TY - JOUR
T1 - Accelerated solution-phase exchanges minimize defects in colloidal quantum dot solids
AU - Zhao, Yong Biao
AU - Liu, Mengxia
AU - Voznyy, Oleksandr
AU - Sun, Bin
AU - Li, Pei Cheng
AU - Kung, Haoting
AU - Ouellette, Olivier
AU - Choi, Min Jae
AU - Lu, Zheng Hong
AU - García de Arquer, F. Pelayo
AU - Sargent, Edward H.
N1 - Publisher Copyright:
© 2019 Elsevier Ltd
PY - 2019/9
Y1 - 2019/9
N2 - Colloidal quantum dots (CQDs) are promising for solar cell applications in view of their low cost, solution processing, and bandgap tuning. Solution-phase ligand exchange is one successful strategy to realize dense CQD solids with minimized inhomogeneous broadening and enhanced carrier transport. In this process, long alkane ligands are replaced with short metal-halide ligands during a phase-transfer from a non-polar to a polar solvent. Unfortunately, these two processes – ligand exchange and phase transfer – possess very different kinetic rates. We hypothesized that this kinetic rate mismatch could lead to CQD surface disruption and to the formation of unpassivated sites and dangling bonds, contributing in turn to sub-bandgap trap states. Here we present a ligand exchange that favors a rapid (seconds, instead of minutes or hours) surface modification and phase transfer that minimizes surface exposure during phase-transfer. We accelerate the exchange dynamics by using highly concentrated alkane-capped CQD solutions, maximizing the contact area of CQD surfaces with incoming halide ligands to achieve enhanced exchange rates. Using this strategy, we achieve improved photoluminescence quantum yield (from 22% to 32%), sharper bandtails (from 38 meV to 17 meV), and realize CQD photovoltaic cells with enhanced open-circuit voltage (0.670 V vs. 0.650 V of control), fill factor, and power conversion efficiency (12.1% vs. 11.0% in the control).
AB - Colloidal quantum dots (CQDs) are promising for solar cell applications in view of their low cost, solution processing, and bandgap tuning. Solution-phase ligand exchange is one successful strategy to realize dense CQD solids with minimized inhomogeneous broadening and enhanced carrier transport. In this process, long alkane ligands are replaced with short metal-halide ligands during a phase-transfer from a non-polar to a polar solvent. Unfortunately, these two processes – ligand exchange and phase transfer – possess very different kinetic rates. We hypothesized that this kinetic rate mismatch could lead to CQD surface disruption and to the formation of unpassivated sites and dangling bonds, contributing in turn to sub-bandgap trap states. Here we present a ligand exchange that favors a rapid (seconds, instead of minutes or hours) surface modification and phase transfer that minimizes surface exposure during phase-transfer. We accelerate the exchange dynamics by using highly concentrated alkane-capped CQD solutions, maximizing the contact area of CQD surfaces with incoming halide ligands to achieve enhanced exchange rates. Using this strategy, we achieve improved photoluminescence quantum yield (from 22% to 32%), sharper bandtails (from 38 meV to 17 meV), and realize CQD photovoltaic cells with enhanced open-circuit voltage (0.670 V vs. 0.650 V of control), fill factor, and power conversion efficiency (12.1% vs. 11.0% in the control).
KW - Colloidal quantum dot solar cells
KW - Exchange dynamics
KW - Halide passivation
KW - Solution ligand exchange
UR - http://www.scopus.com/inward/record.url?scp=85068466308&partnerID=8YFLogxK
U2 - 10.1016/j.nanoen.2019.103876
DO - 10.1016/j.nanoen.2019.103876
M3 - Article
AN - SCOPUS:85068466308
SN - 2211-2855
VL - 63
JO - Nano Energy
JF - Nano Energy
M1 - 103876
ER -