Chen Weijie

In the fabrication of inorganic γ-CsPbI₃ perovskite solar cells with dimethylamine iodide as an additive, annealing at low temperature (≈190 °C) won't completely remove the DMA, Cs_1−x DMA _x PbI₃ is thus formed, but phase-pure CsPbI₃ samples can be obtained by further annealing at high temperatures (≈340 °C). Both compositions show decent efficiencies for solar cell application, but the pure CsPbI₃ device demonstrates higher stability.

Inorganic CsPbI₃ perovskite solar cells (CsPbI₃ PSCs) have attracted extensive attention because of their excellent thermal stability and appropriate bandgap for tandem solar cells. At present, intermediate phase engineering with organic additive, e.g., dimethyl amine iodide (DMAI), is the most common method to obtain high-efficiency CsPbI₃ PSCs. However, it remains controversial whether the intermediate phase is entirely converted into a pure CsPbI₃ phase. By exploring the effect of annealing temperature, herein, it is demonstrated that substantial organic residues remain in the produced films while using the current standard recipes of DMA⁺-assisted synthesis of Cs_1−x DMA _x PbI₃ perovskites. Thermal gravimetric and nuclear magnetic resonance show that DMA⁺ remains in films annealed at a standard annealing temperature of 190 °C. The DMA⁺ does, however, disappear if the annealing temperature is increased to 340 °C, which leads to a larger grain size, a contraction of the lattice, a narrower bandgap, and a redshift of the absorption onset. Though both Cs_1−x DMA _x PbI₃ and CsPbI₃ perovskite solar cells show decent efficiencies, the pure CsPbI₃ perovskite solar cells annealed at a higher temperature demonstrate higher operational stability.

Herein, the factors affecting the stability of perovskite solar cells (PSCs) are analyzed and the encapsulations are divided into inner encapsulation and outer encapsulation based on the encapsulation materials' location and function. Then, the classification and application of encapsulation materials are systematically summarized. In addition, the encapsulation techniques, the characterization technologies, and stability tests of encapsulated PSCs are also proposed.

The poor stability hampers the commercialization of perovskite solar cells (PSCs). Many methods have recently been reported to enhance their stability, among which encapsulation is one of the most effective methods to improve the stability of PSCs. Herein, a summary of the factors influencing the stability of PSCs is provided and the commonly used encapsulation technologies and different types of encapsulation materials in detail are introduced. Then, the characterization technologies of encapsulation and stability tests of encapsulated PSCs are proposed. Finally, current issues and chances for encapsulating material development are considered.

In situ and ex situ investigations on blade-coated all-polymer bulk-heterojunctions give a deeper understanding of the morphology formation process, which demonstrates that polymer chain conformation in solution will persist in solid. The better photovoltaic performance with reduced non-radiative energy loss is the result of the relatively ordered structure. This work provides insights into the correlation between morphology formation and device performance.

All-polymer solar cells (all-PSCs) have achieved impressive progress by employing acceptors polymerized from well performing small-molecule non-fullerene acceptors. Herein, the device performance and morphology evolution in blade-coated all-PSCs based on PBDBT:PF5–Y5 blends prepared from two different solvents, chlorobenzene (CB), and ortho-xylene (o-XY) are studied. The absorption spectra in CB solution indicate more ordered conformation for PF5–Y5. The drying process of PBDBT:PF5–Y5 blends is monitored by in situ multifunctional spectroscopy and the final film morphology is characterized with ex situ techniques. Finer-mixed donor/acceptor nanostructures are obtained in CB-cast film than that in o-XY-cast ones, corresponding to more efficient charge generation in the solar cells. More importantly, the conformation of polymers in solution determines the overall film morphology and the device performance. The relatively more ordered structure in CB-cast films is beneficial for charge transport and reduced non-radiative energy loss. Therefore, to achieve high-performance all-PSCs with small energy loss, it is crucial to gain favorable aggregation in the initial stage in solution.

Two thiophene derivatives, namely 3-cthlorothiophene and 3-thiophene ethylenediamine, are selected for the post-treating the printable mesoscopic solar cells. The different properties of the thiophene derivatives allow them to stay at different depths of the device and play different roles. Ultimately, the defect-assisted recombination is suppressed and the interfacial energy band is regulated. The resulting devices exhibit improved performance.

