Blue Electroluminescent Copolymers by Parylene-Based Chemical Vapor Deposition Kathleen M. Vaeth and Klavs F. Jensen* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received March 7, 2000; Revised Manuscript Received May 31, 2000 ABSTRACT: Copolymerization of chemically vapor deposited poly(p-phenylenevinylene) and parylene-N is explored as a method for color-tuning the polymer photoluminescence. A significant blue shift in peak photoluminescence from that of pristine poly(p-phenylenevinylene) is observed in the vapor-deposited copolymer, the magnitude of which is controlled by the delivery rates of each of the monomers, with peak photoluminescence wavelengths ranging from 525 nm for low parylene-N incorporation to 470 nm for high parylene-N incorporation. These copolymers are successfully integrated into single-layered light- emitting diodes exhibiting blue light output, which is the first demonstration of color-tuning poly(p- phenylenevinylene) prepared by parylene-based chemical vapor deposition. Introduction Conjugated polymers with band gaps in the visible region such as poly(p-phenylenevinylene) (PPV) and its derivatives have generated a large amount of interest over the past decade, due to their potential as low-cost materials for optoelectronic devices such as light-emit- ting diodes (LEDs) 1 and lasers. 2,3 One of several advan- tages of these electroluminescent (EL) polymer systems is that, with the wide variety of solution-based methods known for modifying the chemical structure of polymers, it is fairly easy to control the emission color of the material. 4 Shifting the characteristic green-yellow pho- toluminescence (PL) color of pristine PPV to the red or blue has been realized by two major routes: increasing or decreasing the band gap of the polymer by placing substituents on the polymer backbone that alter the electronic structure of the material 5,6 and reducing the effective conjugation length of the polymer (for a blue shift only). The latter of the two methods can be achieved by forcing the polymer backbone to twist out of plane with bulky side chains 7 or kinked linkages 8,9 or by disrupting the conjugated structure of the polymer through copolymerization with a second unconjugated monomer. 10-12 An alternate method to solution-processing for syn- thesis of PPV is chemical vapor deposition (CVD) from p-xylene derivatives such as 1,9-dihalo[2.2]paracyclo- phanes 13 or R,R′-dihalo p-xylenes, 14 in a manner analo- gous to CVD of parylenes. 15,16 The potential advantages of synthesizing the polymer film by CVD, as compared to solution processing, include better control of impurity incorporation and layer thickness during film deposition and a greater amount of flexibility in the fabrication of complex structures such as graded compositions and stacked layers. To date, the emission colors of EL polymers prepared with parylene-based CVD chemistry have been from the green-yellow part of the visible spectrum. 14,17,18 In principle, the same types of ap- proaches used to tune the emission color of solution- processed EL polymers could also be used for fabrication of CVD PPV derivatives. In practice, it is not straight- forward to implement these schemes, since the chem- istry of vapor polymerization processes is not as well understood as solution polymerization chemistries. Most attempts at CVD of PPV derivatives have focused on fabrication of substituted homopolymers via pyrolysis of R,R′-dihalo-p-xylylenes with ring group substituents, but none have proven successful due to difficulties in monomer sublimation and excessive monomer fragmen- tation. 14 Fabrication of phenyl-substituted PPV with a hybrid CVD/solution process has been demonstrated, but use of solvents is undesirable with CVD since it adds complexity to film processing. 19 Our previous work with PPV CVD revealed that incorporation of a small amount of aliphatic groups (which form as side products during monomer pyrolysis) into the polymer backbone resulted in a small blue shift in peak PL 20 from the polymer. This suggests that a possible approach for blue-shifting the peak photolu- minescence of CVD PPV in a controlled manner is deliberate copolymerization with an unconjugated mono- mer, which would have the effect of shortening the PPV conjugation length. This has been demonstrated previ- ously with solution-processed PPV, 10 but to our knowl- edge, color-tuning of CVD PPV-based EL polymers by copolymerization is relatively unexplored. However, use of copolymerization to tailor the properties of unconju- gated CVD parylenes has been reported previously. 21-23 This approach has the potential to be quite flexible, since the p-xylylene reactive intermediate of the process can be either copolymerized with another p-xylylene or used as a radical initiator for an unsaturated monomer. The resulting material of this deposition is a statistical copolymer, where the polymer composition is controlled by the reactivity and delivery rates of each monomer. 22 In this paper, we report our preliminary results on CVD copolymerization of PPV and parylene-N for fabrication of blue light-emitting polymers (Scheme 1). A significant blue shift in peak luminescence is observed from the copolymer, the magnitude of which is con- trolled by the monomer delivery rates. These copolymers are incorporated into functional single-layered LEDs exhibiting measurable brightnesses. Experimental Section Polymer Synthesis. The system used for polymer CVD has been described previously. 18 The temperatures used for mono- mer pyrolysis and deposition were 675 and 25 °C, respectively. 5336 Macromolecules 2000, 33, 5336-5339 10.1021/ma0004108 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/08/2000