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Use of Surface Photovoltage Spectroscopy to Measure Built-in Voltage, Space Charge Layer Width, and Effective Band Gap in CdSe Quantum Dot Films
journal contribution
posted on 2016-08-09, 00:00 authored by Jing Zhao, Benjamin A. Nail, Michael A. Holmes, Frank E. OsterlohSurface photovoltage spectroscopy
(SPS) was used to study the photochemistry
of mercaptoethanol-ligated CdSe quantum dot (2.0–4.2 nm diameter)
films on indium doped tin oxide (ITO) in the absence of an external
bias or electrolyte. The n-type films generate negative voltages under
super band gap illumination (0.1–0.5 mW cm–2) by majority carrier injection into the ITO substrate. The photovoltage
onset energies track the optical band gaps of the samples and are
assigned as effective band gaps of the films. The photovoltage values
(−125 to −750 mV) vary with quantum dot sizes and are
modulated by the built-in potential of the CdSe–ITO Schottky
type contacts. Deviations from the ideal Schottky model are attributed
to Fermi level pinning in states approximately 1.1 V negative of the
ITO conduction band edge. Positive photovoltage signals of +80 to
+125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal
films are attributed to electron–hole (polaron) pairs that
are polarized by a space charge layer at the CdSe–ITO boundary.
The space charge layer is 70–150 nm wide, based on thickness-dependent
photovoltage measurements. The ability of SPS to directly measure
built-in voltages, space charge layer thickness, sub-band gap states,
and effective band gaps in drop-cast quantum dot films aids the understanding
of photochemical charge transport in quantum dot solar cells.
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ITO conduction band edgemajority carrier injectionSPSSpace Charge Layer WidthSurface Photovoltage SpectroscopyPositive photovoltage signalsspace charge layer thicknessquantum dot sizesEffective Band Gapphotovoltage onset energies trackspace charge layerfilmnmCdSe Quantum Dot Films Surface photovoltage spectroscopyband gaps
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