Study of organic and perovskite solar cells for power conversion efficiency improvement
Creators: name: Yeboah, Douglas
Publisher: name: Charles Darwin University
Year: 2018

description: In recent years, thin film organic solar cells (OSCs) and perovskite solar cells (PSCs) based on solution processable organic semiconductors and perovskites, respectively, have attracted huge research interests as promising alternatives to conventional silicon solar cells, owing to their appealing features such as lightweight, large area production capabilities, low fabrication cost and mechanical flexibility. Although, OSCs have achieved power conversion efficiency (PCE) of about 12 %, it is still lower than 25 % obtained in silicon solar cells. In comparison, the PCE of perovskite solar cells has reached more than 22 %, which is much better but both OSCs and PSCs have a stability problem; their working lifetime is very short. As a result, both OSCs and PSCs have not reached the commercialization stage yet. Hence, the understanding of physical processes that govern the operation and stability of organic and perovskite solar cells is very crucial in order to find pathways to enhance their PCE and stability. One of the most efficient OSCs at present is the bulk heterojunction (BHJ) OSC, which consists of a blend of organic donor material and fullerene acceptor as the active layer sandwiched between two electrodes. In this thesis, in-depth theoretical studies of the device physics in bulk heterojunction organic and perovskite solar cells have been carried out with the aim of enhancing their photovoltaic performance. These studies include exciton diffusion to the donor-acceptor interface, optimization of charge carrier collection and fill factor in bulk heterojunction organic solar cells (BHJ OSCs), and the optimization of the open circuit voltage in both BHJ OSCs and PSCs. The thesis is organized as follows: after the introduction in chapter 1, a comprehensive study of exciton diffusion lengths and diffusion coefficients in BHJ OSCs is presented in chapter 2, where diffusion lengths and diffusion coefficients of singlet and triplet excitons are derived as functions of photophysical parameters such as spectral overlap integral, photoluminescence (PL) quantum yield, dipole moment and refractive index. In chapter 3, the influence of photoexcitation of the donor and acceptor materials separately and simultaneously on the open circuit voltage in BHJ OSCs is studied. The analytical studies of the factors that determine the fill factor of BHJ OSCs are presented in chapter 4, where the influence of material parameters such as effective charge carrier mobility, effective charge carrier concentration, active layer thickness, and dielectric constant, on fill factor is analysed through a newly derived extraction coefficient (훽). Finally, in chapter 5, the open circuit voltage loss is analysed in perovskite solar cells by deriving two analytical expressions that relate the open circuit voltage to other device parameters such as charge carrier mobility, charge carrier concentration, effective density of states of holes at the valence band edge, photogeneration rate, active layer thickness and electron drift velocity. The derived expressions are used to illustrate the influence of the material parameters on open circuit voltage and the origin of the open circuit voltage loss in perovskite solar cells. The results presented in this thesis are expected to provide useful insights in synthesizing novel organic and perovskite materials for the fabrication of high-efficiency bulk heterojunction organic and perovskite solar cells.