The Application Of Conductive Polymer Counter Electrodes To Dye-sensitized Solar Cells Based On TiO₂Naonotube Arrays

Generally, the dye-densitized solar cell (DSSC) consists of a dye-sensitizedtitanium dioxide (TiO2) nanoparticle photoanode, an I/I3electrolyte, and a counterelectrode. The existed numerous boundaries among TiO2nanoparticles result in slowdiffusion rate of electrons and serious charge recombination between electrons andiodide ions in the electrolyte. Highly ordered, one-dimensional TiO2nanotube arrays(TNTs) can provide direct electron transport channel, resulting in slower carrierrecombination, longer electron lifetime, and higher electron transfer rate. Among thecatalytic materials of counter electrodes, Pt is usually used. Whist it is a kind of nobelmetal, which isn’t consistent with low-cost characteristic of DSSCs. Conductivepolymers, such as poly(3,4-ethylenedioxythiophene)(PEDOT), poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)(PEDOT: PSS) and polyaniline (PAn), can beused as substitutes of Pt, owing to their low cost, easy fabrication, and high catalyticactivity. In this paper, the main research contents and results are as follows:(1) A12.5m double-ended open, one-dimensional ordered TNT film wasprepared by a two-step anodization method and then detached from Ti foil with a30%H2O2aqueous solution. Through HF and TiCl4aqueous solution treatments, theroughness of TNTs film can be increased, resulting in the improved dye loadingamounts and light harvesting efficiency. Combining HF with TiCl4treatment, thehighest power conversion efficiency (PCE) of DSSC can attain to7.30%, with35.69%enhancement compared with that of the non-treated one (5.38%).(2) A high transparent PEDOT counter electrode is prepared with a simple methodof in situ chemical polymerization of EDOT on fluorine doped tin oxide over-layerglasses (FTO glasses) at room temperature. The cyclic voltammetry, electrochemicalimpedance spectroscopy, and Tafel polarization are measured to evaluate the catalyticactivity of PEDOT counter electrode for I3-/I-redox couple. The related data of Ptcounter electrode are also tested for comparison. All of the data show that PEDOTcounter electrode has higher catalytic activity than that of Pt counter electrode. The DSSCs with front-side illuminated TiO2nanotube arrays based photoelectrodes andthese counter electrodes are assembled, and their photovoltaic performance aremeasured. PCE of the DSSC with PEDOT counter electrode can attain to7.71%, alittle higher than that of the cell with Pt counter electrode (7.30%). Taking theadvantage of high transparency of PEDOT counter electrode, an Ag mirror is put onthe back side of PEDOT counter electrode of the DSSC to reflect light back for powerconversion. PCE of the DSSC with this special structure can be further enhanced to8.36%.(3) S/rGO/PEDOT-PSS counter electrode was prepared with PEDOT:PSS modifiedby reduced graphene oxide (rGO) and sulfuric acid aquous solution. The influences ofrGO additive and sulfuric acid treatment on the morphology, transparency, catalyticactivity and photovoltaic performance of PEDOT-PSS electrode were investigated.rGO/PEDOT-PSS composite showed an enhanced active surface area for I3reduction.Sulfuric acid treatment improved the conductivity of PEDOT-PSS electrode, resultingin a promoted short circuit current density of the DSSC. PCE of DSSC assembledwith S/rGO/PEDOT-PSS (rGO/PEDOT-PSS treated by sulfuric acid) reached7.03%,with96.9%enhancement comparing with that of the DSSC with pure PEDOT-PSScounter (3.57%).(4) Polyaniline nanobelts with40-70nm width and140-340nm longth werefabricated by a template-free method. Cylic voltammetry and electrochemicalimpedance spectroscopy data showed that, the electrochemical catalytic activity ofPAn counter electrode was comparable to that of Pt counter electrode. DSSCsassembled by the PAn nanobelts counter electrode showed a PCE of6.32%, whichwas87%of power convertion efficiency in Pt counter electrode DSSCs under thesame condition.