1-Decyl phosphonic acid (DPA) proved an effective p-doping additive to oxidize spiro-OMeTAD as a hole-transporting material (HTM) for nanostructured Sb2S3 extremely thin absorber (ETA) solar cells. DPA had the effect of enhancing the hole conductivities of spiro-OMeTAD in the presence of lithium bis(trifluoromethyl- sulfonyl)-imide (LiTFSI) to significantly improve the device performance. Transient photoelectric measurements and electrochemical impedance spectra of devices fabricated with varied additive conditions provided information about the charge- transport and recombination kinetics and hole conductivities for Sb2S3 devices with additives DPA and LiTFSI that exhibited superior performance. Thirty-five identical devices were fabricated to show the excellent reproducibility and long-term stability in air; the best Sb2S3 ETA device attained efficiency of power conversion 6.0%, which is twice that of a device using other typical additives (LiTFSI and 4-tert-butyl-pyridine) for spiro-OMeTAD serving as HTM. (Chemical Equation Presented).