## 2 Experimental Details  
The DLC thin film was deposited using a 13.56 MHz RF-PECVD system on 2 cm \(\times\) 2 cm glass and silicon substrates. To decontaminate the substrates surfaces before deposition, the substrate was cleaned with ultra-sonication, acetone, methanol, and deionized water (DIW). After cleaning, the native oxide film on the substrate was removed using a hydrogen fluoride (HF) solution. The substrates were dried with N\({}_{2}\) gas and then mounted on the holder in PECVD. To improve adhesion between the substrate and DLC thin film, pretreatment was performed in the generated plasma by infusing H\({}_{2}\) gas of 100 sccm at an RF power of 140 W for 2 min. The reaction gas was injected with a mixture of methane (CH\({}_{4}\)) of 25 sccm and H\({}_{2}\) of 100 sccm in a fixed condition; a base pressure of 2 \(\times\) 10\({}^{-3}\) Torr, process pressure of 1 Torr, and RF power for plasma of 120 W at room temperature. The thickness of the DLC thin films was adjusted from 20 to 100 nm with a change in the deposition time.  
Structural properties of the DLC thin films were analyzed using Raman spectroscopy (Jobin Yvon, LabRamHr, excitation wavelength of 514 nm) [16], and surface characteristics were observed using field effect scanning electron microscope (FESEM, Hitachi, Tokyo, Japan, S-4700) and atomic force microscopy (AFM, Park Systems, Suwon, Korea, XE-200). Tribological characteristics including hardness and elastic modulus of the DLC thin films were examined by using a nanoindenter (MTS Systems Corp., Eden Prairie, MN, USA, Nanoindenter II) and nano scratch tester (Anton Paar GmbH, Graz, Austria, NST\({}^{3}\)). Optical properties, such as transmittance, refractive index, reflection, and absorption coefficient of DLC thin films were measured using an ultraviolet-visible spectrometer (SCINCO Co. Ltd., Seoul, Korea, S-2100) and spectroscopic ellipsometry (Horiba, Kyoto, Japan, UVISEL2).  
The working electrode in a DSSC was fabricated by the doctor blade coating method using a slurry with TiO\({}_{2}\) nanoparticles (13 nm, Solarnik SA, Aubonne, Switzerland) on fluorine-doped tin oxide (FTO, Solarnik SA, Aubonne, Switzerland, \(\delta\)\(\Omega\)/sq, 130-nm thickness, 1.5 cm \(\times\) 2 cm)/glass before it was sintered at 550 \({}^{\circ}\)C for 60 min in an active area of 0.25 cm\({}^{2}\) to form TiO\({}_{2}\) nanocrystalline thin films with a thickness of approximately 10 \(\upmu\)m. RF-PECVD DLC thin films were introduced as an ARC on the upper working electrode. The working electrodes were dipped in 0.3 mM ruthenium-complexed dye (Solarnix SA, Aubonne, Switzerland, N719, _cis_-Bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II)) as sensitizers for 24 h at room temperature. They were cleaned with ethanol and then dried with N\({}_{2}\) gas. Counter electrode was fabricated by dropping 50-\(\upmu\)L PtCl\({}_{4}\) mixed with chloroplatinic acid (Sigma-Alldrich Inc., St. Louis, MO, USA) of 10 mM and isopropyl alcohol on the FTO/glass and then sintered for 20 min at 450 \({}^{\circ}\)C in a furnace. The dye-adsorbed TiO\({}_{2}\) working electrodes were bonded with the counter electrodes using a 60 \(\upmu\)m hot-melt Surlin spacer (Solaronix SA, Aubonne, Switzerland, SX 1170) at 100 \({}^{\circ}\)C using a hot plate. Iodolyte (Solaronix SA, Aubonne, Switzerland, AN-50), as a low-viscosity electrolyte with tri-iodide of 50 mM in acetonitrile, was injected into one of the two holes drilled into the counter electrodes, and the holes were then sealed with a cover glass and Surlin. The performance and the current-voltage characteristics of the DSSCs with different thicknesses of DLC ARC were implemented by a solar simulator (Abet Technologies Inc., Milford, CT, USA, Xe Lamps of 30 W, AM1.5).