Mixed Lead Halide Passivation of Quantum Dots

Abstract

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.

PbS Colloidal Quantum Dot Inks for Infrared Solar Cells

“…The CQDs passivated by PbBr 2 had higher charge mobility, but the passivation was worse. Mixed lead halide ligands could combine the advantages of these three ligands and each lead halide could be exchanged to its most energy-supported surface, improving passivation and carrier transport ( Fan et al., 2019 ). Therefore, the superior surface passivation of CQDs could improve device performance and also promotes ink stabilization.…”

Efficient Infrared Solar Cells Employing Quantum Dot Solids with Strong Inter‐Dot Coupling and Efficient Passivation

“…As a result, the PbSe/PbS QD IR solar cells focusing on a bandgap of ≈0.95 eV produce a high V OC which is 1.46 times that of pristine PbSe QD devices due to the improved surface passivation. Furthermore, the PbSe/PbS QD devices demonstrate a much higher IR J SC compared to the previously reported PbS QD devices [ 1,3,4,8,9,20,21 ] due to the strong electronic coupling among adjacent QDs. Consequently, we achieved a high Si‐filtered PCE of 1.24% without any light trapping nanostructure.…”

“…At present, the IR solar cells mainly employ PbS quantum dots (QDs) as the photon absorber due to their solution processability, strong quantum confinement, and widely size‐tuned bandgaps, especially their unique high absorption characteristic in IR spectrum region. [ 5–7 ] The IR efficiency (Si‐filtered) of PbS QD based solar cells has reached 1.17% [ 8 ] by improved surface passivation, and was further pushed to 1.34% [ 9 ] via IR light trapping using a nanostructured back reflector. However, the photovoltaic performance is still limited by the intrinsic characteristics of low electronic coupling and carrier transport in PbS QD solids, deteriorating the photocarrier diffusion and extraction and eventually resulting in a low short circuit current density ( J SC ).…”

A Tuned Alternating D–A Copolymer Hole‐Transport Layer Enables Colloidal Quantum Dot Solar Cells with Superior Fill Factor and Efficiency

“…The EDT‐treated CQD layer as the HTL provides well‐optimized energy alignment and a desirable electric field distribution within the active layer. However, it also introduces drawbacks in view of its high density of trap states, [ 3 ] strong chemical interaction with the materials in the device stack, [ 4 ] and limited stability in encapsulated devices. [ 5 ] Importantly, it has a short diffusion length of tens of nm that precludes significant contribution to device photocurrent.…”