1. photo-=light, Volta = voltage vb cb hνhν S*/S + e-e- S/S + I 3 -/I - e-e- cathodeanode * -0.5 V 0 V 0.5 V 1.0 V Semiconductors and Photovoltaics Neal M. Abrams nmabrams@esf.edu

2. Three Questions about Solar Electricity How much solar electricity could we make? How do solar cells work? What materials are cells made of?

3. Looking Towards Nature “Trying to do what Mother Nature has been doing for thousands of years…only better” - Dr. Raymond Orbach Can we do this? How? Source:Berkeley U

4. Solar Irradiance

5. A (short) historical perspective Photoelectric effect discovered in 1839 by Bequerel – All metals produce a voltage when subjected to light of the correct wavelength (energy) Schokley reports the basis of p-n junctions, 1950 Not pursued until 1954 by Bell Labs – Very expensive, 6% efficiency – Reemerged in late 70’s-early 80’s with gas crisis and history repeats itself… NASA launches 1 kW array, 1966 Science drives society and vice-versa

6. How Much Electricity Do We Make Today? 1.8x10 12 Watts (continuously) – 6x10 9 persons – 300 Watts/person – 3 100W light bulbs per person U.S. – 25% of total – 1,500 Watts/person – 15 100W light bulbs per person – 36 kWhr/day/person http://www.eia.doe.gov/oiaf/ieo/electricity.html http://antwrp.gsfc.nasa.gov/apod/ap030426.html.

7. The Solar Dream: There’s lots of sun energy! Area required for all US electricity assuming ~10% efficiency, ~100 x 100 miles 1000 W/m 2 at High Noon 40,000 EJ of solar energy hits the US each year….more than 400x the total energy consumed per year.

8. THE SUN. ONE BIG ENERGY SOURCE

9. Total energy reserves Uncle Harold says: “It is so darned hot here. We just need some of them solar panels!What is the problem?” Aunt Susie says: “Golly it is windy downtown. They just need to install some of those windmills” My dad says: “Boy, you need to figure out how I can fill my car up the garden hose” What is the problem??

10. Average Irradiance 30 Year Average of “full sun” per year: City, State Hours of full sun (kWh/m 2 /yr) San Diego, CA2044 Phoenix, AZ2336 Syracuse, NY1533 Binghamton, NY1496 New York City, NY1642 Seattle, WA1387 Meaning: Syracuse gets 65% of the sun Phoenix gets and therefore needs more PV modules to get the same number of kWh

11. The Nature of Light  wavelength,  energy

12. Solar Irradiance IR vis uv Energy

13. Wavelength distribution 48% of the extraterrestrial irradiance intensity is in the visible range of 380–780 nm Ultraviolet irradiance (< 380 nm) accounts for 6% of the total intensity 45% is given off in the upper infrared. Above 3000 nm the irradiance is energy-negligible.

14. Power Distribution Total ultraviolet irradiance below 380 nm is about 92.6 W/m 2 The visible area has a total power of 660 W/m 2 The remaining IR has a total irradiance of 1367 W/m 2 Where should we be looking?

15. Where the Energy Goes Ozone absorbs solar irradiance almost completely under λ = 290 nm and more weakly to around 700 nm. Water vapor absorbs in the infrared, with pronounced absorption bands at 1.0, 1.4 and 1.8 μm. Above 2.5 μm almost the entire irradiance is absorbed by CO 2 and H 2 O. Some reflection and scattering

16. SHUTTLING ELECTRONS http://www.jimhillmedia.com/mb/images/upload/Van-de-Graaf-Generator-web.jpg

17. Making Electricity from Light: The Photoelectric Effect Light in (frequency ν) Cathode Anode i Vacuum tube Electrons out

18. Einstein’s Explanation of the Photoelectric Effect Energy Gap Blue Photon Electrons in the Cathode Electron Energy Vacuum Red Photon E photon = hν h – Max Planck’s constant

19. The Science Solar energy comes in the form of photons The photovoltaic effect: E = energy h = Planck’s constant = 6.634 x 10 -34 Js c = speed of light = 3 x 10 8 m/s λ = wavelength of light Likewise, E = mc 2  energy, mass, and wavelength are related Atoms are composed of…

20. Bands Energy states transition from discrete to “smeared” progressing from atom  molecule  solid Electrons fill from the bottom  up Highest filled band is the “valence” band Lowest filled is the “conduction” band Ionization boundary atomDiatomic atomTriatomic atom n atoms E1E1 E2E2 E3E3 E∞E∞

21. Conduction Materials can be separated as insulators, semiconductors, or conductors Based on size of VB-CB transition (bandgap) VB E g * > 5.0 eV E g * < 5.0 eV E g * ≈ 0 eV 1 eV = 1240 nm = 1.6 x 10 -19 J CB Forbidden band http://upload.wikimedia.org/wikipedia/commons/3/3f/BandGap-Comparison-withfermi-E.PNG

22. Solar cells: Photons in, Electrons out i Photons in Electrons out Silicon Crystal + - - - - - - - + + + + + + silicon wafers

