MAXIMISING INCIDENT POWER SOLAR CELLS. Piyush Keshri. IIT Kanpur Mentor: Prof. S.S.K. Iyer,IIT Kanpur

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1 MAXIMISING INCIDENT POWER ABSORPTION IN MULTIJUNCTION SOLAR CELLS Piyush Keshri 4 th year, B.Tech (EE) IIT Kanpur Mentor: Prof. S.S.K. Iyer,IIT Kanpur 1 st Indo US Research Academy, 7-11 th Oct 08, Pune

2 ACKNOWLEDGEMENT I specially, Thank Prof. Gautam Biswas and all the Organizers involved in 1 st Indo US Research Academy 7-11 th Oct 08 who have made it possible. My Sincere Gratitude goes towards all the Professors, Students & Staff who have whole hl heartedly devoted d their hi precious time towards the success of this academy and have shown their dedication & commitment towards the research. 2

3 OUTLINE Motivation Overview of various Solar cell efficiencies. Background Solar Spectrum. Efficiency of Solar cells and the parameters involved. Multi junction Solar Cells. Energy Losses and Useful Energy Harvested. Simulation/ Experimental Plan Step Absorption Spectrum Single junction Cells. 2 & 3 junctions Cells. Mismatch Losses. Box Type Absorption Spectrum Single junction Cells. 2 junctions Cells. Summary of Results Summary Future Outlook 3

4 OUTLINE o Motivation o Overview of various Solar cell efficiencies. Background Solar Spectrum Efficiency of Solar cells and the parameters involved. Simulation/ Experimental Plan Step Absorption Spectrum Single junction Cells 2 & 3 junctions Cells. Mismatch Losses Box Type Absorption Spectrum Single junction Cells. 2 junctions Cells. Summary of Results Summary Future Outlook 4

5 MOTIVATION Energy crisis Renewable source of Energy Green Energy Non-polluting & Environment friendly Useful in remote areas Quiet Reliable Source: Institute of Materials Research & Engineering 5

6 OVERVIEW OF VARIOUS SOLAR CELLS Multijunctions highest efficiency Source: National Renewable Energy Laboratory 6

7 OUTLINE Motivation Overview of various Solar cell efficiencies. Background Solar Spectrum Efficiency of Solar cells and the parameters involved. Multi junction Solar Cells. Energy Losses and Useful Energy Harvested. Simulation/ Experimental Plan Step Absorption Spectrum Single junction Cells 2 & 3 junctions Cells. Mismatch Losses Box Type Absorption Spectrum Single junction Cells. 2 junctions Cells. Summary of Results Summary Future Outlook 7

8 SOLAR SPECTRUM Source: PHD Thesis- K. Petritsch,July July 00. AM1.5G spectrum is standard spectrum for solar cell measurements. 8

9 EFFICIENCY OF SOLAR CELLS The photocurrent of a solar cell depends on: No. of created charges collected at the electrodes i.e. Fraction of photons absorbed (η abs ). Fraction of electron-hole pairs dissociated(η diss ). Fraction of (separated) charges reaching the electrodes (η out ). The overall photocurrent efficiency(η j ) η j = η abs * η diss * η out We are focused on the absorption efficiency 9

10 PHOTON ABSORPTION EFFICIENCY (η abs ) Photon Absorption Efficiency (η abs ) of a matl. depends upon: Material Absorption Spectrum- Absorption coefficient of the material ( α ). Width of the material layer (d). Intensity observed at depth d of material, I=I o e -αd. To increase η abs use Multi junction Cells 10

11 MULTI JUNCTION SOLAR CELLS Single Junction Two Junction Three Junction hν hν hν I 1 V 1 V 1 I 1 V 1 I 1 Load Load I 2 V 2 Load I 2 V 2 I Load I 3 V 3 I Load I Load V Load = V 1 V I Load = I Load = V 1 + V 2 1 I = Load min{i 1,I 2 } V Load = V 1 + V 2 +V 3 Efficiency E eff1 E I Load min{i 1,I 2,I 3 } eff2 E eff3 E eff1 << E eff2 << E 11 eff3

