Giacomo Ciamician, 1906

Transcript

1 Modern civilization is the daughter of oil, for this offers to mankind the solar energy in its most concentrated form; that is, in a form in which it has been accumulated in along series of centuries. Modern man uses it with increasing eagerness and thoughtless prodigality for the conquest of the world and [...] oil is to-day the greatest source of energy and wealth. The earth still holds enormous quantities of it, but oil is not inexhaustible. "The problem of the future begins to interest us... Is fossil solar energy the only one that may be used in modern life and civilization? That is the question. "If our black and nervous civilization, based on oil, shall be followed by a quieter civilization based on the utilization of solar energy, that will not be harmful to progress and to human happiness.

2 Modern civilization is the daughter of coal, for this offers to mankind the solar energy in its most concentrated form; that is, in a form in which it has been accumulated in along series of centuries. Modern man uses it with increasing eagerness and thoughtless prodigality for the conquest of the world and [...] coal is to-day the greatest source of energy and wealth. The earth still holds enormous quantities of it, but coal is not inexhaustible. "The problem of the future begins to interest us... Is fossil solar energy the only one that may be used in modern life and civilization? That is the question. "If our black and nervous civilization, based on coal, shall be followed by a quieter civilization based on the utilization of solar energy, that will not be harmful to progress and to human happiness. Giacomo Ciamician, 1906

3 Thin Film Solar Cell Technology State of the Art Summer Workshop Energia per il domani celle solari dal nano al macro Sesto Pusteria, August 3 rd 4 th, 2011 Vanni Lughi DI 3 Dipartimento di Ingengeria Industriale e dell Informazione, University of Trieste, Italy vlughi@units.it

4 World Energy Consumption Current 15 TW Projected (2050) 30 TW Global incident sunlight TW The only big number out there Nate Lewis

5

6

7 Energy is needed for the most diverse applications - Developing countries -

8 Need for carbon-free energy sources Global temperature seems to be raising

9 Need for carbon-free energy sources Atmospheric phenomena

10 Need for carbon-free energy sources Economy

11 Basic Solar Cell Concept Absorber s Requirements: Bandgap matched with solar spectrum ( 1.5 ev) Good transport properties Full absorption within absorber s thickness + -

12 Energy Photovoltaic Effect 1. Absorption of solar radiation Creation of free carriers Free carrier extraction Electrical energy

13 Photovoltaic Effect Selection of Materials 1. Absorption of solar radiation Materials with good absorption of the solar radiation Creation of free carriers 2. Free carrier extraction Electrical energy Perfect materials (large mean free paths for the carriers) Short carrier paths

14 Current Main Technology: Silicon 200 m 3.5 kg Si /kw p

15

16 Why Thin Film Solar Cells? Absorption Coefficient [cm -1 ] Reduce amount of active material by using high absorption coefficient materials Conduction band Direct Bandgap Conduction band Phonon Absorption Valence band Indirect Bandgap Indirect Absorption Direct Absorption Valence band Photon Energy [ev]

17 Advantages of Thin Film Solar Cells Use of less material ( 2 orders of magnitude) Low and stable cost Sustainability of supply Integrated fabrication Wide variety of substrates available Opportunity for integrated device design (e.g. Building Integrated Photovoltaics - BIPV) Flexibility on the material chemistry optimization of bandgap Short travel distances for the carriers enhanced collection Flexible device design (bandgap engineering)

18 Fraction of Useful Photons Absorbed Designing Film Thickness Lambert Beer I = I0 exp (- z) I0 f ( d) 0 (1 exp( ( ) d) N g 0 N ph ( ) d ph ( ) d z Thickness [ m]

