In the present work, polymer solar cells were fabricated from composite blends of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(3-hexylthiophene)-(P3HT with PCBM and PCBM. The composite blends were used as active layers in an ITO/PEDOT:PSS/active layer/Al device structure. Power conversion efficiencies have been measured from current-voltage (I-V) measurements for each of these different composite blends under simulated AM1.5 illumination. In the case of the MEH-PPV devices, the I-V performance has been measured as a function of polymer molecular weight, type of fullerene derivative (C60 or C70), and PCBM:polymer ratios. The highest efficiencies for the ranges used in this study were obtained using the 150,000 g/mol MEH-PPV molecular weight, the C70 PCBM derivative, and a 1:4 MEH-PPV:PCBM ratio. The effect of thermal cycling on the I-V performance for both MEH-PPV and P3HT devices has also been measured from 77K to 330K. The devices exhibited a positive temperature coefficient for the short-circuit current density (Jsc
), which dominated the overall efficiency of the device over this temperature range. Finally, the use of a combination of parylene and polymethylmethacralate for device passivation was shown to provide a dramatic reduction in device degradation under ambient conditions as compared to non-passivated devices.