Changquan Tang

Two-photon absorption and optical power limiting properties of ladder-type tetraphenylene cored chromophores with different terminal groups

A series of soluble ladder-type tetraphenylene cored chromophores with different terminal groups have been synthesized and their structure–property relationship with regards to various linear optical and nonlinear optical properties has been established. By using the two-photon excited fluorescence method and the nonlinear transmission method, the two-photon absorption (2PA) properties of these chromophores were determined, and they were found to be strongly dependent on the electron-richness of the ladder-type tetraphenylene core, as well as the terminal groups. The introduction of strong electron donors (N-alkyl) in both the central core and the terminals led to a chromophore with a high 2PA cross-section value of 2137 GM. Optical limiting behaviors of the synthesized chromophores in THF were investigated by using a femto-second ultra-fast laser. 2PA coefficients for these chromophores in THF (5 mM) ranged from 0.131–0.256 cm GW−1. The ladder-type tetraphenylene cored chromophore with the highest 2PA cross-section value exhibited the best optical limiting performance, as evidenced by light transmission as low as 19.1% at 770 nm under an intensity of 99.6 GW cm−2. The excellent optical limiting performance of these chromophores makes them useful in photonic or optoelectronic devices for protecting human eyes and optical sensors, as well as for stabilizing light sources in optical communications.

BODIPY-doped silica nanoparticles with reduced dye leakage and enhanced singlet oxygen generation.

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.

Highly soluble heteroheptacene: a new building block for p-type semiconducting polymers.

A facial synthetic route to a new heteroheptacene with the inclusion of carbazole and thiophene units is described. The synthesis of two new semiconducting copolymers with use of the heteroheptacene unit is also reported. The introduction of heteroatoms (sulfur, nitrogen) in the fused-ring system leads to small optical band-gaps of these polymers. The charge carrier mobilities for these polymers are measured in ambient conditions which are sufficient for photovoltaic applications.

Diindenocarbazole-based large bandgap copolymers for high-performance organic solar cells with large open circuit voltages

Three donor–acceptor alternating copolymers abbreviated as PC1, PC2, and PC3, respectively, were designed and synthesized using diindenocarbazole (DIC) and dithienylbenzothiadiazole (DTBT) units. Through backbone manipulation, copolymers with large bandgaps (∼2.0 eV) and deep-lying HOMO energy levels (below −5.41 eV) were obtained. The side chains were also investigated to tune the intermolecular interactions and morphology of the copolymers blended with PC71BM. Polymer solar cells (PSCs) based on PC2 : PC71BM exhibit an outstanding power conversion efficiency (PCE) of 7.26%, which represents one of the highest PCEs ever reported for PSCs while combining an open-circuit voltage (Voc) of 0.93 V and a large optical bandgap of 2.01 eV. Under similar device fabrication conditions, regular PSCs based on PC1 and PC3 achieve PCEs of 2.45% and 6.68%, respectively. Moreover, inverted PSCs derived from PC2 also exhibit an attractive PCE of 6.17% with a high Voc of 0.92 V. In view of its similar optical profiles to P3HT, but a deeper-lying HOMO energy level, PC2 should be a promising candidate as a short wavelength absorbing material for tandem solar cells.

Novel ladder-type heteroheptacene-based copolymers for bulk heterojunction solar cells

Four semiconducting copolymers were designed and synthesized based on a novel ladder-type heteroheptacene building block, which was prepared in four steps from 3,7-dibromodibenzo[b,d]thiophene. The copolymers were characterized by 1H NMR spectroscopy, UV-Vis absorption spectroscopy, cyclic voltammetry and their molecular weights were estimated using gel permeation chromatography. Field effect transistors (FETs) fabricated from these four polymers exhibit semiconducting behavior in air. The highest mobility of 2.21 × 10−4 cm2 V−1 s−1 was found in the polymer copolymerized from the ladder-type heteroheptacene and terthiophene. Low band-gap donor–acceptor copolymers with a deep-lying HOMO energy level were synthesized by a copolymerization between the ladder-type heteroheptacene donor and the 2,1,3-benzothiadiazole acceptor. Polymer solar cells made from one of these polymers gave a power conversion efficiency of 4.17%, a current density of 9.2 mA cm−2, an open-circuit voltage of 0.79 V, and a fill factor of ∼57.4% under 100 mW cm−2 AM 1.5 G sunlight illumination.

