Dopant‐Free,Donor–Acceptor‐Type Polymeric Hole‐Transporting Materials for the Perovskite Solar Cells with Power Conversion Efficiencies over 20%

The rich molecular design of electron donor (D)–acceptor (A) polymers offers many valuable clues to obtain high‐efficiency hole‐transporting materials (HTMs) for use in perovskite solar cells (PVSCs). The fused aromatic or heteroaromatic units can increase the conjugation of the polymer backbone to facilitate electron delocalization, which increases the rigidity of adjacent units to prevent rotational disorder and lower the reorganization energy, leading to improved carrier mobility and optimized film morphology. In this work, fused‐ring ladder‐type indacenodithiophene and indacenodithieno[3,2‐b]thiophene are used as D units, benzodithiophene‐4,8‐dione as the A unit, and thienothiophene as a π‐bridge to form the D–A polymers PBDTT and PBTTT, respectively. Both polymers exhibit favorable properties as HTMs including suitable energy levels, high hole mobility, and excellent film quality. Both dopant‐free HTMs endow n‐i‐p PVSCs with promising performance and stability. A maximum power conversion efficiency of 20.28% is achieved for PBDTT‐based devices, which is among the highest values reported to date.     

Porphyrin‐Based Bulk Heterojunction Organic Photovoltaics: The Rise of the Colors of Life

Organic photovoltaics (OPV) represent a thin‐film PV technology that offers attractive prospects for low‐cost and aesthetically appealing (colored, flexible, uniform, semitransparent) solar cells that are printable on large surfaces. In bulk heterojunction (BHJ) OPV devices, organic electron donor and acceptor molecules are intimately mixed within the photoactive layer. Since 2005, the power conversion efficiency of said devices has increased substantially due to insights in the underlying physical processes, device optimization, and chemical engineering of a vast number of novel light‐harvesting organic materials, either small molecules or conjugated polymers. As Nature itself has developed porphyrin chromophores for solar light to energy conversion, it seems reasonable to pursue artificial systems based on the same types of molecules. Porphyrins and their analogues have already been successfully implemented in certain device types, notably in dye‐sensitized solar cells, but they have remained largely unexplored in BHJ organic solar cells. Very recent successes do show, however, the strong (latent) prospects of porphyrinoid semiconductors as light‐harvesting and charge transporting materials in such devices. Here, an overview on the state‐of‐the‐art of porphyrin‐based solution‐processed BHJ OPV is provided and insights are given into the pathways to follow and hurdles to overcome toward further improvements of porphyrinic materials and devices.     

Blue organic light‐emitting diodes based on fluorene‐bridged quinazoline and quinoxaline derivatives

Two blue emitters based on fluorene‐bridged quinazoline and quinoxaline derivatives were prepared via the Suzuki reaction. Their photoluminescent properties were investigated. Furthermore, theoretical studies on these materials using the density functional theory calculation were conducted. To explore their electroluminescent properties, multilayered organic light‐emitting diodes were fabricated with the following device structure: indium–tin–oxide (180 nm)/4,4′‐bis(N‐(1‐naphthyl)‐N‐phenylamino)biphenyl (50 nm)/blue emitting materials ( 1 and 2 ) (30 nm)/bathophenanthroline (35 nm)/8‐hydroxy‐quinolinato lithium (2 nm)/Al (100 nm). Two devices showed efficient blue emission with the external quantum efficiencies of 1.58% and 1.30%, respectively, at 20 mA/cm², and Commission Internationale dÉclairage coordinates of (0.18, 0.24) and (0.19, 0.27) at 6.0 V. These results suggest that the self‐aggregation properties of emitters would have considerable effects on their photoluminescent and electroluminescent properties.     

Alkyl Chain Engineering of Solution‐Processable Star‐Shaped Molecules for High‐Performance Organic Solar Cells

The impact of alkyl side‐chain substituents on conjugated polymers on the photovoltaic properties of bulk heterojunction (BHJ) solar cells has been studied extensively, but their impact on small molecules has not received adequate attention. To reveal the effect of side chains, a series of star‐shaped molecules based on a triphenylamine (TPA) core, bithiophene, and dicyanovinyl units derivatized with various alkyl end‐capping groups of methyl, ethyl, hexyl and dodecyl is synthesiyed and studied to comprehensively investigate structure‐properties relationships. UV‐vis absorption and cyclic voltammetry data show that variations of alkyl chain length have little influence on the absorption and highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) levels. However, these seemingly negligible changes have a pronounced impact on the morphology of BHJ thin films as well as their charge carrier separation and transportation, which in turn influences the photovoltaic properties of these small‐molecule‐based BHJ devices. Solution‐processed organic solar cells (OSCs) based on the small molecule with the shortest methyl end groups exhibit high short circuit current (J_sc) and fill factor (FF), with an efficiency as high as 4.76% without any post‐treatments; these are among the highest reported for solution‐processed OSCs based on star‐shaped molecules.     

