Investigating Carrier Dynamics in Photovoltaic Materials

Mohammad Mehdi Taheri

doi:10.17918/00000514

Understanding photoexcited carrier dynamics is crucial for designing high-performance optoelectronic devices. Carrier cooling in semiconductors, charge transfer across interfaces, and recombination mechanisms are critical processes in photophysical systems that typically occur on the time scale of less than a picosecond to several nanoseconds. Ultrafast techniques, including ultraviolet-visible-infrared transient absorption (TA), time-resolved terahertz spectroscopy (TRTS), and time-resolved photoluminescence (TRPL), are ideal tools for studying charge carrier dynamics at such timescales. This thesis will focus on the application of complementary spectroscopy techniques and modeling to investigate carrier dynamics within CdSe/CdS core/shell colloidal quantum dots (QDs) and Cu3AsS4 and CdTe thin films. CdTe solar technology has attracted the photovoltaic (PV) community for the past three decades owing to its low production cost and record efficiency of 22.1%. However, some challenges must be overcome to further improve its efficiency to the 25% range. Cu3AsS4 thin film is a promising emerging candidate as a PV absorber material due to its earth-abundant and nontoxic constituent elements, but its optoelectronic properties are not well known. Carrier dynamics reveal important details about the recombination processes that limit PV performance. Improvements in the PV device efficiency require a full understanding of the routes for carrier recombination processes. TRPL, which measures emission, has conventionally been used to evaluate recombination mechanisms in thin film PVs, but carrier redistribution often dominates the response at short times. Here we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both TRTS, which measures photoconductivity, and TRPL combined with numerical modeling of the continuity equations and Poisson's equation. We were able to distinguish and quantify bulk and surface recombination in CdTe and Cu3AsS4 thin films, which is critical for the development of thin film PVs with higher efficiency. We also investigated the carrier dynamics in functionalized CdSe/CdS core/shell QDs using complementary ultrafast TA and TRPL spectroscopies and kinetic modeling. Cd-chalcogenide QDs have been widely studied because of their excellent optical properties and their facile tunability. The Cd-chalcogenide QDs have been studied for more than 20 years, but the ambiguities in the interpretation of the TA spectra are still under debate. For one thing, the photoexcited TA signal in Cd-chalcogenide QDs has been fully attributed to conduction band electrons, neglecting any contributions from valence band holes. In this work, we present a comprehensive picture of the electronic processes in photoexcited CdSe/CdS core/shell QDs. We have demonstrated through complementary spectroscopic experiments and kinetic modeling that holes affect the TA results and can contribute ~ 30% to the visible range and ~ 72% to the mid-IR range. The comprehensive picture of photophysical processes provided by the complementary ultrafast techniques and kinetic modeling in this work can accelerate both the fundamental science and application development of nanostructured and molecular systems. This thesis will focus on the application of spectroscopy techniques and modeling to investigate carrier dynamics in optoelectronic systems including thin film PVs and colloidal CdSe based QDs. The methodologies presented in this thesis can serve as a guideline for the accurate interpretation of spectroscopic measurements not only for the cases studied here but also for other optoelectronic systems.

Investigating Carrier Dynamics in Photovoltaic Materials

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Investigating Carrier Dynamics in Photovoltaic Materials

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