Electricity from the Sun (photovoltaics)

Transcription

1 Electricity from the Sun (photovoltaics) 0.4 TW US Electricity Consumption square kilometers of solar cells could produce all the electricity for the US. But they are still too costly.

2 The required area of solar cells 1 kw/m 2 (Incident solar power) 1/4 (Fraction of useful daylight) 0.16 (Efficiency of a solar cell 16%) m 2 ( km 2 ) = kw (Electric power generation in the US)

3 The Efficiency of Solar Cells Keeps Growing Slowly Can that be accelerated? Where is the energy lost? How fast are the carriers lost? Where do they get trapped?

4 Price Reduction with Increasing Volume % Learning Curve : Module price decreases by 20% for every doubling of cumulative production Silicon Wafer Technologies % Slow x 1/3 power law, not an exponential like Moore s Law

5 Efficiency Demands a Price Goal 1 $/W x 0.4 TW = = 0.4 Trillion $ Low end High end (US electric power consumption) Crabtree and Lewis, Physics Today 60, March 2007, p. 37

6 Semiconductor Solar Cells CBM E F ev open E F VBM Contact Contact + Electron and hole are pulled apart by the electric field between the p- and n-doped regions. Voltage builds up until bands flatten. Avoid losing carriers on their way out. Single crystal semiconductors are good at that but expensive. Polycrystalline thin film materials lose carriers at grain boundaries. Passivate!

7 Molecular Solar Cells LUMO E F ev open E F HOMO Contact Acceptor Dye Donor Contact + Control the sequence of energy levels to separate electron and hole. A large drop in energy facilitates carrier separation, but also reduces the voltage (and energy) output. 4 molecular levels as control parameters (only 2 for a semiconductor).

8 Dye-sensitized Solar Cells Grätzel, Nature 414, 338 (2001) and J. Phys. Chem. C 112 (2008) Split water

9 Efficiency Limits Semiconductors: 30% 70% single junction (Shockley-Queisser limit) multiple junctions Molecules: 20% dye-sensitized, single junction Snaith, Adv. Funct. Mater. 19, 1 (2009) Track down the losses systematically and eliminate them one by one.

10 The Shockley-Queisser Limit Lose excess photon energy beyond the band gap. Photons below the band gap are not absorbed.

11 Nanostructured Solar Cells Better design: Regular array of nanorods Use nanostructured fractal structures to minimize the path of excitons, electrons, holes, to the nearest electrode. Avoid losses.

12 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, (2005).

13 Nanorods Coated with Nanocrystals CdSe nanodots (3 nm) replace the dye. Absorption spectrum tunable by quantum confinement (dot size). Robust against radiation damage. Leschkies et al., Nano Letters 7, 1793 (2007).

14 Polymer Solar Cells Polymer chain with a diffusing polaron (electron + distorted polymer), surrounded by fullerene molecules as acceptors. A fullerene can accept up to six electrons in its LUMO (nanotubes also).

15 How Does Nature Do it? Next slide Plants convert solar energy into chemical energy (e.g. sugar). Less than 2% of the solar energy gets converted. But the initial part of the conversion is very efficient.

16 Light-harvesting proteins Chlorophyll Next slide

17 The Oxygen Evolving Complex 4 Mn + 1 Ca Instead of rare Pt (5d), Rh (4d), nature uses plentiful Mn (3d), Fe (3d), Ca(3d) as catalysts. Can we do that in artificial photosynthesis? What does it take? (3D cage?)