Self‐Doped,n‐Type Perylene Diimide Derivatives as Electron Transporting Layers for High‐Efficiency Polymer Solar Cells

Perylene diimide (PDI) with high electron affinities are promising candidates for applications in polymer solar cells (PSCs). In addition, the strength of π‐deficient backbones and end‐groups in an n‐type self‐dopable system strongly affects the formed end‐group‐induced electronic interactions. Herein, a series of amine/ammonium functionalized PDIs with excellent alcohol solubility are synthesized and employed as electron transporting layers (ETLs) in PSCs. The electron transfer properties of the resulting PDIs are dramatically tuned by different end‐groups and π‐deficient backbones. Notably, electron transfer is observed directly in solution in self‐doped PDIs for the first time. A significantly enhanced power conversion efficiency of 10.06% is achieved, when applying the PDIs as ETLs in PTB7‐Th:PC₇₁BM‐based PSCs. These results demonstrate the potential of n‐type organic semiconductors with stable n‐type doping capability and facile solution processibility for future applications of energy transition devices.     

Comparing the Device Physics and Morphology of Polymer Solar Cells Employing Fullerenes and Non‐Fullerene Acceptors

There is a need to find electron acceptors for organic photovoltaics that are not based on fullerene derivatives since fullerenes have a small band gap that limits the open‐circuit voltage (V_OC), do not absorb strongly and are expensive. Here, a phenylimide‐based acceptor molecule, 4,7‐bis(4‐(N‐hexyl‐phthalimide)vinyl)benzo[c]1,2,5‐thiadiazole (HPI‐BT), that can be used to make solar cells with V_OC values up to 1.11 V and power conversion efficiencies up to 3.7% with two thiophene polymers is demonstrated. An internal quantum efficiency of 56%, compared to 75–90% for polymer‐fullerene devices, results from less efficient separation of geminate charge pairs. While favorable energetic offsets in the polymer‐fullerene devices due to the formation of a disordered mixed phase are thought to improve charge separation, the low miscibility (<5 wt%) of HPI‐BT in polymers is hypothesized to prevent the mixed phase and energetic offsets from forming, thus reducing the driving force for charges to separate into the pure donor and acceptor phases where they can be collected.     

Synthesis and photophysical properties of 1,2‐diphenylindolizine derivatives: fluorescent blue‐emitting materials for organic light‐emitting device

New blue‐emitting materials based on 1,2‐diphenylindolizine were designed and synthesized through a microwave‐assisted Suzuki coupling reaction. The photophysical, electrochemical, and thermal properties of the 1,2‐diphenylindolizine derivatives were investigated using UV–visible and fluorescence spectroscopy, cyclic voltammetry, thermogravimetric analysis, and differential scanning calorimetry. The 1,2‐diphenylindolizine derivatives had band gaps of 3.1–3.4 eV and indicated proper emission of around 450 nm without significant difference between in solution and thin solid film. The indolizine derivatives show an enhanced thermal stability (?T_m > 100 °C), compared with 1,2‐diphenylindolizine. These results suggest the 1,2‐diphenylindolizine derivatives are suitable for blue‐emitting materials in organic light‐emitting devices. Copyright © 2015 John Wiley & Sons, Ltd.     

