High-quality Perovskite Crystals Research Articles

Small molecule induced interfacial defect healing to construct inverted perovskite solar cells with high fill factor and stability

Chemical defects at the surface and grain boundaries of perovskite crystals cause deterioration of conversion efficiency and stability of perovskite solar cells (PSCs). In this study, a multifunctional additive, 5-fluoro-2-pyrimidine carbonitrile (FPDCN) molecule, is added into the perovskite precursor solution in order to passivate the uncoordinated Pb2+ by the cyanogen (–CN) group and pyrimidine N in FPDCN. Interestingly, fluorine (F) atoms interact with FA+ to form hydrogen bonds, which could regulate the perovskite crystallization process for the formation of high-quality perovskite crystals. Besides, the F atoms in FPDCN increase the water contact angle of perovskite films. As a result, the carrier extraction and transport in the perovskite film are significantly enhanced, and the non-radiative recombination is suppressed. The corresponding devices achieve a champion photovoltaic conversion efficiency (PCE) of 20.7 % and a fill factor (FF) of over 83 %. The device based on FPDCN shows long-term stability under a high-humidity atmospheric environment by maintaining 85 % of the initial efficiency after aging of 700 h in the glove box. This study provides a simple and convenient method to prepare stable and efficient PSCs by optimizing the perovskite precursor solution.

Molecular Symmetry of Small-Molecule Passivating Agents Improves Crystal Quality of Perovskite Solar Cells.

Direct interaction with the defect sites of perovskite, functional groups have become the focal point of attention as passivating agents. However, the molecular parent nucleus determines the overall physical properties of the molecule, including the push-pull electronic characteristics of the functional groups, which poses significant challenges in terms of selectivity. Here, we discovered that the binary acid structure based on thiophene as the parent nucleus, due to changes in molecular symmetry, caused significant changes in the molecular dipole moment, resulting in significant changes in the passivation effects on under-coordinated Pb2+ in perovskite solar cells. For the axially symmetric thiophen-2,5-dicarboxylic acid (TPDC), the high dipole moment formed a concentrated surface negative potential on the carboxyl group, showing significant superiority over the centrally symmetric thieno[3,2-b] thiophene-2,5-dicarboxylic acid (TTDC) in forming high-quality perovskite crystals, suppressing charge recombination, enhancing effective charge transport, and raising internal electric fields. The power conversion efficiencies of the fully printable mesoscopic perovskite solar cells based on TPDC and TTDC were 17.15 % and 14.79 %, respectively, exhibiting important research value in the field of small molecule passivation mechanism research.

One-step hydrothermal synthesis of Zr-doped brookite TiO2 nanorods for highly efficient perovskite solar cells

Perovskite solar cells (PSCs) fabricated of TiO2 electron transporting material (ETM) have aroused tremendous attention, but their power conversion efficiency (PCE) needs further improvements. In this study, for the first time, zirconium (Zr)-doped brookite TiO2 nanorods (ZTO) are synthesized by a facile route and subsequently employed as an ETM in PSCs which finally achieves a considerable PCE of 19.42 % by optimizing all operating conditions. Further characterizations have been undertaken to explore in detail the effects of Zr doping on the nature of the perovskite active layer and the photovoltaic performance. As a consequence, the enhanced PCE can be ascribed to the high quality perovskite crystals and a more favorable energy level alignment between the perovskite layer and ZTO mesoporous layer.

Monolithically-grained perovskite solar cell with Mortise-Tenon structure for charge extraction balance

Although the power conversion efficiency values of perovskite solar cells continue to be refreshed, it is still far from the theoretical Shockley-Queisser limit. Two major issues need to be addressed, including disorder crystallization of perovskite and unbalanced interface charge extraction, which limit further improvements in device efficiency. Herein, we develop a thermally polymerized additive as the polymer template in the perovskite film, which can form monolithic perovskite grain and a unique “Mortise-Tenon” structure after spin-coating hole-transport layer. Importantly, the suppressed non-radiative recombination and balanced interface charge extraction benefit from high-quality perovskite crystals and Mortise-Tenon structure, resulting in enhanced open-circuit voltage and fill-factor of the device. The PSCs achieve certified efficiency of 24.55% and maintain >95% initial efficiency over 1100 h in accordance with the ISOS-L-2 protocol, as well as excellent endurance according to the ISOS-D-3 accelerated aging test.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Photon Echo Polarimetry of Excitons and Biexcitons in a CH3NH3PbI3 Perovskite Single Crystal

Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light-matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Here we specifically address the energy level order of optical transitions and demonstrate photon echos in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times $T_2$ for excitons and biexcitons at cryogenic temperature to be 0.79 ps and 0.67 ps, respectively. Most importantly, we have developed an experimental photon-echo polarimetry method that not only identifies the contributions from exciton and biexciton complexes, but also allows accurate determination of the biexciton binding energy of 2.4 meV, even though the period of quantum beats between excitons and biexcitons is much longer than the coherence times of the resonances. Our experimental and theoretical analysis methods contribute to the understanding of the complex mechanism of quasiparticle interactions at moderate pump density and show that even in high-quality perovskite crystals and at very low temperatures, inhomogeneous broadening of excitonic transitions due to local crystal potential fluctuations is a source of optical dephasing.

Antisolvents Treatment of Cs0.15FA0.85PbI3 Boosting Efficiency for Perovskite Solar Cells

One-step spin-coating solution deposition was reported as an excellent processing method in perovskite solar cells (PSCs). Antisolvent treatment has been demonstrated to significantly enhance the power conversion efficiency (PCE) of PSCs owing to the formation of high-quality perovskite crystals and passivation of defects. In this article, we alter different antisolvents in solution deposition process, i.e., chlorobenzene, toluene, and diethyl ether, combined with different measurements of scanning electron microscopy, atomic force microscopy, X-ray diffraction, time-resolved photoluminescence, and UV–vis absorption spectrum (UV–vis) to investigate the evolution of perovskite thin films. These results illustrate that a smooth and compact perovskite layer by antisolvent treatment is beneficial to enhancing device performance. A high coverage, large crystal size and smooth Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> FA <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> PbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film with a long carrier lifetime attained via antisolvent treatment is utilized to fabricate an inverted planar heterojunction solar cell, which exhibits PCE more than 15%.

Synergistic Effect of Defect Passivation and Crystallization Control Enabled by Bifunctional Additives for Carbon-Based Mesoscopic Perovskite Solar Cells.

The emerging carbon-based mesoscopic perovskite solar cells (MPSCs) are known as one of the most promising candidates for photovoltaic applications thanks to their screen-printing process and excellent stability. Unfortunately, they usually suffer from serious defects because it is challenging to realize sufficient mesopore filling of the perovskite precursor solution throughout the triple-mesoporous scaffold. Herein, a bifunctional additive, biuret, endowed with both carbonyl and amino groups, was designed to realize a convenient fabrication approach for controllable crystallization of the precursor solution. Owing to the strong coordination ability with perovskite components, the incorporation of biuret can not only regulate crystallization kinetics allowing for the growth of high-quality perovskite crystals but also associate with uncoordinated ions for defect passivation to enhance the overall photovoltaic performance of MPSCs. A champion power conversion efficiency (PCE) of 13.42% with an enhanced short-circuit current density of 19.49 mA cm-2 and a much higher open-circuit voltage of 0.96 V was achieved for the device doped with 3 mol % biuret, which is 26% higher than that of the control device (10.66%). Moreover, the unencapsulated devices with biuret incorporation demonstrated superior stability, maintaining over 90% of the original PCE after 50 days of storage under ambient conditions. This work helps exploit bifunctional additive strategies for simultaneous defect passivation and crystallization control toward high-efficiency and long-term stability of carbon-based MPSCs.

Lewis Base-Mediated Perovskite Crystallization as Revealed by In Situ, Real-Time Optical Absorption Spectroscopy

The strategy of Lewis base modification has been shown to be rather effective in fabricating high-quality perovskite crystals; however, the underlying mechanisms remain controversial owing to the lack of any systematic characterization of the crystallization process. Herein, we report a novel non-invasive optical technique, termed vertical reflection-type in situ, real-time absorption spectroscopy, to investigate the mechanisms of Lewis base-mediated optimization of perovskite crystallinity by visualizing the entire energetic landscape of crystal growth. We show that by virtue of the urea additive, a prototypical Lewis base, the growth kinetics is accelerated prominently by decreasing the activation energy from 73.7 to 41.7 kJ/mol. In addition, the self-passivation of structural disorder during thermal annealing is identified, which is shown to be further strengthened by urea modification toward a shallower distribution of trap states.

