Low-temperature noninjection approach to homogeneously-alloyed PbSexS1\u2212x colloidal nanocrystals for photovoltaic applications

摘要

Homogeneously alloyed PbSexS1\u2212x 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 (Voc) of 0.49 V, short circuit photocurrent (Jsc) 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 PbSexS1\u2212x NCs were carried out at low temperature (60 \ub0C) 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\u2212P 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\u2212P complex, the higher the Se amount in the resulting homogeneously alloyed PbSexS1\u2212x 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.