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.

1. 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.
2. 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.
3. M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals and Polymers. Oxford University Press, New York, 1999.
4. 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.

Luminescent Solar Concentrators (LSCs) aim to reduce the cost of solar electricity by using an inexpensive collector to concentrate solar radiation without mechanical tracking (Figure 1a) ^[1] . Ideally, the dyes re-emit the absorbed light into waveguide modes that are coupled to solar cells attached to the edges of the collector (black arrows). However, some photons are always lost, re-emitted through the face of the LSC, and coupled out of the waveguide (grey arrows). The trapping efficiency, η_trap, is defined as the fraction of photons emitted from the edge versus photons emitted from the face and edge combined. Assuming the dyes emit their photons isotropically, η_trap is given by η_trap = . If the refractive index of the waveguide n_s is ~ 1.7, then 20% of the re-emitted photons are lost.

Such surface losses become compounded if photons trapped in the waveguide and travelling towards the edges are re-absorbed by other dye molecules in the waveguide; see Figure 1b. When such re-absorbed photons are re-emitted, the LSC will suffer more confinement losses, and the cycle repeats. As the number of re-absorption events increases, the overall efficiency of LSCs drops exponentially.

In this work, we aim to quantify the contribution of self-absorption to the surface losses of an LSC. We vary the amount of self-absorption by tuning the dye concentration of ALq₃ and DCJTB in the waveguide (n_s = 1.7). Figure 2 shows preliminary results for η_trap of an LSC with high and low self-absorption. The LSC with low self-absorption has η_trap of 78%, which approaches the theoretical η_trap of 81%. The slightly lower measured value of η_trap is likely to be explained by scattering losses from the LSC surface.

1. J. S. Batchelder, A.H. Zewail, and T. Cole, “Luminescent solar concentrators. 2: Experimental and theoretical analysis of their possible efficiencies,”Applied Optics, vol. 20, pp. 3733-3754, Nov. 1981.