以昇陳

Low‐cost and efficient organic small molecules are desired as hole transporting materials for high‐performance perovskite solar cells. Two new molecules containing a benzodithiophene core and triphenylamine side chains are synthesized from cheap starting materials by a simple and low‐cost method. Lead‐free, tin‐based perovskite solar cells employing these new benzodithiophene‐based hole transporting materials achieve good efficiencies.

Abstract

Developing efficient interfacial hole transporting materials (HTMs) is crucial for achieving high‐performance Pb‐free Sn‐based halide perovskite solar cells (PSCs). Here, a new series of benzodithiophene (BDT)‐based organic small molecules containing tetra‐ and di‐triphenyl amine donors prepared via a straightforward and scalable synthetic route is reported. The thermal, optical, and electrochemical properties of two BDT‐based molecules are shown to be structurally and energetically suitable to serve as HTMs for Sn‐based PSCs. It is reported here that ethylenediammonium/formamidinium tin iodide solar cells using BDT‐based HTMs deliver a champion power conversion efficiency up to 7.59%, outperforming analogous reference solar cells using traditional and expensive HTMs. Thus, these BDT‐based molecules are promising candidates as HTMs for the fabrication of high‐performance Sn‐based PSCs.

Organic photodetectors (OPDs) are promising for large area image detectors. Minimizing the dark current density (J _d) is crucial in most applications; nevertheless, a wide range of J _d values has been reported. Here, possible reasons that lead to this large variation are discussed. A quantitative analysis of intrinsic J _d processes shows that charge injection from the electrodes is the dominant mechanism.

Abstract

Organic photodetectors (OPDs) have gained increasing interest as they offer cost‐effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible‐light OPDs are proposed for use in indirect‐conversion X‐ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near‐infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (J _d) is crucial to improve important figures of merits such as the signal‐to‐noise ratio, the linear dynamic range, and the specific detectivity (D *). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to J _d in OPDs. J _d reduction is typically addressed by fine‐tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental J _d values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

Recently, sequential deposition of donor and acceptor layers has been demonstrated to be an alternative method to fabricate highly efficient bulk‐heterojunction organic solar cells. A simple “needle” model to simulate its morphology indicates a different morphological requirement which rationalizes the high exciton dissociation efficiency.

Abstract

Bulk heterojunction (BHJ) nonfullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq‐BHJ) have recently been shown to be highly efficient, environmentally friendly, and compatible with large area and roll‐to‐roll fabrication. However, the related photophysics at donor‐acceptor interface and the vertical heterogeneity of donor‐acceptor distribution, critical for exciton dissociation and device performance, have been largely unexplored. Herein, steady‐state and time‐resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlating with the luminescent efficiency of interfacial states and its nonradiative recombination, interfacial trap states are characterized to be about 40% more populated in the sq‐BHJ devices than the as‐cast BHJ (c‐BHJ), which probably limits the device voltage output. Cross‐sectional energy‐dispersive X‐ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly visualize the donor–acceptor vertical stratification with a precision of 1–2 nm. From the proposed “needle” model, the high exciton dissociation efficiency is rationalized. This study highlights the promise of sequential deposition to fabricate efficient solar cells, and points toward improving the voltage output and overall device performance via eliminating interfacial trap states.

Aromatic‐diimide‐based polymers have emerged as the most promising n‐type semiconductors and their photovoltaic performance has been significantly improved in the past decade. The recent exciting progress is highlighted and the structure–property relationship of aromatic‐diimde‐based photovoltaic polymers is revealed, which could provide important guidelines for the further design of n‐type photovoltaic polymers.

Abstract

All‐polymer solar cells (all‐PSCs) have attracted immense attention in recent years due to their advantages of tunable absorption spectra and electronic energy levels for both donor and acceptor polymers, as well as their superior thermal and mechanical stability. The exploration of the novel n‐type conjugated polymers (CPs), especially based on aromatic diimide (ADI), plays a vital role in the further improvement of power conversion efficiency (PCE) of all‐PSCs. Here, recent progress in structure modification of ADIs including naphthalene diimide (NDI), perylene diimide (PDI), and corresponding derivatives is reviewed, and the structure–property relationships of ADI‐based CPs are revealed.

Combining the advantages of aggregation‐induced emission (AIE) and organic room‐temperature phosphorescence (RTP), as‐designed RTP‐AIEgens show a maximum photoluminescence quantum yield of 64%. Devices are then fabricated, and nondoped organic light‐emitting diodes (OLEDs) based on the RTP AIEgens exhibit relatively small efficiency roll‐off and efficient electroluminescence quantum efficiency, breaking the theoretical limit of conventional fluorescent OLEDs.

