Search

Recent advances in light‐responsive materials enabled the development of devices that can wirelessly activate tissue with light. Here it is shown that solution‐processed organic heterojunctions can stimulate the activity of primary neurons at low intensities of light via photochemical reactions. The p‐type semiconducting polymer PDCBT and the n‐type semiconducting small molecule ITIC (a non‐fullerene acceptor) are coated on glass supports, forming a p–n junction with high photosensitivity. Patch clamp measurements show that low‐intensity white light is converted into a cue that triggers action potentials in primary cortical neurons. The study shows that neat organic semiconducting p–n bilayers can exchange photogenerated charges with oxygen and other chemical compounds in cell culture conditions. Through several controlled experimental conditions, photo‐capacitive, photo‐thermal, and direct hydrogen peroxide effects on neural function are excluded, with photochemical delivery being the possible mechanism. The profound advantages of low‐intensity photo‐chemical intervention with neuron electrophysiology pave the way for developing wireless light‐based therapy based on emerging organic semiconductors. [ABSTRACT FROM AUTHOR]

Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 °C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells. Non-fullerene-based organic solar cells generally suffer from poor thermal stability and especially in case of devices with thick active layers. Here, the authors report hexanary blends based on multi-component acceptor mixtures with a device efficiency of 17.6% and thermally stability for 23 days at 130 °C. [ABSTRACT FROM AUTHOR]

Efficiencies of organic solar cells have practically doubled since the development of non‐fullerene acceptors (NFAs). However, generic chemical design rules for donor‐NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor–acceptor interface. It is shown that an acceptor–donor–acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is suggested that the molecular quadrupole moment of ≈75 Debye Å balances the losses in the open circuit voltage and gains in charge generation efficiency. [ABSTRACT FROM AUTHOR]

While photovoltaic blends based on non‐fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene‐based acceptors that readily co‐crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine‐grained ternary blend with nanometer‐sized domains that do not coarsen due to a high Tg ≈ 200 °C. As a result, annealing at temperatures of up to 170 °C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high‐temperature processing protocols, which are needed for upscaling and high‐throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 °C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability. [ABSTRACT FROM AUTHOR]

Fullerenes have formed an integral part of high performance organic solar cells over the last 20 years, however their inherent limitations in terms of synthetic flexibility, cost and stability have acted as a motivation to develop replacements; the so-called non-fullerene electron acceptors. A rapid evolution of such materials has taken place over the last few years, yielding a number of promising candidates that can exceed the device performance of fullerenes and provide opportunities to improve upon the stability and processability of organic solar cells. In this review we explore the structure–property relationships of a library of non-fullerene acceptors, highlighting the important chemical modifications that have led to progress in the field and provide an outlook for future innovations in electron acceptors for use in organic photovoltaics. [ABSTRACT FROM AUTHOR]

Abstract: Organic bulk heterojunction solar cells based on small molecule acceptors have recently seen a rapid rise in the power conversion efficiency with values exceeding 13%. This impressive achievement has been obtained by simultaneous reduction of voltage and charge recombination losses within this class of materials as compared to fullerene‐based solar cells. In this contribution, the authors review the current understanding of the relevant photophysical processes in highly efficient nonfullerene acceptor (NFA) small molecules. Charge generation, recombination, and charge transport is discussed in comparison to fullerene‐based composites. Finally, the authors review the superior light and thermal stability of nonfullerene small molecule acceptor based solar cells, and highlight the importance of NFA‐based composites that enable devices without early performance loss, thus resembling so‐called burn‐in free devices. [ABSTRACT FROM AUTHOR]

Abstract: Nonfullerene acceptors (NFAs) in blends with highly crystalline donor polymers have been shown to yield particularly high device voltage outputs, but typically more modest quantum yields for photocurrent generation as well as often lower fill factors (FF). In this study, we employ transient optical and optoelectronic analysis to elucidate the factors determining device photocurrent and FF in blends of the highly crystalline donor polymer PffBT4T‐2OD with the promising NFA FBR or the more widely studied fullerene acceptor PC71BM. Geminate recombination losses, as measured by ultrafast transient absorption spectroscopy, are observed to be significantly higher for PffBT4T‐2OD:FBR blends. This is assigned to the smaller LUMO‐LUMO offset of the PffBT4T‐2OD:FBR blends relative to PffBT4T‐2OD:PC71BM, resulting in the lower photocurrent generation efficiency obtained with FBR. Employing time delayed charge extraction measurements, these geminate recombination losses are observed to be field dependent, resulting in the lower FF observed with PffBT4T‐2OD:FBR devices. These data therefore provide a detailed understanding of the impact of acceptor design, and particularly acceptor energetics, on organic solar cell performance. Our study concludes with a discussion of the implications of these results for the design of NFAs in organic solar cells. [ABSTRACT FROM AUTHOR]

