Review of Photovoltaic Solar Cells. Op5cs for Energy Course 11/5/13 Liz Lund

Transcription

1 Review of Photovoltaic Solar Cells Op5cs for Energy Course 11/5/13 Liz Lund

2 Outline Solar electricity produc5on How Photovoltaics (PV) work Types of PV Emerging technologies

3 Solar Electricity Produc5on hip://

4 Renewables in US ~10% Renewable Energy in US Solar is a small frac5on 0.1%

5 Global vs. US Renewable Energy Same trends for PV global and in US Wind and solar are fastest growing renewables in US Wind 17% Solar 86% hip://

6 Solar Growth is Exponen5al Solar electricity genera5on grown by a factor of 9 (2000 to 2011)

7 Who s Making It?

8 Where is it Installed?

9 Where in US? States with extensive incen5ves lead PV deployment

10 Solar Prices $.50/waI 99% cost drop since 1977 Solar becoming compe55ve with fossil fuel generated energy Swanson s Law: PV costs drop 20% for every doubling of shipped volume hip:// 25/solar- energy- this- is- what- a- disrup5ve- technology- looks- like

11 How Photovoltaics Work

12 Photovoltaic Solar Cells: Light In, Electricity Out Typical CZTS(Se) Solar Cell Stack 3 Necessary Func-ons: 1. Generate charge carriers from light semiconductor + sunlight 2. Separate charge carriers p- n junc5on (diode) Front Contact Transparent Conduc5ng Oxide 2 Ni/Al nm ZnO 50 nm i- ZnO 50 nm CdS 3. Collect charge carriers to do electrical work carriers reach front and back contacts Absorber Layer 1 1 µm CZTS(Se) 2 Back Contact 3 Mo Glass

13 Solar Cell, Module, Array

14 n- and p- type semiconductors Silicon Si Phosphorous P Boron B Undoped Si Laice n- Type extra electrons P Doped Si p- Type extra holes B Doped Si Extra electron Missing electron Hole

15 Making a Diode n- Type extra electrons p- Type extra holes 1. n- and p- type materials separate are neutral 2. Place materials together Free electrons and holes diffuse and recombine Leave behind charged nuclei (localized) Create charged interface Charge build- up repels complete diffusion At equilibrium, establish space charge region 4. Built in voltage poten5al Diode behavior of pn junc5on Space Charge Region

16 The Band Gap: Energy barrier to genera5ng charge carriers Individual Atoms Discrete Energy States Atoms in a Crystal Energy Bands Band Diagram Energy p s Conduc5on Band Energy Gap Valence Band E g E C E V Valence band (of states): Lower energy state Electrons are localized around atomic nuclei Formed from bonding orbitals in crystal hip://pveduca5on.org/pvcdrom/pn- junc5on/band- gap Conduc-on band (of states): Higher energy state Electrons are delocalized Free to move throughout crystal Formed from an5- bonding orbitals in crystal

17 Genera5ng Charge Carriers- Absorp5on of Light hip://pveduca5on.org/pvcdrom/pn- junc5on/absorp5on- of- light Semiconductor only absorbs light with energy equal or greater than the band gap (lower wavelength) Ideal band gap: 1.1 ev Lower band gaps give lower voltage (current losses)

18 Absorp5on Coefficient- PV Material Property hip:// junc5on/absorp5on- depth

19 Schockley- Quiesser Limit Passing the Limit: Use mul5ple materials Have mul5ple pn junc5ons Concentrate sunlight Harvest heat also Use quantum dots to harvest excess photon energy η = 33.7% theore5cal maximum for single pn junc5on cell with no concentra5on PloIed with actual record efficiencies of different materials What limits this? (33% example) 47% to heat. 18% not absorbed. 2% local recombina5on of holes and electrons.

