ABSTRACT
Usually thin films of different materials in polymer solar cells are prepared by spin coating of their solutions. Spin coating is used almost exclusively but is not suitable for highvolume production, as it causes significant loss of the coating materials. Additionally several unintentional variations in processing may get introduced that are sometime difficult to identify and they affect the cell performance. For cost-effective large-scale production of polymer solar cells, slot-die coating, gravure coating, knife-over-edge coating, offset coating, spray coating, and printing techniques (such as inkjet printing, pad printing, and screen printing) are the most suitable techniques. F. C. Krebs presented a detailed review on various printing and coating techniques and their applications in fabrication of polymer solar cells [1]. For high-speed large-area roll-to-roll (R2R) processing of polymer solar cells, slot-die coating and screen printing are the most useful techniques. In R2R fabrication, the substrate is a roll of long wound flexible sheet, which is known as a web, and is unrolled for coating and rolled again after coating. The solutions being used in printing techniques are normally called ink. After completion of one process on the roll, it is transferred to another unwinder for the next step. The coatings steps could be discrete, which means each step has a separate unwinder and rewinder, or integrated, whereby the steps are followed one by another in a single unwinding and rewinding process. The flexible sheet could be of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polylactic acid (PLA), kapton, polyethylene (PE), polypropylene (PP), or nylon. Kapton has excellent thermal and mechanical properties, but it is too expensive for general purpose. PLA is biodegradable and may be good for disposal of dead organic solar cells (OSCs), but it has low resistivity against organic solvents and poor thermal properties. Compared to other flexible sheets, PET is found to have overall better properties to be a substrate material and is used most often in R2R production of OSCs. In R2R coating machines, the substrates enter from one side and after coating of different layers, the complete solar modules come out from the other side. Screen printing can possibly produce 1000-100,000 m2 OSCs per day on a process line and for silicon (Si) solar cells the same area would take around a year for fabrication [2]. It shows the potential of printing technology and makes OSCs very cost effective. Initially, printed OSCs showed poor performance, but advanced research and development has resulted in high-performance solar cells. The performance of printed devices still remains low compared to spin-coated devices, but the difference is small. Development in R2R processing would allow transfer of laboratory polymer solar cells to large-scale industrial production. Before I discuss the R2R fabrication of OSC modules, it is important to understand some of the related coating techniques.