Organic solar cells that are free of burn-in, the commonly observed rapid performance loss under light, are presented. The solar cells are based on poly(3-hexylthiophene) (P3HT) with varying molecular weights and a nonfullerene acceptor (rhodanine-benzothiadiazole-coupled indacenodithiophene, IDTBR) and are fabricated in air. P3HT:IDTBR solar cells light-soaked over the course of 2000 h lose about 5% of power conversion efficiency (PCE), in stark contrast to [6,6]-Phenyl C61 butyric acid methyl ester (PCBM)-based solar cells whose PCE shows a burn-in that extends over several hundreds of hours and levels off at a loss of ≈34%. Replacing PCBM with IDTBR prevents short-circuit current losses due to fullerene dimerization and inhibits disorder-induced open-circuit voltage losses, indicating a very robust device operation that is insensitive to defect states. Small losses in fill factor over time are proposed to originate from polymer or interface defects. Finally, the combination of enhanced efficiency and stability in P3HT:IDTBR increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene-based acceptor instead. [ABSTRACT FROM AUTHOR]

The photovoltaic performance of blends of a series of 4,4′-Difluoro-4-bora-3a,4a-diaza-s-indacenes)-based (BODIPY) conjugated polymers donors with fullerene electron acceptors is investigated. Despite the high Voc values observed, solar cell device yields relatively low power conversion efficiencies. Our study takes into account the materials’ structure-property relationship, light harvesting capabilities, charge transport, collection properties and morphological characteristics to elucidate factors affecting the photovoltaic performance in this class of polymers. We show that elimination of low molecular weight species and suitable electrodes for hole collection can be used to overcome some of the observed limitations on photovoltaic performance. [ABSTRACT FROM AUTHOR]

Abstract: 4,7-Bis(2,3-dihydrothieno [3,4-b] [1,4] dioxin-5-yl)-2-dodecyl-2H-benzo [1,2,3] triazole (BEBT) was polymerized both electrochemically (ePBEBT) and chemically (cPBEBT). Since chemical polymerization enabled a soluble polymer in common organic solvents, a single layer electrochromic device of ePBEBT was constructed. The polymer cPBEBT was also used in bulk heterojunction (BHJ) solar cells as the active layer in combination with a soluble fullerene derivative, 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 PCBM. [Copyright &y& Elsevier]

The 'phonon-glass electron-crystal' concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly 'phonon glasses'. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being 'electron crystals'. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic 'PGEC' thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S cm−1 and an ultralow thermal conductivity of <0.1 Wm−1K−1, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use 'arm-shaped' double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials. Achieved high thermoelectric figure of merit (ZT) in organic thermoelectric materials remains a challenge due to their low packing order and poor host/dopant miscibility. Here, the authors report side chain-engineered n-doped fullerene derivatives with record ZT >0.3 for organic thermoelectrics. [ABSTRACT FROM AUTHOR]

Small‐molecule‐based non‐fullerene acceptors (NFAs) are emerging as a new field in organic photovoltaics, due to their structural versatility, the tunability of their energy levels, and their ease of synthesis. High‐efficiency polymer donors have been tested with these non‐fullerene acceptors in order to further boost the efficiency of organic solar cells. Most of the polymer:fullerene systems are optimized with solvent additives for high efficiency, while little attention has been paid to NFA‐based solar cells so far. In this report, the effect of the most common additive, 1,8‐diiodooctane (DIO), on PTB7‐Th:PC71BM solar cells is investigated and it is compared with non‐fullerene acceptor 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno‐[1,2‐b:5,6b′]di‐thiophene (ITIC) devices. It is interesting that the high boiling solvent additive does have a negative impact on the power conversion efficiency when PTB7‐Th is blended with ITIC acceptor. The solar cell devices are studied in terms of their optical, photophysical, and morphological properties and find out that PTB7‐Th:ITIC devices with DIO results in coarser domains, reduced absorption strength, and slightly lower mobility, while DIO improves the absorption strength of the PTB7‐Th:PC71BM blend film and increase the aggregation of PC71BM in the blend, resulting in higher fill factor and Jsc. PTB7‐Th:PC71BM and PTB7‐Th:ITIC blends are selected as model systems to analyze the effect of additive in fullerene and non‐fullerene acceptor based devices. A diametrically opposite situation occurs in ITIC‐based devices compared to fullerene devices upon addition of 1,8‐diiodooctane. The reason for the drastic performance drop is the formation of a well‐woven pathway, which increases phase separation of blend matrix and surface roughness. [ABSTRACT FROM AUTHOR]