Solar EnergyPart 2: Photovoltaic

cells

San Jose State UniversityFX RongèreJanvier 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

Si Si

Si Si

Si Si

Si

Si

Si

No mobile carriers

Si Si

Si P

Si Si

Si

Si

Si

extra electron

N-type

Si Si

Si B

Si Si

Si

Si

Si

Hole (missing electron)

P-type

B

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 diffusionElectron diffusion

Electric Field

N

+++

---

Effective radiation on solar cell

Minimum photon energy is required to move an electron from valence to conduction (band gap)

0. h + heatValence

Conduction

Electron

0. h + heatValence

Conduction

Electron Thermalization

Transmission

Effective radiation on silicon based cells

So

lar

Sp

ectr

al I

rrad

ian

ce (

103 W

.m-2.μ

m)

No conversionConversion

eVh 2.1. min

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 currentMaximum Power

Manufacturing

Source: Renewable Energy, Power for a sustainable future. G. Boyle, 2004

(1,900 oC)

Siemens

Manufacturing (1)

Silicon is obtained from Silica (SiO2)

Carbothermic reduction: Temperature 1,900oC Reaction with charcoal

Bulk silicon is already doped with p-type (boron) (2.1016 atoms/cm3) (Silicon: 5.1022 atoms/cm3)

Bulk silicon is sliced in 180-350 μm wafers Czochralski process

Czochralski process Procedures:

High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted down in a crucible (1,500 oC)

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 (1019 atoms/cm3)

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 cellSilicon reflectance: 40%Film treatment reduces it to 3%

Active

Source: Chelikowsky, J. R. and M. L. Cohen, Phys. Rev. B14, 2 (1976) 556-582.

Glass Coating

Transmittance enhancement

vu

Minimal thickness of silicon cell

Absorption coefficient

ia

dx

di

Active

iλ: Light intensityx: depthaλ: Absorption coefficient

xaixi exp0

xiadx

xdi

Transmission through Silicon

0%

20%

40%

60%

80%

100%

0 0.2 0.4 0.6 0.8 1 1.2

Depth

Trn

asm

issi

on

(i/

i0)

a= 1 cm-1

a= 10 cm-1

a= 100 cm-1

Jellison, Jr., G. E. and F. A. Modine, Appl. Phys. Lett- 41, 2 (1982) 180-182.

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 200-300 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-SiMC-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

CuInxGa(1-x)Se2 (x є [0,1])Band gap = fct(x) є [1.0μm,1.7μm])

CdTe Cadmium Tellerium

Other Technologies

3rd generation: Non semi-conductor based Dye cells: photo-electrochemical cells OPV: Organic polymers Nanocrystal

4th 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

InGaPGaAs Ge

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 CuInxGa(1-x)Se2

18

16

14

12

10

Eff

icie

ncy

(%)

1.61.51.41.31.21.11.0

0.3 0.6 0.9

Ga/(Ga+In) =(1-x)

Source: Rommel Noufi High Efficiency CdTe and CIGS Thin Film Solar Cells: Highlights of the TechnologiesChallenges 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 TechnologiesChallenges NREL 2007

ZnO, ITO2500 Å

CdS700 Å

Mo0.5-1 µm

Glass,Metal Foil,

Plastics

CIGS1-2.5 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 ComparisonTechnology Advantages Drawback

Bulk mono-crystalline silicon

EfficiencyReliability

Form factorCost

Bulk multi-crystalline silicon

CostReliability

Form factorEfficiency

Bulk multi-junctions silicon

Efficiency CostComplexity

a-Si:H thin filmEfficiency

Form factorReliability

CIGSEfficiency

Form factor, CostDefault tolerance

ProcessDurability

Indium availability

CdTe

Form factor, CostDefault tolerance

DurabilityToxicityCadmium

availability

Technology ComparisonTechnology Advantages Drawback

Dye cells Form factorCost

ReliabilityEfficiency

OPV CostForm factor

ReliabilityEfficiency

Nanocrystal EfficiencyForm factor

Complexity

Source: Navigant Photovoltaic Manufacturer Shipments & Competitive Analysis 2006/2007 April 2007

Solar cost

Solar module

Source: http://www.solarbuzz.com/

Solar Cost

Example of cost split

Today, installed cost is $7-$10/Wp

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

Shipment by Major Manufacturers Year 2005

SHARP (J P),

427MW, 25%

Q-Cells (G),

165MW, 9%

Kyocera (J P),

142MW, 8%

Sanyo (J P),

125MW, 7%

Schott, 95MW,

5%

BP Solar, 88MW,

5%

SunTech (China),

83MW, 5%

MoTech (TW),

60MW, 3%

Others, 413MW,

24%

Mitsubishi (J P),

100MW, 6%

Shell Solar,

60MW, 3%

Module Conversion Rates

California Solar Initiative recommendationPV Module Conversion Rate CSI 2009

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

20%

0 100 200 300 400 500

Peak Power (W)

Co

nve

rsio

n R

ate

Source: CSI - List of Eligible Inverters http://www.gosolarcalifornia.org/csi/step3.html

Photovoltaics

System

Off grid Grid-tied

Inverter

Converts DC in AC by a commutation device

Issues are:HarmonicsFailure of the

switching deviceEfficiency 90%-

95%Cost: $1/W

Ex: Xantrex GT 3.0 Inverter

Inverter Efficiencies

California Solar Initiative recommendations

Inverter Efficiencies CSI 2009

84

86

88

90

92

94

96

98

100

1,000 10,000 100,000 1,000,000

Power (W)

Eff

icie

ncy

%

Source: CSI - List of Eligible Inverters http://www.gosolarcalifornia.org/equipment/inverter.php

Some PV companies Information:

www.solarbuzz.com Distributors:

www.sunwize.com Local companies to follow

www.Appliedmaterials.com www.Miasole.com www.Calisolar.com www.solyndra.com www.Nanosolar.com www.Sunpowercorp.com www.Akeena.com www.EnergyInnovations.com www.Nanosysinc.com www.solfocus.com

Solfocus PV concentrator