•Four novel D-π-A-π-D-typed DOBT-based dyes have been synthesized for BHJ-OSCs.•PV properties were tuned via molecular design with ethynyl-bridge and terminal D unit.•A high Voc of 1.06 V was obtained for CZEDOBT due to synergistic effect of ethynyl and Cz.•A best PCE was achieved for TPAEDOBT due to triple-bond and high hole-transfer capacity.•The work provides valuable hints for rational design of novel SMs-donor materials.Two novel D-π-A-π-D-typed dioctyloxy-benzothiadiazole (DOBT)-based small molecules (SMs), TPAEDOBT and CZEDOBT, were designed and synthesized, consisting of ethynyl as π-linkage, besides, incorporating triphenylamine (TPA) and alkylated-carbazole (Cz) as terminal electron-donating units respectively. Single-bond compounds, TPADOBT and CZDOBT, were designed as references to fully investigate their photovoltaic (PV) performance of solution-processed devices. As expected, the PV properties were finely tuned via molecular design with ethynyl-linker and terminal electron-donating units. Among them, the best power conversion efficiency (PCE) of 3.21% was obtained for TPAEDOBT-based device due to triple-bond effect and high hole-mobility. Most importantly, a remarkably increased open-circuit voltage (Voc) of 1.06 V was achieved for CZEDOBT-based device due to synergistic effect of ethynyl-linker with electron-withdrawing character and Cz unit with the relatively weakened electron-donating ability, which is among the earliest report for DOBT-Cz based SMs with high Voc so far. These results provide an important guide for rational design of novel SMs PV materials.A series of novel D-π-A-π-D type DOBT-based dyes named TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT have been designed and synthesized successfully for solution-processable BHJ-OSCs. Their PV properties were finely tuned via the molecular design with ethynyl-linker and terminal electron-donating units. By contrast, the relatively high PCE of 3.21% has been achieved for TPAEDOBT-based device due to its triple-bond effect and high hole-mobility capacity. Most importantly, a remarkably increased Voc of 1.06 V has been obtained for CZEDOBT-based device due to the synergistic effect of ethynyl-linker with electron-withdrawing character and Cz units with the relatively weakened electron-donating ability, which is among the earliest report for DOBT-ethynyl-Cz based SMs with high Voc so far. The results provide the valuable hints for rational design of novel SMs-donor materials.Download high-res image (251KB)Download full-size image

A novel D–π–A–π–D-type organic small molecule (OSM) named (TPACN)2Qx was designed and synthesized for solution-processible organic solar cells (OSCs), which contained 2,3-diphenyl-substituted quinoxaline (Qx) as the central acceptor (A) unit, triphenylamine (TPA) as terminal electron-donor (D) moieties, and acrylonitrile as the π-linkage segment. With an effective electron-withdrawing property and relatively stable quinoid geometry of Qx, (TPACN)2Qx exhibits a narrow band-gap of 1.88 eV and strong and broad absorption at 300–700 nm. As a consequence, an excellent power conversion efficiency (PCE) of 6.25% was achieved based on the (TPACN)2Qx:PC61BM blend using a simple spin-coating process in a solution, which is the highest efficiency achieved to date for Qx-core based solution-processed OSM photovoltaic (PV) devices. The impressive result demonstrates that OSMs employing quinoxaline derivative as an electrophilic unit can compete with their polymer counterparts.

Much attention has been paid to the push-pullstructure organic small molecule (OSM) materials for photovoltaic (PV) application in the past decade, due to their facile reduction of energy band gap (Eg) and effective control of PV properties. π-bridge plays an important role in the push-pull-structure OSMs since an appropriate π-linkage is crucial for improving the PV performance of organic solar cells (OSCs). In this review, various π-bridge groups (thiophene, alkene, alkyne, arene and heterocycle) and the pertinent π-linkage effect will be systematically summarized. These results suggest that the in-depth study of the π-linkage effect is essential to deeply understanding the relationship between the molecular structure and property, thus improving PV performance.在过去的十年中, 推拉结构的有机小分子光伏材料由于分子带隙及光伏性能等易于实现有效的调控, 在光伏领域中受到了广泛的关注, 适宜的桥键联接对于提升有机太阳能电池的光伏性能起到了重要的作用, 因此π-桥键在推拉结构的有机小分子设计中扮演着十分重要的角色. 本文重点综述了推拉机构有机小分子的不同π-桥键(包括噻吩, 烯烃, 炔烃, 芳烃和杂环及其相应的衍生物) 对于材料的光伏性能所产生的重要影响, 这些结果表明,深入的研究π-桥键效应对于深刻理解分子结构和材料性能之间的关系以及通过分子的结构优化来提高材料的光伏性能具有十分重要的意义.

