Enhanced Electrical Properties and Air Stability of Amorphous Organic Thin Films by Engineering Film Density

Yu Esaki; Takeshi Komino; Toshinori Matsushima; Chihaya Adachi

doi:10.1021/acs.jpclett.7b02808

Enhanced Electrical Properties and Air Stability of Amorphous Organic Thin Films by Engineering Film Density

J Phys Chem Lett. 2017 Dec 7;8(23):5891-5897. doi: 10.1021/acs.jpclett.7b02808. Epub 2017 Nov 21.

Authors

Yu Esaki^{1

2}, Takeshi Komino^{1

2

3}, Toshinori Matsushima^{1

2

4}, Chihaya Adachi^{1

2

3

4}

Affiliations

¹ Center for Organic Photonics and Electronics Research (OPERA), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan.
² Japan Science and Technology Agency (JST) , ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan.
³ Education Center for Global Leaders in Molecular System for Devices, Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan.
⁴ International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan.

PMID: 29139292
DOI: 10.1021/acs.jpclett.7b02808

Abstract

The influences of film density and molecular orientation on the carrier conduction and air stability of vacuum-deposited amorphous organic films of N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) were investigated. The substrate temperature (T_sub) during vacuum deposition had different effects on the film density and molecular orientation of α-NPD. Film density was a concave function of T_sub; maximum density was attained at T_sub = 270-300 K. α-NPD molecules were randomly oriented at T_sub = 342 K, and their horizontal orientation on the substrate became dominant as T_sub decreased. Hole current and air stability were clearly raised by increasing the film density by 1 to 2%; these effects were, respectively, attributed to enhanced carrier hopping between neighboring α-NPD molecules and suppressed penetration of oxygen and water. These results imply that increasing film density is more effective to enhance the electrical performance of organic thin-film devices with α-NPD films than control of molecular orientation.