Dissertation/Thesis Abstract

Charge Recombination and Transport Dynamics in Organic Solar Cells
by Cowan, Sarah Rachel, Ph.D., University of California, Santa Barbara, 2011, 173; 3456114
Abstract (Summary)

In this thesis, we observe the charge transport and recombination dynamics in thin film bulk heterojunction (BHJ) blends of strongly absorbing donor materials (polymer, small molecule) and electron accepting (fullerene, small molecule) materials.

Transient photoconductivity measurements carried out on bulk heterojunction (BHJ) solar cells demonstrate the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination. The transient photoconductance data imply the existence of a well-defined internal field; carrier sweep-out is proportional to the magnitude of the internal field and limited by the carrier mobility. At external voltages near open circuit where the internal field approaches zero, the photocurrent decays because of the recombination of photogenerated mobile carriers.

Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (JSC) and the fill factor (FF). Light intensity and temperature-dependent current-voltage measurements on a variety of polymer BHJ cells show that the recombination kinetics are universally voltage-dependent and evolve from first-order sweep-out at short circuit to bimolecular recombination at open-circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The "missing 0.3 V" inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open-circuit voltage at room-temperature results from the temperature dependence of the quasi-Fermi levels in the polymer and fullerene domains—a conclusion based on the fundamental statistics of fermions.

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device. Steady state studies show a dramatic increase in the trap-assisted recombination rate when [6,6]-phenyl C84 butyric acid methyl ester (PC84BM) is introduced as a trap site in polymer bulk heterojunction solar cells. The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley-Read-Hall recombination; the latter is dramatically enhanced by the addition of the PC84BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the charge density and trap density dependence of recombination loss.

Indexing (document details)
Advisor: Heeger, Alan J.
Commitee: Chabinyc, Michael, Nguyen, Thuc-Quyen, Wudl, Fred
School: University of California, Santa Barbara
Department: Materials
School Location: United States -- California
Source: DAI-B 72/08, Dissertation Abstracts International
Subjects: Alternative Energy, Optics, Materials science
Keywords: Bulk heterojunction, Charge transport, Photovoltaics, Polymers, Recombination, Solar cells
Publication Number: 3456114
ISBN: 978-1-124-65407-2
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