Advanced Analytical Chemistry Lecture 9. Chem 4631

Transcription

1 Advanced Analytical Chemistry Lecture 9 Chem 4631

2 Solar Cell Research

3 Solar Cell Research

4 Solar Cell Research

5 Solar Cell Research Thin film technologies

6 Candidates for thin-film solar cells: Crystalline Si (c-si), amorphous Si (a-si), low-cost, but low stable efficiency, (6% market share) Cadmium telluride (CdTe), Cd is toxic, covering large surface area with toxic material is not desirable Copper-indium-gallium-selinium (CIGS), Availability of In, 0.08 ppm, cost can go high in case of increased demand CdTe, CdS, C(I, Ga)Se2 (1% market share) Gallium-arsenide ( GaAs), efficiencies above 30%, But very expensive, mainly suited for space application, As is toxic C-Si, abundant raw-material ( ppm) high efficiency, stable efficiency, thin-film technology provides potential to reduce the cost of wafer based cells, very attractive option to explore

7 Thin Film Solar Cell Technologies CdTe, C(I,Ga)Se 2 direct band gap 1.45eV(CdTe), 1.1eV(CIGS) heterojunction with n-cds Solar cells are stable and technology is relatively cost effective material availability Toxicity of Cd

8 Inorganic Thin Film Solar Cells A thin film of semiconductor is deposited by low cost methods. Less material is used. Cells can be flexible and integrated directly into roofing material. CdTe CIGS (CuInGaSe 2 ) amorphous Si Metal P-type CdTe 3~8 um N-type CdS Transparent Conducting Oxide Glass Superstrate 0.1 um 0.05 um ~1000 um

9 Cadmium Telluride Solar Cells glass Direct bandgap, E g =1.45eV High efficiency (Record:16.5%; Industry: 11%) High module production speed Long term stability (20 years) CdS/CdTe

10 CdTe: Industrial Status First Solar is the leader. It takes them 2.5 hours to make a 11% module. Average Manufacturing Cost 2006: $1.40/watt 2007: $1.23/watt 2008: $1.08/watt The energy payback time is 0.8 years.

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12 One reason cells on the roof don t have 16.5 % efficiency The challenge in industry is to implement thin CdS layers without having a pinhole.

13 How much of a problem is the toxicity of Cd? It is probably manageable. First Solar will recycle the panels when the customer is done with them. Is there enough Te? The amount of Te in a cell is (thickness)(density)(mass fraction Te). 2-mm thick cells require (2 mm)(5.7 g/cm 3 )(0.52) = 5.7 g/m 2. The sun gives us 1 kw/m 2, so a 10 % efficient cell produces 100 W m 5.7 g 2 m 2 16 W g Te.

14 The Reserve of Te According to the United States Geologic Survey, the world reserve of Te is 47,000 tons. If all of it was used to make solar cells, we could generate 0.68 TW during peak conditions or about 0.14 TW averaged throughout the day. We want >5 TW. The Reserve is defined as the amount that can be economically recovered.

15 The cost of Te In 2008 Te cost $250/kg. Continuing the example from before, that translates to $/W The cost of Te could go up a lot before affecting the price of solar cells By estimation, First Solar used half of the world s annual production of Te last year. The near future should be interesting.

16 Can we find more Te? Te is a byproduct of Cu mining. As the price goes up, more Cu plants will install equipment to capture the Te. Until recently, no known Te ores were known. We might find a lot more Te when we look for it.

17 Searching for more abundant materials

18 Solar Cells Using Non-Toxic Abundant Materials CuInGaSe2 19.5% efficient thin film architecture Cu2ZnSnS4 (CZTS) 6.7% efficiency (Katagiri et al.) 1.45 ev Eg CZTS has kesterite structure Cu Sn Zn S Raw Material Costs Cu - $3.35/lb Zn - $1.59/lb Sn - $6.61/lb S $0.02/lb Ga - $209/lb In - $361/lb Se $4, 2007 $33/lb Relative Abundance Cu x 10-5 Zn x 10-5 Sn x 10-6 S Ga x 10-5 In x 10-7 Se - 5 x 10-8

19 Cu(InxGa1-x)Se2 World record efficiency = 20.0 %. Many companies are evaporating, printing, sputtering and electrodepositing it. Some are just starting to ship cells. Handling a 4-element compound is tough. Shell Solar, CA Global Solar Energy, AZ Energy Photovoltaics, NJ ISET, CA ITN/ES, CO NanoSolar Inc., CA DayStar Technologies, NY/CA MiaSole, CA HelioVolt, Tx Solyndra, CA SoloPower, CA Wurth Solar, Germany SULFURCELL, Germany CIS Solartechnik, Germany Solarion, Germany Solibro, Sweden CISEL, France Showa Shell, Japan Honda, Japan Bold ½ billion dollars investment

