Henrichsen, Henrik Hartmann

I projektet er der blevet undersøgt forskellige metoder til hvordan syntetiske organiske halvledermaterialer kan kontaktes elektrisk. Et nyt design af top-kontakter til organiske nanofibre blev afprøvet med tre forskellige tilgangsvinkler. En metode blev fundet mere egnet end de to andre, men må yderligere raffineres for at komme til at virke. Elektriske karakteristikker blev målt for arrays af krystallinske organiske nanofibre, og en model for de elektriske karakteristikker udviklet. Fra modellen kunne størrelsen af krystaldomæner i nanofibrene estimeres.I projektet blev grafén, et enkelt atom-lag af kulstof-atomer, afprøvet som elektrodemateriale til for forskellige formål. Dels som elektrode i organiske felt-effekt transistorer, dels som elektroder til dielektroforese-eksperimenter. Ydermere blev det observeret at molekyler af para-hexaphenylen kan forme krystallinske domæner på grafén. Da grafén kan skabes som en frithængende membran, muliggør det undersøgelser af dannelsen af organiske krystaller med atomar opløsning i transmissions elektron mikroskoper.Optoelectronic applications of organic semiconductor materials is a researchfield, which recently came to the large scale consumer market in display technologies.Organic semiconductors are mainly applied in amorphous form offeringfabrication control on a large scale. Crystalline organic semiconductors,where the molecular packing is more crucial, have not yet had a major impactin commercial products. This thesis describes development of new waysto electrically contact organic semiconductors. In particular, crystalline organicpara-hexaphenylene (p6P) nanofibers have been used as a representativecomponent for the organic nanofiber class.Organic light emitting devices based on nanofibers cannot readily be fabricatedby conventional methods developed for thin film devices. A novel designof layered top contacts, separated by an insulating layer, was fabricated usingthree different approaches. Creating the separator by partly oxidizing an Alcathode anodically is considered the most promising implementation, howeverfurther development would be necessary.During the project a group of collaborators managed to obtain electricallystimulated light emission in organic p6P nanofibers, by using an AC-gatedorganic field-effect transistor (OFET) implementation.The electrical properties of arrays of p6P nanofibers were investigated asgrownand modeled theoretically. The developed model, assuming hopping-liketransport of charge carriers, was used to estimate the distance between hoppingsites. A distance of 23±5 nm was extracted and found to be in good agreementwith transmission electron microscopy (TEM) studies.Graphene, a one atom thin 2D crystal of carbon, has several propertiesrelevant for electrodes: it is atomically flat, optically transparent, does notoxidize, and has high electrical and thermal conductivity. In this project theuse of graphene as an electrode material for organic electronics was investigated.For this purpose a fabrication process compatible with contamination sensitivecleanroom equipment was developed. First the process was applied to fabricatearrays of OFET templates and p6P applied as the organic semiconductor. Thetested devices exhibited large injection barriers and significant hysteresis of theelectrical characteristics. Therefore the device design was found unsuitable toelucidate the possible advantages of graphene electrodes in OFETs.Secondly the electrode fabrication method was applied to realize electrodesfor dielectrophoresis experiments. Robust electrodes with multi-layer graphenecontact pads and few-layer graphene electrode edges were made. Carbon nanotubeswere assembled with dielectrophoresis between electrodes. Optimizationof the dispersion prevented the graphitic electrodes from being washed off, andthe same samples could be reused for several experiments. During the experiments it was discovered that thin films of p6P on graphiticsubstrates can form crystalline domains. Molecular orientations on the sampleswere probed by fluorescence and white light polarization experiments. It wasfound that blue reflected light has the same polarization as fluorescence fromthe samples. This can be used to probe molecular orientations in these samplesand completely avoid the bleaching effect of UV-excitation. An investigationof the morphological and molecular orientations within the domains, in relationto the graphitic lattice, showed growth of two different crystalline phases.One of the phases was found comparable to the β-phase typically observed onmica substrates. The morphology of the other phase had formed nanofiber-likeaggregates on the substrates with typical dimensions up to 500×20 nm2. Apossible application was demonstrated by growing nano-aggregates of p6P ona suspended graphene membrane, which could be used for TEM studies of theas-grown crystalline properties of p6P

Online Research Database In Technology

Electrical Interfaces for Organic Nanodevices

http://orbit.dtu.dk/ws/files/5216088/henrik_henrichsen_phdthesis_5_okt2010_gath.pdf

Electrical Interfaces for Organic Nanodevices

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