Confined Synthesis of 2D Nanostructured Materials toward Electrocatalysis

2D nanostructured materials have shown great application prospects in energy conversion, owing to their unique structural features and fascinating physicochemical properties. Developing efficient approaches for the synthesis of well‐defined 2D nanostructured materials with controllable composition and morphology is critical. The emerging concept, confined synthesis, has been regarded as a promising strategy to design and synthesize novel 2D nanostructured materials. This review mainly summarizes the recent advances in confined synthesis of 2D nanostructured materials by using layered materials as host matrices (also denoted as “nanoreactors”). By virtue of the space‐ and surface‐confinement effects of these layered hosts, various well‐organized 2D nanostructured materials, including 2D metals, 2D metal compounds, 2D carbon materials, 2D polymers, 2D metal‐organic frameworks (MOFs) and covalent‐organic frameworks (COFs), as well as 2D carbon nitrides are successfully synthesized. The wide employment of these 2D materials in electrocatalytic applications (e.g., electrochemical oxygen/hydrogen evolution reactions, small molecule oxidation, and oxygen reduction reaction) is presented and discussed. In the final section, challenges and prospects in 2D confined synthesis from the viewpoint of designing new materials and exploring practical applications are commented, which would push this fast‐evolving field a step further toward greater success in both fundamental studies and ultimate industrialization.     

Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well‐Known PBDTTT Polymers

With the advances in organic photovoltaics (OPVs), the invention of model polymers with superior properties and wide applicability is of vital importance to both the academic and industrial communities. The recent inspiring advances in OPV research have included the emergence of poly(benzodithiophene‐co‐thieno[3,4‐b]thiophene) (PBDTTT)‐based materials. Through the combined efforts on PBDTTT polymers, over 10% efficiencies have been realized recently in various types of OPV devices. This review attempts to critically summarize the recent advances with respect to five well‐known PBDTTT polymers and their design considerations, basic properties, photovoltaic performance, as well as device application in conventional, inverted, tandem solar cells. These PBDTTT polymers also make great contributions to the rapid advances in the field of emerging ternary blends and fullerene‐free OPVs with top performances. Addtionally, new challenges in developing novel photovoltaic polymers with more superior properties are prospected. More importantly, the research of highly efficient PBDTTT‐based polymers provides useful insights and builds fundamentals for new types of OPV applications with various architectures.     

Thiophene Rings Improve the Device Performance of Conjugated Polymers in Polymer Solar Cells with Thick Active Layers

Developing novel materials that tolerate thickness variations of the active layer is critical to further enhance the efficiency of polymer solar cells and enable large‐scale manufacturing. Presently, only a few polymers afford high efficiencies at active layer thickness exceeding 200 nm and molecular design guidelines for developing successful materials are lacking. It is thus highly desirable to identify structural factors that determine the performance of semiconducting conjugated polymers in thick‐film polymer solar cells. Here, it is demonstrated that thiophene rings, introduced in the backbone of alternating donor–acceptor type conjugated polymers, enhance the fill factor and overall efficiency for thick (>200 nm) solar cells. For a series of fluorinated semiconducting polymers derived from electron‐rich benzo[1,2‐b:4,5‐b′]dithiophene units and electron‐deficient 5,6‐difluorobenzo[2,1,3]thiazole units a steady increase of the fill factor and power conversion efficiency is found when introducing thiophene rings between the donor and acceptor units. The increased performance is a synergistic result of an enhanced hole mobility and a suppressed bimolecular charge recombination, which is attributed to more favorable polymer chain packing and finer phase separation.     

Diterpenoid biopolymers: New directions for renewable materials engineering

Most types of ambers are naturally occurring, relatively hard, durable resinite polymers derived from the exudates of trees. This resource has been coveted for thousands of years due to its numerous useful properties in industrial processes, beauty, and purported medicinal properties. Labdane diterpenoid‐based ambers represent the most abundant and important resinites on earth. These resinites are a dwindling nonrenewable natural resource, so a new source of such materials needs to be established. Recent advances in sequencing technologies and biochemical engineering are rapidly accelerating the rate of identifying and assigning function to genes involved in terpenoid biosynthesis, as well as producing industrial‐scale quantities of desired small‐molecules in bacteria and yeast. This has provided new tools for engineering metabolic pathways capable of producing diterpenoid monomers that will enable the production of custom‐tailored resinite‐like polymers. Furthermore, this biosynthetic toolbox is continuously expanding, providing new possibilities for renewing dwindling stocks of naturally occurring resinite materials and engineering new materials for future applications. © 2010 Wiley Periodicals, Inc. Biopolymers 95: 71–76, 2011.     

