## 4 Conclusions  
The RF-PECVD system was used to manufacture DLC thin films. The tribological, optical, and structural properties of the DLC thin films were analyzed at varying thicknesses, and the correlation between the properties was considered for the application of DLC thin films to the ARC of DSSCs. As the thickness of the DLC thin films increased, the surface roughness and friction coefficient slightly increased, the hardness and elastic modulus increased, and the contact angle decreased somewhat. The transmittance of visible light at 550 nm decreased, while the refractive index and absorption coefficient increased with an increase in thickness. The strength of the Raman spectrum increased slightly; the G-peak shifted toward a relatively higher wavenumber as a result of Gaussian curve fitting for the Raman spectrum; and the \(\text{I}_{D}/\text{I}_{G}\) ratio decreased, which means that the \(sp^{3}\) bonds in the DLC thin films decreased, and the collision between carbon and hydrogen ions in the plasma increased on the surface as the deposition time increased, the carbon \(sp^{3}\) bonds decreased due to the reduction in hydrogen ions, and thus the DLC thin films became densified due to the increased surface temperature. The optimal \(\text{J}_{sc}\), \(V_{oc}\), and FF values with a change in the thickness of the DLC thin films were chosen for the significantly improved CE from 4.92% to 5.35% in the 60-nm-thick DLC ARC for DSSCs, with a hard and hydrophobic surface for protective and self-cleaning surfaces.  
The following is available online at [[https://www.mdpi.com/2076-3417/11/358/s1](https://www.mdpi.com/2076-3417/11/358/s1)]([https://www.mdpi.com/2076-3417/11/358/s1](https://www.mdpi.com/2076-3417/11/358/s1)), Figure S1: Gaussian fitting of Raman spectra for the RF-PECVD DLC thin films with various film thicknesses: (a) 20, (b) 40, (c) 60, and (d) 80 nm.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Thickness (nm)** & **CE (\%)** & \(\text{\boldmath$V_{oc}$}\) **(V)** & \(\text{\boldmath$J_{oc}$}\) **(mA/cm\({}^{2}\))** & **FF (\%)** \\ \hline
0 & 4.92 & 0.649 & 11.93 & 63.58 \\
20 & 5.00 & 0.691 & 12.11 & 59.70 \\
40 & 5.21 & 0.698 & 11.50 & 65.35 \\
60 & 5.35 & 0.719 & 11.38 & 64.99 \\
80 & 4.60 & 0.713 & 9.79 & 65.97 \\ \hline \end{tabular}
\end{table}
Table 1: Optoelectronic properties of the DSSCs with different thickness of the RF-PECVD DLC ARC compared with the non-ARC cell.  
Figure 6: Dye-sensitized solar cells (DSSCs) (**a**) without and (**b**) with the RF-PECVD DLC anti-reflective coatings (ARCs). (**c**) Cell characteristics (_J-V_ curves) of the DSSCs with different thickness of the RF-PECVD DLC ARC compared with the non-ARC cell.  
**Author Contributions:** Conceptualization, Y.S.P. and N.-H.K.; investigation, J.-S.S.; data curation, Y.S.P.; writing--original draft preparation, J.-S.S. and Y.S.P.; writing--review and editing, N.-H.K.; project administration, N.-H.K.; funding acquisition, N.-H.K. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy(MOTIE) of the Republic of Korea (No. 20184010201650). This study was supported by a research fund from Chosun University, 2017.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data is contained within the article or supplementary material.  
**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.