Zhaofei Chai

High performance organic sensitizers based on 11,12-bis(hexyloxy) dibenzo[a,c]phenazine for dye-sensitized solar cells

Three new metal-free organic sensitizers containing 11,12-bis(hexyloxy) dibenzo[a,c]phenazine (BPz) units were synthesized and used for dye-sensitized solar cells (DSSCs). The broad absorption spectra indicate that the light harvesting abilities were enhanced by the introduction of the BPz unit in the π-conjugated space, which can also cause an anti-aggregation effect and the suppression of charge recombination. Among these sensitizers, LI-39 showed the best photovoltaic performance: a short-circuit photocurrent density (Jsc) of 14.40 mA cm−2, an open-circuit photovoltage (Voc) of 0.74 V, and a fill factor (ff) of 0.67, corresponding to an overall conversion efficiency of 7.18% under standard global AM 1.5 solar light conditions. The result shows that the organic sensitizers based on this bulky fused aromatic rings as well as the auxiliary acceptor are the promising candidates for improvement of the performance of DSSCs.

New triphenylamine-based sensitizers bearing double anchoring units for dye-sensitized solar cells

Two organic sensitizers (LI-33 and LI-34) with double anchoring units were synthesized and utilized for dye sensitized solar cells (DSSCs), which contained thiophene or vinyl thiophene as π-bridge. The introduction of double anchoring units can change their absorption spectra and energy levels in a large degree, thus, the better light-harvesting ability and the convenient electron transfer along the whole molecule can be obtained. The solar cell based on LI-34 exhibited a broad incident photon-to-current conversion efficiency (IPCE) spectrum and high conversion efficiency (η=6.05%) with coadsorbent CDCA.

New D–π–A organic dyes containing a tert-butyl-capped indolo[3,2,1-jk]carbazole donor with bithiophene unit as π-linker for dye-sensitized solar cells

Two new D–π–A organic dyes containing a tert-butyl-capped indolo[3,2,1-jk]carbazole unit as a donor, bithiophene unit as a π-linker and cyanoacrylic acid as an acceptor (CL-10 and CL-11) have been designed and synthesized for dye-sensitized solar cells (DSSCs). By comparison, two dyes exhibit almost the same absorption and electrochemical properties, because the introduction of two hexyl groups at non-ortho β-positions of the 2,2′-bithiophene unit dose not destroy the planarity of the molecular skeleton of CL-11. However, the DSSC devices based on them show very different photovoltaic performances with power conversion efficiency (PCE) of 3.96% for CL-10 and 2.85% for CL-11, which can be attributed to the existence of the hexyl groups in CL-11 that lead to a big decrease of the dye adsorption onto TiO2. The addition of chenodeoxylic acid (CDCA) as a coadsorbant can significantly improve the photovoltaic performances of the DSSCs with a big increase in the PCE for CL-10 (4.68%) and CL-11 (4.66%).

Conjugated or Broken: The Introduction of Isolation Spacer ahead of the Anchoring Moiety and the Improved Device Performance

Acceptors in traditional dyes are generally designed closed to TiO2 substrate to form a strong electronic coupling with each other (e.g., cyanoacrylic acid) to enhance the electron injection for the high performance of the corresponding solar cells. However, some newly developed dyes with chromophores or main acceptors isolated from anchoring groups also exhibit comparable or even higher performances. To investigate the relatively untouched electronic coupling effect in dye-sensitized solar cells, a relatively precise method is proposed in which the strength is adjusted gradually by changing isolation spacers between main acceptors and anchoring groups to partially control the electronic interaction. After an analysis of 3 different groups of 11 sensitizers, it is inferred that the electronic coupling should be kept at a suitable level to balance the electron injection and recombination. Based on a reference dye LI-81 possessing a cyanoacrylic acid as acceptor and anchoring group, both photocurrent and photovoltage are synergistically improved after the properties of isolation spacers were changed through the adjustment of the length, steric hindrance, and push-pull electronic characteristic. Accordingly, the rationally designed dye LI-87 with an isolation spacer of thiophene ethylene gives an efficiency of 8.54% and further improved to 9.07% in the presence of CDCA, showing a new way to develop efficient sensitizers.

