Modeling of Charge Transfer at Dye-Semiconductor Interfaces in p-Type Solar Cells

Abstract

Dye-sensitized solar cells are composed of cheap and recyclable materials. These colorful and flexible cells convert sunlight into renewable energy. However, dye-sensitized solar cells are inefficient due to their low-charge current. The goal of this thesis is thus to create better understanding of the various components of these cells in order to improve their efficiencies. The main focus of dye-sensitized solar energy research lies in charge-transfer reactions between three main components: dye molecule; semiconductor surface, and electrolyte. In dye-sensitized solar cells, the charge is transferred from the excited dye molecule to the semiconductor surface. The charge is then transported to the electrode to create the electric current.<br/><br/>The studied components are derivatives of boron-dipyrromethene, perylene monoimide, and trisphenyl amine as dye molecules. Nickel oxide and titanium dioxide are used as semiconductor surfaces. The studied anchoring groups are carboxylate, 1,2-diol, and pyridine. These are studied in isolation, and then, their interactions in contact are investigated. In this study theoretical modeling is used, which includes the hybrid functional B3LYP and CRYSTAL09 software. The hybrid functional, B3LYP, is not widely used in studies within periodic boundary conditions such as dye-semiconductor interfaces. Thus, the mentioned systems have not been studied earlier at this level of theory.<br/><br/>In the first part of the study, the isolated systems, namely derivatives borondipyrromethene and titanium dioxide, are investigated. Results with BODIPY show that i) the vinylene group makes no difference for the electron transition nor orbital localization of orbitals, ii) phosphonate draws the most and carboxyl the least amount of the electron localization from the anchor, iii) methyl groups block completely the electron localization from the anchoring group, and iv) the absence of donor decreases the energy of HOMO level. BODIPY with vinylene and methyl groups were chosen for the synthesis, because the methyl groups increase the life-time of electron–hole pair. Doping with nitrogen shows that the amount of nitrogen atoms makes difference for electronic structure, yet do not distort the lattice. The electronic structure changes due to the created empty gaps states that enhance electron conductivity.<br/><br/>In the second part of this study, derivates of perylenemonoimide based dye molecules trisphenyl amine based anchoring groups on the NiO(100) surface and their interactions are investigated. The study with complete dye molecules on the nickel oxide surface is straightforward: the dyes’ highest occupied molecular orbital is above nickel oxide’s valence band maximum, thus, the spontaneous charge transfer is hard to obtain. Next it is shown that the anchoring group impacts the energy level alignment because of created dipole moment of the system that creates a shift within an electrostatic potential. Due to the shift, the highest occupied molecular orbitals of the dye molecule are either above (in case of carboxylate) or below (in the cases of pyridine and 1,2-diol) valence band maximum. In the case of complete molecules, which have the carboxylate as an anchoring group, HOMOs are above the VBM.<br/><br/>The results and analysis of the study show the importance of the size of the molecule and the anchoring group. The smaller the dye molecule, the smaller the distances are inside the interacting system, and shorter distances to transfer the charge. In the case of complete molecules, which have the carboxylate as an anchoring group, HOMOs are above the VBM. In conclusion, the largest effect is caused by the anchoring group inside the interacting system.