Perovskite Monocrystals: Ultrathin Perovskite Monocrystals Boost the Solar Cell Performance (Adv. Energy Mater. 34/2020)

In article number 2000453, Weili Yu, Chunlei Guo and co-workers demonstrate that by combining the space-limiting technique and the anti-temperature crystallization method, millimeter sized MAPbI3 perovskite monocrystals with thickness from tens of nanometers to micrometers can be fabricated. Solar cells based on the 300 nm thick perovskite monocrystal are prepared, which show 3% enhancement in power conversion efficiency (PCE) compared to their polycrystalline counterparts. This work provides a scalable way to synthesize high quality perovskite crystals with less grains and grain boundaries, is believed a key step to develop perovskite single crystal solar cells with high performance.

Self-assembled NiO microspheres for efficient inverted mesoscopic perovskite solar cells

Perovskite solar cells (PSCs) with both normal (n-i-p) and inverted (p-i-n) mesoscopic structures usually exhibit higher efficiency than their planar counterparts because the mesoporous charge transport layers can supply heterogeneous nucleation sites for growing high quality perovskite crystals and enlarged charge separation area for better charge extraction. However, comparing with the achieved extremely high or even the certified world record efficiency of mesoscopic PSCs, the significant improvement of inverted mesoscopic PSCs has yet been made, mainly owing to the lack of suitable p-type semiconductors for preparing mesoporous hole transport layers (HTLs). Here, an emulsion-based bottom-up self-assembly strategy is used to prepare NiO microspheres from well-dispersed NiO nanocrystals. The self-assembled NiO microspheres are further used to fabricate mesoporous NiO HTLs of the inverted mesoscopic PSCs. The as-prepared mesoporous NiO HTL with self-assembled NiO microspheres can provide more suitable graded energy alignment, better charge carrier dynamics and reduced dark recombination in the device comparing with the inverted planar PSC with NiO nanocrystal HTL, contributing to obviously enhanced photovoltaic performance and nearly eliminated photocurrent-voltage hysteresis. Due to the general strategy of emulsion-based bottom-up self-assembly for microspheres synthesis, it will overcome the shortage of p-type materials for preparing efficient inverted mesoscopic PSCs.

Solution evaporation processed high quality perovskite films

Organic-inorganic hybrid perovskite solar cells (PSCs), due to their high power conversion efficiency (PCE) and low cost, have been considered as one of the most promising photovoltaic devices. Well-distributed large-grain perovskite crystals play an important role for light adsorption and charge transfer. While, high quality perovskite crystals closely relate to the preparation method. Generally, the preparation of perovskite films requires a nitrogen atmosphere and toxic anti-solvents, which hinder their practical applications. Here, we reported a novel and simple solution evaporation process followed with a methylammonium iodide (MAI) solution immersion to prepare high quality perovskite films. Porous lead iodide (PbI2) films were firstly formed by evaporation of PbI2 precursor solution. The obtained porous PbI2 films were completely transformed into well-distributed large-grain perovskite films after a quick MAI solution immersion. As a result, the corresponding PCE was enhanced by 30% compared to that prepared with a regular sequential deposition. This solution evaporation method provides a convenient and practical way for preparing high-quality perovskite films.

A one-pot method for controlled synthesis and selective etching of organic-inorganic hybrid perovskite crystals

Abstract Organometal halide perovskites have recently emerged with a huge potential for photovoltaic applications. Moreover, preparation of high-quality perovskite crystals with controlled morphology is of great significance for the fundamental studies such as optical and electrical properties, as well as the applications. Here, we report a one-pot solvothermal process to synthesize sheet-shaped CH3NH3PbBr3 single crystals with the lateral size of 100 µm and the thickness of 3–8 µm. Furthermore, a controlled etching behavior on the crystalline surface was demonstrated, which could be the irregular collapse of the crystalline surface caused by the local accumulation of methylammonium cations. Using this technique, CH3NH3PbBr3 single crystal sheets could be used in the various optoelectronic devices, such as nanolaser, optical sensors, photodetectors and field effect transistors.

