Molecular Engineering Of Organic Sensitizers Based On Indoline For Dye-Sensitized Solar Cells

As a potential alternative to conventional inorganic photovoltaic devices, dye-sensitized solar cells (DSSCs) have attracted increasing attention in the past decades due to their high performance/price ratio and environmentally friendly nature. As a light-absorbing media in DSSCs, organic sensitizers are considered as promising candidates because of their facile structural modifications, low cost, and relatively high absorption coefficient with respect to ruthenium complexes. Extensive efforts have been focused on improving the overall efficiency and stablility of the sensitizers through rationalizing molecular structures. In this dissertation, we strived to optimize the donor moiety, conjugated bridge and acceptor/anchoring group of sensitizers with the configuration of D-π-A. Besides, we incorporated phthalimide and benzotriazole as auxiliary electron electron-withdrawing units in indoline based D-A-π-A sensitizers to systematically study their effect on the photophysical and electrochemical as well as photovoltaic properties in DSSCs.The main contents of the manuscript are summarized as follows:In introduction, the research background and basic working principle of DSSCs were introduced. The recent progress of typical dye sensitizers was reviewed, and the research strategy of the dissertation was presented.A novel series of organic dyes containing a fluorene unit as the conjunction bridge (S1-S4) have been designed and synthesized for a potential application in DSSCs. Their absorption spectra, electrochemical and photovoltaic properties have been investigated in detail. The tuning of the HOMO and LUMO energy levels can be conveniently realized by alternating the donor moiety. As demonstrated, the indoline unit can exhibit a stronger electron donating ability, realizing a broader absorption when coated onto TiO2. The incorporation of octyl-substituted fluorene is highly beneficial to prevent close π-π aggregation, thus favorably suppressing charge recombination and intermolecular interaction. In both HOMO and LUMO orbitals, the electron density located at the conjunction bridge (fluorene unit) is quite low, suggesting that the incorporated fluorene unit plays quite small role in the electron delocalization between the donor and acceptor units due to the twist conformation. The overall conversion efficiencies of DSSCs based on these dyes are in the range of3.04to4.26%and highly dependent upon their donor moieties. Among these dyes, S3,which utilizes a methyl-substituted indoline moiety as an electron donor, has shown a maximum short current destiny (JSc), with a relatively high open circuit voltage (Foc) of727mV.For developing panchromatic dyes, we develop sensitizer C-RA, and focus on the effect of rhodanine acceptor on photovoltaic performances in D-π-A indoline dye. Upon changing from cyanoacrylic acid (C-CA) to rhodanine-3-acetic acid (C-RA) as acceptor and anchoring group, C-RA shows broader absorption band, which can overlap with the solar spectrum more preferably. However, the power conversion efficiency of DSSCs based on C-RA (0.57%～0.90%) is one order of magnitude with respect to C-RA (8.49%). We find that the distinct difference of IPCE values between C-CA and C-RA is predominately attributed to the different excited electron injection yield (Φinj). Both the excited electron lifetime and DFT calculation indicate that the serious charge recombination between the photo-induced electron and the oxidized dye state in C-RA results in a low electron injection efficiency, leading to a low Jsc. Moreover, with respect to C-CA, the systematic SLIT and EIS studies demonstrate that C-RA holds the relatively low injection charge density in the TiO2electrode and fast charge recombination rate, leading to a low Voc. Our studies are highly helpful to the design of novel metal-free D-π-A organic sensitizers, especially for those using rhodanine-3-acetic acid as acceptor.Two novel D-π-A organic dyes LS-4and LS-5, containing3,4-ethylenedioxythiophene (EDOT) in the conjugated bridge, are developed for nanocrystalline TiO2-based DSSCs. Systematic investigation is carried out on focus of the joint photophysical and electrochemical analysis on photovoltaic properties. Incorporating EDOT as conjugated bridge brings several characteristics:(i) distinctly increasing the light-harvesting ability such as broadening absorption range and increasing molar extinction coefficients;(ii) improving electron injection with the predominant enhancement of short-circuit photocurrent;(iii) exhibiting little effect on the open circuit voltage. LS-4shows a more negative oxidation potential and a bathochromic shift in absorption spectra with respect to LS-5, indicative of the more powerful electron-donating capability of indoline unit than that of triphenylamine. With coadsorption of2mM chenodeoxycholic acid (CDCA), the LS-4based DSSC exhibits a promising conversion efficiency of6.05%, with a,Jsc of13.23mA cm-2, Voc of642mV, and fill factor (ff) of0.711under AM1.5illumination (100mW cm-2). Moreover, in the system of LS-4, DSSCs is workable with the driving force of150mV for dye regeneration process, paving a road towards minimizing energy losses in dye regeneration process.Two organic D-A-π-A sensitizers LS-2and WS-5containing N-octyl substituted phthalimide and benzotriazole as auxiliary electron withdrawing units with similar dimension and structure architecture were systematically studied on focus of photophysical, electrochemical as well as photovoltaic properties in nanocrystalline TiO2-based dye-sensitized solar cells. Interestingly, with similar five-member benzo-heterocycles, the two auxiliary acceptors of phthalimide and benzotriazole play exact different roles in absorption and intramolecular charge transfer:(ⅰ) in contrast with WS-5delocalized throughout the entire chromophore, the HOMO orbital of LS-2is mainly located at the donor part due to the twist conformation with the existence of two carbonyl groups in phthalimide;(ⅱ) the dihedral angles of "D-A" plane and "A-π" plane in LS-2further suggest that the incorporation of phthalimide moiety results in curvature of electron delocalization over the whole molecule;(ⅲ) in contrast with the beneficial charge transfer of benzotriazole in WS-5, the phthalimide unit in LS-2plays an opposite negative contribution to the charge transfer, that is, blocking ICT from donor to accepter to some extent; and (ⅳ) in electrochemical impedance spectroscopy, the incorporated benzotriazole unit enhances electron lifetime by18.6-fold, the phthalimide only increases electron lifetime by5.0-fold. Without coadsorption of CDCA, the DSSCs based on WS-5exhibited a promising maximum conversion efficiency of8.38%with significant enhancement in all photovoltaic parameters (Jsc=15.79mA cm-2, Voc=791mV,ff=0.67). In contrast, with the very similar D-A-π-A feature changing the additional acceptor from benzotriazole to phthalimide unit, the photovoltatic efficiency based on LS-2was only5.11%, with less efficient photovoltaic parameters (Jsc=10.06mA cm-2, Voc=748mV,ff=0.68). Therefore, our results demonstrate that it is essential to choose proper subsidiary withdrawing unit in D-A-π-A sensitizer configuration for DSSCs.To optimize the photovoltaic performance of LS-1, the π-bridges of EDOT, methylene methine chain and hexyl-thiophene was introduced into indoline based sensitizer to develop dyes LS-3, LS-6and LS-7, and systematically studied the effect of different conjugated bridges on the photophysical, electrochemical as well as photovoltaic properties in DSSCs. The absorption spectra of three dyes show significant red-shift with respect to LS-1. Among them, EDOT in LS-3shows a priority in optimizing the photophysical and electrochemical properties of indoline based sensitizers. I-V and EIS results indicate that the introduction of alkyl chains on thiophene unit is effective in TiO2/dye/electrolyte interface modification, and realizing the enhancement of Voc. Under optimized condition, LS-7based DSSC exhibits a conversion efficiency of5.45%, with the parameters Jsc=11.61mA cm-2, Voc=744mV,ff=0.63.