Influence of structure and C₆₀ composition on properties of blends and bilayers of organic donor-acceptor polymer/C₆₀ photovoltaic devices

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/Cgo composite and improved donor-acceptor proximity, leading to electron conductivity, contribute to the increase in power conversion efficiency E_ff. Two broad peaks were observed in the spectral response of the blended OPVCs, with maximum peaks at ∼490nm (= I_BL1) and ∼350nm (= I_BL2). The intensity ratio of I_BL1 to I_BL2 (I_BL1/I _BL2) decreases with an increase in the C₆₀ concentration. Also, I_BL1 is blue-shifted by 25-30 nm with an increase in the C ₆₀ 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 C ₆₀ 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.