Roll-to-roll gravure printing of organic photovoltaic modules-insulation of processing defects by an interfacial layer

Gravure printing as direct patterning roll-to-roll (R2R) production technology can revolutionize the design of thin-film organic photovoltaic (OPV) devices by allowing feasible manufacturing of arbitrary-shaped modules. This makes a distinction to coating methods, such as slot die coating, in which the pattern is limited to continuous stripes. Here, we analyze the thin-film formation and its influence on OPV module performance as the gravure printing of hole transport and photoactive layers are transferred from laboratory to R2R pilot production environment. Insertion of a 0.8-nm layer of lithium fluoride (LiF) as an interfacial layer between the active layer and the electron contact provided insulation against the detrimental pinholes formed in the R2R printing process. Using this device configuration, we produced well-performing R2Rprinted monolithic modules with a mean efficiency of 1.7%. In comparison, reference modules with an efficiency of 2.2% were fabricated using laboratory-scale bench top sheet-level process. Surface energy and tension measurements together with optical microscopy were used to analyze the printability of the materials. The pinhole insulation was investigated in detail by processing R2R-printed OPV modules with different interfacial layer materials and performing electrical measurements under dark and AM1.5 illumination conditions. Furthermore, we analyzed the LiF distribution using X-ray photoelectron spectroscopy. The insulating nature of the LiF layer to improve module performance was confirmed by manufacturing lithographically artificial pinholes in device structures. The results show the possibility to loosen the production environment constraints and the feasibility of fabricating well-performing thin-film devices by R2R gravure printing.