Oxidative chemical vapor deposition of conductive polymers for use in novel photovoltaic device architectures

Abstract

The conductive polymer poly(3,4-ethylenedioxythiophene), (PEDOT), deposited via oxidative chemical vapor deposition (oCVD) has been investigated for use in organic electronic devices. The oCVD process as well as the application of oCVD PEDOT in photovoltaic devices is described. oCVD enables the synthesis of conjugated conductive films with advantageous properties for organic optoelectronic device applications. The oCVD process of forming the polymer film allows compatibility with a wide range of substrates, including those that are flexible or fragile, and provides a relatively low-energy means of depositing film layers that may not be possible through solution or other processing. Films deposited using varying oCVD process and pre- and post-treatment parameters (e.g. temperature, oxidant exposure, rinsing) were characterized based on their physical and electrical properties. It was found that acid rinsing of the already deposited films led to lower sheet resistance and surface roughness and an improvement in film stability. The oCVD PEDOT has been demonstrated as a replacement for solution-processed PEDOT:PSS as a hole transporting layer as well as for the transparent electrode material (typically ITO) in typical organic photovoltaic structures. Reverse-structure photovoltaic cells were also created using direct deposition of PEDOT electrodes onto small molecule active layer materials yielding fully dry-processed devices. The direct deposition of PEDOT top electrodes has enabled the fabrication of devices on opaque substrates leading to a greater than ten-fold improvement in previous devices fabricated on paper. Compatibility with novel photovoltaic materials has been demonstrated in work done using oCVD PEDOT as HTLs on graphene electrodes to make ITO-free devices.