Nanotechnology in Photovoltaics

Nanotechnology in Photovoltaics ECSAC10 Losinj, August 25 th, 2009 Vanni Lughi vlughi@units.it Dipartimento dei Materiali e delle Risorse Naturali University of Trieste

Our laboratory Two lines of research on photovoltaic materials: Thin film PV (CdS/CdTe-based) Novel nanostructured materials low-cost and high efficiency Part of MEL, Laboratory for Energy Materials research and services in the field of energy materials, including photovoltaic installations (e.g. 20 kw installation on DMRN s building roof, 20 kw installation on the City Hall s Roof in Trieste, )

Doctoral student: Luca Cozzarini Our laboratory Students: Michele Pianigiani Matteo Barbone Alice Orzan Alice Furlan Emanuele Slejko Giulio Pipan Stefania Cacovich Luca Pavan MaXun-Genefinity personnel: Dr. Francesca Antoniolli, Ph.D Andrea Radivo Mauro Del Ben Facilties: Dept. Materials and Natural Resources (DMRN) Spectroscopy Lab DMRN

Introduction Why photovoltaics (PV)? Evolution of PV (economical technical social) Current PV technologies Basic physics Si-based technology Thin films Contents Emerging technologies High efficiency approaches (Tandem) Low-cost approaches (Biomimetics) Third generation technologies Nanostructured materials colloidal solids

Photovoltaics Direct conversion of solar energy to electrical energy (for free!)

Why Photovoltaics?

Growth of the Energy Demand - Particularly in the developing countries-

Growth of the Energy Demand - Particularly in the developing countries-

Need for carbon-free energy sources Temperature rising (?) http://epa.gov/climatechange/science/images/co2-temp.gif http://www.globalwarmingart.com/wiki/image:carbon_dioxide_400kyr_rev_png

Need for carbon-free energy sources Temperature rising (?) Old Data New Data

Need for carbon-free energy sources Ice Melting

Need for carbon-free energy sources Climate change

Need for carbon-free energy sources Economic sanctions

Need for carbon-free energy sources Hoffert, Nature 1998

Photovoltaics vs. other Renewable Resources Allen Barnett

Evolution of Photovoltaics Economical Technical Social acceptance and integration

Economical Evolution: Photovoltaic Market

New Markets Off-grid Rural Applications Off-grid Telecommunications On-grid Residential and commercial Allen Barnett

Market size and growth World Market: 15B Demand and supply growth: 35%/year Demand exceeds supply Annual Installation (GWp) Trends in photovoltaic applications, Survey report of selected IEA countries between 1992 and 2005, report IEA- PVPS T1-15:2006 DOE Solar Energy Technologies Program, Multi-Year Program Plan 2007-2011, U.S. Department of Energy, Energy Efficiency and Renewable Energy

Average price ($/Wp) Price Learning curve: 80% 6 5.5 5 4.5 Silicon technology 4 3.5 3 2.5 Thin film technology 2 Apr-01 Jan-04 Oct-06

Technological evolution

Technology evolution: Efficiency increase NREL - US Dept.of Energy

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

Efficiency (%) 100 80 60 Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit - Economical convenience 0.1 $/W 0.2 $/W Grid parity 0.5 $/W Ultimate thermodynamic limit 40 20 1 $/W SQ limit for single junction 3.5 $/W 0 100 200 300 400 500 Cost ($/m 2 )

Integration of PV

Traditional Photovoltaic Installations

Integration: Aesthetics

Integration: Aesthetics

Integration: Aesthetics & Function

Integration: Aesthetics & Function

Current PV Technologies

Photovoltaic Effect

Photovoltaic Effect

Energiy p-n junction p n E p Conduction band Valence band n Spatial coordinate

Photovoltaic Effect Energy 1. Absorption of solar radiation Conduction Band Creation of free carriers + - 2. Free carrier transport and extraction Valence Band Electrical energy

