Tetracene Single Crystals Research Articles

Photovoltaic Effects in Tetracene Crystals

Abstract Studies on the photovoltaic effect in tetracene single crystals have shown that, in general, two photovoltaic mechanisms yielding signals of opposite sign can be observed. On illuminating the crystal, a fast photovoltage response is obtained which is eventually dominated by a more slowly rising photovoltage of the opposite sign. The action spectrum of the fast response is closely related to the absorption spectrum of the crystal. The slow response is more effectively induced by light in spectral regions where the crystal absorbs light weakly. Temperature higher than room temperature decrease the response time of the slow photovoltage. The results are discussed in terms of photovoltage mechanisms which involve singlet excitons and photoinduced detrapping of trapped charge.

Space-charge-limited currents in tetracene single-crystals

It is established that gold and palladium form ohmic contacts for hole injection into sublimation grown tetracene crystals. Ohmic electron injecting contacts are formed by sodium and barium. The resulting SCL-currents can be described conventionally assuming an exponential trap distribution. Since the total trap density H 0 = 3.10 15cm −3 and the distribution parameter l = ( T c / T) = 4.0±0.3 turn out to be equal for electron and hole traps, we conclude that the same defects are able to trap both electron and holes. Temperature variation of the SCL-currents yields the energetic distance of the Quasi-Fermi-level of electrons respectively holes from the lowest conduction respectively valence band. Its dependence on the applied voltage is in accordance with theory. The dependence of the SCL-currents on voltage and spacing of the electrodes can be described by j ∼ ( F l+1 / d l ).

Effect of Magnetic Field on the Fluorescence of Tetracene Crystals: Exciton Fission

The fluorescence efficiency of tetracene single crystals may be enhanced by as much as 38% in a magnetic field $H>2000$ G. The enhancement is anisotropic with respect to the orientation of $H$ in the $\mathrm{ab}$ plane. It is shown that a magnetically sensitive coupling of singlet states to a double-triplet-exciton state (${T}_{1}{T}_{1}$) at \ensuremath{\sim}2.40 eV is an important channel for radiationless decay in crystalline tetracene above 160\ifmmode^\circ\else\textdegree\fi{}K.

Space-charge-limited currents in tetracene single-crystals

Space-charge-limited currents in tetracene single-crystals

Low-Lying Valence Band States and Intrinsic Photoconductivity in Crystalline Anthracene and Tetracene

The wavelength dependence of the intrinsic photoconductivity of anthracene and tetracene single crystals has been measured with photon energies up to 6.5 eV. Both materials exhibit three peaks or shoulders in the 3.0–6.5-eV range. The photoconductivity spectra are interpreted by comparing them to the kinetic energy distributions of external photoemitted electrons. The energy sequence of the photoconductivity peaks is in good agreement with characteristic energy losses suffered by the externally emitted photoelectrons. The decreases in the photocurrent, as the photon energy is increased, are shown to arise at energies at which transitions from lower-lying valence states set in. When transitions from lower-lying valence states are involved, the energy of the final state may be below the photoconductivity threshold energy and the current decreases. The energy sequences of the valence states are compared to theoretical HMO and extended-Hückel-method calculations. Finally, the absorption spectra of these crystals are related to the photoconductivity action spectra.

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.

Double-Quantum External Photoelectric Effect in Organic Crystals

Illumination of anthracene, tetracene, and perylene single crystals with strongly absorbed light of energy several electron volts smaller than the reported threshold for electron emission, produces electron emission at a rate proportional to the square of the light intensity, indicating the occurrence of a biphotonic process. Evidence is presented in the case of anthracene indicating that a singlet exciton—exciton interaction is involved. The efficiency of photoemission in anthracene is 10−25 I0, where I0 is the intensity of 3650-Å light. The escape depth of the electrons is estimated as equal to or greater than 40 Å.

The Polarized Ultraviolet Spectrum of Tetracene Single Crystals by a Photoconductance Method

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