lizhendong

A novel non-fullerene acceptor, possessing a very low bandgap of 1.34 eV and a high-lying lowest unoccupied molecular orbital level of −3.95 eV, is designed and synthesized by introducing electron-donating alkoxy groups to the backbone of a conjugated small molecule. Impressive power conversion efficiencies of 8.4% and 10.7% are obtained for fabricated single and tandem polymer solar cells.

The morphology and the photovoltaic characteristics of a polymer–fullerene bulk-heterojunction solar cell processed with a processing additive are probed simultaneously. The processing additive reversibly enhances the solar cell performance which decays when the additive is lost.

After rapid progress over the past five years, organic–inorganic perovskite solar cells (PSCs) currently exhibit photoconversion efficiencies comparable to the best commercially available photovoltaic technologies. However, instabilities in the materials and devices, primarily due to reactions with water, have kept PSCs from entering the marketplace. Here, laser beam induced current imaging is used to investigate the spatial and temporal evolution of the quantum efficiency of perovskite solar cells under controlled humidity conditions. Several interesting mechanistic aspects are revealed as the degradation proceeds along a four-stage process. Three of the four stages can be reversed, while the fourth stage leads to irreversible decomposition of the photoactive perovskite material. A series of reactions in the PbI₂-CH₃NH₃I-H₂O system explains the interplay between the interactions with water and the overall stability. Understanding of the degradation mechanisms of PSCs on a microscopic level gives insight into improving the long-term stability.

State-of-the-art perovskite solar cells are examined under accelerated aging conditions using fast laser beam induced current imaging. The results demonstrate that the degradation of perovskite solar cells in the presence of water is a four-stage process involving phase transformations of the perovskite material. The work allows a detailed understanding of the evolution and mechanisms of moisture-induced perovskite degradation on a microscopic scale.

Photodetectors are designed, which operate in the broadband regime upon bottom illumination (from the indium tin oxide (ITO) side) and in the narrowband regime upon top illumination (from the air/perovskite side). The narrowband photodetectors show high external quantum efficiency of above 10⁴%. The operational spectrum of the photodetectors can also be tuned by adjusting the halide composition in the active material.

High-performance MoS₂ transistors are developed using atomic hexagonal boron nitride as a tunneling layer to reduce the Schottky barrier and achieve low contact resistance between metal and MoS₂. Benefiting from the ultrathin tunneling layer within 0.6 nm, the Schottky barrier is significantly reduced from 158 to 31 meV with small tunneling resistance.

The time evolution of the current–voltage characteristic of planar heterojunction perovskite solar cell (PSC) is studied within an operating temperature range of 200–325 K. The photovoltaic (PV) performance of PSC is found to be influenced by five carrier transport pathways, which strongly depend on operating temperature and light illumination. At low temperature, a severe degradation of PV performance is presented but temporary. This is attributed to ion accumulation at the TiO₂/CH₃NH₃PbI₃ and hole transport material/CH₃NH₃PbI₃ interfacial regions, as an origin of screening effect of built-in field, evidenced by the low external quantum efficiency (EQE). By light illumination at open-circuit, a steady PV performance will be reached and the stabilization time increases with decreasing temperature. The recovery of PV performance is attributed to ion diffusion in CH₃NH₃PbI₃ layer in the absence of electric field. The EQE observations indicate that photogenerated carriers are separated and collected efficiently after a long time light illumination due to a reduction of the screening effect. At high temperature, because of the low ion density at interfacial regions, the PV performance shows a quick response to light. These findings may help understanding of the mechanism of temperature-dependent photogenerated carrier transport in the PSC.

The degradation of photovoltaic performance in perovskite solar cells is observed when the operating temperature goes down to 200 K. After light illumination at open-circuit, the photovoltaic performance is significantly improved. A difference in carrier separation and collection efficiency is shown by external quantum efficiency measurement. This is attributed to the reduced screening effect of built-in field.

