Mixed Lead Halide Passivation of Quantum Dots

• Published in: Advanced materials (Weinheim) Vol. 31; no. 48; pp. e1904304 - n/a
• Main Authors: Fan, James Z., Andersen, Nigel T., Biondi, Margherita, Todorović, Petar, Sun, Bin, Ouellette, Olivier, Abed, Jehad, Sagar, Laxmi K., Choi, Min‐Jae, Hoogland, Sjoerd, de Arquer, F. Pelayo García, Sargent, Edward H.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.11.2019
• Subjects: Charge transport; Circuits; Energy conversion efficiency; infrared photovoltaics; Lead chlorides; Lead compounds; ligand exchange; Materials science; Metal halides; nanomaterials; Passivity; Perovskites; Photovoltaic cells; Quantum dots; Quantum efficiency; Solar cells; quantum dots; ligand exchange; nanomaterials; infrared photovoltaics
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.201904304

Infrared‐absorbing colloidal quantum dots (IR CQDs) are materials of interest in tandem solar cells to augment perovskite and cSi photovoltaics (PV). Today's best IR CQD solar cells rely on the use of passivation strategies based on lead iodide; however, these fail to passivate the entire surface of IR CQDs. Lead chloride passivated CQDs show improved passivation, but worse charge transport. Lead bromide passivated CQDs have higher charge mobilities, but worse passivation. Here a mixed lead‐halide (MPbX) ligand exchange is introduced that enables thorough surface passivation without compromising transport. MPbX–PbS CQDs exhibit properties that exceed the best features of single lead‐halide PbS CQDs: they show improved passivation (43 ± 5 meV vs 44 ± 4 meV in Stokes shift) together with higher charge transport (4 × 10‐2 ± 3 × 10‐3 cm2 V‐1 s‐1 vs 3 × 10‐2 ± 3 × 10‐3 cm2 V‐1 s‐1 in mobility). This translates into PV devices having a record IR open‐circuit voltage (IR Voc) of 0.46 ± 0.01 V while simultaneously having an external quantum efficiency of 81 ± 1%. They provide a 1.7× improvement in the power conversion efficiency of IR photons (>1.1 µm) relative to the single lead‐halide controls reported herein. Infrared colloidal quantum dot (IR CQD) solar cells harvest solar power beyond the band edge of crystalline silicon. Using single lead halides for ligand exchange improves IR CQD passivation or transport, but not both. A mixed halide ligand exchange method improves both metrics, resulting in IR solar cells with improved power conversion efficiencies.