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The structure-property relationships of small molecular acceptors (SMAs) are a key issue in the molecular design of new-generation acceptor materials for further improving the device efficiencies of polymer solar cells (PSCs). Herein, three couples of SMA isomers were synthesized, based on three central fused ring units and two 1,1-dicyanomethylene-3-indanone (IC) isomer electron-withdrawing terminal units with chlorine substitutions in different positions of its benzene ring: Cl-1 with chlorine on the same side as a CO group of IC and Cl-2 with chlorine on the same side as the CN groups of IC. Through systematical investigation, we found that the chlorine substitution position of the terminal groups has a regular and significant influence on the molecular packing and photovoltaic performance of the SMAs. The molecular packing behavior of the SMAs is closely related to and determined by the configuration of their terminal groups, no matter which central fused ring of the SMAs is used. In particular, the Cl-1-based SMAs possess a stronger crystallinity with long range ordering packing in their molecular plane direction, while the more abundant and stereoscopic π-π intermolecular interaction in the Cl-2-based SMAs promotes the molecules to form three-dimensional charge transporting channels and leads to their red-shifted absorption and higher electron mobilities. Therefore, the Cl-2-based PSCs exhibit a higher power conversion efficiency (PCE) compared to that of the Cl-1-based devices, and the best PCE of a Cl-2 SMA-based PSC reached 16.42%. These results highlight the importance of the investigation of intermolecular interactions, packing and the arrangement of the SMAs in the solid-state, which may provide direct insights for exploring the relationship between the molecular structure and property of the photovoltaic materials. Moreover, we envision that if fragments such as end groups or side chains with more diverse molecular interactions are added into the design and the subsequent synthesis of the SMAs, this may be beneficial to promoting molecular π-π accumulation and further improving the molecular order, forming suitable molecular packing and morphology in the resulting blend films, and finally affecting the efficiency of the PSCs. This journal is

In developing organic semiconductor polymers for photovoltaic applications, chlorine substitution has become an effective strategy in replacing fluorine substitution to overcome the drawbacks of low yield and high cost, commonly associated with fluorination. In general, several molecular positions are available for chlorination. To obtain a clear understanding of the impact of chlorine substitution on the intrinsic polymer properties, an investigation of structure-property relationships is necessary. Herein, four donor-acceptor type polymers with the same conjugated backbone and flexible alkyl chains, but with chlorine atoms in different positions, are employed to systematically investigate the effect of the site of chlorination on the optoelectronic properties and photovoltaic performance. Substitution of fluorine by chlorine in the backbone slightly increases open circuit voltage (Voc) and fill factor (FF) of the solar cells but causes a loss of short-circuit current density (Jsc). The introduction of chlorine in the conjugated side chains, however, significantly improves Voc, FF, and power conversion efficiency, benefiting from a lower HOMO energy level, efficient and well-balanced transport properties, and superior nanoscale morphology. This journal is

Building on our previous works that compared the efficacy of terpolymers vs. ternary blends in improving the performance of bulk heterojunction organic solar cells, the final piece of this series of studies focuses on comparing terpolymer and ternary blends constructed with two polymers with structurally similar backbones (monoCNTAZ and FTAZ) yet markedly different open circuit voltage (Voc) values. Terpolymers and ternary blends of five different ratios were studied and the results demonstrate that while the overall performance of both the systems is similar, the ternary blends exhibit higher short circuit current (Jsc) values, while the terpolymers exhibit higher Voc values. Investigation of the charge transfer state using low-energy external quantum efficiency (EQE) indicates that the ternary blends are governed by a parallel-like mechanism, while the terpolymer does not follow this mechanism. The key morphological difference between the systems, as elucidated by resonance soft X-ray scattering (RSoXS), is the slightly smaller size (∼60 nm) of domains in the ternary blends compared to that of the terpolymer (∼80 nm), which may affect exciton harvesting in the terpolymer system and lead to lower Jsc values. In addition, a lower driving force for the formation of charge transfer (CT) state is also likely to contribute to the lower Jsc values in the terpolymer system. All together, the data show that structurally similar (perhaps even miscible) polymers still exhibit key differences in performance when paired in terpolymers vs. ternary blends and allow us to further illuminate the underlying mechanisms of such complex systems.