High Quality Perovskite Crystals for Efficient Film Photodetectors Induced by Hydrolytic Insulating Oxide Substrates

AbstractA high quality perovskite film is a key factor in determining the device performance, such as photovoltaic cells, light‐emission diodes, lasers, and photodetectors. Here, a method is presented to improve the crystalline quality of perovskite films on surface‐oxygen‐rich insulating oxide substrates, which can promote the growth of both the polycrystalline and single crystals and enhance the adhesion between the perovskites and the substrates. A much longer carrier diffusion length of exceeding 5 µm together with significantly reduced trap density and nonradiative recombination is achieved for the film. These perovskite films show much better lasing and photodetector performance, indicating promising applications for the light emitting, lasing, and detector devices.

(Invited) Real Time Analysis of the Crystallization Dynamics of Organilead Halide Perovskite

Organolead-halide perovskite solar cells have been attracting much attention because of the rapid improvement of its power conversion efficiency. Simple fabrication process can be advantageous for the cost reduction, where just mixing lead halide and amine halide results in the perovskite crystal. Although there have been published numbers of method for the fabrication of perovskite solar cells, reproducibility of the device characteristics is great issue. The fundamentals of crystallization process is important to improve the device performance as well as reproducibility, however, such studies are still underway. We have been conducted in-situ analysis of the crystallization process of organolead-halide perovskite by means of grazing incident wide angle X-ray scattering (GIWAXS) [1]. The measurement was performed at BL46XU in SPring-8 and 2 dimensional X-ray detector (PILATUS 300K) was used, which enables the real time GIWAXS measurement. Perovskite film was synthesized on the measurement stage of the beamline to observe the crystallization dynamics. A PbI2 thin film was placed and CH3NH3I solution was dropped onto the PbI2. The reduction in the amount of PbI2 raw material over time was observed as well as the formation process of the perovskite crystal. Analysis of the rate of progression of this reaction revealed that reaction progression did not follow the normal diffusion phenomenon but instead followed an anomalous diffusion process. This can be originated from the anomalous diffusion of CH3NH3I into the PbI2thin film medium, which shows the heterogeneous structure. Next we focus on the angle dependent X-ray diffraction patterns. In the early stage of the reaction the formed crystals were oriented in two specific directions, but with the progress of time they changed to a random orientation. This finding suggests that the crystals changed continuously during the process of formation of the perovskite crystal. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells. [1] T. Miyadera et al., Nano Lett., 2015, 15 (8), pp 5630–5634. This work is financially supported by the new energy and industrial technology development organization (NEDO) and Japan science and technology agency (JST) through its funding program for Precursory Research for Embryonic Science and Technology (PRESTO). Figure 1

In-situ fabrication of dual porous titanium dioxide films as anode for carbon cathode based perovskite solar cell

We develop a dual porous (DP) TiO2 film for the electron transporting layer (ETL) in carbon cathode based perovskite solar cells (C-PSCs). The DP TiO2 film was synthesized via a facile PS-templated method with the thickness being controlled by the spin-coating speed. It was found that there is an optimum DP TiO2 film thickness for achieving an effective ETL, a suitable perovskite/TiO2 interface, an efficient light harvester and thus a high performance C-PSC. In particular, such a DP TiO2 film can act as a scaffold for complete-filling of the pores with perovskite and for forming high-quality perovskite crystals that are seamlessly interfaced with TiO2 to enhance interfacial charge injection. Leveraging the unique advantages of DP TiO2 ETL, together with a dense-packed and pinhole-free TiO2 compact layer, PCE of the C-PSCs has reached 9.81% with good stability.

Crystallization Dynamics of Organolead Halide Perovskite by Real-Time X-ray Diffraction.

We analyzed the crystallization process of the CH3NH3PbI3 perovskite by observing real-time X-ray diffraction immediately after combining a PbI2 thin film with a CH3NH3I solution. A detailed analysis of the transformation kinetics demonstrated the fractal diffusion of the CH3NH3I solution into the PbI2 film. Moreover, the perovskite crystal was found to be initially oriented based on the PbI2 crystal orientation but to gradually transition to a random orientation. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells.