One-step processing of multilayers in organic solar cells†
An important virtue of polymer-based electronic devices is solution processing that offers large area, cheap roll-to-roll manufacturing. However, all organic electronic devices require a multilayered structure of successive thin layers stacked one on top of the other with defined interfaces and little to no layer intermixing. Solution processing of this type of structure is challenging, requiring careful formulation of the solution and selection of solvents, and is often avoided by turning to molecular films that are deposited by complicated and energy consuming deposition processes. Therefore, self-forming interlayers, where molecules segregate from the semiconducting layer to spontaneously form a distinct thin layer, show great potential for fast and simple processing of polymer-based devices. Until now, self-forming interlayers were demonstrated as a tool to tune the energy-level alignment at either the top or bottom interface in organic devices. Here we show that two judiciously selected additives can be collectively mixed into the active layer blend and each spontaneously migrates to a different interface to self-form interlayers at both organic/contact interfaces. The driving forces for additive migration, surface and interface energy considerations, induce interlayer formation during spin coating and top electrode deposition. Both interlayers modify the interfacial energy level alignment leading to improved device performance. In this case we show that a tri-layer stack composed of electron selective layer/active layer/hole selective layer is formed in one spin coating process. The methodology demonstrated here for organic solar cells is general and versatile, suitable for all polymer electronic devices and can be translated to roll-to-roll fabrication processes such as printing and coating.