Theoretical Study On Dye-Sensitized And Perovskite Solar Cells And Photocatalytic Properties Of Mixed-Phase Materials

Our environment is getting worse due to economic development of the world, which requires the human being to use the renewable energy. Energy harvesting direct-ly from sunlight offers a promising approach for the request of clean energy, whereas it has little negative influence for environment. Generally, there are two ways to uti-lize solar energy. One is to convert solar energy into electrical energy directly, such as dye-sensitized cell. The other is to convert solar energy into chemical energy firstly, such as splitting water into hydrogen and oxygen using light, then utilizing hydrogen as chemical energy. In last decades, experimental works in these two areas have made great progress and power conversion efficiency (PCE) has been greatly improved. For example, dye-sensitized solar cell (DSSC) based on zinc porphyrin dye molecules and cobalt redox recently reached the best PCE of 12.3%, while PCEs claimed in the recent reports about perovskite solar cells also have exceeded 20%. Meanwhile, a large num-ber of semiconductors with photocatalytic activities have been designed and synthesized for water splitting, such as oxides and nitrogen oxides, exhibiting good performance for photocatalytic water splitting. However, there are many ambiguities need to be clari-fied, particularly on the theory. To further improve PCE and photocatalytic efficiency, it is essential to reveal the microscopic physical and chemical mechanisms. Our stud-ies are mainly focused on theoretical investigation of the microscopic mechanism of dye-sensitised solar cells and the geometric and electronic properties of photocatalytic materials, providing theoretical guide for the experimental study in the related field. This dissertation contains the following chapters.Chapter 1 gives a brief introduction on the related background.Chapter 2 presents the basic theory and framework of the first-principles method, including Hartree-Fock method, the fundamental theorem of DFT, exchange-correlation functional, plane wave basis set as well as pseduopotential. At last, we introduce com-putational packages adopted in this paper.Chapter 3 shows the first-principles theoretical investigation on the mechanism of YDoc sensitized TiO2 solar cell, where a step-by-step theoretical protocol based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) at both the molecular and periodic levels have been performed to study a Zn-porphyrin complex named YDoc sensitized TiO2 solar cell including dye excitations, electron injection, the regeneration of photo-oxidized dyes and the effect of electrolyte additives. Our study reveals the possibility of a favorable electron transfer from the excited dye to the semiconductor conduction band (CB) and suggests three possible pathways of the electron injection from the dye to nanoparticle (TiO2)38. One is the direct one-step injection by photoexcitation and the other two are from the different parts of the excited dye to the nanoparticle. The influence of the electrolyte composition on the geometrical and electronic features of the dye/TiO2 system has also been studied. It is found that with the additive of the lithium ion, the energy gap between the LUMO of dye and the TiO2 CB edge increases which subsequently increases the driving force for the ultrafast excited-state electron injection, contrary to the effect of 4-tert-butylpyridine additive. The computational results of the oxidized dye interacting with I- and I2- reveal that there are a few of possible mechanisms for regeneration of oxidized dye. The effective mechanisms of the regeneration are suggested.Chapter 4 presents a computational view on the change of geometric and electron-ic properties of perovskites arisen by the partial substitution of Pb by Sn. Recently, the solar cells with hybrid organic-inorganic lead halide perovskites have achieved a great success and their power conversion efficiency reaches to about 17.9%. For prac-tical applications, one has to avoid the toxicology issue of lead, devoting to develop the lead-free perovskite solar cells by using metal substitution. It has been shown that tin is one of possible candidates as a replacement for lead. Herein, a step-by-step proto-col based on the first-principles calculations is performed to investigate the geometrical and electronic properties of mixed Sn and Pb perovskite MAPbxSn1-xI3 with different crystal symmetries. Hybrid functional PBEO together with the inclusion of spin-orbit coupling effect is used to perform electronic-structure calculations. The results reveal that the band gaps of MAPbxSn1-xI3 decrease with the x decreases. Further investiga-tions show that the crystal symmetry can also modify the band gap, smaller with the lower symmetry. The random rotation of organic cations makes the band alignments in the compounds, which facilitates the separation and transfer of hole and electron. In-terestingly, the computed binding energies of the unrelaxed exciton has the same trend with band gaps, which decreases with decreasing of x and the binding energies of MAP-bo.5Sno.5I3 also decrease as the crystal symmetry lows, implying a faster exciton disso-ciation with lower x and lower symmetry at an ambient temperature.Chapter 5 presents the first-principles investigation on the origin of high photocat-alytic properties in the mixed-phase TiO2 and Ga2O3. Rational design and fabrication of mixed-phase oxide junctions is an attractive strategy in the photocatalytic applications. A new tuneable α/β mixed-phase Ga2O3 has recently been discovered to have the high activity in the photocatalytic water splitting. Here we perform a first-principles study to reveal the nature of efficient separation of photogenerated carriers achieved by the mixed-phase Ga2O3. It is found that the tensile strain and lattice misfitting at the inter-face junctions significantly tune their energy bands. As the interior angles between two components change, the characteristics of valence band-edge states can be significantly different. Through the analysis on the bonding strength of the bonds near the inter-faces, and the comparison of calculated and experimentally-observed carrier migration directions, we suggest a favorable junction for the efficient separation of photogener-ated carriers. Meanwhile, we also present a step-by-step theoretical protocol based on the first-principles methods to reveal the insight into the high photocatalytic activity achieved by the mixed-phase TiO2, consisting of anatase and rutile. The interface ge-ometries, density of states, charge density, optical absorption spectra and band offsets have been calculated. The most stable interface structures have been identified and the interfacial strain-dependent band offsets have been found. We find that the geometri-cal reconstruction around the interfacial area plays a negligible influence on the light absorption of the heterojunction and the interfacial sites seem not dominant the con-tribution to the band-edge states. For the most stable heterostructure, the calculated valence-band maximum and conduction-band minimum of rutile respectively lie 0.52 and 0.22 eV above those of anatase, which agree well with the experimental measure-ments and other theoretical predications.Chapter 6 gives a brief summary and perspective on the related filed.