Synthesis and Characterization of PbSe Nanocrystals by …
Synthesis and properties of PbSe xS 1-x alloyed nanocrystals
Colloidal nanocrystal quantum dots (NQDs) of narrow band gap materials are of substantial general interest because of their unparalleled potential as infrared fluorophores. While PbSe NQDs are a promising class of infrared-active nanocrystals due to high emission quantum yields and a wide useful spectral range, typical synthetic methods are sensitive to a variety of factors, including the influence of solvent/ligand impurities that render reproducibility difficult. In this work, we specifically examine the effects of diphenylphosphine and 1,2-hexadecanediol, as surrogates for putative trioctylphosphine-based reducing impurities, on the synthesis of PbSe NQDs. Specifically, we compare their influence on NQD size, chemical yield, and photoluminescence quantum yield. While both additives substantially increase the chemical yield of the synthesis, they demonstrate markedly different effects on emission quantum yield of the product NQDs. We further examine the effects of reaction temperature and oleic acid concentration on the diolassisted synthesis. Increased oleic acid concentration led to somewhat higher growth rates and larger NQDs but at the expense of lower chemical yield. Temperature was found to have an even greater effect on growth rate and NQD size. Neither temperature nor oleic acid concentration was found to have noticeable effects on NQD emission quantum yield. Finally, we use numerical simulations to support the conjecture that the increased yield is likely a result of faster monomer formation, consistent with the activation of an additional reaction pathway by the reducing species.
Journal of Nanoscience and Nanotechnology
N2 - We investigated the effect of PbSe quantum dot size on the performance of Schottky solar cells made in an ITO/PEDOT/PbSe/aluminum structure, varying the PbSe nanoparticle diameter from 1 to 3 nm. In this highly confined regime, we find that the larger particle bandgap can lead to higher open-circuit voltages (∼0.6 V), and thus an increase in overall efficiency compared to previously reported devices of this structure. To carry out this study, we modified existing synthesis methods to obtain ultrasmall PbSe nanocrystals with diameters as small as 1 nm, where the nanocrystal size is controlled by adjusting the growth temperature. As expected, we find that photocurrent decreases with size due to reduced absorption and increased recombination, but we also find that the open-circuit voltage begins to decrease for particles with diameters smaller than 2 nm, most likely due to reduced collection efficiency. Owing to this effect, we find peak performance for devices made with PbSe dots with a first exciton energy of ∼1.6 eV (2.3 nm diameter), with a typical efficiency of 3.5%, and a champion device efficiency of 4.57%. Comparing the external quantum efficiency of our devices to an optical model reveals that the photocurrent is also strongly affected by the coherent interference in the thin film due to Fabry-Pérot cavity modes within the PbSe layer. Our results demonstrate that even in this simple device architecture, fine-tuning of the nanoparticle size can lead to substantial improvements in efficiency.