Cyanation is a common chemical modification strategy to fine‐tune the energy levels and molecular packing of organic semiconductors, especially materials used in organic solar cells (OSCs). Generally, cyanation is used to modify the end groups of high‐performance small‐molecule acceptors (SMAs). However, the cyanation strategy has not been investigated on the central backbone of SMAs, which could introduce stronger intermolecular interaction and enhance the π–π stacking for rapid charge transport. This paper, for the first time, reports a new cyanation strategy on the central benzo‐quinoxaline core and synthesizes two novel A‐DA'D‐A type SMAs, named BQx‐CN and BQx‐2CN, with mono‐ and di‐cyanide groups, respectively. Through tailoring the number of CN groups, the BQx‐CN‐based OSC exhibits the best device performance of 18.8%, which is significantly higher than the non‐cyano BQx‐based one. The reason for the superior performance of BQx‐CN‐based devices can be attributed to the fine‐tuned energy level, stronger packing, and ideal phase segregation, which lead to superior exciton dissociation, faster charge transport, and suppressed recombination, therefore the highest fill factor (FF) and power conversion efficiencies (PCE). The research demonstrates the effectiveness of the cyanation strategy on the central core of SMAs for enhanced molecular packing and better performance of OSCs. [ABSTRACT FROM AUTHOR]

Orienting polymer semiconductors is desirable to optimize device characteristics, provide insight into microstructure, and magnify subtle phase behavior. Here, a combination of uniaxial strain and subsequent heating of the donor–acceptor (DA) polymer PBnDT‐FTAZ is discovered to lead to exceptional optical dichroic ratios of up to 38 (and close to 50 near the polymer's absorption edge). This alignment is achieved due to the existence of a previously undetected thermotropic liquid crystal (LC) mesophase. The LC transition, not discernable through calorimetry, is uncovered through a combination of in situ UV–vis spectroscopy, X‐ray scattering, and dynamic mechanical analysis (DMA). Comparing PBnDT‐FTAZ to the non‐fluorinated PBnDT‐HTAZ and the homo polymer PBnDT, all of which show similar thermal transitions, reveals that exceptional alignment is only found in PBnDT‐FTAZ. This is attributed to the PBnDT‐FTAZ film having two distinct liquid crystal populations, and the polymer templating to a highly aligned, high‐clearing temperature population when heated. The DMA thermal relaxation behavior observed here is also seen in other DA conjugated polymers suggesting that such thermotropic LC mesophases may be common in these materials. These findings demonstrate a polymer semiconductor with remarkable alignment and uncover phase behavior with broad implications for process‐structure‐property relationships in polymer semiconductors. [ABSTRACT FROM AUTHOR]

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.

Owing to the use of cost-effective materials and excellent stability, nonfullerene polymer solar cells (PSCs) have great potential for realizing large-area industrial production. In contrast to fullerene-based devices, non-fullerene PSCs have exhibited a superior photovoltaic performance with up to 12% efficiency and long-term thermal stability. Presently, one of the major factors hindering industrial production is the high sensitivity of the power conversion efficiency (PCE) to thickness variations, which can significantly affect the manufacturing yields and production costs of roll-to-roll processing. Specifically, the device fill factors and PCEs of many high-efficiency nonfullerene PSCs show a significant loss when the thickness of the active layer is over 100 nm. In order to achieve high output capabilities earlier, there is an urgent need to find a processing method to fabricate high-efficiency thick-film nonfullerene PSCs. Controlling the morphology and performance sensitivity in thick-film non-fullerene devices is a great challenge in the field. Here, we present a simple morphology optimization method via thermal annealing to fabricate highly efficient thickness-insensitive non-fullerene PSCs. After this treatment, PBDB-T/IT-M-based nonfullerene PSCs can afford an impressive PCE of up to ∼9.4% at an active layer thickness of 250 nm. In addition, the devices with an active layer thickness of 400 nm still maintain a high efficiency close to 9%. The photovoltaic properties and morphology parameters resolved from hard and soft X-ray scattering clearly indicate that thermal annealing plays a key role in improving the film thickness insensitivity for non-fullerene PSCs.

Conjugated block copolymers provide a pathway to achieve thermally stable nanostructured thin films for organic solar cells. We characterized the structural evolution of poly(3-hexylthiophene)-block-poly(diketopyrrolopyrrole-terthiophene) (P3HT-b-DPPT-T) from solution to nanostructured thin films. Aggregation of the DPPT-T block of P3HT-b-DPPT-T was found in solution by small-angle X-ray scattering with the P3HT block remaining well-solvated. The nanostructure in thin films was determined using a combination of wide and small-angle X-ray scattering techniques as a function of processing conditions. The structure in solution controlled the initial nanostructure in spin-cast thin films, allowing subsequent thermal annealing processes to further improve the ordering. In contrast to the results for thin films, nanostructural ordering was not observed in the bulk samples by small-angle X-ray scattering. These results suggest the importance of controlling solvent induced aggregation in forming nanostructured thin films of conjugated block copolymers.