Improving the photovoltaic performance of ladder-type dithienonaphthalene-containing copolymers through structural isomerization

A ladder-type angular-shaped dithienonaphthalene (aDTN), an isomer of ladder-type linear-shaped dithienonaphthalene (DTN), was designed and synthesized as an electron-rich unit to construct donor–acceptor copolymers with deep-lying highest occupied molecular orbital (HOMO) energy levels. Benzo[c][1,2,5]thiadiazole (BT) with various substituents were used as electron deficient units for synthesizing the target copolymers (PaDTNBTO, PaDTNBTH, and PaDTNBTF) via the Stille coupling reaction. Incorporating different substituents onto the BT moiety has significant effects on the photophysical and electrochemical properties of the copolymers, as well as on the roughness of the polymer/PC71BM blends. With four solubilizing alkyl chains on the aDTN unit, all its three copolymers have good solubility in common solvents. The synthesized copolymers exhibit deep-lying HOMO energy levels, leading to high open circuit voltages (Voc ≥ 0.90 V) of the resulting polymer solar cells. The bulk heterojunction solar cell based on the aDTN-containing copolymers (PaDTNBTO) shows an improved efficiency of 6.44% and an increased Voc of 0.92 V compared to that based on the linear-shaped DTN containing counterpart (efficiency = 4.78%, Voc = 0.86 V). Whereas, under the same device fabrication conditions, PaDTNBTH- and PaDTNBTF-based devices exhibit efficiencies of 5.22% and 1.73%, respectively. Our results demonstrate that aDTN is a better building block in constructing p-type copolymers for high open circuit voltage devices compared to the linear-shaped DTN.

Side-chain engineering of diindenocarbazole-based large bandgap copolymers toward high performance polymer solar cells

Five wide bandgap conjugated polymers based on diindenocarbazole (DIC) and dithienylbenzothiadiazole (DTBT) alternating units have been designed and prepared to investigate the effects of side chains on the photovoltaic performance of the polymers. All the polymers are soluble in common organic solvents and they show similar optical bandgaps of around 2.0 eV as well as deep-lying highest occupied molecular orbital (HOMO) energy levels (below −5.48 eV). Bulk heterojunction (BHJ) polymer solar cells (PSCs) using phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor material were fabricated. The side chains on the polymer backbone have a strong impact on the film morphology of the polymer:PC71BM blends. A phase separation with a relatively larger domain size was found for the polymers with longer side chains, while those with relatively shorter alkyl chains could form uniform films featuring a phase separation with a smaller domain size. Finally, PSCs based on PC1BT6:PC71BM exhibited an outstanding power conversion efficiency of 7.34% with a high open-circuit voltage (Voc) of 0.95 V. Our results demonstrate that the judicious design of side-chains is effective in improving the photovoltaic performance of DIC-based polymers. With a larger bandgap than P3HT but an improved power conversion efficiency (PCE) with a large Voc, this type of DIC-based polymer should be a promising bottom layer material for tandem solar cells.

