The formation of interfacial wrinkle and corresponding electronics structures at organic/metal interface
Mei-Hsin Chen1*
1Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien, Taiwan
* presenting author:Mei-Hsin Chen,
The basic mechanism leading to the morphology change in the poly(3-hexylthiophene) mixed with [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) active layer was explored systematically. We found that the wrinkles appearing at the cathode surface under post-annealing processes are more prominent than those under pre-annealing processes, which is strongly related to the roughness increase of the active layer. Due to thermal-induced instability from the aluminum (Al) layer with the device under post-annealing processes, a net compressive strain develops from the thermal expansion mismatch between the layers. Furthermore, an analysis of the mechanical properties shows that the enlarged nano-dimension roughness of the active layer is dominated by the final-bottom morphology of the compressed Al layer which depends on the annealing sequence. Moreover, the phase segregations and the evolutions of energy levels in bulk-heterojunction solar cells during annealing were also investigated via ultraviolet and x-ray photoemission spectroscopy (UPS and XPS). Upon aluminum deposition onto top of the active layer, the highest occupied molecular orbital (HOMO) of P3HT exhibits a great downward shift after annealing, resulting in the larger energy difference between donor and acceptor and Voc in solar cell devices. Besides, both UPS and XPS results indicate the annealing treatment will induce the out-diffusion of PCBM toward cathode, which can provide more ideal hetero structures to improve device performances.

Keywords: organic, interface