ME 498 Fundamentals of Modern Photovoltaics. Discussion 33: Thin Film Photovoltaics 7 November 2016

Transcription

1 ME 498 Fundamentals of Modern Photovoltaics Discussion 33: Thin Film Photovoltaics 7 November 2016

2 Summary of the Most Common Commercial (or close) PV Technologies Today 4 PV Market Wafer- Based Monocrystalline Silicon % of market Typically use CZ grown wafers Key players: SunPower, REC, Sanyo Commercial Cell Efficiencies: 18-22% Thin Film 15% of market Key players: First Solar (CdTe), Energy Conversion Devices (a- Si) CdTe, a- Si becoming established CIGS: emerging (start- ups: Nanosolar, Heliovolt) Commercial Cell Efficiencies: 6-11% Polycrystalline Silicon % of market Typically use Bridgman wafers Key players: Q- Cells, Suntech, REC Commercial Cell Efficiencies: 16-18% Source: PV News, v 29 (2010)

3 Not All is Well in the Thin Films World

4 Readings on Thin Film Solar Luque and Hegedus, Handbook of Photovoltaic Science & Engineering: Chapter 11: Crystalline Silicon Thin Films Chapter 12: Amorphous Silicon Thin Films Chapter 13: Cu(InGa)Se 2 Thin Films Chapter 14: CdTe Thin Films Poortmans and Arkhipov, Thin Film Solar Cells: FabricaHon, CharacterizaHon, and ApplicaHons.

5 Lots of Opcons for Thin Film PV Mulcjunccon System, Spectrolab Amorphous Silicon, Inverter- China Poly- Crystalline Silicon Helmholtz Insctute, Germany CIGS, Greentech Media Dye- Sensiczed Solar Cells, Src: celcias.com Organic, Src: ScienceDaily.com CdTe, UTEP

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7 Materials properces to make thin films Direct band gap, or use light- trapping Absorpcon High Charge separacon Efficient Charge transport Can be slow(er)

8 Thin- Film Photovoltaics 10 n p 100µm Tradiconal n p µm Thin film - Use less material - Lighter - Flexible - Can use more expensive materials! - Can put places we wouldn t otherwise - Easier to install

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10 General Consideracons Advantages Thin layers (few µm) less material used potencal for cost savings Potencal for low- temperature processing potencal cost savings Potencal for roll- to- roll deposicon good for BIPV, ease of installacon Disadvantages Lower cell and module efficiencies potencally larger costs Elements used: some are scarce Low temperature processing film stoichiometry, uniformity issues konarka PolyIC

11 Thin Film Modules 13 Building Integrated Photovoltaics (BIPV) Modules serve both as construccon materials and power generators Thin Film Modules Are typically encapsulated into flexible arrays for ease of installacon. BaOery Park Wintergarden, NYC CIS Tower, Manchester England Parisienne Hotel

12 Market Share in 2010 Source: Green Tech Media, PV News NB: Between 2009 and 2010, thin film PV market share actually declined from 17% to 13%.

13 Forecascng Future Role of Thin Film PV Arguments For Increased Market Share: Cheaper modules (only mamers if we can get the efficiency up) The success of First Solar demonstrates ability to scale produccon Less reliance on silicon, it s availability Shorter produccon process than wafer Si more throughput Ideal for BIPV, one of the fastest growing segments of PV market Solar for vehicles is a huge opportunity for thin film PV Arguments Against Increased Market Share: Silicon wafer technologies are too established, mature The recent increase in thin film market share was due to lack of polysilicon supply; now those silicon supply issues have been addressed Efficiencies are never going to get good enough Encapsulacon and degradacon issues too big Source: Thin Film Solar Technology Market Shares, Strategies, and Forecasts, Worldwide, 2011 to 2017

14 Lots of Opcons for Thin Film PV Mulcjunccon System, Spectrolab Amorphous Silicon, Inverter- China Poly- Crystalline Silicon Helmholtz Insctute, Germany CIGS, Greentech Media Dye- Sensiczed Solar Cells, Src: celcias.com Organic, Src: ScienceDaily.com CdTe, UTEP

15 Recall: Shockley- Quiesser Limit for Cell Efficiency For small band gaps, efficiency is limited by thermalizacon losses For large band gaps, efficiency is limited by losses due to non- absorpcon of the solar spectrum The tradeoff between thermalizacon and non- absorpcon losses results in the opcmal band gap of a semiconductor of approximately ev, and a maximum theoreccal efficiency of close to 40% for concentrated light, and around 31% for standard light.

16 Beacng the Shockley- Queisser Limit The SQ limit applies to single semiconductor junccons A number of ideas have been suggested for beacng the limit Mulcjunccon solar cells Mulcband ( Intermediate Band ) solar cells Hot Carrier Cells Mulcple Excitacon Generacon Only one of these has been demonstrated to work so far: mulljunclon solar cells

17 Lots of Opcons for Thin Film PV Mulcjunccon System, Spectrolab Amorphous Silicon, Inverter- China Poly- Crystalline Silicon Helmholtz Insctute, Germany CIGS, Greentech Media Dye- Sensiczed Solar Cells, Src: celcias.com Organic, Src: ScienceDaily.com CdTe, UTEP

18 What is a mulljunclon solar cell? Mulcple semiconductors, with different band gaps, are used simultaneously Each semiconductor absorbs a certain part of the solar spectrum, allowing for high overall absorpcon and low thermalizacon losses

19 TheoreLcal Efficiency Limits E g,1 E g,2 E g,3 Dividing up the spectrum allows us to absorb a large porcon of the solar spectrum while minimizing thermalizacon losses e - h + E g,1 e - h + E g,2 e- h + E g,3

