Mechanism of Carrier Photogeneration and Carrier Transport in Molecular Crystal Tetracene

Models for the carrier photoexcitation mechanism in molecular crystals have been established initially on the bases of measurements on oligoacenes and later applied to conjugated polymers as well. These models emphasize the localized nature of photoexcitations and describe carrier generation as a secondary process involving exciton dissociation. The results of our photoconductivity studies of single crystal tetracene are at variance with these widely accepted models, and in fact indicate that the photocarrier quantum efficiency appears independent of temperature, photon energy, and light intensity, thus featuring the hallmarks of direct interband carrier photogeneration and coherent carrier transport at band states.

Figure 1

Transient PC waveforms measured at various temperatures (listed from top to bottom): 181, 221, 262, 301, 341, and 378 K; $h\nu =3.10\mathrm{eV}$, $I=53\mu \mathrm{J}/{\mathrm{cm}}^2$ per pulse and $F=83\mathrm{kV}/\mathrm{cm}$; the inset depicts a logarithmic plot of the field dependence of the peak transient PC (dots) measured at room temperature with $I=53\mu \mathrm{J}/{\mathrm{cm}}^2$ per pulse as well as a fit to the data (dotted line) from which a power law dependence of $\mathrm{PC}\sim F^{1.6}$ was deduced.

Figure 2

Temperature dependence of the steady-state PC measured at $h\nu =2.71\mathrm{eV}$ using $F=10\mathrm{kV}/\mathrm{cm}$, $I=2.7\mathrm{mW}/{\mathrm{cm}}^2$, and modulation frequency of 166 Hz (a) and of the peak transient PC measured using $h\nu =3.10\mathrm{eV}$, $I=53\mu \mathrm{J}/{\mathrm{cm}}^2$ per pulse, and $F=83\mathrm{kV}/\mathrm{cm}$ (b); the dashed line represents the best power law fit to the data from which a $T^{-2.09}$ dependence was deduced.

Figure 3

The transient PC waveforms displayed in Fig. 1 (181, 301, and 378 K) normalized at the peak PC; the inset shows the excitation spectrum of the peak transient PC at few excitation photon energies measured, normalized to the number of incident photons per unit area.

Figure 4

Normalized transient PC waveforms obtained at various excitation intensities ($h\nu =3.10\mathrm{eV}$, $T=330\mathrm{K}$), $F=83\mathrm{kV}/\mathrm{cm}$); the pulse intensities used from top to bottom were: $33\mu \mathrm{J}/{\mathrm{cm}}^2$, $83\mu \mathrm{J}/{\mathrm{cm}}^2$, and $264\mu \mathrm{J}/{\mathrm{cm}}^2$; the inset shows the dependence of the peak transient PC on excitation intensity (dots) and a linear fit to the data (dotted line).