Abstract

Aggregation‐induced emission (AIE) is a beneficial strategy for generating highly effective solid‐state molecular luminescence without suffering losses in quantum yield. However, the majority of reported AIE‐active molecules exhibit only strong fluorescence, which is not ideal for electrical excitation in organic light‐emitting diodes (OLEDs). By introducing various substituent groups onto the biscarbazole compound, a series of molecular materials with aggregation‐induced phosphorescence (AIP) is designed, which exhibits two distinctly different phosphorescence bands and an absolute solid‐state room‐temperature phosphorescence quantum yield up to 64%. Taking advantage of the AIE feature, the AIP molecules are fabricated into OLEDs as a homogeneous light‐emitting layer, which allows for relatively small efficiency roll‐off and shows an external electroluminescence quantum yield of up to 5.8%, more than the theoretical limit for purely fluorescent OLED devices. The design showcases a promising strategy for the production of cost‐effective and highly efficient OLED technology.

An ideal materials combination based on the electron donor BSFTR and acceptor Y6 is selected to construct small‐molecule solar cells (SMSCs). By morphology optimization, an extraordinary power conversion efficiency of 13.69% with a remarkably low energy loss of 0.48 eV is achieved, which is beneficial from the matched photoelectric properties, the favorable blend morphology, and is the best binary SMSC performance reported so far.

Abstract

Compared with the quick development of polymer solar cells, achieving high‐efficiency small‐molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open‐circuit voltage of 0.85 V, a high short‐circuit current of 23.16 mA cm⁻² and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high‐performance SMSCs.

A highly transparent near‐infrared (NIR)‐absorbing isobutyl‐substituted diimmonium salt (IDI) selectively absorbs light in the NIR range to provide a transparent photothermal film, IDI‐doped polysulfone (IDIS) film. The IDIS film with 16.7 wt% IDI shows high thermal/moisture resistance, photothermal conversion efficiency, and photodistillation conversion efficiency. By combining this layer with poly(dimethylsiloxane), it becomes a transparent soft actuator that can generate a transparent hot Venus flytrap reversibly.

Abstract

While numerous near‐infrared (NIR) materials have emerged, most of them are strongly colored or black due to the absorption band or tails in the visible region. Here, a highly transparent and soluble NIR‐absorbing ionic salt, isobutyl‐substituted diimmonium borate (IDI), is synthesized and fabricated, through a solution process, as a thin film that shows a transmittance of over 93% in the whole visible region. A transparent photothermal (PT) film heater is fabricated with the IDI‐doped polymer solution, which shows a photothermal conversion efficiency (η_PT) of 75.2%. Additionally, the prepared PT heater shows a high water evaporation conversion efficiency (η_w) of 68.8% upon exposure to a 1064 nm laser. Furthermore, the transparent IDI film affords the development of a wireless transparent actuator for the first time, generating a bending angle over 75°, with over 2700 bending cycles. The transparent IDI film creates a hot transparent Venus flytrap and a colorful or fluorescent actuator upon the addition of colorants without losing the actuation properties.

Thanks to the strong electron‐donating capability of carbon–oxygen‐bridged (CO‐bridged) ladder‐type building blocks, CO‐bridged nonfullerene acceptors (NFAs) present low bandgaps and strong light‐harvesting capability, delivering high short‐circuit current density (>28 mA cm⁻²) and high power conversion efficiency (>14% for single‐junction and >17% for tandem) in organic solar cells.

Abstract

Recently, acceptor–donor–acceptor (A–D–A) small molecules have emerged as promising nonfullerene acceptors (NFAs) for organic solar cells and have attracted great attention. The carbon‐bridged (C‐bridged) ladder‐type D unit plays a crucial role in developing high‐performance A–D–A NFAs. However, the medium electron‐donating capability of C‐bridged units is unfavorable for making NFAs with strong light‐harvesting capability. In this regard, carbon–oxygen‐bridged (CO‐bridged) ladder‐type units present advantages in developing strong light‐absorbing NFAs. Here, recent progress in the newly emerging CO‐bridged NFAs is highlighted. The synthetic methods for the polycyclic CO‐bridged building blocks are introduced. The photovoltaic performance for CO‐bridged NFAs is summarized and discussed. Perspectives on developing high‐performance CO‐bridged‐NFA‐based solar cells are made.