20 TheoreLcal Efficiency Limits ATer MarH and Arroyo, # of Layers TheoreLcal Efficiency 1 31% % % % % % Infinite 86.8% (black body limit)

21 Two typical approaches to making mulljunclon solar cell. Spectrum SpliUng Use of dichroic mirrors to split incoming spectrum and guide light to separate semiconductors. Stacked Cells Stacked semiconductors integrated in series on a single substrate SC 1 SC 2 SC 3 Advantages: - No need to integrate all semiconductors together on a single substrate Disadvantages: - Integracng metal conneccons to the individual cells is complicated Advantages: - Conneccons and integracon more straighqorward Disadvantages: - Need to idencfy semiconductors that can readily be grown on top of each other - Manufacturing is slow, expensive; requires epitaxial growth

22 Spectrum SpliUng First mulcjunccon system constructed in 1978 was a spectrum splirng system (Moon et. al.) A long- cme record- holding solar cell was a spectrum splirng mulcjunccon solar cell (University of Delaware, 2007), efficiency of 43% ATer: Science 317 p583 (2007)

23 Stacked Cells GaInP 2 (gap ev) GaAs (gap 1.42 ev) Ge (gap 0.66 ev) Most commercially available mulcjunccon cells exhibit the monolithic two terminal stack geometry Typically the semiconductors are GaInP 2, GaAs, and Ge The GaInP layer absorbs highest energy photons, and transmits the remainder to the GaAs layer The GaAs layer absorbs the medium energy photons, and transmits whats les to the Ge Germanium layers absorbs the lowest energy photons Although these semiconductors are not opcmal based on band gap alone, they can be integrated together using expitaxial growth processes

24 Stacked Cells Here is what a typical cell structure looks like Everything is integrated onto a germanium substrate The individual semiconductors are connected in series using tunnel junccons

25 Stacked Cells The actual structure is even a limle more complicated, since each semiconductor should exhibit a pn junccon The payback for the effort is ultra- high efficiencies, for example the IQE curve for the stack is:

26 What is a tunnel junclon? We can think of a stacked mulcjunccon cell as a bunch of individual solar cells that are lined up in series. e e h h h e J J P N P N P N V = V 1 + V 2 + V 3 Thus, current through each cell should be the same (the thickness of each semiconductor is selected for current matching), while voltages add. Thanks to the tunnel junccon, there is no voltage drop between the pn junccons (electrons and holes recombine without energy loss).

27 What is a tunnel junclon? A tunnel junccon is a heavily doped PN junccon that introduces an ohmic contact between the P- terminal of one cell and the N- terminal of another Effeccvely allows for electrons and holes to recombine without a difference in Fermi levels This is achieved by: - Fermi energy above the conduccon band edge on the N side - Fermi energy below the valence band edge on the P side - And a small, narrow intrinsic region that carriers can tunnel through Note that without the tunnel junchon, the p- type region of the InGaP layer would be directly connected to the n- type region of the GaAs layer (a pn junchon with opposite direchon to the others would appear), and the photovoltage would be zero. In a tunnel junchon, the carriers can cross the junchon without loss of voltage.

28 Stacked Cells Sept : Former record- holding solar cell from Fraunhofer- ISE and Soitec at 44.7% efficiency, using four III- V based subcells

29 Where are mulljunclon cells used? 1. In spaaaace kw mulcjunccon solar array on the Internaconal Space Stacon (Aug 2008), 29% efficiency cells Note: The figure of merit in space is not energy conversion efficiency or dollars per wam. Rather, it is power generated/kg.

30 Where are mulljunclon cells used? 2. Terrestrial applicahons in conjunchon with concentrators These are osen used in tracking concentrator systems, because for these systems the PV cells themselves are a small part of the overall cost. Goal: the improved efficiency under concentracon should make up for the extra cost of the cells. Fresnel lens concentrate sunlight 500x to cells that are a few mm 2 ; average module efficiencies around 30%

31 How ConcentraLon Improves Efficiency Recall the expression for the JV curve of a pn- junccon solar cell. ( " J = J sc J o exp qv % * # $ kt & ' ) 1 + -, Serng the current to zero, we can solve for V OC. V OC = kt q ln! J $ sc # +1& " J o % η = P out P in = (FF)(V 0 OC)(J 0 SC ) φ 0 η = P out P in = Concentracon Factor C (FF)(V 0 OC + kt q lnc)(cj 0 SC ) Cφ 0

32 MulLjuncLon solar cells are widely commercially available Example: ZTJ Photovoltaic Cell from Emcore Corporacon (Albuquerque, NM)

33 MulLjuncLon solar cells are widely commercially available Example: ZTJ Photovoltaic Cell from Emcore Corporacon (Albuquerque, NM)

34 MulLjuncLon solar cells are widely commercially available Example: Spectrolab (Boeing)

35 MulLjuncLon solar cells are widely commercially available Example: SolFocus CPV Systems Use 1/1000 th the solar cell materials in tradiconal PV Cell efficiencies near 40% Module efficiencies over 25% dual- axis tracking on arrays refleccve opccs give 650x concentracon

36 MulLjuncLon solar cells are widely commercially available Example: Solar Junccon currently the world s most efficient solar cell at 43.5% Size : (5.5 x 5.5) mm2 produces 20W under 400x concentracon

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42 MulLjuncLon solar cell layers are typically grown by MOCVD (metal- organic chemical vapor deposilon) Compound semiconductors are grown epitaxially by thermochemical reaccons Organic compounds or metal- organic compounds introduced to chamber Pyrolysis of the chemical elements occurs at the surface/substrate