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Using molecules to capture solar energy
Timothy Schmidt
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The solar spectrum 1000 W/m2
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Light is made of photons
Planck constant Speed of light Energy of photon Wavelength
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The solar spectrum High energy photons Low energy photons
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Electrons absorb photons, get energy
unoccupied energy levels E electrons diamond
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Electrons absorb photons, get energy
unoccupied energy levels electrons silicon
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The solar spectrum 1000 W/m2
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single threshold solar cells
unoccupied energy levels ~32% max V electrons
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Shockley-Queisser limit
35% SQ limit 6000K sun limit Si 25% GaAs efficiency CdTe CdS 15% Ge a-Si:H 5% 0.5 1.0 1.5 2.0 2.5 Band gap /eV
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the 3 generations of solar cell
Thermodynamic Limit Single threshold (Shockley-Queisser limit)
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routes to circumventing Shockley-Queisser limit
using the low energy photons upconversion intermediate band cells using all the photon’s energy downconversion multiple exciton generation singlet Fission hot carrier cells
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Electrons have “spin” Ground state S0 Excited state S1
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Electrons have “spin” Ground state S0 Excited state S1
Excited state T1
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Molecular photophysics!
fluorescence (fast, allowed) absorption
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Molecular photophysics!
Intersystem crossing absorption Phosphorescence (slow, forbidden)
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what is wrong with this picture?
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triplet-triplet annihilation (TTA)
sensitizer sensitizer S1 S1 S1 S1 ISC ISC T1 TTA T1 T1 T1 S0 S0 S0 S0 emitter emitter
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Up-conversion cell Up-conversion unit Limiting efficiency of an
Upconversion cell is about 50% cell Up-conversion unit
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tetrakisquinoxalinoporphyrin palladium (II)
select a sensitizer tetrakisquinoxalinoporphyrin palladium (II)
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ultrafast photoluminescence
The compound only fluoresces while in S1 (only 10ps!) Measured fluorescence lifetime = 5ps Calculated fluorescence lifetime = 20ns Fluorescence yield = 0.025% Triplet yield may be %!
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100% fluorescence yield used by others for TTA upconversion
select an emitter 100% fluorescence yield used by others for TTA upconversion
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TTA emission kinetics
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prompt fluorescence vs upconversion
laser laser upconversion fluorescence
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TTA yields mixture ref pure rubrene ref 33% 25%
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Nothing wrong with this picture!
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or this…
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ummmm?
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Dr Nicholas J. Ekins-Daukes Now at Imperial College
Dr Raphael “femtofingers” Clady Dr Nicholas J. Ekins-Daukes (NED) Now at Imperial College our man in London Murad Tayebjee Dennis Cheng
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$$ acknowledgements $$
University of Sydney University of NSW ARC LIEF ARC DP GCEP
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Application to a-Si realistic a-Si cell 1mm up-conversion unit
100nm, 200nm or 300nm 1mm up-conversion unit
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Application to a-Si Modelled porphyrin absorption spectra
Spectra transmitted through cell
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Application to a-Si +34% Bare a-Si Jsc Voc FF Eff 100nm 84.2 0.832
54.8% 3.8% 200nm 114 0.869 57.4% 5.7% 300nm 131 0.884 58.1% 6.8% 667 Porphyrin Jsc Voc FF Eff 100nm 88.8 0.838 55.3% 4.1% 200nm 120 0.874 57.6% 6.1% 300nm 136 0.889 58.2% 7.1% Monster porphyrin Jsc Voc FF Eff 100nm 105 0.859 56.8% 5.1% 200nm 141 0.893 58.4% 7.6% 300nm 160 0.906 58.8% 8.8% +34% 780 Porphyrin Jsc Voc FF Eff 100nm 89.4 0.839 55.4% 4.2% 200nm 122 0.876 57.7% 6.2% 300nm 139 0.891 58.3% 7.4%
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