Studies of Interfaces between Organic and Inorganic Semiconductors and Applications of Their Charge Transfer Complexes in Organic Optoelectronic Devices

有機與無機半導體界面的研究及其電荷轉移配合物在有機光電器件中的應用

Student thesis: Doctoral Thesis

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Award date31 Aug 2020

Abstract

Interfaces between donors (D) and acceptors (A) play significant roles in optoelectronic devices. Charge-transfer complexes (CTCs) formed by mixing D and A molecules have drawn much attention in recent years. With special charge distributions and electronic interactions at the D/A interfaces, unexpected properties such as metallic conductivity, ambipolar transport and ferroelectric property, etc. are discovered. Most of these properties are demonstrated in organic-organic CTCs. In this thesis, we extend our exploration to hybrid organic-inorganic interfaces and their CTCs.

Molybdenum (VI) oxide (MoO3) is selected as the inorganic acceptor because of its deep lying conduction band minimum, which can facilitate electron transfer from organic D materials. When interfaces are formed between MoO3 and pentacene, α-Sexithiophene (6T) and C60, strong interface dipoles and/or band bendings are observed with ultraviolet photoelectron spectroscopy (UPS). Their properties and applications are further studied.

With strong charge transfer characteristics in these MoO3:D CTCs, different novel properties are observed. 1) By controlling with a pre-bias voltage in a MoO3:pentacene CTC based device, current changes over two orders of magnitude are observed. It is shown for the first time that a dipole with controllable direction can be set up inside CTC by pre-biasing. The mechanism and its application are discussed. 2) Carrier transport in MoO3:6T CTC thin film are studied. While CTC thin films with both electron and hole transport properties are still rare, MoO3:6T CTC thin film shows bipolar transport properties. Influences of intermolecular interaction on the charge transport properties are investigated and application of this MoO3:6T film as a buffer layer is explored. 3) By modulating the band structure of a device with a layer of MoO3 between C60 and Al, an organic photodetector (OPD) with two working modes is fabricated. The mechanism of dual charge separation modes is further discussed, and applications are explored.

In this study, novel properties are obtained from simple co-evaporated organic-inorganic CTC thin films and organic/inorganic heterojunctions. These methods enable further applications in functionalization of interfaces and performance enhancement in organic electronic devices.