Charge carrier dynamics in hybrid systems for solar energy conversion
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Abstract
This thesis covers the fundamental understanding of key processes; specifically, charge carrier dynamics, in hybrid systems with relevance in energy production (via solar cells) and energy storage (via water splitting). Charge carrier dynamics, including charge generation, interfacial charge transfer and charge transport, are interrogated by using time-resolved terahertz spectroscopy (TRTS) technique. The thesis is organized as follows:
Chapter 1: Introduction
Here we briefly introduce intrinsic loss mechanisms and efficiency limits in photoconversion, and some examples of photoconversion processes as: perovskite solar cells and dye sensitized solar cells for energy production and photocatalytic hydrogen generation for energy storage.
Chapter 2: Experimental technique, materials and sample preparations
In this chapter, we present time-resolved terahertz spectroscopy (TRTS) as a technique to investigate charge carrier dynamics and the nature of the charge transport in hybrid systems. Specifically, we present how to extract the frequency resolved photoconductivity from THz data and how to monitor charge carrier dynamics by TRTS measurements in a pump-probe scheme. A description of materials and sample preparation protocols used throughout the thesis are also presented.
Chapter 3: Phonon-Electron Scattering Limits Free Charge Mobility in Methylammonium Lead Iodide Perovskites
In this chapter, we study the temperature dependent charge carrier dynamics and complex valued photoconductivity in solution-processed films of methylammonium lead iodide perovskite to explain the nature of photo-generated charges (whether free carriers, excitons or both) and the nature of the charge transport. We find that the main photo-generated species are free carriers (rather than excitons). The nature of the conductivity is Drude-like and free charge carrier motion is characterized by an average scattering time. The scattering time determined from Drude fits shows a ~T -3/2 dependence (temperature range 150 < T < 300 K); this figure approaches the theoretical limit for pure acoustic phonon (deformation potential) scattering. Hence, we conclude that electron-phonon, rather than impurity scattering, sets the upper limit on free charge transport for this hybrid perovskite material.
Chapter 4: Electron transfer from 4,4′-dicarboxy-2,2′-bipyridine emerges upon dye photodegradation in N3 sensitized TiO2
In this chapter, we study the evolution of interfacial electron transfer (ET) dynamics in N3 dye sensitized mesoporous TiO2 electrodes upon controlled sensitized electrode photodegradation. We demonstrate that a sub-ps ET signal emerges and becomes practicaly time-invariant upon full photodegradation of the N3 dye. Notably, the amplitude signal corresponds to ~50 % of the original TRTS signal prior sample photodegradation. Frequency-resolved complex photoconductivity measurements confirm that the signal emerging upon full dye decomposition into fragments refers to free carriers populating the oxide conduction band. From Laser Desorption Ionization Mass Spectrometry (LDI-MS), ATR-FTIR spectroscopy and time-resolved terahertz spectroscopy (TRTS) we analyze the chemical identity of the compound from which this sub-ps ET component originates; we find that 4,4′-dicarboxy-2,2′-bipyridine remains anchored to the oxide surface upon full N3 dye photodegradation.
Chapter 5: Correlating Carrier Dynamics and Photocatalytic Hydrogen Generation in Pt Decorated CdSe Tetrapods
In this chapter, we study platinum cocatalyst decorated CdSe tetrapods as a function of Pt nanoparticle size by correlating carrier dynamics and photocatalytic hydrogen generation efficiency. We find that while the capture rate of photogenerated electrons from CdSe to the Pt nanoparticles (co-catalyst) increases as a function of Pt size, the photocatalytic H2 generation is reduced for tetrapods decorated with larger Pt particles. Our results demonstrate the presence of a kinetic competition between electron transfer at the CdSe/Pt interface and electron transfer at the Pt / H2O interface. This trade-off defines an optimum Pt size for photocatalytic H2 generation in Pt decorated CdSe tetrapods.
Chapter 6: Conclusions and Outlook
In this chapter, we briefly summarize the general conclusions and outlook relevant for the studies in this thesis.