Abstract
Colloidal quantum dots are attractive materials for efficient, low-cost and facile implementation of solution-processed optoelectronic devices. Despite impressive mobilities (1-30 cm2 V(-1) s(-1)) reported for new classes of quantum dot solids, it is--surprisingly--the much lower-mobility (10(-3)-10(-2) cm2 V(-1) s(-1)) solids that have produced the best photovoltaic performance. Here we show that it is not mobility, but instead the average spacing among recombination centres that governs the diffusion length of charges in today's quantum dot solids. In this regime, colloidal quantum dot films do not benefit from further improvements in charge carrier mobility. We develop a device model that accurately predicts the thickness dependence and diffusion length dependence of devices. Direct diffusion length measurements suggest the solid-state ligand exchange procedure as a potential origin of the detrimental recombination centres. We then present a novel avenue for in-solution passivation with tightly bound chlorothiols that retain passivation from solution to film, achieving an 8.5% power conversion efficiency.
Highlights
Colloidal quantum dots are attractive materials for efficient, low-cost and facile implementation of solution-processed optoelectronic devices
Colloidal quantum dots (CQDs) are nanocrystal semiconductor particles dispersed in solution that can readily be cast into electronically conducting solids
Despite numerous reports of field effect[23,29,30] and terahertz[22,31] mobilities on the order of 1–30 cm[2] V À 1 s À 1, which should in principle result in much greater efficiencies than seen today[32], a PV device has yet to be made that benefits from these increased charge carrier mobilities
Summary
Colloidal quantum dots are attractive materials for efficient, low-cost and facile implementation of solution-processed optoelectronic devices. Recent efforts have concentrated on eliminating trap states detrimental to carrier lifetime[5,14,15], investigating the impact of size polydispersity on an ensemble of CQDs16,17, improving charge collection[13,18,19], characterizing field-effect mobility in these materials[20,21,22,23] and developing novel doping strategies to enable new high-efficiency device architectures[6,24,25,26]. It is desirable to eliminate surfacedangling bonds and chemical impurities to avoid formation of deep electronic trap states[5,28] that increase the rate of carrier recombination If these features can be combined to produce a high mobility-lifetime product, increased transport lengths should permit the construction of thick absorber layers capable of absorbing the available solar light while maintaining the efficient harvesting of the resultant photocarriers. Using a targeted strategy of in-solution passivation with chlorinated thiols to improve CQD protection during solid-state exchange, we achieve the reduction in trap densities, resulting in an 8.5% PCE using a standard planar device architecture
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