The Effects of [6,6]-Phenyl-C61 Butyric Acid Methyl Ester (PCBM) in Polymer Solar Cells
Date Issued
2011
Date
2011
Author(s)
Wang, Jeng-Yu
Abstract
The thesis focuses on the research of “Vertical Phase Segregation”, which occurs in the active layer of polymer solar cells.
Typically, for conventional polymer solar cells which have bottom anode and top cathode, the active layer solution is composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as electron donor and acceptor materials, respectively, and is fabricated on the patterned ITO substrate by spin coating method. Because of the difference of physical and chemical properties between P3HT and PCBM, as the blend film is fabricated, the “Phase Segregation” occurs. That is, the free (air) surface of the active layer is donor-enriched whereas the bottom of the active layer is acceptor-enriched.
The phase segregation is unfavorable for the solar cells with conventional structure. The accumulated P3HT and PCBM near the cathode and anode, respectively, are just like energy barriers for photo-induced electrons and holes. The phenomenon may cause the recombination of photo-induced charges, which reduces the photocurrent and further affect the power conversion efficiency.
The first topic of this thesis focuses on enhancing the photocurrent in polymer solar cells. Before thermally depositing metal cathode, PCBM is deposited onto the active layer in advance. After finishing the thermal evaporation process, the post annealing is performed to let the PCBM molecules diffuse into the active layer to mix with the accumulated P3HT on the top active layer. That way, the barrier between the active layer and cathode disappears, which increases the photocurrent as well as the power conversion efficiency.
Recently, A. Kumar et al have proposed that the phase segregation is related to the surface energy difference between P3HT and PCBM. We suspect that the phase segregation may be related to the density difference between the two materials, too. Therefore, in the second topic, we adopt a novel process to fabricate polymer solar cells in order to demonstrate whether the phase segregation is related to the density difference or not.
Subjects
Polymer Solar Cells
Organic Solar Cells
P3HT
PCBM
Phase Segregation
Type
thesis
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