Review
=== Quantum dot solar cells [1] === Background: Hot carrier and impact ionization Hot carrier @ solar cells: Electrons/holes receives photons with energy much higher than their band gap will carry kinetic energy create effective temperature much higher than lattice temperature (3000K of carrier @ 300K of lattice etc.). Primary loss for this:
- Heat through scattering (KN is transferred into band vibration?)
- Photon emission (electrons occupied holes to let out photons?). Approach to stop this loss:
- Stacked cascaded multiple p-n junctions to match multiple band of solar spectrum. This reduce carrier relaxation via photon emission. Efficiency increase to 66%.
- Reduce thermal relaxation – utilize hot carriers before their relaxation.
- Enhanced photo-voltage: being extracted before cooling; Requirement: transportation of hot carriers faster compare to the cool down rate – which should be related to the material itself.
- Enhanced photo-current: create second or more electron-hoe pair(s) by impact ionization (Auger effect: one electron hit a band create one e-h pair. What will happen to this electron?); Requirement: impact ionization rate faster compare to the cool down rate & e-h pair transport rate faster than cool rate?.
- Quantization confinement will dramatically reduce hot carrier’s cooling rates. (So this confinement will also reduce the cool down rate in Auger process which is not desired?)
- What about some part of solar spectrum is absorbed to stimulate electron from En to En+1 or higher and the left spectrum will cause these electrons to emission from En+1 to En-1 to let out more photons with same lambda. Then one layer will be used to specifically to absorb these electrons.
Hot electrons and hot holes cool down rate are different because:
- Mass difference (Which one is faster?)
- Hot carrier cool rate depends on density of photo-generated hot carriers (How this makes diff?).
Predicted way for QDs: hot electrons with slowed cooling rate in QD --- Auger process, fast cool down hole due to mass & closer quantized space --- photo-currents. Bottleneck due to fast hole trap at the surface will slow cooling rate (7 ps at CdSe, InP QDs compare to 0.3 ps cooling rate without hole trap). This will prevent Auger process.
Reference
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