Oxygen-Related Band Gap State in Single Crystal Rubrene

A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Two-photon excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene.

Figure 1

A part of the absorption spectrum showing the lowest energy molecular exciton transition (0-0) and the PL spectra of crystalline rubrene at 20 K in vacuum. An inset in the upper right corner shows the relation between the absorption and the PL peaks. The surface luminescence spectrum is measured after cw excitation ($h\nu =3.815\mathrm{eV}$, the penetration depth $d\sim 1\mu \mathrm{m}$) and the bulk spectrum is measured after 2P excitation (100 fs, $h\nu =1.65\mathrm{eV}$, $\mathrm{NA}=0.3$) at a depth of $100\mu \mathrm{m}$ from the surface. An inset in the lower left corner shows the temperature dependence of intensity of the main band I and two higher order vibronic bands, corresponding to transitions (0-1) and (0-2).

Figure 2

PL characteristics of (a) as-grown and (b) oxidized rubrene crystals at different temperatures. The as-grown sample is held in vacuum, and the oxidized sample is sealed in a capsule containing oxygen at a normal pressure. PL bands I, II, and III, and their higher order vibronic bands ${\mathrm{I}}_1$ and ${\mathrm{I}}_2$, characteristic of as-grown samples are indicated by arrows in (a), PL band ${\mathrm{I}}^{\prime }$, vibronic band ${\mathrm{I}}_1^{\prime }$, and band O characteristic of oxidized samples are indicated by arrows in (b). The color maps show spectral characteristics normalized for every temperature.

Figure 3

(a) Normalized room temperature PL characteristics measured using 2P excitation (100 fs, $h\nu =1.65\mathrm{eV}$, $\mathrm{NA}=0.8$) at various depths $z$ inside a crystalline rubrene sample naturally oxidized in air. (b) The bulk rubrene spectrum (top) measured with $\mathrm{NA}=0.3$ at $z=100\mu \mathrm{m}$ is compared to normalized room temperature PL characteristics of a vacuum-held sample (center) and an oxidized sample after single-photon excitation (bottom).

Figure 4

(a) Dark current-voltage characteristics and (b) photocurrent spectral response of an oxidized rubrene sample and a sample held in vacuum (solid line). The photocurrent $\Delta I=I_{\mathrm{photo}}-I_{\mathrm{dark}}$ is measured using a grating monochromator and a broadband white light source modulated at 10 Hz.