Oxygen Effect on The photoconductivity Behavior of C₆₀ Thin Films

S. Kazaoui, and N. Minami
[Trans. Mat. Res. Soc. Jpn., Vol.14B, p.1243, 1994]

Conductivity (sigma_d) and photoconductivity (sigma_ph) measurements have been performed on polycrystalline C₆₀ thin films. C₆₀ films are characterized by an extended optical absorption edge (E_gopt~1.7eV) and photoconductivity onset (E_gel=1.7~2.1eV). sigma_d and sigma_ph in the high temperature regime (T>270K) are thermally activated (Ea(sigma_d)=0.50 and Ea(sigma_ph)=0.20±0.05eV) whereas at low temperature regime (T<270K) there are weakly temperature dependent, for all excitation wavelength above the photoconductivity edge. Oxygen, in the octahedral interstitial sites of fcc lattice of C₆₀, has those effects in this material: decrease in sigma_ph and sigma_d by orders of magnitude, increase in sigma_ph/sigma_d ratio; decrease in photoconductivity response time. Moreover the oxygen effect on those properties is mostly reversible. Here carrier photogeneration and transport processes are invoked in the context of localized electronic states and oxygen effects are interpreted. Uv-visible absorption and photoconductivity spectroscopies reveal some aspect of the electronic structure of the C₆₀ film. For the purpose of comparison Uv-visible absorption of C₆₀ in solution (hexane), thin film (air exposed) and photoconductivity spectra (air exposed) are represented in fig.1. Uv-visible absorption spectrum of C₆₀ thin film retains all the characteristic absorption bands observed in solution. However, the latter one shows neither a shoulder at 600 nm nor an extended tail in the region 700-800nm as clearly observed for films. In the range 260-800nm corresponding peaks are observed in both absorption and photoconductivity spectra of thin film confirming that photoconductivity is generated via the excitation of C₆₀ molecules.