Thermodynamics and Solar Energy By Edmund Blackadder in Edmund Blackadder's Diary Fri Oct 31, 2014 at 09:55:05 PM EST Tags: solar energy, delft, thermodynamics, shockley-queisser limit, heat losses, recombination losses, up converters, down converters, quantum dots (all tags)
	From the Delft Solar Energy MOOC, Video 6.1 "Third Generation PV Technologies": The Shockley-Queisser limit as discussed in week 3, is a kind of thermodynamic approach to estimate the maximum performance of a single junction solar cell. The AM1.5 spectrum is incident on a solar cell. We don't allow the solar cell to increase in temperature. This means that all energy in the incoming AM1.5 spectrum can escape the system of the solar cell by either the current density generated or by the radiative recombination of charge carriers. As a result the maximum efficiency which can be achieved is around 33% in the band gap range from 1.0 eV up to 1.8 eV as indicated by the black area in the shown graph. Now we are going to look at a very fundamental limitation of the photovoltaic effect as we have discussed it so far. What physical principles are limiting the extent of photogeneration?

ADVERTISEMENT Sponsor: rusty This space intentionally left blank ...because it's waiting for your ad. So why are you still reading this? Come on, get going. Read the story, and then get an ad. Alright stop it. I'm not going to say anything else. Now you're just being silly. STOP LOOKING AT ME! I'm done! comments (24) active | buy ad ADVERTISEMENT [...] Tackling these fundamental limitations means that we can develop PV concepts with conversion efficiencies that can surpass the Shockley-Queisser limit. [...] As a result, the highest conversion efficiencies of 44% have been achieved. This exceeds the Shockley-Queisser limit by more than 10%. --- Professor Arno Smets goes on to describe how single photons incident upon a solar cell can result in the generation of more than one electron-hole pair. A high-energy photon can produce two, or more, electron-hole pairs, thus raising the External Quantum Efficiency over 100%. IBM's solar concentrator reports 80% overall efficiency. Combined with some of the theoretical gains from up or down conversion of photons (so that each photon creates more than one electron-hole pair in the solar cell) can create overall efficiencies that exceed the power of the solar input. Was Solyndra working on advanced solar technologies such as up-conversion and down-conversion? We should be investing in such research. Market signals might prefer to allocate all resources to cheap panels that get 20% efficiency; government should create the money to research the technology that will get us greater than 100% efficiency. The reason thermodynamic-derived limits can be overcome is that the new technologies rely on nanoscale exploitation of quantum effects. If charge carriers can be multiplied, i.e. a single photon can produce more than one charge carrier, why can't the output of the solar cell exceed the input? Do the charge carriers care where they came from when they create a current? If you can increase the EQE over 1, can't you increase the current density without limit? From one of the Solar Energy MOOC's questions: The short-circuit current density can be calculated from the EQE and the photon flux by: Jsc=q∫λ2λ1ΦAM1.5(λ)EQE(λ)dλ The bounds of the first integral, and the "sc" and "AM1.5" subscripts, are mangled by the copy-paste (such is the brittleness of mathematics), but basically if you increase the EQE, can't you increase the Jsc (short-circuit current density), even if the photon flux remains the same?

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Delft Solar Energy MOOC Shockley-Q ueisser limit External Quantum Efficiency over 100% IBM's solar concentrator quantum effects Edmund Blackadder's Diary