Dissertation

Characterisation and Lifetime Studies of CPDT- and BT-Based Photovoltaic Cells

Waters, Richard Huw Robert

2015

Overview

Organic photovoltaic (OPV) devices provide an opportunity for low cost, printable solar cells. This thesis focuses upon improving power conversion efficiencies (PCE) and lifetimes of OPV devices, with an emphasis on studying materials made from cyclopentadithiophene (CPDT) and benzothiadiazole (BT) monomers. The first part of the work focuses on characterising the optical and electrical properties of new materials based on this material system. A donor-acceptor-donor (D-A-D) small molecule, with the monomer order of CPDTBT-CPDT (C2B1) was trialled and demonstrated an optical band gap of 1.8 eV. OFET mobility was measured as 5 x 10-3 cm 2 .V-1.s-1 in the saturation region, and when blended with phenyl-C71-butyric acid methyl ester (PC71BM), gave a PCE of 1.54% under AM1.5G illumination. The moderate performance directed research towards donor-acceptor (D-A) polymers (PCPDTBT), synthesised using direct heterolytic arylation. OPV devices made with this material blended with PC71BM gave a maximum PCE of 4.23%, when tested under AM1.5G. OPV device performance is slightly higher than for PCPDTBT synthesised using more established techniques. This is the first known report of a working device with an active layer polymer synthesised using the direct heterolytic arylation route. The stability of the PCPDTBT material was tested using a combination of OPV device data and analytical instruments. From device data, PCPDTBT was shown to be less stable than the more commonly reported P3HT material. Significantly, processing additives used to optimise the active layer morphology, are shown to be detrimental to long-term performance, approximately halving the device half-life (T50%). The physical changes are examined using Atomic Force Microscopy (AFM) and Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and show that the inclusion of processing additives leads to greater morphological changes during ageing experiments. The chemical changes occurring in PCPDTBT were examined using XPS and show that light soaking leads to observations of severe oxidation, with a break-up of the aromatic rings, formation of sulphates at the thiophene ring, chain scission in the polymer backbone and loss of side chains. However, it is concluded that morphological changes are mostly responsible for the observed decrease in OPV device PCE. PCPDTBT with thermally initiated cross-linking behaviour is characterised and used to fabricate OPV cells. Cross-linkable PCPDTBT demonstrates a PCE of 3.65%, which is similar to its non-cross-linkable analogue, however, improved stability is observed from ageing experiments. This increase in stability, investigated further using AFM and GISAXS, is a result of fewer morphological changes in the active layer. While the work has focussed on PCPDTBT, many of the conclusions regarding the analysis of material degradation could be of wider interest to the field. The analysis could provide some new insights, on the degradation and stability of conjugated polymers and fullerene derivatives.

• Publisher: ProQuest Dissertations & Theses
• Subject: Additives / Alternative Energy / Annealing / Applied physics / Condensed matter physics / Electrical engineering / Electrodes / Lifetime / Morphology / Optical properties / Photovoltaic cells / Polymers / Solid state physics / Topography / X-rays
• ISBN: 1088387225, 9781088387221