Nanostructured transparent metal electrodes for organic solar cells

Cost-effective and highly efficient renewable-energy generation is becoming ever more important in this age of rising energy prices and global climate change. Organic solar cells (OSCs) represent a promising energy-conversion technology for clean and carbon-neutral energy production because of their potential for low-cost and simple fabrication methods and their compatibility with large-area processing on flexible substrates. To date, most OSCs have been built on indium tin oxide (ITO) substrates because this material offers both optical transparency and good electrical conductivity. However, ITO is not an optimum electrode, chemically,¹ mechanically,² or economically.

In an effort to replace ITO, nanotube networks³ and random silver (Ag)-wire grids⁴ were recently investigated. However, the former cannot simultaneously achieve high transparency and conductivity, while the latter suffer from current shunt due to the random nature of Ag nanowires. We therefore developed a new type of transparent and conductive electrode based on metallic nanostructures⁵ that could potentially lead to the development of low-cost and high-performance large-area OSCs.⁶

The transparent metal electrode is composed of a nanoscale wire grid, as shown in the inset in Figure 1, and made through nanoimprint lithography.⁷ This emerging technique is well suited for organic electronics requiring low-cost and high-throughput production. The resulting metal electrodes have high optical transparency and good electrical conductivity. Both properties can be adjusted relatively independently by changing the metal linewidth and thickness in the periodic metal-grid structure.⁵

Figure 1. Comparison of the optical transmittance of the nanoimprinted copper (Cu) electrode and a conventional high-performance indium tin oxide (ITO) electrode in the visible regime.

We have developed highly transparent metal electrodes with an average transmittance of 83% in the visible range, made of copper (Cu) nanowires with a linewidth of ∼70nm and a period of 700nm. Figure 1 shows a scanning-electron-microscope image and optical-transmittance spectra of the high-transparency Cu electrode mounted on a glass substrate. The 40nm-thick Cu electrode has a sheet resistance of about 28Ω per square (a unit exclusively used for sheet resistance). However, we can further increase the electrical conductivity by a factor of three using thicker metal, without significantly sacrificing transmittance.⁵

Figure 2. (a) Schematic of the fabrication of a nanopatterned metal electrode on a poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)-coated glass substrate using a flexible polydimethylsiloxane (PDMS) stamp. (b, c) Scanning-electron-microscope images of (b) top and (c) tilted view of the Cu electrode transferred onto the PEDOT:PSS-coated substrate.

We have also developed a fabrication method that can be adapted to roll-to-roll processing.⁸ As shown in Figure 2, a flexible polydimethylsiloxane (PDMS) stamp^9,10 is made, and a 40nm-thick layer of Cu deposited on top of the stamp is transferred onto poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS)-coated flexible (e.g., PET, or polyethylene terephthalate) or rigid (e.g., glass) substrates from the protrusions of the PDMS stamp with low pressure (10psi) and temperature (80°C). The results imply that fabrication of nanopatterned metal electrodes can be extended to roll-to-roll processing.

To evaluate the feasibility of using nanopatterned metal electrodes as high-transparency conducting electrodes for organic optoelectronic devices, we made bulk-heterojunction—a blend of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid in a one-to-one ratio by weight—OSC devices with nanopatterned Cu as transparent anode. Control samples using conventional high-quality ITO anodes were fabricated at the same time. The current-voltage characteristics of both types of solar cells are very similar (see Figure 3), indicating that these structures are interchangeable.

Figure 3. Current density versus voltage characteristics of OSCs with Cu and conventional ITO electrodes. (Intensity: AM1.5G 100mW/cm².)

In summary, we have demonstrated that wire-grid electrodes made of various metals fabricated using nanoimprint lithography can replace conventional ITO electrodes in solar-cell production with equal performance. The metal grating structures could also be exploited for both light trapping and energy concentration through excitation of surface plasmons. We expect that these effects will lead to enhanced light absorption by the organic semiconductors and therefore to higher energy-conversion efficiencies. In addition, nanostructured transparent metal electrodes can potentially be used in organic LEDs (OLEDs) for display and lighting applications. Currently, the poor light extraction is one of the limitations to the external quantum efficiency of OLEDs.¹¹ Previous approaches to address this issue include forming periodic patterns in the device structure, such as 2D hole arrays and Bragg gratings with proper periodicity.^12,13 Our nanostructured metal electrodes effectively prevent the waveguiding effect encountered in devices using ITO electrodes that results from its high refractive index, and therefore increase the light-outcoupling efficiency. Moreover, using inexpensive Cu as a transparent electrode and possibly roll-to-roll fabrication could help to realize low-cost, large-area organic-optoelectronic devices such as OSCs and LEDs.