Printing and in situ Morphology Detection of Organic Thin Film Photovoltaic Devices

The drying process of organic films directly affects final morphology of blends and through in situ scattering technique, the details of morphology inside thin films can be monitored. By designing and running in situ detection system, the crystallization behaviour of polymer molecules in PTB7-th: PC71BM blend, which shows a high efficiency, and the effect of additive (DIO) on the morphology evolution during the process of drying can be investigated. It was observed that the packing style of polymer molecules inside the blend was similar to that of spin-coated samples, indicating that the change of coating mode would not affect the orientation of crystallization. From 2D GIXD profile, it was found that at the beginning of film-forming process, aggregates were formed in the solution, ordered but highly swollen by solvent. With the evaporation of the solvent, the degree of order increased and the solvent was expelled, causing a tighter arrangement of chains and a smaller spacing. With the removal of solvent, the effective size and ordered length scale of the aggregates decreased. The interaction between PTB7-th and DIO was different from that with solvent. As a poor solvent, DIO can slow down the drying process of thin films. Thus the addition of DIO led to the increase of crystal coherence length of PTB7-th. The crystallization of polymer molecule was completed at the beginning of film forming process, which was prolonged by the addition of DIO, but the final crystallinity of the blend was not affected. At the same time, through the printing equipment integrated in the system, the power conversion efficiency (PCE) of our printed devices reached nearly 9%, very close to the efficiency of spin-coated devices. This work strengthened the understanding of morphology inside organic photovoltaic thin films, promoted the progress of industrial production technology of organic photovoltaic devices and had great scientific and commercial value.