A Cryogenic Process for Antisolvent-Free High-Performance Perovskite Solar Cells

Positive Effects of Guanidinium Salt Post-Treatment on Multi-Cation Mixed Halide Perovskite Solar Cells

As one of the most promising photovoltaic technologies, perovskite solar cells (PSCs) exhibit high absorption coefficients, tunable bandgaps, large carrier mobilities, and versatile fabrication techniques. Nevertheless, the commercialization of the technology is hindered by poor material stability, short device lifetimes and the scalability of fabrication techniques. To address these technological drawbacks, various strategies have been explored, with one particularly promising approach involving the formation of a low-dimensional layer on the surface of the three-dimensional perovskite film. In this work, we demonstrate the use of guanidinium tetrafluoroborate, CH6BF4N3, (GATFB) as a post-treatment step to enhance the performance of PSCs. Compared with the control sample, the application of GATFB improves the film surface topology, reduces surface defects, suppresses non-radiative recombination, and optimizes band alignment within the device. These positive effects reduce recombination losses and enhance charge transport in the device, resulting in PSCs with an open-circuit voltage (VOC) of 1.18 V and a power conversion efficiency (PCE) of 19.7%. The results obtained in this work exhibit the potential of integrating low-dimensional structures in PSCs as an effective approach to enhance the overall device performance, providing useful information for further advancement in this rapidly evolving field of photovoltaic technology.

High-Quality Solution-Processed Quasi-2D Perovskite for Low-Threshold Lasers

Spin-coated quasi-two-dimensional halide perovskite films, which exhibit superior optoelectronic properties and environmental stability, have recently been extensively studied for lasers. Crystallinity is of great importance for the laser performance. Although some parameters related to the spin-coating process have been studied, the in-depth understanding and effective control of the acceleration rate on two-dimensional perovskite crystallization during spin-coating are still unknown. Here we investigate the effect of solvent evaporation on the microstructure of the final perovskite films during the spin-coating process. The crystallization quality of the film can be significantly improved by controlling solvent evaporation. As a result, the prepared quasi-2D perovskite film exhibits a stimulated emission threshold (pump: 343 nm, 6 kHz, 290 fs) of 550 nm as low as 16.2 μJ/cm2. Transient absorption characterization shows that the radiative biexciton recombination time is reduced from 738.5 to 438.3 ps, benefiting from the improved crystallinity. The faster biexciton recombination significantly enhanced the photoluminescence efficiency, which is critical for population inversion. This work could contribute to the development of low-threshold lasers.

Seeding Agents in Metal Halide Perovskite Solar Cells: From Material to Mechanism

Metal halide perovskite solar cells (PSCs) have been showing up in the commercial field, with an inspiring power conversion efficiency (PCE) of over 26 % in the laboratory. The quality of perovskite films is still a bottleneck due to the random and fast crystallization of ionic perovskite materials. Seeding agent-mediated crystallization has consistently been recognized as an efficient method for preparing bulk single crystals and high-quality films. Herein, we summarized the seeding mechanism, characterization techniques, and seeding agents working in different locations during PSC device fabrication. This Review could further facilitate researchers with a deeper understanding of seeding agents and enhance more choices for seeding crystallization to improve the performance further and the device's large-scale fabrication toward commercialization.

Importance of Electric-Field-Independent Mobilities in Thick-Film Organic Solar Cells

