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)(2)WS4 precursor under air ambient. The thermal gravimetric analysis curve shows that WS3, WS2, and WOx are formed by the thermal decomposition of (NH4)(2)WS4 above 160 degrees 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 degrees C suggest that (NH4)(2)WS4 decomposed into WOx. The power conversion efficiency (PCE) of the OPV improved from 1.51% to 3.14% after the insertion of 250 degrees C-annealed (NH4)(2)WS4 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 degrees C-annealed (NH4)(2)WS4 (15.76 cd/A) was higher than that of the device based on PEDOT:PSS (1234 cd/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)(2)WS4 is a promising candidate for fabrication of the HIL in optoelectronic devices using a facile solution process under air ambient. (C) 2015 Elsevier B.V. All rights reserved.