20 Cu(In x Ga 1-x )Se 2

21 Cu(InxGa1-x)Se2 Nanosolar s Roll-to-Roll Coating 11-16% efficiency

22 Solar Cell Research

23 Organic Semiconductors Attractive properties: Abundant: ~100,000 tons/year Mature industry/markets Low materials cost: ~1$/g 17 /m 2 Low-cost manufacturing Non-toxic CuPc Copper Phthalocyanine Example

24 Solid State Organic Solar Cells High absorption in the visible spectrum Have relaxed deposition requirements Can be manufactured in a low cost process Can be grown on thin and flexible substrate Can add value to existing product Challenge! Current power conversion efficiencies are too low for commercial implementation

25 Current Challenges The lower photocurrent is due to poor light absorption, generation and transport. The fill factor is due to poor transport and recombination. Improving light harvesting Small band gap polymer, dye-sensitized materials, light-trapping structures Improving charge transport Carrier mobility (10-2~10-5 cm2/vs) is low Control morphology Processing condition, self organization, synthesis of D-A block copolymer, use of porous films as template Addressing manufacturing issue and improving stability By encapsulating cells and more stable materials Understanding device function and limits to performance

26 Structures and Principles of LED & Photovoltaic mode LED mode PV mode A PV mode is the reverse of a LED. In PVs electrons are collected at the metal electrode and holes are collected at the ITO electrode.

27 Organic Solar Cells: Three Types Dye sensitized solar cells: Electrochemical cells Small molecule organic solar cells: made by vacuum deposition Polymer solar cells: made by solution, low temperature processing All face the same challenges: Increase power conversion efficiency Increase stability Large area technology

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30 Organic Semiconductors Used in Solar Cells

31 Polymer solution processed cells

32 Solar Cells The photocurrent produces a voltage drop across the resistive load, which forward biases the pn junction.

33 Importance of Polymer Morphology on Photovoltaic Efficiency Morphology determining parameters: The spin casting solvent The composition between polymer and fullerene The solution concentration The controlled phase separation and crystallization induced by thermal annealing The chemical structure of the materials

34 ~200 nm thick Polymer-Fullerene Bulk Heterojunction Cells Donor polymer (i.e. P3HT) absorbs light generating an exciton (i.e. bound electron hole pair). Exciton must diffuse to the Donor/Acceptor (e.g. PCBM) interface to split. Ca/Al Electrons travel to the back electrode. Holes travel to the front electrode. PEDOT

35 6.1 % Eg = 1.9 ev The LUMO-LUMO offset is 0.7 ev. Heeger, LeClerc et al Nature Photonics 3 (2009) p. 297

36 6.77 % Yang Yang, Luping Yu et al Nature Photonics, 3 (2009) p. 649

37 Heliatek and IAPP had 8.3% efficiency for organic photovoltaic cells First products enter the market at the beginning of 2012

38 3rd generation cells Up- and down conversion Intermediate band Hot carriers Superlattices Quantum dots Nanotubes

39 Multiple Exciton Generation in nanocrystals Quantum Dots if high energy photon comes in will excite the carrier into higher level the exciton (ehp) can split and give multiple pairs. This starts to happen at 3xs the bandgap but does not get efficient until 4-5xs the bandgap. R. D. Schaller, V. I. Klimov, Physical Review Letters, 2004, Vol. 92.

40 MEG has been observed Shaller and Klimov, Phys. Rev. Lett. 92 (2004) p No decent solar cells have been made. Art Nozik et al., Nanoletters 5 (2005) p. 865.

41 Advantages of Nanowires 6.4 % efficient cells have been made.

42 Primary challenge for PV Cost reduction of factor 5 to become competitive with conventional electricity Today PV module price: /Wp (Wp = Watt peak) Integral approach: Reducing module costs raw materials & labor, investments efficiency, lifetime Optimizing systems integration area and power related costs Note: overall optimum highest efficiency

43 Future

44 (Courtesy of Sarah Kurtz, NREL)

45 Thanks to: Source: NREL Solar Radiation Data Manual Sarah Kurtz, NREL Outline of research paper Due Today Homework: 3rd generation solar cell

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