Using Terahertz Spectroscopy to Study Systems with Solar Energy Applications Rebecca L. Milot, Gary F. Moore, Gary W. Brudvig, Robert H. Crabtree, and Charles A. Schmuttenmaer Yale University Department of Chemistry June 21, 2013

Semiconductors Silicon Silicon – 400 nm (3.2 eV) excitation Conduction Band Valence Band e- Non-contact electrical probe E g = 1.1 eV

Recombination and trapping dynamics Injection dynamics Injection efficiency Time-Resolved THz Spectroscopy t Pump Monitor change in THz maximum as a function of optical pump delay 400 nm Sub-picosecond time resolution

Solar Cells Traditional Photovoltaic Solar Cells Dye-Sensitized Solar Cells M. Gratzel, Inorganic Chemistry, Conduction Band Valence Band e- Silicon N719 on TiO 2

Solar Fuels The Yale Solar Group Our Plan K. J. Young et al., Coord. Chem. Rev., Photosynthesis 6 CO H 2 O + light → C 6 H 12 O O 2 Water oxidation: 2 H 2 O + light → O H e - CO 2 reduction: 6 CO H e - → C 6 H 12 O H 2 O

Porphyrins ChlorophyllPF 15 M = H 2, Zn 2+, Cu 2+, Ni 2+, or Pd 2+

Steady State Optical Properties S0S0 S1S1 S2S2 T1T1 Possible Excited States and Deactivation Pathways π-π* transitions H 2 PF 15 in DCM

Energy Levels fs-ps ps-ns μs-ms

TiO 2

SnO 2 ΔTHz 0 A1A1 τ 1 (ps)A2A2 τ 2 (ps)τ r (ns) H 2 PF ZnPF CuPF PdPF

SnO 2 Summary

Conclusions PF 15 Sensitizers TiO 2 : Inefficient electron injection into TiO 2 SnO 2 : Different mechanisms of electron injection No clear winner Future Work Make devices Study recombination – Nanosecond transient absorption G. F. Moore et al., Energ. Environ. Sci., 2011.

Acknowledgements thz.yale.edu Schmuttenmaer Group Charles Schmuttenmaer Michael Williams Daniel Aschaffenburg Stafford Sheehan Yale Solar Energy Collaboration Gary Moore Victor Batista Gary Brudvig Robert Crabtree