In organic solar cells (OSCs), a thick active layer usually yields a higher photocurrent with broader optical absorption than a thin active layer. In fact, a ∼300 nm thick active layer is more compatible with large-area processing methods and theoretically should be a better spot for efficiency optimization. However, the bottleneck of developing high-efficiency thick-film OSCs is the loss in fill factor (FF). The origin of the FF loss is not clearly understood, and there a direct method to identify photoactive materials for high-efficiency thick-film OSCs is lacking. Here, we demonstrate that the mobility field-dependent coefficient is an important parameter directly determining the FF loss in thick-film OSCs. Simulation results based on the drift-diffusion model reveal that a mobility field-dependent coefficient smaller than 10^-3 (V/cm)^-1/2 is required to maintain a good FF in thick-film devices. To confirm our simulation results, we studied the performance of two ternary bulk heterojunction (BHJ) blends, PTQ10:N3:PC₇₁BM and PM6:N3:PC₇₁BM. We found that the PTQ10 blend film has weaker field-dependent mobilities, giving rise to a more balanced electron-hole transport at low fields. While both the PM6 blend and PTQ10 blend yield good performance in thin-film devices (∼100 nm), only the PTQ10 blend can retain a FF = 74% with an active layer thickness of up to 300 nm. Combining the benefits of a higher J_SC in thick-film devices, we achieved a PCE of 16.8% in a 300 nm thick PTQ10:N3:PC₇₁BM OSC. Such a high FF in the thick-film PTQ10 blend is also consistent with the observation of lower charge recombination from light-intensity-dependent measurements and lower energetic disorder observed in photothermal deflection spectroscopy.

A Morphological Study of Solvothermally Grown SnO2 Nanostructures for Application in Perovskite Solar Cells

Tin(IV) oxide (SnO₂) nanostructures, which possess larger surface areas for transporting electron carriers, have been used as an electron transport layer (ETL) in perovskite solar cells (PSCs). However, the reported power conversion efficiencies (PCEs) of this type of PSCs show a large variation. One of the possible reasons for this phenomenon is the low reproducibility of SnO₂ nanostructures if they are prepared by different research groups using various growth methods. This work focuses on the morphological study of SnO₂ nanostructures grown by a solvothermal method. The growth parameters including growth pressure, substrate orientation, DI water-to-ethanol ratios, types of seed layer, amount of acetic acid, and growth time have been systematically varied. The SnO₂ nanomorphology exhibits a different degree of sensitivity and trends towards each growth factor. A surface treatment is also required for solvothermally grown SnO₂ nanomaterials for improving photovoltaic performance of PSCs. The obtained results in this work provide the research community with an insight into the general trend of morphological changes in SnO₂ nanostructures influenced by different solvothermal growth parameters. This information can guide the researchers to prepare more reproducible solvothermally grown SnO₂ nanomaterials for future application in devices.

Solvent Engineering of the Precursor Solution toward Large-Area Production of Perovskite Solar Cells

Solar cells based on emerging organic-inorganic hybrid perovskite materials have reached certified power conversion efficiency as high as 25.5%, showing great potential in the next generation of photovoltaics toward large-scale industrialization. The most competitive feature of perovskite solar cells (PSCs) is that the perovskite light absorber can be fabricated by a low-cost solution method. For the solution method, the characteristics of the solvent play a key role in determining the crystallization kinetics, growth orientation, and optoelectronic properties of the perovskite film. Although significant progress has been made in the field of solvent engineering in PSCs, it is still challenging for the solution method to sustainably produce industrial-scale PSCs for future commercialization applications. Herein, the advanced progress of solvent engineering of precursor solution in terms of coordination regulation and toxicity reduction is highlighted. The physical and chemical characteristics of different solvents in reducing the toxicity of the solvent system, regulating the coordination property of the precursor solution, controlling the film-forming process of the perovskite film, and adjusting the photovoltaic performance of the PSC are systematically discussed. Lastly, important perspectives on solvent engineering of the perovskite precursor solution toward future industrial production of high-performance PSCs are provided.

Printing High-Efficiency Perovskite Solar Cells in High-Humidity Ambient Environment-An In Situ Guided Investigation

Extensive studies are conducted on perovskite solar cells (PSCs) with significant performance advances (mainly spin coating techniques), which have encouraged recent efforts on scalable coating techniques for the manufacture of PSCs. However, devices fabricated by blade coating techniques are inferior to state-of-the-art spin-coated devices because the power conversion efficiency (PCE) is highly dependent on the morphology and crystallization kinetics in the controlled environment and the delicate solvent system engineering. In this study, based on the widely studied perovskite solution system dimethylformamide-dimethyl sulfoxide, air-knife-assisted ambient fabrication of PSCs at a high relative humidity of 55 ± 5% is reported. In-depth time-resolved UV-vis spectrometry is carried out to investigate the impact of solvent removal and crystallization rate, which are critical factors influencing the crystallization kinetics and morphology because of adventitious moisture. UV-vis spectrometry enables accurate determination of the thickness of the wet precursor film. Anti-solvent-free, high-humidity ambient coatings of hysteresis-free PSCs with PCEs of 21.1% and 18.0% are demonstrated for 0.06 and 1 cm2 devices, respectively. These PSCs exhibit comparable stability to those fabricated in a glovebox, thus demonstrating their high potential.