Abstract

The printable mesoscopic perovskite solar cells consisting of a double layer of metal oxides covered by a porous carbon film have attracted attention due to their industrialization advantages. However, the tens-of-micrometer thickness of the triple scaffold leads to a challenge for perovskite to crystallize and for the charge carriers to separate and travel to the electrode, which limits the open circuit voltage (V _OC) of such devices. In this work, a depth-dependent post-treatment strategy is demonstrated to synergistically passivate defects and tune interfacial energy band alignment. Two thiophene derivatives, namely 3-chlorothiophene (3-CT) and 3-thiophene ethylenediamine (3-TEA), are selected for the post-treatment. Energy-dispersive X-ray spectroscopy proves that 3-CT is uniformly distributed throughout the triple scaffold and effectively passivates the defects of the bulky perovskite, while 3-TEA reacts rapidly with the loose perovskite in the carbon layer to form 2D perovskite, forming a type II energy band alignment at the perovskite/carbon interface. As a result, the defect-assisted recombination is suppressed and the interfacial energy band is regulated, increasing the V _OC to 1012 mV. The PCE of the devices is enhanced from 16.26% to 18.49%. This depth-dependent post-treatment strategy takes advantage of the unique structure and provides a new insight for reducing the voltage loss.

A rigorous opto–electro–thermal simulation is performed to address the microscopic energy conversion processes and disclose the heat generation, dissipation, and manipulation mechanisms behind a photovoltaic device. Various thermal manipulation strategies, including external (cooling effect) and internal (transport layer materials, photoluminescence colorants, and tandem strategy) methods, are proposed to regulate heat generation, accelrate heat dissipation, and reduce device temperature.

Abstract

Perovskite-based single-junction and tandem solar cells have recently attracted considerable attention due to their remarkable advantages in power conversion efficiency (PCE) and fabrication cost; however, their commercialization remains challenging. One crucial limiting factor is the incompetent thermal management, which is inclined to degrade the PCE and stability of the device. Here, a rigorous opto–electro–thermal (OET) simulation is performed to disclose the internal energy conversion and heat mechanisms within devices. Taking a low-bandgap PSC as an example, the microscopic energy conversion processes concerning the contributions from thermalization, Joule, Peltier, and bulk/interface recombination heats are quantitatively identified. Then various thermal manipulation strategies are proposed, including external (cooling effect) and internal (transport layer materials, photoluminescence colorants, and tandem strategy) methods with the purposes of reducing the heat generation and device temperature. Through the joint OET optimization, the predicted temperature of the considered single-junction (tandem) PSC is reduced to 44.3 °C (33.5 °C) with the possible PCE up to 22.35% (29.08%). Based on the simulation, a tandem PSC (under two-terminal configuration) is fabricated and a PCE of 25.03% is realized. This study offers an effective approach for energy analysis and manipulation to realize higher-performance PSCs with lower operation temperatures.

This paper takes the emergence of ADA'DA-type NFAs as a clue to review the development of organic photovoltaic materials, interface materials, and device fabrication technology in recent years. Finally, it is believed that the commercialization of organic photovoltaics still needs to develop novel materials to meet the demand for highly stable, pollution-free, large-area, and low-cost fabrication.

Abstract

With the emergence of ADA'DA-type (Y-series) non-fullerene acceptors (NFAs), the power conversion efficiencies (PCEs) of organic photovoltaic devices have been constantly refreshed and gradually reached 20% in recent years (19% for single junction and 20% for tandem device). The acceptors possess specific design concept, which greatly enrich the NFA types and have excellent compatibility with many donor materials. It is gratifying to note that the previously underperforming donor materials combine with these regulated acceptors to shine again. Nowadays, the concept of modular design is widely used in the research of acceptors and donors, injecting new vitality into the field of organic photovoltaics. Furthermore, these acceptors also promote the research of multicomponent devices, tandem devices, bilayer devices, processing solvent engineering, and additive engineering. Herein, the latest progresses of polymer solar cells with efficiency over 17% are briefly reviewed from the aspects of active material design, interface material development, and device technology. At last, the opportunities and challenges of organic photovoltaic commercialization in the future are discussed.

The spatial and temporal evolution in the recombination of back-contact perovskite solar cells is studied using intensity-dependent photoluminescence. Maps of the photoluminescence intensity and ideality factor resemble the periodic structure of the back-contact electrodes and exhibit interesting transient behavior. Adding a mesoporous TiO₂ layer drastically reduces recombination and lowers the ideality factor, improving the open-circuit voltage (V _OC) by 120 mV.