12 PHOTONS ABSORBED & ENERGY LOSSES e - E 1 E o =hν E o E c E c E g E g E v E v E o =E g E 2 E o -E g = E 1 + E 2 e - hole Optical Loss = E 0, if E o < E g Thermal Loss = E o E g, if E o E g 12

13 USEFUL ENERGY HARVESTED FROM SOLAR SPECTRUM Fraction of Harvested Energy( η HE ) from Solar spectrum depends upon: Fraction of photons absorbed bed ( η ph ) η ph Photon Absorption Efficiency (η abs ) Fraction of useful energy obtained from absorbed photons (η ue ) η ue = Useful energy available after photo absorption Total energy absorbed from photons absorption η ue = E o -E g E o η HE = η ph * η ue 13

14 OUTLINE Motivation Overview of various Solar cell efficiencies. Background Solar Spectrum Efficiency of Solar cells and the parameters involved. Simulation/ Experimental Plan Step Absorption Spectrum Single junction Cells 2 & 3 junctions Cells. Mismatch Losses Box Type Absorption Spectrum Single junction Cells. 2 junctions Cells. Summary of Results Summary Future Outlook 14

15 SIMULATION/EXPERIMENTAL PLAN Step Absorption Spectrum e.g. Inorganic Semiconductors Single Junction Cells 2 junction cells 3 junction cells α Box Type Absorption Spectrum e.g. Organic Solar cells Single Junction Box Type 2 junction Box type λ o Wavelength, λ 15

16 SINGLE JUNCTION CELLS hν α λ o Wavelength, λ I 1 V 1 Load I Load 16

17 SINGLE JUNCTION SOLAR CELLS Efficiency,η = 47.63% Eg 0 = ev λ = 1197nm 17

18 DOUBLE JUNCTION CELLS For Simulation: α 1 = α 2 hν Load I 1 I 2 V 1 V 2 I Load V Load = V 1 + V 2 E E eff2 18

19 ENERGY FOR 2 JN. SOLAR CELLS Efficiency,η = 66.85% Eg 1 = ev/780nm Eg 2 = ev/1700nm 19

20 TRIPLE JUNCTION CELLS hν For Simulation: α 1 = α 2 = α 3 I 1 V 1 Load I 2 V 2 I 3 V 3 I Load V Load = V 1 + V 2 I Load min{i 1,I 2 } E eff2 20

21 TRIPLE JUNCTION SOLAR CELLS Maximized Efficiency of 3 junction cell corresponds to: Efficiency,η = 75.79% Eg 0 = ev / 680nm Eg 1 = ev / 980nm Eg 2 = ev / 1700nm 21

22 SUMMARY (MAXIMIZED EFFICIENCIES OF VARIOUS MULTIJUNCTION CELLS) Parameters Single junction 2 junction 3 junction Solar Solar Cells Solar Cells Cells Efficiency (in %) 47.63% 66.85% 75.79% Eg 1 / λ ev/ 1197nm ev/ 780nm ev / 680nm Eg 2 / λ ev/ ev / 980nm 1700nm Eg 3 / λ ev / 1700nm 22

23 MISMATCH LOSSES hν For Simulation: α 1 = α 2 Load I 1 V 1 λ Wavelength, λ o I 2 V 2 I Load V Load = V 1 + V 2 No. of Photons absorbed, k 1 Considering Mismatch Losses (k 1 k 2 ) o Leads to the Mismatch Loss in the Material. oi Load min{i 1,I 2 } No. of Photons absorbed, k 2 Mismatch Loss= ( k 1 -k 2 ) * E g (1/2) 23

24 ENERGY FOR 2 JN. SOLAR CELLS Photons absorbed, k 1 Photons absorbed, k 2 Including MISMATCH Losses Efficiency,η η = 66.20% Eg 0 = ev/860nm Eg 1 = ev/1692nm K 1 /k 2 = Excluding MISMATCH Losses Efficiency,η = 66.85% Eg 1 = ev/780nm Eg 2 = ev/1700nm 2 24