Aromatic‐diimide‐based polymers have emerged as the most promising n‐type semiconductors and their photovoltaic performance has been significantly improved in the past decade. The recent exciting progress is highlighted and the structure–property relationship of aromatic‐diimde‐based photovoltaic polymers is revealed, which could provide important guidelines for the further design of n‐type photovoltaic polymers.

Abstract

All‐polymer solar cells (all‐PSCs) have attracted immense attention in recent years due to their advantages of tunable absorption spectra and electronic energy levels for both donor and acceptor polymers, as well as their superior thermal and mechanical stability. The exploration of the novel n‐type conjugated polymers (CPs), especially based on aromatic diimide (ADI), plays a vital role in the further improvement of power conversion efficiency (PCE) of all‐PSCs. Here, recent progress in structure modification of ADIs including naphthalene diimide (NDI), perylene diimide (PDI), and corresponding derivatives is reviewed, and the structure–property relationships of ADI‐based CPs are revealed.

Upconverted circularly polarized luminescence (UC‐CPL) via triplet–triplet annihilation‐based photon upconversion in chiral systems is an emerging topic in photochemistry and photophysics. The concept of this topic is described, and recent advances in the construction and application of UC‐CPL materials are highlighted. In addition, new functions emerging from UC‐CPL materials are presented.

Abstract

Circularly polarized luminescent materials are of increasing attention due to their potential applications in advanced optical technologies, such as chiroptical devices and optical sensing. Recently, in all reported circularly polarized luminescent materials, high‐energy excitation results in low‐energy or downconverted circularly polarized luminescence (CPL) emission. Although photon upconversion—i.e., the conversion of low‐energy light into higher‐energy emission, with a wide variety of applications—has been widely reported, the integration of photon upconversion and CPL in one chiral system to achieve higher‐energy CPL emission has never been reported. Herein, a brief review is provided of recent achievements in photon‐upconverted CPL via the triplet–triplet annihilation mechanism, focusing on the amplified dissymmetry factor g _lum through energy transfer process and dual upconverted and downconverted CPL emission through chirality and energy transfer process.

As described by Zhixiang Wei and co‐workers in article number https://doi.org/10.1002/adma.2018050891805089, for large‐area organic solar cells, high active‐layer thickness tolerability is generally required, the methods to reduce power conversion efficiency losses are critical, and printing methods suitable for roll‐to‐roll printing are highly important. By combining material requirements, modular designs, and printing methods, the application of organic solar cells will be successfully realized in the near future.

Device model simulation is a macroscopic computer‐assisted tool for modeling organic and organic–inorganic hybrid perovskite solar cells. It simulates the underlying physical mechanisms of the electrical characteristics, such as space‐charge‐limited current, injection‐limited current, ohmic contact, short‐circuit current density, open‐circuit voltage, J–V hysteresis phenomena, power conversion efficiency in the present of surface recombination, trap/defect dependent recombination, or direct band recombination.

Abstract

Device model simulation is one of the primary tools for modeling thin film solar cells from organic materials to organic–inorganic perovskite materials. By directly connecting the current density–voltage (J–V) curves to the underlying device physics, it is helpful in revealing the working mechanism of the heatedly discussed organic–inorganic hybrid perovskite solar cells. Some distinctive optoelectronic features need more phenomenological models and accurate simulations. Herein, the application of the device model method in the simulation of organic and organic–inorganic perovskite solar cells is reviewed. To this end, the ways of the device model are elucidated by discussing the metal–insulator–metal picture and the equations describing the physics. Next, the simulations on J–V curves of organic solar cells are given in the presence of the space charge, interface, charge injection, traps, or exciton. In the perovskite section, the effects of trap states, direct band recombination, surface recombination, and ion migration on the device performance are systematically discussed from the perspective of the device model simulation. Suggestions for designing perovskite devices with better performance are also given.

A simple structure, nonchlorinated solvent processable donor–acceptor conjugated polymer based on a denser alkyl side chains strategy is obtained and exhibits reliable high hole mobility of up to 9.24 cm² V⁻¹ s⁻¹ in organic thin film transistor (OTFT) using a bar coating technique, which has great potential for large‐scale material and device manufacturing of high mobility OTFTs.

