Platinum-free, carbon-based materials as efficient counter electrodes for dye-sensitized solar cells Hendri Widiyandari 1 *, Adi Prasetio 2 , Agus Purwanto 3 , Agus Subagio 2 , and Rachmat Hidayat 4 1 Department of Physics, Faculty of Mathematics and Natural Science, Sebelas Maret University, Surakarta 57126, Indonesia 2 Department of Physics, Faculty of Science and Mathematics, Diponegoro University, Semarang 50275, Indonesia 3 Department of Chemical Engineering, Faculty of Engineering, Sebelas Maret University, Surakarta 57126, Indonesia 4 Department of Physics, Institut Teknologi Bandung, Bandung 40132, Indonesia *E-mail: hendriwidiyandari@staff.uns.ac.id Received February 2, 2018; accepted March 23, 2018; published online May 22, 2018 The electrocatalytic potential of carbon materials makes them the most viable candidate to replace Pt as a counter electrode (CE) in dye-sensitized solar cells (DSSCs). In this research, we report our study using graphite, CNT/graphite composite, CNT, and Pt-based CEs in DSSCs. The electrochemical impedance spectroscopy (EIS) measurement showed that the CNT-based CE (CNT-CE) has the lowest charge transport resistance (R ct ) compared with graphite and the CNT/graphite composite. The photovoltaic performance measurement showed that the CNT-CE resulted in a short-circuit photocurrent density (J sc ) of 3.59 mA&cm %2 whereas the Pt-based CE (Pt-CE) resulted in a J sc of 2.76 mA&cm %2 . © 2018 The Japan Society of Applied Physics Dye-sensitized solar cells (DSSCs) have attracted attention because of their high conversion efficiency, low-cost, and easy fabrication. 1–3) Basically, a DSSC consists of transparent conducting oxide (TCO) glass coated with a wide-band-gap semiconductor, a Ru-based sensitizer, an electrolyte contain- ing an I - =I 3 - redox couple, and a counter electrode (CE). 4) Over the past two decades, the seemingly constant develop- ment of DSSC technology has markedly improved the photo- voltaic performance. One of the most important develop- ments has resulted in an increase in the performance of CEs. The regeneration process has been accelerated from I 3 - to I - , and the electron transfer activity has been increased from an external load to an electrolyte solution. 5,6) Platinum (Pt) is the most widely used CE material owing to its excellent electric conductivity and electrocatalytic activity. 7) Pt, however, is an expensive noble metal with high processing temperature and is prone to corrosion from electrolytes, which necessitates its replacement. 8,9) Carbon materials are the most promising materials to replace Pt because of their high conductivity, high electro- catalytic activity, and corrosion resistance. 10–13) Graphite is an inexpensive carbon material with excellent intrinsic conduc- tivity. However, graphite has a very poor catalytic activity when used as a CE material for DSSC application. 14) There are many attempts to improve the photovoltaic properties of carbon-CE-based DSSCs. Li et al. reported solely a natural graphite material with different structures (nanofiber, nano- sheet, and nanoball). 15) Veerappan et al. used sub-micrometer- sized graphite as a conducting film and a CE simultane- ously. 16) Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure, which are classified into two types, i.e., single-walled CNTs and multiwalled CNTs. The conductivity and corrosion resistance of the CNTs, however, have continued to fascinate researchers who continue to search for what gives these materials their unique properties. In several attempts, CNTs have improved the photovoltaic per- formance of DSSCs. 17–19) The specific surface area, stability in general electrolytes including the redox couple in DSSCs, and the electrocatalytic activity of the catalyst play important roles in improving the working capability of the counter electrode. The electrocatalytic activity of the catalyst as the CE for DSSC application is measured by electrochemical impedance spectroscopy (EIS). Chiba et al. reported the internal resist- ance of DSSCs measured by EIS to investigate DSSC mech- anisms as well as propose an equivalent circuit for modeling DSSCs based on the results of EIS analysis. 3) On the basis of the modeling, the internal resistance of DSSCs consists of three resistance elements, i.e., the sheet resistance of the TCO, the resistance of ionic diffusion in the electrolyte, and the resistance at the interface between the counter electrode and the electrolyte. The internal resistance affects FF and J sc , that is, a lower internal resistance in a device increases FF and J sc . In this paper, we present our investigation of the relation- ship between the charge transport resistance at the CE=elec- trolyte interface and photovoltaic parameters, such as short- circuit current density and open-circuit voltage that affects the conversion efficiency of DSSCs using carbon-based CEs consisting of graphite, CNT=graphite composite, and CNTs. Commercial Pt was also used in this study for comparison. In this experiment, CNTs were synthesized by spray pyrolysis. The precursor of CNTs was prepared by mixing 0.6 g of ferrocene (Merck) in 10 mL of benzene (Merck). The precursor solution was dispersed into fine droplets using a syringe pump. The droplets flowed into the electrical tubular furnace and heated at 900 °C. The collected CNTs were refluxed using 65% HNO 3 for 4 h at 100 °C. The amounts of CNTs and HNO 3 solution are 0.1 g and 50 mL, respectively. CNT powder was obtained by filtering and drying the refluxed CNTs at 120 °C overnight. Carbon-based CEs were prepared from CNTs, graphite (Merck), and CNT–graphite composite with 1:1 of %wt. The preparation of carbon- based CEs was carried out in two steps: preparing the carbon paste and then coating the substrate with the paste by the doctor-blade method. The carbon paste was prepared by mixing 0.2 g of ethyl cellulose (Sigma Aldrich) in 2 ml of ethanol with stirring for 5 min, followed by mixing 0.8 g of terpineol (Sigma Aldrich) in the solution used as the binder. The carbon powder was dispersed in the binder with stirring for 5 min. The resulting carbon paste was deposited on a 1 cm 2 area of a fluorine-doped tin oxide (FTO) glass substrate (15 Ω=□, Dyesol) via the doctor-blade method, followed by drying at 80 °C for 5 min and annealing at 450 °C for 1 h. The thickness of the carbon film depends on that of scotch tape. DSSCs were assembled firstly by preparing a TiO 2 -based Japanese Journal of Applied Physics 57, 068001 (2018) https://doi.org/10.7567/JJAP.57.068001 BRIEF NOTE 068001-1 © 2018 The Japan Society of Applied Physics brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Diponegoro University Institutional Repository