Optoelectronic and spectroscopic characterisation of polymer-cadmium sulfide nanocomposite solar cells
Creators: name: Dowland, Simon
Publisher: name: Imperial College London
Year: 2013

description: Reducing the band gap of organic materials to accommodate absorption in the near IR has been a long standing problem of fully organic systems. In contrast, the size control of nanoparticles has enabled the tuning of the electronic band gap such that near IR absorption is achievable by a variety of inorganic materials. Furthermore, electron and hole mobilities in single crystal inorganic materials have been reported consistently well in excess of those in organic materials. However, these advantages have not translated into superior performances in devices based on solution-processed nanocrystals, which rely on good charge generation properties and an effective transport network in the photoactive layer to extract a current. To achieve an effective dispersion of nanoparticles throughout an organic matrix, traditionally, capping ligands have been employed on the surfaces of the nanoparticles. These have been shown to hinder both interfacial charge transfer, between the organic material and nanoparticle, and charge transport between nanoparticles. As a result much effort has been directed at facilitating good mixing between nanoparticles and organic materials without the use of ligands. One method that has shown promise utilises a single-source xanthate organometallic precursor, which is soluble in common organic solvents and thus easily dispersed throughout a polymer thin film. The application of heat results in the decomposition of this compound yielding volatile organic side products and a percolating metal sulfide nanostructure. The research presented in this thesis focuses on the design and function of solar cells employing photoactive layers of the semiconducting polymer, P3HT, and nanostructured CdS fabricated employing the in-situ strategy described above. Firstly, investigation of the role of the architecture of the solar cell is undertaken. The materials involved in the electron extraction process are scrutinised, followed by those responsible for hole extraction. The introduction of new materials and layers as well as their method of fabrication are considered in order to ascertain the optimum combination to support the effective operation of the active layer. The focus is then shifted on to the optimisation of the photoactive layer. Study into the effects of changing composition and processing conditions on the behaviour of devices is conducted and the potential routes for further improvement of the CdS/P3HT system are established. Next, the relationship between morphology and the photophysical properties of CdS/P3HT blends is subjected to a comprehensive spectroscopic investigation. Altering the morphology of the blends is achieved through a change in the composition ratio of the samples and confirmed by TEM. Transient and steady state absorption techniques as well as photoluminescence are used to monitor the behaviour of excited states and identify the drawbacks of the CdS/P3HT combination. Finally, the environmental stability of typical CdS/P3HT devices is examined. Devices and films are subjected to inert and ambient conditions both in the light and dark and the change in performance and charge separation yield (as determined by transient absorption measurements) is monitored over time. Furthermore a comparison is made between the stability of CdS/P3HT systems with their fully organic P3HT/PCBM counterparts. These are subjected to oxygen and water separately in order to shed light on the mechanism of degradation in both systems.