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Side‐chain symmetry‐breaking strategy plays an important role in developing photovoltaic materials for high‐efficiency all‐small‐molecule organic solar cells (ASM OSCs). However, the power conversion efficiencies (PCEs) of ASM OSCs still lag behind their polymer‐based counterparts, which can be attributed to the difficulties in achieving favorable morphology. Herein, two asymmetric porphyrin‐based donors named DAPor‐DPP and DDPor‐DPP were synthesized, presenting stronger intermolecular interaction and closer molecular stacking compared to the symmetric ZnP‐TEH. The DAPor‐DPP:6TIC blend afforded a favorable morphology with nanoscale phase separation and more ordered molecular packing, thus achieving more efficient charge transportation and suppressed charge recombination. Consequently, the DAPor‐DPP:6TIC‐based device exhibited superior photovoltaic parameters, yielding a champion PCE of 16.62% higher than that of the DDPor‐DPP‐based device (14.96%). To our knowledge, 16.62% can be ranked as one of the highest PCE values among the binary ASM OSC filed. This work provides a prospective approach to address the challenge of ASM OSCs in improving film morphology and further achieving high efficiency via side‐chain symmetry‐breaking strategy, exhibiting great potential in constructing efficient ASM OSCs. [ABSTRACT FROM AUTHOR]

The conjugated small‐molecule materials of organic solar cells have always played a crucial role in light‐harvesting, charge transport, morphology optimization, and the attainment of efficient devices. The advancement of novel materials and the understanding of underlying molecular design rules serve as the driving force for furthering efficient and stable photovoltaic devices. Among a variety of design principles, the symmetry‐breaking strategy, which is well developed in 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:5,6‐b']dithiophene (ITIC)‐series acceptors, recently demonstrates great potential in small‐molecule acceptors and donors for realizing high power conversion efficiency. In this review, in order to give a deep insight on asymmetric strategy, the small‐molecule acceptors and donors are systematically summarized with asymmetric structure to elucidate structure‐performance relationship, molecular packing behaviors, and morphology evolution. Not only the delicate balance between open circuit voltage and short‐circuit current density, but also the reductions in charge recombination and non‐radiative recombination are considered to play the key points in improving the photovoltaic performance when the asymmetric molecule is used as host or guest materials. Finally, concise challenges and outlooks are provided for the future development and application of asymmetric molecules and symmetry‐breaking strategies. [ABSTRACT FROM AUTHOR]

Abstract Side‐chain symmetry‐breaking strategy plays an important role in developing photovoltaic materials for high‐efficiency all‐small‐molecule organic solar cells (ASM OSCs). However, the power conversion efficiencies (PCEs) of ASM OSCs still lag behind their polymer‐based counterparts, which can be attributed to the difficulties in achieving favorable morphology. Herein, two asymmetric porphyrin‐based donors named DAPor‐DPP and DDPor‐DPP were synthesized, presenting stronger intermolecular interaction and closer molecular stacking compared to the symmetric ZnP‐TEH. The DAPor‐DPP:6TIC blend afforded a favorable morphology with nanoscale phase separation and more ordered molecular packing, thus achieving more efficient charge transportation and suppressed charge recombination. Consequently, the DAPor‐DPP:6TIC‐based device exhibited superior photovoltaic parameters, yielding a champion PCE of 16.62% higher than that of the DDPor‐DPP‐based device (14.96%). To our knowledge, 16.62% can be ranked as one of the highest PCE values among the binary ASM OSC filed. This work provides a prospective approach to address the challenge of ASM OSCs in improving film morphology and further achieving high efficiency via side‐chain symmetry‐breaking strategy, exhibiting great potential in constructing efficient ASM OSCs.

