INTERFACIAL ENGINEERING IN ORGANIC ELECTRONICS

Abstract

The interfaces between organic semiconductors and metals, metal oxides, or dielectric surfaces in polymer solar cells and organic thin-film transistors strongly influence charge transport, injection, extraction, and recombination, ultimately affecting organic device characteristics. In this thesis, we elucidate the structural and electronic characteristics of the interfaces between organic semiconductors and metals, metal oxides, or dielectric surfaces, and study how such interfaces affect the device characteristics of polymer solar cells and organic thin-film transistors, using the following three examples.

In poly(3-hexyl thiophene): [6,6]-phenyl-C61-butyric acid methyl ester, P3HT:PCBM, bulk-heterojunction solar cells, the surface energy difference between P3HT and PCBM as well as the surface energy of the substrate can induce composition variation in the vertical direction of the P3HT:PCBM film. We observed the accumulation of P3HT at the P3HT:PCBM-cathode (LiF/Al) interface. By delaminating, transferring, and flipping the P3HT:PCBM layer, we built devices having the opposite preferential segregation, where a blend of P3HT and PCBM segregated to the cathode. We found the device characteristics to be insensitive to the preferential segregation of P3HT.

In order to tune the open-circuit voltage (Voc) of polymer solar cells, we adsorbed fluoro-alkyl and hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide (ITO) for forming self-assembled monolayers (FmSAMs and HmSAMs, respectively). Polymer solar cells having FmSAM-, HmSAM-treated ITO, and bare ITO as anodes display significant differences in Voc. The SAM adsorption can set up energy barrier for minority carrier transport to the anode, which accordingly suppresses recombination at the anode and thereby increases Voc.

Bias stress instability in organic thin-film transistors originates from mobile charges trapped in the bulk of organic semiconductor and at the organic semiconductor-dielectric interface. We present an idea of qualitatively decoupling the trapping contribution from the bulk of organic semiconductor and from the dielectric surface, by fabricating organic single-carrier diodes having active layers of different thicknesses and organic thin-film transistors with different dielectrics. In P3HT, in which a broad distribution of tail states is present, bias stress instability originates from trapping in the bulk. On the other hand, traps at the organic semiconductor-dielectric interface dominate bias stress instability in PCBM transistors.