Low Temperature Fabrication of an Indium-Free Dye-Sensitized Solar Cell: Based on Commercially Available Polymer Substrates

Abstract

Photovoltaic (PV) devices that convert sunlight to electrical energy consist of inorganic PVs (conventional silicon cell) and organic PVs (bulk heterojunction and dye-sensitized solar cell). Motivated by the current drive for affordable PV devices, this study investigates potential polymers as a cheaper and flexible alternative substrate to glass. Using sputtered ultrathin Au films as an alternative to standard transparent conducting films such as indium doped tin-oxide (ITO) and fluorine doped tin-oxide (FTO), allows the fabrication of a flexible PV. The chosen PV device for the prospective flexible Au films is a dye-sensitized solar cell (DSSC) consisting of a nanoporous semiconducting metal oxide, TiO2 sandwiched between a working electrode and a counter-electrode. Potential polymers that were sputtered with ultrathin Au films were evaluated based on Au film crystallinity, morphology and adhesion that is closely related to the film’s optical transparency and conductivity. The usage of these polymers typically limits the annealing temperature of the TiO2 film that is needed to promote good interparticle connection for efficient charge transport. In order to accommodate for the low thermal stability of the substrates, various low temperature TiO2 deposition techniques such as microwaved hydrothermal assisted method, spin-coating and doctor-blading was used. The microwave method provided a quick, mild temperature technique for growing TiO2 films with the anatase phase on Au coated polymers. The quality of the differently deposited TiO2 films was evaluated based on surface morphology, crystallinity and cyclic voltammetry (CV) to give us an indication of the film’s integrity and quality. Au-DSSCs were fabricated by using the standard N719 dye and iodide/triiodide electrolyte. Due to unavoidable surface defects in the deposited TiO2 film, the electrolyte could potentially infiltrate through the TiO2 pores and come into contact with the underlying electrode. Conversely, this creates a problem for the Au electrode that is easily etched by the corrosive electrolyte. One way to combat this is by passivating the Au electrode with selected alkanethiol self-assembled monolayers (SAMs) that consist of different alkyl chain lengths and terminal functional groups. The most suitable SAM exhibits considerable passivation as demonstrated by electrochemical studies and affinity between the terminal functional group and the TiO2 film. This leads to fabricating an Au-DSSC whereby its efficiency is compared with a FTO-DSSC as the benchmark. However, the Au-DSSC’s performance was considerably lower than FTO using all three TiO2 deposition techniques. Interestingly, the choice of TiO2 deposition technique also influenced the cell’s efficiency using both FTO and Au electrodes. The degree of influence was closely related to the TiO2’s blocking behaviour as reported by the CV results. In addition, the presence of a SAM did not improve the Au-DSSC’s efficiency which we suspect is due to alterations to the Au electrode’s work function and additional resistive properties. This study has successfully demonstrated low temperature fabrication of Au films and TiO2 films on commercially available polymers; bypassing the standard thermal annealing procedure. It also revealed that various modifications to the prospective Au working electrode is necessary if it were to be on par with currently available DSSCs. However, it is worth considering Au films on polymers for other PV devices such as organic PVs not necessarily limited to DSSCs.