Argent Dearden, Chloe (2015) The use of zinc oxide in hybrid inorganic-organic photovoltaic devices. PhD thesis, University of Warwick.

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Abstract

Organic photovoltaics (OPV) and hybrid organic-inorganic photovoltaics (HOPV) have the potential to provide alternative and economical energy sources; with the long-term goal of delivering renewable resources with longevity. Recent improvements in cell design and material combinations have revealed the true potential of this field. For this to be reached, continuous advancements in materials, concept development, encapsulation and scientific understanding are necessary.

This thesis focuses on the use of zinc oxide (ZnO) in the field of both HOPVs and OPVs. ZnO had been successfully implemented for decades in a range of applications, including light emitting diodes and biological sensors due to its diverse chemical and physical properties along with the ease of fabrication. Initially ZnO is investigated as a direct replacement for a fullerene acceptor offering the potential of improved energetic matching to the donor material used. The latter stages of this thesis looks at the use of ZnO as an electron extracting layer for a polymeric active layer.

Chapter 1 provides a brief introduction to the field of photovoltaics and the materials used in this thesis. In Chapter 2 an overview of the experimental techniques used is given. In Chapter 3, inverted HOPV devices are fabricated. The potential of ZnO as a promising electron acceptor is shown, utilizing the donor material boron subphthalocyanine chloride (SubPc), a typical small molecule (SM) organic semiconductor. X-ray photoelectron spectroscopy (XPS) shows subtle differences in the electronic structure of ZnO films in terms of Zn:O ratio when the processing temperature is varied, and Kelvin Probe (KP) revealed a significant difference in the surface work function. Variation in annealing temperature is shown to improve the open circuit voltage (from 0.82 V to 1.23 V) of the device and therefore enhance the performance.

Chapter 4 compares two methods used to probe energy levels. The chapter compares the differences between the data obtained for identical ZnO samples using ultra-violet photoelectron spectroscopy (UPS) and KP. The surface composition is also monitored throughout by XPS. The chapter reveals that ZnO is susceptible to UV irradiation and the impact on the measurements is discussed.

One of the main limitations of the planar HOPV is photocurrent. Chapter 5 looks to improve this through the implementation of a molybdenum oxide (MoOx) optical spacer layer. Optical modelling is initially used to predict the impact of varying the layer thickness of SubPc and MoOx. The model is developed further by including the diffusion length (LD) of the SubPc donor material. The improved estimates are compared to an experimental data set of 40 different thickness combinations. Optical optimisation resulted in a 62 % improvement in device performance, compared to the layer thicknesses used in Chapter 3.

The use of ZnO as an electron extracting layer with a polymeric active layer is investigated in Chapter 6. Two methods for ZnO layer formation, electrodeposition (ED) and sol-gel (SG) are compared using two different transparent electrodes, indium tin oxide (ITO) and transparent gold (tAu). ED ZnO layers have issues with transparency and reproducibility lowering the overall averaged performance.

This thesis highlights the important role ZnO can play in the development of OPV and HOPV devices. The research provides an important step for understanding the fundamental principles governing the operation of hybrid solar cells and helps to close the gap between TMO/polymer and TMO/SM devices. The performances of these TMO/SM devices reach efficiencies exceeding 0.70 %, compared to previous published devices only reaching 0.017 %.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Photovoltaic power generation, Zinc oxide
Official Date:	January 2015
Dates:	Date Event January 2015 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Jones, T. S. (Tim S.)
Sponsors:	Engineering and Physical Sciences Research Council (EPSRC)
Extent:	xx, 214 leaves : illustrations, charts
Language:	eng

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