π‐Extended Narrow‐Bandgap Diketopyrrolopyrrole‐Based Oligomers for Solution‐Processed Inverted Organic Solar Cells

A series of narrow‐bandgap π‐conjugated oligomers based on diketopyrrolopyrrole chromophoric units coupled with benzodithiophene, indacenodithiophene, thiophene, and isoindigo cores are designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of these different central cores on the optoelectronic and morphological properties, carrier mobility, and photovoltaic performance are investigated. These π‐extended oligomers possess broad and intense optical absorption covering the range from 550 to 750 nm, narrow optical bandgaps of 1.52–1.69 eV, and relatively low‐lying highest occupied molecular orbital (HOMO) energy levels ranging from ?5.24 to ?5.46 eV in their thin films. A high power conversion efficiency of 5.9% under simulated AM 1.5G illumination is achieved for inverted organic solar cells based on a small‐molecule bulk‐heterojunction system consisting of a benzodithiophene‐diketopyrrolopyrrole‐containing oligomer as a donor and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as an acceptor. Transmission electron microscopy and energy‐dispersive X‐ray spectroscopy reveal that interpenetrating and interconnected donor/acceptor domains with pronounced mesoscopic phase segregation are formed within the photoactive binary blends, which is ideal for efficient exciton dissociation and charge transport in the bulk‐heterojunction devices.     

On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

Fullerene‐based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor–acceptor (D–A) interface. Model systems comprised of polythiophene‐based donor and rylene diimide‐based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct‐contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.     

Hierarchical Structures Based on Two‐Dimensional Nanomaterials for Rechargeable Lithium Batteries

Two‐dimensional (2D) nanomaterials (i.e., graphene and its derivatives, transition metal oxides and transition metal dichalcogenides) are receiving a lot attention in energy storage application because of their unprecedented properties and great diversities. However, their re‐stacking or aggregation during the electrode fabrication process has greatly hindered their further developments and applications in rechargeable lithium batteries. Recently, rationally designed hierarchical structures based on 2D nanomaterials have emerged as promising candidates in rechargeable lithium battery applications. Numerous synthetic strategies have been developed to obtain hierarchical structures and high‐performance energy storage devices based on these hierarchical structure have been realized. This review summarizes the synthesis and characteristics of three styles of hierarchical architecture, namely three‐dimensional (3D) porous network nanostructures, hollow nanostructures and self‐supported nanoarrays, presents the representative applications of hierarchical structured nanomaterials as functional materials for lithium ion batteries, lithium‐sulfur batteries and lithium‐oxygen batteries, meanwhile sheds light particularly on the relationship between structure engineering and improved electrochemical performance; and provides the existing challenges and the perspectives for this fast emerging field.     

2D Materials for Large‐Area Flexible Thermoelectric Devices

The rapid development of the concept of the “Internet of Things (IoT)” requires wearable devices with maintenance‐free batteries, and thermoelectric energy conversion based on large‐area flexible materials has attracted much attention. Among large‐area flexible materials, 2D materials, such as graphene and related materials, are promising for thermoelectric applications due to their excellent transport properties and large power factors. In this Review, both single‐crystalline and polycrystalline 2D materials are surveyed using the experimental reports on thermoelectric devices of graphene, black phosphorus, transition metal dichalcogenides, and other 2D materials. In particular, their carrier‐density dependent thermoelectric properties and power factors maximized by Fermi level tuning techniques are focused. The comparison of the relevant performances between 2D materials and commonly used thermoelectric materials reveals the significantly enhanced power factors in 2D materials. Moreover, the current progress in thermoelectric module applications using large‐area 2D material thin films is summarized, which consequently offers great potential for the use of 2D materials in large‐area flexible thermoelectric device applications. Finally, important remaining issues and future perspectives, such as preparation methods, thermal transports, device designs, and promising effects in 2D materials, are discussed.     

What is Killing Organic Photovoltaics: Light‐Induced Crosslinking as a General Degradation Pathway of Organic Conjugated Molecules

In view of a rapid development and increase in efficiency of organic solar cells, reaching their long‐term operational stability represents now one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light‐induced dimerization of the fullerene‐based acceptor materials discovered recently is considered to be one of the main causes for burn‐in degradation of organic solar cells. In this work, it is shown that not only the fullerene derivatives but also different types of conjugated polymers and small molecules undergo similar light‐induced crosslinking regardless of their chemical composition and structure. In the case of conjugated polymers, crosslinking of macromolecules leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Results of this work, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics.     

