Photophysics of Organic Molecular Systems – A Study of Excited State Dynamics

Abstract

This thesis is dedicated to studies of the excited-state dynamics in organic molecular systems for solar energy conversion by employing time-resolved experimental techniques. Organic photovoltaic (OPV) devices have received significant attention in the past decade and reaching record high power conversion efficiencies (PCE) above 17%. An essential step towards reaching the predicted PCE limit of 25.5% is to develop a comprehensive picture of the photophysical processes, specifically the loss processes, in OPV devices. It is the aim of this thesis to investigate and understand the fate of excited-states in organic electron donor/acceptor systems by ultrafast spectroscopic techniques, specifically, to reveal the interplay between energy and charge transfer processes.

The first part deals with the identification of different polymorphs in a diketopyrrolopyrrole-based (DPP) polymer. Applying time-resolved photoluminescence (TRPL) measurements to the polymer dissolved in different solvent mixtures and using multivariate curve resolution (MCR) to deconvolute the ground-state absorption spectra reveals the co-existence of an amorphous (α) and two semi-crystalline (β1 and β2) polymer phases. The OPV device performance is shown to increase by the additional absorption of the β2 phase.

The second part compares the efficiency of direct and energy transfer-mediated charge generation in prototypical donor-acceptor dyads that use as the electron donor triangulene derivatives chemically linked to the electron acceptor perylenediimide (PDI) block via oligophenylene spacers of different lengths. Charge generation efficiencies are found to be similar and increase with the donor-acceptor spatial separation. A combination of transient absorption (TA) measurements and computation of the dyad’s excited-state landscape revealed the presence of “optically-dark” excited-states that are populated by ultrafast donor-acceptor energy transfer prior to hole (back) transfer.

The last part of the dissertation uses TRPL, TA, and time-delayed collection field (TDCF) measurements alongside MCR analysis to provide a comprehensive analysis of the yield of individual photophysical processes in OPV devices. A systematic methodology is proposed and tested on two all-polymer BHJ devices used as model systems. The experimental findings are supported by successful simulation of the solar cells’ JV characteristics using the spectroscopically-determined kinetic parameters. More generally, this approach can be used to quantify efficiency-limiting processes in other donor-acceptor BHJs.