Dye-sensitized solar cell technology (DSSC) has been intensively studied because of its simple structure, low fabrication costs, promising light harvesting efficiency and environmental friendliness. DSSC is considered the next generation solar cell technology. It is a potential replacement for conventional silicon solar cells. The high energy conversion efficiency of dye-sensitized solar cells is accomplished through the use of a highly porous semiconductor film coated with a monolayer dye-sensitizer as the working electrode. This technology was developed by O'Regan and Grätzel in 1991 . TiO2 nanoparticle is normally used as a semiconductor because it delivers the highest energy conversion efficiency among available semiconductors (ZnO, Nb2O5, WO3, In2O3, SnO2) [, ,  and ]. Platinum (Pt) film is commonly used as the cell counter electrode in reducing tri-iodide to iodide (I3−+ 2e−→3I−). This is because Pt has a good catalytic activity with I3−. However, Pt is an expensive substance. Thus, cheaper alternative catalysts have been intensively investigated. These included compounds such as carbon black, carbon nanotubes or conductive polymers [ , , ,  and ].
Poly (3,4-Ethylendioxythiophene)–Poly (Styrene Sulfonate) (PEDOT–PSS) is one of the conductive polymers which is considered for use as a DSSC counter electrode. This is because of its unique properties: good catalytic activity, good conductivity and comparatively lower price than Pt. Many researchers observed that the pure PEDOT–PSS counter electrodes generate lower cell efficiency than Pt-based DSSC [, ,  and ]. In order to enhance polymer based DSSC performance, other materials are incorporated with polymer to increase the film surface area, conductivity and/or catalytic activity. For example, Wenjing Hong et al.  incorporated graphene into PEDOT–PSS. They obtained an increased efficiency of ~ 4.5% compared to ~ 2.3% for pure PEDOT–PSS based DSSC. Benhu Fan et al.  improved PEDOT–PSS DSSC performance by adding carbon nanotube to polymer. An efficiency of ~ 6.5% was attained by the nanotube incorporation. It is well known that nanoparticles have a high surface area to volume aspect ratio. Thus, by mixing nanoparticles (other than graphene or carbon nanotube) into PEDOT–PSS, film roughness and exposed polymer area can be increased. This could lead to the enhancement of DSSC performance in the same manner as graphene or carbon nanotube incorporation. Muto et al.  and  employed many types of nanoparticles (ZnO, NiO, Al2O3 and TiO2) to increase roughness and solar cell efficiency. Efficiency of ~ 4.38% was obtained by mixing TiO2 nanoparticles (~ 50 nm) with PEDOT–PSS. This suggests that incorporation of semiconductors promotes dye-sensitized solar cell performance. However, the effect of nanoparticle sizes on the dye-sensitized solar cell performance has not yet been fully characterized. In the present work, we further investigated the influence of TiO2 nanoparticle sizes (~ 25 nm and ~ 100 nm) mixed with PEDOT–PSS on DSSC performance. TiO2 nanoparticles were used rather than other metal-oxide nanoparticles because TiO2 is non-toxic and inexpensive. This well serves our purpose of minimizing DSSC cost. In addition, the effect of incorporating small (~ 25 nm) and large (~ 100 nm) TiO2 particles at several ratios upon film structure and solar cell performance was also explored. The change in counter electrode catalytic activity and conductively was analyzed by Cyclic Voltammogram and Electrochemical Impedance Spectroscopy, respectively.
2.1. Working electrodes
Conductive glass (fluoride doped tin oxide glass, FTO, with sheet resistance of 8 Ω/sq, Solaronix), was used as a substrate. The FTO glass was treated with TiCl4 aqueous solution (40 mM) at 70 °C for 30 min. The transparent and scattered TiO2 films, with area of 0.25 cm2, were coated on the TiCl4 layer by a screen printing technique using the TiO2 paste PST-18NR and PST-400C, respectively (Catalysts & Chemicals Ind. Co., Ltd). TiO2 films were annealed at 500 °C for 1 h, and then were immersed in the dye solution, cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)-bis-tetrabutylammonium (N719, Solaronix, 5 × 10− 4 M in acetonitrile:tert-butanol at a volume ratio of 1:1) for 24 h at room temperature.
2.2. Counter electrodes
Two different TiO2 sizes (~ 25 nm (P25, Aldrich), referred to as small TiO2 and ~ 100 nm (Aldrich), referred to as large TiO2) were used for preparing five TiO2 compositions on the basis of weight percentage: 1) 100% large TiO2:0% small TiO2 (100L), 2) 70% large TiO2:30% small TiO2 (70L), 3) 50% large TiO2:50% small TiO2 (50L), 4) 30% large TiO2: 70% small TiO2 (30L) and 5) 0% large TiO2:100% small TiO2 (0L). A slurry of PEDOT–PSS (Aldrich) and TiO2 was prepared by mixing 0.4 g composite large–small TiO2 in 0.5 ml PEDOT–PSS. It was stirred by using a magnetic stirrer for 30 min. Composite polymer–TiO2 slurries were coated onto FTO using a glass rod and followed by drying at ~ 80 °C for 6 h. For a comparison, Pt counter electrodes were prepared by a sputtering method.