Abstract
The conversion of solar energy into electricity through the photovoltaic effect could be a viable approach to supply the global energy requirements with minimal detrimental effects on the environment. Recent research on photovoltaics has focussed on organic photovoltaics (OPVs) which utilises organic materials due to their ease of processing, high light absorption coefficient and potential for low cost thin film device fabrication compared to its inorganic counterparts. The work presented in this Thesis is focussed on enhancing the photo-generated current of OPVs based on conjugated polymers and fullerene derivatives through the incorporation of multi wall carbon nanotubes (MWCNTs). Initial studies focus on the identification of suitable conditions for the fabrication of efficient reference devices (photo-active layer composed of poly(3-hexylthiphene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (C60-PCBM)) in order to observe any improvements upon addition of MWCNTs into the active layer. Deteriorated device characteristics are achieved with pristine MWCNTs (p-MWCNTs) incorporated P3HT:PCBM active layer devices due to the poor thin film morphology of the P3HT:p-MWCNTs:PCBM active layer films. This is identified as being due to the poor dispersion of nanotubes in organic solvents that hampers the formation of good interpenetrating networks at the nano-scale. Therefore, to improve the dispersion of MWCNTs in 1,2 dichlorobenzene (DCB) solvent acid functionalisation of MWCNTs is carried out. These dispersible acid functionalised MWCNTs (O-MWCNTs) in DCB are characterised by Raman and infrared spectroscopy, for verification of attachment of functional groups. An enhanced device performance is achieved through the incorporation of O-MWCNTs in to the P3HT:PCBM system. Optimisation of P3HT:O-MWCNTs:PCBM solar cells is observed to lead to devices with higher short circuit current densities (JSC). Investigations towards understanding the enhancement of JSC from P3HT:O-MWCNTs:PCBM is carried out through external quantum efficiency (EQE) measurements and photoluminescence (PL) spectroscopy. Higher EQE values of P3HT:O-MWCNTs:PCBM active layer devices, in particularly the higher collection probability in the blue and green region of the electromagnetic spectrum suggests an efficient charge separation at donor/acceptor (D/A) heterojunction, reduced recombination, and improved charge carrier mobility as the reasons affecting the increase in the net photo-generated current. The JSC is shown to increase from 8.0 mAcm-2 in the reference device to 8.34 mAcm-2 in the O-MWCNTs incorporated device. Furthermore, the drastic PL quenching suggests enhanced exciton dissociation in the system and is in agreement with the results achieved from EQE measurements. Analysis of these observations lead to the conclusion that O-MWCNTs are responsible for creating additional charge generating sites (exciton dissociation centres, EDCs) in the active layer. Favourable charge generation at P3HT/O-MWCNTs is observed due to the efficient exciton dissociation at the nanoheterojunction. A hypothesis of “O-MWCNTs as EDCs” is formulated on considering these observations and further investigation of the hybrid OPV system is carried out. The dielectric constant (e) for the composite systems of P3HT:PCBM and P3HT:O-MWCNTs:PCBM are estimated from a space charge limited current (SCLC) model and compared. A 1.23 times increase in the e of P3HT:O-MWCNTs:PCBM composite compared to P3HT:PCBM indicates a lower Coulombic attraction which leads to a lower binding energy. Charge transport analysis conducted through a SCLC model is used to understand the effect of addition of the tertiary component to the existing system and hole mobility is estimated. The addition of O-MWCNTs is favourable in lowering the e, minimising the exciton binding energy and enhancing the exciton dissociation process in the P3HT:O-MWCNTs:PCBM system. The JSC achieved for this system is higher than the existing P3HT:PCBM system and the calculated higher hole mobility of this system supports the improved charge transport process. As a result of these achievements, the role of O-MWCNTs in the OPV system is recognised as beneficial for charge generation.