Spectroscopic studies of the charge-transfer state and device performance of hybrid and organic solar cells

The last few years have witnessed a remarkable explosion in advances in the field of photovoltaics (PV) based on organic semiconductors as the light absorbing material, both in terms of device efficiency and in terms of the physical understanding of the underlying processes. This thesis aims to contribute to this field by examining two different types of solar cells based on light-absorbing organic semiconductors. It consists of two main parts connected through their focus on the role of charge transfer (CT) states on device performance, and more specifically on a focus on effect of CT states on open-circuit voltage losses. The first two results chapters focus on the fabrication and analysis of novel hybrid heterojunction solar cells based on the molecular inorganic semiconductor copper thiocyanate (CuSCN) in conjunction with light absorbing organic semiconductors, which show surprisingly high charge generation efficiencies. In the first results chapter I show that the formation of a hybrid CT-state formed at the interface between CuSCN and organic semiconductors is key to the charge generation process in these cells. In in the second results chapter I present the discovery of a remarkable nanostructured interface formed between CuSCN and PC70BM via solution processing with which it is possible to fabricate semi-transparent devices with power conversion efficiencies (PCEs) exceeding 5%, which is amongst the highest reported efficiencies for hybrid heterojunction solar cells. The final two results chapters focus on the role of CT-states in affecting the voltage losses of bulk-heterojunction (BHJ) solar cells based on non-fullerene acceptors. In the third results chapter I present the device optimization and characterization of a solar cell based on a novel polymer:non-fullerene acceptor combination, and show that it is possible to fabricate solar cells with PCEs exceeding 13%, amongst the highest reported efficiencies at the time. I extend the analysis of the role of the CT-state in achieving this high efficiency in the last results chapter, where I present a new model that helps to explain the observation of very low non-radiative voltage losses in blends with a low driving energy for charge separation through the processes of hybridization of CT and local excited states.Open Acces