Optimizing the alkyl chains in a terminal group is one of the most efficient strategies to enhance the power conversion efficiency (PCE) of all small molecule organic solar cells (ASM‐OSCs). Oligothiophene, as an easily synthesized small molecule (SM) donor with an explicit structure, can influence intermolecular packing, planarity, and crystallinity through optimization of the alkyl chain in the terminal group. Herein, two SMs, Tz6T‐P and Tz6T‐iP, are synthesized using the n‐propyl and isopropyl isomers, respectively, as alkyl chains in the terminal group. Compared with the linear chain (n‐propyl) of Tz6T‐P, the rigid branch chain (isopropyl) in Tz6T‐iP exhibits an almost complanate configuration in the ground state and higher crystallinity. Consequently, Tz6T‐iP features lower levels of bimolecular recombination and trap‐assisted recombination, higher ability for exciton dissociation and charge collection, and longer carrier lifetimes. Tz6T‐iP achieves a PCE of 15.7% for oligothiophene‐based ASM‐OSCs. These results provide innovative strategies, demonstrating that branching the terminal alkyl chain can adjust planarity, crystallinity, and separation morphology for the structural design of photovoltaic materials. [ABSTRACT FROM AUTHOR]

All‐small‐molecule organic solar cells (ASM‐OSCs) are challenging for their inadequate efficiency and device stability due to their more susceptive morphology. Herein, a family of isomeric small molecule donors (SMDs) is synthesized based on the benzodithiophene–terthiophene core with linear, 1st carbon, and 2nd carbon position branched butyl‐based rhodanine for ASM‐OSCs, respectively. The single crystal of thiophene‐substituted model T‐s‐Bu forms a more compact intermolecular packing with herringbone structure than slip‐layered packing‐based T‐n‐Bu and T‐i‐Bu. SM‐i‐Bu and SM‐s‐Bu show slightly blue‐shifted absorption and deepened HOMO levels in the neat film compared to SM‐n‐Bu. SM‐s‐Bu:BO‐4Cl blend films have distinct face‐on packing orientations and suitable fibrous phase separation along with more apparent microcrystals. Finally, SM‐s‐Bu:BO‐4Cl‐based device yields an improved power conversion efficiency of 16.06% compared to 15.12% and 8.22% for SM‐n‐Bu:BO‐4Cl and SM‐i‐Bu:BO‐4Cl, which is one of the top‐ranked results for BTR‐series SMDs in binary ASM‐OSCs. More importantly, the excellent storage stability with a T80 lifetime of over 1700 h and decent thermal stability is realized in SM‐s‐Bu:BO‐4Cl. This work highlights that the isomeric terminal alkyl with a branching point directly connected to the backbone for SMDs is a promising strategy for improving the crystal packing and film morphology and achieving highly efficient and stable ASM‐OSCs. [ABSTRACT FROM AUTHOR]

All small molecule organic solar cells (ASM‐OSCs) have numerous advantages but lower power conversion efficiencies (PCEs) than their polymer equivalents, which is largely due to the suboptimal nanoscale network structure in a bulk heterojunction (BHJ). Herein, new small molecule donors with symmetric/asymmetric hybrid cyclopentyl‐hexyl side chains are designed, accounting for manipulated intermolecular interactions and BHJ morphology. Theoretical and experimental results reveal that the asymmetric cyclopentyl‐hexyl side chains modification has a significant influence on potential energy surface and intermolecular interaction that can ensure preferable molecular assembly and regulate the D/A interfacial energetics, thus boosting the exciton dissociation and charge transport when pairing with a wide‐used acceptor L8‐BO. Concurrently, a nanoscale bicontinuous interpenetrating network with optimal domain size can be fully evolved in the BHJ layer. As a consequence, the As‐TCp‐based binary device achieves a superior PCE of 16.46% in comparison to that of the controlled symmetric counterparts S‐BF (14.92%) and A‐TCp (15.77%), and ranks one of best performance among ASM‐OSCs. This study demonstrates that precise manipulation of the cyclo‐alkyl chain in combination with the asymmetric 2D side chain strategy is an effective synergistic approach to control intermolecular interaction and nanoscale bicontinuous phase separation for achieving high‐performance ASM‐OSCs. [ABSTRACT FROM AUTHOR]

