## 3 Results
### Preparation and Characterization of Graphene Films  
Next, for transparent conductive electrodes in thin-film carbon-based solar cells, large-area and high-quality carbon-based films are needed. Thus, we used the CVD method to synthesize large-area graphene and CNT films. Because there is a trade-off between transparency and conductivity for graphene films, to gain more insight about this balance, few-layer and multi-layer graphene were grown by adjusting the feed rate of the carbon precursor. To conveniently distinguish between the two kinds of graphene films, few-layer (layer numbers from two to about five) and multi-layer (layer number of ~10) samples [53] are labeled as G and MG, respectively. Figure S2 shows the photographs of as-grown G films on copper foil, G and MG films transferred onto PEN substrates, and G and MG films protected by polymethyl methacrylate (PMMA) floating on the etching solution. Large-area graphene films with different thicknesses and transparency levels were obtained. The surface morphology of G films are demonstrated in Figure S3. From the optical images of the films on copper and transferred onto a SiO\({}_{2}\)/Si substrate, it can be seen that the large-area films are uniform and clean. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images  
Figure 1: **(a)** Schematic illustration and **(b)** energy level diagram of the solar cells fabricated with carbon nanotube (CNT) or graphene (G) films as anodes and graphene film as cathode. PEDOT:PSS denotes poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). P3 HT and PCBM denote poly(3-hexyl)thiophene and phenyl-C61-butyricacid methyl ester, respectively; **(c)** Photograph of a semitransparent solar cell with all-carbon electrodes; **(d)** Good flexibility of as-obtained solar cell device. (Figure S4) show typical wrinkles of the thin films resulting from the different expansion coefficients of the substrates. Figure 2a shows the large-area G film transferred onto a plastic wafer with high transparency. In order to investigate the thickness and crystallization of the G films, high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SEAD) patterns were collected and are shown in Figure 2b. The SEAD pattern shows one complete set of bright diffraction spots, which indicates the high crystallization and single-crystalline feature of the G films. At the edge of the G films there are two clear stripes, which suggests that the two-layer films have been obtained. Furthermore, the film thickness can be inferred from the Raman and transmittance spectra. As shown in Figure 2c, the typical Raman characteristic peaks involving D, G, and 2D types are demonstrated. Since the intensity ratio of the 2D and G band is greater than 2, it can be verified that the G film has few layers [54]. There is a low peak at the D band (\(\sim\)1350 cm\({}^{-1}\)) associated with defects, which suggests that the G films contain few impurities (e.g., amorphous carbon). The average optical transparency at 550 nm is 91.5% (Figure 2d), which corresponds to about three layers, according to the 2.3% visible light absorbance of single-layer graphene [55]. The sheet resistance varies from 1.0 to 1.5 k\(\Omega\)/sq. with the slight change of the thickness. Meanwhile, the MG films were also characterized by Raman spectrometry, as shown in Figure S5. On the contrary, the intensity ratio of the 2D and G band is less than 2, which implies that the MG films have more than five layers. Accordingly, the MG films have a transmittance of 83% with sheet resistances of 450-710 \(\Omega\)/sq. (Figure S5b). It is critical to achieve a smooth surface on the bottom electrodes in thin film solar cells. The surface roughness of G films (on SiO\({}_{2}\)/Si) was observed with atomic force microscopy (AFM). Figure S6a,b show the planar and three-dimensional surface images, from which one can see many micro-waves caused by the transfer process of nanoscale thin films.