Chen Weijie

Chemical complexity in mixed cesium-formamidinium metal halide perovskites (MHP) is mainly determined by the Cs concentration. Excessive Cs incorporation leaves substantial local phase inhomogeneities that are not completely converted to the photoactive α-phase by thermal annealing, whereas moderate Cs incorporation results in clean functional MHP.

Abstract

Mixed cesium- and formamidinium-based metal halide perovskites (MHPs) are emerging as ideal photovoltaic materials due to their promising performance and improved stability. While theoretical predictions suggest that a larger composition ratio of Cs (≈30%) aids the formation of a pure photoactive α-phase, high photovoltaic performances can only be realized in MHPs with moderate Cs ratios. In fact, elemental mixing in a solution can result in chemical complexities with non-equilibrium phases, causing chemical inhomogeneities localized in the MHPs that are not traceable with global device-level measurements. Thus, the chemical origin of the complexities and understanding of their effect on stability and functionality remain elusive. Herein, through spatially resolved analyses, the fate of local chemical structures, particularly the evolution pathway of non-equilibrium phases and the resulting local inhomogeneities in MHPs is comprehensively explored. It is shown that Cs-rich MHPs have substantial local inhomogeneities at the initial crystallization step, which do not fully convert to the α-phase and thereby compromise the optoelectronic performance of the materials. These fundamental observations allow the authors to draw a complete chemical landscape of MHPs including nanoscale chemical mechanisms, providing indispensable insights into the realization of a functional materials platform.

Intermolecular interactions are ubiquitous and elusive in organic solar cells. Herein, the complicated relationship between multidimensional intermolecular interactions in active layers and photovoltaic performance is disclosed. The results show that complex interactions are critical to synergistically regulating comprehensive properties, including molecular assembly, stacking orientations, charge transport, and morphology management, which helps to achieve an efficiency over 19% in ternary devices.

Abstract

Research on organic solar cells (OSCs) has progressed through material innovation and device engineering. However, well-known and ubiquitous intermolecular interactions, and particularly their synergistic effects, have received little attention. Herein, the complicated relationship between photovoltaic conversion and multidimensional intermolecular interactions in the active layers is investigated. These interactions are dually regulated by side-chain isomerization and end-cap engineering of the acceptors. The phenylalkyl featured acceptors (LA-series) exhibit stronger crystallinity with preferential face-on interactions relative to the alkylphenyl attached isomers (ITIC-series). In addition, the PM6 and LA-series acceptors exhibit moderate donor/acceptor interactions compared to those of the strongly interacting PM6/ITIC-series pairs, which helps to enhance phase separation and charge transport. Consequently, the output efficiencies of all LA series acceptors are over 14%. Moreover, LA-series acceptors show appropriate compatibility, host/guest interactions, and crystallinity relationships with BTP-eC9, thereby leading to uniform and well-organized “alloy-like” mixed phases. In particular, the highly crystalline LA23 further optimizes multiple interactions and ternary microstructures, which results in a high efficiency of 19.12%. Thus, these results highlight the importance of multidimensional intermolecular interactions in the photovoltaic performance of OSCs.

For perovskites, it is challenging to quantitatively study the concentration of defects. A theoretical relationship between defect concentration, conductivity, and oxygen partial pressure can be established based on the defect chemistry equilibria. The type and concentration of defects are reflected through the conductivity variation with oxygen partial pressure.

Abstract

Recent advances in perovskite ferroelectrics have fostered a host of exciting sensors and actuators. Defect engineering provides critical control of the performance of ferroelectric materials, especially lead-free ones. However, it remains a challenge to quantitatively study the concentration of defects due to the complexity of measurement techniques. Here, a feasible approach to analyzing the A-site defect and electron in alkali metal niobate is demonstrated. The theoretical relationships among defect concentration, conductivity, and oxygen partial pressure can be established based on the defect chemistry equilibria. The type and concentration of defects are reflected through the conductivity variation with oxygen partial pressure. As a result, the variation of defect concentration gives rise to defect-driven interfacial polarization, which further leads to distinct properties of the ceramics. e.g., abnormal dielectric behavior. Furthermore, this study also suggests a strategy to manipulate defects and charges in perovskite oxides for performance optimization.

