The use of UV/ozone-treated MoS2 nanosheets for extended air stability in organic photovoltaic cells

Abstract

MoS₂ nanosheets obtained through a simple sonication exfoliation method are employed as a hole-extraction layer (HEL) to improve the efficiency and air stability of organic photovoltaic cells (OPVs). The reduction in the wavenumber difference, appearance of a UV-vis peak, and atomic force microscopy images indicate that MoS₂ nanosheets are formed through the sonication method. The OPVs with MoS₂ layers show a degraded performance with a power conversion efficiency (PCE) of 1.08%, which is lower than that of OPVs without HEL (1.84%). After performing the UV/ozone (UVO) treatment of the MoS₂ surface for 15 min, the PCE value increases to 2.44%. Synchrotron radiation photoelectron spectroscopy data show that the work function of MoS₂ increases from 4.6 to 4.9 eV upon UVO treatment, suggesting that the increase in the PCE value is caused by the bandgap alignment. Upon inserting poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) between MoS₂ and the active layer, the PCE value of the OPV increases to 2.81%, which is comparable with that of the device employing only PEDOT:PSS. Furthermore, the stability of the OPVs is improved significantly when MoS₂/PEDOT:PSS layers are used as the HEL. Therefore, it is considered that the use of UVO-treated MoS₂ may improve the stability of OPV cells without degrading the device performance.