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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)


Electrical Engineering and Computer Science

First Advisor

Qiquan Qiao


Increasing energy demand, faster depletion of fossil fuels and adverse impacts to environment lead to search for renewable energy sources. Solar energy is one the most promising alternatives to replace fossil fuels. Sophisticated fabrication procedures and costly raw materials of crystalline silicon (c-Si) solar cells have limited their large scale application. Organic solar cells (OSCs) having both low material cost and simple fabrication processes emerged as one of the most probable candidates to replace c-Si solar cell. Tandem device structure shows the potential to reach the targeted efficiency of 15-20%. Most high performing tandem polymer solar cells deployed high bandgap polymer poly (3-hexylthiophene) (P3HT) as front cell material as its bandgap is nearly 1.9 eV and absorbs only high energy photons. P3HT, with HOMO energy level ~ -5.0 eV, mixed with fullerene derivative PCBM is capable to generate a Voc of ~ 0.6 V. So polymers with lower HOMO than that of P3HT have the potential to increase Voc and enhance photovoltaic performance. A wide bandgap polymer PBDT-ABT-2 with HOMO of -5.17 eV was reported which can yield higher Voc than P3HT but needed optimized fabrication conditions. xv In this work, the effects of solvents and additives on nano-morphology and performance of PBDT-ABT-2 inverted solar cells were investigated. The fabrication conditions were optimized and an efficiency of 5.47% was achieved with a short circuit current density of 13.81 mA/cm2. The polymer/fullerene blending ratio was optimized using UV-Vis spectroscopy, atomic force microscopy (AFM) and current density – voltage (J-V) characterization. The optimum ratio was found to be 1:2 with efficiency of 3.48%, Jsc of 7.18 mA/cm2 and FF of 70.36% when device was fabricated from a solution of 10 mg/mL in chlorobenzene (CB). Addition of 1,8-diiodooctane (DIO) as additive into the 1:2 blend solution of PBDT-ABT-2:PC60BM improved both Jsc (9.81 mA/cm2) and FF (71.28%), leading to an increased efficiency to 4.75%. Change of solvent from CB to chloroform (CF) with DIO increased the Jsc from 9.80 mA/cm2 to 12.1 mA/cm2, but reduced the FF from 71.28% to 62.85%, leading to an efficiency to 5.17%. The highest efficiency of 5.47% was obtained with a Jsc of 13.81 mA/cm2 and a FF of 59.1% by switching fullerene derivative from PC60BM to PC70BM. Performance of PBDT-ABT-2 overweighs that of P3HT in terms of Jsc and Voc. Therefore, future work could be to use PBDT-ABT-2 as front cell material to replace the commonly used P3HT in tandem structure to enhance photovoltaic performance.

Library of Congress Subject Headings

Solar cells


Includes bibliographical references (pages 81-90)



Number of Pages



South Dakota State University


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