Understanding the impact of molecular design on singlet fission for potential photovoltaic applications

• Main Author: Xiao, Xingchi
• Other Authors: Lam Yeng Ming
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Chemistry; Engineering; Other; Single fission; Perovskite; Transient absorption; Anthrathiadiazole; Organic synthesis; Thin film; Transient dynamics
• Online Access: https://hdl.handle.net/10356/174738
• Institution: Nanyang Technological University

Singlet fission (SF) is a multiple exciton generation process, where two or more exciton pairs can be generated with one absorbed photon. By involving the SF process into photovoltaic (PV) devices, the power conversion efficiency (PCE) of the single-junction solar cell is believed to be able to circumvent the theoretical Shockley-Queisser limit. As one of the most promising PV devices, perovskite solar cell (PSC) has achieved a remarkable progress in terms of the PCE during the last decade. However, due to the insufficient understanding of the electronic dynamics at the SF/perovskite interface, the SF-enhanced PSC has never been truly realized. In this thesis, the charge transfer dynamics of SF/perovskite heterojunctions has been comprehensively studied by combining the commercial SF material and perovskite with well-matched energy structures, and the impact of molecular geometries on SF process has been systematically investigated by synthesizing a series of new nitrogen and sulfur-substituted polyacenes. All these findings will contribute to the development of future SF-enhanced PV devices. First, the charge transfer dynamics of TIPS-pentacene/Cs0.05(FA0.85MA0.15)0.95PbI2.55Br0.45 heterojunction was investigated. The well-matched energy structures of two layers fulfilled the energy requirements of charge transfer process from the triplet state of TIPS-pentacene to the conduction band of perovskite in 1.2 ps, along with an effective hole transfer process from the valence band of perovskite to the ground state of TIPS-pentacene in nanoseconds. The efficient charge transfer process at the interface resulted in an increase by 20% in the free carrier density of perovskite. These results validate the possibility of augmentation in the carrier density of perovskite by the sensitization of SF process, thus shedding a light on the improvement in the PCE of SF-enhanced solar cells. Next, a series of novel SF materials consisting of nitrogen and sulfur incorporated polyacenes has been successfully synthesized to investigate the impact of molecular structures on their electronic properties. The energy structure results demonstrated the stabilization effect of substituted nitrogen atoms on the frontier molecular orbitals, which thus would improve the chemical stability. More importantly, the SF process that was only observed in the dimers, presented a significant structure-function relationship. For the anthrathiadiazole (ATDA) dimers, the directly-linked geometry delivered the fastest triplet generation rate of 1.09 ps, while the phenylene-linked meta geometry showed the lowest triplet generation rate of 270.2 ps. This work reveals the relationship between the SF property and the molecular structures and thus provides a design guide for the novel SF materials applied in the PV devices. Last, the ATDA-based thin films were fabricated by spin-coating method to investigate their SF properties in solid states. All SF films showed ultrafast SF process occurred in picoseconds with high triplet yield over 100%. The huge improvement of SF dynamics for ATDA monomer and meta-ATDA-dimer from solution to thin film demonstrates the crucial role of intermolecular SF process. While the resemblance of SF dynamics for ortho-ATDA-dimer in both solution and thin film indicates the insignificant impact of intermolecular interaction on the SF process, highlighting the dominance of intramolecular SF pathway in the ortho dimer. Thes results support the hypothesis that SF dimers are more suitable to be applied to the SF-enhanced PV devices.