Abstract
Organic photovoltaics (OPV) is one of the most dynamic and rapidly developing solar cell technologies leading to renewable / green energy. OPVs are based on organic semiconductors such as conjugated polymers, fullerenes and other small molecules. Such devices can be fabricated by low-cost, roll-to-roll printing techniques by layering extremely thin photoactive coatings on lightweight, flexible substrates which maybe organic in nature as well. Various OPV technologies are being developed in industrial and academic research fields where OPVs are on its way to broad commercialisation. To improve the power conversion efficiencies (PCEs) further researchers have looked into embedding nano-scale materials in OPVs over the last few years. The leading potential candidates being, carbon nanotubes (CNTs) and graphene, due to their extraordinary electronic properties and quantum dots with their tunable size dependant electronic properties. In this thesis, the effect of utilising single walled carbon nanotubes (SWNTs) as hole transport layers in bulk-hetero junction (BHJ) OPVs has been studied using a polymer wrapping technique of SWNTs for dispersion purposes. Supramolecular interactions based on pi-pi stacking between poly(3-hexylthiophene-2,5-diyl)poly(3-hexylthiophene) (rr-P3HT) and SWNTs are exploited to prepare self-assembled networks of nanohybrid structures for hole extraction in OPVs. The effectiveness of such a network for hole transport is demonstrated, with the hole extraction capability shown to be associated with the potential of SWNTs being hole-doped by the surrounding rr-P3HT sheath. The OPV device performance parameters obtained using the rr-P3HT/s-SWNT nanohybrid structures is comparable to devices fabricated using the much optimised Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layers. Therefore, the nanohybrids are proposed as a potential replacement of the widely utilised and standard PEDOT:PSS hole extraction layers in OPVs which are acidic and known to affect the transparent conductive oxide. The maximum PCE of devices employing nanohybrid HTLs is the best reported for OPVs utilising CNT HTLs to date (PCE ~ 7. 6%). In addition to the use of SWNTs as HTLs, their use for the fabrication of hole collecting electrodes in OPVs is also discussed. A simple method of drop casting, thermal annealing and acid treatment is introduced in fabricating SWNT electrodes that are chemically and mechanically robust. rr-P3HT wrapped electronic type separated SWNTs (i. e. semiconducting / s-SWNTs, metallic / m-SWNTs) are originally used to fabricate the films, due to the formation of uniform, well-dispersed, percolated networks of SWNTs resulting from polymer wrapping. The original sheet resistance of the polymer wrapped SWNTs has been brought down by ~ 80-90% using HNO3 acid treatment resulting in hole doping of the SWNTs. The results indicate an electronic type selective doping mechanism where the s-SWNTs show a higher affinity towards hole doping relative to m-SWNTs. Subsequently, the doped, highly conductive SWNT films have been employed as semitransparent positive electrodes in organic BHJ solar cells for hole collection. Performances of the devices indicate the importance of electrode film morphology over the electronic type of the doped SWNTs used in films with similar sheet resistance and optical transmission. The device performance characteristics relative to reference ITO electrodes suggest that a decrease in roughness of the SWNT electrodes could lead to better device performance in future.