Colloidal quantum dot solar cells exploiting hierarchical structuring

Abstract
Extremely thin-absorber solar cells offer low materials utilization and simplified manufacture but require improved means to enhance photon absorption in the active layer. Here, we report enhanced-absorption colloidal quantum dot (CQD) solar cells that feature transfer-stamped solution-processed pyramid-shaped electrodes employed in a hierarchically structured device. The pyramids increase, by up to a factor of 2, the external quantum efficiency of the device at absorption-limited wavelengths near the absorber band edge. We show that absorption enhancement can be optimized with increased pyramid angle with an appreciable net improvement in power conversion efficiency, that is, with the gain in current associated with improved absorption and extraction overcoming the smaller fractional decrease in open-circuit voltage associated with increased junction area. We show that the hierarchical combination of micron-scale structured electrodes with nanoscale films provides for an optimized enhancement at absorption-limited wavelengths. We fabricate 54.7° pyramid-patterned electrodes, conformally apply the quantum dot films, and report pyramid CQD solar cells that exhibit a 24% improvement in overall short-circuit current density with champion devices providing a power conversion efficiency of 9.2%.

Citation
Labelle, A. J., Thon, S. M., Masala, S., Adachi, M. M., Dong, H., Farahani, M., … Sargent, E. H. (2015). Colloidal Quantum Dot Solar Cells Exploiting Hierarchical Structuring. Nano Letters, 15(2), 1101–1108. doi:10.1021/nl504086v

Acknowledgements
This publication is based in part on work supported by Award KUS-11-009-21, made by 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. We thank Angstrom Engineering, Inc. and Innovative Technology, Inc. for useful discussions regarding material deposition methods and control of the glovebox environment, respectively. H.D. would like to acknowledge financial support from China Scholarship Council (CSC). The authors thank Larissa Levina for the assistance of CQDs synthesis and E. Palmiano, R. Wolowiec, G. Koleilat, and D. Kopilovic for their technical help over the course of this study. We also thank Professor Heman Miguez of the Institute of Materials Science of Seville for very helpful discussions regarding electrode structuring. We would finally like to thank Professor Peter Herman and Dr. Kitty Kumar, both of the Electrical and Computer Engineering Department at the University of Toronto, for materials and assistance in preparing the Silicon master.

Publisher
American Chemical Society (ACS)

Journal
Nano Letters

DOI
10.1021/nl504086v

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