Synthesis And Photovoltaic Properties Of Organic Sensitizers With A Thiophene-Based Π-Conjugating Spacer

Generally, metal-free organic sensitizers possess donor-π spacer-acceptor conformation. Thiophene and its derivatives were investigated and applied extensively for photoelectric devices owing to their excellent optoelectronic properties. Thiophene and its derivatives were incorporated into sensitizers with such a push-pull configuration as a π spacer in DSSC. Broadening the UV-Vis absorption bands and shifting the maximum absorption wavelength bathochromically can be achieved by modification and optimization of thiophene-based π spacer, for example, fusing aromatic unit into thiophene ring, which can also tune the HOMO and LUMO energy levels to match the redox potential of the redox couple in the electrolyte and the conduction band of the titania felicitously, respectively, thus resulting in an increased electron injection efficiency and dye regeneration rate. In addition, functionalization of dye molecules is expected to suppress dye aggregates and charge recombination via introduction of varied alkyl chain, changing the location of the alkyl chain substituted on the thiphene rings, which is favorable for improving photovoltaic performance of DSSCs. Based on the above points, we have designed and synthesized a series of functionalized photosensitizers based on thiophene for DSSC application. The main reseach contents are summarized as follows.(1) The effects of the linear and branched alkyl chains on the photophysical, electrochemical properties and photovoltaic properties are investigated. By comparison, branched alkyl chains are superior to linear alkyl chains in suppression of intermolecular interactions and the electron recombination between the injected electrons and the electron acceptors in the electrolyte. Consequently, a more significant improvement of the open-circuit photovoltage can be achieved by the introduction of branched alkyl chains to the π-conjugated bridge of the organic dye in comparison to the incorporation of linear alkyl chains. Conversely, linear alkyl chains are better than branched alkyl chains in broadening photoresponse and hence photocurrent generation. As a result, a power conversion efficiency of 8.12% was achieved for the DSSC based on FNE29 with linear alkyl chains.(2) Two sets of isomeric organic dyes with bithiophene (L2 and R2) or terthiophene (L3 and R3) bridging the electron donor and acceptor groups have been designed and straightforwardly synthesized via a facile and selective synthetic route. The structural difference between the isomers stands at the position of the incorporated alkyl chains which are introduced into the "left" (L) or "right" (R) side of the oligothiophene spacer. The relationship between the isomeric structures and the optoelectronic properties are systematically investigated. It is found that in the L series dyes, the alkyl group is much closer to the aromatic donor moiety, which brings about strong steric hindrance and therefore causes a remarkable twist in the molecular skeleton. By contrast, a more planar chemical structure and more effective π-conjugation are realized in the R series dye isomers. Consequently, the R series isomeric dyes demonstrate bathochromically shifted maximum absorption bands, resulting in the improved light-harvesting capability and enhanced photo-generated current. However, the L series isomeric dyes with more twisted molecular skeleton show weaker intermolecular interactions and slower charge recombination rate, which induces higher open-circuit photovoltage. Taking all factors into consideration, the R dye exhibits higher power conversion efficiency (η) than the corresponding L dye. Promisingly, a η of 8.41% has been achieved for R3 based DSSC with liquid electrolyte containing Co(Ⅱ)/(Ⅲ) redox couple. The isomer R3 based DSSC with quasi-solid-state electrolyte displays the highest η of 7.10%, which remains 98% of the initial value after continuous light soaking for 1000 h. This work presents the crucial importance of molecular engineering and paves the way to designing organic sensitizers for highly efficient and stable DSSCs.(3) Novel organic sensitizers containing naphtho[2,1-b:3,4-b’]dithiophene group with triarylamine as the electron donor have been designed and synthesized for DSSCs. It is found that the incorporation of an electron acceptor is superior to an electron donor on broadening the absorption spectrum and improving the DSSC performance. As a result, the η value significantly increases from 5.2% for FNE50 based DSSC to 8.2% for FNE52 based DSSC with a liquid electrolyte. Most importantly, the DSSC based on FNE52 with a quasi-solid-state electrolyte displays a η of 7.1%, which remains 98% of the initial value after continuous light soaking for 1000 h.(4) Novel organic sensitizers containing thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety with triphenylamine or julolidine as the electron donor have been designed and synthesized for quasi-solid-state DSSCs. The incorporation of TPD is highly beneficial to broadening the absorption spectra of the organic sensitizers and diminishing the intermolecular interaction. Therefore, the charge recombination is slowed down, which is revealed by the controlled intensity modulated photovoltage spectroscopy. A quasi-solid-state DSSC based on sensitizer FNE38 with TPD and triphenylamine moieties demonstrates the best solar energy conversion efficiency at standard AM 1.5G sunlight without the use of coadsorbant agents.(5) Thieno[3,4-c]pyrrole-4,6-dione based organic sensitizers with triphenylamine (FNE38 and FNE40) or julolidine (FNE39 and FNE41) as electron donating unit have been designed and synthesized. Linear alkyl chain, i.e., hexyl group, or branched alkyl chain, i.e.,2-ethylhexyl group, is introduced into the dye molecular skeleton to minimize the intermolecular interactions. It is found that the sensitizers containing either hexyl or 2-ethylhexyl group demonstrate similar photophysical and electrochemical properties, such as absorption spectra and energy levels, due to their similar chemical structures. However, the sensitizers FNE40 and FNE41 with branched 2-ethylhexyl chain exhibit higher open-circuit voltage (Voc), short-circuit current (Jsc) and η than FNE38 and FNE39, respectively, with the linear hexyl group for the quasi-solid-state DSSCs. The Voc differences can be well explained by the different charge recombination rate. This work presents a design concept for considering the crucial importance of the branched alkyl substituent in novel metal-free organic sensitizers.