Charge recombination studies in polymer solar cells

Abstract

This thesis presents the study of charge recombination within high performance organic solar cell materials. It focuses on the early nanosecond timescale dynamics after charge generation but before charge extraction occurs. This is achieved by monitoring the change in sample absorption using the technique of transient absorption spectroscopy. This work begins with the need for organic solar cells then discusses the background theory of organic semiconductors and the main operational concepts of organic solar cells. The sample preparation and basic characterisation techniques are described and the main technique of transient absorption explained. The building of the experimental apparatus is discussed in detail with key challenges and solutions noted. The results and system capabilities are then compared against those in recent publications.

Two polymer:fullerene blends were studied. P3HT:PC₆₁BM was investigated with transient absorption anisotropy which allowed simultaneous investigation of charge motion and recombination. This blend shows a threshold charge density under which charges are trapped and slow to move, and above which charges are extremely mobile. Different probe wavelengths observe different subsets of charges with an order of magnitude difference 5.7 × 10⁻¹² - 2.38 × 10⁻¹¹ cm³s-¹ in their bimolecular recombination rates.

The other blend studied was PTB7:PC₇₁BM and here the influence of solvent additive was explored using broadband transient absorption. The absorption spectra of different excited state species were measured. This allows the separation of the polaron and triplets kinetics which are then modelled by rate equations. The addition of DIO decreases the trap density by ~ 30 %, and increases the rate of dissociation by ~ 30 %.

However more than 95 % of the charges still recombine through the triplet exciton even with the addition of DIO.

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