Perovskite Quantum Wells Formation Mechanism for Stable Efficient Perovskite Photovoltaics-A Real-Time Phase-Transition Study

The combination of a bulk 3D perovskite layer and a reduced dimensional perovskite layer (perovskite quantum wells (PQWs)) is demonstrated to enhance the performance of perovskite solar cells (PSCs) significantly in terms of stability and efficiency. This perovskite hierarchy has attracted intensive research interest; however, the in-depth formation mechanism of perovskite quantum wells on top of a 3D perovskite layer is not clearly understood and is therefore the focus of this study. Along with ex situ morphology and photophysical characterization, the time-resolved grazing-incidence wide-angle X-ray scattering (TS-GIWAXS) technique performed in this study provides real-time insights on the phase-transition during the organic cation (HTAB ligand molecule) coating and PQWs/3D architecture formation process. A strikingly strong ionic reaction between the 3D perovskite and the long-chain organic cation leads to the quick formation of an ordered intermediate phase within only a few seconds. The optimal PQWs/3D architecture is achieved by controlling the HTAB casting, which is assisted by time-of-flight SIMS characterization. By controlling the second ionic reaction during the long-chain cation coating process, along with the fluorinated poly(triarylamine) (PTAA) as a hole-transport layer, the perovskite solar cells demonstrate efficiencies exceeding 22% along with drastically improved device stability.

Progress in the synthesis of imide-based N-type polymer semiconductor materials

Based on the development situation and challenge of organic photovoltaics (OPVs) and organic field-effect transistors (OFETs), it is necessary to develop N-type polymer building blocks with specific structures and performance. After decades of development, some excellent polymer receptor building blocks have been developed to construct N-type organic semiconductors, which have been applied in OFETs and OPVs. In this paper, four kinds of imide (bisthiophene imide BTI, bisthiazolimide BTz, naphthalimide NDI, and perylene imide PDI)-based N-type polymer semiconductor materials are introduced, and their applications in OFETs and OPVs are analyzed, too. The molecular structure design and the performance of corresponding materials are summarized to provide further guidance and reference for the design and development of high performance N-type polymer semiconductors.

Aged sol-gel solution-processed texture tin oxide for high-efficient perovskite solar cells

Mesoporous n-i-p perovskite solar cells (PeSCs) demonstrate attractive potential for obtaining high power conversion efficiencies (PCEs), by employing inorganic electron transport layers (ETLs). However, these ETLs composed of dual layers (a condense layer and a mesoporous layer) suffer the composite process and high sintering temperature. Here, we demonstrate a simple and efficient process to improve the device performance of PeSCs by using a textured SnO₂ film. Self-aged sol-gel SnO₂ solution after spin coating results in a textured structure without sacrificing the surface coverage. Excellent light trapping ability is achieved by optimizing the aged time of sol-gel SnO₂ solution, which mimics the evolution of the conventional mesoporous layer. Such a SnO₂ textured structure provides a large contact area for rapid charge extraction, and alleviates interfacial recombination loss. Therefore, this PeSC yields an optimal PCE of 19%, which is prominent in state-of-the-art SnO₂-based devices. These results indicate that one-step solution processed SnO₂ with a textured structure offers a simple and efficient way to improve the device performance of PeSCs without a complex process.

