Solar Energy Part 2: Photovoltaic cells San Jose State University FX Rongère Janvier 2009

Photovoltaic effect Discovered by Edmond Bequerel in 1839 First Solar cell was built by Charles Fritts in 1883 Russel Ohl patented the first modern solar cell in 1946 Bell Laboratories found that doped silicon may have high photovoltaic properties in 1954

Photovoltaic Effect Photovolatic effect is generated when Photons hit a semi-conductor material With a higher energy than the gap between its Valence and Conduction bands Free electrons move on one side (n-side) while holes move on the other side (p-side) A difference of potential is created between n-side and p-side allowing current through a load outside of the semi-conductor

Silicon

Silicon based photovoltaic cells Silicon is a metalloid of the Group IV and Period 3 It has 14 electrons on 3 orbits (2,8,4) Its crystalline structure is Face-centered Cubic; each Si atom is surrounded by four other atoms Every atom of the Valence band is bonded with one atom of a neighbor saturating the valence band

Silicon based photovoltaic cells Its band gap (at 300 K) is 1.2 eV Doping with Boron and Phosphor dramatically improves its photovoltaic properties Phosphore has 5 variance electrons Boron has 3 variance electrons Silicon has 4 variance electrons Charge Carriers

Silicon based photovoltaic cells N-P Jonction P P N Hole diffusion Electron diffusion Electric Field N

Effective radiation on solar cell Minimum photon energy is required to move an electron from valence to conduction (band gap) + heat Valence Conduction Electron + heat Valence Conduction Electron Thermalization Transmission

Effective radiation on silicon based cells Solar Spectral Irradiance (10 3 W.m -2.μm) No conversion Conversion λ (m)

The Shockley-Queisser limit The Shockley-Queisser limit is a measure of the upper obtainable conversion rate of a perfect solar cell based on only one solar cell material with only one electronic band gap Conversion rate of a perfect Si based cell is 33% Thermalization: 47% Transmission: 18% Recombination: 1.5%

Structure of a silicon Solar Cell contacts P-layer N-P junction N-layer E=0.5 Volt, 3 Amp. (typically) Volts Amp. Open circuit voltage Short circuit current Maximum Power

Manufacturing Source: Renewable Energy, Power for a sustainable future. G. Boyle, 2004 (1,900 o C) Siemens

Manufacturing (1) Silicon is obtained from Silica (SiO 2 ) Carbothermic reduction: Temperature 1,900 o C Reaction with charcoal Bulk silicon is already doped with p-type (boron) ( atoms/cm 3 ) (Silicon: atoms/cm 3 ) Bulk silicon is sliced in μm wafers Czochralski process

Procedures: High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted down in a crucible (1,500 o C) A seed crystal, mounted on a rod, is dipped into the molten silicon The seed crystal’s rod is pulled upwards and rotated at the same time Precise control of temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single- crystal, cylindrical ingot from the melt. Inert atmosphere, such as argon

Si based Solar cells Mono-crystalline Solar cell Conversion rate (panel): 15-20% Major Manufacturers: SunPower, SunTech, Sharp 35% of the market Poly-crystalline Solar cell Conversion rate (panel): 11-15% Major Manufacturers: Kyocera, Sharps, Q-cell, SunTech, BP Solar, Photowatt 60% of the market

Energy for silicon based solar panels About 2,000 to 5,000 MJ/m2 (2005) leading to an energy payback of 1.5 to 2.5 years. Source: M. Asema, M. de Wild-Scholten THE REAL ENVIRONMENTAL IMPACTS OF CRYSTALLINE SILICON PV MODULES: AN ANALYSIS BASED ON UP-TO-DATE MANUFACTURERS DATA

Manufacturing (2) Source: Renewable Energy, Power for a sustainable future. G. Boyle, 2004

Manufacturing (2) N-type elements (Phosphor) are injected by surface diffusion (10 19 atoms/cm 3 ) Anti-reflection 100+ nm coating with silicon nitride deposited by PECVD Electrodes are then put in place: In the back aluminum base layer on the all surface On the front silver base layer with “fingers” and “bus-bars” in order to reduce the shaded area on the cells

Manufacturing (3) Coating to reduce the absorption of the photons by the cell Silicon reflectance: 40% Film treatment reduces it to 3% Active Source: Chelikowsky, J. R. and M. L. Cohen, Phys. Rev. B14, 2 (1976)

Glass Coating Transmittance enhancement vu

Minimal thickness of silicon cell Absorption coefficient Active i λ : Light intensity x: depth a λ : Absorption coefficient Jellison, Jr., G. E. and F. A. Modine, Appl. Phys. Lett- 41, 2 (1982)

Minimal thickness of silicon cell Theoretically, 1-3 mm of bulk silicon would be needed to absorb photons Practically, rays are reflected by a layer of aluminum on the back of the cell and trapped in the layer of silicon by texturing the upper surface Typically, silicon solar cell wafer are μm thick

