High-Performance Transparent Copper Grid Electrodes Fabricated by Microcontact Lithography for Organic Photovoltaics

Abstract

We report high-performance transparent copper grid electrodes on glass and plastic substrates that offer a higher Haacke figure-of-merit than conventional indium tin oxide electrodes and are well-matched to the requirements for organic photovoltaics (OPVs). The electrode is fabricated using microcontact lithography with a combination of molecular resist and low toxicity etchant, namely, hexadecanethiol and aqueous ammonium persulfate. This approach to electrode fabrication is much faster than conventional lithography because it takes <2 s to print the molecular resist layer and tens of seconds to etch the copper film, with both processes performed in ambient air. The grid line width achieved is >20 times narrower than is possible using conventional metal printing methods and so a grid pitch <30 μm is easily achieved without increasing metal coverage. The very small grid-line spacing relaxes the requirement to use highly conductive films to span the gaps between grid lines, reducing parasitic absorption losses. This is demonstrated using an extremely thin (10 nm) poly­(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer. Additionally, we present evidence that it is not always necessarily to embed the metal grid into the substrate or to planarize with a charge-transport layer, to avoid leakage current across the OPV device.

Advances in Flexible Metallic Transparent Electrodes

“…[3,367] Metal-based ordered grids, such as Ag or Cu grids, can be deposited on glass or polymeric substrates, and can be associated with very high Haacke's FoM; they can match fairly well the requirements, e.g., for organic photovoltaics. [368] Oxide/metal/oxide have also been largely investigated in recent years since they can exhibit a very low sheet resistance (0.1-0.3 Ω sq −1 ), and an optical transmittance over 82% with uniform properties over a very large area (≈1 m 2 ). [369] They can therefore be efficiently used as transparent automobile windshield heaters.…”

A Microcontact-Printed Nickel-Passivated Copper Grid Electrode for Perovskite Photovoltaics

“…However, the fill-factor ( FF ) and open circuit voltage ( V oc ) are lower in the former ( FF : 0.55 ± 0.04 and 0.60 ± 0.04 and V oc : 0.49 ± 0.03 and 0.52 ± 0.04 V). PEDOT:PSS with imidazole and the GOPS cross-linker has a conductivity equal to that of PEDOT:PSS (PH 1000), which at the thickness used here (70 nm) is more than sufficient to span the 27.2 micron gaps between Cu grid lines without degrading the device FF , and so, the lower FF and V oc are not due parasitic resistance associated with the layer spanning the gaps between grid lines. It is evident from the dark current characteristic, shown in Figure , that the current leakage across the device results from the earlier onset of forward injection.…”

“…Despite these advantages, there are very few reports pertaining to using this technique to make grid electrodes. To the best of our knowledge, to date there has been only two reports: Xia et al and Bellchambers et al have shown that μ-CP can be used to fabricate Ag grids and Cu grids, respectively, the latter on both rigid glass and flexible plastic substrates. The economic case for using Cu instead of Ag is clear, since Cu is <1% the cost of Ag with comparable electrical conductivity .…”

A touchless user interface based on a near-infrared sensitive transparent optical imager using printed Cu grid electrodes.

“…The effect of both the Cu grid and the patterned OPD design on the optical transmittance and its design optimization are discussed below. Electro-optical simulations and design rules for NIR OPDs with a printed Cu grid TCE The effect of the metal grid design on the performance of organic photovoltaics was studied experimentally [16][17][19][20][21] and simulated [39][40][41] . These studies however only provide the optimal metal grid's design for speci c device architectures and for metal grids with much wider line width (≥10 µm) than employed in this work 39,40 .…”

“…To make the NIR-sensitive OPD array visually transparent, it must have transparent conductive electrodes (TCEs). One of the most promising TCEs are metal grids [14][15][16][17][18][19][20][21] , since the electrooptical performance is easily tunable by changing the grid design (e.g., line width, pitch, and thickness) 22 . A metal grid with a line width less than 3 µm becomes invisible to the naked eye over 1 cm away from it based on human eye's angular resolution 23 , providing transparency without noticeable haze.…”