Rare-earth doped fluorides for silicon solar cell efficiency enhancement Diana Serrano Garcia A.Braud, P.Camy, J-L.Doualan, A.Benayad, V.Menard, R.Moncorge CIMAP, University of Caen, France

Summary Limitations of solar energy conversion Downconversion: Quantum cutting Experimental results and models Conclusion Quantum cutting with Rare-earth doped fluorides

Photovoltaics (PV) - Conversion of solar energy into electricity Silicon as the most famous semiconductor for solar cell development* Si doped n Si doped p +²+² - - Electron-hole pairs creation *R. Singh; Journal of Nanophotonics, 3 (2009)

I) Silicon limitations for solar spectrum conversion Short wavelenghts BC BV Si Gap =1,12eV E - Energy loss due to carrier thermalization - Absorption of photons h >1,12 eV Energy lost

CB VB 1,12eV E - Silicon transparent for h <1,12 eV. Low efficiency for Silicon solar cells (a-Si 9%,c-Si 25%*) Silicon limitations for solar spectrum conversion Long wavelenghts *M.Green, Progress in Photovoltaics : Research and Applications 17, 320 (2009)

Stacking of semiconductors with different bandgaps SC 2SC 1SC 3 Decreasing bandgap Semicond. 1 Semicond. 2 Semicond. 3 Solution I: multi junction solar cells The larger bandgap at the surface of the device BUT: expensive and difficult to produce Aerospace Applications High efficiency (World record 40% * ) *M.Green, Progress in Photovoltaics : Research and Applications 17, 320 (2009)

Si 2 e - Quantum cutting Solution II: Frequency Conversion hνhν hνhν hνhν Efficiency enhancement Quantum Cutting Low cost production *T. Trupke, M. Green; Journal of applied physics 92, 3 (2002) 1668 Ideal converter  36,6%* c-Si Solar Cell Si Quantum cutting layer

Downconversion: Quantum cutting by energy transfer Donor Acceptor E D0D0 A0A0 D1D1 D2D2 A1A1 A0A0 A1A1 D 2  D 1 and A 0  A 1 D 1  D 0 and A 0  A 1 Donor excitation D 0  D 2 Acceptor relaxation A 1  A 0 2 photons emission Energy transfer 1 Energy transfer 2 hv/2 hv Getting 2 photons from 1 photon? 1 2 From 1 photon we get 2 photons

Quantum cutting with rare-earth doped fluorides: ►Why Yb? E~10000 cm -1 ~1,2eV ~ Si Gap Yb 3+ Pr 3+ Yb 3+ Donor Acceptor Pr 3+ /Yb 3+ system E( 3 P 0 – 1 G 4 )~E( 2 F 5/2 – 2 F 7/2 ) E( 1 G 4 – 3 H 4 )~E( 2 F 5/2 – 2 F 7/2 ) Acceptor E ►Why Pr ? Resonant Energy Transfer 1 2

Host matrix: Fluorides - Low phonon energy High fluorescence quantum yield - Large transparency range Differences: RE 3+ doping Short distance between ions Very efficient energy transfer KY 3 F 10 Uniform distribution of dopants CaF 2 Formation of complexes (clusters)

CaF 2 :Pr/Yb First energy transfer Pr  Yb Intensity ratio as a function of Yb concentration Pr excitation Increase of first transfer Pr ( 3 P 0 )  Yb ( 2 F 5/2 ) with Yb concentration Pr 3+ Donor Yb 3+ Acceptor Transfer Ytterbium emission under Pr ( 3 P 0 ) excitation Energy transfer from Pr to Yb (%Pr constant at 0.5%) Experimental Results

3 P 0 fluorescence decay in CaF 2 and KY 3 F 10 Decrease of  ( 3 P 0 ) with Yb concentration for both hosts Experimental Results Energy transfer rate : Energy transfer efficiency :

Experimental Results 97% eficiency with 4% Yb in CaF 2 96% eficiency with 20% Yb in KY 3 F 10 Transfer in CaF 2 more efficient than transfer in KY 3 F 10 Efficiency (%)

Modeling I: Classical model 3 energy transfer - Possible interaction between all Pr and Yb ions - Uniform ion distribution

6 possible states 3H43H4 2 F 5/2 3P03P0 1G41G4 2 F 7/2 Pr 3+ Yb 3+ Limited interaction within the pair P 1 +P 2 +P 3 +P 4 +P 5 +P 6 =1 Each state has a probability P i Modeling II: Pair model

Conclusion Possible QC with CaF 2 :Pr/Yb and KY 3 F 10 : Pr/Yb Transfer in CaF 2 more efficient! Implementation in Si solar cells Very efficient Pr  Yb first transfer (97%) Conclusions New models (Three or more ions model??)

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