Abstract

The efficiency of back-contact perovskite solar cells has steadily increased over the past few years and now exceeds 11%, with interest in these devices shifting from proof-of-concept to viable technology. In order to make further improvements in the efficiency of these devices it is necessary to understand the cause of the low fill factor, low open-circuit voltage (V _OC), and severe hysteresis. Here a time-dependent Suns-V _oc and Suns-photoluminescence (PL) analysis are performed to monitor the transient ideality factor spatially. Two sets of quasi-interdigitated back-contact perovskite solar cells are studied; cells with and without a mesoporous TiO₂ layer. Maps of the PL intensity and ideality factor resemble the periodic structure of the back-contact electrodes and the transient behavior exhibit distinct features such as a temporary variation in the periodicity of the modulation, spatial phase shifting, and phase offsets. It is shown that the presence of the mesoporous layer greatly reduces recombination, increasing the V _OC by 0.12 V. Coupled 2D time-dependent drift-diffusion simulations allow the experimental results to be modeled, and replicate the key features observed experimentally. They reveal that non-uniform ion distribution along the transport layer interfaces can drastically alter the PL intensity and ideality factor throughout the device.

Vacuum evaporation of lead iodide and solution processing of organic ammonium halide are combined to produce large-area homogeneous perovskite films with high reproducibility. The resulting PSCs achieve a power conversion efficiency (PCE) of 24.0% (certified PCE 23.7%) on large area (1 cm²) under AM 1.5G illumination, which is currently the highest PCE for large area perovskite photovoltaics.

Abstract

Organic–inorganic hybrid perovskites exhibit outstanding performances in perovskite solar cells (PSCs). However, the complex solution chemistry of perovskites precursors renders it difficult to prepare large-area devices in a reproducible way, which is a prerequisite for the technology to make an impact beyond lab scale. Vacuum processing, instead, is an established technology for large-scale coating of thin films. However, with respect to the hybrid perovskites it is highly challenging due to the high vapor pressure of the organic ammonium halide. In this work, vacuum evaporation of lead iodide and solution processing of organic ammonium halide is combined to produce large-area homogeneous perovskite films with large grains in a highly reproducible way. The resulting PSCs achieve a power conversion efficiency (PCE) of 24.3% (certified 23.9%) on small area (0.10 cm²), 24.0% (certified 23.7%) on large area (1 cm²) and 20.0% for minimodule (16 cm²), and maintain 90% of its initial efficiency after 1000 h 1-sun operation. The vacuum evaporation prevents advert environmental effects on lead halide formation and guarantees a reproducible fabrication of high-quality large-area perovskite films, which opens a promising way for large-scale fabrication of perovskite optoelectronics.

Two analogous asymmetric polymerized small-molecule acceptors of PYFCl-T and PYF&PYCl-T are synthesized based on small-molecule acceptors with different halogenated end groups to explore their effects on the photovoltaic performance of all-polymer solar cells (all-PSCs). The ternary all-PSC based on PM6:PY-IT:PYFCl-T reaches a high power conversion efficiency of 18.12%, benefitting from the miscibility-driven optimization of active layer morphology by asymmetric PYFCl-T.

Abstract

High-performance all-polymer solar cells (all-PSCs) deeply rely on the joint contributions of desirable optical absorption, adaptive energy levels, and appropriate morphology. Herein, two structural analogous polymerized small-molecule acceptors (PSMAs), PYFCl-T and PYF&PYCl-T, are synthesized, and then incorporated into the PM6:PY-IT binary blends to construct ternary all-PSCs. Due to the superior compatibility of PY-IT and PYFCl-T, the ternary all-PSC based on PM6:PY-IT:PYFCl-T with 10 wt% PYFCl-T, presents higher and more balanced charge mobility, suppressed charge recombination, and faster charge-transfer kinetics, resulting in an outstanding power conversion efficiency (PCE) of 18.12% with enhanced J _sc and FF, which is much higher than that (PCE of 16.09%) of the binary all-PSCs based on PM6:PY-IT. Besides, the ternary all-PSCs also exhibit improved photostability. The conspicuous performance enhancement principally should give the credit to the miscibility-driven phase optimization of the donor and acceptor. These findings highlight the significance of polymer-backbone configuration modulation of PSMAs in morphology optimization toward boosting the device properties of all-PSCs.

Grain-boundary grooves are found to have important effects on the properties of perovskite heterointerfaces and thus the device performance of perovskite solar cells (PSCs). Especially, flattening grain-boundary grooves elevates the (opto-)mechanical reliability, contributing to device efficiency and stability enhancement.

