Solar Photovoltaics & Energy Systems

Lecture 4. Structure and performance of crystalline semiconductor solar cells

ChE-600

Kevin Sivula, Spring 2014

Presentation schedule

22nd 27thAndrea Pisoni Aiman RahmanudinXiaoyun Yu Bornoz PaulineCarlos Morales Martin MuellerJelena Vukajlovic Plestina Paulo and ZhiXavier Jeanbourquin Sadig and Kasparas

• PowerPoint (or equivalent) based presentation • Two possibilities

With a partner – 25 min Independent – 15 min

• Select one of the given publications• Presentation scope

Background for the work • Introduce the field• Define the motivation for the work

Describe the concept/methodology/results in detail Critically comment on the work

• Significance of the result and impact on the field• Other similar approaches?

Follow-up work needed (or already performed) to fulfill the promise of the concept

Shockley–Queisser limit

Tp µ Eg

V

Net electron flux through device:

𝐽𝐽 = −𝑞𝑞 �𝐸𝐸𝑔𝑔

∞

𝐵𝐵 𝐸𝐸 𝑑𝑑𝐸𝐸

𝐽𝐽 = −𝑞𝑞 �𝐸𝐸𝑔𝑔

∞2𝜋𝜋𝑐𝑐2ℎ3

𝐸𝐸2𝑑𝑑𝐸𝐸

𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸 − 𝜇𝜇𝑘𝑘𝐵𝐵𝑇𝑇

− 1

𝐽𝐽 = −𝑞𝑞Φ

0

2

4

6

8

10

12

14

0.95 1 1.05 1.1Eg 1.05Eg

B(E)

µ = 99.9 % Eg

µ = 99.0 % Eg

µ = 90.0 % Eg

Shockley–Queisser limit

T1

Tp

Q1

Q2

W

Tp, µ, Eg

k1

Maximum solar energy conversion efficiency for planet earthwith a semiconductor of band gap Eg

Tp µ Eg

V

Ts

Then:

𝐽𝐽 = −𝑞𝑞𝑘𝑘 𝐶𝐶𝐶𝐶 �𝐸𝐸𝑔𝑔

∞𝐸𝐸2𝑑𝑑𝐸𝐸

𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸𝑘𝑘𝐵𝐵𝑇𝑇𝑠𝑠

− 1− 1 − 𝐶𝐶𝐶𝐶 �

𝐸𝐸𝑔𝑔

∞𝐸𝐸2𝑑𝑑𝐸𝐸

𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸𝑘𝑘𝐵𝐵𝑇𝑇𝑝𝑝

− 1− �

𝐸𝐸𝑔𝑔

∞𝐸𝐸2𝑑𝑑𝐸𝐸

𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸 − 𝑞𝑞𝑞𝑞𝑘𝑘𝐵𝐵𝑇𝑇𝑝𝑝

− 1

Losses in semiconductor solar cells

Useful photons

ℎ𝜈𝜈 < 𝐸𝐸𝑔𝑔

Electron hole relaxation

Radiative recombination

𝑞𝑞𝑞𝑞 < 𝐸𝐸𝑔𝑔

The position of the Fermi level in a semiconductor

The number of electrons in the conduction band:

𝑛𝑛𝑒𝑒 = 𝑁𝑁𝐶𝐶 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝐶𝐶 − 𝐸𝐸𝑓𝑓𝑘𝑘𝐵𝐵𝑇𝑇

Similarly for holes:

𝑛𝑛ℎ = 𝑁𝑁𝑉𝑉 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝑓𝑓 − 𝐸𝐸𝑉𝑉𝑘𝑘𝐵𝐵𝑇𝑇

Then we can define :

𝑛𝑛ℎ𝑛𝑛𝑒𝑒 = 𝑛𝑛𝑖𝑖2 = 𝑁𝑁𝐶𝐶𝑁𝑁𝑉𝑉 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝐶𝐶 − 𝐸𝐸𝑉𝑉𝑘𝑘𝐵𝐵𝑇𝑇

𝑛𝑛𝑖𝑖2 = 𝑁𝑁𝐶𝐶𝑁𝑁𝑉𝑉 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝐺𝐺𝑘𝑘𝐵𝐵𝑇𝑇

Under global equilibrium the number of carriers in a semiconductor always equals 𝑛𝑛𝑖𝑖2

