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Design of thin film organic solar cells for optimum photovoltaic performance

Narayan, Monishka Rita (2013). Design of thin film organic solar cells for optimum photovoltaic performance. PhD Thesis, Charles Darwin University.

Document type: Thesis
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Author Narayan, Monishka Rita
Title Design of thin film organic solar cells for optimum photovoltaic performance
Institution Charles Darwin University
Publication Date 2013
Thesis Type PhD
Subjects 0599 - Other Environmental Sciences
Abstract Organic solar cells are a class of thin film solar cells that offer advantages of low-cost, flexibility, inexpensive fabrication techniques and large-scale production. However, the performance and stability of organic solar cells limit their commercialization. In this thesis, a theoretical study is carried out on the various operation mechanisms, including photo-generation of excitons, diffusion and dissociation of excitons and separation of charge carriers to their respective electrodes, in bulk-heterojunction organic solar cells. The analysis of results obtained from the above theoretical works is used in simulating optimal designs for two types of organic solar cells; bulk-heterojunction and hybrid structures. In this thesis, after the introductory chapter 1, the rates of excitation, diffusion and dissociation of excitons in bulk-heterojunction solar cells are derived, respectively, in the next three chapters of this thesis using the corresponding interaction operators, transition matrix elements and Fermi’s golden rule. The rate of excitation of singlet excitons is derived using the usual exciton-photon interaction operator as perturbation and that of triplet excitons is obtained using the new time-dependent exciton-spin-orbit-photon interaction operator which depends on the atomic number of the constituting atoms. Thus, the influence of incorporation of heavy metal atoms on the photo-generation of triplet excitons is also studied. The effect of excitonic binding energy and diffusion length on the performance of bulk-heterojunction solar cells is presented. A dissociation mechanism for the Frenkel excitons into free charge carriers at the donor-acceptor interface is proposed and possible loss mechanisms in bulk-heterojunction organic solar cells are discussed. In addition, the study of the optical properties of bulk-heterojunction and hybrid organic solar cells is presented in Chapter 5, together with their designs, which are optimized using the semiconducting thin film optics simulation software (SETFOS). The layer thicknesses have been optimized with respect to maximum photon absorption in the active layer and the optimal designs thus obtained produce high power conversion efficiencies of 12.87% and 4.70% in the bulk-heterojunction and hybrid organic solar cells, respectively. These efficiencies are found to be notable enhancements from their corresponding current experimental power conversion efficiencies. The results of this investigation are expected to contribute towards the fabrication of high-efficiency bulk-heterojunction and hybrid organic solar cells.

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