• A novel electron-donating building unit "PCz" was synthesized. • A series of 3D "dual-blade-propeller" SMDs were synthesized based on "PCz". • Thecompound PCz(3TDPP) 2 achieved a narrow electrochemical bandgap of only 1.37 eV. • PCz(3TDPP) 2 :PC 71 BM-based device produced a PCE of 5.31 % and a J sc of 18.75 mA cm-2. • These studies provide important references for the development of new PV materials. The recent development of organic photovoltaics provides a promising solution to the challenge of global energy demand. Obviously, the design of novel active layer molecules is crucial. Furthermore, the relationship between photovoltaic performance and molecular structure regulation has become more important and deserves further research. Here, a series of 3D "dual-blade-propeller" small molecule donors (SMDs) were designed and synthesized based on the novel "PCz" and diketopyrrolopyrrole (DPP) units. A detailed study was conducted on the ground state geometry and orbital energy levels of molecules through density functional theory (DFT) calculations. It is worth mentioning that the π-bridge regulation strategy has achieved remarkable results, with the HOMO energy level of thiophene oligomer-bridged compound PCz(3TDPP) 2 reaching -5.05 eV and a narrow bandgap of only 1.37 eV. Considering the basic requirements for solution-processed photovoltaic devices, alkyl chains are introduced into π-bridged thiophene oligomers to improve the solubility and film-forming properties of the materials. The PCz(3TDPP) 2 :PC 71 BM-based device obtained the short-circuit current density (J sc) of 18.75 mA cm-2, which is one of the highest currents in reported thiophene oligomer-bridged SMDs. Both experimental and theoretical studies have revealed the important effects of molecular design and π-bridge regulation strategy on photovoltaic performance. These studies provide important references for the development of new photovoltaic materials with novel configurations. A novel 3D electron-donating building block "PCz" was designed and synthesized based on 2,7-substituted carbazole, and three novel A-D-A typed SMDs were synthesized using diketopyrrole (DPP) as the terminal electron-withdrawing units. The synergistic effect of thiophene-oligomer and DPP leads to the formation of a 3D "dual-blade-propeller" configuration, prolonging molecular conjugation, promoting ICT processes, and effectively controlling excessive aggregation between molecules. Consequently, this unique molecular configuration and strong intermolecular π-π interactions give the conjugated system special photovoltaic properties. By comparing experimental results, the compound PCz(3TDPP) 2 among them has the strongest photon absorption. The PCz(3TDPP) 2 :PC 71 BM-based device produced a PCE of 5.31 % and a J sc of 18.75 mA cm-2, which is one of the highest currents in reported thiophene oligomer-bridged SMDs. These results will guide the molecular design of carbazole derivatives and optimize the molecular structure of SMDs. These studies provide important references for the development of new photovoltaic materials with novel configurations. [Display omitted] [ABSTRACT FROM AUTHOR]

Organic solar cells are famous for their cheap solution processing. Their industrialization needs fast designing of efficient materials. For this purpose, testing of large number of materials is necessary. Machine learning is a better option due to cheaper prediction of power conversion efficiencies. In the present work, machine learning was used to predict power conversion efficiencies. Experimental data were collected from the literature to feed the machine learning models. A detailed data visualization analysis was performed to study the trends of the dataset. The relationship between descriptors and power conversion efficiency was quantitatively determined by Pearson correlations. The importance of features was also determined using feature importance analysis. More than 10 machine learning models were tried to find better models. Only the two best models (random forest regressor and bagging regressor) were selected for further analysis. The prediction ability of these models was high. The coefficient of determination (R2) values for the random forest regressor and bagging regressor models were 0.892 and 0.887, respectively. The Shapley additive explanation (SHAP) method was used to identify the impact of descriptors on the output of models. [ABSTRACT FROM AUTHOR]

Intramolecular Cl−S non‐covalent interaction is introduced to modify molecular backbone of a benzodithiophene terthiophene rhodamine (BTR) benchmark structure, helping planarize and rigidify the molecular framework for improving charge transport. Theoretical simulations and temperature‐variable NMR experiments clearly validate the existence of Cl−S non‐covalent interaction in two designed chlorinated donors and explain its important role in enhancing planarity and rigidity of the molecules for enhancing their crystallinity. The asymmetric isomerization of side‐chains further optimizes the molecular orientation and surface energy to strike a balance between its crystallinity and miscibility. This carefully manipulated molecular design helps result in increased carrier mobility and suppressed charge recombination to obtain simultaneously enhanced short‐circuit current (Jsc) and fill factor (FF) and a very high efficiency of 15.73 % in binary all‐small‐molecule organic solar cells. [ABSTRACT FROM AUTHOR]