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

Silicon technology

Silicon technology Record performance Single crystal silicon Polycrystalline Cell Record 24.7% 20.3% Module record 22.7% 15.3% Commercial modules 15-18% 12-15

Efficeincy Silicon Technology 24.7% 30% (theoretical limit) Silicon 2009 Time

Silicon Technology - Summary Efficeincy Mature Technology (max efficiency > 80% thermodynamic limit) Silicon 24.7% 30% (theoretical limit) 2009 Time Large quantities of pure silicon are needed high cost of raw material Limitation in raw material supply Kazmersky, 2006

Efficiency (%) 100 80 60 Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit - Economical convenience 0.1 $/W 0.2 $/W Grid parity 0.5 $/W Ultimate thermodynamic limit 40 20 Silicon technology 1 $/W SQ limit for single junction 3.5 $/W 0 100 200 300 400 500 Cost ($/m 2 )

Thin Film Solar Cells Materials with: high absorption coefficient good match with the solar spectrum (1.5 ev) Less active material is needed (thin films: 0.1 10 um) Lower cost Less problems with supply of raw materials Short carrier paths (perfection requirements are relaxed)

Thin Film Solar Cells Medium efficiency Low Cost CdTe CIGSS Amorphous Si Cell Record 16.5% 19.5% 9.5% Module record 10.7% 13.4% -- Commercial modules 8-10% -- 4-7%

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

Efficiency (%) 100 80 60 Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit - Economical convenience 0.1 $/W 0.2 $/W Grid parity 0.5 $/W Ultimate thermodynamic limit 40 1 $/W SQ limit for single junction 20 Thin film technology Silicon technology 3.5 $/W 0 100 200 300 400 500 Cost ($/m 2 )

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% Nanotechnology-based films are currently reaching the market

PV efficiency: Thermodynamic limit Energy Conduction Band + - Valence Band

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

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

Tandem Cells Current efficiency record > 40% Multijunction cells very expensive Aerospace applications or terrestrial concentration

Efficiency (%) 100 80 60 Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit - Economical convenience 0.1 $/W 0.2 $/W Grid parity 0.5 $/W Ultimate thermodynamic limit 40 1 $/W Multijunctions SQ limit for single junction 20 Thin film technology Silicon technology 3.5 $/W 0 100 200 300 400 500 Cost ($/m 2 )

Why nanotechnology? Morphological advantages (surface area) Phenomena that govern optoelectronic properties of materials occur at the nanoscale Phenomena at the nanoscale are governed by the laws of quantum mechanics new opportunities for controlling material properties

Dye Sensitized Solar Cell (DSSC) Graetzel Cell

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

Photosynthesis 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%

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 packing on the surface

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

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

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

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

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

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

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

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

Intermediate Band Materials Beyond single junction limits (at low cost!)

Making an Intermediate Band Material Quantum dots embedded in a semiconductor Nanotechnology at the service of PV Conduction band Valence Band Intermediate Band

Photovoltaic Devices Based on Intermediate Band Materials Max efficiency 47% Current record for single-junction cell: 28.3% Scalable, low-cost production approaches Diversity of substrates excellent integration (nanoinks and photovoltaic paints )

Efficiency (%) 100 80 60 Tecno-Economical Evolution - Cost and Efficiency PV Figures of Merit - Economical convenience 0.1 $/W 0.2 $/W Grid parity 0.5 $/W Ultimate thermodynamic limit 40 IB Materials 1 $/W Multijunctions SQ limit for single junction 20 Thin film technology Silicon technology 3.5 $/W 0 100 200 300 400 500 Cost ($/m 2 )

Efficiency 47% (IB material limit) 24.7% 30% (single junction limit) Silicio 16.5% CdTe 2010 Time

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%

Properties of nanomaterials Phenomena at the nanoscale are governed by the laws of quantum mechanics. new opportunities for controlling material properties Phenomena that govern optoelectronic properties of materials occur at the nanoscale