Flexible electronics have received considerable attention in the past decades due to their promising application in rollable display screens, wearable devices, implantable devices, and other electronic applications. In particular, conjugated polymers are favored for flexible electronics due to their mechanical flexibility and potential for solution‐processed fabrication techniques, such as blade‐coating, roll‐to‐roll printing, and high‐throughput printing allowing for high‐performance transistor devices. Thiophene is the prevailing conjugated unit to construct these conjugated polymers due to its favorable electronic properties. On the other hand, furans are among the few conjugated moieties that are easily derived from bio renewable resources. To promote sustainability, we selectively introduced furan into the conjugated backbone of a high‐mobility polymer scaffold and systematically studied the effect on the microstructure and charge transport. We show that partially and selectively replacing thiophene units with furan can yield nearly comparable performance compared to the all‐thiophene polymer. This strategy offers an improvement in the sustainability of the polymer by incorporating bio‐sourced furan without sacrificing the high‐performance characteristics. Meanwhile, polymers with incorrect or complete furan incorporation show reduced mobilities. This work serves to develop coherent structure–morphology–performance relationships; such knowledge will establish guidelines for the future development of sustainable, furan‐based conjugated materials. [ABSTRACT FROM AUTHOR]

Unfused‐ring acceptors with different electron‐deficient central units, dithieno[3,2‐f:2′,3′‐h]quinoxaline (KS40) and dithieno[3,2‐e:2′,3′‐g]‐2,1,3‐benzoxadiazole (KS41) are synthesized and characterized. When blended with PM6, KS40 with a weaker electron‐accepting quinoxaline exhibits a much higher open‐circuit voltage (VOC) of 0.945 V. As a result, the KS40‐based device achieves a power conversion efficiency (PCE) of 12.2%. PM6:KS41‐based devices, in contrast, afford improved charge mobilities as well as suppressed recombination behavior, which eventually leads to an improvement in the fill factor (72.4%). Consequently, the device is able to achieve a PCE of 11.6%. [ABSTRACT FROM AUTHOR]

Discrepancies in reported values of exciton binding energy (Eb) for organic semiconductors (OSs) necessitate a comprehensive study. Traditionally, Eb is defined as the difference between the transport gap (Et) and the optical gap (Eopt). Here, the Eb values of PBnDT‐TAZ polymer variants are determined using two commonly employed methods: a combination of ultraviolet photoemission spectroscopy and low‐energy inverse photoemission spectroscopy (UPS‐LEIPS) and solid‐state cyclic voltammetry (CV). Eb values obtained by UPS‐LEIPS show low dispersion and no clear correlation with the polymer structure and thedielectric properties. In contrast, CV reveals a larger dispersion (200 meV‐1 eV) and an apparent qualitative Eb‐molecular structure correlation, as the lowest Eb values are observed for oligo‐ethylene glycol side chains. This discrepancy is discussed by examining the implications of the traditional definition of Eb. Additionally, the impact of both intrinsic and extrinsic factors contributing to the derived experimental values of Et is discussed. The differences in intrinsic and extrinsic factors highlight the context‐dependent nature of measurement when drawing global conclusions. Notably, the observed Eb trend derived from CV is not intrinsic to the pure materials but likely linked to electrolyte swelling and associated changes in dielectric environment, suggesting that high‐efficiency single‐material organic photovoltaics with low Eb may be possible via high dielectric materials. [ABSTRACT FROM AUTHOR]

Chemical degradation and morphology failure of ink-processed organic solar cells are now extensively studied. In contrast, the general problem that inks prepared via thermal and mechanical agitation degrade and age rapidly at room temperature has yet to be delineated as a commercialization bottleneck and resolved. This study unveils the intrinsic aging of common polymer:nonfullerene acceptor (NFA) binary inks and the impact of electro-optically active component additives on ink shelf life. As a result, we developed an effective approach to slow down the ink aging by employing an additive (i.e. PCBM variants) with high miscibility with the polymer and NFA. It is inferred that the PCBMs in the inks acts as a co-solvent and slows down the polymer and possibly the NFA pre-aggregation, preventing the formation of large domains in the films. At the same time, the PCBMs dissolved in the polymer-rich phase of the devices can maintain the electron percolations and hence benefit charge creation and collection. The method of introducing a hyper-miscible third component, that is, with concentration above the percolation threshold, to improve ink shelf life and maintain the percolation is delineated for the first time. It represents a synergistic approach to promote the scale-up of ink-processed organic solar cells. [ABSTRACT FROM AUTHOR]