The effects of processing on organic photovoltaic materials

Abstract

The work of this thesis has two main areas of research: firstly, combining morphological and photophysical measurements to better understand the high efficiency blend of PTB7:PC₇₁BM and the manner in which 1,8-diiodooctane (DIO) affects its structure and properties; and secondly, in developing a completely solution-processable method for creating a controlled zinc oxide nanorod array.

The crystal structure of PTB7 was found to comprise broad, overlapping diffraction peaks, where previously there was thought to be only one diffraction peak. It was also discovered that an intimately mixed, ordered phase exists in PTB7:PC₇₁BM which gives rise to a diffraction peak of its own. This is the first report of such a peak in this blend.

The PTB7:PC₇₁BM blend was prepared with the solvent additive DIO in a range of concentrations. It was discovered that there is a broad range of DIO concentrations over which this blend will perform well, and it is only when 20 vol% of DIO was used that the crystallinity of PTB7 changes. From 3 vol% to 8 vol% there were no significant changes, with solar cell performance almost unchanged.

The PTB7:PC₇₁BM blend was prepared with a range of different solvent additives, to try and determine the mechanism by which DIO improves this blend’s performance. The important features of DIO appear to include its wettability, its boiling point, and its two iodine atoms at opposite ends of the molecule.

Nanorod arrays of zinc oxide were prepared by a combination of spray pyrolysis and hydrothermal growth. Changing the concentration and volume of the spray pyrolysis precursor solution controlled the thickness and grain size of the deposited zinc oxide layer, which in turn controlled the dimensions of the resulting nanorod array which grew from the spray pyrolysed layer.

Date of Award	22 Jun 2016
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Ifor David William Samuel (Supervisor)