Polymer-metal hybrid transparent electrodes for flexible electronics

Abstract

Despite nearly two decades of research, the absence of ideal flexible and transparent electrodes has been the largest obstacle in realizing flexible and printable electronics for future technologies. Here we report the fabrication of 'polymer-metal hybrid electrodes' with high-performance properties, including a bending radius <1 mm, a visible-range transmittance>95% and a sheet resistance <10 Ω sq(-1). These features arise from a surface modification of the plastic substrates using an amine-containing nonconjugated polyelectrolyte, which provides ideal metal-nucleation sites with a surface-density on the atomic scale, in combination with the successive deposition of a facile anti-reflective coating using a conducting polymer. The hybrid electrodes are fully functional as universal electrodes for high-end flexible electronic applications, such as polymer solar cells that exhibit a high power conversion efficiency of 10% and polymer light-emitting diodes that can outperform those based on transparent conducting oxides.

Figure 1. Schematic illustration of the PAP electrode.

(a) Flexible PAP electrode consisting of the ultra-thin Ag film between the PEI and PEDOT:PSS supporting layers. (b) Conceptual diagram for the growth mechanism of the Ag film with the PEI nucleation inducer. (c) Conceptual diagram for the destructive interference in the Ag film with the PEDOT:PSS anti-reflective layer. (d) Images of the large-area, flexible PAP. (e) Cross-sectional and surface morphology images of the PEI-Ag and bare-Ag electrodes taken using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Scale bars, left 50 nm, right 500 nm.

Figure 2. Performance of the PAP electrode.

(a) Variations in R_sh of the bare-Ag (black square), PEI-Ag (blue pentagon) and PAP (red circle) electrodes as a function of Ag thickness. The PEI layer efficiently reduces the τ_p of the Ag films to less than 10 nm via a coordination bond-induced nucleation process. The inset shows the four-point probe system used to measure R_sh. (b) Optical transmittance of the bare-Ag (black square), PEI-Ag (blue pentagon) and PAP (red circle) electrodes. Through destructive interference, the PEDOT:PSS layer leads to a 20% greater transmittance of the PAP electrode than that of the PEI-Ag electrode. The inset shows images of the electrodes located on top of our school logos. (c,d) Increase in resistance versus the bending radius (c) or number of bends (d) for the flexible PAP (red circle) and ITO (black square) electrodes on the PEN substrates. The insets show images of the bending test machine used (left) and the optical microscope images of the ITO and PAP electrodes after bending (right). Scale bar, 100 μm.

Figure 3. PSC applications using the PAP electrodes.

(a) Device architecture of the PSCs, which is comprised of the PEI:PTB7-Th:PC₇₀BM photoactive layer between the bottom PAP cathode and top Ag anode. (b) Energy level diagram of the PSCs. The self-assembled PEI layer induces a work function reduction in the PAP cathode by forming interfacial dipoles, which leads to favourable ohmic contact between the PAP and PC₇₀BM. (c,d) J-V characteristics (c) and EQE spectra (d) of the PSCs based on different electrodes and substrates. The devices using the PAP electrodes outperform the devices using conventional ITO electrodes. Black solid square, red solid circle, black open square and black open circle represent PAP on PEN, PAP on Glass, ITO on PEN and ITO on Glass, respectively.

Figure 4. PLED applications using the PAP electrodes.

(a) Device architecture of the PLEDs using the P-PPV emissive layer between the PAP anode and Ca/Al cathode. (b,c) L-V characteristics (b) and C-V characteristics (c) of the PLEDs. The performances of the PAP-based PLEDs are slightly greater than or comparable with those of the ITO-based PLEDs. The insets show the energy level diagram and J-V characteristics of the PLEDs. (d) Electroluminescent (EL) spectra of the PLEDs. The EL spectrum of the PAP-based device is nearly identical to that of the ITO-based device. The inset shows an image of the flexible PLEDs with the PAP electrode. Black solid square, red solid circle, black open square and black open circle represent PAP on PEN, PAP on Glass, ITO on PEN and ITO on Glass, respectively.