Synthesis Of Thiophene Based Organic Dyes And Their Applications In Dye-Sensitized Solar Cells

Dye-sensitized solar cells as the sole efficient solar cells imitate photosynthesis, arecurrently attracting a great deal of both scientific and industrial interest, owing to their greatsignificance to solve problems of clean renewable energy. A photosensitizer plays a crucialrole in the conversion of solar to electricity in dye-sensitized solar cells (DSCs). Thecombination of some non-iodine one-electron outer-sphere redox shuttles withhigh-absorption-coefficient organic D-π-A dyes has very recently brought forth a newopportunity towards efficiency enhancement of dye-sensitized solar cells (DSCs). Thereby acomprehensive understanding on structure-property relationship of metal-free organicphotosensitizers will play a pivotal role in further development of high-efficiency devices.Based on this, the main research work are as follows:On the basis of three simple organic dyes bearing the thiophene (C213),3,4-ethylenedioxythiophene (C230) and dihexyl-substituted cyclopentadithiophene (C218)π-conjugated linkers, we insert one double-bond between a triphenylamine electron donor andthiophene heterocycles to construct their counterparts (C236, C237and C238), with a clearmotivation to investigate the general influence of one double-bond difference in these organicdyes upon the photovoltaic behaviours via a joint photophysical and electricalcharacterization, by taking a close look at the dye-correlated multi-channel charge transferdynamics and energetics at the titania/dye/electrolyte interface. One double-bond insertionbetween a triphenylamine electron donor and π-conjugated heterocycles endows abathochromic effect in terms of electronic absorption of a dye-coated titania film, but leads toa relatively lower electron injection yield as indicated by transient emission measurements.Moreover, this style of one more double-bond incorporation brings forth a lower open-circuitphotovoltage, owing to the occurrences of a downwards shifted titania conduction band edgeand an augmented rate constant of charge recombination at the titania/electrolyte interface asrevealed by electrical impedance analysesWe designing three organic dyes (C225, C226and C227), wherein thedihexyl-substituted cyclopentadithiophene moiety is stepwise shifted from the electronacceptor side to the donor one, along with the additional use of two3-hexylthiophene rings as the conjugated spacing unit. With respect to C225and C226, C227presents a relativelyinefficient photo-induced electron injection as indicated by photoluminescence measurements,which accounts for its lower efficiencies of converting incident monochromatic photons tocollected electrons. Transient absorption measurements suggest that the charge recombinationbetween oxidized dye molecules and titania electrons gradually decelerates from C225toC227, whilst the interception of oxidized dye molecules by iodide ions exhibits an apparentdriving force-dependent, Marcus normal region behavior.We synthesized two simple model photosensitizers T1and T3, bearing the thiophene andterthiophene as π-conjugated linkers to demonstrate the redox couple dependent trait ofmetal-free dyes in DSCs, and further take a close look at its intrinsic origins through a jointelectrical, photophysical, and computational analysis. A cobalt electrolyte has been identifiedto offer an advantageous impact of π-conjugation extension in push-pull organic dyes, uponthe interfacial charge recombination kinetics and thus the open-circuit photovoltage of DSCs,in sharp contrast with the conventional iodine congener. The usage of a solelyvisible-light-absorption dye has generated a DSC free of corrosive iodine, exhibiting animpressive overall power conversion efficiency of8.0%at the AM1.5G conditions.Base on the dye sensitizers C213, C230and C218, we increase the number of electronacceptor to fabricate their congeners C231, C232and C233. We scrutinize some generalinfluences of the cyanoacrylic-acid-anchoring-group number of organic dyes upon theoptoelectronic features of mesoscopic titania solar cells based on thetris(1,10-phenanthroline)cobalt(II/III) redox shuttle, by employing three pairs of uniped andbiped dyes with thiophene,3,4-ethylenedioxythiophene and4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene as the π-conjugated spacers. It is foundthat regardless of the spacer selection, a biped dye confers an improved light absorptioncoefficient of a stained titania film in comparison with its uniped analogue, which is highlydesirable for the cells with an one-electron outer-sphere redox shuttle, because in this type ofcells a very thin titania film is always needed to satisfy the mass transport and electroncollection requirements. However, our electrical impedance analyses have revealed that thealteration from a uniped chromophore to its biped congener evokes not only a downwarddisplacement of the titania conduction band edge but also an acceleration of interfacial chargerecombination of the titania electrons with cobalt(III) ions, leading to an attenuated open-circuit photovoltage, highlighting the future necessity to design a sterically bulkyorganic dye featuring a superior light-harvesting capacity.