Abstract

Optomechanical reliability has emerged as an important criterion for evaluating the performance and commercialization potential of perovskite solar cells (PSCs) due to the mechanical-property mismatch of metal halide perovskites with other device layer. In this work, grain-boundary grooves, a rarely discussed film microstructural characteristic, are found to impart significant effects on the optomechanical reliability of perovskite–substrate heterointerfaces and thus PSC performance. By pre-burying iso-butylammonium chloride additive in the electron-transport layer (ETL), GB grooves (GBGs) are flattened and an optomechanically reliable perovskite heterointerface that resists photothermal fatigue is created. The improved mechanical integrity of the ETL–perovskite heterointerfaces also benefits the charge transport and chemical stability by facilitating carrier injection and reducing moisture or solvent trapping, respectively. Accordingly, high-performance PSCs which exhibit efficiency retentions of 94.8% under 440 h damp heat test (85% RH and 85 °C), and 93.0% under 2000 h continuous light soaking are achieved.

Halogenated propeller-shaped ammonium is synthesized and applied to reduce the surface energy and prompt the formation of the α-phase Cs _x FA_1−x PbI₃ perovskite structure, which generates a record efficiency of 23.6% for Cs _x FA_1−x PbI₃-based inverted structural perovskite solar cells with excellent stability.

Abstract

Methylammonium (MA)-free formamidinium (FA)-dominated Cs _x FA_1−x PbI₃ is rising as the most promising candidate for highly efficient and stable perovskite solar cells. However, the growth of high-quality Cs _x FA_1−x PbI₃ black-phase perovskite structure without ion doping in the lattice remains a challenge. Herein, propeller-shaped halogenated tertiary ammonium is synthesized, showing high binding energy on the perovskite surface and large steric hindrance. This molecule can significantly reduce the barrier of high surface energy that suppresses the growth of the α-phase Cs _x FA_1−x PbI₃ structure. As a result, the α-phase structure can be formed at room temperature, which can further act as a seed for the growth of high-quality film. Solar cells based on the film show a record efficiency up to 23.6% for MA free Cs _x FA_{1− x} PbI₃ solar cells with inverted structure and excellent stability at 85 °C over 200 h.

Herein, the novel asymmetric guest acceptor was rationally designed by combining the structural fusion and unidirectional extension strategies. The optimized ternary device with the incorporation of this guest acceptor achieved considerably improved power conversion efficiency (PCE) of 18.77 % with a boosted open-circuit voltage (V _oc) of 0.896 V.

Abstract

The design and selection of a suitable guest acceptor are particularly important for improving the photovoltaic performance of ternary organic solar cells (OSCs). Herein, we designed and successfully synthesized two asymmetric silicon–oxygen bridged guest acceptors, which featured distinct blue-shifted absorption, upshifted lowest unoccupied molecular orbital energy levels, and larger dipole moments than symmetric silicon–oxygen-bridged acceptor. Ternary devices with the incorporation of 14.2 wt % these two asymmetric guest acceptors exhibited excellent performance with power conversion efficiencies (PCEs) of 18.22 % and 18.77 %, respectively. Our success in precise control of material properties via structural fusion of five-membered carbon linkages and six-membered silicon–oxygen connection at the central electron-donating core unit of fused-ring electron acceptors can attract considerable attention and bring new vigor and vitality for developing new materials toward more efficient OSCs.

Nature Communications, Published online: 25 January 2023; doi:10.1038/s41467-023-36118-7

Quasi-2D halide perovskites are attracting increasing attention for light-emitting devices. Here, the authors demonstrated efficient and stable quasi-2D perovskite LEDs enabled by suppressed phase disproportionation with newly designed organic ligands.

Nature Materials, Published online: 26 January 2023; doi:10.1038/s41563-022-01448-2

The study of the inherent charge transport behaviour of 3D lead halide perovskite is challenging, owing to entanglement with ionic migration effects and dipolar disorder instabilities. Here, the authors circumvented both challenges and found that ion migration is much suppressed in mixed metal perovskite compositions relative to pure-Pb counterparts.

Nature Energy, Published online: 26 January 2023; doi:10.1038/s41560-022-01189-1

Perovskite photovoltaics are promising for space applications, but their reliability needs to be addressed. Now, Kirmani et al. present a 1-μm-thick silicon oxide that affords protection against protons, alpha particles and atomic oxygen.

Nature Energy, Published online: 26 January 2023; doi:10.1038/s41560-022-01191-7

Efficient mechanical energy harvesting approaches are needed. Here, Zhang et al. develop a plied carbon nanotube yarn that harvests mechanical energy upon stretching and lateral deformations, achieving 17.4 and 22.4% efficiencies for tensile and torsional harvesting, respectively.