At present, high-performance small molecule donors for organic solar cells (OSCs) are almost entirely composed of the electron donating core benzodithiophene (BDT), which has resulted in a lack of material diversity and further improvement. In this article, we constructed an A-π-A′-π-A conjugated structure using electron withdrawing group benzobisthiazole (BBT) core and rhodanine end group, and synthesized two small molecule donors BBTSM-3 and BBTSM-4. Under the dual electron withdrawing effects of BBT and rhodanine, the HOMO energy levels of BBTSM-3 and BBTSM-4 reached extremely low levels (−5.60 eV and −5.64 eV), but still matched well with the acceptor Y6. In addition, through reasonable side chain regulation, BBTSM-3 exhibited stronger crystallinity and weaker aggregation at the same time. This provides a reference point for the use of the electron withdrawing core and the regulation of side chains among small molecule donors. Benzobisthiazole and rhodanine construct small molecule donors with A-π-A′-π-A molecular charge transport system and really low HOMO energy levels. [Display omitted] • Benzobisthiazole and rhodanine were used to synthesize small molecule donors with A-π-A′-π-A system. • BBTSM reached really low HOMO energy levels, while still exhibited good exciton dissociation with Y6. • Stronger crystallinity and weaker aggregation can be achieved simultaneously by regulating the side chains. [ABSTRACT FROM AUTHOR]

The power conversion efficiencies (PCEs) of non-fullerence organic solar cells (OSCs) have exceeding 20 %, largely attributable to the availability of Y-series acceptor materials. Most current high-efficiency OSCs utilize polymer donors, in comparison with polymer donors, small molecule (SM) donor materials offer distinct advantages such as facile synthesis, high yields, and minimal batch-related issues, thereby facilitating the commercialization of cost-effective OSCs. Currently, the PCE of OSCs employing SM donors have surpassed 20 %. This review elucidates significant recent advancements of SM donors, focusing particularly on the benzodithiophene (BDT) unit derivatives, and investigates how the chemical structure of SM donors impacts molecular conformation and stacking patterns. Additionally, we explore criteria for donor-acceptor materials compatibility and methods for morphological control. Finally, we outline persistent challenges and provide insights into the future development of small molecule donor-based OSCs. The PCE of OSCs employing SM donors have surpassed 20 %. This review elucidates significant recent advancements of SM donors, focusing particularly on the benzodithiophene (BDT) unit derivatives, and investigates how the chemical structure of SM donors impacts molecular conformation and stacking patterns. Additionally, we explore criteria for donor-acceptor materials compatibility and methods for morphological control. Finally, we outline persistent challenges and provide insights into the future development of small molecule donor-based OSCs. [Display omitted] • The review outlines the new advancements of SM donor materials based on the benzodithiophene (BDT) center core. • The review categorize these structures base on BDT unit, underscoring their potential in enhancing the performance of OSCs. • The review outline persistent challenges and provide insights into the future development of SM donor-based OSCs. [ABSTRACT FROM AUTHOR]

Organic solar cells are famous for their cheap solution processing. Their industrialization needs fast designing of efficient materials. For this purpose, testing of large number of materials is necessary. Machine learning is a better option due to cheaper prediction of power conversion efficiencies. In the present work, machine learning was used to predict power conversion efficiencies. Experimental data were collected from the literature to feed the machine learning models. A detailed data visualization analysis was performed to study the trends of the dataset. The relationship between descriptors and power conversion efficiency was quantitatively determined by Pearson correlations. The importance of features was also determined using feature importance analysis. More than 10 machine learning models were tried to find better models. Only the two best models (random forest regressor and bagging regressor) were selected for further analysis. The prediction ability of these models was high. The coefficient of determination (R2) values for the random forest regressor and bagging regressor models were 0.892 and 0.887, respectively. The Shapley additive explanation (SHAP) method was used to identify the impact of descriptors on the output of models.

