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Document Type

Thesis - University Access Only

Award Date

2013

Degree Name

Master of Science (MS)

Department / School

Electrical Engineering and Computer Science

First Advisor

Qiquan Qiao

Second Advisor

David Galipeau

Abstract

Organic solar cells have attractive aspects like flexibility, light-weight, environment-friendliness and low-cost due to manufacturing-ease on large area substrates using roll-to-roll processing, solution casting or screen printing. Most high performance organic solar cells utilize tandem structure with wide band gap polymer as a front cell material and low band gap polymer as a rear cell material. Since poly(3-hexylthiophene) (P3HT) has higher band gap ~1.9 eV and absorbs the higher energy photons, it is the most frequently used front cell material for tandem solar cells. However, the HOMO of P3HT is -5.0 eV, generating a Voc around 0.6 V. This limits the device performance of an organic solar cell. In this work, a new wide band gap copolymer poly{2-Octyldodecyloxy -benzo [1,2-b;3,4-b]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-bis(dithiophen-2-yl)-benzo [c][1,2,5]-thiadiazole} (PBDT-ABT-2) synthesized by a post- doctoral student, Dr Qiliang Chen was studied and applied in an organic solar cell with a device structure of glass/ITO/PEDOT:PSS/PBDT-ABT-2:PCBM/Ca/Al. Donor/acceptor ratio, solution concentration and spin-speed for active layer deposition were optimized. PBDT-ABT-2 had lower HOMO energy level, as evidenced from cyclic voltammetry and higher Voc than P3HT, as evidenced from current density vs voltage characterization. The 1:1 donor/acceptor ratio with a solution concentration of polymer (10 mg) + PCBM (10 mg) in 1 mL of DCB had the highest device performance with a power conversion efficiency of 3%, short-circuit current of 7.63 mA/cm2, open-circuit voltage of 0.71 V and a fillfactor of 53.74%. Since PBDT-ABT-2 has higher Voc and wider band gap, it can be used as front cell material in tandem device structures. Future work can include blending of copolymer PBDT-ABT-2 with acceptor materials having higher LUMO energy levels to increase Voc. Optimization of processing conditions using additives, solvent annealing and thermal annealing to improve active layer morphology and current density.

Library of Congress Subject Headings

Copolymers
Solar cells
Heterojunctions

Description

Includes bibliographical references (pages 79-88)

Format

application/pdf

Number of Pages

101

Publisher

South Dakota State University

Rights

Copyright 2013 Purna P. Maharjan

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