UV/ozone-treated WS₂ hole-extraction layer in organic photovoltaic cells

WS₂ nanosheets obtained through a simple sonication exfoliation method are employed as a hole-extraction layer to improve the efficiency of organic photovoltaic cells (OPVs). A reduction in the wavenumber difference in the Raman spectra, the appearance of a UV absorption peak, and atomic force microscopy images indicate that WS₂ nanosheets are formed through the sonication method. The power conversion efficiency (PCE) values of OPVs with and without untreated WS₂ layers are both 1.84%. After performing a UV-ozone (UVO) treatment on the WS₂ surface for 15 min, the PCE increases to 2.4%. Synchrotron radiation photoelectron spectroscopy data show that the work function of WS₂ increases from 4.9 eV to 5.1 eV upon UVO treat- ment, suggesting that the increase in the PCE value is caused by the band alignment. Upon inserting poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT: PSS) between the WS₂ and the active layer, the PCE value of the OPV increases to 3.07%, which is superior to that of the device employing only PEDOT:PSS (2.87%). Therefore, it is considered that the use of UVO-treated WS₂ is able to improve the performance of OPV cells.