Rapid Microwave-Annealing Process of Hybrid Perovskites to Eliminate Miscellaneous Phase for High Performance Photovoltaics

Rapid processing technologies of perovskite solar cells (PSCs) offer an exciting approach to raise the rate of production. Herein, a rapid microwave-annealing process (MAP) is reported to replace the traditional hotplate annealing process (HAP) and the processing period of perovskite is reduced to less than 1 min. Benefiting from the penetrability and simultaneity of microwave irradiation, the MAP method can effectively eliminate miscellaneous phases and thus achieve >1 µm large-size crystal grains in perovskite films. These MAP treated perovskite films exhibit pure crystalline phase, long charge-carrier lifetime, and low defect density, which can substantially improve the PSC efficiency without requiring an additional enhancer/passivation layer. The inverted planar PSCs present enhanced power conversion efficiency from 18.33% (HAP) to 21.59% (MAP) and good stability of >1000 h lifetime without encapsulation under ambient conditions. In addition, MAP can be applied to a large-size (10 cm × 10 cm) perovskite film fabrication as well as a broader tolerance in environmental temperature and precursor concentration, making it a reliable method for repeatably practical fabrication of perovskite photovoltaics.

High-Quality MAPbBr3 Cuboid Film with Promising Optoelectronic Properties Prepared by a Hot Methylamine Precursor Approach

Though CH₃NH₃PbBr₃ single crystals are frequently applied in various optoelectronic devices due to their favorable cuboid geometry, superior optoelectronic properties, and better stability than CH₃NH₃PbI₃, CH₃NH₃PbBr₃ polycrystalline films normally show poorer morphology with scattered crystals than their iodide counterparts, inherently due to their different crystallization habits. In this work, a facile process based on a hot methylamine-based precursor with high viscosity and concentration is demonstrated to counteract rapid ion diffusion. The precursor also has special features including a large colloidal size, a solid form at room temperature, and fast crystallization offered by the easy evacuation of methylamine. CH₃NH₃PbBr₃ films composed of tightly aligned CH₃NH₃PbBr₃ cuboids on micron scale are obtained. Wide channel (100 μm) photodetectors made from the CH₃NH₃PbBr₃ films show promising photoresponse and fast response speeds on par with those based on single crystals, suggesting high film quality and good optoelectronic connections between neighboring cuboids.

Modifying Mesoporous TiO2 by Ammonium Sulfonate Boosts Performance of Perovskite Solar Cells

Mesoporous-structure perovskite solar cells (meso-PVKSCs) have been widely utilized due to the achieved high efficiency for which the TiO₂ layer usually suffers from sufficient electron trap states, low electron mobility, and inavoidable catalytic activity. Herein, a mesoporous TiO₂ (m-TiO₂) layer is modified by tetraethylammonium p-toluenesulfonate (abbreviated as TEATS) for the first time, leading to a significant photoelectric conversion efficiency enhancement from 19.14 to 20.69% for Cs_0.05MA_0.12FA_0.83PbI_2.55Br_0.45 (abbreviated as CsMAFA) meso-PVKSCs. In particular, the obtained champion open-circuit voltage (V_oc) is 1.18 V, which is a record high value for meso-PVKSCs with CsMAFA triple cation mixed perovskite. A series of measurements were employed to investigate the influences of TEATS modification on the energy band structures of TiO₂ as well as the CsMAFA perovskite layer atop, unveiling that TEATS modification benefits defect passivation of the TiO₂ film along with a decrease in the work function of TiO₂. Besides, TEATS modification helps to improve the wettability of perovskite precursors on the m-TiO₂ substrate, affording improved film quality of perovskite with enhanced crystallinity and grain size. Consequently, the trap states existed in the perovskite film can be passivated, and the interfacial charge recombination is suppressed. This further benefits the improvement of the ambient stability of devices.

PMMA Thin Film with Embedded Carbon Quantum Dots for Post-Fabrication Improvement of Light Harvesting in Perovskite Solar Cells

Perovskite solar cells (PSCs) with a standard sandwich structure suffer from optical transmission losses due to the substrate and its active layers. Developing strategies for compensating for the losses in light harvesting is of significant importance to achieving a further enhancement in device efficiencies. In this work, the down-conversion effect of carbon quantum dots (CQDs) was employed to convert the UV fraction of the incident light into visible light. For this, thin films of poly(methyl methacrylate) with embedded carbon quantum dots (CQD@PMMA) were deposited on the illumination side of PSCs. Analysis of the device performances before and after application of CQD@PMMA photoactive functional film on PSCs revealed that the devices with the coating showed an improved photocurrent and fill factor, resulting in higher device efficiency.

