(NH4)2WS4 precursor as a hole-injection layer in organic optoelectronic devices

Abstract

This paper presents a facile method to fabricate the hole injection layer (HIL) for organic photovoltaic cells (OPVs) and organic light-emitting diodes (OLEDs) using a thermally annealed (NH4)2WS4 precursor under air ambient. The thermal gravimetric analysis curve shows that WS3, WS2, and WOx are formed by the thermal decomposition of (NH4)2WS4 above 160°C. The disappearance of the S 2p peak in the photoemission spectra and of the W–S peak in the Raman spectra as well as the decrease in the water contact angle after annealing at 250°C suggest that (NH4)2WS4 decomposed into WOx. The power conversion efficiency (PCE) of the OPV improved from 1.51% to 3.14% after the insertion of 250°C-annealed (NH4)2WS4 as the HIL, which is comparable to the PCE of the OPV based on poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) HIL (3.23%). Furthermore, the luminance efficiency of the OLED with 250°C-annealed (NH4)2WS4 (15.76cd/A) was higher than that of the device based on PEDOT:PSS (12.34cd/A). The results of the in situ deposition experiments revealed that the improved device performance originates from the energy level alignment and electron–hole balance. These data demonstrate that 250°C-annealed (NH4)2WS4 is a promising candidate for fabrication of the HIL in optoelectronic devices using a facile solution process under air ambient.