Structural, Optical, and Photovoltaic Properties of Tetracene Thin Films

Abstract

The structure, absorption spectra, and photovoltaic response of tetracene (Tc) thin films 120 and 200 nm in thickness deposited in a vacuum of 6.5 MPa onto different substrates at room temperature have been studied. The photovoltaic response is measured by the method of capacitor cell. It is found that island films are formed on glass and quartz substrates, whereas solid polycrystalline films on copper ones, which is a result of the different interaction energies of Tc molecules with one another and with substrate molecules, molecules of ITO layers, and carbon atoms on the substrate surface. In quasi-amorphous films at a temperature of 4.2 K, excited molecules are deformed stronger as compared to those in polycrystalline films and free molecules at 300 K. The photo-emf f at the front (illuminated) free surfaces of Tc films is negative in the spectral interval of 1.637-3.258 eV, which testifies to the hole photoconductivity in the films. The photo-emf r at the rear surface of 120-nm Tc films (illumination through the ITO electrode, which directly contacted with the Tc film) changed its sign in the spectral intervals of 2.193-2.494 and 2.927-3.153 eV as the oxygen concentration in the measuring cell decreased, which testifies to a change of the energy band bending in the Tc films near their rear and front (free) surfaces. K e y w o r d s: tetracene thin films, Al-Tc-Au heterostructures, photovoltaic response.

Donor-Acceptor Interaction in Films of Tetracene–Tetracyanoquinodimethane Heterostructures and Composites

“…4 and 5, curve 1 ) and formula (2), the following minimum values of were obtained: 200 nm for = 120 nm, 146 nm for = 170 nm, and 144 nm for = 200 nm. It should be noted that the absorption spectra of the TC film 200 nm in thickness were given in work [4]. Hence, for the TC film 120 nm in thickness, exceeds the film thickness in the spectral interval 1.6-3.5 eV.…”

“…Holes are the majority charge carriers in TC crystals and films [1,4,32]. The conductivity of the crystals of TCNQ ion-radical salts is realized by means of the electron transfer along the linear stacks of TCNQ − anion-radicals and holes along the alternative stacks of cation-radicals of various nature [35].…”

“…It was suggested [4] that, in TC films, and are the algebraic sums of three photo-emfs: D , , and . The Dember photo-emf D is associated with the diffusion of nonequilibrium holes toward the dark surface of the film.…”

“…The wide structural absorption band for the TC film corresponds to the first electron transition in the TC molecule. The long-wave bands at 2.340 and 2.460 eV are the components of the Davydov splitting of the 0 → 0 transition, and they testify to the presence of a crystalline phase in the TC films [4] [17,18]. At the same time, the band at 2.714 eV can be a vibrational repetition of the band at 2.627 eV.…”

“…Tetracene (TC) is a high-resistance organic semiconductor of the -type with a wide energy gap ≥ 3.0 eV [1]. The photovoltaic effect in TC crystals and films was studied in works [2][3][4]. The effective coefficient of conversion of the light energy into the electric one in solar cells with a TC/C60 (fullerene) heterostructure was found to equal 1.7-2.3% [5,6] or 0.34% [7].…”