23. Solar Cells: Photoelectric Effect in a Semiconductor Band Gap Conduction Band Electron Energy Valence Band Green Photon Infrared Photon free electron free hole = Cell Voltage

24. Mechanism of Electron Generation a Goldilocks problem Photons with an energy >E g collide with the material Energy is conserved and electrons are excited from the VB to the CB CB electrons travel through a circuit, powering a device ---------- - - - - Valence Band Conduction Band E g too small E g too large E g just right

25. How do we get the right bandgap? Doping Increases conductivity (lowers VB-CB threshold) by adding electrons or holes Adding electrons: n-type (negative); P, As, Sb Adding holes: p-type (positive); B, Al

26. The p-n junction Electrons diffuse to border of p-type region Holes diffuse to border of n-type region + - + + + + + + + + + + - - - - - - - - - - - Space charge regionp region n region

27. Solar Cell Processes Charge separation Reflection Transmission Recombination Charge separation Reflection n-region - + - + p-region Transmission

28. The Magic in the Panel Photons in sunlight hit the solar panel and are absorbed creating a dc source (a battery) An array of solar panels converts solar energy into usable DC electricity. Inverters convert the DC to 60 Hz AC to feed the grid. n-layer p-layer back contact anti-reflective coating front contact Cover glass e-

29. Anatomy of PV cell n-layer p-layer back contact anti-reflective coating front contact Cover glass e-

30. Electron Generation and Movement

31. FLAVORS OF PHOTOVOLTAICS

32. Photovoltaic types and benefits Silicon – Single crystal silicon (c-Si) – Multicrystalline silicon (mc-Si) – Amorphous silicon (a-Si) Thin-film – Silicon – Cadmium telluride, CdTe – Copper indium gallium diselenide, CIGS Very efficient in diffuse light conditions Dye-sensitized

33. Efficiency: How high? Maximum measured efficiencies under lab conditions as of 2008

34. Limits to Ideal Solar Cell Efficiencies William Shockley nAssumed that recombination is “radiative”

35. Recall: – 37% of sunlight is in the visible, 400-700 nm – 32 % of sunlight is in the low-IR, 700-1200 nm – Silicon does not convert photons to electrons above ~1200 – Most of the energy above the bandgap (low wavelengths) is converted to heat Limits to Solar Cell Efficiency

36. Single Crystal Silicon First commercial solar cell High efficiency (Theoretical 27 %) – Practical ~10-15 % Expensive to produce – Cleanroom environment, ultrahigh purity required = 1.12 eV = 1100 nm Max efficiency = 27 %

37. Silicon – what PV is made of (for now) Silicon is the dominant materials in PV production 26% of the Earth’s crust, second most abundant element by weight (oxygen is #1) Melting point: 1410 C Production of pure PV-grade silicon – not easy!

38. Polycrystalline Silicon Lower cost Lower efficiency – Grain boundaries cause electron- hole recombination Easier to produce Also amenable to thin film or multicrystalline cells + - Grain boundary

39. Czochralski method for obtaining single crystal silicon from polycrystalline Goal: Turn high-purity polycrystalline into high-purity single-crystal Small single-crystal seed is produced Used to grow remaining single crystal silicon http://en.wikipedia.org/wiki/File:Czochralski_Process.svg

40. Thin-film/heterojunctions Direct-bandgap semiconductors (silicon is indirect) Very thin layers of high-efficiency PV material – Silicon cells need to be 87.5x thicker to absorb same amount of light – Lower manufacturing costs, less purity Multiple bandgaps possible (solar lasagna) Issues with junctions between layers (grain boundaries, current limiting) Examples: GaInAs, CuInGaSe 2 (CIGS), CdTe Materials tend to be toxic (or just not good)

41. Dye cells Use molecular dyes as light absorber Inject electrons into a semiconductor Inexpensive, flexible materials Relatively low efficiency (8-12 %) AM1.5 N719 Ruthenium 535-bisTBA (N719)

42. Dye Cells TiO 2 particles (13 nm) * Kalyanasundaram, K.; Grätzel, M. Coord. Chem. Rev. 1998, 77, 347. vb cb hνhν S*/S + e-e- S/S + I 3 -/I - e-e- cathode anode * I3-I3- I-I- Dye e-e- dye dye*/ dye + ~ 10μm e-e- e-e- e-e- e-e- load Pt counter transparent conductive oxide (TCO) 13nm -0.5 V 0 V 0.5 V 1.0 V

43. Maximum Solar Cell Efficiencies National Renewable Energy Lab (NREL)

45. EVALUATING PV CELLS http://www.udel.edu/iec/NREL_IBC_SHJ_IV_Curve.gif

46. Specifications for PV modules Abb. TermMeaning V oc Open circuit voltagemax voltage with no load V max Voltage at maximummax voltage at max power I sc Short circuit currentmax current with no load I max Current at maximummax current at max power P Maximum powerP = I max x V max

47. Revisiting bandgaps Extra energy leaves as heat ---------- - - - - Valence Band Conduction Band E g too small E g just right heat

