Bendable Solar Cells from Stable, Flexible, and Transparent Conducting Electrodes Fabricated Using a Nitrogen‐Doped Ultrathin Copper Film

Abstract

Copper has attracted significant interests as an abundant and low‐cost alternative material for flexible transparent conducting electrodes (FTCEs). However, Cu‐based FTCEs still present unsolved technical issues, such as their inferior light transmittance and oxidation durability compared to conventional indium tin oxide (ITO) and silver metal electrodes. This study reports a novel technique for fabricating highly efficient FTCEs composed of a copper ultrathin film sandwiched between zinc oxides, with enhanced transparency and antioxidation performances. A completely continuous and smooth copper ultrathin film is fabricated by a simple room‐temperature reactive sputtering process involving controlled nitrogen doping (<1%) due to a dramatic improvement in the wettability of copper on zinc oxide surfaces. The electrode based on the nitrogen‐doped copper film exhibits an optimized average transmittance of 84% over a spectral range of 380 −1000 nm and a sheet resistance lower than 20 Ω sq−1, with no electrical degradation after exposure to strong oxidation conditions for 760 h. Remarkably, a flexible organic solar cell based on the present Cu‐based FTCE achieves a power conversion efficiency of 7.1%, clearly exceeding that (6.6%) of solar cells utilizing the conventional ITO film, and this excellent performance is maintained even in almost completely bent configurations.

Ultrathin Metal films for Transparent Electrodes of Flexible Optoelectronic Devices

“…Therefore, a quantitative comparison between optical transmittance results in the literature must be processed with utmost caution. The same authors further validated that the improvement in wetting utilizing either an oxygen or nitrogen gas additive, is also highly applicable to the fabrication of 2D‐Cu ultrathin films exhibiting highly efficient optoelectrical performance (Figure c) . The most critical issue in this method is the precise control of the gas dose in the metal films within the minimal level, which is sufficiently high to influence wetting behavior, but sufficiently low to avoid the formation of any defined oxide or nitride phase.…”

“…There are numerous original research results and reviews that support the influence of oxide‐supporting materials and their surface states on wetting of the objective metals subsequently deposited on the oxides . Even if the choice of oxide materials is limited to highly transparent semiconductors and conductors for OMO applications, the oxide supporting materials still include many candidates, such as ITO and SnO x :M (where M is a metal dopant), ZnO and ZnO:M, MoO 3 , TiO 2 , WO 3 , and ZnS . It is beyond the scope of this article to compare and discuss all candidates, and this article therefore attempts to highlight some notable results for capturing a broad outline of some critical aspects for choosing the superior oxide support, as summarized in Table 1 .…”

“…The cost aspect is especially critical to OSCs that exhibit inferior power conversion efficiencies compared to any of their inorganic competitors, and thus desperately need short payback times, which are the times required for devices to produce amounts of energy equivalent to those consumed during their fabrication . Recently, numerous trials with OMO structures and their derivatives employing an ultrathin metal film have demonstrated a distinct potential for use in flexible OSCs and OLEDs, either as front or back electrodes. The superiority of the electrical conductance of the OMO structure facilitates large‐area devices with competitive efficiencies that are not attainable from devices employing amorphous ITO or other TCOs supported on polymers.…”

Enhanced Oxidation Stability of Transparent Copper Films Using a Hybrid Organic‐Inorganic Nucleation Layer

“…[11][12][13] It has also been shown that the higher optical losses in Cu, as compared to Ag, can be mitigated by electrode and/or device design, including using a metal oxide overlayer to increase transparency. [14][15][16] Due to the high surface energy of Cu and Ag these metals interact only weakly with glass and other technologically important transparent plastic substrates, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), and so the formation of robust and continuous films of these metals with thickness < 10 nm is notoriously difficult to achieve using thermal evaporation. [3,17] Metal atoms condensing on the substrate diffuse over the surface and aggregate into particles which only form a continuous network for nominal thicknesses > 10 nm.…”

Structural Optimization of Oxide/Metal/Oxide Transparent Conductors for High‐Performance Low‐Emissivity Heaters

“…Increasing the metal film thickness improves its continuity and conductivity, however, this also causes a drastic reduction in transmittance. Considerable improvements have been made in the quality of ultrathin metal films by using wetting inducers such as oxides, metal seeds, polymers, and gas additives . These conventional wetting inducers are thought to provide dense nucleation centers, which suppresses 3D metal‐island growth.…”