19 Cost Stability The active material counts for about 5 10% of the overall cost (in Si technology, it is about 50%) COSTO DELLE MATERIE PRIME Modulo fotovoltaico Distinta Materie prime Costo per pezzo Vetro1 43,20 Antiriflesso 0,30 ITO 3,30 CdS 0,03 CdTe 2,50 CdCl2 0,30 Cu 0,01 EVA 1,04 Vetro2 10,80 Collegamento 1,35 Contatto oro 3,00 ZnTe 0,50 Tot 66,33 Costo Imballaggi Cartoni 0,50 Tot 0,50 Costo Totale Prodotto 1 66,83

20 Advantages of Thin Film Solar Cells Use of less material ( 2 orders of magnitude) Low and stable cost Sustainability of supply Integrated fabrication Wide variety of substrates available Opportunity for integrated device design (e.g. Building Integrated Photovoltaics - BIPV) Flexibility on the material chemistry optimization of bandgap Short travel distances for the carriers enhanced collection Flexible device design (bandgap engineering)

21 PV modules thin film technology - Structure and fabrication - Continuous process Sequential deposition of thin films on glass (superstrate structure) or on the back electrode (substrate structure) The output is an integrated module, where the cells in series are formed a posteriori via laser cutting

22 + - Basic Solar Cell Design

23 Thin Film Solar Cell (superstrate) 200 nm 50 nm 5000 nm 200 nm 200 nm

24 200 nm Thin Film Solar Cell (substrate) 50 nm 5000 nm 200 nm

25 Photovoltaic modules Colored modules ( mimetic ) Semi-transparent modules Flexible modules

26 Building Integration

27 Advantages of Thin Film Solar Cells Use of less material ( 2 orders of magnitude) Low and stable cost Sustainability of supply Integrated fabrication Wide variety of substrates available Opportunity for integrated device design (e.g. Building Integrated Photovoltaics - BIPV) Flexibility on the material chemistry optimization of bandgap Short travel distances for the carriers enhanced collection Flexible device design (bandgap engineering)

28 a-si:h (amorphous silicon) nm Direct bandgap: 1.7 ev (tunable with Ge alloying) Convenient large scale deposition methods (mainly CVD and PE-CVD at C) X Need to stabilize performance (Staebler-Wronsky) Key design issue: i-layer quality and thickness (tradeoff btw absorption and collection) Need to minimize absorption within the p-level (e.g. alloying with carbon -> window layer (2eV) nm glass TCO p-sic:h i-si:h n-si:h ZnO Ag/Al

29 Tandem cell Thin Film Silicon - variants c-si:h Bandgap 1.1 ev No degradation poly-si Bandgap 1.1 ev No degradation Good conduction

30 CdTe Thin Film Solar Cells Perfection requirements of materials are relaxed

31 Cadmium Telluride Thin Film Solar Cells Ideal bandgap High absorption coefficient Fast e-h separation (e.b.e. 10 mev) High mobility: 1000 cm 2 /V s - electrons 80 cm 2 /V s - holes Band bending at grain boundary -> reduced recombination Current limitations: Contacts and surface recomb. Variety of deposition processes (sputtering, CSS, electrochemical, evaporation) Key to process: activation treatment at 500 C (crystal quality improves, intermixing at CdS/CdTe interface, p-doping established, defects annealed, )

32 PV modules: Datasheets

33 PV modules: Datasheets

34 CIGS Thin Film Solar Cells Tunable bandgap Deposition by co-evaporation (!) Many aspects similar to CdTe datasheet

35 A new player: CZTS (Copper Zinc Tin Sulfide) Bandgap 1.4 High absorption coefficient ( > 10 4 in the visible range) Abundant, environmentally friendly elements Many aspects similar to CIGS and CdTe Variety of deposition methods available (nanoinks, electrochemistry, sputtering, EB-PVD, PLD, sol gel.)

