(809c) Oxidative Chemical Vapor Deposition of Semiconducting Polymers For Integration Into Polymer Solar Cells

We demonstrate the use of oxidative chemical vapor deposition (oCVD), a vacuum-based processing technique, for the deposition of semiconducting polymers for facile integration into polymer solar cells (PSCs). Proceeding from volatile monomer compounds, polymer synthesis and film deposition occur simultaneously at modest vacuum and temperature without the need to consider the resultant polymer solubility. Hence, this technique extends the library of materials available for optimizing PSCs to insoluble and infusible polymers, which are typically considered difficult to process.

By controlling the monomer precursors and deposition conditions, we are able to successfully tune the resulting semiconducting polymer bandgap. oCVD unsubstituted polythiophene (PT) is shown to have an optical bandgap of 2.0 eV. To extend absorption into the infrared, oCVD unsubstituted polyisothianaphthene (PITN) was deposited, which has an optical bandgap around 1.1 eV. PITN’s optical and electrical properties are shown to be highly dependent on the deposition conditions used. Increasing the substrate temperature during deposition from 70 °C to 130 °C results in a significant increase in conjugation length, as demonstrated by UV-Vis-NIR, Fourier Transform Infrared, and Raman spectroscopy. The absorption spectra show a red-shift of over 100 nm in λ_max, resulting in a lowering of the bandgap from 1.14 eV at 70 °C to 1.05 eV at 130 °C. Cyclic voltammetry measurements showed that PITN’s highest occupied molecular orbital (HOMO) increased by about 0.1 eV with increasing temperature.

Bilayer heterojunction photovoltaic cells were fabricated on patterned ITO-coated glass substrates. The vapor-deposited semiconducting polymer was directly deposited onto the ITO to serve as the electron donor layer. The electron acceptor was deposited by either vacuum thermal evaporation of fullerene C₆₀ or spin-coating various fullerene derivatives, such as PCBM. Bathocuproine (BCP), an exciton blocking layer, and a silver top cathode were then thermally evaporated to complete the devices. Power conversion efficiencies greater than 1% under AM 1.5G (100 mW cm^-2) were achieved.

This work demonstrates that oCVD is a viable technique for the processing and design of polymer active layers for PSCs without solubility or substrate considerations. 

This work was supported by Eni SpA under the Eni-MIT Solar Frontiers Center.