No abstract

Two novel small molecule acceptors (DTNIC6 and DTNIC8) based on a ladder-type dithienonaphthalene (DTN) building block with linear (hexyl) or branched (2-ethylhexyl) alkyl substituents are designed and synthesized. Both acceptors exhibit strong and broad absorption in the range from 500 to 720 nm as well as appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Replacing the linear hexyl chains with the branched 2-ethylhexyl chains has a large impact on the film morphology of photoactive layers. In the blend film based on DTNIC8 bearing the branched alkyl chains, morphology with well-defined phase separation was observed. This optimal phase morphology yields efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced charge carrier mobilities. Benefited from these factors, organic solar cells (OSCs) based on PBDB-T:DTNIC8 deliver a highest power conversion efficiency (PCE) of 9.03% with a high fill factor (FF) of 72.84%. This unprecedented high FF of 72.84% is one of the highest FF values reported for nonfullerene OSCs. Our work not only affords a promising electron acceptor for nonfullerene solar cells but also provides a side-chain engineering strategy toward high performance OSCs.

Two novel copolymers incorporating N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide (DI) and benzo[1,2-b:4,5-b′]dithiophene (BDT) units have been designed, synthesized, and characterized. By the incorporation of the DI unit, both polymers show a bathochromically shifted absorption with a deep lying highest occupied molecular orbital (HOMO) energy level. The polymer based on thienyl group substituted BDT exhibits an intense absorption in the longer-wavelength region, a deeper lying HOMO energy level, and a higher carrier mobility, all of which contribute to the resulting polymer solar cells with a higher power conversion efficiency (PCE) of 5.19% and an increased V oc of 0.91 V.

Ternary organic photovoltaics (OPVs) were constructed with one wide-band-gap donor PM6 and two A–D–A-type acceptors (M-series M36 and MQ5) with similar chemical structures. Power conversion efficiency (PCE) of the optimal ternary OPVs reaches 17.24% with 20 wt % MQ5 content, arising from a simultaneously increased short circuit current density (J SC) of 25.36 mA cm–2 and a fill factor (FF) of 76.02% as compared to those of two binary OPVs. The photon harvesting of ternary active layers can be maximized by adjusting the MQ5 content by reason of the complementary absorption spectra of M36 and MQ5. The molecular arrangement of PM6 and M36 can be collectively optimized by introducing an appropriate amount of MQ5 as a morphology regulator for facilitating effective charge transportation in ternary active layers. The improved photon harvesting and charge transport in active layers should be two important factors responsible for J SC and FF improvement of optimal ternary OPVs, respectively. More than an 8.8% improvement of PCE is achieved in ternary OPVs with an appropriate amount of MQ5 as the photon-harvesting enhancer and morphology regulator. The huge potential of A–D–A-type materials in constructing highly efficient OPVs can be further exploited based on a ternary strategy.

Nonfullerene acceptors (MQ3, MQ5, MQ6) are synthesized using asymmetric and symmetric ladder‐type heteroheptacene cores with selenophene heterocycles. Although MQ3 and MQ5 are constructed with the same number of selenophene heterocycles, the heteroheptacene core of MQ5 is end‐capped with selenophene rings while that of MQ3 is flanked with thiophene rings. With the enhanced noncovalent interaction of O⋅⋅⋅Se compared to that of O⋅⋅⋅S, MQ5 shows a bathochromically shifted absorption band and greatly improved carrier transport, leading to a higher power conversion efficiency (PCE) of 15.64 % compared to MQ3, which shows a PCE of 13.51 %. Based on the asymmetric heteroheptacene core, MQ6 shows an improved carrier transport induced by the reduced π–π stacking distance, related with the increased dipole moment in comparison with the nonfullerene acceptors based on symmetric cores. MQ6 exhibits a PCE of 16.39 % with a VOC of 0.88 V, a FF of 75.66 %, and a JSC of 24.62 mA cm−2.