One of the factors limiting the performance of organic solar cells (OSCs) is their large energy losses (E _loss) in the conversion from photons to electrons, typically believed to be around 0.6 eV and often higher than those of inorganic solar cells. In this work, a novel low band gap polymer PIDTT-TID with a optical gap of 1.49 eV is synthesized and used as the donor combined with PC₇₁BM in solar cells. These solar cells attain a good power conversion efficiency of 6.7% with a high open-circuit voltage of 1.0 V, leading to the E _loss as low as 0.49 eV. A systematic study indicates that the driving force in this donor and acceptor system is sufficient for charge generation with the low E _loss. This work pushes the minimal E _loss of OSCs down to 0.49 eV, approaching the values of some inorganic and hybrid solar cells. It indicates the potential for further enhancement of the performance of OSCs by improving their V _oc since the E _loss can be minimized.

Polymer solar cells with minimal energy losses below 0.5 eV are realized based on a blend of a low band gap (1.49 eV) polymer PIDTT-TID and PC₇₁BM, exhibiting a high open-circuit voltage of 1.0 V and a decent efficiency of 6.7%. Solid proof shows that the good performance stems from the high-energy charge transfer state and low-energy loss, but sufficient driving force.

The structure evolution of oligomer fused-ring electron acceptors (FREAs) toward high efficiency of as-cast polymer solar cells (PSCs) is reported. First, a series of FREAs (IC-(1-3)IDT-IC) based on indacenodithiophene (IDT) oligomers as cores are designed and synthesized, effects of IDT number (1–3) on their basic optical and electronic properties are investigated, and more importantly, the relationship between device performance of as-cast PSCs and donor(D)/acceptor(A) matching (absorption, energy level, morphology, and charge transport) of IC-(1-3)IDT-IC acceptors and two representative polymer donors, PTB7-Th and PDBT-T1 is surveyed. Then, the most promising D/A system (PDBT-T1/IC-1IDT-IC) with the best D/A harmony among the six D/A combinations, which yields a power conversion efficiency (PCE) of 7.39%, is found. Finally, changing the side-chains in IC-1IDT-IC from alkylphenyl to alkyl enhances the PCE from 7.39% to 9.20%.

A series of fused-ring electron acceptors (FREAs) based on indacenodithiophene (IDT) oligomers as cores are designed and synthesized, and effects of IDT number (1–3) on their basic optical and electronic properties are investigated. High-performance as-cast polymer solar cells based on FREAs with power conversion efficiency as high as 9.2% are achieved through structure evolution of oligomer FREAs.

Three acceptor–acceptor (A–A) type conjugated polymers based on isoindigo and naphthalene diimide/perylene diimide are designed and synthesized to study the effects of building blocks and alkyl chains on the polymer properties and performance of all-polymer photoresponse devices. Variation of the building blocks and alkyl chains can influence the thermal, optical, and electrochemical properties of the polymers, as indicated by thermogravimetric analysis, differential scanning calorimetry, UV–vis, cyclic voltammetry, and density functional theory calculations. Based on the A–A type conjugated polymers, the most efficient all-polymer photovoltaic cells are achieved with an efficiency of 2.68%, and the first all-polymer photodetectors are constructed with high responsivity (0.12 A W⁻¹) and detectivity (1.2 × 10¹² Jones), comparable to those of the best fullerene based organic photodetectors and inorganic photodetectors. Photoluminescence spectra, charge transport properties, and morphology of blend films are investigated to elucidate the influence of polymeric structures on device performances. This contribution demonstrates a strategy of systematically tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.

Three n-type conjugated polymers are synthesized to achieve the most efficient acceptor–acceptor type polymer-based all-polymer photovoltaic cells with an efficiency of 2.68% and the first all-polymer photodetectors with high responsivity (0.12 A W⁻¹) and detectivity (1.2 × 10¹² Jones). This contribution provides a strategy of tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.

Organic materials for near-infrared (NIR) photodetection are in the focus for developing organic optical-sensing devices. The choice of materials for bulk-type organic photodetectors is limited due to effects like high nonradiative recombination rates for low-gap materials. Here, an organic Schottky barrier photodetector with an integrated plasmonic nanohole electrode is proposed, enabling structure-dependent, sub-bandgap photodetection in the NIR. Photons are detected via internal photoemission (IPE) process over a metal/organic semiconductor Schottky barrier. The efficiency of IPE is improved by exciting localized surface plasmon resonances, which are further enhanced by coupling to an out-of-plane Fabry–Pérot cavity within the metal/organic/metal device configuration. The device allows large on/off ratio (>1000) and the selective control of individual pixels by modulating the Schottky barrier height. The concept opens up new design and application possibilities for organic NIR photodetectors.

