Ligang Yuan

Tri-cation and dual-anion mixed perovskites have been widely utilized in perovskite solar cell (PSC) applications due to their novel properties such as high absorption, high stability, and low cost. To commercialize the PSCs, further improving the device performance without detrimentally changing the device configuration is important at present. Herein, Au@SiO₂ nanoparticles (NPs) are introduced to modify the interface between mesoporous TiO₂ (mp-TiO₂) and mixed perovskite with increased main photovoltaic parameters of the device, resulting in a ≈29% enhancement of power conversion efficiency (PCE) from 15.8% to 20.3%. The origins of the enhancement have been studied by exploring the optical absorption, optical power distribution, and charge carrier behaviors within the system. The small perturbation transient photovoltage measurement exhibits prolonged charge carrier lifetimes after the Au@SiO₂ NPs incorporation, and time of flight photoconductivity measurement shows that charge carrier mobilities of this system are also enhanced. These characteristics make metallic nanostructures a promising functional material in facile tuning of the charge carriers transport and further boosting the PCE of the PSCs.

Au@SiO₂ core−shell nanoparticles are incorporated into perovskite solar cells based on tri-cation (cesium, methylammonium, and formamidinium) and dual-anion (Br and I) mixed perovskite. Significant enhancements are observed in nearly all photovoltaic parameters after Au@SiO₂ nanoparticles incorporation. The enhanced device performance can be ascribed to the light absorption enhancement, more efficient carrier injection/transport, and the enhanced charge carriers mobilities.

Recent synthetic developments have generated intense interest in the use of cesium lead halide perovskite nanocrystals for light-emitting applications. This work presents the photoluminescence (PL) of cesium lead halide perovskite nanocrystals with tunable halide composition recorded as function of temperature from 80 to 550 K. CsPbBr₃ nanocrystals show the highest resilience to temperature while chloride-containing samples show relatively poorer preservation of photoluminescence at elevated temperatures. Thermal cycling experiments show that PL loss of CsPbBr₃ is largely reversible at temperatures below 450 K, but shows irreversible degradation at higher temperatures. Time-resolved measurements of CsPbX₃ samples show an increase in the PL lifetime with temperature elevation, consistent with exciton fission to form free carriers, followed by a decrease in the apparent PL lifetime due to trapping. PL persistence measurements and time-resolved spectroscopies implicate thermally assisted trapping, most likely to halogen vacancy traps, as the mechanism of reversible PL loss.

High-temperature photoluminescence properties of CsPbX₃ nanocrystals are important to the viability of this family of emitters in lighting and display applications. The persistence of photoluminescence at temperatures above 400 K depends strongly on the halide composition, and photoluminescence loss occurs through thermally assisted trapping to halogen vacancies.

A new acceptor–donor–acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V _OC of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.

A new acceptor–donor–acceptor-structured nonfullerene acceptor, ITCC, is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V _OC of over 1 V is recorded in photovoltaic devices, suggesting great potential for applications in tandem organic solar cells.

The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.

The development of wide-bandgap (WBG) polymers is of significant importance in boosting the power conversion efficiency of organic solar cells. Recent advances in WBG polymers and their applications in different types of organic photovoltaic devices are reviewed. In addition, empirical regularities and invigorating perspectives are provided to help guide future designs of WBG photovoltaic materials.

Semiconducting photocatalytic solar–hydrogen conversion (SHC) from water is a great challenge for renewable fuel production. Organic semiconductors hold great promise for SHC in an economical and environmentally benign manner. However, organic semiconductors available for SHC are scarce and less efficient than most inorganic ones, largely due to their intrinsic Frenkel excitons with high binding energy. In this study the authors report polymer heterojunction (PHJ) photocatalysts consisting of polyfluorene family polymers and graphitic carbon nitride (g-C₃N₄) for efficient SHC. A molecular design strategy is executed to further promote the exciton dissociation or light harvesting ability of these PHJs via alternative approaches. It is revealed that copolymerizing electron-donating carbazole unit into the poly(9,9-dioctylfluorene) backbone promotes exciton dissociation within the poly(N-decanyl-2,7-carbazole-alt-9,9-dioctylfluorene) (PCzF)/g-C₃N₄ PHJ, achieving an enhanced apparent quantum yield (AQY) of 27% at 440 nm over PCzF/g-C₃N₄. Alternatively, copolymerizing electron-accepting benzothiadiazole unit extended the visible light response of the obtained poly(9,9-dioctylfluorene-alt-benzothiadiazole)/g-C₃N₄ PHJ, leading to an AQY of 13% at 500 nm. The present study highlights that constructing PHJs and adapting a rational molecular design of PHJs are effective strategies to exploit more of the potential of organic semiconductors for efficient solar energy conversion.

