Photogeneration of Charge Carriers in Tetracene

The photocurrent and photovoltage of tetracene single crystals with aqueous electrodes were studied as a function of the excitation wavelength in the 220–560-mμ region. The fluorescence efficiency of these crystals was studied in the same wavelength region. At wavelengths longer than 410 mμ, the photocurrent is due to injection of holes at the illuminated electrode. A bulk-generated (electron) photocurrent is shown to be produced with excitation energies in excess of 3 eV, i.e., at wavelengths less than 410 mμ. The relative fluorescence efficiency Φ(λ) begins to decrease with decreasing wavelengths at 410 mμ. The drop in Φ(λ) at excitation energies greater than 3 eV is attributed to the appearance of a nonradiative competitive process. This process involves mostly the formation of nonconducting and nonfluorescent (or weakly fluorescent) charge-transfer states. Formation of separated charges with low quantum efficiency (φi≲5.10−3) also occurs which gives rise to the observed bulk-generated negative current. Since this current appears only for excitation energies ≳3 eV, we associate this value with the smallest band gap for conductivity in tetracene. The wavelength dependence of the bulk-generated negative photocurrent is related to varying probabilities of transition from the different excited electronic states of the crystal to ionized states. The positive hole current is mostly due to the diffusion of the lowest-energy singlet excitons to the surface and their dissociation at the electrode. The wavelength dependence of this current can be adequately described by a diffusion equation and an exciton diffusion length of 2000 Å is calculated from the experimental data and this equation. The wavelength dependence of the photovoltage is strongly dependent on the negative charge density and exhibits minima in wavelength regions where this density is highest.