Real-Time Tracking of Singlet Exciton Diffusion in Organic Semiconductors

Exciton diffusion in organic materials provides the operational basis for functioning of such devices as organic solar cells and light-emitting diodes. Here we track the exciton diffusion process in organic semiconductors in real time with a novel technique based on femtosecond photoinduced absorption spectroscopy. Using vacuum-deposited ${\mathrm{C}}_{70}$ layers as a model system, we demonstrate an extremely high diffusion coefficient of $D\approx 3.5\times {10}^{-3}{\mathrm{cm}}^2/\mathrm{s}$ that originates from a surprisingly low energetic disorder of $<5\mathrm{meV}$. The experimental results are well described by the analytical model and supported by extensive Monte Carlo simulations. The proposed noninvasive time-of-flight technique is deemed as a powerful tool for further development of organic optoelectronic components, such as simple layered solar cells, light-emitting diodes, and electrically pumped lasers.

Figure 1

Experimental concept. (a) Schematic of the time-of-flight experiment, and molecular structures of TPTPA (donor) and ${\mathrm{C}}_{70}$ (acceptor). (b) Energetics of ${\mathrm{C}}_{70}$ exciton (shown as the brown oval) dissociation via hole transfer process [19]. Energy levels of ${\mathrm{C}}_{70}$ and TPTPA are taken from Refs. [20, 21].

Figure 2

(a) Absorption spectra of 48 nm ${\mathrm{C}}_{70}$ (purple line) and 12 nm TPTPA (yellow line) samples. (b) Polaron absorption spectra for the 6 nm sample at 0.4 ps (black squares) and 1.5 ns (green diamonds) delays; for other polaron spectra, see Fig. S4 in Ref. [25]. Solid lines are fits with Gaussian functions. The excitation wavelength is 530 nm.

Figure 3

Measured (dots) and fitted with analytical model (solid lines) transients for different samples. Each transient is offset by $\mathrm{\Delta }T/T=2\times {10}^{-4}$ to the corresponding dashed line; the thickness of each ${\mathrm{C}}_{70}$ layer is indicated to the right. All transients are normalized by the number of absorbed photons and corrected for the ${\mathrm{C}}_{70}$ contribution (see Ref. [25] for details).

Figure 4

(a) Exciton diffusion coefficients obtained from independent fits of each transient. The inset shows the temperature dependence of the diffusion coefficient for the 24 nm thick sample. (b) Exciton harvesting efficiency versus ${\mathrm{C}}_{70}$ layer thickness. The experimental values are shown by symbols while the solid line is obtained from a fit to Eq. (3). The data are corrected for the exciton annihilation. For the thinnest sample (6 nm), a 100% exciton harvesting efficiency is assumed.