Influence of Structure and C60 Composition on Properties of Blends and Bilayers of Organic Donor-Acceptor Polymer/C60 Photovoltaic Devices

Abstract

We have fabricated both blended and bilayered organic photovoltaic cells (OPVCs) using C60 and poly[2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV). Improvements in photovoltaic performance are seen in blended OPVCs when the C60 concentration is increased. It is believed that the optimized surface morphology of MEH-PPV/C60 composite and improved donor-acceptor proximity, leading to electron conductivity, contribute to the increase in power conversion efficiency Eff. Two broad peaks were observed in the spectral response of the blended OPVCs, with maximum peaks at ∼490 nm (=IBL1) and ∼350 nm (=IBL2). The intensity ratio of IBL1 to IBL2 (IBL1/IBL2) decreases with an increase in the C60 concentration. Also, IBL1 is blue-shifted by 25–30 nm with an increase in the C60 concentration. The significant improvement in the performance was observed in the bilayered OPVCs on the thermally induced interdiffusion of C60 into the MEH-PPV network, leading to the existence of C60 molecules within the exciton diffusion radius of the MEH-PPV network. The spectral response of bilayered OPVCs reveals two peaks at 535 nm and 345 nm. The former peak is red-shifted by 45 nm compared to that in blended OPVCs. We also investigate the effect of top electrode materials on the photovoltaic performances. To the best of our knowledge, we have obtained the best performances of blended and bilayered OPVCs fabricated with the Al and Mg electrodes, respectively. In both OPVC structures, the nanoscale composition control of the two materials, the choice of metal electrode, and the device processing techniques all play an important role in determining or enhancing the solar cell performance.