Selenium and Tellurium
G&E Ch. 16 and Supplements
Batteries vs. Electrolytic Cells
An electrochemical cell that releases energy is called a galvanic cell (battery, power
generation, ΔG<0, anode(−), cathode(+)). An electrochemical cell that requires the
input of energy is called an electrolytic cell (electrolysis, power consumption, Δ G>0,
anode (+), cathode (−)).
Tellus and Selene
Tellurium was the first of these two elements to be
discovered. It was observed in the ores mined in the
gold districts of Trannsylvania and was referred to as
“metallum problematicum” or “aurum paradoxum”
because it didn’t behave like antimony, which is what
they thought it was. Tellurium is named after the word
tellus, meaning earth, and has only one allotrope.
Selenium was isolated 35 years later and was named
after selene, meaning moon, because it behaved a lot
like tellurium. Selenium has several known allotropes,
many of which have the common 8-membered ring
structures we identified with sulfur. The most stable
and common form is gray selenium (shown bottom,
left).
Marie Curie and Po
Polonium was discovered by Marie Curie in the
act of processing huge amounts of Uranium
ore and following its separation by
radioactivity.
Together with the parallel isolation of radium,
Marie Curie won her 2nd Nobel Prize in 1911.
The discovery of Po was the first time invisible
quantities of an element were identified,
separated, and investigated based solely on its
radioactivity – but by no means the last!
Po has no stable isotopes.
Se and Te from Cu Slime
Se and Te are comparatively rare. Se occurs
in crustal rock at 0.05 ppm (similar to Ag and
Hg) and Te is 0.002 ppm (similar to Au and
Ir).
Both occur in small quantities as the pure
element, together with sulfur, or as metal
chalcogenides (in pure or partially oxidized
form).
The main source of Se and Te is the anode
slime deposited during the electrolytic
refining of copper – this mud also contains
commercial quantities of Ag, Au, and the Pt
metals. The direct recovery from mineral
tellurides and selenides is not generally
economically viable because of their rarity.
Se Ruby Glass and Xerography
Selenium is produced on a scale of 2000 metric
tons per year. 35% of it is incorporated into
glass as a decolorizing agent (1-2 kg/ metric
ton of glass). At higher levels, the selenium
causes a pink to red color. The vibrant red
associated with Se ruby glass comes from
Cd(S/Se) incorporation.
Xerography was historically another huge
application of Se, which is a photoconductive
semiconductor material.
Photovoltaic Effect
In fact the photovoltaic effect was discovered in Se
by Becquerel in 1839. The first paper on the topic
appeared in Nature in 1873: “Effect of Light on
Selenium during the Passage of an Electric Current”.
An actual working solid state photovoltaic cell was
constructed in 1883 by Fritts, who coated selenium
with a thin gold layer to form the junctions – device
had about 1% efficiency.
Modern solar cells use a p-n junction design to
control the flow of electrons in a single direction. A
single layer of a semiconducting material would
never expect to show great efficiency.
Band Structure of Selenium
Gray selenium has a direct band gap of 1.74 eV and is
composed of chains of Se atoms as shown above. Se is
always intrinsically p-type doped and the prevailing
hypothesis is that this arises due to acceptor states
generated from dangling Se bonds because the chains are
not infinitely long (same principle as in CdSe QDs).
Semiconducting Trends in Group 16
Eg
O2
α-S
Gray Se
Te
~ 5 eV
2.6 eV
1.74 eV
0.33 eV
The bandgap decreases, the lattice constant increases, the bond strength decreases, and
the orbital overlap decreases as we proceed from O to S to Se to Te.
This trend is also observed in group 14 – C (~5 eV), Si (1.1 eV), Ge (0.7 eV), and Sn (~0
eV).
One way to understand this is that the valence band is composed of bonding electrons
and the conduction band will contain antibonding electrons (as in HOMO/LUMO in
molecules). Short, strong bonds are difficult to break, meaning its difficult to populate
antibonding orbitals, meaning there is a large bandgap. Long, weak bonds are easy to
break, meaning its easy to populate antibonding orbitals, meaning there is a small
bandgap.
Tellurium and CdTe
Production of Te is on a much
smaller scale, ~350 metric tons per
year. 70% of this goes into steel
production (makes steel more
machineable).
One emerging use of tellurium is in
CdTe thin-film solar cells, as
commercialized by First Solar.
CdTe has a direct bandgap of 1.49 eV,
making it ideally suited to absorbing
sunlight (well matched to the solar
spectrum maximum).
Thin-Film CdTe Solar Cells
CdTe solar cells are constructed with a ptype layer of CdTe and an n-type layer of
CdS.
In 1981 Kodak introduced a CVD strategy
known as close space sublimation to
fabricate the CdTe layers. Monosolar and
AMETEK introduced electrodeposition.
This led to 10% efficiency cells.
The next breakthrough was the
introduction of a transparent conducting
oxide between the substrate and the CdS
to help the movement of current across
the top of the cell (SnO2).
Thinning the CdS layer allowed more light
to reach the CdTe, allowing 15% efficiency,
and the potential for commercialization.
First Solar now produces half a gigawatt
annually.
Issues with CdTe
Two major challenges to wide-scale CdTe use are the availability of tellurium and the
toxicity of cadmium.
Other Thin-Film Technologies: CIGS
Red (Cu), Yellow (Se), Blue (In/Ga)
CIGS thin-film solar cells address issues of earth
abundance and toxicity.
CuInxGa1-xSe2; x = 0-1; Eg = 1 (CuInSe2) to 1.7 eV
(CuGaSe2)
Record efficiencies of 20% have been measured –
the highest of any thin film technology.
Cell construction mirrors the CdTe architecture
closely.
Flexible CIGS and Global Solar
Polycrystalline CIGS films are typically made by co-sputtering (bombarding solid target with
high energy particles to eject material to be deposited on substrate) Cu, Ga, and In onto a
substrate and then annealing (heating) in Se vapor to generate the solid solution CIGS
structure.
CIGS cells printed on flexible substrates are being commercialized by Global Solar.
Main Group (13-16) Survey: Recap
Boron: Electron deficiency, boranes, borides, superconductivity, boron nitride, dielectrics
Al, Ga, In: conductivity and properties of metals, phonons/plasmons, band theory basics,
thermal conduction, lasers, metalloid clusters, superatoms
C: fullerenes, nanotubes, and graphene – properties and uses
Si: intro to semiconductors and doping, p-n junction, transistors
Ge, Sn, Pb: band engineering, lead acid batteries
N, P, As: bonding in group 15, P4, spherical aromaticity, binary semiconductors, doping
(amphoteric, isoelectronic), excitons, solar cells, LEDs
O: simplifying band structure concepts, DSSCs, non-stoichiometry, ion conduction, excited
state catalysis, semiconductor to metal transition in transition metal oxides, Mott
insulators, mixed valency
S, Se, Te: allotropy, sodium/sulfur batteries and grid storage, metal sulfides (pyrite, galena,
molybdenite), photovoltaic effect, CdTe and CIGS thin film solar
Last Topic: Nanoscale Inorganics
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