Abstract

Herein, a simple structure, nonchlorinated solvent processable high mobility donor–acceptor conjugated polymer, poly(2,5‐bis(4‐hexyldodecyl)‐2,5‐dihydro‐3,6‐di‐2‐thienyl‐pyrrolo[3,4‐c]pyrrole‐1,4‐dione‐alt‐thiophene) (PDPPT3‐HDO), is reported. The enhanced solubility in nonchlorinated solvent is realized based on a denser alkyl side chains strategy by incorporating small size comonomer thiophene. An associated benefit of thiophene comonomer is the remarkable structural simplicity of the resulting polymer, which is advantageous for industrial scaling up. The alkyl side chain density and structure of PDPPT3‐HDO can efficiently control the self‐assembly properties in solution and film. By bar coating from o‐xylene solution, PDPPT3‐HDO forms aligned films and exhibits high hole mobility of up to 9.24 cm² V⁻¹ s⁻¹ in organic thin film transistors (OTFTs). Notably, the bar‐coated OTFT based on PDPPT3‐HDO shows a close to ideal transistor model and a high mobility reliability factor of 87%. The multiple benefits of increased side chain density strategy may encourage the design of high mobility polymers that meet the requirements of mass production of OTFT materials and devices.

The formation of an inversion layer within n‐Si near the interface with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) based conductive thin films is evidenced. High power conversion efficiency in solar cells is correlated with a large contact‐induced band bending in Si, high polymer conductivity, and proper Si interfacial passivation.

Abstract

Heterojunctions formed by ultrathin conductive polymer [poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate)—PEDOT:PSS] films and n‐type crystalline silicon are investigated by photoelectron spectroscopy. Large shifts of Si 2p core levels upon PEDOT:PSS deposition provide evidence that a dopant‐free p–n junction, i.e., an inversion layer, is formed within Si. Among the investigated PEDOT:PSS formulations, the largest induced band bending within Si (0.71 eV) is found for PH1000 (high PEDOT content) combined with a wetting agent and the solvent additive dimethyl sulfoxide (DMSO). Without DMSO, the induced band bending is reduced, as is also the case with a PEDOT:PSS formulation with higher PSS content. The interfacial energy level alignment correlates well with the characteristics of PEDOT:PSS/n‐Si solar cells, where high polymer conductivity and sufficient Si‐passivation are also required to achieve high power conversion efficiency.

A nonfullerene acceptor based active layer with high halogen contents is designed to fabricate efficient thick‐film organic solar cells. The conventional structure device using chlorinated acceptor F–2Cl and fluorinated donor PM6 exhibits a power conversion efficiency over 10% with an active layer thickness of 600 nm.

Abstract

Developing efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick‐film organic solar cells employing a nonfullerene acceptor F–2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achieved under optimized conditions. PCE of 9.03% is obtained for the inverted device with active layer thickness of 500 nm. It is worth noting that the conventional structure device still maintains the PCE of over 10% when the film thickness of the active layer is 600 nm, which is the highest value for the NF‐OSCs with such a large active layer thickness. It is found that the performance difference between the thick active layer films based conventional and inverted devices is attributed to their different vertical phase separation in the active layers.

An open‐shell donor–acceptor conjugated polymer enhances charge delocalization in the reduced state and is demonstrated as a highly stable anode in supercapacitors, with 90% capacitance retention after 2000 redox cycles. Asymmetric supercapacitors using this n‐dopable polymer operate with a wide 3 V potential window, with a best‐in‐class energy density and a long cycle life critical to energy storage and management.

Abstract

Supercapacitors have emerged as an important energy storage technology offering rapid power delivery, fast charging, and long cycle lifetimes. While extending the operational voltage is improving the overall energy and power densities, progress remains hindered by a lack of stable n‐type redox‐active materials. Here, a new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated polymer is demonstrated, which exhibits an open‐shell ground state, intrinsic electrical conductivity, and enhanced charge delocalization in the reduced state. These attributes afford very stable anodes with a coulombic efficiency of 99.6% and that retain 90% capacitance after 2000 charge–discharge cycles, exceeding other n‐dopable organic materials. Redox cycling processes are monitored in situ by optoelectronic measurements to separate chemical versus physical degradation mechanisms. Asymmetric supercapacitors fabricated using this polymer with p‐type PEDOT:PSS operate within a 3 V potential window, with a best‐in‐class energy density of 30.4 Wh kg⁻¹ at a 1 A g⁻¹ discharge rate, a power density of 14.4 kW kg⁻¹ at a 10 A g⁻¹ discharge rate, and a long cycle life critical to energy storage and management. This work demonstrates the application of a new class of stable and tunable redox‐active material for sustainable energy technologies.

