Characterizing Bismuth Doping of Colloidal Germanium Quantum Dots for Energy Conversion Applications

The numerous electronic and optoelectronic applications that rely on semiconductors require tuning their properties through doping. Germanium quantum dots (Ge QDs) were successfully doped with bismuth up to 1.5 mol %, which is not achievable in the bulk Ge system. The structures of oleylamine- and dodecanethiol-capped Ge QDs were probed with EXAFS, and the results are consistent with Bi dopants occupying surface lattice sites. Increasing the amount of Bi dopant from 0.50 to 1.5 mol % results in increasing disorder. In particular, the nearest-neighbor Bi-Ge bond length is much longer than the Ge-Ge bond length in Ge QDs. Oleylamine to dodecanethiol ligand exchange was shown to partially restore order in doped QDs. Transport measurements of the Bi-doped Ge QD thin films revealed that Bi doping leads to a significant increase in dark current and photocurrent. These results indicate that doping can provide a pathway for improving the performance of group IV quantum dots for energy conversion applications including photodiodes and photovoltaic cells.