Photophysical Studies of Active Layer Compositions in Organic Solar Cells

Abstract

For the last few decades, inorganic solar cells, such as gallium arsenide and silicon, have dominated research and commercial ventures. Commercial silicon-based solar cells have particularly expanded in recent years, including significant rises in small-scale home solar installations and solar farms spanning several acres. Organic solar cells (OSCs), based on organic semiconductors, have also recently attracted much attention within the Internet-of-Things community because of their ability to be integrated into low-energy off-grid devices and because of their spectral sensitivity making them good candidates for indoor solar applications. OSCs possess a unique quality in that they are solution processed. This enables them to be deposited on flexible substrates which can be scaled-up easily and cheaply. OSCs rely on a donor-acceptor configuration in the active layer. An inherent characteristic of OSCs is that they form tightly bound electron-hole pairs (excitons) following photoexcitation and as these excitons separate, they form charge-transfer (CT) states. The formation and characteristics of excitons and CT states are heavily dependent on the active layer composition and morphology, and they can affect device performance by promoting charge transport processes or loss pathways. In this thesis, we explore methods for suppressing loss pathways arising from these excited states. We explore the effectiveness of increasing spacing between donor-acceptor active layer components to suppress intermolecular interactions, via an encapsulation process. We use encapsulation to ‘sheath’ a donor polymer sub-unit which has been associated with the formation of a particular type of CT state. Morphological and photophysical techniques are used to rationalise changes in device performance following encapsulation. Macrocyclic molecules are also explored as a method to control packing and we apply this to the acceptor component of the active layer. The unique cavity that forms following macrocyclisation offers interesting packing types upon addition of a polymer. We explore the morphological and photophysical implications of this structural change in the neat material and when blended with a widely used donor material. OSCs also have a high tendency to form triplet states which contribute to non-radiative voltage losses and limit charge extraction in working solar cells. We finally propose integrating photon upconversion via triplet-triplet annihilation (TTA-UC) into an OSC to recycle the low-energy triplets. This should effectively give these excitations a second chance to contribute to the photocurrent. We show the presence of TTA-UC in an organic solar cell, propose a mechanism through which the process occurs and investigate the factors which currently limits its contribution to the overall device efficiency.