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20151211_PJR_FINAL Thesis.pdf (7.49 MB)

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Charge Carrier Processes in Photovoltaic Materials and Devices: Lead Sulfide Quantum Dots and Cadmium Telluride

Roland, Paul Joseph
http://orcid.org/0000-0001-5594-8024
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1449857685

Abstract Details

2015, Doctor of Philosophy, University of Toledo, Physics.
Charge separation, transport, and recombination represent fundamental processes for electrons and holes in semiconductor photovoltaic devices. Here, two distinct materials systems, based on lead sulfide quantum dots and on polycrystalline cadmium telluride, are investigated to advance the understanding of their fundamental nature for insights into the material science necessary to improve the technologies. Lead sulfide quantum dots QDs have been of growing interest in photovoltaics, having recently produced devices exceeding 10% conversion efficiency. Carrier transport via hopping through the quantum dot thin films is not only a function of inter-QD distance, but of the QD size and dielectric media of the surrounding materials. By conducting temperature dependent transmission, photoluminescence, and time resolved photoluminescence measurements, we gain insight into photoluminescence quenching and size-dependent carrier transport through QD ensembles. Turning to commercially relevant cadmium telluride (CdTe), we explore the high concentrations of self-compensating defects (donors and acceptors) in polycrystalline thin films via photoluminescence from recombination at defect sites. Low temperature (25 K) photoluminescence measurements of CdTe reveal numerous radiative transitions due to exciton, trap assisted, and donor-acceptor pair recombination events linked with various defect states. Here we explore the difference between films deposited via close space sublimation (CSS) and radio frequency magnetron sputtering, both as-grown and following a cadmium chloride treatment. The as-grown CSS films exhibited a strong donor-acceptor pair transition associated with deep defect states. Constructing photoluminescence spectra as a function of time from time-resolved photoluminescence data, we report on the temporal evolution of this donor-acceptor transition. Having gained insight into the cadmium telluride film quality from low temperature photoluminescence measurements, we now turn to completed devices, evaluating the influence of back contact transport versus temperature. Cadmium telluride photovoltaic devices are known to form a Schottky junction when simply using a metal back contact. Our group previously reported on the attempted application of iron pyrite nanocrystals as a back contact material due to their high conductivity and doping concentration. These devices, however, exhibited non-ideal current-voltage curves where an S-Kink restricted current collection and reduced efficiency. Here we employ temperature dependent current-voltage measurements to gain insight into the S-Kink behavior and attempt to replicate the current-voltage curves using circuit modeling. We develop a modified diode circuit model where an anti-parallel diode pair serves to limit the current flow at voltages near VOC. This model successfully reproduces the experimental data and provides a means to extract diode parameters from current-voltage plots exhibiting S-Kink behavior.
Randall Ellingson (Committee Chair)
Sanjay Khare (Committee Member)
Yanfa Yan (Committee Member)
Terry Bigioni (Committee Member)
Michael Cushing (Committee Member)
124 p.
Physics; Solid State Physics
Temperature-dependence; Charge Carrier Transport; Semiconductor Point Defects; Lead Sulfide Quantum Dots; Cadmium Telluride; Photoluminescence; Time Correlated Single Photon Counting; Photovoltaics

Recommended Citations

Citations

  • Roland, P. J. (2015). Charge Carrier Processes in Photovoltaic Materials and Devices: Lead Sulfide Quantum Dots and Cadmium Telluride [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1449857685

    APA Style (7th edition)

  • Roland, Paul. Charge Carrier Processes in Photovoltaic Materials and Devices: Lead Sulfide Quantum Dots and Cadmium Telluride. 2015. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1449857685.

    MLA Style (8th edition)

  • Roland, Paul. "Charge Carrier Processes in Photovoltaic Materials and Devices: Lead Sulfide Quantum Dots and Cadmium Telluride." Doctoral dissertation, University of Toledo, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1449857685

    Chicago Manual of Style (17th edition)

toledo1449857685
488
© 2015, some rights reserved.Creative Commons License Charge Carrier Processes in Photovoltaic Materials and Devices: Lead Sulfide Quantum Dots and Cadmium Telluride by Paul Joseph Roland is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Toledo and OhioLINK.