Photogeneration of Charge Carriers in Tetracene

Abstract

The photocurrent and photovoltage of tetracene single crystals with aqueous electrodes were studied as a function of the excitation wavelength in the 22Q-560-mlL region. The fluorescence efficiency of these crystals was studied in the same wavelength region. At wavelengths longer than 410 mIL. 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 filL. The relative fluorescence efficiency <1>(>') begins to decrease with decreasing wavelengths at 410 mIL. The drop in <1>(>') 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 (<I>.;S 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 A 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.

Evolution of Photophysical and Photovoltaic Properties of Perylene Bis(phenethylimide) Films upon Solvent Vapor Annealing

“…The increase in exciton diffusion length, l, with order can be roughly estimated by assuming that the fraction of excitons reaching the illuminated surface is R l/(1 + Rl) 19,20 and that the Corrected fluorescence emission spectra of two 270 nm thick PPEI films at different degrees of order. Each film had a small square of TiOPc evaporated on it.…”

Organic Solar Cells: An Overview Focusing on Active Layer Morphology

“…While they could not completely explain the phenomenon, they postulated that different exciton dissociation mechanisms must occur at the light and dark electrodes. Later, they also observed a photovoltaic effect in a tetracene-water system (Geacintov et al 1966). Since this device was also completely symmetrical, except for illumination, they thought that exciton dissociation via electron injection into the water, and hole transport by the organic material away from the interface, could explain the observed behavior.…”

Interfacial processes in small molecule organic solar cells

“…Thus, in the inorganic samples the main process of charge separation is the drift of free carries (electrons and holes) which result in the photovoltage. Different from such a process, what are dominantly produced in the photoexcitation process of an organic material are not free carriers but strongly bound electron-hole pairs in one molecule (excitons) [21,22]. Excitons have no direct contribution to the photovoltage because of their electric neutrality.…”