Transient spectroscopic studies of charge carrier dynamics in organic solar cells

Abstract

Organic solar cells (OSCs) are one of the promising third-generation lightweight and flexible photovoltaics. Since the 1990s, the development of OSCs can be described in three phases. In the first phase, the ‘bulk heterojunction’ concept was established. From the 2000s, the second phase focused on pairing different polymers with fullerene derivatives, following the observation of ultrafast electron transfer from polymer donors to fullerene acceptors. Most recently, in the third phase, the development of non-fullerene acceptors has led to a surge in the efficiency. While the material design has been driving such developments, photophysical understandings of the mechanisms are still lagging yet are crucial towards the evolution of OSCs. This thesis describes an experimental investigation into the functional mechanisms of charge generation, separation, and recombination processes in non-fullerene based OSCs by steady-state and transient spectroscopies. Chapter 3 and 4 investigate the carrier dynamics in polymer:non-fullerene bulk heterojunction OSCs with small driving forces for charge separation, probing their particular photophysics. Specifically, Chapter 3 shows that charge-transfer state recombination becomes more severe in a low-offset system. This analysis has been extended to six other systems in Chapter 4, suggesting that the lifetime of charge-transfer states becomes shorter as their energetics are suppressed. Chapter 5 and Chapter 6 focus on the carrier dynamics in homojunction OSCs employing small molecules. While bulk heterojunction OSCs are currently leading the efficiency, homojunction OSCs are still attractive due to their enhanced device stability and high photovoltages. Chapter 5 shows that the molecular electrostatics arising from different molecular orientations drives charge separation in a sexithiophene pristine film, leading to efficient charge photogeneration in devices. In contrast, Chapter 6 attributes the origin for charge generation in naphthalocyanine based devices to the built-in field, meaning no sign of charge generation is observed in pristine films. Finally, the impact of these two charge generation pathways on the carrier dynamics in planar heterojunction OSCs employing the aforementioned two materials is investigated. Overall, this thesis examines the impacts of the driving force, molecular electrostatics and electric field on charge generation, separation, and recombination processes in both heterojunction and homojunction OSCs. The unique findings within this thesis have unlocked greater understandings of the fundamental mechanisms of charge generation in OSCs and it is hoped that this will aid in the design of future material systems.