Abstract
Semiconductive organic polymer and small molecule materials are widely researched for the fabrication of low cost, large area and flexible organic photovoltaic devices. Semiconductor nanocrystals which demonstrate size tuneable optical bandgaps, can be incorporated with organic materials to enable wider spectral absorption and consequently improve the power conversion efficiency (n) of organic photovoltaics. Hybrid systems fabricated with wide bandgap (CdSe, CdTe) nanocrystals have reported promising results to this end. However, in order to further increase the spectral absorption of solar irradiation, particularly in the energy rich near infrared region, narrow bandgap nanocrystal systems must be utilised in hybrid photovoltaic fabrication. Attempts of using Pb chalcogenide (PbS and PbSe) nanocrystals in hybrid architectures have not yet been completely successful. The aim of this project was to design, fabricate and characterise novel organic (fullerene) and PbS-nanocrystal based hybrid photovoltaic systems for broadband light harvesting. Small molecule organic materials (pentacene and tetracene) and C60 were used as the organic and fullerene materials together with PbS-nanocrystals synthesised in-house. Three primary device architectures were investigated, where PbS-nanocrystals were used as an electron donor, acceptor and also as a tandem layer. A PbS-nanocrystal/C60 hybrid photovoltaic device in which the nanocrystals act as charge donors demonstrated a thirty fold increase in short circuit current density (Jsc) upon removal of the as-synthesised oleic acid ligands studied using X-ray photoelectron spectroscopy (XPS) and thermogravimetric methods. XPS analysis demonstrated a shift in the binding energy of Pb4f7/2 orbital, attributed to the removal of oleic ligands. Furthermore, photosensitivity up to 1600 nm was demonstrated with an external quantum efficiency (EQE) of 0.025%, together with a maximum EQE of 3.3% at 450 nm. The PbS-nanocrystal/C60 hybrid photovoltaic architecture was optimized by exchanging the as- synthesised oleic ligands in PbS-nanocrystals to shorter butylamine ligands, characterised by photoluminescence and infrared spectroscopy, which improved charge carrier mobility. It was seen using atomic force microscopy that butylamine capped nanocrystals formed smooth non-porous films on the conductive substrates which enabled deposition of thinner nanocrystal films (100 nm) further improving charge extraction. An eight fold improvement of n was observed as compared to oleic capped based hybrid nanocrystal photovoltaics. Furthermore, it was found that carrier mobility of a PbS-nanocrystal film was improved by soaking in anhydrous methanol. Consequently, the hybrid photovoltaic fabricated after methanol treatment demonstrated a Jsc of 5 mAcm-2 and an n of 0.44%, which is the highest reported for a hybrid photovoltaic incorporating narrow bandgap nanocrystals, A maximum EQE of 35% at 400 nm and up to 5% EQE within the near infrared region was also demonstrated from this device. Further optimization of this photovoltaic system is discussed by modelling the maximum expected Jsc and by inferring properties controlling the open circuit voltage and fill factor. PbS-nanocrystals were also used as electron acceptors incorporated with acenes. Photoinduced electron transfer between tetracene/PbS-nanocrystals was seen to be more efficient as opposed to pentacene/PbS-nanocrystals, studied using photoluminescence quenching and EQE measurements. This phenomenon was explained using an interfacial effect between the pentacene/PbS-nanocrystals attributed to the permanent dipole moment of the nanocrystals. Furthermore, the possibility of using PbS-nanocrystals as a donor-acceptor tandem layer was investigated by fabrication of a multifunctional hybrid photovoltaic using tetracene and C60, which demonstrated up to two orders greater EQE in both the ultraviolet and near infrared. Device operations for all above architectures were justified by direct measurement of the PbS-nanocrystal energy levels using ultraviolet photoelectron spectroscopy and cyclic voltammetry measurements. It is concluded that with suitable optimisations the novel photovoltaic systems studied here could be explored as a viable thin film photovoltaic technology.