36 Energy PV efficiency: Thermodynamic limit Conduction Band Valence Band Max efficiency: 30%

37 Beyond The Single Junction Limit: Tandem cells More efficient use of the solar spectrum

38 Tandem Cells Thermodynamic limiting efficiency: 86.8% Current efficiency record > 40% Multijunction cells very expensive Aerospace applications or terrestrial concentration

39

40

41

42 Thin Film Solar Cells Medium efficiency Low Cost a-si:h 10.1 Cell Record Module Record a-si:h - tandem CdTe CIGS CZTS 9.7 N/A III-V multijunction 33.8 (41.6) N/A DSSC (s) Organic (s) Green, J. Prog. Photov Techno-economical analysis will follow

43 Thin Film Solar Cells - Production Potential isupply Corp., 2011

44 Thin Film Solar Cells Shipments and Pricing isupply Corp., 2011

45 Thin Film Solar Cells Market Shares isupply Corp., 2011

46 Thin Film Solar Cells Slight slowdown isupply Corp., 2011

47 Disponibilità delle materie prime e sostenibilità degli approvvigionamenti Nota: Il cadmio è un sottoprodotto dell estrazione dello zinco Disponibile principalmente in Cina, Australia ed USA Data from NREL, USA Si tende a non utilizzare più il cadmio nelle nuove tecnologie (es: Pile Ni-Cd) attesa grande disponibilità a basso prezzo La disponibiltà di cadmio non limita la produttività di moduli al CdTe

48 Disponibilità delle materie prime e sostenibilità degli approvvigionamenti Tellurio è l elemento più scarso tra quelli richiesti per produrre moduli al CdTe Gli attuali ritmi di estrazione assicurano una produzione annua di 29 GWp/anno: 5 volte la produzione complessiva annuale di fotovoltaico 100 volte l attuale produzione basata su CdTe L estrazione di tellurio può venir incrementata di 15 volte senza costi aggiuntivi Dati NREL, USA

49 CdTe e l ambiente CdTe è un composto stabile e non rilascia Cd nemmeno in condizioni estreme I quantitativi di Cd nei moduli sono minimi Anche altre tecnologie fotovoltaiche contengono Cd I moduli al CdTe rappresentano un modo per fissare il Cd I moduli al CdTe sono quasi interamente riciclabili Separazione dell hardware strutturale ed elettrico dal modulo Comminuzione e separazione tra vetro e solidi contenenti cadmio Riciclaggio del vetro Estrazione e raffinazione dei metalli

50 CdTe e l ambiente Table 1. Sources and Relative Contributions of Cd Exposure to Humans (in Europe) Phosphate fertilizers 41.3 % Fossil fuel combustion 22.0 % Iron and steel 16.7 % production Natural sources 8.0 % Non-ferrous metals 6.3 % Cement production 2.5 % Cadmium products 2.5 % Incineration 1.0 % When PV replaces burning coal for electricity generation, it will prevent a minimum of 2 g of Cd in gaseous emissions and about 140 g of Cd in ash form for each GWh of electricity produced

51 Riuso del CdTe

52 Concluding Remarks There is a number of commercially available thin film technologies - excellent alternatives to silicon in some applications (e.g. for area-based design of installations) The thin film PV market is potentially vast and untapped There is a variety of potentially viable new technologies (e.g. CZTS)

53 Thin Film Solar Cell Technology State of the Art Summer Workshop Energia per il domani celle solari dal nano al macro Sesto Pusteria, August 3 rd 4 th, 2011 Vanni Lughi DI 3 Dipartimento di Ingengeria Industriale e dell Informazione, University of Trieste, Italy vlughi@units.it

54

55 Photovoltaic modules Photovoltaic modules are the fundamental element of the photovoltaic generator Commercial crystalline silicon modules

56 Photovoltaic modules Concentration: Requires tracking systems In principle efficiency is higher

57 Efficeincy Technology evolution: Efficiency increase 24.7% 30% (theoretical limit) Silicon CdTe Thin film 16.5% 2009 Time

58 Efficiency (%) Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit $/W 0.2 $/W Grid parity 0.5 $/W Economical convenience Ultimate thermodynamic limit 40 1 $/W SQ limit for single junction 20 Thin film technology Silicon technology 3.5 $/W Cost ($/m 2 )

