Theoretical Investigations On The Spectral Characteristic Of Dye-sensitized Solar Cell Systems And The Design About Dye Molecules

Due to the high incident photo-to-current efficiency (IPCE) and unique thermalstability, dye-sensitised solar cell (DSSC) have been widely used in mechanica, bionic,aerospace and other fields. Compared with the traditional silicon solar cell. DSSCs arelower in cost, simpler in preparation. Though lower in efficiency, DSSCs wouldchallenge the market of traditional silicon cells with further increasing in efficiency.The remarkable improvements in DSSCs's overall efficiency contributed by thechemistry commucity from every corner of the world have been enlargeing thispossibility in the last two decades. In experiment, the attentions are focused onincreasing efficiency, synthesizing dye molecule and optimizing device structure,while physical chemist devote more effort on the electronic structure of molecule andsimulation of photoelectric process related with DSSC efficiency.The current PhD thesis focuses on the electronic structure and spectral characterof the separate dye molecules and the dye which adsorbed TiO2surface. Based on thetheoretical design and calculation,the dye molecules with strong and wide absorptionband in the visible region were obtained. A small TiO2surface model which can beused to explore the electronic structure and spectral properties of dye onheterogeneous interface contributed by dye and TiO2surface was designed and the rationality has been validated. The detailed findings are summarized in the followingseven chapters. In Chapter 1, we provided the fundmental background about DSSC,the goal of the PhD project, and the principle conclusions I have got. The technicaldetails are summaried in Chapter 2 with possible clarity. The rest of the chapters focuson the exact work I have contributed.1. A variety of heteroleptic ruthenium sensitizers have been widly applied inDSSC due to their higher light-harvesting efficiency and lower charge-recombinationpossibility than the well known homoleptic N3 dye. Hence, a great deal of attentionhas been focused on sensitizers with the general formulaRu(ancillary-ligand)(anchoring-ligand)(NCS)2, for example, Ru(4,4'-bis(5-hexylthio-phen-2-yl)-2,2'-bipyridine)(4,4'-carboxylic acid-4'-2,2'-bipyri-dine)(NCS)2(C101) andRu(N-(4-butoxyphenyl)-N-2-pyridinyl-2-pyridinamine)(4,4'-carboxylic acid-4'-2,2'-bi-pyridine)(NCS)₂(J13). In order to simulate the real conformation of dye whenadsorbed onto the TiO₂surface under the experimental conditions, thephotosensitizing processes of these sensitizers within different degrees ofdeprotonation (2H, 1H to 0H) have been explored theoretically. The ground/excitedstate geometries, electronic structures and spectroscopic properties are calculatedusing density functional theory (DFT) and time-dependent DFT (TDDFT). Theabsorption and emission spectra of all the complexes in acetonitrile solution are alsopredicted at the TDDFT (B3LYP) level. The calculated results show that the ancillaryligand contributes to the molecular orbital (MO) energy levels and absorptiontransitions. It is intriguing to observe that the introduction of a thiophene group intothe ancillary ligand increased the absorption transitions in the 380–450 nm region.The calculations reveal that although deprotonation destabilizes the overall frontierMOs of the chromophores, it tends to exert a greater influence on the unoccupiedorbitals than on the occupied orbitals. Consequently, an obvious blue shift wasobserved for the absorptions and emissions from 2H, 1H to 0H. Finally, the optimaldegree of deprotonation for C101 and J13 has been evaluated, which is expected tolead further improvements in the performance of DSSCs coated with these sensitizers. 2. Azoles and their derivatives were adopted as ancillary ligand to improve theperformance of N3 in DSSC. DFT based approaches were applied to explore theelectronic structures and properties. The calculation reveals that dye molecule with 1,2, 4-triazole groups would show very high intensity of absorption in visible range.TDDFT results indicate that the ancillary ligand dominates the molecular orbital (MO)energy levels and masters the absorption transition nature. The deprotonation ofanchoring ligand not only affects the frontier MO energy levels but also controls theenergy gaps of HOMO–LUMO and LUMO–LUMO+1. Small gap between LUMO–LUMO+1 contributes positively to the overall efficiency of DSSC.3. Special efforts were devoted to improve the absorption behavior of AR20 invisible region. DFT based approaches were applied to explore the electronic structureproperties of AR20 and its derivatives. TDDFT results indicate that the ancillaryligand controls the molecular orbital (MO) energy levels and masters the absorptiontransition nature. The deprotonation of anchoring ligand not only affects the frontierMO energy levels but also determines the energy gaps of highest occupiedMO–lowest unoccupied MO (LUMO) and LUMO–LUMO+1. Introducing thiophenegroups into ancillary ligands would enhance the efficiency of the final DSSC. Theabsorption intensity of the thiophene substituted derivatives of AR20 is irrelevantwith environment circumstance change, such as pH value. This special natureprognosticates the thiophene-substituted derivatives of AR20 which would have abroad application in DSSC.4. Electron injection from a photoexcited chromophore into the surface ofmesoscopic semiconductor TiO₂nanoparticles is one of the key electron transferprocesses for DSSC. A reasonable and reliable TiO₂surface model (taken from theanatase (101) and rutile (110) crystals a slab model) was designed, which wasemployed to investigate the absorption behavior of dye molecules such as C101, J13and N749 on TiO₂surface with under DFT method. According to the calculationresults, the detailed orbital components and absorption transition are obtained;furthermore, the ultrafast, excited-state charge injection and emission charege injection was discriminated. Generally, both the ways by which the dye is adsorbedinto TiO₂and degree of deprotonation of dye molecules could affect absorptionspectrum remarkably. Our calculations show that the more efficient DSSCs shouldhave larger conjugation degree for ancillary ligands or the whole system, which isbeneficial to photon absorption from the visible to near-IR region.5. N749, brings about outstanding IPCE in long wavelength range in DSSC.Depending on rational design of TiO₂(101) surface model and connecting typesbetween N749 and TiO₂film, the experimental absorption peaks have been wellreproduced within DFT and TDDFT approaches. According to the calculations, thedifference in connecting models in DSSC affects the final utilization of solar energydirectly. The way of electron injection in DSSC was discriminated within the resultsof charge-transfer transition. In addition, the degree of deprotonating about dyemolecules could also influence absorption spectrum obviously. It was demonstratedthat the dyes with closely-lying low energy unoccupied orbitals, which are conduciveto orbital-coupling, would improve the overall photo-to-current efficiency in DSSC.