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ P5+ Si4+

Si4+ Si4+ Si4+ Si4+

e-

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ B3+ Si4+

Si4+ Si4+ Si4+ Si4+

h+

n-type

p-type

Controlling the Fermi level in a semiconductor

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ P5+ Si4+

Si4+ Si4+ Si4+ Si4+

e-

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ Si4+ Si4+

Si4+ Si4+ B3+ Si4+

Si4+ Si4+ Si4+ Si4+

h+

𝑛𝑛𝑒𝑒 𝑛𝑛ℎ 𝐸𝐸𝐹𝐹

n-type 𝑛𝑛𝑒𝑒 ≈ 𝑛𝑛𝐷𝐷 𝑛𝑛ℎ =

𝑛𝑛𝑖𝑖2

𝑛𝑛𝑒𝑒≈𝑛𝑛𝑖𝑖2

𝑛𝑛𝐷𝐷𝐸𝐸𝐹𝐹 = 𝐸𝐸𝐶𝐶 − 𝑘𝑘𝐵𝐵𝑇𝑇𝑇𝑇𝑛𝑛

𝑁𝑁𝐶𝐶𝑛𝑛𝐷𝐷

p-type 𝑛𝑛𝑒𝑒 =

𝑛𝑛𝑖𝑖2

𝑛𝑛ℎ≈𝑛𝑛𝑖𝑖2

𝑛𝑛𝐴𝐴𝑛𝑛ℎ ≈ 𝑛𝑛𝐴𝐴 𝐸𝐸𝐹𝐹 = 𝐸𝐸𝑉𝑉 − 𝑘𝑘𝐵𝐵𝑇𝑇𝑇𝑇𝑛𝑛

𝑁𝑁𝑉𝑉𝑛𝑛𝐴𝐴

Metal contact

φM

Ohmic junction

n-type Semiconductor

+ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +

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rgy Ef

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rgy Ef

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rgy Ef

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rgy Ef

Conduction band

Valence band

Eg

Metal contact

φM

Schottky junction

n-type Semiconductor

+ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +

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rgy

Conduction band

Ef

Valence band

Eg

Elec

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rgy

Conduction band

Ef

Valence band

Eg

Elec

tron

ene

rgy

Conduction band

Ef

Valence band

Eg

Elec

tron

ene

rgy

Conduction band

Ef

Valence band

Eg

Elec

tron

ene

rgy

Conduction band

Ef

Valence band

Eg

n-type Schottky junction diode

V

J

B

D

C

n-type Schottky junction solar cell

V

J

pn-junction diode

pn-junction diode solar cell

V

J

Forward biasReverse bias

Forward bias

Reverse bias

Equilibrium

The J-V curve and power conversion

Potential (volts)

Curr

ent d

ensit

y (m

A) J

J Power production (m

W)

-

-

-

-

-

-

-

𝜂𝜂𝑠𝑠𝑠𝑠𝑇𝑇𝑠𝑠𝑠𝑠 =𝑃𝑃𝑠𝑠𝑃𝑃𝑒𝑒𝑠𝑠 𝑠𝑠𝑜𝑜𝑜𝑜𝑃𝑃𝑠𝑠𝑃𝑃𝑒𝑒𝑠𝑠 𝑖𝑖𝑛𝑛 =

𝐽𝐽𝑞𝑞𝐼𝐼𝑇𝑇𝑜𝑜𝐼𝐼𝑖𝑖𝑛𝑛𝑠𝑠𝑜𝑜𝑖𝑖𝑠𝑠𝑛𝑛 𝑖𝑖𝑛𝑛𝑜𝑜𝑒𝑒𝑛𝑛𝑠𝑠𝑖𝑖𝑜𝑜𝑖𝑖

𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚 =( 𝐽𝐽𝑞𝑞 )𝑀𝑀𝑀𝑀𝑀𝑀

𝑃𝑃𝑖𝑖𝑖𝑖=𝐽𝐽𝑆𝑆𝐶𝐶 𝑞𝑞𝑂𝑂𝐶𝐶 𝐹𝐹𝐹𝐹

𝑃𝑃𝑖𝑖𝑖𝑖𝐹𝐹𝐹𝐹 =

( 𝐽𝐽𝑞𝑞 )𝑀𝑀𝑀𝑀𝑀𝑀𝐽𝐽𝑆𝑆𝐶𝐶 𝑞𝑞𝑂𝑂𝐶𝐶

Timeline of photovolatic development

1883 - Charles Fritts develops a solar cell using selenium on a thin layer of gold to form a device giving less than 1% efficiency.