Polymer heterojunctions (PHJs) constructed via intermolecular π–π interactions between polyfluorene family polymers and graphitic carbon nitride are successfully developed for stable and efficient photocatalytic solar–hydrogen conversion. A molecular design strategy is executed to further promote the exciton dissociation or light-harvesting ability of these PHJs toward higher photocatalytic activities.

In article number 1604695, Wallace C.H. Choy and co-workers propose a novel room-temperature scheme of pyridine-promoted formation of perovskite films featuring large grain-sizes, and high crystallinity, while being free of residues. This new approach enables the fabrication of all room-temperature, solution-processed perovskite solar cells (PVSCs) with a record efficiency of 17.10% and 14.19%, with no hysteresis on rigid and flexible substrate respectively.

High performance organic photovoltaic devices typically rely on type-II P/N junctions for assisting exciton dissociation. Heremans and co-workers recently reported a high efficiency device with a third organic layer which is spatially separated from the active P/N junction; but still contributes to the carrier generation by passing its energy to the P/N junction via a long-range exciton energy transfer mechanism. In this study the authors show that there is an additional mechanism contributing to the high efficiency. Some bipolar materials (e.g., subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc)) are observed to generate free carriers much more effectively than typical organic semiconductors upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than those of reported single-layer devices. In addition, internal quantum efficiencies (IQEs) of bilayer devices with opposite stacking sequences (i.e., SubNc/SubPc vs SubPc/SubNc) are found to be the sum of IQEs of single layer devices. These results confirm that SubNc and SubPc can directly generate free carriers upon photoexcitation without assistance from a P/N junction. These allow them to be stacked onto each other with reversible sequence or simply stacking onto another P/N junction and contribute to the photocarrier generation.

Subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc) are identified as novel bipolar organic semiconductors with unusually high freecarrier generation upon photoexcitation. Functioning of devices with opposite stacking order (SubNc/SubPc vs SubPc/SubNc) confirms their operation does not rely on charge dissociation at the interfaces. These enable a new class of organic photovoltaic devices with non-p–n-junction-based architectures.

Phototransistors with a structure of a nitrogen-doped graphene quantum dots (NGQDs)–perovskite composite layer and a mildly reduced graphene oxide (mrGO) layer are fabricated through a solution-processing method. This hybrid phototransistor exhibits broad detection range (from 365 to 940 nm), high photoresponsivity (1.92 × 10⁴ A W⁻¹), and rapid response to light on–off (≈10 ms). NGQDs offer an effective and fast path for electron transfer from the perovskite to the mrGO, resulting in the improvement of photocurrent and photoswitching characteristics. The high photoresponsivity can also be ascribed to a photogating effect in the device. In addition, the phototransistor shows good stability with poly(methyl methacrylate) encapsulation, and can maintain 85% of its initial performance for 20 d in ambient air.

A solution-processed phototransistor based on the composite of a perovskite modified by nitrogen-doped graphene quantum dots (NGQDs) and mildly reduced graphene oxide shows a high photoresponsivity of up to 1.92 × 10⁴ A W⁻¹, a broad photodetection range of UV–vis–NIR, and good stability in air.

Though various efforts on modification of electrodes are still undertaken to improve the efficiency of perovskite solar cells, attributing to the large scope of these methods, it is of significance to unveil the working principle systematically. Herein, inverted perovskite solar cells based on indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/CH₃NH₃PbI₃/phenyl-C61-butyric acid methyl ester (PC₆₁ BM)/buffer metal/Al are constructed. Through the choice of different buffer metals to tune work function of the cathode, the contact nature of the active layer with the cathode could be manipulated well. In comparison with the device using Au/Al as the electrode that shows an unfavorable band bending for conducting the excited electrons to the cathode, the one with Ca/Al presents a dramatically improved efficiency over 17.1%, ascribed to the favorable band bending at the interface of the cathode with the active layer. Details for tuning the band bending and the corresponding charge transfer mechanism are given in a systematic manner. Thus, a general guideline for constructing perovskite photovoltaic devices efficiently is provided.

A systematic investigation on the impact of the work function of the cathode on the performance of inverted structure perovskite solar cells is provided. Lowering the work function of the cathode largely improves performance, including promoting the separation and conduction of charge carriers and even increasing the generation rate of excitons.