Recently, sequential deposition of donor and acceptor layers has been demonstrated to be an alternative method to fabricate highly efficient bulk‐heterojunction organic solar cells. A simple “needle” model to simulate its morphology indicates a different morphological requirement which rationalizes the high exciton dissociation efficiency.

Abstract

Bulk heterojunction (BHJ) nonfullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq‐BHJ) have recently been shown to be highly efficient, environmentally friendly, and compatible with large area and roll‐to‐roll fabrication. However, the related photophysics at donor‐acceptor interface and the vertical heterogeneity of donor‐acceptor distribution, critical for exciton dissociation and device performance, have been largely unexplored. Herein, steady‐state and time‐resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlating with the luminescent efficiency of interfacial states and its nonradiative recombination, interfacial trap states are characterized to be about 40% more populated in the sq‐BHJ devices than the as‐cast BHJ (c‐BHJ), which probably limits the device voltage output. Cross‐sectional energy‐dispersive X‐ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly visualize the donor–acceptor vertical stratification with a precision of 1–2 nm. From the proposed “needle” model, the high exciton dissociation efficiency is rationalized. This study highlights the promise of sequential deposition to fabricate efficient solar cells, and points toward improving the voltage output and overall device performance via eliminating interfacial trap states.

Today, the open‐circuit voltage (V _OC) of state‐of‐the‐art quantum dot photovoltaics (QD‐PVs) remains low relative to its bandgap. Herein, a bilayer hole‐transport layer (HTL) is developed and adapted to suppress undesired interfacial dipoles and electron leakage. The results reveal the relationship between the electron‐blocking barrier at HTL and V _OC, which enables a QD‐PV cell with a decreased V _OC deficit of 450 mV.

Abstract

Quantum‐dot (QD) photovoltaics (PVs) offer promise as energy‐conversion devices; however, their open‐circuit‐voltage (V _OC) deficit is excessively large. Previous work has identified factors related to the QD active layer that contribute to V _OC loss, including sub‐bandgap trap states and polydispersity in QD films. This work focuses instead on layer interfaces, and reveals a critical source of V _OC loss: electron leakage at the QD/hole‐transport layer (HTL) interface. Although large‐bandgap organic materials in HTL are potentially suited to minimizing leakage current, dipoles that form at an organic/metal interface impede control over optimal band alignments. To overcome the challenge, a bilayer HTL configuration, which consists of semiconducting alpha‐sexithiophene (α‐6T) and metallic poly(3,4‐ethylenedioxythiphene) polystyrene sulfonate (PEDOT:PSS), is introduced. The introduction of the PEDOT:PSS layer between α‐6T and Au electrode suppresses the formation of undesired interfacial dipoles and a Schottky barrier for holes, and the bilayer HTL provides a high electron barrier of 1.35 eV. Using bilayer HTLs enhances the V _OC by 74 mV without compromising the J _SC compared to conventional MoO₃ control devices, leading to a best power conversion efficiency of 9.2% (>40% improvement relative to relevant controls). Wider applicability of the bilayer strategy is demonstrated by a similar structure based on shallow lowest‐unoccupied‐molecular‐orbital (LUMO) levels.

Nature Photonics, Published online: 28 October 2019; doi:10.1038/s41566-019-0528-x

Effect of side chain length of central fused‐ring on the physicochemical, self‐assembly, and photovoltaic properties of the small molecule acceptors (SMAs) is investigated. Single‐crystal structure and grazing incidence wide‐angle X‐ray scattering results reveal that the longer side chains lead to different long‐range ordering in the molecular aggregation, which improves the molecular ordering in films and increases photovoltaic performance of the SMAs.

Abstract

The effects of central alkoxy side chain length of a series of narrow bandgap small molecule acceptors (SMAs) on their physicochemical properties and on the photovoltaic performance of the SMA‐based polymer solar cells (PSCs) are systematically investigated. It is found that the ordered aggregation of these SMAs in films is enhanced gradually with the increase of alkoxy chain length. The single‐crystal structures of these SMAs further reveal that small changes in the side chain length can have a dramatic impact on molecular self‐assembly. The short‐circuit current density and power conversion efficiency values of the corresponding PSCs increase with the increase of the side chain length of the SMAs. The π–π coherence length of the SMAs in the active layers is increased with the increase of the side chain length, which could be the reason for the increase of the J _sc in the PSCs. The results indicate that small changes in side chain length can have a dramatic impact on the molecular self‐assembly, morphology, and photovoltaic performance of the PSCs. The structure–performance relationship established in this study can provide important instructions for the side chain engineering and for the design of efficient SMAs materials.