An investigation of liquid crystalline and semiconducting blends for applications in photovoltaics

Myers, Steven A.

Abstract

We investigated a number of novel liquid crystalline semiconductors which have been synthesised in house at the University of Hull. The electrochemical and optical properties of these materials were studied, in order to assess their suitability as donor or acceptor materials for photovoltaic (PV) devices. We investigated the HOMO and LUMO energies of materials with similar chemical structures and hence identified the effect of adding or removing certain molecular groups. Compounds with a benzothiadiazole-thiophene structure were found to be potentially low band gap electron donor materials with perylene based compounds as electron acceptors. Fluorene-thiophene based compounds were also identified as potential electron donors. Charge mobility was studied for several electron donor compounds with similar chemical structures. Donor and acceptor (D/A) blends were investigated and compared to the pure compounds. The Time of Flight (TOF) technique was used to measure mobility. For the pure donor a reduction in mobility is seen when the molecule chain is extended with two additional thiophene groups. The electron mobility of the blends was higher by 2 orders of magnitude than that of the pure acceptor. This provides the blends with balanced charge transport for PV devices.

We fabricated and characterised bulk heterojunction PV devices mixing various donor materials with the same perylene acceptor. The highest device efficiency was produced by a fluorene-thiophene structured donor compound. The fill factor (FF) for all devices was poor and this may be attributed to the acceptor material. We investigated the phase transitions of several D/A blends. An improved device efficiency was produced when we annealed within the liquid crystal phase. We investigated the thin film morphology using an atomic force microscope (AFM) and correlated domain size to PV device performance.

Citation

Myers, S. A. (2012). An investigation of liquid crystalline and semiconducting blends for applications in photovoltaics. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4213411

• Thesis Type: Thesis
• Deposit Date: Nov 20, 2012
• Publicly Available Date: Feb 22, 2023
• Keywords: Physics
• Public URL: https://hull-repository.worktribe.com/output/4213411
• Additional Information: Department of Physics, The University of Hull
• Award Date: May 1, 2012