48. The Heat Problem in Silicon Efficiency (%) Spectral Range White 100 mW/cm 2 Transmitted (visible) 100 mw/cm 2 Reflected (NIR) 100 mw/cm 2 Efficiency19.512.523.7 V oc decreases 2.3 mV/°C for silicon V oc vs. time for a Si cell at ~7x white light concentration, with wavelength- selective mirrors placed in the beam path. V oc losses are lowest using NIR light - less power is thermalized

49. Heat increases level of valence band electrons Decreases band gap; distance between E c and E v is smaller Lowers cell voltage Why might this be? e- EgEg EVEV ECEC heat e- EgEg EVEV ECEC

50. The Heat Problem – a real example Voc decreases 136.8 mV/°C Solar arrays typically put out ~40 DCV Arrays can heat to 65% above ambient – 90 °F day  140 °F panel (60 °C) Voc at 25 °C = 40 V, now 35.2 V – 12% loss in power (assuming no change in current) Take home message: Cooling is very important Passive works well

51. The Heat Problem isn’t a problem… (sometimes) Example: Operating temperature of 10 °F = - 12 °C Then, with Voc decreasing 136.8 mV/°C – a 40 VDC cell could produce 45V, or 13% increase over standard conditions When and where might this happen?

52. Measuring Power Always less than 100 % Some definitions V oc : Open circuit Voltage - Maximum voltage when there is no current draw. I sc : Short circuit Current - Maximum current when there is no voltage draw. ff: fill-factor - The ratio between the maximum power and theoretical maximum (A/B). Indicates ‘quality’ of the cell. i sc V oc B A

53. Measuring Power PV power dependent on: – Incident energy – Type of module – Module temperature – Angle of incidence VB CB VB CB E1E1 E 2 < E 1 ni.com

54. Calculating Efficiency Area1.44 cm 2 Lamp power176.9 mW/cm 2 V oc 0.616 V I sc 45.7 mA UnitCalculation Power max V max x I max = 21.3 mW Current density (J sc )I sc /area Fill factorP max /(I sc x V oc ) = 75.7% Efficiency(J sc x V oc x ff)/Irradiance P max I sc x V oc η = 8.3 %

55. PV panels, ESF Walters Grid

56. Science in Practice ESF PV array on Walters

57. Science into Practice ESF PV array on Walters

58. PV wiring Using backup power with batteries

59. Series vs. Parallel Series – voltage adds, current constant Parallel – current adds, voltage constant High current  resistive lossesHigh voltage, but current limited http://www.sustainableenergy.com/typo3temp/pics/fadf66b23f.jpg

60. Anatomy of a PV installation

61. What does PV depend on? Photovoltaic Power Distance from the sun MaterialAngle/TiltTemperature

62. What is next in PV? New Materials So-called “3 rd generation” photovoltaics – Thin films – Mixed semiconductors – Organic PVs – Multiple bandgaps New Architectures Increase light absorption – Scattering Improve the electron pathway – Inexpensive single crystal materials Nanowire arrays Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications, Nature Materials 9, 239 - 244 (2010)

63. The Ideal Solar Cell A multi-wavelength absorber where all energy is absorbed, none is wasted. EG1EG1 EG2EG2 EG3EG3 EG4EG4 EG5EG5 UVIR vis

64. Helios Unpiloted prototype aircraft for flight at 30 km (18.5 miles)

65. The Sun: A periodic (but predictable) energy source Energy output is not constant This needs to be addressed at a system-wide level

66. The biggest limit on how much useful energy is panel efficiency. – The energy not converted to electricity is about 85%! Cost – think about economy of scale Periodic and intermittent nature of sunlight – Storage – batteries, capacitors, water, hydrogen Electricity only While optimizing the system’s efficiency is important, be aware that it may be less expensive, more aesthetic or more convenient to sacrifice some efficiency. Limits to Real World PV

67. Storing Solar Energy e-e- h+h+ e-e- h+h+ h+h+ e-e- H2H2 O2O2 - + What to do when the sun goes down? Solar thermal capacitors fuel cells batteries direct on grid solar cell

68. PEM * Fuel Cell and Electrolyzer * Polymer electrolyte membrane H H H H H H H O O O O O O O O H H H Hydrogen Oxygen Anode Cathode – + H2H2 H2OH2O Electrolyte H3O+H3O+ H2OH2O e -’ s H2H2 O2O2 - + Your favorite PV …but that is for another time

69. References and Resources US DOE, Energy Efficiency and Renewable Energy (EERE) – http://www.eere.energy.gov/ National Renewable Energy Lab (NREL) – http://www.nrel.gov NY State Energy Research and Development Authority (NYSERDA) – http://www.nyserda.org School Power Naturally – http://www.powernaturally.org/programs/SchoolPowerNaturally/default. asp Handbook of Photovoltaic Science and Engineering, Luque and Hegedus, Eds. V. Quaschning, “Understanding Renewable Energy Systems”, 2005. PVCDROM – http://pvcdrom.pveducation.org/