Thermal stability of poly[2-methoxy-5-(2’-phenylethoxy)-1,4-phenylene vinylene] (MPEPPV):fullerene bulk heterojunction solar cells
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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 (Tg) of 111 °C. The thermal characteristics and phase behavior of MPE-PPV:[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends were investigated by means of modulated temperature differential scanning calorimetry and rapid heating–cooling calorimetry. The thermal stability of MPE-PPV:PCBM solar cells was compared with devices based on the reference MDMO-PPV material with a Tg of 45 °C. Monitoring of the photocurrent–voltage characteristics at elevated temperatures revealed that the use of high-Tg 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:PCBM blend. Both observations indicate that the use of high-Tg 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 C71-butyric acid methyl ester (PCBM) resulted in a substantial increase of current density and power conversion efficiency, up to 3.0% for MDMO-PPV:PCBM and 2.3% for MPE-PPV:PCBM.