Molecular orientation and charge transfer dynamics of modified GO:PEDOT:PSS composites for optoelectronic applications

Organic Solar Cells (OSC) are photovoltaic devices that convert solar radiation into electric current. In these systems, the anode usually consists of glass or plastic covered by a thin layer of transparent conductive oxide, such as indium tin oxide (ITO), acting as a hole transport layer. The cathode is made of low work function metals, such as aluminum that acts as electron transport layer. The photoactive layer consists of donor materials (p-type semiconductor polymers) and electron acceptors (such as fullerene derivatives). The most common OSCs can be constructed in bi-layer or bulk heterojunction architectures, which differ in the way the donor and acceptor units are distributed along the photoactive layer. One aspect that causes concern in the architecture of the current OSCs is the ITO anode, which presents some disadvantages (mainly for being expensive and easy to break when deflected), resulting in an unwanted setting of device upscaling. Therefore, the substitution of this material is of great relevance for the continuity of the research and production of the OSCs. The conducting polymer poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT:PSS) is certainly one of the most important substitute material reported for indium tin oxide in organic devices. Although graphene oxide (GO) is not a good conductor, it can form high-quality thin films and can be transparent, and additionally, GO is an inexpensive material and can be easily synthesized. PEDOT: PSS blended with GO films were evaluated by X-ray photoabsorption (NEXAFS) for molecular orientation studies and Resonant Auger Electron Spectroscopy (RAS) using the Core-Hole-Clock method (CHC) for charge transfer dynamics investigations. The opposite behavior of the pi* and sigma* transitions in the NEXAFS spectra suggests that the thiophene-unit in PEDOT is preferably oriented with the molecular plane perpendicular to the substrate surface. Furthermore, no clear angular dependence was observed for PSS, which indicates that the sulfur units of this polymeric dopant film have a more amorphous nature. The charge transfer times showed a decrease from about 5 fs for GO:PEDOT:PSS 5% to 4.3 fs for the GO:PEDOT:PSS 5% treated in ethylene glycol nanocomposite. These spectroscopic results suggest that the simple solvent treatment using ethylene glycol proved to be efficient and able to increase the conductivity of the nanocomposite films. (author)