Photodynamic therapy (PDT) is a promising modality for cancer treatment. The essential element in PDT is the photosensitizer, which can be excited by light of a specific wavelength to generate cytotoxic oxygen species (ROS) capable of killing tumor cells. The effectiveness of PDT is limited in part by the low yield of ROS from existing photosensitizers and the unwanted side effects induced by the photosensitizers toward normal cells. Thus the design of nanoplatforms with enhanced PDT is highly desirable but remains challenging. Here, we developed a heavy atom (I) containing dipyrromethene boron difluoride (BODIPY) dye with a silylated functional group, which can be covalently incorporated into a silica matrix to form dye-doped nanoparticles. The incorporated heavy atoms can enhance the generation efficiency of ROS. Meanwhile, the covalently dye-encapsulated nanoparticles can significantly reduce dye leakage and subsequently reduce unwanted side effects. The nanoparticles were successfully taken up by various tumor cells and showed salient phototoxicity against these cells upon light irradiation, demonstrating promising applications in PDT. Moreover, the incorporated iodine atom can be replaced by a radiolabeled iodine atom (e.g., I-124, I-125). The resulting nanoparticles will be good contrast agents for positron emission tomography (PET) imaging with their PDT functionality retained.

A novel ladder-type dithienonaphthalene (DTN) was designed and synthesized as an electron-rich unit for constructing donor–acceptor copolymers. Different acceptor moieties, including benzo[c][1,2,5]thiadiazole (BT), 5,6-bis(hexyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TBT), and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDPP), were used as electron-deficient units for the target copolymers. These copolymers (PDTNBT, PDTNTBT, and PDTNTDPP) were obtained via the Stille coupling reaction and characterized by 1H NMR spectroscopy, UV–vis absorption spectroscopy, cyclic voltammetry, and gel permeation chromatography (GPC). Owing to the four solubilizing alkyl chains on the DTN unit, all the three copolymers have good solubility in common solvents. Among these polymers, PDTNTBT exhibits the highest space-charge limit current (SCLC) hole mobility of 2.13 × 10–5 cm2 V–1 s–1, which is beneficial for achieving high performance solar cells. Under the simulated AM 1.5G illumination condition (100 mW/cm2), solar cells based on PDTNTBT:PC71BM (1:3, w/w) exhibit a power conversion efficiency (PCE) of 4.8% with a current density of 10.3 mA cm–2, an open-circuit voltage of 0.86 V, and a fill factor of 54%. With the same device fabrication method, PDTNTDPP:PC71BM (1:3, w/w) and PDTNBT:PC71BM (1:3, w/w) based devices exhibit efficiencies of 1.52% and 2.79%, respectively. Furthermore, inverted solar cells based on these copolymer blends are also fabricated. The inverted devices based on PDTNTDPP:PC71BM (1:2, w/w) and PDTNBT:PC71BM (1:2, w/w) exhibit PCEs of 1.60% and 2.89%, respectively, which are similar to their corresponding conventional devices. And the inverted devices based on PDTNTBT:PC71BM (1:2, w/w) show a higher PCE of 5.0%, and more importantly, they are quite stable as demonstrated by the 4.75% PCE after ambient storage for two months.

Two wide bandgap copolymers based on bulky indacenodithiophene (IDT) and alkoxylated benzothiadiazole units (PIDTBTO-T and PIDTBTO-TT) with the thiophene or thieno[3,2-b]thiophene (TT) π-bridge are designed and synthesized. The effect of π-bridges on the π-π packing, optical, carrier transport, nano-sized phase separation and photovoltaic properties of the copolymers are investigated in depth. In comparison with the PIDTBTO-T-based counterpart, the best performance solar cell based on PIDTBTO-TT exhibits a higher power conversion efficiency (PCE) of 8.15% which is mainly attributed to the formation of a fibrous network for the active layer based on PIDTBTO-TT. Furthermore, when a novel hybrid electron transport layer (PDIN:PFN) is introduced into a tandem solar cell using the PIDTBTO-TT-based device and a PTB7-Th-based device as the bottom and top cell components, respectively, the resulting solar cell exhibits an outstanding PCE of 11.15% with a large open circuit voltage of 1.70 V. To the best of our knowledge, the PCEs of 8.15% and 11.