An organic Schottky photodetector is combined with a plasmonic nanohole electrode to enable sub-bandgap photodetection in the NIR. The responsivity can be modulated by an external field affecting the injection barrier for charge carriers from metal to organic. This allows >1000 on/off ratio and the selective control of individual pixel in detector matrix/arrays. The concept opens up new possibilities for organic optical-sensing devices.

Semiconductor micro/nano-cavities with high quality factor (Q) and small modal volume provide critical platforms for exploring strong light-matter interactions and quantum optics, enabling further development of coherent and quantum photonic devices. Constrained by exciton binding energy and thermal fluctuation, only a handful of wide-band semiconductors such as ZnO and GaN have stable excitons at room temperature. Metal halide perovskite with cubic lattice and well-controlled exciton may provide solutions. In this work, high-quality single-crystalline cesium lead halide CsPbX₃ (X = Cl, Br, I) whispering-gallery-mode (WGM) microcavities are synthesized by vapor-phase van der Waals epitaxy method. The as-grown perovskites show strong emission and stable exciton at room temperature over the whole visible spectra range. By varying the halide composition, multi-color (400–700 nm).WGM excitonic lasing is achieved at room temperature with low threshold (~ 2.0 μJ cm⁻²) and high spectra coherence (~0.14–0.15 nm). The results advocate the promise of inorganic perovskites towards development of optoelectronic devices and strong light-matter coupling in quantum optics.

High-quality cesium lead halide nanoplatelets functioning as whispering-gallery-mode microcavities are synthesized by vapor-phase van der Waals epitaxy method. Multicolor, low-threshold excitonic lasing action with a high spectra coherence of 0.14–0.15 nm is realized at room temperature. The findings are not only important for developing on-chip small lasers and high-speed exciton devices but also promising for fundamental studies in cavity quantum electrodynamics.

Effective singlet fission solar cells require both fast and efficient singlet fission as well as favorable energetics for harvesting the resulting triplet excitons. Notable progress has been made to engineer materials with rapid and efficient singlet fission, but the ability to control the energetics of these solar cells remains a challenge. Here, it is demonstrated that the interfacial charge transfer state energy of a rubrene/C₆₀ solar cell can be modified dramatically by the morphology of its constituent films. The effect is so pronounced that a crystalline system is able to dissociate and collect triplets generated through singlet fission whereas an as-deposited amorphous system is not. Furthermore, a novel technique for studying the behavior of this class of devices using external quantum efficiency (EQE) measurements in the presence of a background light is described. When this method is applied to rubrene/C₆₀ solar cells, it is shown that triplet–triplet annihilation makes significant contributions to photocurrent in the amorphous device—enhancing EQE by over 12% at relatively low intensities of background light (4 mW cm⁻²)—while detracting from photocurrent in the crystalline device. Finally, the conclusions on how the material system is affected by its morphology are strengthened by time-resolved photoluminescence experiments.

The interfacial charge transfer state energy of a crystalline rubrene/C₆₀ solar cell is over 300 meV lower than that of an amorphous rubrene/C₆₀ device. Despite that both amorphous and crystalline rubrene undergo singlet fission, this shift means that only the crystalline device can dissociate the low energy triplets generated by singlet fission and operate as a singlet fission solar cell.

On page 4464, M. J. Ko, H. J. Son, and co-workers develop a flexible perovskite solar cell by applying a novel low-temperature solution-processable polymer (PhNa-1T) as a hole-transport material. Compared with conventional PEDOT:PSS, PhNa-1T effectively improves solar cell efficiency and device stability due to the pH-neutral property of PhNa-1T and efficient charge extraction and suppressed charge recombination in solar cell devices.