Properties of nanomaterials What does nano mean? A material is nano when a selected property starts to differ from its bulk behavior

EnergiY Electrons in a (spherical) box E 2 2 2m R e 2 n 2 n = 1, 2, 3, Electrons can be described by waves Confined waves can only have a discrete set of wavelengths In confined systems, electrons can only assume discrete values of energy Position of energy levels depends upon confinement (size of the box)

Quantum Dots Artificial atoms nanocrystal atom As for atoms, quantum dots have discrete electron energy levels

Nanocrystals 2.5 nm Typical size: 1 100 nm (10 2-10 5 atoms) Most atoms are at the surface (75% for 1 nm nanocrystals) Essentially free of lattice defects

Nanocrystals

Semiconductor nanocrystals (quantum dots): Optical properties

Energia Quantum dot absorption and emission Dependence on size E g ( Q. D.) E g0 2 2 2m R eh 2

Semiconductor Nanocrystals: Synthesis

Semiconductor Nanocrystals: Synthesis Colloidal synthesis of cadmium selenide Cd-oleate + Se-TOP CdSe + products Cadmium precursor: Cd-oleate Selenium precursor: Se-TOP (trioctylphosphine-selenium) mixing Precipitation of CdSe

Semiconductor Nanocrystals: Colloidal Synthesis Key role of NUCLEATION and GROWTH TIME, TEMPERATURE and CONCENTRATION control reaction kinetics Surfactant are needed to avoid cluster formation

Nucleation

Nanocrystal shapes Paul Alivisatos, UC Berkeley

Nanostructured materials Colloidal solids A chance to design novel materials with entirely new properties 8 nm

Methods for assembling colloidal solids

Bio-templated nanomaterials E. Hu, UCSB A. Belcher, MIT

C.B. Murray et al., Annu. Rev. Mater. Sci 30, 545 (2000) Assembly and structural control D.J. Norris, Adv. Mater. 16, 1394 (2004) 8 nm diameter CdSe Glass Sticking coefficient Particle arrival speed Crystalline (ordered)

QD-solid assembly: QD Distance Engineering Curve Capping agent Atoms per chain Interparticle distance (Å) 50 nm h Trihexadecylphosphate 16 17 i Trioctylphosphine calchogenide 8 11 j Tributylphosphine oxide 4 7 Y. Yin, A.P. Alivisatos, Nature 437, 664 (2005) C.B. Murray et al., Science 270, 1335 (1995)

Electronic properties of colloidal solids

QD-solid assembly: Bi-component QD-Solid PbSe Fe 2 O 3 {110} SL F.X. Redl et al., Nature 423, 968 (2003)

xxx Materials engineering CB IB VB

Energiy Our approach for sinthesizing an IB material Matrice di ZnSe matrix CdSe QDs Electron affinity Bandgap ZnSe 4.09 2.58 CdSe 4.95 1.74 ZnSe 4.09 2.58

Absorption Peak Position [nm] NCs Growth Kinetics Control of Size 595 590 Cd:OA 1:19 585 580 575 Cd:OA 1:10 570 565 200 220 240 260 280 Temperature [ C]

Assembly: Control of interparticle forces

Intensity [a.u.] Peak Intensity [a.u.] Assembly: Ordering 0 500 1000 1500 2000 Time (s) 0.00 0.50 1.00 1.50 2.00 q-spacing (A -1 )

Uniform Thin Films of NC assemblies

Counts Densification of NC assemblies 1200 1000 Core/shell 800 600 400 200 0 ITO ZnO ITO ZnO ZnO Core/shell ITO CdSe CdSe CdSe 21 26 31 36 41 46 51 56 2theta [ ] As deposited Core/shell Sintered

