Characterisation of carbon nanotube variability and application in solar cells

Abstract

Photovoltaic solar cells (PVs) are now recognised as the world’s fastest growing energy technology, yet, they only account for a mere fraction of current global renewable power capacity. It is acknowledged that this inadequate market penetration has been largely a result of the technology’s excessive cost. Researchers have therefore endeavoured to find innovative, economic solutions with the aim of either cutting back on the active material quantity or improving power efficiencies to abate operating expenditure. Both approaches, however, have presented a cost vs. performance trade-off, which some believe may be surmounted by the employment of Nanotechnology. Amongst the many potential nano-materials proposed for PV conversion is the Carbon Nanotube (CNT) due to its low material utilisation, superior carrier transport properties and most notably; a tunable band-gap. This thesis investigates the theoretical performance of a range of CNT based solar cells, and in doing so, novel computational methodologies are formulated towards characterising the related electronic and optical properties with respect to the CNT structural variability.

The first part of this thesis addresses the issue concerning the differentiation of metallic and semiconducting CNTs. As an outcome, a simulation-efficient and experimentally validated analytical model is developed to distinguish the nanotubes and predict the band-gap of semiconducting CNTs. In addition, a model that approximates the semiconducting CNT’s carrier effective mass is presented.

The key challenge affecting CNT’s at present resides in the uncertainty of the structural characteristics realised using existing synthesis techniques. Thus, the second part of this thesis aims to statistically model the variation in band-gap and carrier effective mass as a function of typical geometric distributions. This work offers a valuable insight into the optimisation of CNT diameter related process parameters towards suppressing electronic variability.

The final part of this thesis initially focuses on modelling the optical absorption of CNTs where the photo-generated current and quantum efficiency responses are derived for various tube geometries when exposed to laser illumination. The established models are later exploited in combination with an equivalent PV circuit model to evaluate the performance metrics of a variety of isolated CNT based PV devices under solar radiation. A proposed set of multi-band-gap CNT PV devices are also analysed where the optimized CNT structures for PV conversion are outlined.

Within the confines of the assumptions made in this study, it is concluded that only specific types of CNTs may yield competitive PV conversion efficiencies compared to other nanotechnology based solar cells. However, reservations are maintained on whether CNTs could outperform bulk PV materials, even when a multiple band-gap scheme is considered

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