Hot-Carrier Interactions in Semiconductor Nanomaterials Designed for High-Efficiency Solar Energy Conversion

In a conventional solar cell the excess energy of an absorbed high-energy photon is wasted as heat through the thermalization of the photogenerated hot carrier, thereby limiting the energy conversion efficiency of the photovoltaic device. Novel physics arising under strong quantum confinement in semiconductor nanocrystals enables the high-efficiency conversion of this excess energy toward generating additional carriers through a process called carrier multiplication (CM). While CM is significantly enhanced in quantum dots compared to bulk semiconductors, further improvements are necessary in order to have a significant impact on real-world device performance.

Guided by our recently developed phenomenological model for CM,¹ we have designed nanoscale heterostructures in which intentionally slowed intraband cooling of hot carriers gives a greater “window of opportunity” for competing CM events to occur. Our refined synthetic methods have enabled near-atomic scale precision in the control of the core (PbSe) and shell (CdSe) dimensions of these heterostructures over a very large range of shell thicknesses, allowing us to fine-tune electronic wavefunctions and consequently maximize CM efficiencies. By targeting a precise range of core radius/shell thickness ratios, we were able to achieve a four-fold increase in CM yield over conventional monocomponent PbSe QDs, accompanied by a considerable reduction of the CM threshold almost down to the fundamental two-energy-gap limit². The record CM quantum yield is a combined result of multiple factors: (i) effective capture of “hot” energetic holes in a long-lived, shell-localized valence band CM channel; and (ii) significant spatial and energy mismatch between these states and relatively sparse core levels that dramatically slow phonon-assisted cooling, increasing the likelihood of hole relaxation via impact-ionization-like scattering.

These nanostructures are the first demonstration of the generalizable concept of "CM-engineering" in which consideration of the mechanism of CM and other competing cooling processes is used to rationally design nanomaterials featuring CM yields approaching the ideal thermodynamic limit. Potentially, efficiencies very close to the thermodynamic limit can be achieved by combining this approach with shape control^3,4 (e.g. elongation, as in nanorods) and the use of different material compositions^1,4 (e.g. PbTe or Si).

References

1.       J. T. Stewart, J. T.; L. A. Padilha; W. K. Bae; W.-k Koh; J. M. Pietryga; V. I. Klimov, “Carrier Multiplication in Quantum Dots within the Framework of Two Competing Energy Relaxation Mechanisms”, J. Phys. Chem. Lett. 4, 2061-2068 (2013).

2.       C. M. Cirloganu, L. A. Padilha, Q. Lin, N. S. Makarov, K. A. Velizhanin, H. Luo, I. Robel, J. M. Pietryga, V. I. Klimov, “Enhanced carrier multiplication in engineered quasi-type-II quantum dots”, Nat. Commun. 5, 4148 (2014)

3.       L.A. Padilha, J.T. Stewart, R.L. Sandberg, W.K. Bae, W. Koh, J.M. Pietryga, V.I.Klimov, Aspect Ratio Dependence of Auger Recombination and Carrier Multiplication in PbSe Nanorods, Nano Lett. 13, 1092–1099 (2013)

4.       L.A. Padilha, J.T. Stewart, R.L. Sandberg, W.K. Bae, W. Koh, J.M. Pietryga, V.I.Klimov, Carrier Multiplication in Semiconductor Nanocrystals: Influence of Size, Shape and Composition, Acc. Chem. Res., 46, 1261-1269 (2013)