In this thesis, theoretical studies on the various operation mechanisms of bulk heterojunction solar cells have been carried out, with the view of enhancing their photovoltaic performance. The processes considered in this thesis include mechanism of intersystem crossing of excitons, diffusion of excitons, probability of recombination of free charge carriers into their excitonic states, and charge carrier transport. The results obtained from the above theoretical works have inspired the optimisation of two types of bulk heterojunction solar cells by simulation, using the optical admittance analysis method (OAAM). After the introduction in chapter 1, the mechanism of intersystem crossing in organic materials is described and the rate of intersystem crossing is derived in chapter2, by using the first order perturbation theory and Fermi’s golden rule. A newly derived exciton-spin-orbit-vibrational interaction operator which depends on the atomic number of the constituent atoms in the material is used as a perturbation operator. Thus, the influence of incorporation of heavy metal atoms on the rate of intersystem crossing is also studied. The diffusion length and diffusion coefficient of excitons, and recombination of free charge carriers into their excitonic states are presented in chapter 3. In chapter 4, an analytical expression for charge carrier mobility dependent open-circuit voltage in organic and hybrid solar cells is derived by optimising the current density obtained using the drift-diffusion model. By using the derived expression, the calculated open-circuit voltage in organic and hybrid solar cells is used to illustrate the effects of charge carrier mobilities on the open-circuit voltage. In chapter 5, the optimisation of design of two BHJ OSCs of structures: (1) ITO/PEDOT:PSS /P3HT:PCBM/LiF/Al and (2) ITO/ZnO/PTB7:PCBM /LiF /Ag is carried out. The thicknesses of the active layers and metal electrodes are optimised using OAAM simulation. It is found that by optimising the thicknesses of the two BHJ OSCs structures, one can increase the short-circuit current density of ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al BHJ OSC by 4.8% and that of ITO/ZnO/PTB7:PCBM /LiF/Ag by 13.3%. Thus, an increase of more than 2 and 6 % in power conversion efficiency can be achieved in ITO/PEDOT:PSS/ P3HT:PCBM/LiF/Al and ITO/ZnO /PTB7:PCBM /LiF /Ag OSCs, respectively. The results of this investigation are expected to be useful in the fabrication of high-efficiency bulk heterojunction solar cells.
|Date of Award||Jan 2017|
|Supervisor||Jai Singh (Supervisor)|