•Four novel D-π-A-π-D typed OSMs have been designed and synthesized for BHJ-OSCs.•PV properties were tuned via molecular design incorporating of TT- and F-unit.•A significantly increased Jsc was observed for BDCTTMBT with narrow Eg.•A high Voc of 1.11 V was obtained for BDCTFBT due to F-effect with lowest HOMO.•A best PCE was achieved for BDCTTFBT due to double effect of TT- and F-unit.A series of novel D-π-A-π-D typed organic small molecules (OSMs) have been designed and synthesized successfully for solution-processed bulk-heterojunction (BHJ) solar cells, consisting of thiophene (T) or bithiophene (TT) as a segment of π-bridge, besides, incorporating benzothiadiazole (BT) or fluorinated benzothiadiazole (FBT) as the electron-withdrawing core (A), respectively. The photovoltaic (PV) performance was finely tuned via molecular design. A significantly increased short-circuit current density (Jsc) was observed for BDCTTMBT with narrow energy gap (Eg) due to the extended conjugation-length with TT-linkage, and a high open-circuit voltage (Voc) of 1.11 V was obtained for BDCTFBT due to the F-substitution with the lowest highest occupied molecular orbital (HOMO). Among them, a best power conversion efficiency (PCE) of 4.86% was achieved for BDCTTFBT based devices due to the double effect of TT-linkage and F-substitution. These results provide valuable information on design of novel PV materials.A series of novel D-π-A-π-D typed OSMs have been designed and synthesized successfully for solution-processed BHJ solar cells. The PV performance was finely tuned via molecular design. A significantly increased Jsc was observed for BDCTTMBT with narrow Eg due to the extended conjugation-length with TT-linkage, and a high Voc of 1.11 V was obtained for BDCTFBT due to the F-substitution with the lowest HOMO. Among them, a best PCE of 4.86% was achieved for BDCTTFBT based devices due to the double effect of TT-linkage and F-substitution, which is among the best performance for BT-TPA based BHJ OSMs-SCs ever reported to date. These results provide valuable information on the design of novel PV materials.

Two new D–π–A–π–D-type diketopyrrolopyrrole (DPP) based organic small molecules (OSMs), M3 and M4, were synthesized successfully, consisting of ethynyl as π-linkage, and containing alkylated carbazole and fluorene as terminal electron-donating groups, respectively. To investigate the triple-bond effect on the optical electronic properties, non-ethynyl analogues M1 and M2 with a single-bond-linkage were designed and synthesized as well. The relationship between molecular structure and property was thoroughly investigated by experimental and theoretical studies. In contrast, the ethynyl-linkage structural design could not only lower the highest occupied molecular orbital (HOMO) levels, but also delicately balance the relationship between the deep-lying HOMO and narrow band gap, thus improving the photovoltaic (PV) performance. As a result, compounds M3 and M4 exhibited relatively deep-lying HOMO levels relative to M1 and M2, resulting in the corresponding PV devices with an increased open-circuit voltage (VOC) of 0.84 V and 0.98 V, with power conversion efficiency (PCE) of 1.99% and 3.10%, respectively. Whereas M1 and M2 based devices showed a VOC of 0.46 V and 0.89 V, and a PCE of 1.48% and 2.23%, respectively. The best PV performance of M4 was primarily attributed to the deep-lying HOMO level and reasonably high hole mobility caused by the structural design with an ethynyl-linkage and a fluorene end-capping group. We found that the systematic investigation of the triple bond effect on novel OSMs could be critical for a deep understanding of the relationship between molecular structure and property. This work provided an important guide for the rational design of novel PV materials.

Four novel D–π–A–π–D-type small molecules (SMs) were synthesized and their π-linkage effects were investigated. Among them, SM2 with vinylene as the π-linkage exhibited better molecular coplanarity, reaching a relatively higher power conversion efficiency (PCE) of 3.76%. SM3 and SM4 with acetylene and acrylonitrile as π-linkages exhibited relatively higher open-circuit voltages (VOC) of 0.93 V and 0.90 V, respectively, owing to their deep-lying HOMO levels. These results gave an important guide for developing new materials in solution-processed small molecule solar cells.

