Impedance Spectroscopy for Annealing-Induced Change of Molybdenum Oxide in Organic Photovoltaic Cell

Abstract

Organic photovoltaic cells (OPVs) have been investigated for a long time, and practical realizations have been also in progress due to their high photovoltaic performance over 12%. In this study, we fabricated normal OPVs and investigated the reason of an improved optical-to-electrical conversion efficiency by annealing the MoO x layer. The photoconversion efficiency was improved up to 5.65% from 2.05% after annealing at 160˚C for 5 min, and the external quantum efficiency also increased for all the measurement wavelength ranging from 300 to 900 nm. Especially, the short circuit current density increased among photovoltaic parameters. The carrier transport resistance of photoactive layer was found to be reduced by evaluating the impedance measurement. These results indicate that defects at the MoO x /organic interface were successfully reduced by the thermal annealing process of MoO x layer. The efficient carrier transport was realized for the annealed-device, resulting in the high device performance. In addition, the relaxation and electron recombination times were also reduced by the annealing process, which lead to the improved photovoltaic performance.