High power conversion efficiency (PCE) organic solar cells (OSC) with planar DII‐A‐DI‐A‐DII structured organic molecule with alkylthiophene‐bezodithiophene (SBDT) a midway donor unit DI with varied number of fluorine atoms like benzoselenadiazole (SBTSe and SFBTSe) a acceptor unit and alkyl‐bithiophene a end group and donor unit DII, SFBTSe and SBTSe, were designed and synthesized for the appliance as donor moieties in OSC. It showed an intense absorption in visible expanse (582, 577 nm) and frontier orbital levels of HOMO (−5.49 and−5.50 eV), and LUMO level of (−3.57 and−3.59 eV), for SBTSe and SFBTSe respectively. The acceptor units were assigned using the results obtained from experiments and DFT/TD‐DFT simulations. This study reveals that all the acceptors are not identical in influencing the energy levels. Besides acceptors, molecular framework also plays a significant role in this regard. In addition, we have found that the acceptor's performance depends on its electron withdrawing ability. This manuscript provides a better understanding about the acceptor's property relationship and vital insights on the design of novel terminal acceptor units with greater number of hetero‐functionalities to developing the PCE of the OSC. [ABSTRACT FROM AUTHOR]

Relocating the Se-containing alkyl side chain from the para- to meta- position improves the efficiency of charge generation, suppress the trap-assisted charge recombination, enhances the charge mobilities, promotes the D-A miscibility, and yields favourable morphology. Therefore, T2:BTP-eC9 based devices afford a impressive PCE of 14.3 % and a J SC of 26.73 mA/cm2. [Display omitted] • Two new selenium alkyl chain-containing small-molecule donors T1 and T2 were synthesized for all small molecule solar cells. • The D-A miscibility is tuneable through the side chain engineering, and ultimately modulate the PCE of solar cells. • The T2:BTP-eC9 based device delivers a remarkable J SC of 26.73 mA/cm2. Substituting sulfur with selenium in photoactive materials emerges as a promising strategy to enhance the performance of organic solar cells (OSCs). Typically, selenophene was incorporated into the conjugated backbone to replace thiophene. Conversely, the selenium-containing alkyl chain has garnered less attention in the context of OSCs. In this study, we fine-tune the active layer morphology by shifting the selenium-containing side chain from the para -position (T1) to the meta -position (T2). Notably, in conjunction with the nonfullerene acceptor BTP-eC9 , the T2:BTP-eC9 blend demonstrates a significantly higher power conversion efficiency (PCE) compared to the T1:BTP-eC9 blend (14.30 % vs. 10.63 %). Analysis using Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) reveals that the T2:BTP-eC9 blend exhibits a smoother surface and larger domain size, resulting in enhanced exciton dissociation efficiency (96.6 % for T2:BTP-eC9 vs. 90.9 % for T1:BTP-eC9). Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) indicates that the T1:BTP-eC9 blend is less textured than the T2:BTP-eC9 blend, and space-charge limited current tests confirm lower and unbalanced hole/electron mobility in the T1:BTP-eC9 blend compared to T2:BTP-eC9. Charge recombination dynamics and donor–acceptor miscibility analysis further support these findings. The T2:BTP-eC9 OSCs demonstrate an impressive J SC of 26.73 mA/cm2, ranking among the reported to date for all small molecule OSCs. [ABSTRACT FROM AUTHOR]

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has increased dramatically, making a big step toward the industrial application of OSCs. Among numerous OSCs, benzodithiophene (BDT)-based OSCs stand out in achieving efficient PCE. Notably, single-junction OSCs using BDT-based polymers as donor materials have completed a PCE of over 19%, indicating a dramatic potential for preparing high-performance large-scale OSCs. This paper reviews the recent progress of OSCs based on BDT polymer donor materials (PDMs). The development of BDT-based OSCs is concisely summarized. Meanwhile, the relationship between the structure of PDMs and the performance of OSCs is further described in this review. Besides, the development and prospect of single junction OSCs are also discussed., (© 2023 Wiley-VCH GmbH.)