Optical properties of NC assemblies PL of the nanocrystals PL of the matrix

Concluding Remarks PV is at a tipping point breakthrough technology is just around the corner Bio- and Nanotechnology will likely be the key to high performance, low cost PV devices Colloidal routes to fabricate PV materials are extremely promising: The structure can be finely engineered at the nanoscale Cost can be reduced (ambient process conditions)

Doctoral student: Luca Cozzarini Aknowledgements Students: Michele Pianigiani Matteo Barbone Alice Orzan Alice Furlan Emanuele Slejko Giulio Pipan Stefania Cacovich Luca Pavan MaXun-Genefinity personnel: Dr. Francesca Antoniolli, Ph.D Andrea Radivo Mauro Del Ben Facilties: Dept. Materials and Natural Resources (DMRN) Spectroscopy Lab DMRN

State of the art

Stato dell arte

Moduli basati su film sottile Materiali con eccellente assorbimento della radiazione solare Piccoli spessori Stabilità dei costi Sostenibilità degli approvvigionamenti Flessibilità nella scelta del substrato Moduli integrati Esempi: Silicio amorfo; CIGS; CdTe

Moduli basati su film sottili di CdTe Proprietà elettroniche ideali (bandgap 1.5 ev massimo utilizzo della radiazione solare) Disponibilità delle materie prime Robustezza e varietà di processi produttivi disponibili Provata fattibilità industriale (First Solar; Antec)

Economia dell industria fotovoltaica 1200 PV Cell/Module Production (MW) 1000 800 600 400 200 0 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Rest of World Europe Japan U.S 6 5 4 World Watch Institute Fatturato complessivo ~ 10 Mld. /anno Crescita media ~ 35%/anno Installazione annuale (GW) 3 2 1 0 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Vendite First Solar 80 70 60 50 MW 40 30 20 x 10 0 2003 2004 2005 2006 2007

Costo dell energia fotovoltaica

Altre applicazioni

Schermi piatti Basso consumo Alta efficienza Un solo materiale per vari colori

QD-LED Quantum Dot-Based Light-Emitting Diodes La selezione della dimensione dei nanocristalli consente la scelta del colore da uno spettro continuo Applicazioni: Display più luminosi, con miglior riproduzione del colore, a basso consumo

Dyes biocompatibili Studio del comportamento di cellule, ed in generale degli esseri v

Diagnosi e cura dei tumori Le nanoparticelle vengono funzionalizzate in modo da venir accumulate presso il tumore Le proprietà di emissione vengono utilizzate per individuare il tumore Le proprietà di assorbimento vengono

Diagnosi e cura dei tumori

Inchiostri a fluorescenza Sicurezza e tecnologie anti-contraffazione

Main Losses in PV Devices Metal contact Metal contact

Sommario Tecnologia Rendimento Costo /Wp Tempo di ritorno dell energia [anni] Ritorno investimento energetico (EROI) Silicio monocristallo Silicio policristallo 15-22% 4-5 2-5 5-15 12-15% 4 1.7-3.5 8-17 Film sottili 5-13% 2-3 1-1.5 17-30 Celle tandem 30-42% alto?? DSSC 5-10% (2-3) Materiali organici 3-6% (1-2) <1 >50 Nanomateriali? (<1)

The bandgap dilemma

Absorption coefficients

Minimum thickness for full absorption Legge di Lambert Beer I = I0 exp (- z) z I0 g d N d N d d f ph ph 0 0 ) ( ) ( ) ) ( exp( (1 ) (

Minimum thickness for full absorption Legge di Lambert Beer I = I0 exp (- z) z I0 g d N d N d d f ph ph 0 0 ) ( ) ( ) ) ( exp( (1 ) (

250 245 240 Sintesi per test di reversibilità Temperatura Dimensione 4 3.9 Temperatura [ C] 235 230 225 220 215 3.8 3.7 3.6 Diametro NC [nm] 210 205 3.5 200 0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960 1020 1080 1140 1200 Tempo [s] 3.4