Improved Charge Transport in PbS Quantum Dot Thin Films following Gel Permeation Chromatography Purification

We report a systematic investigation of the electronic properties of lead sulfide quantum dot (QD) films prepared by gel permeation chromatography (GPC) or conventional precipitation/redispersion (PR) to identify the effect of these different purification methods. Following spin coating and ligand exchange with ethanedithiol, metal top contacts were patterned by evaporation through a shadow mask. A transmission line model (TLM) approach was employed to separate the bulk resistivity from the contact resistance. The GPC-purified film showed a resistivity as low as 1.5 k Omega-cm, similar to 3x lower than for PR, as well as decreased contact resistance with Au contacts. These differences in electronic properties were accompanied by decreased surface roughness. By tuning the workfunction of metal contacts to these films, a transition from ohmic to rectifying behavior characteristic of p-type conductivity was observed. The contrasting behavior of the ohmic- and Schottky-contacted devices permits estimation of majority carrier (hole) and minority carrier (electron) properties, respectively. With ohmic contacts, the GPC-purified film displayed an increased responsivity (>10 mA/W) and photoconductive gain along with a decrease in photocurrent lifetimes, indicating improved mobility. Scanning photocurrent microscopy (SPCM) was used to characterize the minority carrier diffusion length in Schottky-contacted devices, which was found to be comparable in all three films studied, but a much shorter photocurrent lifetime in the GPC-purified film indicates a much greater electron mobility, indicative of diminished trapping. The results indicate that film properties are sensitive to preparation conditions in addition to ligand density and that GPC is a highly effective route to QDs suited for optoelectronic applications.

Automated summary

The electronic properties of lead sulfide quantum dot films varied significantly depending on the purification method used, with the GPC-purified films showing lower resistivity and decreased contact resistance with gold contacts compared to films purified by conventional precipitation/redispersion. By tuning the workfunction of metal contacts, a transition from ohmic to rectifying behavior characteristic of p-type conductivity was observed in the GPC-purified films. The results suggest that film properties are sensitive to preparation conditions and that GPC is a highly effective route to quantum dots suited for optoelectronic applications.