Supplementary information on the paper:
“Investigation of aged organic solar cell stacks by cross-sectional transmission electron microscopy coupled with elemental analysis”
P. Favia1, E. Voroshazi1,2,, P. Heremans1,2, H. Bender1
1. imec, Kapeldreef 75, B-3001 Leuven, Belgium
2. Katholieke Universiteit Leuven, ESAT, Arenberg Kasteelpark 10, B-3001 Leuven, Belgium
Published in Journal Materials Science
Optical and scanning electron microscopy
The organic solar cell devices (ITO/PEDOT:PSS/P3HT:PCBM/Yb/Al) are aged at room temperature (RT), 85ºC, 120ºC and 150ºC in dark. The sample annealed at 85ºC is close to real-life conditions in an operating solar cell, while the 150ºC can be considered as an extreme condition aimed to accelerate aging by heating. The samples stored for 1000 h at room temperature (sample A), annealed at 85ºC for 120h (sample B) and 150ºC for 120h (sample C), and a sample annealed at 120ºC for 350h (sample D) are selected for further analysis.
The evolution of the ageing process is monitored by optical microscopy through the glass substrate (Figure S1). The formation of clusters with annealing is already evident at lower temperature (sample B). These clusters increase in size with annealing time and temperature while their density decreases as smaller particles gradually aggregate in larger clusters. Annealing at considerably higher temperatures than the Tg of the blend triggers the diffusion of the material even in confined layers. For long annealing time at high temperature the clustering reaches an equilibrium where large clusters of PCBM crystallize and form “needle”-like structures. Additionally, weak contrast lines appear in the metal layers as observed by backside optical microscopy as well as top view SEM, suggesting that these are cracks in the cathode metal.
Figure S1: Optical images of the samples seen through the glass substrate: sample A: RT, 1000h, sample B: 85ºC for 120h, sample C: 150ºC for 120h, sample D: 120ºC for 350h. The upper row of images shows the edge of the electrode (all scale bars are 100µm).The lower row of images are taken at higher magnification (scale bar 20 µm) in the electrode region.
Top view SEM imaging is applied to study the structure of the needle-like features and cracks in more detail (Figure S2). After 120h annealing at 150ºC elongated protrusions start to appear at the surface (Figure S2a). These regions further elongate after extended annealing forming the needle-like structures seen under the optical microscope in Figure S1 (sample D). These overgrown structures show facetted shapes indicating the presence of crystals (Figure S2b) and are surrounded by material with smooth surface appearing on the SEM images very similar to the surface of the fresh or room-temperature (sample A) aged samples. The weak contrast lines on the optical images correspond to lines with small topography in the SEM image (Figure S2b).
Figure S2: Scanning electron microscopy images of the start of needle formation at 150ºC after 120 h (a), and long needles with crystals and cracks formed at 120ºC after 350 h (b).
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