Significance: Hydrogen sulfide (H2S) is an important biological signaling molecule involved in many physiological processes. These diverse roles have led researchers to develop contemporary methods to deliver H2S under physiologically relevant conditions and in response to various stimuli. Recent Advances: Different small-molecule donors have been developed that release H2S under various conditions. Key examples include donors activated in response to hydrolysis, to endogenous species, such as thiols, reactive oxygen species, and enzymes, and to external stimuli, such as photoactivation and bio-orthogonal chemistry. In addition, an alternative approach to release H2S has utilized the catalyzed hydrolysis of carbonyl sulfide (COS) by carbonic anhydrase to generate libraries of activatable COS-based H2S donors. Critical Issues: Small-molecule H2S donors provide important research and pharmacological tools to perturb H2S levels. Key needs, both in the development and in the use of such donors, include access to new donors that respond to specific stimuli as well as donors with well-defined control compounds that allow for clear delineation of the impact of H2S delivery from other donor byproducts. Future Directions: The abundance of reported small-molecule H2S donors provides biologists and physiologists with a chemical toolbox to ask key biological questions and to develop H2S-related therapeutic interventions. Further investigation into different releasing efficiencies in biological contexts and a clear understanding of biological responses to donors that release H2S gradually (e.g., hours to days) versus donors that generate H2S quickly (e.g., seconds to minutes) is needed. [ABSTRACT FROM AUTHOR]

Abstract: A growing number of recent studies have demonstrated the substantial impact of the alkyl side chains on the device performance of organic semiconductors. However, detailed investigation of the effect of side‐chain engineering on the blend morphology and performance of ternary organic solar cells (OSCs) has not yet been undertaken. In this study, the performance of ternary OSCs is investigated in a given poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)):[6,6]‐phenyl‐C71‐butyric acid methyl ester (PTB7‐Th:PC71BM) host set by introducing various small molecule donors (SMDs) with different terminal side‐chain lengths. As expected, the performance of binary OSCs with SMDs depends greatly on the side‐chain length. In contrast, it is observed that all SMD‐based ternary OSCs exhibit almost identical and high power‐conversion efficiencies of 12.0–12.2%. This minor performance variation is attributed to good molecular compatibility between the two donor components, as evidenced by in‐depth electrical and morphological investigations. These results highlight that the alloy‐like structure formed due to the high compatibility of the donor molecules has a more significant effect on the overall performance than the side‐chain length, offering a new guideline for pairing donor components for achieving high‐performance ternary OSCs. [ABSTRACT FROM AUTHOR]

Two new acceptor–π(donor)–donor–π(donor)-acceptor small molecules SM1 and SM2 with benzodithiophene (BDT) donor core unit linked via phenothiazine (PTZ) donor units with 1,3–indanedione and malononitrile end capping units, respectively, have been designed and synthesized and used as electron donor along with PC 71 BM as acceptor for solution-processed bulk heterojunction organic solar cells. The influence of these end-capping groups on their thermal, optical, electrochemical properties and photovoltaic performance was investigated. After the optimization of the donor to acceptor weight ratio and solvent vapor annealing, the organic solar cells based on SM1 :PC 71 BM and SM2 :PC 71 BM active layers showed the power conversion efficiency (PCE) of 6.20% and 7.45%, respectively. Hence, the end-capping acceptor units for the two small molecules revealed a significant influence on the PCE of the organic solar cells, which resulted from differences in film absorption, molecular packing and charge transport properties. [ABSTRACT FROM AUTHOR]

In organic solar cells (OSCs), it is very important to investigate the relationship between the structure and performance of materials by rational design of small molecule donors (SMDs). In this work, we use thiazole replace a thiophene unit in the π bridge, which could strengthen the connection between π bridges and end groups (EGs). Due to the N–S non-covalent interaction between EG and thiazole unit, the negative charge region of SMDs is expanded, the electron-withdrawing ability of EGs is also enhanced, and the corresponding molecular framework is twisted into a unique linear type. This can effectively enhance intermolecular interaction, promote molecular crystallization, and help to improve hole mobility of related SMDs. This work enriches design strategy that strengthens the interaction between π bridges and EGs, and provides a reference for studying the relationship between molecular structure and crystallinity. [Display omitted] • Thiophene π bridge in the small molecule donor is replaced by thiazole. • N–S interaction leads to linear conformation of conjugated skeleton. • Thiazole unit expand the negative electron distribution region of end groups. • N–S interaction improves hole mobility of small molecular donor. [ABSTRACT FROM AUTHOR]

