Organic semiconductors: exciton and charge carrier dynamics on macroscopic and microscopic levels

Oksana Ostroverkhova, Oregon State University, DMR 0748671

Our work explores optoelectronic properties of small-molecular weight organic semiconductors from macroscopic (thin films) to microscopic (single-molecule) levels. This year’s highlights include: In functionalized anthradithiophene (ADT) thin

films and their blends (macroscopic level), we:

• Developed a physical picture of exciton and charge carrier dynamics;

• Fully characterized H-aggregates responsible for optical and photoluminescent (PL) properties of films;

• Established mechanisms and conditions for energy transfer and exciplex formation in ADT mixtures and their effect on optoelectronic properties

On the single-molecule (microscopic) level, we:• Imaged ADT molecules in various

environments on a single-molecule level and characterized their properties

W. E. B. Shepherd et al., J. Phys. Chem. Lett. 2, 362 (2011), W. E. B. Shepherd et al., Proc. of SPIE 7935, 79350G (2011)

R = TIPS or TESR’ = F or CN

(b)

(f)

(d)

(e)

Figure: ADT derivatives under study (top left). (a): Effect of ADT solid-state packing on photobleaching. (b) and (c): Depending on the distance between ADT molecules in ADT blends, either FRET (b) or exciplex formation (c) dominate. (d) and (e): Changes in PL and transient photoconductive properties of ADT blends due to exciplex formation. (f): Stability of ADT single molecules with respect to blinking.

0 5 10 15 20 25 30 350.0

0.2

0.4

0.6

0.8

1.0

1.2

Pho

tocu

rren

t (a

rb.

u.)

Time (ns)

0%

0.2%

2.0%

R

R

R’R’

(c)

(a)

Involvement of undergraduate students in research and development of research instrumentation and software and in outreach activities

Oksana Ostroverkhova, Oregon State University, DMR 0748671

Undergraduate students, including under-represented women students, are actively involved in both research and educational activities carried out by our group. Four undergraduate students (one female) were involved in the project this year (two graduated in June 2011). Currently, Alex Robertson (junior, nuclear engineering major) performs experiments in and continues to develop image analysis software for single-molecule fluorescence spectroscopy, and Afina Neunzert (junior, physics major) explores time-resolved photoconductivity of organic semiconducting films. Alex and Afina are working under the supervision of graduate students Whitney Shepherd and Mark Kendrick. Our group regularly participates in outreach events demonstrating various optical effects to freshmen.

The PI developed and taught a novel graduate-level course on Nanoscience and Nanotechnology in Fall 2010. Currently, the PI is developing a new graduate-level course on Modern Optics (to be taught in Fall 2011) and is editing a book entitled “Handbook of organic materials for optical and optoelectronic devices: properties and applications” (Woodhead Publishing, Cambridge, UK), to be published in 2012.

Undergraduate student Alex Robertson is analyzing single molecule spectroscopy data

Undergraduate student Afina Neunzert is measuring time-resolved photocurrent in organic semiconducting films