Germanium nanocrystal solar cells.
2010-08
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Germanium nanocrystal solar cells.
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2010-08
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Abstract
Greenhouse gas concentrations in the atmosphere are approaching historically
unprecedented levels from burning fossil fuels to meet the ever-increasing world energy
demand. A rapid transition to clean energy sources is necessary to avoid the potentially
catastrophic consequences of global warming. The sun provides more than enough
energy to power the world, and solar cells that convert sunlight to electricity are
commercially available. However, the high cost and low efficiency of current solar cells
prevent their widespread implementation, and grid parity is not anticipated to be reached
for at least 15 years without breakthrough technologies.
Semiconductor nanocrystals (NCs) show promise for cheap multi-junction
photovoltaic devices. To compete with photovoltaic materials that are currently
commercially available, NCs need to be inexpensively cast into dense thin films with
bulk-like electrical mobilities and absorption spectra that can be tuned by altering the NC
size. The Group II-VI and IV-VI NC communities have had some success in achieving
this goal by drying and then chemically treating colloidal particles, but the more abundant and less toxic Group IV NCs have proven more challenging. This thesis reports thin films
of plasma-synthesized Ge NCs deposited using three different techniques, and
preliminary solar cells based on these films.
Germanium tetrachloride is dissociated in the presence of hydrogen in a
nonthermal plasma to nucleate Ge NCs. Transmission electron microscopy and X-ray
diffraction indicate that the particles are nearly monodisperse (standard deviations of 10- 15% the mean particle diameter) and the mean diameter can be tuned from 4-15 nm by
changing the residence time of the Ge NCs in the plasma.
In the first deposition scheme, a Ge NC colloid is formed by reacting
nanocrystalline powder with 1-dodecene and dispersing the functionalized NCs in a
solvent. Films are then formed on substrates by drop-casting the colloid and allowing it to
dry. As-deposited films are electrically insulating due to the long hydrocarbon molecules
separating neighboring particles; however, mass spectrometry shows that annealing
treatments successfully decompose these molecules. After annealing at 250 °C, Ge NC
films exhibit conductivities as large as 10-6 S/cm.
In the second film deposition scheme, a Ge NC colloid is formed by dispersing
Ge NCs in select solvents without further surface modification. While these “bare” NCs
quickly agglomerate and flocculate in nearly all non-polar solvents, they remain stable in
benzonitrile and 1,2-dichlorobenzene, among others. Thin-film field-effect transistors have been fabricated by spinning Ge NC colloids onto substrates and the films have been
subjected to various annealing procedures. The devices show n-type, p-type, or ambipolar
behavior depending on the annealing conditions, with Ge NC films annealed at 300 °C
exhibiting electron saturation mobilities greater than 10-2 cm2/Vs and on-to-off ratios of
104.
The final film deposition scheme involves the impaction of Ge NCs onto
substrates downstream of the synthesis plasma via acceleration of the NCs through an
orifice. This technique produces highly uniform films with densities greater than 50% of
the density of bulk Ge. By varying the size of the Ge NCs, we have measured films with band gaps ranging from the bulk value of 0.7 eV to over 1.1 eV for films of 4 nm Ge
NCs.
Having deposited dense thin films with tunable band gaps and respectable
mobilities, we have begun fabricating bilayer solar cells consisting of heterojunctions
between Ge NC films and P3HT, Si NCs, or Si wafers. Preliminary devices exhibit opencircuit
voltages and short-circuit currents as large as 0.3 V and 4 mA/cm2, respectively.
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University of Minnesota Ph.D. dissertation. September 2010. Major: Mechanical Engineering. Advisor: Uwe R. Kortshagen. 1 computer file (PDF); xiii, 205 pages, appendix p. 203-205.
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Holman, Zachary Charles. (2010). Germanium nanocrystal solar cells.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/116701.
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