Investigation of aged organic solar cell stacks by cross-sectional transmission electron microscopy coupled with elemental analysis

Polymer solar cells are of great interest as candidates for future low-cost and lightweight energy sources. One of the major reliability problems of these devices is the thermal instability of the blend morphology typically composed of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT and PCBM, respectively). Phase segregation of the blend has been extensively investigated by transmission electron microscopy (TEM) on free-standing films. In this study, we investigate in cross-section the morphology reorganization of P3HT:PCBM layers confined between poly(3,4-ethylenedioxythiophene)poly-(styrenesulfonate) (PEDOT:PSS) and a metal electrode similar to functional solar cell devices. The strengths of different TEM imaging and compositional analysis modes for the investigation of organic solar cells is illustrated by studying the evolution of the material stack with ageing conditions. Combining TEM imaging of the layer stack with energy-dispersive X-ray and energy loss electron spectroscopy, we not only gain insight into the phase segregation process but also explore the interdiffusion in the layer stack. More than 100 °C annealing leads to the formation of elongated protrusions ranging 100–500 nm. Thinning of the neighboring areas indicates lateral diffusion in the stack. Interestingly, the metal cathode remains still conformal over these large aggregates. The particles protrude through the metal layer only after prolonged (>100 h) annealing at higher temperatures when they reach several micrometer in height and are identified as crystalline PCBM-like material. Hence, almost full phase separation occurs by PCBM agglomeration and diffusion over large distances. Elemental analysis confirms that diffusion of the electrode materials (In, Sn and Yb) into the P3HT:PCBM stack remains below the detection limit.