Feb 18, 2009
Polymer composite nanorod arrays for solar cells
In a recent study, which was published in Nanotechnology, researchers used melt-assisted wetting of porous anodic alumina oxide (AAO) templates to prepare ordered core–shell structured nanorod arrays of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) composite for use in solar cells. The dimension of this ordered structure is determined by the pores of the AAO template and melt-assisted wetting condition under vacuum.
As shown in the image, the average diameter of the nanorods was equal to that of the nanopores in the ordered AAO membrane (figure a). Figure b provides a transmission electron microscopy image, revealing the core–shell-like structures; the dark central region of the nanorods represents the PCBM-rich region (n-type rich), which has a higher electron density than that of the P3HT-rich region (scale bar: 50 nm).
Additionally, conducting mode atomic force microscopy was used to characterize the electrical variation between core and shell regions. Figure d shows the contrast between the current images of the core and shell regions. The current resulted mainly from hole transport because the AFM tip Pt and bottom electrode Indium Tin Oxide (ITO) have high work functions of ca. 5.7 and 4.8 eV, respectively. Because the Pt-coated tip was biased, the influence of the surface electrical properties of ITO played only a minor role.
The phase separation of P3HT/PCBM blends in the wetting of the porous AAO membrane is determined by the flow induced shear stress, which is the largest along the AAO pore wall and is the lowest in the center of AAO pore. Since the modulus of PCBM is larger than that of P3HT at 120deg, the maximum stress along the AAO pore wall will induce lower viscosity part of the blend, i.e. P3HT rich region to flow along. Whereas, the minimum stress in the center of AAO pore will have higher viscosity part of the blend, PCBM rich region, to flow along.
Melt-assisted wetting of porous anodic alumina oxide (AAO) templates can be an efficient method for producing core-shell structured nanorod arrays that consisting of polymer and nanoparticles. This nanorod array structure has a great potential for being used in ordered bulk hetrojunction polymer solar cell device because it provides straight and independent charge pathways from interfaces to the electrode to minimize the probability of back electron transfer upon illumination.
About the author
The work was performed at the National Chiao Tung University and supported by the National Science Council through Project NSC 96-2120-M-009-005 . Hsu-Shen Wang is a PhD student studying in Department of Materials Science and Engineering at the National Chiao Tung University. Shih-Yung Chen is a PhD student studying in the Institute of Atomic and Molecular Science, Academia Sinica. Prof. Yuh-Lin Wang is the Director of the Institute of Atomic and Molecular Science, Academia Sinica Taipei, Taiwan. Prof. Kung-Hwa Wei is the Chairman of the Department of Materials Science and Engineering at the National Chiao Tung University, Hsinchu, Taiwan.