25 ENERGY FOR 3 JN. SOLAR CELLS hν Including MISMATCH Losses Efficiency,η η = 73.58% Eg 1 = ev/680nm Eg 2 = ev/1031nm Eg 3 = ev/1632nm K 1 /k 2 = K 2 /K 3 = I 1 I 2 I 3 V 1 V 2 V 3 No. of Photons absorbed, k 1 Excluding MISMATCH Losses Efficiency,η = 75.79% Eg 1 = ev/680nm Eg 2 = ev/980nm Eg 3 = ev/1700nm No. of Photons absorbed, k 2 No. of Photons absorbed, k 3 25

26 COMPARISON OF EFFICIENCIES ( WITH/WITHOUT CONSIDERING MISMATCH LOSSES) Complete absorption by 2 jn. Solar Cells Parameters Model does not include Model include MISMATCH Losses MISMATCH Losses Efficiency (in %) 66.85% 66.20% Eg 1 / λ ev/ 780nm ev/ 860nm Eg 2 / λ ev/ 1700nm ev/ 1692nm Current Ratio, n Complete absorption by 3 jn. Solar Cells Parameters Model does not include MISMATCH Losses Efficiency (in %) 75.79% 79% 73.58% Model include MISMATCH Losses Eg 1 / λ ev/ 680nm ev/ 680nm Eg 2 / λ ev/ 980nm ev/ 1031nm Eg 3 / λ ev/ 1700nm ev/ 1632nm 26

27 REALISTIC ABSORPTION SPECTRUM Materials do not absorb for complete energy spectrum (> E g ). Inorganic materials have typically wide absorption spectrum. However, organic materials have narrow absorption spectrum. Hence, box type absorption spectrum( ) has been taken as 1 st approximation for further simulations. 27

28 SINGLE JUNCTION CELLS (BOX TYPE) hν I 1 V 1 Load I Load 28

29 1 JUNCTION SOLAR CELLS (BOX TYPE) Efficiency,η = 24.98% Eg 0 = ev λ = 435nm 635nm 29

30 DOUBLE JUNCTION CELLS (BOX TYPE) hν For Simulation: α 1 = α 2 I 1 V Load I 2 V 2 I Load V Load = V 1 + V

31 2 JUNCTION SOLAR CELLS (BOX TYPE) Efficiency = 43.70% λ 2 = nm λ 1 = nm Eg 2 = ev Eg 1 = ev Current Ratio,n =

32 SUMMARY (BOX TYPE ABSORPTION SPECTRUM) Parameters Single junction Solar Cells 2 junction Solar Cells Efficiency (in %) 24.98% 47.00% Eg 1 / λ 1 λ 1 = 200nm ev/ ev/ nm nm Eg 2 / λ 2 λ 2 = 200nm ev/ nm Maximized i values for complete absorption spectrum Parameters Single junction Solar Cells Efficiency (in %) 47.63% 66.85% 2 junction Solar Cells Eg 1 / λ ev/ 1197nm ev/ 780nm Eg 2 / λ ev/ 1700nm 32

33 OUTLINE Motivation Overview of various Solar cell efficiencies. Background Solar Spectrum Efficiency of Solar cells and the parameters involved. Simulation/ Experimental Plan Step Absorption Spectrum Single junction Cells 2 & 3 junctions Cells. Mismatch Losses Box Type Absorption Spectrum Single junction Cells. 2 junctions Cells. Summary of Results Summary Future Outlook 33

34 SUMMARY Very high efficiency achieved in Multijunction cells. Complete Absorption (e.g. Inorganic Solar Cells) Single junction Cell 2 junction Cell 3 junction Cell.. (47.63%) (66.22%) (73.58%) With Mismatch (66.85%) (75.79%) Fraction of Useful Harvested Energy, η HE Box Type Absorption Spectrum ( ) (e.g. Organic Materials) Single junction Cell 2 junction Cell 3 junction Cell.. (24.98%) (43.60%).. Practically, 40.8% efficiency has been achieved by 3 junction Solar cell. 34

35 FUTURE OUTLOOK Using Gaussian absorption Spectrum Choice of Appropriate Materials Include other loss parameters Thickness To be adjusted d to have same current Lattice Mismatch more realistic. optimum value of η HE. more accurate efficiency. Exploring Structure Blend, Bilayer, Tandem further 35

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37 BAND GAP OF DIFFERENT MATERIALS 37