Improved synthesis and photovoltaic performance of donor–acceptor copolymers based on dibenzothiophene-cored ladder-type heptacyclic units

We have developed a facile synthetic route to a ladder-type donor unit (SDCT) wherein two outer thiophene subunits are covalently fastened to the central dibenzothiophene core through two sp3-hybridized bridging carbons. An innovative transformation from an aryl ketone group to an aryl ester group was applied to construct the ladder-type molecular skeleton, and the overall synthetic yield toward the donor unit has been significantly improved by choosing aryl side chains instead of aliphatic ones to avoid competing dehydration reactions. To reveal the effects of π-spacers and heteroatom substituents, three donor–acceptor (D–A) copolymers containing SDCT and acceptor units of 2,1,3-benzothiadiazole (BT), 2,1,3-benzoxadiazole (BO), or 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DTBT) were synthesized, characterized and used for polymer solar cells (PSCs). All polymers exhibit blue-shifted absorption spectra and deeper-lying HOMO energy levels compared to the previous carbazole-based skeleton analogues. In comparison with its analogous polymer with the same π-conjugated backbone, the polymer with alkoxy-substituted BT as the acceptor unit (PSBT) shows an order of magnitude higher OFET mobility (1.8 × 10−4versus 1.25 × 10−5 cm2 V−1 s−1). An optimal device based on PSBT : PC71BM (1/3 in wt%) delivers a respectable PCE of 5.18% and a high Voc of 0.94 V, all of which are superior to those of the carbazole-based analogue (PCE = 3.7%, Voc = 0.80 V) and greatly surpass the values of a previously reported dibenzothiophene-based polymer (PCE = 0.76%, Voc = 0.64 V). These results demonstrate that SDCT is a promising building block for constructing photovoltaic polymers and the synthetic strategy developed herein can be used to prepare other dibenzothiophene-cored ladder-type heptacyclic units.

Ladder-type tetra-p-phenylene-based copolymers for efficient polymer solar cells with open-circuit voltages approaching 1.1 V

Side-chain engineering of polymer backbones can induce subtle variations in polymer properties, resulting in a significant impact on their photovoltaic performance. In this work, four ladder-type tetra-p-phenylene containing copolymers with different alkyl side chains (P3FTBT1, P3FTBT1F, P3FTBT8O6 and P3FTBT1O6) were designed and synthesized. These copolymers have large bandgaps (∼2.0 eV) and deep-lying highest occupied molecular orbital (HOMO) energy levels (from −5.44 eV to −5.53 eV). The substitution of two hexyl groups with two methyl groups on the ladder-type tetra-p-phenylene unit afforded polymer P3FTBT1 which exhibits an enhanced power conversion efficiency (PCE) of 5.39%. Incorporation of fluorine into the benzo[c][1,2,5]thiadiazole (BT) unit gave polymer P3FTBT1F which exhibits a PCE of 4.50% with an open circuit voltage (Voc) of 1.09 V. By introducing two alkoxy groups to the BT unit, P3FTBT1O6 was synthesized, and it exhibits a PCE of 5.73% with a Voc of 1.02 V. The results suggest that the ladder-type tetra-p-phenylene is an excellent building block to construct donor–acceptor copolymers with high PCEs and large Vocs.

Asymmetric indenothiophene-based non-fullerene acceptors for efficient polymer solar cells

Three new asymmetric acceptor-donor-acceptor structured molecules are designed and synthesized by incorporating indenothiophene as the central core. Their bandgaps and energy levels can be easily tuned by varying the electron withdrawing ability of the terminal groups such as dicyanovinyl, 3-ethylrhodanine, and 2-(1,1-dicyanomethylene)-3-ethyl-rhodanine. Inverted polymer solar cells using these molecules as acceptors and PTB7-Th as a donor material afford a highest power conversion efficiency of 7.49% with a high open circuit voltage of 1.02 V as well as a low energy loss of 0.59 eV.摘要本文设计合成了三个新型含茚并噻吩的“受体-给体-受体”型不对称非富勒烯受体材料. 通过使用具有不同吸电子能力的末端基团(如:二氰乙烯基、3-乙基绕丹宁、2-二氰亚甲基-3-乙基绕丹宁)实现了目标材料的带隙和能级调控. 以这些非富勒烯受体材料与PTB7-Th给体材料共混制备的倒置聚合物太阳电池, 实现了高达7.49%的光电转换效率和1.02伏的高开路电压以及0.59电子伏的低能量损失.