Fabrication and characterisation of high efficiency inverted P3HT:PCBM polymer solar cells.

Among many photovoltaic (PV) technologies polymer-fullerene based inverted bulk heterojunction (BHJ) solar cells have drawn lot attention in recent years due to low cost fabrication over a large area by using simple solution-processed methods. This thesis presents a study of inverted organic solar cells (OSCs) on indium tin oxide (ITO) coated glasses and metal substrates using spin coating technique. Zinc oxide (ZnO) and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) were used as the electron transport layer (ETL) and hole transport layer (HTL) respectively. Poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methylester (P3HT:PCBM) was used as an active layer. The ZnO layers deposited using nanoparticles (NPs) and sol-gel route at a temperature of 150 °C. The poor wettability of aqueous PEDOT:PSS on the hydrophobic P3HT:PCBM layer was improved with the addition of surfactant Triton X-100. The P3HT:PCBM photoactive layer was optimised in terms of solvents, concentrations and layer thickness. However, the thickness of the active layer in BHJ devices need to be very thin (-200 nm) which causes poor light absorption and low carrier mobilities. Therefore, it is important to introduce new approaches to enhance the photon absorption efficiency of the active layer under the film thickness limitation. Among all the approaches plasmonic nanostructures have recently emerged as an expanding area to enhance light absorption in organic photovoltaic (OPV) devices. A low vacuum Plasma assisted physical vapour deposition (PAPVD) method was used to deposit Gold (Au) NPs thin film onto the PEDOT:PSS layer in the inverted P3HT:PCBM OSC devices. The Au NPs incorporated into the PEDOT:PSS layer and reaching the active P3HT:PCBM layer can provide a significant plasmonic broadband light absorption enhancement to the active layer. An improvement in the short circuit current density of 50-90% has been achieved compared with those OSC devices without the plasmonic light absorption enhancement. The enhanced current density is directly related to the enhancement of light absorption in the active P3HT:PCBM layer due to the creation of localized surface plasmonic resonance (LSPR) by Au NPs or nanometer-sized Au clusters. OSC devices were fabricated with various thicknesses of Au NPs films; the OSC device with -5.0 nm Au NPs film exhibited an efficiency of - 5.01% with a fill factor of -66.61%. The obtained improvement in power conversion efficiency (PCE) is mainly assigned to the noticeable increase of photocurrent and the improvement in the fill factor.Top illuminated inverted OSC devices were fabricated on metal substrates, namely: polished stainless steel (SS), titanium coated steel (Ti-S), chromium coated aluminium (Cr-Al) and polished aluminium (Al). OSC devices on metals had the simple structure of (Metal substrate/sol-gel ZnO/P3HT:PCBM/PEDOT:PSS/hcPEDOT:PSS). Good performance OSC devices were achieved on both the SS and Cr-Al substrates with an efficiency of -3.10% while no effective devices were produced on Ti-S and Al substrate. Electrical impedance spectroscopy experiments showed that the native oxide layer on top of different metal substrates causes significantly different performance in PCE for the inverted OSC devices.