20 Device Parameters hip:// cell- opera5on/ light- generated- current JV Curve Jsc Voc Resistances Series- contact and current movement Shunt- manufacturing defects Fill Factor Max Powerpoint Efficiency IQE EQE hip:// cell- opera5on/ iv- curve

21 JV Curve Series Resistance Slope of Line Shunt Resistance Slope of Line Maximum power point

22 Types of PV

23 hip:// NEED MORE BULLET POINTS

24 Classes of PV Thin Film 1-2 um thick dirty deposi5on methods Op5cally thin- use op5cs to compensate More defect tolerant CIGS, CdTe, CZTS, a- Si Deposit on flexible substrates Thick 100 s of um thick Typically c- Si Must be very pure for carrier diffusion Monocrystalline Con5nuous crystal laice Expensive produc5on Higher efficiencies in cells Polycrystalline Many grains (um- cm size) More grain boundaries=more defects Most thin films Projected Cell Produc5on Market Shares 2011 hip://

25 Comparing PV Technologies Energy Payback Time 10% CIGS, 4% OPV 1 yr c- Si 2.7 yrs Commercial vs. Record % Efficiencies Technology Super Moncrystalline Silicon Monocrystalline Silicon Mulitcrystalline Silicon CdTe CIGS µc- Si, a- Si a- Si (3- j) a- Si (1- j) Organic Record Cell n/a n/a

26 Emerging PV

27 hip:// NEED MORE BULLET POINTS

28 Perovskite Solar Cells hip://news.sciencemag.org/physics/2013/09/flat- out- major- advance- emerging- solar- cell- technology

29 Dye- Sensi5zed Solar Cells Organic/Inorganic Hybrid Cheap to make Life5me? 3 different materials: 1. Light absorp5on Sensi5zing (organic) dye 2. Electron transport Wide band- gap (inorganic) semiconductor 3. Hole transport/electron regenera5on Electrolyte Hardin et al., Nature Photonics (2012)

30 Mul5- junc5on PV Thin top layers absorb shorter wavelengths Longer wavelengths in boiom cell Tunnel diode allows current flow but electrical field isola5on of top and boiom cells Expensive to make Crystalline laice and currents must match- really difficult and constrains materials! Currently only used in space World Record: 43% with 5- tandem cell UNSW

31 Concentra5ng PV (CPV) Focus light using mirrors, lenses Use small area, high efficiency cells Addi-onal requirements Non- imaging op5cs- focus on radia5ve rather than image transfer Trackers- 2D east to west each day, north to south each season Cooling system hip://newenergynews.blogspot.com/2009/07/concentra5ng- photovoltaics.html hip://amonix.com/content/low- energy- produc5on- costs- 0

32 CPV cont d Advantages Very efficient, less space Low water usage, advantage over tradi5onal solar thermal concentrators (4 vs. 850 gal/ MWhr) Lower cost of materials, majority of infrastructure is not PV Well- suited for large power plant arrays Nearing grid parity now! Disadvantages Requires direct sunlight New technology- late to the funding game Trackers are expensive Suited for desert climates especially

33 Earth- Abundant Thin Films Abundance in Earth s Crust World Annual Produc-on (tons) Phillip Dale We use these currently We use these currently Large- scale PV should use earth- abundant and industrially- available elements to avoid boilenecks commodity elements

34 PV materials cost- Red Herring Typical Module = few hundred $ $$ First Solar: Module shipping box cost higher than CdTe material cost Glass for panel much more expensive than 1 cm 3 semiconductor film (>100x) Absolute semiconductor materials costs insignificant!

35 Winning Argument- TW Scale- Up Poten5al Sulfur: byproduct of Cu, Zn ores & fossil fuel produc5on In or Te produc9on limits: CIGSe & CdTe: ~150 GW P /yr More cm 3 /kg from light elements Many sulfides have right E Gap Fthenakis, Ren. Sus. Ener. Rev (2009)

36 CZTS- CIGS with no Indium Kesterite structure (like chalcopyrite) E Gap = 1.5 ev, direct gap First PV inves5ga5ons in late 1990 s Katagiri Friedlmeier η = 11% for evaporated cells* (IBM) Chen APL (2009) *Wang APL (2010)

37 Summary Photovoltaic solar cells needs 3 things Genera5on of charge carriers- semiconductor with band gap Separa5on of carriers- pn junc5on Collec5on of current to do electrical work Variety of technologies that get the job done C- Si s5ll dominate the market It s geing cheaper faster