1904 - Wilhelm Hallwachs makes a semiconductor-junction solar cell (copper and copper oxide).

1932 - Audobert and Stora discover the photovoltaic effect in Cadmium selenide (CdSe).

1954 - Bell Labs announces the invention of the first modern silicon pn junction solar cell with about 6% efficiency.

Modern Si p-n Junction PVs

• Thin p-type layer (2.5 μm) formed over a n-type base.• Assuming Eg = 1.02 eV and μ = 0.5 eV, 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚 = 22% was

considered possible• Losses were reported as reflection (50%), e-h recombination,

and resistance in the surface layer and the contacts.

Modern Si p-n Junction PVs

Si growth methods

Polycrystalline silicon made from the Siemens process

2 HSiCl3 → Si + 2 HCl + SiCl4

Si growth methods

Single crystalline silicon by the Czochralski process

Si growth methodsDopant incorporation during crystal growth

• Dopants are added to the melt to provide a controlled N or P doping level in the wafers.

• However, the dopant incorporation process is complicated by dopant segregation.

• Generally, impurities “prefer to stay in the liquid” as opposed to being incorporated into the solid.

• This process is known as segregation. The degree of segregation is characterized by the segregation coefficient, ko, for the impurity.

CS

CL

kO = CS

CL

CS and CL are the impurity concentration just on the either side of the solid/liquid interface.

Si growth methods

Dopant incorporation during crystal growth

kO = CS

CL

Most k0 values are <1 which means the impurity prefers to stay in the liquid.Thus as the crystal is pulled, dopant concentration will increase.In other words, the distribution of dopant along the ingot will be graded.

Si growth methods

Dopant incorporation during crystal growthNote the relatively flat profile produced by boron with a ko close to 1. Dopants with ko << 1 produce much more doping variation along the crystal.

Si growth methods

Si processing

Structure of a modern Si pn junction PV

alkaline etching

diffusion furnace

Structure of a modern Si pn junction PV

Timeline of Si pn junction development

Ribbon silicon reduces waste

Structure of a modern Si pn junction PV

Modern Si PV installations

7.7MW in Rion-des-Landes

Proliferation and price of c-Si PVs

Data: Navigant Consulting Graph. PSE AG 2013

Proliferation and price of c-Si PVs

Data: Navigant Consulting Graph. PSE AG 2013

Global Cumulative PV Installation until 2012

Data: from 2000 to 2011: EPIA; for 2012: IHS. Graph: PSE AG 2013

All percentages are related to the total global installation

Global Cumulative Silicon PV Installation

Data: Navigant. Graph: PSE AG 2013

About 40 GWp of Silicon PV installed until 2012

Global Cumulative Silicon PV Installation

Data: Navigant. Graph: PSE AG 2013

Electrical Capacity of Renewable Energy Sources in Germany

In 2012 about 23% of the electricity in Germany has been generated by renewable energy (RE) sources according to BDEW

Proliferation and price of c-Si PVs

Future convergence with Grid?

C. Wolden et al. J. Vac. Sci. Technol. A 29, 030801 (2011)

Crystalline silicon PV overview

• Modern silicon pn junction invented in 1954 by Bell labs• Achieved 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚 = 25%• Standard commercial cells are at 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 =20 – 23% with

module efficiency of 15 – 20% • Total installed capacity is around 40 GW and c-Si

accounts for about 90% of all installed PV.*• Price is now as low as 0.20$/kWh (or 1.30 $/Wp)• Energy/CO2 payback time is now about 2.5 years**• Current trends will see this technology reach 1 TW

installed capacity by 2020

*Renewables 2011 GLOBAL STATUS REPORT http://www.ren21.net/Portals/97/documents/GSR/GSR2011_Master18.pdf

**Solar Energy Volume 85, Issue 8, 2011, Pages 1609–1628

𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚

𝐶𝐶𝑚𝑚𝑠𝑠𝑚𝑚

𝐶𝐶 = 1

Multi“color” conversion

i = 1 Eg = Eg1

i = 2 Eg = Eg2 < Eg1

i = 3 Eg = Eg3 < Eg2

.

