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Document Type

Dissertation - University Access Only

Award Date

2013

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical Engineering and Computer Science

First Advisor

Mahdi Farrokh Baroughi

Second Advisor

David Galipeau

Abstract

Organic solar cells have a potential for low cost energy conversion due to their low material cost, high absorption coefficient, flexibility, and low production process cost compared to their silicon based solar cell counterparts. However, the usage of organic solar cell was hindered by their low conversion efficiency. The loss mechanisms that lead to lower efficiency in organic solar cells have not been clearly understood. This thesis addresses two major needs in the field of organic solar cells: 1. Need for a comprehensive model that describes transport and coupling of energy carrying particles in the bulk and interfaces of organic and organic/inorganic material systems and comprehensive interface model allowing recombination and cross-coupling of energy carrying particles. 2. Need for a comprehensive study on the physical phenomena and material/device parameters on the performance of organic solar cells. The objectives of this dissertation were to 1) develop a comprehensive model to simulate transport of energy-carrying particles in organic/inorganic solar cell including interfaces. 2) Study the impact of material and device parameters on the performance characteristics of organic solar cells. 3) Design highly efficient (12-14%) organic - plasmonic solar cell structures with potential for cost effective implementation. A generalized charge transport model for simulation of electronic charge transport in solar cells with organic and organic/inorganic structures was introduced. The model uses conservation law and contains exciton dissociation and recombination as well as exciton injection processes. Interaction of energy carrying particles (electrons, holes, singlet excitons, and triplet excitons) with each other and their transformation in the bulk of the donor and acceptor media as well as the donor/acceptor interfaces are incorporated in form of coupling matrices into the continuity equations and interface boundary conditions. The comprehensive interface boundary conditions developed in this thesis enabling simulation of devices with multilayer light absorbers. Based on this model, the analytical solution for exciton distribution in bi-absorber solar cell was developed. The model was implemented on a bilayer PCBM-P3HT device as a case study. The effect of different device and material parameters on the performance characteristics of the device was studied. The studied parameters were surface recombination velocity for excitons, exciton lifetime, flux of dissociated exciton, bulk recombination coefficient, charge mobilities and energy barrier at the contact. Almost all free carrier recombination in the bilayer organic solar cell occurred at the vicinity of the donor/acceptor interface. Hence, the quality of the interface had a major effect on the Voc of the solar cells. The results also showed that the Voc can vary in a wide range from 0.5 V to 1.3 V by modifying the electronic structure of the donor/acceptor interface. This dependence the Voc on the interface energy band structure can explain the high Voc of organic solar cell, reported by Heeger et al. Increasing the direct recombination rate ( ) decreased the Voc of an organic solar cell with the rate of 60mV per decade. According to the simulation, a novel organic solar cell with a cone array as a back contact, absorbed more than 80% of photons in the absorption spectrum (350 nm up to 800 nm) of the AM 1.5. As a result, using a high quality interface electronic structure and the proposed novel nano-cone cell structure, the performance of an organic solar cell can reach more than 17%.

Library of Congress Subject Headings

Solar Cells
Energy conversion

Description

Includes bibliographical references (pages 127-141).

Format

application/pdf

Number of Pages

156

Publisher

South Dakota State University

Rights

In Copyright - Educational Use Permitted
http://rightsstatements.org/vocab/InC-EDU/1.0/

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