Copper Oxide Thin Films through Solution Based Methods for Electrical Energy Conversion and Storage
Zhu, Changqiong.
2016
-
Abstract: Copper
oxides (Cu2O and CuO), composed of non-toxic and earth abundant elements, are promising
materials for electrical energy generation and storage devices. Solution based
techniques for creating thin films of these materials, such as electrodeposition, are
important to understand and develop because of their potential for realizing substantial
energy savings compared to traditional ... read morefabrication methods. Cuprous oxide (Cu2O), with
its direct band gap, is a p-type semiconductor that is well suited for creating
solution-processed photovoltaic devices (solar cells); several key advancements made
toward this application are the primary focus of this thesis. Electrodeposition of
single-phase, crystalline Cu2O thin films is demonstrated using previously unexplored,
acidic lactate/Cu2+ solutions, which has provided additional
understanding of the impacts of growth solution chemistry on film formation. The
influence of pH on the resulting Cu2O thin film properties is revealed by using the same
ligand (sodium lactate) at various solution pH values. Cu2O films grown from acidic
lactate solutions can exhibit a distinctive flowerlike, dendritic morphology, in
contrast to the faceted, dense films obtained using alkaline lactate solutions. Relative
speciation distributions of the various metal complex ions present under different
growth conditions are calculated using reported equilibrium association constants and
experimentally supported by UV-Visible absorption spectroscopy. Dependence of thin film
morphology on the lactate/Cu2+ molar ratio and applied potential
is described. Cu2O/eutectic gallium-indium Schottky junction devices are formed and
devices are tested under monochromatic green LED illumination. Further surface
examination of the Cu2O films using X-ray photoelectron spectroscopy (XPS) reveals the
fact that films grown from acidic lactate solution with a small
lactate/Cu2+ molar ratio, which exhibit improved photovoltaic
performance compared to films grown from basic lactate solution with a large
lactate/Cu2+ molar ratio, are sodium-free. This finding stands in
contrast to the observation that films grown in basic solution contain a significant
amount of sodium impurity at their top surfaces. Therefore, it is concluded that the
sodium impurities present in films grown from basic lactate solutions are detrimental to
overall photovoltaic device performance by introducing interface traps and recombination
centers for charge carriers, which suggests that removing these impurities may be a
promising strategy for improving Cu2O based solar cells. It has been found that
impurities at the surface of electrodeposited p-Cu2O films can be efficiently removed
through the use of concentrated aqueous ammonia solution as a wet etching agent. The
performance of Cu2O homojunction photovoltaic devices incorporating etched p-Cu2O as the
bottom layer is higher compared to devices with as-deposited p-Cu2O layers due to an
improvement of the homojunction interface quality. Reducing the density of defect states
that act as carrier recombination centers is found to lead to larger open circuit
voltages. Zinc-doped cuprous oxide (Zn:Cu2O) thin films have also been prepared via
single step electrodeposition from an aqueous solution containing sodium perchlorate.
The Zn/Cu molar ratio in the Cu2O films can be tuned by adjusting the magnitude of the
applied potential and the sodium perchlorate concentration. Electrical characterization
reveals that zinc dopants increase the Fermi level in Zn:Cu2O films, enabling a
three-fold improvement in the power conversion efficiency of a fully electrodeposited
Cu2O homojunction photovoltaic device. Complementary to the development of Cu2O based
photovoltaic devices, the use of solution deposited cupric oxide (CuO) thin films for
capacitive energy storage has also been investigated. A seed layer-assisted chemical
bath deposition (SCBD) method has been developed to create high quality CuO thin films
on transparent conductive electrode (ITO)/glass substrates. A CuO seed layer is formed
by the electrodeposition of Cu2O on ITO electrode for 10 s, followed by a brief (15 min)
heating step to convert the Cu2O to CuO. The seed layer is found to be essential for the
growth of micrometer-thick, adherent CuO thin films on ITO-coated glass, as no films
were observed to form on substrates without a seed layer. The addition of sodium lactate
to the SCBD solution can be used to tune the morphology and relative crystallinity of
the CuO films. A highly crystalline CuO film has been deposited from a solution without
sodium lactate, while a largely amorphous CuO film was realized using
lactate/Cu2+ molar ratio equal to 1.0. The CuO film with greater
amorphous character exhibited a significantly larger specific capacitance as a redox
active electrode compared to the crystalline film (2700 mF/g vs. 96
mF/g).
Thesis (Ph.D.)--Tufts University, 2016.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Matthew Panzer.
Committee: Maria Flytzani-Stephanopoulos, Elizabeth Podlaha-Murphy, and Elena Rybak-Akimova.
Keywords: Chemical engineering, Materials Science, and Energy.read less - ID:
- tm70n6654
- Component ID:
- tufts:21339
- To Cite:
- TARC Citation Guide EndNote