Tuning Energy Levels without Negatively Affecting Morphology: A Promising Approach to Achieving Optimal Energetic Match and Efficient Nonfullerene Polymer Solar Cells

One advantage of nonfullerene polymer solar cells (PSCs) is that they can yield high open‐circuit voltage (V_OC) despite their relatively low optical bandgaps. To maximize the V_OC of PSCs, it is important to fine‐tune the energy level offset between the donor and acceptor materials, but in a way not negatively affecting the morphology of the donor:acceptor (D:A) blends. Here, an effective material design rationale based on a family of D–A1–D–A2 terthiophene (T3) donor polymers is reported, which allows for the effective tuning of energy levels but without any negative impacts on the morphology of the blend. Based on a T3 donor unit combined with difluorobenzothiadiazole (ffBT) and difluorobenzoxadiazole (ffBX) acceptor units, three donor polymers are developed with highly similar morphological properties. This is particularly surprising considering that the corresponding quaterthiophene polymers based on ffBT and ffBX exhibit dramatic differences in their solubility and morphological properties. With the fine‐tuning of energy levels, the T3 polymers yield nonfullerene PSCs with a high efficiency of 9.0% for one case and with a remarkably low energy loss (0.53 V) for another polymer. This work will facilitate the development of efficient nonfullerene PSCs with optimal energy levels and favorable morphology properties.     

Bis(Naphthalene Imide)diphenylanthrazolines: A New Class of Electron Acceptors for Efficient Nonfullerene Organic Solar Cells and Applicable to Multiple Donor Polymers

Unlike universally applicable fullerene derivatives, current nonfullerene electron acceptors are rarely effective with more than one donor polymer in bulk heterojunction (BHJ) solar cells. A novel class of nonfullerene electron acceptors, bis(naphthalene imide)‐3,6‐diphenyl‐trans‐anthrazolines (BNIDPAs), that is applicable and yields efficient photovoltaic devices with multiple donor polymers, including a thiazolothiazole–dithienosilole copolymer (PSEHTT) and benzodithiophene copolymers (PBDTT‐FTTE and PTB7) is reported. Photovoltaic devices composed of the BNIDPA‐butyloctyl (BO) acceptor with PSEHTT, PBDTT‐FTTE, and PTB7, respectively, have power conversion efficiencies of 3.0%–3.1% with high open‐circuit voltages of ≈1.0 V. In contrast, BHJ devices composed of BNIDPA‐DT acceptor with larger 2‐decyltetradecyl chains and the same donor polymers have substantially reduced bulk electron mobility and reduced photovoltaic efficiencies of 1.3%–1.7%, which highlight the critical role of the size of alkyl chains appended onto nonfullerene electron acceptors. The present results provide a rare example of nonfullerene electron acceptors that are capable of pairing with multiple donor polymers to achieve efficient BHJ solar cells.     

Imidazolium‐Substituted Polythiophenes as Efficient Electron Transport Materials Improving Photovoltaic Performance

In the field of polymer solar cells, improving photovoltaic performance has been the main driver over the past decade. To achieve high power conversion efficiencies, a plethora of new photoactive donor polymers and fullerene derivatives have been developed and blended together in bulk heterojunction active layers. Simultaneously, further optimization of the device architecture is also of major importance. In this respect, we report on the use of specific types of electron transport layers to boost the inherent I–V properties of polymer solar cell devices, resulting in a considerable gain in overall photovoltaic output. Imidazolium‐substituted polythiophenes are introduced as appealing electron transport materials, outperforming the currently available analogous conjugated polyelectrolytes, mainly by an increase in short‐circuit current. The molecular weight of the ionic polythiophenes has been identified as a crucial parameter influencing performance.     

Xylene‐Bridged Phosphaviologen Oligomers and Polymers as High‐Performance Electrode‐Modifiers for Li‐Ion Batteries

Lithium‐ion batteries are one of the most common forms of energy storage devices used in society today. Due to the inherent limitations of conventional Li‐ion batteries, organic materials have surfaced as potentially suitable electrode alternatives with improved performance and sustainability. Viologens and phosphaviologens in particular, are strong electron‐accepting materials with excellent kinetic properties, making them suitable candidates for battery applications. In this paper, new polymeric species of the latter moieties are reported that lead to improved electrode stability and device performance. The performance of the phosphaviologen is further enhanced through the utilization of both redox steps, allowing for good performance proof‐of‐concept hybrid organic/Li‐ion batteries. This opens the potential for more sustainable and improved battery performance for use in current energy applications.