The solution-phase chemistry of infrared-emitting colloidal quantum dots (CQDs) has attracted much attention thanks to the utility of these materials in both biological [1, 2] and optoelectronic [3–5] applications. PbS CQDs, for example, are of interest in solar applications owing to their small bandgap and large Bohr exciton radius, which together allow bandgap tuning over the full solar spectral range.[4] Among other third-generation photovoltaics, such as copper indium gallium selenide (CIGS)[5, 6] and copper zinc tin sulfide (CZTS),[7, 8] colloidal quantum dots specifically offer the opportunity to create tandem or multi-junction solar cells on the basis of the same material, because of its widely tunable bandgap. The solution processability of CQD materials could in principle enable highly scalable and flexible manufacturing processes to be developed, such as roll-to-roll processing.

Advances in CQD photovoltaics have recently resulted in solar cell power conversion efficiencies exceeding 7%.[9–11] Although these performance levels are promising, all high-performing device results to date have relied on a multiple layerby-layer strategy for film fabrication (Figure 1a) rather than a single-layer deposition process. The layer-by-layer approaches use a solid-state exchange of the long aliphatic ligands, typically required in and after synthesis for colloidal stability; shorter linkers are necessary for film densification and conduction. This solid-state exchange process suffers the dual drawbacks of low efficiency in the utilization of quantum dots—typically below 1%—combined with incompatibility with roll-to-roll processing. An ideal fabrication process would instead be lossless from a materials utilization perspective and would allow fast, serial, large-area deposition of an electronic-transport-compatible film.