This review summarizes the structural modification strategies of organic small-molecule semiconductors with high electron mobilities, a promising candidate for the construction of next-generation complementary organic logic-digital circuits, to achieve chemical stability and high electron transport properties. In addition, the applications of n-type small-molecule semiconductor materials based on high mobility in organic electronic devices, such as organic field-effect transistors, organic light-emitting transistors, organic photodetectors, and gas sensors, are introduced.

Abstract

Organic electronics has made great progress in the past decades, which is inseparable from the innovative development of organic electronic devices and the diversity of organic semiconductor materials. It is worth mentioning that both of these great advances are inextricably linked to the development of organic high-performance semiconductor materials, especially the representative n-type organic small-molecule semiconductor materials with high electron mobilities. The n-type organic small molecules have the advantages of simple synthesis process, strong intermolecular stacking, tunable molecular structure, and easy to functionalize structures. Furthermore, the n-type semiconductor is a remarkable and important component for constructing complementary logic circuits and p-n heterojunction structures. Therefore, n-type organic semiconductors play an extremely important role in the field of organic electronic materials and are the basis for the industrialization of organic electronic functional devices. This review focuses on the modification strategies of organic small molecules with high electron mobility at molecular level, and discusses in detail the applications of n-type small-molecule semiconductor materials with high mobility in organic field-effect transistors, organic light-emitting transistors, organic photodetectors, and gas sensors.

A simple small molecule 10-(4-(3,6-dimethoxy-9H-carbazol-9-yl)phenyl)-3,7-bis(4-vinylphenyl)-10H-phenoxazine is developed for in situ fabrication of polymer hole conductor (CL-MCz) via a facile and low-temperature cross-linking technology. The device with CL-MCz yields a champion power conversion efficiency of 23.9% along with an extremely low energy loss down to 0.41 eV.

Abstract

Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) represents the state-of-the-art hole transport material (HTM) in inverted perovskite solar cells (PSCs). However, unsatisfied surface properties of PTAA and high energy disorder in the bulk film hinder the further enhancement of device performance. Herein, a simple small molecule 10-(4-(3,6-dimethoxy-9H-carbazol-9-yl)phenyl)-3,7-bis(4-vinylphenyl)-10H-phenoxazine (MCz-VPOZ) is strategically developed for in situ fabrication of polymer hole conductor (CL-MCz) via a facile and low-temperature cross-linking technology. The resulting polymer CL-MCz offers high energy ordering and improved electrical conductivity, as well as appropriate energy-level alignment, enabling efficient charge carrier collection in the devices. Meanwhile, CL-MCz synchronously provides satisfied surface wettability and interfacial functionalization, facilitating the formation of high-quality perovskite films with fewer bulk iodine vacancies and suppressed carrier recombination. Significantly, the device with CL-MCz yields a champion efficiency of 23.9% along with an extremely low energy loss down to 0.41 eV, which represents the highest reported efficiency for non-PTAA-based polymer HTMs in inverted PSCs. Furthermore, the corresponding unencapsulated devices exhibit competitive shelf-life stability under various operational stressors up to 2500 h, reflecting high promises of CL-MCz in the scalable PSC application. This work underscores the promising potential of the cross-linking approach in preparing low-cost, stable, and efficient polymer HTMs toward reliable PSCs.

An efficient passivator 3,4,5,6-tetrafluorophthalic acid (TFPA) is designed and applied to enhance the efficiency and stability of perovskite solar cells (PSCs). The efficiency of TFPA-modified PSCs improves to 23.70% from 21.46%. Additionally, the device without encapsulation maintains 90% of its initial efficiency after storing for 5200 h in ambient condition, whereas the control device declined beyond 30%.

