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Nano Lett. 2018 Jul 11;18(7):4417-4423. doi: 10.1021/acs.nanolett.8b01470. Epub 2018 Jun 21.

Acid-Assisted Ligand Exchange Enhances Coupling in Colloidal Quantum Dot Solids.

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Department of Electrical and Computer Engineering , University of Toronto , 10 King's College Road , Toronto , Ontario M5S 3G4 , Canada.
Pohang Accelerator Laboratory , Kyungbuk, Pohang 37673 , Republic of Korea.
Department of Chemistry , University of Toronto , 80 St. George Street , Toronto , Ontario M5S 3G4 , Canada.
Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy , University of Toronto , Toronto , Ontario M5S 3M2 , Canada.


Colloidal quantum dots (CQDs) are promising solution-processed infrared-absorbing materials for optoelectronics. In these applications, it is crucial to replace the electrically insulating ligands used in synthesis to form strongly coupled quantum dot solids. Recently, solution-phase ligand-exchange strategies have been reported that minimize the density of defects and the polydispersity of CQDs; however, we find herein that the new ligands exhibit insufficient chemical reactivity to remove original oleic acid ligands completely. This leads to low CQD packing and correspondingly low electronic performance. Here we report an acid-assisted solution-phase ligand-exchange strategy that, by enabling efficient removal of the original ligands, enables the synthesis of densified CQD arrays. Our use of hydroiodic acid simultaneously facilitates high CQD packing via proton donation and CQD passivation through iodine. We demonstrate highly packed CQD films with a 2.5 times increased carrier mobility compared with prior exchanges. The resulting devices achieve the highest infrared photon-to-electron conversion efficiencies (>50%) reported in the spectral range of 0.8 to 1.1 eV.


Colloidal quantum dots; infrared; narrow bandgap; photovoltaics; solution-phase ligand exchange; surface passivation

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