Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electrong-"hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device. © 2012 Macmillan Publishers Limited. All rights reserved.
KAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): KUS-11-009-21
Acknowledgements: This publication is based in part on work supported by an award (KUS-11-009-21) from the King Abdullah University of Science and Technology (KAUST), by the Ontario Research Fund Research Excellence Program and by the Natural Sciences and Engineering Research Council (NSERC) of Canada. The authors thank Angstrom Engineering and Innovative Technology for useful discussions regarding material deposition methods and control of the glovebox environment, respectively. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy (contract no. DE-AC02-05CH11231). The authors thank L. Goncharova, M. T. Greiner, E. Palmiano, R. Wolowiec and D. Kopilovic for their help during the course of the study. A. H. I. acknowledges support from the Queen Elizabeth II Graduate Scholarship in Science and Technology. D. Z. acknowledges support from the NSERC CGS D scholarship.
- Biomedical Engineering
- General Materials Science
- Atomic and Molecular Physics, and Optics
- Electrical and Electronic Engineering
- Condensed Matter Physics