Novel D–A−π–A-Type Organic Dyes Containing a Ladderlike Dithienocyclopentacarbazole Donor for Effective Dye-Sensitized Solar Cells

A novel ladderlike fused-ring donor, dithienocyclopentacarbazole (DTCC) derivative, is used to design and synthesize three novel donor–acceptor−π–acceptor-type organic dyes (C1–C3) via facile direct arylation reactions, in which the DTCC derivative substituted by four p-octyloxyphenyl groups is served as the electron donor and the carboxylic acid group is used as the electron acceptor or anchoring group. To fine-tune the optical, electrochemical, and photovoltaic properties of the three dyes, various auxiliary acceptors, including benzo[2,1,3]thiadiazole (BT), 5,6-difluorobenzo[2,1,3]thiadiazole (DFBT), and pyridal[2,1,3]thiadiazole (PT), are incorporated into the dye backbones. The results indicate that all of the three dyes exhibit strong light-capturing ability in the visible region and obtain relatively high molar extinction coefficients (>31 000 M–1 cm–1) due to their strong charge transfer (CT) from donor to acceptor. Moreover, theoretical model calculations demonstrate fully separated highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels for the three dyes, which is helpful for efficient charge separation and electron injection. Using the three dyes as sensitizers, conventional dye-sensitized solar cells (DSSCs) based on liquid iodide/triiodide redox electrolytes are fabricated. Our results indicate that the BT-containing dye C1 affords the highest power conversion efficiency of up to 6.75%, much higher than that of the DFBT-containing dye C2 (5.40%) and the PT-containing dye C3 (1.85%). To our knowledge, this is the first example reported in the literature where the DTCC unit has been used to develop novel organic dyes for DSSC applications. Our work unambiguously demonstrates that the ladderlike DTCC derivatives are the superb electron-donating blocks for the development of high-performance organic dyes

Recyclable mechanoluminescent luminogen: different polymorphs, different self-assembly effects of the thiophene moiety and recovered molecular packing via simple thermal-treatment

We report a purely organic recyclable mechanoluminescent luminogen (tPE-5-MeTh) with the combination of an AIEgen and a thiophene group as the self-assembly unit. As a result of the different self-assembly effects, two crystalline polymorphs were cultured from different solvents, which possess totally different packing modes and various inter/intramolecular interactions with opposite ML properties. Excitingly, the molecular packing status of tPE-5-MeTh could be adjusted through simple thermal-treatment, offering recoverability and controllability of its ML activity. These results provide a new avenue for rational molecular design, convenience of potential applications and deep understanding of the inherent mechanism.

Significantly Improved Performance of Dye-Sensitized Solar Cell by Optimizing Organic Dyes with Pyrrole as the Isolation Space and Utilizing Alkyl Chain Engineering

The optimization of interfacial and intramolecular charge transfer of organic dyes is crucial to the photovoltaic performance of dye-sensitized solar cells. In this study, an efficient strategy was afforded by the introduction of alkylated pyrrole as the isolation spacer into organic dyes. The interfacial charge transfer can be organized by the tunable alkyl chains as a barrier layer to inhibit electron recombination, and the pull–push system can be strengthened by the incorporation of electron-withdrawing units (benzothiadiazole) and electron-rich moieties (pyrrole) into the conjugated bridge simultaneously, resulting in excellent light-harvesting capability. Therefore, the high conversion efficiency of 9.70% was achieved by LI-124 with suitable alkyl chains and device optimization.

Some new molecular design strategy to improve the performance of organic dyes in dye sensitized solar cells

With the aim to obtain good metal free organic dyes with high performance, different series of dyes were designed by optimizing their structures. Based on the experimental results, the relationship between the structure and property is summarized, to provide some useful information for the further development of new organic dyes.