59 Thin Film Solar Cells Young Technology (max efficiency is 30-40% of thermodynamic limit) Films can be deposited on various substrates (flexible, architectural elements for building integration, etc.) Low Cost Thermodynamic limit is still 30%

60 Efficiency (%) Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit $/W 0.2 $/W Grid parity 0.5 $/W Economical convenience Ultimate thermodynamic limit 40 1 $/W Multijunctions SQ limit for single junction 20 Thin film technology Silicon technology 3.5 $/W Cost ($/m 2 )

61 Dye-sensitized solar cell (DSSC) Grätzel cell - Decreasing cost -

62 Dye-sensitized solar cell (DSSC) Grätzel cell Efficiency ~ 5-10% Liquid electrolyte DSSC c-si

63 Photosynthesis - mastering charge separation - Energy is absorbed by the antenna complexes Cascade of energy transfer between donors and acceptors embedded in antennas (FRET) Energy is funneled to the reaction center Subsequent multistep electron transfer This creates a charge-separated state which lasts for 10s of s or more, with a ~100% QE Limitations Absorption is only efficient at specific wavelenghts To reach the charge-separated state the electron needs to spend energy Energy converted in chemical energy ~ 9%

64 Artificial Photosynthesis: Basic Example Porphyrin-fullerene couples self-assembled on surfaces Donors and acceptors are bonded covalently Thiol Porphyrin C 60 Methil Viologen (MV 2+ ) Quantum yield: 0.5% Lifetime 0.77 s Loose packaging on the surface

65 Artificial Photosynthesis Requirements Appropriate redox potentials and excitation energy for donors and acceptors (quantum yield) Small reorganization energy (- G cs ~ and - G CR >> ) to favor forward electron transfer (charge separation time) Distance between donors and acceptors Solvent characteristics Vibrational modes of the molecules

66 Artificial Photosynthesis: A more efficient scheme Ferrocene Porphyrin C 60 Quantum yield > 25% Lifetime ~ 16 s Dense packaging on the surface Pentads do even better H. Imahori et al., Adv. Func. Materials 14, 525 (2004)

67 Artificial Photosynthesis: Mimicking energy transfer Boron Dipyrrin Thiol Ferrocene Porphyrin Quantum yield: up to 50% Enhanced spectral absorption Dense packaging on the surface H. Imahori et al., Adv. Func. Materials 14, 525 (2004)

68 Artificial Photosynthesis on Transparent Conductive Oxide (ITO) Avoids quenching by electrode Transparent substrate H. Imahori et al., Adv. Func. Materials 14, 525 (2004)

69 Artificial Photosynthesis: Hierarchical assembly for enhancing absorption C 60 Porphirine H. Imahori et al., Adv. Func. Materials 14, 525 (2004)

70 Artificial Photosynthesis: Hierarchical assembly for enhancing absorption Gold nanoparticle H. Imahori et al., Adv. Func. Materials 14, 525 (2004)

71 Stealing from Nature Photoactive reaction centers on metal substrates e - L. Forlov et al., Adv. Materials 17, 2434 (2005)

72 Making an Intermediate Band Material Quantum dots embedded in a semiconductor Nanotechnology at the service of PV Banda di conduzione Banda di valenza Banda intermedia

73 Photovoltaic Devices Based on Intermediate Band Materials Max efficiency 47% Scalable, low-cost production approaches Diversity of substrates excellent integration (nanoinks and photovoltaic paints)

74 Efficiency (%) Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit $/W 0.2 $/W Grid parity 0.5 $/W Economical convenience Ultimate thermodynamic limit 40 IB Materials 1 $/W Multijunctions SQ limit for single junction 20 Thin film technology Silicon technology 3.5 $/W Cost ($/m 2 )

