Electrical Impedance Studies of the E ect of a Bu er Layer on Organic Bulk Hetrojunction Solar Cells

The e ect of a bu er layer on organic bulk hetrojunction solar cells of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester fullerene derivative (PCBM) has been studied using impedance spectroscopy in the frequency range between 100 Hz and 1 MHz. The bu er layer interfaces with a metallic contact and with the organic bulk materials in uence the e ciency and the stability of devices. Thermal annealing of the devices improves the interface with the bu er layer, thereby lowering the majority and the minority carrier injection voltage. The photocurrent density-voltage (J-V) characteristics show that the ll factor of the device with a molybdenum-oxide (MoO3) bu er layer is higher than that of the device with a poly (3, 4ethylenedioxy-thiophene):polystyrenesulfonate (PEDOT:PSS) bu er layer. The equivalent circuit analysis for the PEDOT:PSS and the MoO3 bu er-layer devices shows that the device's capacitance and resistance for MoO3 is lower than that for PEDOT:PSS under illumination. The e ciency of the thermally-annealed PEDOT:PSS and MoO3 bu er-layer devices are 2.95 and 3.44 %, respectively. The lower e ciency of the PEDOT:PSS bu er-layer devices can be attributed to the formation of a thin insulator layer at the PEDOT:PSS/P3HT:PCBM interface under illumination.