Module Conversion rates Electrical efficiency and module optimization increase overall conversion rate Source: B. von Roedern and H.S. Ullal The Role of Polycrystalline Thin-Film PV Technologies in Competitive PV Module Markets. NREL 33rd IEEE Photovoltaic Specialists Conference San Diego, California May 11–16, 2008 Nota: CZ-Si: mono-Si MC-Si: multi-Si

Silicon Manufacturing Capacity Worldwide evolution of the capacity of major manufacturers

Other cell technologies First generation: bulk silicon Mono-crystalline silicon multi-junctions Mono-crystalline silicon Multi-crystalline silicon Second generation: thin films a-Si:H amorphous hydrogenated silicon CIGS Copper-Indium-Gallium-Selenium CuIn x Ga (1-x) Se2 (x є [0,1]) Band gap = fct(x) є [1.0μm,1.7μm]) CdTe Cadmium Tellerium

Other Technologies 3 rd generation: Non semi-conductor based Dye cells: photo-electrochemical cells OPV: Organic polymers Nanocrystal 4 th generation: Composite technologies

Solar PV cell efficiency

GaAs Multijunction Capture more solar radiation bandwidth by combining semi-conductors with different band gaps GaAs Gallium Arsenide Band gap: 1.43 eV λ=.87 μm Ge Germanium band gap: 0.67 eV λ=1.85 μm InGaP Indium Gallium Phosphide Band gap: 2.26 eV λ=.55 μm InGaPGaAsGe GaAs crystal

Future of Multijunction

Solfocus Started in 2005 at PARC in Palo Alto Received $150M in VC funding

Amorphous Silicon a-Si:H Mainly used for small devices like calculators Si atoms are not arranged in an organized crystal Some atoms are partially not bonded to others Hydrogen atoms are used to fill the defects May be degraded by high energy light Source: DOE

CIGS Copper Indium Gallium Selenium Solid solution of copper indium selenide ("CIS") and copper gallium selenide, Chemical formula of CuIn x Ga (1-x) Se Efficiency (%) Ga/(Ga+In) =(1-x) Source: Rommel Noufi High Efficiency CdTe and CIGS Thin Film Solar Cells: Highlights of the Technologies Challenges NREL 2007 Best efficiency for 1-x=.31 Absorber band gap (μm)

CIGS Cell structure Source: Rommel Noufi High Efficiency CdTe and CIGS Thin Film Solar Cells: Highlights of the Technologies Challenges NREL 2007 ZnO, ITO 2500 Å CdS 700 Å Mo µm Glass, Metal Foil, Plastics CIGS µm [n] [p]

CdTe Cadmium Telluride – Cadmium Sulfide Band Gap: 1.5 eV Cell structure CdS ZnTe:Cu 2µm CdTe CdS ZnTe:Cu [n] [p]

Thin-film Companies Source: H.S. Ullal and B. von Roedern Thin Film CIGS and CdTe Photovoltaic Technologies: Commercialization, Critical Issues, and Applications NREL 2007

Applications CdTe panels in Walpolentz (Germany) 40 MW

Applications GIGS panels in Wales (84 kW)

Technology Comparison TechnologyAdvantagesDrawback Bulk mono-crystalline silicon Efficiency Reliability Form factor Cost Bulk multi-crystalline silicon Cost Reliability Form factor Efficiency Bulk multi-junctions silicon EfficiencyCost Complexity a-Si:H thin film Efficiency Form factor Reliability CIGS Efficiency Form factor, Cost Default tolerance Process Durability Indium availability CdTe Form factor, Cost Default tolerance Durability Toxicity Cadmium availability

Technology Comparison TechnologyAdvantagesDrawback Dye cellsForm factor Cost Reliability Efficiency OPVCost Form factor Reliability Efficiency NanocrystalEfficiency Form factor Complexity Source: Navigant Photovoltaic Manufacturer Shipments & Competitive Analysis 2006/2007 April 2007

Solar cost Solar module Source:

Solar Cost Example of cost split Today, installed cost is $7-$10/W p Source: Meng TAO Inorganic Photovoltaic Solar Cells: Silicon and Beyond The Electrochemical Society Interface Winter 2008

Cost reduction First Solar (CdTe) cost reduction road map Source: First Solar Corporate Overview Q3 2008

Major Manufacturers

Module Conversion Rates California Solar Initiative recommendation Source: CSI - List of Eligible Inverters

Photovoltaics System Off grid Grid-tied

Inverter Converts DC in AC by a commutation device Issues are: Harmonics Failure of the switching device Efficiency 90%-95% Cost: $1/W Ex: Xantrex GT 3.0 Inverter

Inverter Efficiencies California Solar Initiative recommendations Source: CSI - List of Eligible Inverters

Some PV companies Information: Distributors: Local companies to follow Solfocus PV concentrator