Efficient Organic Solar Cells with Solution‐Processed Silver Nanowire Electrodes

Conducting polymers, [ 2–4 ] metal inks, [ 5 ] nanoparticulate metal oxides, [ 6 ] carbon nanotubes, [ 7–9 ] and graphene [ 10–12 ] have been investigated as potential alternatives to brittle indium tin oxide (ITO), but none can yet compete in terms of transparency and sheet resistance. Thin meshes of silver nanowires (AgNWs) [ 13–18 ] have recently emerged as promising electrodes due to their ability to provide transmittances greater than 85% at sheet resistances less than 20 Ω sq − 1 . [ 13 , 14 ] Their application to printed electronics, however, is challenging due to a highly non-uniform topography, which can cause shorting through other layers. This is especially problematic for devices using AgNWs as the lower (substrate-facing) electrode since the mesh presents an extremely rough base layer on which to build the device, leading to signifi cant interelectrode shorting. This in turn leads to low shunt resistances, high dark currents, and poor device effi ciencies. For instance, Peumanns and co-workers reported vacuum-deposited organic solar cells (OSCs) on AgNW-coated glass with low shunt resistances of less than 1 k Ω cm 2 and power conversion effi ciencies (PCEs) less than 0.5%. [ 13 ] They subsequently reported polythiophene/ fullerene OSCs with PCEs of 2.5%, [ 16 , 17 ] using the AgNWs in a top-electrode confi guration where their rough morphology is less detrimental to other layers. However to achieve these effi ciencies they fi rst had to pulse the devices at 10 V to burn-out localized shorts with potentially adverse implications for device lifetimes, suggesting a need for alternative device architectures that better suppress shunt formation. In practice most organic devices utilize a transparent substrate through which light passes, and hence they require a transparent lower electrode. To address this need Zeng et al. embedded AgNWs in a thick fi lm of polyvinyl alcohol, ensuring the exposed nanowires sat fl ush with the top of the fi lm and so provided a planar surface on which to deposit further layers. [ 18 ]