Solar Photovoltaics & Energy Systems

Transcription

1 Solar Photovoltaics & Energy Systems Lecture 3. Crystalline Semiconductor Based Solar Cells ChE-600 Wolfgang Tress, March

2 Photovoltaic Solar Energy Conversion 2

3 Outline Recap: Thermodynamics of semiconductor-based energy conversion From chemical to electrical energy pn and metal-semiconductor junctions Silicon solar cell technology Photovoltaic installations 3

4 Semiconductor vs. Heat Engine spectral irradiance [W/(m 2 nm)] energy [ev] O 3 O 2 H 2 O H 2 O AM 1.5g blackbody at 5800 K 4 AM 1.5g photon σt S flux η rad = σ T S 4 T A 4 Sun and absorber: Black body a E = e 3E = 1 H 2 O H 2 O CO [nm] x = 1 T A photon flux density [photons/(s m 2 nm)] 4 T S 4 η C = T A T 0 T A = 1 T 0 T A surroundings sun absorber Carnot engine 4

5 Semiconductor vs. Heat Engine η rad = σ T S 4 T A 4 σt S 4 = 1 T 4 A 4 T S Sun and absorber: Black body a E = e E = 1 η C = T A T 0 T A = 1 T 0 T A sun absorber Carnot engine Absorber is at elevated temperature surroundings 5

6 Semiconductor vs. Heat Engine Band gap: a E = e E = 0 ; E < E g 1 ; E E g E E C E V CB - E g + VB E F n E F p E F n = N C exp E C E F n k B T Quasi-Fermi level splitting chemical energy potential p = N V exp E p F E V k B T Charge flow electrical current Absorber is at surrounding temperature Radiation is converted into chemical energy 6

7 Shockley-Queisser Limit efficiency [%] Photovoltage Photocurrent J ph η SC = P el = J ph V ph P sun P sun V ph hν bandgap E g [ev] E C CB n E F E n C E F E g p E F E V VB thermalization hν transmission CB E g p E F E V VB 7

8 Losses in Single-Bandgap Material thermalization transmission Used photon energy 8

9 Shockley-Queisser Limit efficiency [%] Photovoltage (b) absorbed photon flux density j [m -2 s -1 ] 4 Photocurrent J ph η SC = P el = J ph V ph P sun bandgap E g [ev] 0 x P 0 P sun V ph bandgap E [ev] g short-circuit current density J [ma/cm 2 ] J ph,max = e a E Φ AM1,5g E de = e E g Φ AM1,5g E de V ph,max =? 9

10 Detailed Balance Limit Boltzmann approx. φ BB E, T exp ev k B T In the dark: thermal equilibrium between solar cell and surroundings: B 0 E = a E φ BB T 0 10

11 Detailed Balance Limit Boltzmann approx. φ BB E, T exp ev k B T In the dark: thermal equilibrium between solar cell and surroundings: B 0 E = a E φ BB T 0 Balance under light: J ph V = e a E Φ AM1,5g E de + e B 0 E de e B 0 E de exp J ph,max J em,0 ev k B T Open circuit (J ph V = 0): V oc,rad = k BT e ln J ph J em,

12 Detailed Balance Limit Boltzmann approx. φ BB E, T exp ev k B T In the dark: thermal equilibrium between solar cell and surroundings: B 0 E = a E φ BB T 0 Balance under light: J ph V = e a E Φ AM1,5g E de + e B 0 E de e B 0 E de exp J ph,max J em,0 ev k B T Open circuit (J ph V = 0): V oc,rad = k BT e ln J ph J em,

13 JV-Curve of Ideal Solar Cell with E g =1.6 ev P el = J ph V ph V ph Open-circuit voltage V oc Maximum power point short circuit current density J sc Fill factor η SC,max = P el,max = max J ph V ph V ph = J scv oc FF P sun P sun P sun 13

14 JV-Curve of Ideal Solar Cell with E g =1.6 ev P el = J ph V ph V ph Open-circuit voltage V oc dark Maximum power point short circuit current density J sc Diode behavior 14

15 From Chemical to Electrical Energy E C CB E V E g VB E F n E F p + - collection + - Absorber - + Requirement: Charge selective contacts 15

16 From Chemical to Electrical Energy Ideal solar cell structure (Würfel) CB E C - E g n E F p E F E V VB + Energy barrier selective contact 16

17 Metal Contacts E E Vac CB work function E C E g E F E F E V VB semiconductor metal When put in contact in equilibrium Fermi levels will align 17

