Parylene-based chemical vapor deposition of electroluminescent polymer films used in polymer light emitting diodes

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1999.Includes bibliographical references (leaves 136-143).The effects of reaction and deposition conditions on the properties of the conjugated electroluminescent polymer poly(p-phenylene vinylene) (PPV) prepared by parylene-based chemical vapor deposition (CVD) are explored in this thesis. A reactor for CVD of PPV is designed and constructed, and both a,a'-dibromo p-xylene and a,a.'-dichloro p-xylene are tested as source monomers. By sufficiently baking out the CVD system, reproducible fabrication of unoxidized CVD PPV is achieved for the first time. The optical characteristics of the CVD PPV are in good agreement with solution-processed material, but the polymer does exhibit enhanced aliphatic hydrocarbon incorporation, which can influence the polymer bandgap and therefore emission color. For the same deposition conditions, these hydrocarbon defects are more prominent in polymer prepared from the bromine monomer. By correlating changes in polymer composition with the CVD reaction and deposition conditions, the source of the aliphatk hydrocarbon incorporation is determined to be fragmentation of the starting monomer during the pyrolysis step. Tuning the peak emission color of the CVD polymer through copolymerization is also addressed. Fabrication of device-quality CVD PPV films is achieved for the first time through a greater understanding of how the deposition conditions influence the polymer film structure. Several different growth morphologies are observed below the critical surface polymerization temperature, such as island, transitional, and coalesced or 'layered' growth. By re-designing the reactor configuration to allow film deposition at room temperature, single-layered CVD PPV devices with reasonable tum-on voltages and light output easily seen with the naked eye in a well-lit room are realized. A highly novel, parallel method for selective deposition of CVD PPV is also developed through use of chemical surface modification, achieving one-step patterned growth of the deposited material. Treatment of the substrate with evaporated iron, iron salts, or organo-iron complexes is found to inhibit polymer growth. Spatial control of the inhibitor through microcontact printing of carboxylic-acid terminated alkanethiols used in conjunction with metal salts, or photolithographic patterning of evaporated metal films, allows fabrication of selectively grown features as small as 5 μm and films as thick as 3500 A. This is more than sufficient for use as the active element in LEDs, and the selectively grown PPV is successfully incorporated into a functional device. Iron treatment also inhibits the deposition of other parylene-based CVD polymers such as parylene-N and parylene-C, resulting in selectively grown structures on the order of several microns in thickness. For the parylene systems, a wide range of transition metal elements, salts, and organometallic complexes are found to produce the same growth inhibition effect, which suggests that the chemical surface modification approach presented here may be a general technique for controlling the growth of vapor-processed parylene-based polymers.by Kathleen Michelle Vaeth.Ph.D