## 1 Introduction  
Carbon materials including zero-dimensional fullerenes [1], one-dimensional carbon nanotubes (CNTs) [2], and two-dimensional graphene [3] have attracted increasing attention due to their unique physical, chemical, and electrical properties. They inspire scientists to attempt to collect them to assembly all-carbon optoelectronic devices, including photodetectors [4; 5] and photovoltaics [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16], which have many potential advantages, such as low cost, simple processing, and long-term stability. Currently, in the field of photovoltaics, carbon nanomaterials are mainly employed as additives [17; 18; 19; 20; 21; 22; 23] or transparent electrodes [24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38]. Organic photovoltaics (OPVs) have special advantages including light weight, good flexibility, convenient fabrication, and large-area feasibility, which make them versatile in niche places unfit for Si-based inorganic solar cells, such as power-generating windows and flexible electronic devices [39; 40]. [PERSON]'s group reported visibly transparent OPVs that have a maximum transparency of 66% at 550 nm, aimed to be add-on components of multijunction photovoltaics, smart windows, and building-integrated photovoltaics (PVs) [41]. However, almost all published OPV devices require the use of a metal oxide (e.g., indium tin oxide, ITO) and noble metals(e.g., Ag, Au) as electrode materials, both of which have some drawbacks, as the former is brittle and expensive, the latter is reflective and opaque. These material issues are being addressed by the development of transparent and flexible thin films made of carbon nanostructures including CNTs and graphene. Some studies have been carried out with CNTs and graphene as anodes in OPV cells. In 2006, [PERSON] et al. used printed films of single-walled carbon nanotube networks on plastic as transparent and flexible anodes of OPVs with a power conversion efficiency (PCE) close to the performance of its ITO/glass counterpart [42]. CNT films produced by chemical vapor deposition (CVD) [31; 43], polyaniline interfacial polymerization [29], and brush-painting [44] were also employed as anodes for building ITO-free OPVs, showing more flexible properties than their ITO-based counterparts. Meanwhile, researchers also tried to apply graphene as anodes. For example, large-area graphene films synthesized via CVD were successfully used as anodes in OPV cells, showing the potential of graphene to replace ITO [45]. Chemically reduced graphene oxide was another material employed to form graphene flexible electrodes through spin-casting onto substrates [33; 46; 47]; however, its low optical transmittance and high sheet resistance were generally not favorable to the fabrication of solar cells with high efficiency. Thus, the CVD process is preferable for the preparation of thin films with a large area, intact structure, and low number of defects for either CNT or graphene. Concerning cathodes, CNT and graphene films were used to substitute the conventional metal grid combined with ITO; consequently, it became possible to fabricate semi-transparent OPVs that could be illuminated at both sides [26; 31; 48]. However, carbon-based flexible electrodes are applied only on one side of abovementioned OPVs, which still involve brittle ITO or noble metals. Recently, graphene anode- and cathode-based OPVs have been demonstrated, paving the way to flexible solar cells with full graphene electrodes [35; 37; 38]. To date, there are relatively few studies devoted to all-carbon electrode applications in OPVs. In this context, it is promisingly cost-effective to replace metal oxide- and metal-based electrode materials with flexible transparent carbon-based nanomaterials as electrodes in OPVs [49; 50]. In this work, CNT and graphene films are used as both anodes and cathodes in conventional OPVs. The large-area, continuous, and highly transparent CNT and graphene films were synthesized and transferred onto flexible substrates.