Perovskite solar cells have become stars in photovoltaics due to their rapidly increased efficiency. However, their stability is still below par due to moisture permeation from grain boundaries and defects. To conquer both problems at once, a passivation agent 3,4,5,6-tetrafluorophthalicacid (TFPA) is rationally designed to heal both for not only improved cell efficiency but also better stability. It is found that the TFPA is prone to distribute along grain boundaries and has little influence within the bulk of the perovskite film. In addition, it appears that the TFPA helps to reduce the film roughness, to adjust the energy level, to facilitate hole transporting from perovskite to spiro-OMeTAD, and to increase the hydrophobicity of the perovskite film, as it is demonstrated by the inhibited nonradiative recombination and prolonged carrier lifetime. Owing to strong interactions between F, -COOH, and Pb, the device with TFPA shows outstanding efficiency and stability. A perovskite solar cell with TFPA modification delivers a champion efficiency of 23.70% and a significantly enhanced stability that the device maintains 90% of its initial efficiency after 5200 h, among the best ambient stability. Herein, an effective strategy of grain boundary passivation is provided to improve the stability of perovskite solar cells.

Publication date: March 2023

Source: Journal of Energy Chemistry, Volume 78

Author(s): Nikolai A. Belich, Andrey A. Petrov, Pavel A. Ivlev, Natalia N. Udalova, Alla A. Pustovalova, Eugene A. Goodilin, Alexey B. Tarasov

The implementation of one-step electrospun polymer encapsulated chiral perovskites strategy opens vast possibilities to access high-quality low-dimensional chiral perovskite nanomaterials with superior blue-emissive circularly polarized luminescence and stability, which can be readily extended to a rich diversity of stable chiral perovskite with tailorable dimensions, compositions, architecture, surface chemistry, and chirality for advanced chiroptical devices.

Abstract

Chiral perovskite materials have intrigued enormous interests because of their appealing chiroptical properties and tailorable non-centrosymmetric structures. However, it remains challenging to realize high-efficiency blue emissive circularly polarized luminescence (CPL) of intrinsic chiral perovskite nanomaterials at room temperature. Herein, a robust and versatile electrospinning strategy is reported for in situ construction of chiral 2D and quasi-2D perovskite nanosheets (PNSs) protected in polymer hybrid nanofibers. It is found that quasi-2D chiral PNS/polymer possesses inherent chirality and enhanced CPL properties at room temperature compared to 2D counterparts. Notably, CPL emission color of chiral quasi-2D PNS/polymer can be tuned from deep blue to sky blue, and a high luminescence dissymmetry values up to −8.0 × 10⁻³ can be achieved. Different perovskites, polymers, and nanofibrous structures are expanded to explore the universality of polymer protected PNSs. Significantly, compared to spin-coated film, the stabilities of quasi-2D PNS/polymer film are greatly improved due to the effective protection of polymer. The obtained PNS/polymer hybrid nanofiber films can be conveniently implemented for circularly polarized light emitting diode devices. This study may open up a new avenue for the scalable fabrication of chiral perovskite nanomaterials of interest and their applications in the CPL related fields.

Publication date: 21 December 2022

Source: Joule, Volume 6, Issue 12

Author(s): Felix Utama Kosasih, Enkhtur Erdenebileg, Nripan Mathews, Subodh G. Mhaisalkar, Annalisa Bruno

A volatile solid additive 2,5-diiodothiophene (DIT) introduced to fine tune the morphology of PM6:Y6 blends shows a power conversion efficiency (PCE) of 17.48%, much higher than 1,4-diiodobenzene (DIB) devices. Notably, the thiophene-based additives evaporate completely without post-treatment; however, DIB retains in blends. The additive strategy post-treatment free provides simple processability for the fabrication of organic solar cells.

Active layer morphology is critical to determine the photovoltaic performance of organic solar cells (OSCs), and additive-assisted morphology optimization is one of the most widely used strategies to fine tune the donor–acceptor network in the active layer. However, the postprocessing procedures and additive residues require additional efforts to maintain the device performance. Herein, a low-cost, simple structural design and volatile solid additive 2,5-diiodothiophene is demonstrated to effectively optimize the active layer morphology and prompt the photoelectric conversion efficiency (PCE) of OSCs without any post-treatment, outperforming the devices fabricated with the prototypical volatile additive 1,4-diiodobenzene. The 2,5-diiodothiophene (DIT)-optimized PM6:Y6 binary OSCs obtain a PCE of 17.48%, accompanied with reduced trap-assisted charge recombination and balanced charge carrier mobility. In addition, we notice that the bromide and chlorine analogues of DIT can enhance the device performance to a certain extent as well. The findings suggest that the post-treatment-free nature of the thiophene-based processing additives are promising candidates for tuning the active layer morphology, providing simple processability for large-scale fabrication of OSCs.

