Print This Page
Download as PDFTriplet Exciton Dynamics in Tetracene Versus Rubrene Crystals
Figure 1: Triplet exciton transport in tetracene crystals with varying time delays. a, The prompt fluorescence profile. The image was taken with camera integration with a time delay of 0–0.5 μs upon laser excitation. b, c, d, e, Delayed fluorescence profiles with time delays of 0.5–1 μs, 1–2 μs, 2–3 μs, and 3 μs– upon laser excitations, respectively. f, Cross-section of images in a, b, c, d, e. The area of cross-section is denoted in image a.
Exciton transport is universal in every kind of organic optoelectronic devices – including organic light-emitting diodes (OLEDs) and organic solar cells [1] . In organic bilayer photovoltaic devices, exciton diffusion limits donor/acceptor thicknesses, restricting sufficient absorptions of photovoltaic materials. Exciton diffusion of singlet excitons (total spin of 0) is usually limited to tens of nanometers, significantly smaller than the absorption length at the visible spectrum (a ~ 1mm) [2] . However, triplet excitons (total spin of 1), having disallowed-transition to ground states, are capable of moving much longer distances, up to several mm [3] . The long-range triplet exciton transport can allow us to build more efficient solar cells.
Despite their long intrinsic lifetime, triplet diffusion is disorder-limited in amorphous or polycrystalline organic semiconductors [2] . Organic single crystals, however, provide defect-free environment where triplet excitons can diffuse over long distances without being quenched by defects [3] .
Long-range triplet exciton transport has been reported before in organic acene crystals. However, in previous studies, triplet exciton diffusion was measured using indirect methods, such as probing polarization- and wavelength-dependent photoconductivity [4] . In this work, we perform direct imaging of triplet excitons by monitoring delayed fluorescence in two archetypical organic single crystals: tetracene and rubrene crystals. The comparison between tetracene and rubrene crystals is interesting since they exhibit similar molecular structures but differ in crystal structures. Our study will contribute to a better understanding of long-range exciton transport and benefit the power conversion capability of organic solar cells by overcoming exciton diffusion bottlenecks.
- P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, pp. 3693, 2003. [↩]
- R. R. Lunt, N. C. Giebink, A. A. Belak, J. B. Benziger, and S. R. Forrest, “Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching,” Journal of Applied Physics, vol. 105, pp. 053711, 2009. [↩] [↩]
- M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals and Polymers. Oxford University Press, New York, 1999. [↩] [↩]
- H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, and V. Podzorov, “Observation of long-range exciton diffusion in highly ordered organic semiconductors,” Nature Materials, vol. 9, pp. 938, 2010. [↩]