ABSTRACT
Solar cells have been attracting great attention due to energy and environmental concerns. Although the crystalline silicon solar cells are still the main player in practical application today, they have a problem of too long payback time because of the high fabrication cost. Organic solar cells (OSCs) are regarded as the next-generation solar cells due to their merits, including low fabrication cost, light weight, high mechanical flexibility, tunable chemical and physical properties via molecular design, and fast module installation.1,2 An OSC usually has a photoactive layer 100-200 nm thick sandwiched between two electrodes. At least one electrode must be transparent to harvest light. OSCs can be semitransparent or transparent in the visible range when both electrodes are transparent.3-6 A transparent electrode is needed not only for solar cells, but also for other optoelectronic devices. Indium tin oxide (ITO) is the most popular material for transparent electrodes now. ITO is a wide bandgap degenerate semiconductor. It has superior optical and electrical properties. For example, a 150-nm-thick ITO thin film can have a sheet resistance of below 15 Ω/sq and an optical transmittance of above 90% in the visible range. Nevertheless, there are several severe problems for ITO.7,8 Indium is a scarce element on Earth, which has caused the skyrocketing of the indium price. ITO has poor mechanical flexibility so that it cannot be used for flexible electronic devices. Flexible electronic devices are regarded as the next-generation electronic devices.