The multifunctional additive 4-amino-2,3,5,6-tetrafluorobenzoic acid (ATFBA), with terminal amino and carboxyl groups, passivates the surface defects and enhances the stability of the Sn–Pb perovskite, therefore resulting in high sensitivity and highly stable Sn–Pb perovskite photodetectors. In addition, the ATFBA-based photodetector has been integrated with a pulse oximetry visualization system, successfully yielding accurate blood oxygen saturation and heart rate.

Abstract

Low-bandgap tin (Sn)-lead (Pb) halide perovskites can achieve near-infrared response for photodetectors. However, the Sn-based devices suffer from notorious instability and high defect densities due to the oxidation propensity of Sn²⁺. Herein, a multifunctional additive 4-amino-2,3,5,6-tetrafluorobenzoic acid (ATFBA) is presented, which can passivate surface defects and inhibit the oxidation of Sn²⁺ through hydrogen bonds and chelation coordination from the terminal amino and carboxyl groups. The perfluorinated benzene ring structure of ATFBA affords the passivator assembled at the grain boundaries to enhance the water resistance. With the synergistical passivation of these functional groups, the Sn–Pb perovskite photodetector exhibits a remarkable responsivity of 0.52 A W^-1 and an excellent specific detectivity of 5.34 × 10¹² Jones at 850 nm, along with remaining 97% of its initial responsivity over 450 days. Benefitting from high sensitivity, the photodetector is integrated into a pulse oximetry sensor visualization system, yielding accurate blood oxygen saturation and heart rate with less than 2% error. This work paves the avenue toward constructing high-performance and stable Sn–Pb perovskite photodetectors for practical applications.

Nature Communications, Published online: 13 December 2022; doi:10.1038/s41467-022-35441-9

Poly(vinylidene fluoride) (PVDF) as the polymer template used in perovksite solar cells enables slow crystal growth and efficient defect passivation, which effectively reduce non-radiation recombination and minimize E _LOSS of V _OC. PVDF-based PSCs achieve a champion efficiency of 24.21% with an excellent voltage of 1.22 V, which is the highest V _OC values reported for FAMAPb(I/Br)₃-based PSCs.

Abstract

Recently, organic–inorganic metal halide perovskite solar cells (PSCs) have achieved rapid improvement, however, the efficiencies are still behind the Shockley–Queisser theory mainly due to their high energy loss (E _LOSS) in open-circuit voltage (V _OC). Due to the polycrystalline nature of the solution-prepared perovskite films, defects at the grain boundaries as the non-radiative recombination centers greatly affect the V _OC and limit the device efficiency. Herein, poly(vinylidene fluoride) (PVDF) is introduced as polymer-templates in the perovskite film, where the fluorine atoms in the PVDF network can form strong hydrogen-bonds with organic cations and coordinate bonds with Pb²⁺. The strong interaction between PVDF and perovksite enables slow crystal growth and efficient defect passivation, which effectively reduce non-radiation recombination and minimize E _LOSS of V _OC. PVDF-based PSCs achieve a champion efficiency of 24.21% with a excellent voltage of 1.22 V, which is one of the highest V _OC values reported for FAMAPb(I/Br)₃-based PSCs. Furthermore, the strong hydrophobic fluorine atoms in PVDF endow the device with excellent humidity stability, the unencapsulated solar cell maintain the initial efficiency of >90% for 2500 h under air ambient of ≈50% humid and a consistently high V _OC of 1.20 V.

In situ absorption measurement is used to investigate the aggregation behavior of acceptors during slot-die-coating. The 1 cm² flexible device can reach a power conversion efficiency of 13.70%, with excellent shelf stability and upscaling ability. The connected modules (180 cm²) can effectively power a smartphone, showing great potential for future applications.

