Energy Loss and Transport in Nanomaterials

Moving Energy Towards Functional Targets

The efficient transport of energy is fundamental to the function of nearly all biological, chemical, and material systems. For materials, electronic energy transport is central to both basic science and the performance of nearly all modern electronic and optoelectronic technologies. Energy transport in materials can take many different forms such as free charges, neutral excitons, photons, and even emerging quasi-particles such as exciton-polaritons. In efficient devices, this energy is controllably shuttled to functional targets, but, more commonly, these particles rapidly lose energy, change momentum, and scatter off defects. In emerging nanostructured materials, transport is especially random and tortuous, where the high surface and interface densities confine energy to small length scales. Satisfactory control of energy flow in materials therefore requires a detailed understanding of how local structure, composition, and morphology affect energy loss, momentum, and transport.

Energy Loss in Nanostructured Semiconducting Materials

The passivation of defective semiconductor surfaces has been a critical manufacturing step for nearly all semiconductors used in the electronics and optoelectronics industry. Early on in my graduate studies at the University of Washington, we discovered that emerging perovskite semiconductors are no exception and, interestingly, they exhibit grain-to-grain heterogeneous emission that limits photovoltaic device performance (deQuilettes, D.W. et al., Science, 2015, 348, 683-686). Importantly, this study directly identified regions that could be further improved even in what many considered were exceptional photovoltaic devices.

In this regard, we explored a wide range of passivating agents including small thiols, phosphines, phosphine-oxides, and amines which led to the identification of the electron donating species, n-trioctylphosphine oxide, that led to a record 8.8 microsecond photoluminescence lifetime, which has now become a benchmark for evaluating new passivation strategies (deQuilettes, D.W. et al. ACS Energy Lett. 2016, 1, 2, 438-444). Excitingly, this strategy has yielded some of the highest quality solution-processed semiconductors to date, achieving a quasi-Fermi level splitting (i.e. voltage) that was 97% of the thermodynamic limit (Braly, I.; deQuilettes, D.W. et al. Nat. Photon. 2018, 12, 355-361)

Although some of these chemical routes have been effective, a fundamental understanding of the nature of the defects in perovskites and how they can be selectively targeted, especially in device stacks with interfaces, is an ongoing challenge. My future goals are to extend these findings to interfacial layers critical to the performance of photovoltaics, light emitting diodes, lasers, other optoelectronic devices.

Energy Transport in Nanostructured Semiconducting Materials

In addition to local energy loss, energy flow in nanostructured materials is heterogeneous. Using fluorescence microscopy, we utilized a new technique using local excitation (solid circles in Figure) paired with widefield detection to directly image anisotropy in energy carrier transport. Specifically, we found that grain boundaries can often serve both as non-radiative recombination centers, but also as energy barriers, which impede carrier transport (deQuilettes, D.W. et al. ACS Nano, 2017, 11, 11, 11488-11496).

In order to quantify time-dependent diffusion using optical microscopy in a wide range of materials, we developed and improved upon previous mean squared displacement (MSD) models which track small changes in the carrier distribution profiles as a function of time. We derived analytical solutions to complex non-linear partial differential equations, which now allow for the direct extraction of diffusion coefficient while taking into account non-linear recombination terms that artificially broaden the distribution profile (deQuilettes, D.W. et al. Phys. Rev. Appl. 2020, under review).

Additional Related Publications

deQuilettes, D.W.; Zhang, W.; Burlakov, V. M.; Graham, D.J.; Leijtens, T.; Osherov, A. Bulović, V.; Snaith, H.J.; Ginger, D.S.; Stranks, S.D. “Photo-induced Halide Redistribution in Organic-Inorganic Perovskite Films,” Nature Communications.,2016, 7 (11683). DOI: 10.1038/ncomms11683
deQuilettes, D.W.; Laitz, M.; Brenes, R.; Dou, B.; Motes, B.T.*; Stranks, S.D.; Snaith, H.J.; Bulović, V.; Ginger, D.S. Invited Review: “Maximizing the External Radiative Efficiency of Hybrid Perovskite Solar Cells,” Pure Applied Chemistry, 2020, 92, 697-706. DOI: 10.1515/pac-2019-0505
Zuo, L.; Guo, H.; deQuilettes, D.W.; Jariwala, S.; De Marco, N.; Dong, S.; DeBlock, R.H.; Ginger, D.S.; Dunn, B.; Wang, M.; Yang, Y. “Polymer-Modified Halide Perovskite Films for Efficient and Stable Planar Heterojunction Solar Cells,” Science Advances, 2017, 3, e1700106. DOI: 10.1126/sciadv.1700106
Wang, J.T.W.; Wang, Z.; Pathak, S.; Zhang, W.; deQuilettes, D.W.; Wisnivesky, F.; Huang, J.; Nayak, P.; Patel, J.; Yusof, H.; Vaynzof, Y.; Zhu, R.; Ramirez, I.; Zhang. J.; Ducati, C.; Grovenor, C.; Johnston, M.B.; Ginger, D.S.; Nicholas, R.J.; Snaith, H.J. “Efficient Perovskite Solar Cells by Metal Ion Doping,” Energy Environmental Science, 2016, 9, 2892-2901. DOI: 10.1039/C6EE01969B