.

.

Eg = Egn < Egn-1i = n

IV

IV

IV

IV

𝑖𝑖→∞68.2 %

𝑖𝑖→∞86.8 %

III-V semiconductors

III-V semiconductors

A modern III-V multijunction cell

Advances in OptoElectronicsVolume 2007 (2007), Article ID 29523

Band diagram of a multijunction solar cell

Proc. of SPIE Vol. 6339 633909-1

Multi junction cells for high 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

Aerospace Industry

Spectolab 15.5 mGa,InP/GaAs/Ge

Multi junctionSolar panels𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 30%

Concentrated PV power plants

Concentrated PV power plants

x-Si approaches grid parity

Continuing trends with C-Silicon

• Low efficiency Standard commercial cells are at 15-18% Need at least 22.5% cell efficiency

• Poor use of material Current cells use 3-9 g Si/Wp Should be under 0.6 g/Wp

• Current manufacturing processes are too complex and expensive

x-Si approaches grid parity

Without a significant paradigm shift in fabrication

“Generations” of solar cells

Amorphous silicon

DOI: 10.1143/JJAP.50.030001

H

H H

H

H H

HH

H

Staebler–Wronski (SW) effect

a-Si devices and modules 2MW Solar Model Power (Amorphous Silicon)Location: Bangbu City, Anhui Province

a-Si based tandem cells

DOI: 10.1143/JJAP.50.030001

a-Si PV costs and outlook• Price is currently about 20-30% less

than x-Si in $/Wp• Relatively low efficiencies• Slow deposition rates

High capital costs• Only 4.2% of global sales

Switzerland-based Oerlikon Solar $0.47/Wp in 2014

The search for a new material

The search for a new material

Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43, 2072.

The search for a new material

Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43, 2072.

CdTe photovoltaics

• CdTe Eg = 1.5 eV• Absorption coefficient

greater than 105 cm-1

• Thickness of only 2 µm is required for 99% absorption

• Zincblende crystal structure

• PVD or CVD methods used for thin film formation

Glass

p-CdTe

n-CdS

CdTe photovoltaics

• Best cell 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 =17 % (Normally modules are around 10%) → 0.9$/Wp(first cell below 1$/W)

• High-Rate Vapor Transport Deposition

Waldpolenz Solar Park (40MW)550’000 First solar panelsLocation:GermanyCompleted in 2008

Drawbacks:• CdTe is toxic• Tellurium is an

extremely rare element

CuInxGa(1-x)Se2 (CIGS)

• I-III-VI2 semiconductor material

• General material class of Chalcopyrites

• Eg = 1.0 eV – 1.7 eV• Deposition methods:

Selenization of metal precursors that are in the form of either stacked layers (Cu-Ga/In), nanoparticles

or inks consisting of Cu, In,andGa

Sputtering from metal selenide targets (Cu₂Se, In₂Se₃, (In,Ga)₂Se₃);

thermal evaporation from pure

CIGS device structure

CIGS device performance

• 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 15-16 % typical (20.3 % champion cells)• Solar Frontier and Miasolé using PVD techniques 0.75 $/Wp

• Nanosolar actively producing modules using solution based approaches.

Thin Film performance overview

Thin film PV installations overview

Thin film PV installations overview

Thin film PV installations overview

Data: Navigant. Graph: PSE AG 2013

About 50 GWp of Silicon + thin film PV installed until 2012

Energy Payback time

Data: Mono- and multi- Silicon data: ISE 2011; CPV data: “Environmental Sustainability of Concentrator PV Systems: Preliminary LCA Results of the Apollon Project“ 5th World Conference on PV Energy Conversion. Valencia, Spain, 6-10 September 2010; all other data: Wild-Scholten(ECN),Sustainability Dec. 2009. Graph:PSE AG 2013

Energy Payback time

Data: ISE 2011 (for mono, multi); de Wild-Scholten (ECN),Sustainability Dec. 2009. Graph: PSE AG 2013

Energy Payback time