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

O2 α-S Gray Se Te

Eg ~ 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 p- type 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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