One of the main goals outlined in our PRF proposal was understanding charge trapping in organic electronic materials. In the summer of 2007, Michael Jaquith used electric force microscopy to image long-lived trapped charge in a transistor made from a molecular cousin to pentacene -- a triisopropylsilyl pentacene derivative("TIPS" pentacene) given us by Professor John Anthony of the University of Kentucky. The Anthony synthesis leads to samples which are far purer than the best commercially available organic electronic materials (which decompose if they purified by repeated sublimation).

The preliminary (unpublished, confidential) results of this summer's electric force microscope study of charge trapping in TIPS pentacene were rather surprising:

1) Charge-trap sites in the TIPS pentacene transistor were distributed far more

uniformly than in a similar pentacene transistor (see the attached nugget).

2) Trapped charge in TIPS pentacene takes much longer to clear than in pentacene.

3) Electrons can be injected from a gold electrode into TIPS pentacene, which is not

the case in pentacene. Moreover, the electron mobility in TIPS is small but

measurable.

4) In TIPS pentacene, injected electrons can be used to erase trapped positive charge,

essentially instantaneously.

We can draw some preliminary conclusions from these findings. A mechanism of charge trapping involving a chemical reaction of pentacene cation radicals (e.g. holes) with hydrogenated and oxygenated pentacene defects has been proposed by Northrup and Chabinyc. Since the relevant defect-carbon sites in TIPS pentacene are unavailable to bind with oxygen or hydrogen, our Findings (1) and (2) suggest that the charge trapping mechanisms at play in TIPS pentacene are different from those at play in pentacene. We conclude that other possible charge-trapping chemical reactions must be considered for the acenes. Our Finding (4) would seem to rule out atom-transfer chemistry of the sort proposed by Northrup and Chabinyc as the mechanism of charge trapping in TIPS pentacene. Future theoretical work on charge trapping reactions in organic electronic materials will be required to explain how the positively-charged products of the charge-trapping reaction can be so quickly converted to neutral non-trapping species by introduction of free electrons.

Sirringhaus and Friend, et al., have shown that electron trapping in organic transistors fabricated on silicon dioxide dielectrics (as ours are) is facile because electrons react with the silicon dioxide surface. Our finding of finite electron mobility in a TIPS pentacene transistor fabricated with a silicon dioxide dielectric -- Finding (3) -- suggests that this conclusion should probably be reexamined.

We can glean many additional insights into charge trapping in organic electronic materials by irradiating the samples with variable wavelength light. Light of the proper wavelength creates electron-hole pairs in the organic material, some of which split into free electrons and holes which then help clear long-lived traps. This light-induced clearing of traps greatly decrease the time required to complete our electric force microscope studies of trapped charge. Sweeping the wavelength of the irradiation and monitoring either local capacitance or the rate of elimination of trapped charge should allow us to locally image the absorption spectra of molecular species in the film. Coffey and Ginger have recently shown that photogeneration of charge can be locally mapped in solar cell materials by adding irradiation to electric force microscopy experiments.

In the summer of 2007, first-year graduate student Justin Luria began to modify our custom electric force microscope to accept a fiber-optic-coupled variable wavelength light source. The variable-wavelength light source was specified and parts were ordered. A modified inertial coarse approach mechanism was designed, developed, and tested which can accommodate not only a cantilever and its displacement-detecting single-mode optical fiber but also a second multimode optical fiber for irradiating the sample underneath the cantilever.

In last year's report I described work underway in our laboratory to use custom-fabricated silicon microcantilevers to measure the local charge mobility in an organic electronic material -- a Holy Grail of the organic semiconductor community. In support of this effort, first-year graduate student Sarah Wright worked on a new approach to fabricating high-compliance silicon microcantilevers with integrated ~100-nm diameter metallic nanorod tips. In the summer of 2007, Ms. Wright was trained on instrumentation at the Cornell NanoScale Science and Technology Facility, and explored using xenon difluoride to etch silicon in increments as small as ~100 nm. Her preliminary results suggest that this will be possible, but only by carefully calibrating the xenon difluoride dose and exposure time.