Dependence of opto-electric properties of (semi-)conducting films in polymer based solar cells on viscous shear during the coating process

Abstract

Organic photovoltaic is a promising technology for low-cost energy conversion. One of its major challenges is the transfer of the manufacturing process to a continuous roll-to-roll process. Previous research showed that the coating method has a significant impact on film properties, which may be explained by a shear-rate induced crystallization of the polymer–fullerene-blend.In this paper we report on a controlled variation of the shear-rate during slot-die coating of photoactive and conductive layers for polymer solar cells. Light absorption of photoactive layers increased towards higher coating speed and thus higher shear-rate by up to 28% from 0.6m/min to 12m/min. The currently lower performance of roll-to-roll processed solar cells, compared to laboratory scale devices may be increased by intentionally applying a high shear rate during the coating process. In contrast, a shear induced crystallization is insignificant for conductive (PEDEOT:PSS and Ag-nanoparticle) films, where conductivity decreased when the operating point approached the stability limit. Thus, a low capillary number is desirable for PEDOT:PSS layers, whereas the performance of the photoactive layer increased within the investigated velocity range. These tendencies, shown here for a standard material system (P3HT:PCBM), are substantial for the design of a roll-to-roll process for efficient polymer solar cells.