Nanocarbon Growthed By Chemical Vapor Deposition And Its Application On Dye Sensitized Solar Cells Counter Electrode

Dye sensitized solar cells(DSSCs) have aroused extensive attentions due to the easy fabrication, low cost and relatively high efficiency to convert solar energy into electricity. However, improving the photoelectric conversion efficiency of the DSSC has been existing a bottleneck. The concept of irradiation from both side provides a new route for enhancing the photovoltaic performance of solar cells. That means the counter electrode(CE), as an indispensable part of the DSSC, must be transparent. Carbon nanotubes(CNTs) and graphene(graphene) are all new carbon materials, with large surface area, high transmittance and high electron mobility. Meanwhile nanotubes can offer scattering-free ballistic transport, and graphene is able to as the platform for accelerating the ballistic transport. Combining the advantages of both, the composite material electrode can achieve high catalytic activity and high transmittance. For this, nanocarbon growthed by chemical vapor deposition(CVD) was applied on DSSC transparent counter electrode and the DSSC performance was studied by front- and rear-side illumination.Firstly,a facile in-situ grown nanocarbon composite materials including one dimensional(1D) carbon nanotubes(CNTs) and two dimensional(2D) graphene nanosheets were directly deposited on FTO conductive glass substrates. The growth processes were optimized by time and gas flows, which for concerning the optimal growth condition. Then, the growthed nanocarbon composite was characterized by Raman spectrum, Scanning Electron Microscope(SEM), Transmission Electron Microscope(TEM) ect. The sheet resistance is around 750 Ω/□. Transmittance is up to 79.8%.Secondly, the growth mechanism of the CNTs/G nanocarbon composite was investigated through X-ray Photoelectron Spectroscopy(XPS) and X-ray diffraction(XRD). It is found that when the temperature reached 900℃, Cu vapor was generated and C2H2 when came in contact with the “hot” metal Cu vapor, decomposed into carbon and hydrogen species. When the carbon interacted with the surface of the FTO substrate, the original Sn O2 nanoparticles on the surface were reduced to metallic Sn. Meanwhile, the Sn droplets acted as the catalytic centers to decompose C2H2 and grow CNTs. The low-melting-point Sn nanoparticles formed liquid Sn droplets and the Sn droplets around the CNT were absorbed into the interior of the CNTs by the capillary force, forming the 1D Sn/CNTs core-shell structure. The other carbon species crystallized to 2D graphene nanosheets and deposited on the rough surface of the FTO substrate.Last but not least, the electrocatalytic ability of the CNTs/G CE and the photovoltaic performance of the DSSC were studied by Cyclic Voltammograms(CVs), Electrochemical Impendence Spectra(EIS) and Photocurrent-voltage(I-V) test. CV curves showed that the cathodic current density of the CNTs/G CE was smaller than Pt CE(1.12 m A cm-2 vs. 2.86 m A cm-2), but the CNTs/G CE have lower overpotential than Pt CE(452 m V vs. 570 m V). The EIS also confirmed the CNTs/G CE exhibit high electrocatalytic activity. Charge transfer resistance(RCT) is 2.13 Ω cm2. While the larger sheet resistance(Rs=192 Ω cm2) lower the hole cell performance. I-V test was carried out under front- and rear-side illumination respectively with and without reflector. The results demonstrated using transparent CE can improve the photovoltaic performance of DSSC. The energy conversion efficiency is 5.16% under bifacial illumination.