Martin Pope

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...

Charge‐Transfer Exciton State and Energy Levels in Tetracene Crystal

An experimental study of the double‐quantum external photoelectric effect in tetracene reveals evidence of an exciton state of energy 2.9 eV different from the normal exciton. This mobile, bulk‐generated state is considered to be a charge‐transfer (CT) exciton and is most efficiently produced at an energy ≥Ec=3.7 eV. We associate Ec with the lower edge of a relatively broad upper conductivity band. Presumably, there are other conducting states Ec′ such that ECT<Ec′<Ec. The calculated value for the polarization energy of the free electron in the crystal, Pe=Ic—Ec=1.6 eV, where Ic is the threshold energy for external electron emission. This is the same as the polarization energy of the positive hole Ph=Ig—Ic, where Ig is the ionization energy of the tetracene molecule. The electron affinity of the isolated molecule must be less than 0.8 eV. The diffusion length of the charge‐transfer exciton is less than 5000 A.

Singlet Exciton Fission and Tri plet-Tri plet Exciton Fusion in Crystalline Tet racene

Abstract The dominant radiationles decay channel in crystalline tetracene at 300 °K is a fission of an excited singlet into two triplet excitons with a rate constant γS = 1.5 × 10−12 ± 5% cm3 -sec−...

Charge-transfer Exciton State and Ionic Energy Levels in Anthracene Crystal

Abstract By means of external electron emission produced by double-exciton annihilation inside anthracene crystal, evidence is produced for the existence of a charge-transfer (CT) exciton in anthracene at 3.45 eV, with a lifetime conjectured to be 2 × 10−7 sec. The efficiency of generating the CT exciton increases markedly at an incident light energy greater than 4 eV. This value is associated with that of an upper conductivity band. The lowest conductivity level in anthracene must be larger than 3.45 eV, and is estimated to lie at 3.7 eV. Delayed fluorescence produced by α-particle bombardment of anthracene is discussed in terms of CT excitons.

Magnetic field effect on the chlorophyll fluorescence in Chlorella.

Abstract The chlorophyll a fluorescence in Chlorella pyrenoidosa can be enhanced by 4–9% if the excitation light beam is parallel to an external magnetic field or decreased by 4–9% if the light beam is oriented perpendicular to a magnetic field of about 16 kgauss or more. These effects cannot be explained in terms of the small changes in light absorption which are also observed. It is suggested that these observations are due to a reorientation of pigment molecules in the magnetic field.

GENERATION OF CHARGE CARRIERS IN ANTHRACENE WITH POLARIZED LIGHT

Abstract : In order to ascertain the dependence of bulk generated photocurrent on the absorption coefficient in anthracene, a study was made of the relative quantum efficiency of the photocurrent at fixed wavelength as a function of the angle, Theta, between the electric vector of polarized light and the b crystalline axis.

Heterofission of pentacene excited singlets in pentacene-doped tetracene crystals

Abstract The magnetic field dependence of the fluorescence of pentacene-doped tetracene single crystals indicates that following energy transfer to pentacene, the trapped pentacene singlet fissions into one tetracene triplet exciton and one pentacene triplet (estimated energy 0.81 ± 0.02 eV).

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‐A light. The escape depth of the electrons is estimated as equal to or greater than 40 A.

Preparation of Thin Anthracene Single Crystals

Anthracene single crystals ranging in thickness from 4 to 80 μ and from 0.5 to 2 cm2 in area have been prepared by crystallization from a selected solution system. The method is general and employs a density and temperature gradient within the solution which permits the crystal to grow while suspended in the solvent.

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‐Huckel‐method calculations. Finally, the absorption spectra of these cryst...