| AVS 54th International Symposium | |
| Surface Science | Friday Sessions |
| Session SS2+EM+TF-FrM |
| Session: | Organic Films II: Semiconductors and C@sub 60@ |
| Presenter: | D.B. Dougherty, National Institute of Standards and Technology |
| Authors: | D.B. Dougherty, National Institute of Standards and Technology W. Jin, University of Maryland at College Park W.G. Cullen, University of Maryland at College Park G. Dutton, University of Maryland at College Park J.E. Reutt-Robey, University of Maryland at College Park S.W. Robey, National Institute of Standards and Technology |
| Correspondent: | Click to Email |
The most efficient small-molecule organic solar cells employ heterojunctions between donor and acceptor materials.1 In order to push the performance of such devices into a regime of cost-effective power generation, it is necessary to develop a detailed understanding of organic-organic interface morphology and its relationship to electronic band alignment. We have approached this question by studying the technologically relevant2 system of C60 deposited onto pentacene, using STM/STS to provide clues to the relative importance of competing intermolecular interactions. When C60 is deposited onto a pentacene bilayer on a Ag(111) surface,3 STM measurements show two unique structures in the first layer. The pentacene bilayer forms a well-ordered structure on Ag(111) with the long molecular axis nearly parallel to the surface. At the lowest coverages, C60 forms linear chains whose direction is templated by the underlying pentacene rows, with C60 molecules located between rows of pentacene molecules. The details of the observed structural arrangement are related to electrostatic interactions between C60 and the pentacene bilayer structure. Information on local relative band alignment for these structures is measured using constant-current distance-voltage spectroscopy.4 The local transport gap for C60 linear chains is 4.4±0.2 eV compared with a gap of 3.7±0.2 eV for the surrounding pentacene bilayer. The magnitudes of the gaps are influenced by local polarization energies in each structure. At higher coverage, domains of C60 with no discernable long range order dominate the first layer. This disorder probably arises from frustrated intermolecular interactions between the two different chemical species. The lateral interactions between C60 molecules (favoring hexagonal ordering) cannot be optimized simultaneously with the comparable strength interactions between C60 and the underlying pentacene film (favoring an oblique unit cell). *This work has been partially supported by the Dept. of Commerce through the NIST Center of Nanomanufacturing and Metrology and the NSF-funded MRSEC via DMR-05-20471.
1P. Peumans, J. Appl. Phys. 93, 3693 (2003).
2S. Yoo et al., Appl. Phys. Lett. 85, 5427 (2004).
3Eremtchenko et al., Phys. Rev. B 72, 115430 (2005).
4S.F. Alvarado, et al., Phys. Rev. Lett. 81, 1082 (1998).