75 Efficiency 47% (IB material limit) 24.7% 30% (single junction limit) Silicio 16.5% CdTe 2008 Time

76 Beyond The Single Junction Limit: Other approaches Multiple carrier generation using high energy photons to extract more than one electron per photon Hot electron extraction extracting high energy photogenerated electrons before they thermalize Thermodynamic limiting efficiency (both cases): 86.8%

77 DMRN: Ricerca su celle a film sottile di CdTe Tipica sezione trasversale Modulo fotovoltaico al CdTe Cella al CdTe su substrato flessibile

78 counts DMRN: Ricerca su celle a film sottile di CdTe 30W PET (220) (311)?? θ

79 Celle multigiunzione Superare il dilemma del bandgap Efficienza record > 42% Estremamente complesse e costose

80 Materiali a banda elettronica intermedia Un modo elegante per superare il dilemma del bandgap

81 Realizzare un materiale a banda intermedia: Punti quantici immersi in un semiconduttore Banda di conduzione Banda di valenza Banda intermedia

82 Dye-sensitized solar cell (DSSC) Grätzel cell Rendimento ~ 5-10% Uso di elettrolita liquido DSSC c-si

83 Need for carbon-free energy sources Hoffert, Nature 1998

84 Fotovoltaico vs. altre fonti alternative Allen Barnett

85 Show pictures of thin film modules). -An idea of current costs (breakdown); energy payback -First solar and other thin film companies -Commercial techs state of the art (see datasheets) -Thin film techs (from the book) -New: CZTO -Multijunctions (can only be made by thin films) -Upconverters, downconverters -Quantum dot based (include Nanosolar etc.)

86 Film sottili La questione energetica il ruolo del fotovoltaico Introduzione generale Fabbisogno domanda (suddivisi per regione del mondo) Proiezioni al futuro Il portafoglio energetico attuale proiezioni future Considerazioni economiche ambientali sociali sulle conseguenze della distribuzione del portafoglio energetico attuale La questione del costo dell energia costi nascosti, incentivi, tasse Introduzione alle tecnologie per l energia rinnovabile Generazione centralizzata e distribuita dell energia Il ruolo del fotovoltaico Cenni sulla tecnologia rendimento e valore aggiunto Considerazioni sulla fattibilità (bilancio tra energia solare disponibile e fabbisogno umano; payback time energetico; payback time monetario) fattibilità di installazione (importanza della geometria del campo pv, delle condizioni al contorno e delle condizioni ambientali) Dati sull industria Tecnologie attuali (cenni) e indicazione dei limiti e problemi attuali (per preparare il terreno per i seminari successivi). Tecnologie fotovoltaiche Introduzione Effetto fotovoltaico Funzionamento di una cella I punti deboli del meccanismo fotovoltaico perdite Limiti fisici I dispositivi a silicio (breve storia e poi partire da un modulo ed analizzarlo) Celle di silicio (giunzione p-n) Fabbricazione e caratteristiche di celle e moduli I film sottili (cenni) Perché film sottili (miglior assorbimento, meno materiale, etc.) Analisi di un dispositivo a film sottile CIGS, CdTe, a-si Tandem Nuovi concetti e tecnologie (cenni) La cella di Graetzel Polimeri Fullereni Nanotubi Biotecnologie Nanotecnologie Nota: Film sottili e nuove tecnologie saranno oggetto di seminari successivi Sistemi fotovoltaici Sistemi grid connected, stand alone e mixed Cenni di impianti elettrici: costituzione di un campo pv per applicazioni grid-connected Analisi energetica economica ambientale (CO2 e effetto serra) Sistemi FV ed edilizia Sistemi fissi ad inseguimento Concentrazione no concentrazione

87 Current Technologies a-si Microcrystalline Si Multijunction Si CIGS CdTe -New: CZTO -High performance Multijunctions (can only be made by thin films) -Upconverters, downconverters -Quantum dot based (include Nanosolar etc.) -Biomimetics -Other technologies (PEC, O-PV) -Commercial techs