Developing microwave probes for investigating opto-electronic processes in photovoltaic materials

Motivated by the need to understand the underlying opto-electronic processes which limit the efficiency of photovoltaic materials, we develop two complimentary microwave capabilities: Time-resolved Microwave Conductivity (TRMC) as a probe for charge carrier dynamics, and shaped pulse Electron Paramagnetic Resonance (pEPR) as a coherent probe for spin-dependent processes, such as Singlet-Fission, Triplet-Triplet annihilation, and spin-dependent recombination in organic semiconductors. We investigate the charge carrier mobilities and lifetimes of CH3NH3PbI3 lead halide perovskite thin films using TRMC. The spectrally resolved TRMC apparatus enabled the investigation of charge decay dynamics at a range of excitation energies, mimicking the solar spectrum. We differentiate between direct and trap-mediated recombination processes by comparing conductivity and polarizability transients. We find that the relatively high mobilities of lead halide perovskites (10-2 - 102 cm2/V s) result in diffusion length in the order of 1-10 um, and are maximized when the photon excitation energy matches the bandgap. Given the large discrepancies in material properties (e.g. mobilities, lifetimes, trap densities) reported in literature for CH3NH3PbI3, we investigate the effect of sample preparation methods and surface passivation on the charge carrier recombination dynamics. We find that both the charge carrier mobilities and decay dynamics depend significantly on the sample preparation method. Pulsed electron paramagnetic resonance (EPR) is a powerful tool for investigating spin-dependent processes. It has been well-established in NMR spectroscopy that the use of shaped radiofrequency pulses can increase the fidelity, excitation bandwidth and sensitivity of NMR techniques. Technical challenges associated with implementing shaped microwave pulses have inhibited the widespread application of these techniques in EPR spectroscopy. In order to exploit the availability of shaped pEPR, we a) develop ShapedPulseSimulator, a computational research tool for designing and optimizing shaped pulses for EPR; b) design a custom-built quadrature-amplitude modulation instrument to implement shaped pulses into an existing Bruker E580 spectrometer; and c) demonstrate an increased sensitivity of the Rabi frequency spectroscopy technique using shaped pulses applied to P3TP, a singlet-fission material. We find that the increased Rabi frequency fidelity of shaped Rabi pulses results in a more constrained estimate of the remnant coupling between triplet pairs generated during the singlet-fission process.

Grantor dc:publisher

UNSW, Sydney

Year dc:date

2018

Author dc:creator

• Guse, Joanna A.

• Electron paramagnetic resonance (EPR)
• Opto-electronic processes
• Time-resolved Microwave Conductivity (TRMC)

Statement dc:rights

• open access
• CC BY-NC-ND 3.0
• free_to_read

Licence

• https://purl.org/coar/access_right/c_abf2
• https://creativecommons.org/licenses/by-nc-nd/3.0/au/

Language dc:language

EN

Identifier

https://doi.org/10.26190/unsworks/21088

OAI identifier oai:identifier

oai:unsworks.library.unsw.edu.au:1959.4/61521