18 Doping amitngroup.blogspot.com 18

19 Doping Fermi Level Shift E C CB E C CB E F E C CB E F E V E g VB E V E g VB E V E g VB E F 19

20 pn Junction J V = J 0 exp ev k B T 1 20

21 Metal Semiconductor Junctions I E E Vac WF metal > WF n-sc in contact E Vac E C CB E F work function (WF) E g E F E C E F E V VB E V n-semiconductor metal n-semiconductor metal Schottky Contact Diode 21

22 Metal Semiconductor Junctions II E E Vac WF metal < WF n-sc E Vac in contact CB work function (WF) E C E F E F E C E F E g E V E V VB n-semiconductor metal n-semiconductor metal Ohmic Contact 22

23 Metal Semiconductor Junctions III E E Vac WF metal > WF n+ SC in contact E Vac E C CB E F work function (WF) E g E F E C E V E F VB E V n + -semiconductor metal n + -semiconductor metal Tunneling junction Ohmic contact, not selective 23

24 Diode under Illumination Superposition of dark and photocurrent (J ph V = J sc ): J V = J 0 exp ev k B T 1 J sc V oc = k BT e ln J sc J J ph V = J ph,max + J em,0 exp ev k B T 1 V oc,rad = k BT e ln J ph J em,

25 Equivalent Circuit Diode Current source controlled by light Resistances account for: R s : Voltage loss due to charge transport resistance R p : Current loss due to shunt paths 25

26 Sketch of a Solar Cell 26

27 Realization: Which material? Silicon: Why? - E g is suitable - Doping is possible - Mostly used in microelectronics - Native oxide as passivation layer - Abundant and non-toxic efficiency [%] bandgap E g [ev] 27

28 Crystalline Silicon Solar Cells inhabitat.com 28

29 Basic Structure of Silicon Solar Cell 120 µm 2-3 mm 400 µm 13 µm 29

30 From Sand to Solar: The Ingot SiO 2 Reduction of SiO 2 with carbon in an electric arc furnace Purification of Silicon (Siemens Process) metallurgical grade silicon (> 98 % pure) Si + 3 HCl -> HSiCl 3 + H 2 Czochralski (CZ) Alternative: FZ crystalline ingot Electronic grade poly silicon (impurities < cm -3 ) 30

31 From Sand to Solar: The Ingot SiO 2 Reduction of SiO 2 with carbon in an electric arc furnace Purification of Silicon (Siemens Process) metallurgical grade silicon (> 98 % pure) Si + 3 HCl -> HSiCl 3 + H 2 Czochralski (CZ) Alternative: FZ crystalline ingot Electronic grade poly silicon (impurities < cm -3 ) 31

32 From Sand to Solar: The Wafer Production steps lapping, polishing sawing wafer 32

33 From Sand to Solar: The Solar Cell KOH Newport Corp./Spectra-Physics 33

34 Multicrystalline Solar Cell Martin Green 34

35 From Sand to Solar: The Module 35

36 Modules SolarWorld 36

37 From Sand to Solar: The PV Installation wikipeda 37

38 Different Architectures 38

39 Different Architectures: PERC 39

40 Different Architectures: PERC 40

41 Different Architectures: Bifacial Module 41

42 Different Architectures Laser-doped selective contact Interdigitated back contact design (IBC) percent-on-mass-produced-solar-cell---whatnext_ /#axzz47sduypzsa 42

43 History and Record Lab Efficiencies 1839: Becquerel: discovery of PV effect 1883: Fritts: Cu/Se/Au module, η=1% 1905: Einstein: Photoeffect 1954: Bell Labs: Si solar cell, η=6% 26.6% 25.8% 43

44 Technical Breakthroughs Reduce Costs History, verschiedene typen, NREL Chart 44

45 PV Installations Worldwide ISE Freiburg,

46 PV Installations Worldwide ISE Freiburg,

47 PV Installations Worldwide ISE Freiburg,

48 PV Production ISE Freiburg,

49 Price 49

50 Costs and Prices: The Learning Curve 50

51 Average Price PV Rooftop System In D ISE Freiburg,

52 Incentives in Germany ISE Freiburg,

53 Electricity Costs: Feed in Tariffs in D ISE Freiburg,

54 Energy Payback Time (EPBT) ISE Freiburg,

55 Energy Payback Time (EPBT) ISE Freiburg,

56 Example: Rooftop Installation Irradiation: 1200 kwh/m 2 a 20% module 240 kwh/m 2 a 0.2 kw p /m 2 20 m kwh/a 4 kw p (20 modules) Investment costs 5200 Euro Household electricity cost 1600 Euro/a 4 kw continuous power kwh/a 14% capacity factor volatile (storage capacity and costs) 56

57 Summary Crystalline Silicon Solar Cells 35 efficiency [%] J sc = 41.8 ma cm -2, V oc = 0.74 V, FF = bandgap E g [ev] 57