MoS₂ spirals grown by the chemical vapor deposition method, driven by a threading dislocation, has a peculiar rhombohedral-like structure. This threading dislocation can carry helical current in the vertical direction and greatly enhances the vertical conductance in the MoS₂ multilayer samples.

Lead-free perovskite infrared light-emitting diodes are achieved by using a halide perovskite CsSnI₃ as an emissive layer. The film shows compact micrometer-sized grains with only a few pinholes and cracks at the grain boundaries. The device exhibits maximum radiance of 40 W sr⁻¹ m⁻² at a current density of 364.3 mA cm⁻² and maximum external quantum efficiency of 3.8% at 4.5 V.

A simple but novel method is designed to study the characteristics of the exciplex state pinned at a donor–acceptor abrupt interface and the effect an external electric field has on these excited states. The reverse Onsager process, where the field induces blue-shifted emission and increases the efficiency of the exciplex emission as the e–h separation reduces, is discussed.

Power conversion efficiency (PCE) of organic photovoltaics (OPVs) lags behind of inorganic photovoltaics due to low dielectric constants (ε _r) of organic semiconductors. Although OPVs with high ε _r are attractive in theory, practical demonstration of efficient OPV devices with high-ε _r materials is in its infancy. This is largely due to the contradiction between the requirements of high ε _r and good donor:acceptor blend morphology in the bulk heterojunction. Herein, a series of fullerene acceptors is reported bearing a polar cyano moiety for both high ε _r and good donor:acceptor blend morphology. These cyano-functionalized acceptors (ε _r = 4.9) have higher ε _r than that of the widely used acceptor, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) (ε _r = 3.9). The high ε _r is realized without decrease of electron mobility and change of the lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy levels. Although the cyano-functionalized acceptors have increased polarity, they still exhibit good compatibility with the typical donor polymer. Polymer solar cells based on the cyano-functionalized acceptors exhibit good active layer morphology and show better device performance (PCE = 5.55%) than that of PC₆₁BM (PCE = 4.56%).

Fullerene adducts bearing a cyano moiety with large dipole moment show not only a high dielectric constant, but also good compatibility with the donor polymer for good blend morphology, and exhibit good polymer solar cell device performances.

An ultrahigh performance MoS₂ photodetector with high photoresponsivity (1.94 × 10⁶ A W^–1) and detectivity (1.29 × 10¹² Jones) under 520 nm and 4.63 pW laser exposure is demonstrated. This photodetector is based on a methyl-ammonium lead halide perovskite/MoS₂ hybrid structure with (3-aminopropyl)triethoxysilane doping. The performance degradation caused by moisture is also minimized down to 20% by adopting a new encapsulation bilayer of octadecyltrichlorosilane/polymethyl methacrylate.

Graphene is the strongest and stiffest material ever identified and the best electrical conductor known to date, making it an ideal candidate for constructing nanocomposites used in flexible energy devices. However, it remains a great challenge to assemble graphene nanosheets into macro-sized high-performance nanocomposites in practical applications of flexible energy devices using traditional approaches. Nacre, the gold standard for biomimicry, provides an excellent example and guideline for assembling two-dimensional nanosheets into high-performance nanocomposites. This review summarizes recent research on the bioinspired graphene-based nanocomposites (BGBNs), and discusses different bioinspired assembly strategies for constructing integrated high-strength and -toughness graphene-based nanocomposites through various synergistic effects. Fundamental properties of graphene-based nanocomposites, such as strength, toughness, and electrical conductivities, are highlighted. Applications of the BGBNs in flexible energy devices, as well as potential challenges, are addressed. Inspired from the past work done by the community a roadmap for the future of the BGBNs in flexible energy device applications is depicted.

Nacre provides an inspiration for constructing graphene-based nanocomposites through interfacial interactions, including hydrogen, ionic, and covalent bonding plus π–π interactions, resulting in ultrastrong 1D fiber and ultratough 2D film nanocomposites. Functional ternary graphene-based nanocomposites feature excellent fatigue and are fire-retardant. Many others are fabricated through synergistic toughening. These integrated high-performance bioinspired graphene-based nanocomposites show promising applications in flexible energy devices.