Design of planar waveguide based on patterning substrate and oriented polymer film
Authors: Fang Yue-Ting  , Yi Jian-Peng  , Chen Jin-Shan  , Wang Hong-Jie  , Chi Lang  , Xia Rui-Dong  ,   Key Laboratory for Organic Electronics and Information Displays, Jiangsu National Synergetic Innovation Center for Advanced Materials, Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, China
Published: December 2020

Semiconducting conjugated polymersused for light emitting devices (LEDs), lasers and amplifiers have received considerable attention due to their low cost and easy fabrication through spin-coating and photochemical processing. A promising material for LED and laser applications is poly(9, 9-dioctylfluorene-co-benzothiadiazole) (F8BT). F8BT has a low stimulated emission threshold and exhibits a large net optical gain at 570 nm. It also shows liquid crystallinity and can be readily aligned into a monodomain by using an alignment layer, polyimide (PI). Oriented film of F8BT exhibits that its charge carrier mobility is increased by more than one order of magnitude compared with isotropic film. The refractive index of the material is also greatly affected by the orientation of the polymer chain. Furthermore, it has been reported that low threshold laser can be achieved by blending P3 HT or red-F solution into F8BT via energy transfer.Here we report a planar waveguide structure obtained via patterning chain oriented area on F8BT: red-F (9 : 1) blend polymer film. The blend solution is obtained by mixing the F8BT solution with red-F solution (with the same concentration, 20 mg/ml in toluene) with a ratio of 9 : 1. The designed waveguide patterns are obtained by inkjet-printing the PI solution onto the pre-cleaned quartz substrates. Thin films (150-200~nm thick) of F8BT: Red F are deposited onto PI by spin coating (2000 rpm). The chain alignment treatment is performed by the following procedure: the films are kept in N2 at 265 ℃ for 2 min, then they are cooled down to 235 ℃ at a rate of 1 ℃/min, finally they are cooled down to room temperature sharply. The PI contacted area on the film becomes anisotropic, while the area without PI keeps isotropic. The refractive index parallel (perpendicular) to the chain direction is significantly increased (reduced) in the PI contacted area compared with outside the PI area. Therefore, the waveguide confinement could be achieved without changing the thickness of the film. Experimental investigations, including AFM images, polarized microscopy images, polarized absorption, and PL spectra of the patterned samples, clearly show the difference between the aligned area and isotropic area.The large percentage of overlap between the emission spectrum of F8BT and the absorption spectrum of red-F solution leads to an efficient energy transfer from F8BT (host) to red-F solution (guest), resulting in a red emission at a wavelength between 600-670 nm from the blend. The polarized absorption and PL spectra of the aligned F8BT: red-F film demonstrate that the absorption intensity of the polarized light parallel to the aligned chain is 5.9 times that perpendicular to the aligned chain at a wavelength of 477 nm, and their ratio is 5.5 at a wavelength of 631 nm.Our demonstration suggests that patterning chain oriented area can be a promising approach to achieving planar waveguide devices by utilizing the refraction index contrast within and beyond the chain oriented region, and the substrate of polyimide (PI) could be patterned with various widths and shapes by the use of inkjet printing technology.