Well-defined small molecule (SM) donors can be used as alternatives to π-conjugated polymers in bulk-heterojunction (BHJ) solar cells with fullerene acceptors (e.g., PC61/71BM). Taking advantage of their synthetic tunability, combinations of various donor and acceptor motifs can lead to a wide range of optical, electronic, and self-assembling properties that, in turn, may impact material performance in BHJ solar cells. In this report, it is shown that changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2- b:4,5- b′]dithiophene-6,7-difluoroquinoxaline SM donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in BHJ solar cells. In these systems ( SM1-3), it is found that 6,7-difluoroquinoxaline ([2F]Q) motifs directly appended to the central benzo[1,2- b:4,5- b′]dithiophene (BDT) unit yield a lower-bandgap analogue ( SM1) with favorable molecular packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%. 1H-1H DQ-SQ NMR analyses indicate that SM1 and its counterpart with [2F]Q motifs substituted as end-group SM3 possess distinct self-assembly patterns, correlating with the significant charge transport and BHJ device efficiency differences observed for the two analogous SM donors (avg. 6.3% vs 2.0%, respectively). [ABSTRACT FROM AUTHOR]

Liquid-crystal small molecule donor (LC-SMD) is a new type organic semiconductor, which is attractive not only for the easy synthesis and purification, well-defined chemical structures, etc., but also for the LC state that makes the crystallinity and aggregation state of molecules adjustable. Here, one new LC-SMD (a-BTR-H4) is synthesized with 1D alkoxyl and 2D thiophene-alkylthiol side-chained benzo[1,2-b:4,5-b']dithiophene core, trithiophene π-bridge, and 3-(2-ethylhexyl) rhodanine end group. a-BTR-H4 shows low LC transition temperature, 117 °C, however, counterpart material (a-BTR-H5) with the same main structure but 3-ethyl rhodanine terminal group does not show LC properties. Although a-BTR-H4/H5 show similar Ultraviolet-visible absorption spectrum and energy levels, a-BTR-H4 affords relatively high photovoltaic performances due to favorable blend morphology produced by the consistent annealing temperature of Y6-based accepters and liquid crystal temperature of donors. Preliminary results indicate that a-BTR-H4 gains a power conversion efficiency (PCE) of 11.36% for Y6-based devices, which is ascribed to better light harvest as well as balanced carrier generation and transport, while a-BTR-H5 obtains 7.57% PCE. Therefore, some materials with unique nematic LC phase have great application potential in organic electronics, and further work to utilize a-BTR-H4 for high-performance device is underway., (© 2022 Wiley-VCH GmbH.)

Active layer material plays a critical role in promoting the performance of an organic solar cell (OSC). Small-molecule (SM) materials have the merits of well-defined chemical structures, few batch-to-batch variations, facile synthesis and purification procedures, and easily tuned properties. SM-donor and non-fullerene acceptor (NFA) innovations have recently produced all-small-molecule (ASM) devices with power conversion efficiencies that exceed 17% and approach those of their polymer-based counterparts, thereby demonstrating their great future commercialization potential. In this review, recent progress in both SM donors and NFAs to illustrate structure-property relationships and various morphology-regulation strategies are summarized. Finally, ASM-OSC challenges and outlook are discussed., (© 2022 Wiley-VCH GmbH.)

[Display omitted] • Four scaffolds containing 3,4,5-triflourophenyl end cap groups have been designed using highly potent donor moieties. • The modified donor molecules expose remarkable photovoltaic properties as compared to that of reference model. • Reorganization energies of designed molecules are found smaller which in turn demonstrate faster transfer of charge carriers. • The photovoltaic potential in terms of V oc was investigated w.r.t fullerene derivative (PC 61 BM) which revealed that designed donor molecules are excellent choice to attain high PCE. Central core modification is convenient strategy to adjust the optoelectronic properties that allows significant utilization of small fullerene free donor materials in photovoltaic technology. Herein, four push–pull conjugated molecules in acceptor–donor-acceptor (A-D-A) configuration are investigated through quantum chemical simulations. A comprehensive DFT study of reference R and designed donor materials (D1 - D4) in which core is linked with thiophene spaced end-cap acceptors is conducted via selected functional MPW1PW91 and basis set (6-31G). Various critical parameters required to upgrade the efficiency of small molecule donors are electronic energy gaps, optical properties and exciton dynamics that are characterized by frontier molecular orbital diagram, absorption graph, transition density matrix (TDM), density of states (DOS), open-circuit voltage (Voc), and reorganization energy of charge carriers. Red shift occurs in all studied fluorine bearing scaffolds which absorb strongly in visible range 634–690 nm as compared to previously reported non-fluorinated molecule (598 nm). Amongst all donor molecules, D3 , elucidates desirable photovoltaic characteristics i.e., highest absorption peak (690 nm), appropriate energy gap (2.17 eV) and considered to be the ideal aspirant in photovoltaics. Furthermore, reduction in reorganization energy and increment in Voc values illustrate that all molecules possess faster charge carrier mobilities than reference. This conceptual study clarified that we could amend the optoelectronic parameters by replacing core with variable electron rich components. Hence, these novel molecules containing fluorine should be recommended for construction of high-performance organic solar cells in future. [ABSTRACT FROM AUTHOR]

