Design and Deposition of Electroactive Polymers for Light-emitting Diodes and Photovoltaics: DMR-0208786

Mary Galvin, University of Delaware and Lewis Rothberg, University of Rochester

Goals: The goal of this research is to develop a model that explains why a mixture of conjugation lengths in a polymer affects the amount

of light emitted by light-emitting diodes (PLEDs). Research: Our past work has showed that polydispersity of conjugation lengths can dramatically impact the efficiency of organic LEDs fabricated from polyparaphenylenevinylenes. To explain the impact, we have modeled these effects using a detailed quantitative theory that takes into account trapping effects on charge transport, charge injection and the location of the recombination region. Experiments were performed by deliberate engineering of trap distributions in organic LEDs fabricated from blends of conjugated oligomers and polymers synthesized in the Galvin group. Theory was done by modifying code written in collaboration with Darryl Smith at Los Alamos National Labs. The result is a general theory that provides a cost-benefit analysis for reducing polydispersity and extrinsic traps in polymer light-emitting diodes. Given a material’s electron and hole mobilities plus HOMO and LUMO levels, the theory allows device engineers to optimize contact selection and trap density. We have now shown experimentally that traps can either increase or decrease device efficiency.

NNO

C8H17OH3C

OC8H17

OC8H17

CH3

C8H17O

Oxadiazole pentamer

NN

OOC12H17

C12H17O

( )

OXAPPV

n

Fig. 1. Chemical structure of some of polymers and oligomers used to test model.

Fig. 2. (a) Model energy level scheme of a PPV-oxadiazole pentamer Device with long chains blended in. Internal quantum (b) and power (c) efficiency versus current for various trap depths at 0.5% trap concentration.

Design and Deposition of Electroactive Polymers for Light-emitting Diodes and Photovoltaics: DMR-0208786

Mary Galvin, University of Delaware and Lewis Rothberg, University of Rochester

EducationDuring the last year 1 student (Steve Konezny) and 1 Postdoctoral Fellow (Dr. Zhenjia Wang) received some financial support from this grant at the University of Rochester. Two students at the University of Delaware were also at least partially supported on this grant (Yashpal Bhandari and Changheui Ko). As part of their professional development all of the students were encouraged to present their research at national meetings. Yashpal presented a poster at the Spring MRS meeting. Steve presented his work at the APS meeting.

Changheui Ko and Galvin also used flexible PLEDS made in the Galvin lab as a demonstration in an undergraduate engineering class, Introduction to Materials Science Class. The demonstration was integrated into a section on semiconducting materials.

OutreachThis year Galvin was involved in teas and socials aimed at mentoring women graduate students and faculty at the University of Delaware. Additionally she gave a presentation on the important role mentors play in career development to a meeting of the Delaware Chapter of the Society of Women Engineers.

Figure 1: PLED made on a plastic substrate used as a class demonstration.