Coordinated Optical Matching of a Texture Interface Made from Demixing Blended Polymers for High-Performance Inverted Perovskite Solar Cells

The continuing increase of the efficiency of perovskite solar cells has pushed the internal quantum efficiency approaching 100%, which means the light-to-carrier and then the following carrier transportation and extraction are no longer limiting factors in photoelectric conversion efficiency of perovskite solar cells. However, the optimal efficiency is still far lower than the Shockley-Queisser efficiency limit, especially for those inverted perovskite solar cells, indicating that a significant fraction of light does not transmit into the active perovskite layer to be absorbed there. Here, a planar inverted perovskite solar cell (ITO/PTAA/perovskite/PC₆₁BM/bathocuproine (BCP)/Ag) is chosen as an example, and we show that its external quantum efficiency (EQE) can be significantly improved by simply texturing the poly[bis (4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) layer. By washing the film prepared from a mixed polymer solution of PTAA and polystyrene (PS), a textured PTAA/perovskite interface is introduced on the light-input side of perovskite to inhibit internal optical reflection. The reduction of optical loss by this simple texture method increases the EQE and then the photocurrent of the ITO/PTAA/perovskite/PC₆₁BM/BCP/Ag device with the magnitude of about 10%. At the same time, this textured PTAA benefits the band edge absorption in this planar solar cell. The large increase of the short-circuit current together with the increase of fill factor pushes the efficiency of this inverted perovskite solar cell from 18.3% up to an efficiency over 20.8%. By using an antireflection coating on glass to let more light into the device, the efficiency is further improved to 21.6%, further demonstrating the importance of light management in perovskite solar cells.

Engineering Halide Perovskite Crystals through Precursor Chemistry

The composition, crystallinity, morphology, and trap-state density of halide perovskite thin films critically depend on the nature of the precursor solution. A fundamental understanding of the liquid-to-solid transformation mechanism is thus essential to the fabrication of high-quality thin films of halide perovskite crystals for applications such as high-performance photovoltaics and is the topic of this Review. The roles of additives on the evolution of coordination complex species in the precursor solutions and the resulting effect on perovskite crystallization are presented. The influence of colloid characteristics, DMF/DMSO-free solutions and the degradation of precursor solutions on the formation of perovskite crystals are also discussed. Finally, the general formation mechanism of perovskite thin films from precursor solutions is summarized and some questions for further research are provided.

Hot-Air-Assisted Fully Air-Processed Barium Incorporated CsPbI2Br Perovskite Thin Films for Highly Efficient and Stable All-Inorganic Perovskite Solar Cells

Replacement of conventional organic cations by thermally stable inorganic cations in perovskite solar cells (PSCs) is one of the promising approaches to make thermally stable photovoltaics. However, conventional spin-coating and solvent-engineering processes in a controlled inert atmosphere hamper the upscaling. In this study, we demonstrated a dynamic hot-air (DHA) casting process to control the morphology and stability of all-inorganic PSCs which is processed under ambient conditions and free from conventional harmful antisolvents. Furthermore, CsPbI₂Br perovskite was doped with barium (Ba²⁺) alkaline earth metal cations (BaI₂:CsPbI₂Br). This DHA method facilitates the formation of uniform grain and controlled crystallization that makes stable all-inorganic PSCs which enables an intact black α-phase under ambient conditions. The DHA-processed BaI₂:CsPbI₂Br perovskite photovoltaics shows the champion power conversion efficiency (PCE) of 14.85% (reverse scan) for a small exposure area of 0.09 cm² and 13.78% for a large area of 1 × 1 cm² with excellent reproducibility. Interestingly, the hot-air-processed devices retain >92% of the initial efficiency after 300 h. This DHA method facilitates a wide processing window for upscaling the all-inorganic perovskite photovoltaics.