Abstract

Slot-die coating is recognized as the most compatible method for the roll-to-roll (R2R) processing of large-area flexible organic solar cells (OSCs). However, the photovoltaic performance of large-area flexible OSC lags significantly behind that of traditional spin-coating devices. In this work, two acceptors, Qx-1 and Qx-2, show quite different film-formation kinetics in the slot-die coating process. In situ absorption spectroscopy indicates that the excessive crystallinity of Qx-2 provides early phase separation and early aggregation, resulting in oversized crystal domains. Consequently, the PM6:Qx-1-based 1 cm² flexible device exhibits an excellent power conversion efficiency (PCE) of 13.70%, which is the best performance among the slot-die-coated flexible devices; in contrast, the PM6:Qx-2 blend shows a pretty poor efficiency, which is lower than 1%. Moreover, the 30 cm² modules based on PM6:Qx-1, containing six 5 cm² sub-cells, exhibit a PCE of 12.20%. After being stored in a glove box for over 6000 h, the PCE remains at 103% of its initial values, indicating excellent shelf stability. Therefore, these results show a promising future strategy for the upscaling fabrication of flexible large-area OSCs.

The triple-cation A₃Sb₂X₉-based perovskite-inspired material (PIM), with cesium, methylammonium and formamidinium occupying its A-site, possesses a suitable band gap for indoor photovoltaics (IPVs). Reduced trap-assisted recombination and high external quantum efficiency of triple-cation Sb-based PIM IPVs ensure an indoor power conversion efficiency of 6.4%, which is the highest among pnictohalide based IPVs.

Abstract

Antimony-based perovskite-inspired materials (PIMs) are solution-processable halide absorbers with interesting optoelectronic properties, low toxicity, and good intrinsic stability. Their bandgaps around 2 eV make them particularly suited for indoor photovoltaics (IPVs). Yet, so far only the fully inorganic Cs₃Sb₂Cl _x I_{9− x} composition has been employed as a light-harvesting layer in IPVs. Herein, the first triple-cation Sb-based PIM (CsMAFA-Sb) in which the A-site of the A₃Sb₂X₉ structure consists of inorganic cesium alloyed with organic methylammonium (MA) and formamidinium (FA) cations is introduced. Simultaneously, the X-site is tuned to guarantee a 2D structure while keeping the bandgap nearly unchanged. The presence of three A-site cations is essential to reduce the trap-assisted recombination pathways and achieve high performance in both outdoor and indoor photovoltaics. The external quantum efficiency peak of 77% and the indoor power conversion efficiency of 6.4% are the highest values ever reported for pnictohalide-based photovoltaics. Upon doping of the P3HT hole-transport layer with F4-TCNQ, the power conversion efficiency of CsMAFA-Sb devices is fully retained compared to the initial value after nearly 150 days of storage in dry air. This work provides an effective compositional strategy to inspire new perspectives in the PIM design for IPVs with competitive performance and air stability.

The impact of post-annealing temperature on the defect properties and photovoltaic performance of hydrothermally grown antimony sulfoselenide solar cells is studied in this work. Post-annealing is beneficial in increasing the grain size, crystallinity, crystal orientation, and compactness of the films and suppressing defects. Furthermore, a power conversion efficiency of 8.48% is obtained after optimizing the annealing process.

Herein, antimony sulfoselenide (Sb₂(S, Se)₃) thin-film solar cells are fabricated by a hydrothermal method followed by a post-deposition annealing process at different temperatures and the impact of the annealing temperature on the morphological, structural, optoelectronic, and defect properties of the hydrothermally grown Sb₂(S, Se)₃ films is investigated. It is found that a proper annealing temperature leads to high-quality Sb₂(S, Se)₃ films with large crystal grains, high crystallinity, preferred crystal orientation, smooth and uniform morphology, and reduced defect density. These results show that suppressing deep-level defects is crucial to enhance solar cell performance. After optimizing the annealing process, Sb₂(S, Se)₃ solar cells with an improved power conversion efficiency 2.04 to 8.48% are obtained.