Thin metal films as simple transparent conductors

Large-area transparent conductors are essential in many important applications, such as thin-film solar cells, traditional LCDs, and organic LEDs (OLEDs). The widely used transparent conducting oxides (TCOs), such as indium tin oxide (ITO), are typically deposited using plasma sputtering or sol-gel methods. There is a natural tradeoff between transparency and conductivity, with the best films exceeding 90% transparency in the visible part of the spectrum at sheet resistances below 15Ω/square. This level of performance is suitable for thin-film solar-cell applications, where a sparse metal grid can be added to the TCO film as an auxiliary conductor to minimize ohmic losses during charge collection.

However, TCOs typically exhibit a combination of shortcomings (e.g., brittleness, expensive source materials, processing problems, or availability of suitable flexible substrates).¹ They are particularly problematic in reel-to-reel processing of thin-film, flexible devices because they are susceptible to cracking, which raises the film's electrical resistance and makes it permeable to oxygen and moisture that accelerate device degradation. An acute need exists for transparent conductors that are fundamentally different from TCOs in their mechanical, processing, and cost characteristics.²

Figure 1. Metal-organic-metal photovoltaic (PV) cells with thin nonpatterned metal films achieve the same power-conversion efficiency as those with conventional indium tin oxide (ITO) electrodes. hν: Light energy.

The search for TCO replacements for organic photovoltaic (OPV) devices has focused on carbon nanotubes,³ graphene,⁴ highly conductive polymers,⁵ and metallic microgrids combined with conducting polymers.⁶ But few of these approaches have yielded devices that perform as well as those using ITO, and fewer can be scaled up cost-effectively.

Instead, we considered using a very thin, unpatterned metal film. Metals are malleable and can be deposited relatively cheaply and rapidly onto continuously spooled substrate. In organic optoelectronics, thin metal films have been investigated as stand-alone transparent electrodes^7–11 and in conjunction with conducting oxides.^12–14 Generally, the transparency of a metal film drops exponentially with increasing thickness, while the sheet resistance rises rapidly. This tradeoff between transparency and electrical conductivity limits the range of feasible metal thicknesses to 10–20nm. At the low extreme, it can be difficult to maintain film continuity because the metal tends to aggregate into droplets on glass and plastic, while at the high end transparency suffers. As a result, OPV cells using continuous metal films as transparent electrodes have not achieved parity with ITO-based cells.

We recently examined¹⁵ how the sheet resistance varies with thickness and found that 9–10nm-thick silver films exhibit 15Ω/square sheet resistance. This is comparable to device-grade ITO films. We mitigated silver aggregation by co-evaporating it with magnesium, producing films with roughness below 4nm (compared to 7nm roughness for ITO). Using this film as the anode, we vacuum deposited an archetypal copper phthalocyanine (CuPc)/fullerene (C₆₀) planar heterojunction solar cell, completing it with a silver cathode. The metal-organic-metal device exhibited a power-conversion efficiency of 1.88% at AM1.5 (typical daylight) illumination, on par with the 1.86% efficiency of an ITO-based control device (see Table 1 for device characteristics.)

The reason that the device achieved this efficiency is twofold. First, the open-circuit voltage was higher than in the ITO control device (0.55 versus 0.48V). Second, despite the 20% lower overall transmittance of the metal anode than its ITO counterpart, the short-circuit photocurrent suffered only by ∼13%. This was attributable to optical microcavity effects in thin-film OPV stacks, which we explained using optical and transport modeling.¹⁵ The modeling results were also extended to bulk-heterojunction polymer solar cells, suggesting that metal-organic-metal architectures could slightly exceed the efficiency of ITO-based cells.

The optical behavior of thin metal electrodes is reciprocal, suggesting that these results obtained in OPV cells should translate well to OLEDs, benefiting in particular large-area solid-state lighting applications. They could be used in conjunction with dielectric capping layers to further boost device efficiency.¹⁶ These nonpatterned films could be superior to almost any alternative transparent conductor from the standpoint of manufacturing scalability and cost-effectiveness. We plan to verify the long-term reliability of thin metal electrodes and their compatibility with a wider range of substrates. We also plan to apply such electrodes more effectively in the various nonplanar energy-conversion devices (such as solar cells and OLEDs on fibers or on atomic-force-microscope probe tips) we demonstrated previously,^17–20 in addition to the conventional planar devices.