Electrical Impedance Studies of the Effect of a Buffer Layer on Organic Bulk Hetrojunction Solar Cells

The effect of a buffer 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 buffer layer interfaces with a metallic contact and with the organic bulk materials influence the efficiency and the stability of devices. Thermal annealing of the devices improves the interface with the buffer layer, thereby lowering the majority and the minority carrier injection voltage. The photocurrent density-voltage (J-V) characteristics show that the fill factor of the device with a molybdenum-oxide (MoO3) buffer layer is higher than that of the device with a poly (3, 4-ethylenedioxy-thiophene):polystyrenesulfonate (PEDOT:PSS) buffer layer. The equivalent circuit analysis for the PEDOT:PSS and the MoO3 buffer-layer devices shows that the device's capacitance and resistance for MoO3 is lower than that for PEDOT:PSS under illumination. The efficiency of the thermally-annealed PEDOT:PSS and MoO3 buffer-layer devices are 2.95 and 3.44 %, respectively. The lower efficiency of the PEDOT:PSS buffer-layer devices can be attributed to the formation of a thin insulator layer at the PEDOT:PSS/P3HT:PCBM interface under illumination.