The molecular planarity of a small molecule donor is one of the key factors affecting its molecular packing behavior, the morphology of the related active blend and thus the photovoltaic properties of all small molecule organic solar cells (ASM-OSCs). In this contribution, a series of new benzodithiophene (BDT)-based A-π-D-π-A type small molecules were developed, namely SD1, SD2 and SD3, by combining the chemical modification of side chain (sulfuration), π-bridge (alkoxy groups introducing) and end group (different type or different alkyl chain). The sulfuration of the BDT side chains and introducing alkoxy group to the π-bridge could induce intermolecular S⋯S and intramolecular S⋯O non-covalent interactions for higher planarity of these small molecules. The rhodanine and cyanoacetate end groups showed less effect on the optical and electronic properties but significantly affected the molecular packing behavior and blend morphology. From SD1:Y6-T to SD2:Y6-T and SD3:Y6-T, the face-to-face packing was gradually reduced, while the face-to-edge packing was increased, which resulted in lowered exciton dissociation efficiency, further decreased the device performance. SD1:Y6-T blend exhibited more favorable face-to-face packing and ideal morphology for efficient charge dissociation and transport, contributing to a champion PCE of 10.12%. This work provided a deep insight of the donor and acceptor packing behavior effect on the photovoltaic property by molecular design. [Display omitted] [ABSTRACT FROM AUTHOR]

5,5′-bibenzothiadiazole (BBT), which is a dimer of widely used electron deficient building block benzothiadiazole (BT), offers great promise for developing highly efficient electron donors because of its unique structural features. However, a small quantity of work has been reported on BBT based donors, and their overall performances are still far behind those of their BT counterparts. In order to make a contribution in this regard, the novel small molecules were synthesized and applied as donor materials in the bulk heterojunction solar cells, in which the BBT was used as the central node. Compared with the corresponding linear BT based molecule, the swivel-cruciform BBT based molecule exhibited significantly enhanced photovoltaic performance with an impressive power conversion efficiency of 7.21%, which is the best reported device performance based on BBT-core materials in organic solar cells. These results indicate the great potential of BBT for high performance solar cells. Image 1 • Novel BBT-based small molecule donors were designed. • The swivel-cruciform donors exhibited excellent physical and electronic properties. • A record PCE of 7.21% was achieved by simple solution processing. • BBT-based materials have great potential for high-performance photovoltaic devices. [ABSTRACT FROM AUTHOR]

Solution-processable small molecules (SMs) have attracted intense attention due to their definite molecular structures, less batch-to-batch variation, and easier structure control. Herein, two new SM donors based on substituted isatin unit (DI3T, DI3T-2F) are synthesized and applied as electron donors with the mixture of PC 71 BM to construct organic photovoltaics. As a result, 5,6-difluoro isatin derivative (DI3T-2F) obtains a power conversion efficiency of 7.80% by a simple solution spin-coating fabrication process without any additives, solvent, or thermal annealing process. More intuitively, due to stronger intermolecular interaction and higher hole mobility after the incorporation of fluorine atoms in end units, the devices present good tolerance to active layer thickness. The results indicate that DI3T-2F shows promising potential for large-scale printing processes and flexible application of efficient small molecule organic solar cells., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)