive for high-performance PSCs with large-scale production capability.

(Invited) Improvement of Perovskite Films and the Perovskite/TiO2 Interface

The power conversion efficiency of perovskite solar cells has increased from 3.8% to today’s 22.1% for the past several years. However, the properties of perovskite films and interface engineering are still challenging issues. In this study, we carried out a solvent annealing method to improve the crystallization of perovskite films. With this modified sequential deposition method, the solar cells based on the dimethyl sulfoxide annealing exhibited the best efficiency of 18.5%. Furthermore, to modify the TiO2/perovskire interface, Li doped TiO2 instead of pure TiO2 was used as electron transport material. Comparing with the undoped compact TiO2 based solar cells, the efficiency of the Li-doped compact TiO2 film based solar cells is improved from 14.2% to 17.1% with a negligible hysteresis.

Manipulation on Crystallization of Perovskite Thin-films during Blade Coating Fabrication in Air

Organometal trihalide perovskite solar cells (PSCs) have drawn more and attention in the past few years due to the merits of low-cost preparation processes and rapidly improved power conversion efficiency (PCE) with latest certified efficiency of 22.1%.1-4 Commonly, solution processes, including one- or two-step spin-coating methods, are often employed to produce perovskite films.5, 6 Recently, the blade coating technique, with the advantages of large scale fabrication and mass production besides the well-accepted low-cost manner, has also attracted researchers’ interest for the rapid growth of perovskite films.7-8 However, nucleation and crystal growth during the film’s crystallization by blade coating are still lack of controlling, generally resulting in relatively poorer crystal quality and film uniformity. So, further strategies are required to improve the crystallization of perovskite films and ultimate device efficiencies. In this work, different strategies were introduced during the blade-coating process to control and manipulate the crystallization of perovskite films. In depth understanding in the nucleation mechanisms and growth dynamics of films were well accomplished. As a result, the rapid growth of perovskite films with high quality and good optical performances were successfully produced. Correspondingly, efficient PSCs were correspondingly fabricated.

High Crystallization of Perovskite Film by a Fast Electric Current Annealing Process.

High-efficiency organic-inorganic hybrid perovskite solar cells have experienced rapid development and attracted significant attention in recent years. Crystal growth as an important factor would significantly influence the quality of perovskite films and ultimately the device performance, which usually requires thermal annealing for 10 min or more. Herein, we demonstrate a new method to get high crystallization of perovskite film by electric current annealing for just 5 s. In contrast to conventional thermal annealing, a homogeneous perovskite film was formed with larger grains and fewer pinholes, leading to a better performance of the device with higher open-circuit voltage and fill factor. An average power conversion efficiency of 17.02% with electric current annealing was obtained, which is higher than that of devices with a conventional thermal annealing process (16.05%). This facile electric current annealing process with less energy loss and time consumption shows great potential in the industrial mass production of photovoltaic devices.

Solvent vapor annealing of oriented PbI2 films for improved crystallization of perovskite films in the air

The photovoltaic performance of perovskite solar cells is extremely dependent on the crystallization and morphology of the perovskite film, which are affected by the deposition method. Here, we demonstrate a simple approach to form a microporous PbI2 film, with subsequent conversion to a compact, highly crystalline perovskite film. The PbI2 and corresponding perovskite films were further probed by two-dimensional X-ray diffraction. The resultant perovskite exhibited improved photovoltaic performance under ambient conditions with about 50% humidity. The PbI2 microporous structure was formed by exchanging residual DMSO with DMF vapor in the PbI2 film, which facilitated contact with the methylammonium iodide (MAI) solution. The process resulted in the formation of compact, smooth, pinhole-free perovskite films having no residual PbI2. Solar cells fabricated using this methodology exhibited power conversion efficiencies over 16% with negligible photocurrent hysteresis.

Controllable intermediates by molecular self-assembly for optimizing the fabrication of large-grain perovskite films via one-step spin-coating

Abstract In the past two years, the introduction of dimethylsulfoxide (DMSO) has recognized as an effective way to prepare high-quality pervoskite films. During the developed DMSO process, the formation of an intermediate phase has proven to uniform crystal growth rates between CH 3 NH 3 I (MAI) and PbI 2 in common solvents. In this work, controllable intermediates were obtained in dimethylformamide (DMF) by additional solvent of DMSO, which tend to be closely packed by means of intermolecular self-assembly. In addition, the bond strength and chemical composition of intermediate phase were investigated with FTIR and TGA. The use of different ratios of DMSO leads to four different complexes of MAI·PbI 2 ·DMF and MAI·PbI 2 ·(DMSO) y (y = 0.6, 1.5, 1.9), thus dramatically influencing the crystallization of perovskite films. Of these complexes, optimized MAI·PbI 2 ·(DMSO) 1.5 intermediate adduct enables highly dense perovskite thin films with large grain size and enhances efficient perovskite solar cells with high charge carrier lifetime.

Fast and Controllable Crystallization of Perovskite Films by Microwave Irradiation Process.

The crystal growth process significantly influences the properties of organic-inorganic halide perovskite films along with the performance of solar cell devices. In this paper, we adopted the microwave irradiation to treat perovskite films through a one-step deposition method for several minutes at a fixed output power. It is found that the specific microwave irradiation process can evaporate the solvent directly and heat perovskite film quickly. In comparison with the conventional thermal annealing process, a microwave irradiation process assisted fast and controllable crystallization of perovskite films with less energy-loss and time-consumption and therefore resulted in the enhancement in the photovoltaic performance of the corresponding solar cells.

Annealing-free perovskite films by instant crystallization for efficient solar cells

Ethyl acetate (EA) is introduced in a one-step spin-coating method and can induce instant crystallization of perovskite films to create efficient thermal annealing-free perovskite solar cells.