D. Peebles

The band structure and electronic properties of pure and heavily doped polyacetylene (both as grown and stretch oriented) have been investigated by a combination of optical-absorption and -reflection measurements in the frequency range from the middle ir (0.1 eV) through the visible (4.0 eV). The absorption data are consistent with a direct gap of approximately 1.4 eV in the trans-${(\mathrm{CH})}_{x}$. A Kramers-Kronig analysis of the reflection data has been carried out to obtain $\ensuremath{\sigma}(\ensuremath{\omega})$ and $\ensuremath{\epsilon}(\ensuremath{\omega})$. We find that for the undoped semiconducting polymer, the strong transition observed in the visible exhausts the oscillator strength sum rule for $\ensuremath{\pi}$ electrons consistent with an interband transition. The frequency-dependent conductivity obtained from Kramers-Kronig analysis of the metallic polymer reflection data suggests "interrupted-strand" behavior. Application of effective-medium theory implies an intrinsic dc conductivity for metallic ${[\mathrm{CH}{(\mathrm{As}{\mathrm{F}}_{5})}_{0.15}]}_{x}$ of $\ensuremath{\sigma}>2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The measured dc values have thus far been limited by the low-density fibril morphology.

A variety of rectifying junctions have been fabricated using doped and undoped (CH)x. Schottky diodes formed between metallic AsF5-doped (CH)x and n-type semiconductors indicate high [CH(AsF5)y]x electronegativity. The p-type character of undoped trans- (CH)x is confirmed by Schottky-barrier formation with low-work-function metals. An undoped p- (CH)x : n-ZnS heterojunction has been demonstrated with an open-circuit voltage of 0.8 V. These results point to the potential of (CH)x as a photosensitive material for use in solar-cell applications.

T r a n s:(CH)x:n-CdS heterojunctions have been fabricated and used to study the properties of undoped trans-polyacetylene. The I-V data show rectifying behavior characteristic of a p-n junction diode, thus confirming that as-grown polyacetylene is p type. From C-V characteristics we infer a residual acceptor concentration in as-grown films of p-(CH)x of NA?2×1018 cm−3. Depletion has been studied in the polymer by using CdS of different doping concentrations. Detailed studies of the photovoltaic response at energies below Eg for (CH)x imply the existence of a meta-stable deep trap in the polymer with energy near the center of the gap. The threshold energy for pumping into this level provides an independent measurement of the energy gap, Eg=1.45 eV.

Photoelectrochemical photovoltaic cells have been fabricated using polyacetylene, (CH)x, as the active photoelectrode. Using a sodium polysulfide solution as electrolyte, Voc∼0.3 V and Isc∼40 μA/cm2 were obtained under illumination of approximately 1 sun. Under present conditions, the cell efficiency is limited by the series resistance, the small effective area of the electrode configuration, and the absorbance of the solution.