•Novel small molecular PV materials BDPTBT and BDATBT were designed and synthesized.•BDCTBT and BDETBT were further investigated and the devices were optimized.•The study of linkage effect revealed the relationship between structure and property.•BDATBT and BDCTBT based devices showed high Voc of 1.03 V and 1.08 V, respectively.•An impressive PCE of 4.84% was obtained for optimized BDCTBT-based device.Two novel small molecular photovoltaic (PV) materials, BDPTBT and BDATBT were designed and synthesized, consisting of 5,6-bis-(octyloxy)benzo[c][1,2,5]thiadiazole (DOBT) as electron-withdrawing core (A), and triphenylamine (TPA) as electron-donating side group (D). Moreover, the benzene and ethynylbenzene as π-linkage were introduced to form donor–π-acceptor–π-donor (D–π-A–π-D) typed molecular structures, respectively. To fully investigate the linkage effect of a series of small molecules, two reference compounds BDCTBT and BDETBT were also studied systematically, consisting of 2-phenylacrylonitrile and styrene as π-linkage, respectively. As a result, the π-linkage units, benzene, styrene, ethynylbenzene and 2-phenylacrylonitrile played an important role in modifying molecular structure and improving PV performance. Bulk heterojunction (BHJ) solar cells based on BDPTBT/PC61BM and BDATBT/PC61BM yielded the power conversion efficiencies (PCEs) of 2.99% and 2.03%, respectively. Notably, BDATBT based device showed a high open-circuit voltage (Voc) of 1.03 V. Compared to the results we have reported previously, the reference devices based on BDCTBT/PC61BM and BDETBT/PC61BM with the optimized weight ratio showed dramatically enhanced PCEs of 4.84% and 3.40%, respectively, and BDCTBT based device showed a high Voc of 1.08 V. To our knowledge, the Voc of 1.08 V is the highest voltage reported to date for devices prepared from solution-processed small-molecule-donor materials, and the PCE of 4.84% is the highest efficiency reported so far for D–A–D-typed benzothiadiazole (BT)–TPA based solution-processed small molecules PV devices.Graphical abstract

A series of novel A–D–A structured small molecule photovoltaic (PV) materials [CZ(TDPP)2, DPA(TDPP)2, PTZ(TDPP)2 and FL(TDPP)2] with diketopyrrolopyrrole (DPP) as an electron-withdrawing group were synthesized and characterized. These small molecular donors exhibit excellent solubility in common organic solvents. The density functional theory (DFT) calculations demonstrated the intramolecular charge transfer (ICT) behavior of the synthesized PV materials and an efficient charge separation was observed by a fluorescence quenching experiment. In addition, their photophysical and electrochemical properties show that they harvest sunlight over the entire visible spectrum range and keep appropriate energy levels to satisfy the requirement of solution-processable OSCs. Therefore, we explored the PV properties of the synthesized donors by fabricating BHJ solar cells with a typical structure of ITO/PEDOT:PSS/Donors:PC61BM/LiF/Al. Among them, CZ(TDPP)2 revealed a promising performance in PV devices with a power conversion efficiency (PCE) of 1.50%, along with an open-circuit voltage (VOC) of 0.66 V, a short-circuit current density (JSC) of 4.12 mA cm−2, and a fill factor (FF) of 0.44, under an illumination of AM 1.5G (80 mW cm−2).

Two D–A–D-type low band gap organic dyes based on triphenylamine and benzoxadiazole/benzothiadiazole, 4,7-Bis{5-{4-{2-[4-(N,N-diphenylamino)phenyl]-1-nitrilethenyl}phenyl}-2-thienyl}-2,1,3-benzoxadiazole (BDNTBX) and 4,7-Bis{5-{4-{2-[4-(N,N-diphenylamino)phenyl]-1-nitrilethenyl}phenyl}-2-thienyl}-2,1,3-benzothiadiazole (BDNTBT) were successfully synthesized. The properties of two compounds were investigated by density functional theory (DFT) calculations, UV–vis absorption spectroscopy, cyclic voltammetry and fluorescence quenching experiment. The calculated ground-state geometries demonstrate intramolecular charge transfer (ICT) occurs in both molecules during the procedure of charge excitation from HOMO to LUMO. From the data in electrochemistry and fluorescence quenching experiments, the molecules reveal lower HOMO energy levels compared with that of P3HT and proper LUMO energy levels to obtain efficient charge separation with PCBM. Two synthesized compounds exhibit broad absorption range covering the whole visible spectral region. These photophysical and electrochemical properties call attention to that our materials are prospective candidates as donor materials for solution-processable organic photovoltaic cells.Graphical abstractHighlights► Two D–A–D-type low band gap organic dyes (BDNTBX and BDNTBT) were synthesized. ► The properties were investigated by DFT, UV–vis, CV and EM quenching experiment. ► Intramolecular charge transition (ICT) occurs in both molecules. ► Efficient charge separation was observed by fluorescence quenching experiment. ► Two dyes are candidates as donor for solution-processable organic PV cells.

Four novel D–π–A–π–D-type small molecules (SMs) were synthesized and their π-linkage effects were investigated. Among them, SM2 with vinylene as the π-linkage exhibited better molecular coplanarity, reaching a relatively higher power conversion efficiency (PCE) of 3.76%. SM3 and SM4 with acetylene and acrylonitrile as π-linkages exhibited relatively higher open-circuit voltages (VOC) of 0.93 V and 0.90 V, respectively, owing to their deep-lying HOMO levels. These results gave an important guide for developing new materials in solution-processed small molecule solar cells.