In Situ Photochemical Surface Passivation of CdSe/ZnS Quantum Dots for Quantitative Light Emission and Enhanced Photocurrent Response in Solar Cells

Suppression of energy-wasting nonradiative carrier relaxation in photo-activated semiconductor quantum dots (QDs) is an essential requirement for applications such as QD-sensitized solar cell (QDSSC). Here we report that in situ surface passivation of CdSe/ZnS QDs using dithiothreitol (DTT) results in the 4-fold enhancement of photoluminescence (PL) quantum efficiency of QDs. By the analyses of PL intensities and blinking statistics of single QDs, the PL enhancement is assigned to a combination of processes such as unexpected appearance of bright QDs, gradual increase in the PL intensity of QDs, and substantial suppression of blinking. These processes not only suggest the suppression of nonradiative carrier recombination in QDs passivated with DTT molecules but also enable us to classify the nonradiative carrier recombination centers in QDs into low-density defects in which the rates of nonradiative recombination largely exceed the rates of radiative relaxation and high-density defects in which the rates of nonradiative carrier recombination are comparable to the rates of radiative relaxations. In the former case, passivation of the defects by the spontaneous addition of one or a few DTT molecules to QDs with PL intensity below the detection limit results in the formation of highly luminescent QD-(DTT)(n) adducts. In the latter case, highly luminescent QD-(DTT)(n) adducts are formed from already luminescent QDs by the relatively slow addition of DTT molecules. These two types of surface passivation reactions are governed by both chemical and photochemical processes. Overall, the passivation of defects in QD by DTT results in the suppression of energy-wasting nonradiative relaxations, which enables us to drive ca. 33% excess photocurrent in a photoelectrochemical cell constructed using QD-(DTT)(n) as the sensitizer.