The broad application of solar energy harvesting in everyday products, e. g. by integration in clothing, requires mechanically flexible photovoltaic (PV) cells to be available in large volumes and at low costs. PV devices with indium tin oxide (ITO) free transparent electrodes printed on plastic foils are promising candidates for this approach, because they can be produced on large scale in an industrial roll-to-roll fabrication process. In order to compensate for the rather high resistive losses in ITO-free transparent conductive materials, current collecting grids comprising high electrical conductivities need to be included to reduce the sheet resistances. Printing and subsequent sintering of conductive inks or pastes based on metal nanoparticle dispersions is a convenient deposition method for these structures.
The traditional approach of sintering by prolonged heating has the decisive disadvantage of being rather slow, thereby requiring long processing times and being difficult for application in R2R manufacturing. This is especially true if inexpensive commodity polymers are used as the substrate materials, which cannot withstand high temperatures. In addition, inks containing nanoparticles based on non-noble metals are prone to oxidation during thermal sintering, which demands working under protective atmosphere. Sintering methods which are fast enough to allow R2R processing at reasonable speeds and avoid oxidation even when operating in air are therefore in high demand.
In this contribution, photonic flash sintering is presented as a fast and highly efficient processing technology for improving the conductivities of printed metal nanoparticle structures on plastic foils. Due to the selective heating of the metal deposits by the absorption of visible light flashes, for which the substrate itself is transparent, high temperatures can be achieved locally for very short times. Using this approach, resistivity drops of more than six orders of magnitude are achieved within fractions of a second. For silver based inks, both integration in printed organic solar cells with improved power conversion efficiencies and R2R application have been demonstrated. Because the photonic sintering process occurs much faster than the oxidation of copper nanoparticles, the technology can also be applied to copper inks without the need of a protective atmosphere.