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MEEN 489/689: Entrepreneurship & Nanomaterials for Energy Applications

Lecture 10 Nanomaterials in Emerging Energy Sectors & Applications

James Donnell, Andreas Polycarpou,Tanil Ozkan, Haejune Kim

Fall 2015

10/1/2015

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MEEN 489/689 Source : Hessen Nanotech 2008

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Photovoltaics

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https://www.youtube.com/watch?v=JBtEckh3L9Q

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www.youtube.com/watch?v=K76r41jaGJg

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Solar Cell – Operating Principles

A photon from the Sun generates an electron-hole pair in a semiconductor. The electron is pulled to the front, the hole to the back of the solar cell, thereby creating a battery.

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Energy diagram of a solar cell

The electron and hole are pulled apart by the electric field between the p- and n-doped regions. It is critical not to lose electrons and holes on their way out. Crystalline semiconductors are good at that, but expensive.

Energy

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Efficiency limits: 30% for a single junction (Shockley-Queisser limit) 70% for multiple junctions

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Electrons, Holes, and Excitons

A photon excites an electron across the band gap of a semiconductor.

That leaves a hole among the occupied levels in the valence band and an electron among the unoccupied levels in the conduction band.

Electron and hole are attracted electrically and can form an exciton (similar to a hydrogen atom). In organic semiconductors it takes a significant amount of energy to break them apart into free carriers.

PhotonElectron

Hole

Energy

Band Gap

exciton

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Thin film solar cells

• Compound semiconductors: CdTe, CIGS = Cu(InGa)Se• Less material, less energy by low temperature processing• Print solar cells like newspaper (roll-to-roll)

Solar cell printed on plastic

Nanosolar (San Jose, Berlin)

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Nanostructured solar cells

Use nanostructured “fractal” structures to minimize the path of excitons, electrons, holes, to the nearest electrode. Avoid losses.

Better design: Regular array of nanorods

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ZnO nanorods as electrode

Growth time increases from left to right.

(a)-(c) side view (500 nm bar), (d)-(f) top view (100 nm bar). Baxter et al., Nanotechnology 17, S304 (2006) and Appl. Phys. Lett. 86, 053114 (2005).

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Nanorods coated with nanocrystals

CsSe nanodots (3 nm) replace the dye. Absorption spectrum tunable via the size of the dot. More robust against radiation

damage.Leschkies et al., Nano Letters 7, 1793 (2007).

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Solar thermalConvert solar energy to steam, then to electricity

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Thermoelectrics

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Efficiency of Thermoelectric Materials(figure of merit ZT)

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What materials have high ZT?

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Nanoscale effects with thermoelectric conversion

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Nanoscale effects with thermoelectric conversion

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Thermoelectric waste heat recovery systems

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Thermoelectric waste heat recovery systems

https://gsuryalss.wordpress.com/tag/thermoelectric-material/