Impedance analysis of the multilayered organic solar cells with and without hole buffer layer

Abstract

We studied the effects of a buffer layer [molybdenum oxide (MoO3) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)] and the thickness of a p-type tetraphenyldibenzoperiflanthene (DBP) layer on the performance of multilayered organic solar cells in indium–tin oxide (ITO)/buffer/DBP/fullerene C60/bathocuproine (BCP)/Al multilayered photovoltaic devices. The insertion of a hole buffer layer reduced the resistance across the cell in forward biasing under illumination and in the dark, and the negative capacitance behaviors were observed at a voltage higher than VOC, probably owing to the enhancement of the double injection. On the other hand, a hole-blocking ITO/DBP interface reduced the hole injection and extraction, and DBP became similar to a dielectric layer in the dark, as determined from the capacitance–frequency measurement. The photogenerated carriers caused an increase in the capacitance C and the conductance G in the devices with and without the MoO3 buffer layers. Both C and G reached their local minima at a voltage close to VOC, and increased with a decrease in external voltage probably owing to the enhancement of charge extraction efficiency. At a lower voltage, however, C and G reached their peak values and then decreased again with a decrease in external voltage, probably owing to the reduction (ejection) in the number of photogenerated space charges in active layers. The increase in the thickness of the dielectric DBP layer resulted in a reduction in charge extraction efficiency, and an S-shaped curve was observed for the thick DBP cell. The peak voltages in C and G moved toward negative values with an increment in thickness, and this was discussed using the simplified rate equation model.