Chemical vapor deposition of conjugated polymeric thin films for photonic and electronic applications

Abstract

(cont.) Conjugated polymers have delocalized electrons along the backbone, facilitating electrical conductivity. As thin films, they are integral to organic semiconductor devices emerging in the marketplace, such as flexible displays and plastic solar cells, as well as next-generation microphotonic chips. A major processing challenge is that these materials are relatively insoluble. Chemical vapor deposition (CVD) is presented as a synthesis technique for conjugated polymers as an alternative to electrochemical and liquid dispersion methods. CVD will continue to be an essential component of the materials toolset for manufacturers of semiconductor devices. Polysilanes, with a backbone consisting of silicon atoms instead of carbon, have delocalized electrons due to the presence of d-orbitals. Plasma-enhanced CVD (PECVD) of polysilane films was achieved, but they did not exhibit electrical conductivity. Branching resulting from the energetic plasma process quenches the conjugation. However, photo oxidation was used to convert Si-Si bonds into Si-O-Si, reducing the refractive index up to 5%. This led to the direct patterning of tunable waveguides in PECVD hexamethyldisilane (6M2S).

(cont.) Other essential devices for microphotonics are microring resonators used for filtering an individual wavelength from "multicolor" light. Photo oxidation of 6M2S, deposited as the cladding material on ring resonators, allows one to shift the resonant wavelength an order of magnitude more than conventional thermal trimming techniques. Microphotonics will ultimately increase computing speeds with chips that operate using light instead of electricity. A CVD technique was also developed for poly-3,4-ethylenedioxythiophene or PEDOT. Among conducting polymers, PEDOT has superior conductivity (up to 300 S/cm) and excellent stability. CVD PEDOT has a conductivity of about 5 S/cm, while 1 S/cm is the figure-of-merit for a good conducting polymer film. As a charge-injecting layer in organic light-emitting diodes (OLEDs), PEDOT increases the overall power efficiency 30-50%. CVD can further enhance this efficiency gain in organic devices by more conformally coating PEDOT on high-area surfaces. CVD PEDOT films also exhibit reversible electrochromic behavior changing color from their as- deposited sky blue color to a darker blue when they are reduced with an applied voltage. A 50-nm film had a contrast of 16.5% with a switching speed of 27 ms.