## 3 Results
### Device Performance  
Using the abovementioned carbon-based films, we designed six pairs of carbon-based electrodes for OPV devices, including CNT films (G or MG films) as anodes and G films (or ITO) as cathodes (the corresponding devices are labeled as CNT-G, G-G, MG-G, CNT-ITO, G-ITO, MG-ITO, respectively), as shown in Figure 4a. Current density-voltage (\(J\)-\(V\)) characteristics of the OPVs with all-carbon  
Figure 3: Photographs of (**a**) the as-grown CNT film on nickel foil and (**b**) the CNT film transferred onto a PEN substrate; (**c**) scanning electron microscopy (SEM) and (**d**) TEM images of the as-synthesized CNT film; (**e**) Raman spectrum and (**f**) visible transmittance spectrum of the as-synthesized CNT film. electrodes and control devices with ITO-carbon electrodes are shown in Figure 4b-d. The photovoltaic parameters of all kinds of devices are extracted and summarized in Table 1. Figure 4b shows that similar values for short-circuit current density (\(J_{\text{sc}}\)) were achieved for CNT-G and G-G, despite the much lower sheet resistance of CNT films compared to few-layer graphene films. The reason for this is that the relatively higher optical transmittance of graphene films offsets this deficiency in conductivity. However, the open-circuit voltage (\(V_{\text{oc}}\)) of CNT-G (0.47 V) is lower than G-G (0.62 V), which can be ascribed to the inferior heterojunction induced by the increased surface roughness and inhomogeneous cover of CNT networks. The results are identified by the dark \(J\)-\(V\) curves shown in Figure S8. Compared to CNT-G, G-G exhibits a more pronounced diode behavior and negligible reverse leakage current, which indicates the smoother and better interface formed between G electrodes and the adjacent layer. The performance of control cells with one-sided carbon electrodes could be investigated to further examine the competence of carbon-based electrodes. Figure 4c shows that G-ITO has a similar \(V_{\text{oc}}\) (\(\sim\)0.50 V) to that of CNT-ITO, but a lower \(J_{\text{sc}}\), which suggests that the conductivity is more contributive than transparency for carbon-based films as top anodes, in accordance with the reported conclusion by [PERSON] et al. [31]. Moreover, the overall performance of MG-G is decreased seriously due to the low transmittance and high roughness of MG films, which is further identified by the results of MG-ITO (Figure 4d). Although the better conductivity of MG films makes the corresponding devices generate moderate or even higher \(J_{\text{sc}}\), the lower \(V_{\text{oc}}\) and fill factor (FF) illustrate the increased destruction of active layer and carrier recombination caused by the roughness of the thicker graphene films. The PCEs of all-carbon electrodes devices are approximately one-third or half of those of the controls assembled  
Figure 4: (a) Schematic illustration of flexible solar cell devices with all-carbon electrodes and their indium tin oxide (ITO) analogues; (b) \(J\)–\(V\) characteristics of solar cells with CNT (or G) as the anode and G as the cathode; (c) \(J\)–\(V\) characteristics of control devices using ITO as the cathode and CNT (or G) as the anode; (d) \(J\)–\(V\) characteristics of solar cells with MG as the anode and G (or ITO) as the cathode. Here “G” and “MG” denote few-layer graphene films and multi-layer graphene films, respectively. with ITO-carbon electrodes, as shown in Table 1, in addition to G-G and G-ITO couples. The better performance of the ITO control cells could be attributed to the low sheet resistance (~10 \(\Omega\)/sq.) and smoother surfaces (ceramic nature), facilitating small series resistance and optimized interface, which ensure the reasonable values of \(f_{\text{sc}}\) and \(V_{\text{oc}}\). As for the G-G cell, its performance is close to that of its G-ITO counterpart, showing the great potential of graphene to substitute the traditional metal grid.