Accelerated solution-phase exchanges minimize defects in colloidal quantum dot solids

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).