Description
Date/Time: 04-05-2018 - Thursday - 05:00 PM - 07:00 PM
Jordan Dull1 Barry Rand1 Michael Fusella1

1, Princeton University, Princeton, New Jersey, United States

Organic semiconductors, and in particular organic LEDs (OLEDs), have found commercial outlets in the cell phone and television display markets, even though their full potential has not yet been realized. Current technology uses disordered films even though organic materials, like rubrene, have been shown to crystallize over large areas [1]. Furthermore, it has been demonstrated that when these materials crystallize both the charge mobility and diffusion length significantly improve [2,3]. One reason silicon and III-V semiconductors perform so well in modern device applications is because enormous amounts of time and effort were put into making highly crystalline wafers with very few structural and chemical impurities. This history of inorganic semiconductors and the fact that organic materials can crystallize indicate that understanding and controlling how organic materials form ordered films is crucial for improving the performance of organic electronic devices. However, there exists major gaps in understanding of how these crystals form and grow, how defects impact electrical and optical properties, and how interfaces between crystalline layers effect device physics. Yet other than rubrene, there are no known materials that are able to form large area crystalline films. We therefore attempt to search for and understand how other organic semiconductors make long-range-ordered films.

One such material we have found to crystallize is 2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) which is an electron transport layer commonly used in OLEDs. Here we show that a thermal annealing process on thin films (20 nm) of TPBi results in large crystalline domains (~2-4 mm). We will discuss our efforts to control nucleation density, and thus crystalline domain size, as well as peculiarities (voids and wavelike features) that form on the TPBi crystal surface upon longer annealing times. Additionally, we will present optical, structural, and electrical measurements, highlighting the differences between amorphous and crystalline thin films.

References:
[1] M.A. Fusella, et al. Chem. Mater., 29 (16), 6666–6673 (2017)
[2] I.G. Lezama, A.F. Morpurgo, MRS Bull. 38, 51-56 (2013).
[3] H. Najafov, B. Lee, Q. Zhou, L.C. Feldman, V. Podzorov. Nature Mater. 9, 938-943 (2010).

Meeting Program
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5:00 PM–7:00 PM Apr 5, 2018

PCC North, 300 Level, Exhibit Hall C-E