Homogeneously alloyed PbSe(x)S(1-x) nanocrystals (NCs) with their excitonic absorption peaks in wavelength shorter than 1200 nm were developed for photovoltaic (PV) applications. Schottky-type solar cells fabricated with our PbSe₀.₃S₀.₇ NCs as their active materials reached a high power conversion efficiency (PCE) of 3.44%, with an open circuit voltage (V(oc)) of 0.49 V, short circuit photocurrent (J(sc)) of 13.09 mA/cm², and fill factor (FF) of 0.54 under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm². The syntheses of the small-sized colloidal PbSe(x)S(1-x) NCs were carried out at low temperature (60 °C) with long growth periods (such as 45 min) via a one-pot noninjection-based approach in 1-octadecene (ODE), featuring high reaction yield, high product quality, and high synthetic reproducibility. This low-temperature approach employed Pb(oleate)₂ as a Pb precursor and air-stable low-cost thioacetamide (TAA) as a S source instead of air-sensitive high-cost bis(trimethylsilyl)sulfide ((TMS)₂S), with n-tributylphosphine selenide (TBPSe) as a Se precursor instead of n-trioctylphosphine selenide (TOPSe). The reactivity difference of TOPSe made from commercial TOP 90% and TBPSe made from commercial TBP 97% and TBP 99% was addressed with in situ observation of the temporal evolution of NC absorption and with ³¹P nuclear magnetic resonance (NMR). Furthermore, the addition of a strong reducing/nucleation agent diphenylphosphine (DPP) promoted the reactivity of the Pb precursor through the formation of a Pb-P complex, which is much more reactive than Pb(oleate)₂. Thus, the reactivity of TBPSe was increased more than that of TAA. The larger the DPP-to-Pb feed molar ratio, the more the Pb-P complex, the higher the Se amount in the resulting homogeneously alloyed PbSe(x)S(1-x) NCs. Therefore, the use of DPP allowed reactivity match of the Se and S precursors and led to sizable nucleation at low temperature so that long growth periods became feasible. The present study brings insight into the formation mechanism of monomers, nucleation/growth of colloidal composition-tunable NCs, and materials design and synthesis for next-generation low-cost and high-efficiency solar cells.