A-COE 2012, Yamagata, Japan, Dec. 19-21, 2012

Control of molecular orientation and refractive index- New degrees of freedom for advanced optical design of OLEDs and OPVs Daisuke Yokoyama*, Ziruo Hong, Junji Kido
Department of Organic Device Engineering and Research Center for Organic Electronics (ROEL),
Yamagata University, Yamagata 992-8510, Japan
Email: d_yokoyama@yz.yamagata-u.ac.jp
In device design of organic optoelectronic devices, such as OLEDs and OPVs, optical design is very importantfor deriving the full potential of organic semiconductor materials. Since the optical thickness of these organic
devices has the same order as the wavelength of visible light, the device performance is sensitive to the opticalmultilayer structures. However, we have not had many degrees of freedom for the optical design of organic
semiconductor devices because we have long assumed random molecular orientation, isotropic optical properties,
and similar refractive indices of around 1.7–1.8 in vacuum-deposited amorphous organic semiconductor films.
In this presentation, we will demonstrate control of molecular orientation and refractive index for advanced
optical design of the devices with amorphous films. First, we will discuss the effects of molecular orientation on
optical properties of OLEDs and OPVs. We have demonstrated the generality of horizontal molecular orientationin amorphous organic semiconductor films1,2 and first proposed the guideline that horizontal orientation ofemitters can be used to enhance outcoupling efficiency even of small-molecule OLEDs.2 This is important for
further enhancing OLED efficiency and also for discussing the upper limit of OLED efficiency.3 Also, horizontal
orientation of dipoles of OPV materials is preferable for efficient absorption of vertically incident light. We havedemonstrated horizontal orientation of some amorphous OPV materials in neat and co-deposited films.4
Next, we will introduce some results of refractive index control of vacuum-deposited organic films.5 Using
birefringence caused by molecular orientation, we can control refractive indices for horizontally and verticallypolarized light. Moreover, the introduction of structures with high and low molar refractions makes it possible tofurther control refractive index efficiently. An organic semiconductor DBR (distributed Bragg reflector) with alarge refractive index difference of 0.58 was achieved, demonstrating light control by organic films themselves.
(1) D. Yokoyama, J. Mater. Chem. 21, 19187 (2011).
(2) D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Org. Electron. 10, 127 (2009).
(3) (a) J. Frisheisen, D. Yokoyama, A. Endo, C. Adachi, W. Brütting, Org. Electron. 12, 809 (2011). (b) D.
Yokoyama, Y. Park, B. Kim, S. Kim, Y.-J. Pu, J. Kido, J. Park, Appl. Phys. Lett. 99, 123303 (2011).
(4) (a) D. Yokoyama, Z. Wang, Y.-J. Pu, K. Kobayashi, J. Kido, Z. R. Hong, Sol. Energy Mater. Sol. Cells 98,472 (2012); (b) Z. Wang, D. Yokoyama, X.-F. Wang, Z. R. Hong, Y. Yang, Junji Kido, Energy Environ. Sci. (inpress) DOI: 10.1039/C2EE22952H.
(5) D. Yokoyama, K. Nakayama, T. Otani, J. Kido, Adv. Mater. 24, 6368 (2012).

Daisuke Yokoyama obtained his BS and MS in Physical Chemistry from The
University of Tokyo in 1998 and 2000, respectively. After working in Fuji Film
Corporation for six years, he joined Kyushu University and obtained his Ph.D degree inEngineering in 2009. He is currently working at Yamagata University as an assistantprofessor. His research interest is in molecular physics in organic semiconductor
devices, especially analysis and control of molecular orientation and refractive index.