Thermal Stability of Poly[2-methoxy-5-(2′-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV):Fullerene Bulk Heterojunction Solar Cells

To improve the thermal stability of polymer:fullerene bulk heterojunction solar cells, a new polymer, poly[2-methoxy-5-(2'-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV), has been designed and synthesized, which showed an increased glass transition temperature (T-g) of 111 degrees C. The thermal characteristics and phase behavior of MPE-PPV:[6,6]-phenyl C-61-butyric acid methyl ester ([60]PCBM) blends were investigated by means of modulated temperature differential scanning calorimetry and rapid heating-cooling calorimetry. The thermal stability of MPE-PPV: [60] PCBM solar cells was compared with devices based on the reference MDMO-PPV material with a T-g of 45 degrees C. Monitoring of the photo-current-voltage characteristics at elevated temperatures revealed that the use of high-T-g MPE-PPV resulted in a substantial improvement of the thermal stability of the solar cells. Furthermore, a systematic transmission electron microscope study of the active polymer:fullerene layer at elevated temperatures likewise demonstrated a more stable morphology for the MPE-PPV: [60] PCBM blend. Both observations indicate that the use of high-T-g MPE-PPV as donor material leads to a reduced free movement of the fullerene molecules within the active layer of the photovoltaic device. Finally, optimization of the PPV:fullerene solar cells revealed that for both types of devices the use of [6,6]-phenyl C-71-butyric acid methyl ester ([70]PCBM) resulted in a substantial increase of current density and power conversion efficiency, up to 3.0% for MDMO-PPV:[70]PCBM and 2.3% for MPE-PPV:[70]PCBM.