Photogeneration, Diffusion and Decay The life and fate of optical excitations in quantum-dot solids

Abstract

In recent years we have studied the photoconductivity in quantum-dot solids with a combination of terahertz spectroscopy, transient absorption spectroscopy and time-resolved microwave conductivity. Appropriate surface treatments induce controlled necking between Quantum Dots resulting in significant electronic coupling and charge carrier mobilities as high as ~10 cm2/Vs. We demonstrate that at room temperature photogenerated excitons readily dissociate into mobile charge carriers.1-3 Carrier Multiplication (CM) is observed in the photoconductivity of these films.4 The number of surviving free charges that result from CM increases with increasing charge carrier mobility as a result of the competition between Auger recombination and multi-exciton dissociation.5, 6 Finally I will discuss charge trapping at surface sites of IV-VI QD films. Using a combination of electrochemically controlling the Fermi level in QD films with ultrafast transient absorption and photoluminescence spectroscopy we demonstrate that electron trapping can be controlled and even avoided altogether.7 This also allows us to determine the density of trap states in the band gap of the quantum dots and, by comparison with density function theory calculations, to identify the chemical nature of these traps as well as the physical mechanism of charge trapping.8, 9 (1) Gao, Y.; Aerts, M.; Sandeep, C. S. S.; Talgorn, E.; Savenije, T. J.; Kinge, S.; Siebbeles, L. D. A.; Houtepen, A. J. ACS Nano 2012, 6, 9606-9614. (2) Talgorn, E.; Gao, Y.; Aerts, M.; Kunneman, L. T.; Schins, J. M.; Savenije, T. J.; van Huis, M. A.; van der Zant, H. S. J.; Houtepen, A. J.; Siebbeles, L. D. A. Nat. Nanotechnol. 2011, 6, 733-739. (3) Sandeep, C. S. S.; Azpiroz, J. M.; Evers, W. H.; Boehme, S.-C.; Moreels, I.; Kinge, S.; Siebbeles, L. D. A.; Infante, I.; Houtepen, A. J. ACS Nano 2014, 8, 11499-11511. (4) Aerts, M.; Suchand Sandeep, C. S.; Gao, Y.; Savenije, T. J.; Schins, J. M.; Houtepen, A. J.; Kinge, S.; Siebbeles, L. D. A. Nano Lett. 2011, 11, 4485-4489. (5) Suchand Sandeep, C. S.; Cate, S. t.; Schins, J. M.; Savenije, T. J.; Liu, Y.; Law, M.; Kinge, S.; Houtepen, A. J.; Siebbeles, L. D. A. Nat. Commun. 2013, 4, 2360. (6) Gao, Y.; Sandeep, C. S. S.; Schins, J. M.; Houtepen, A. J.; Siebbeles, L. D. A. Nat. Commun. 2013, 4. (7) Boehme, S. C.; Walvis, T. A.; Infante, I.; Grozema, F. C.; Vanmaekelbergh, D.; Siebbeles, L. D. A.; Houtepen, A. J. ACS Nano 2014, 8, 7067-7077. (8) Boehme, S. C.; Mikel Azpiroz, J.; Aulin, Y. V.; Grozema, F. C.; Vanmaekelbergh, D.; Siebbeles, L. D. A.; Infante, I.; Houtepen, A. J. Nano Lett. 2015, 15, 3056-3066. (9) Houtepen, A. J.; Hens,