Effects of mechanical deformations on P3HT:PCBM layers for flexible solar cells

The P3HT:PCBM mixture is the most promising system for absorber layers in flexible polymer-based photovoltaic solar cells. Nowadays, an increased number of studies have been devoted to improving electrical and optical properties of this absorber layer in order to rise the power conversion efficiency. However, studies dealing with its mechanical behavior are scarce, even though they are important on the fabrication processes for flexible devices and operation. Therefore, this work aims to elucidate the effect of tensile and bending deformations on P3HT:PCBM layers deposited on PET/ITO substrates. Mechanical and electrical characteristics were in-situ monitored while strains were applied. Tensile experiments showed that the layers are able to support a unitary strain up to 0.684%, when crack onset strain (COS) is reached, and crack propagation begins until the formation of a grid pattern observed in micrographs. Bending tests were performed with two span-lengths (50.8 mm and 25.4 mm) and two strain conditions on the P3HT:PCBM layers, i.e. tensile and compressive. For the tensile state, micrographs did not show any cracking on the layers regardless of the span-length, even when the bending radius was as low as 6.13 mm. On the other hand, the COS was observed when the layer was subjected to the compressive state with the lowest span and the same curvature radius.