This document discusses thin-film photovoltaics research and opportunities. It covers several topics: - Thin-film solar cell technologies like CIGS, CdTe, and emerging materials like CZTS have higher efficiencies than earlier generations and lower production costs. Research aims to further improve efficiency and reduce costs. - The Helmholtz-Zentrum in Berlin conducts R&D on thin-film photovoltaics including advanced materials, device concepts, and characterization techniques to develop more efficient and cost-effective solar cells. - Issues like material scarcity for some thin-film technologies are being addressed through research into alternative materials and processes to produce solar cells on flexible substrates using less raw

1. Thin-Film Photovoltaics R&D: Innovation, Opportunities
Ahmed Ennaoui
Helmholtz-Zentrum Berlin für Materialien und Energie
ennaoui@helmholtz-berlin.de
IRESEN ´s Event for the launch of calls for proposals 2013 Casablanca, January 30th, 2013
This material is intended for use in lectures, presentations and as handouts to students, it can be provided in Powerpoint format to allow
customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage.
Flexible PV OPV Nanoparticles
Tandem Solar cellSilicon Solar cell
http://www.iresen.org/index.php
Thin Film Solar Cell
DSSC

2. Advanced Thin-Film Devices
Novel Materials and Device Concepts
Advanced Analytics and Modelling
Solar Fuels
• Chalcopyrite-Type Semiconductors
• Silicon Photovoltaics
• Directed toward long-term goal of
producing cost-effective and more
efficient devices
• Advanced interface analysis
• Charge carrier dynamics
• Microstructure an defect analysis
• Device and material characterisation
• Development cost-effective PV
hybrid systems directly convert
sunlight into stored chemical
energy producing hydrogen via
water splitting
Solar Energy Division in Helmholtz-Zentrum Berlin für Materialien und Energie
Kompetenzzentrum Dünnschicht- und
Nanotechnologie für Photovoltaik Berlin
 Thin-film module production.
 R&D education and training
 R&D of industrial processes
 R&D of promising high-risk concepts
 Up-scaling of successful R&D of HZB
 International Summer University on Energy
4 Research Topics

3. Thin-Film Photovoltaics: Innovation and Opportunities
Third Generation: Molecular devices:
 Dye sensitised DSSC
 Organics OPV
 Quantum structured solar cells
Innovation
Thinner
Efficient
FasterSecond Generation
 Cu-chlacopyrite compounds (CIGSSe)
 Emerging structure compounds (CZTSSe)
 Cadmium telluride (CdTe)
 Amorphous and µ-crystalline silicon
Scarcity of materials
Monolithic integration
Lower production costs
Large area deposition
Energy pay back time
Implementation in building
Highlight

4. Introduction: VLSI Technology vs. Solar Cell and Moore´s law
The first practical photovoltaic cell was invented
at Bell Laboratories in 1954 (few %)
1941, first silicon solar cell was reported
(US Patent 240252, filed 27 March 1941)
http://www.intel.com/technology/silicon/mooreslaw/
Solar Cells: Efficient, Thinner , Cheaper, Faster
Pentium 4 has around 55 million components per chip (2003)
Number of transistors doubles every two years
Computer: Faster, cheaper, storing more data
Quelle: G. Willeke, ISE
First transistor

5. Introduction: PV Module production cost evolution
CdTe
Record Cell
EFFiciency (%)
17.3
Record Module
EFFiciency (%)
15.5
Aver. Module
EFFiciency (%)
12.5
Prod. Capacity
2011 (MWp/yr)
2200
Prod. Capacity
2012 (MWp/yr)
2700
CIGSS
Minimodule
Cell (0.5cm2)
17.8
19.7%
Record Module 14.5
Aver Module 12.6
Prod. Capacity
2011 (MWp/yr)
500
Prod. Capacity
2012 (MWp/yr)
1000
 Crystalline -Si PV prices dropped by over 40% EFFICIENCY ≈15 % 0.8 - 0.6 €/W
 CdTe (First Solar) / EFFICIENCY ≈12.2 % 0.67 €/W
 CIGSS Solar Frontier EFFICIENCY ≈ 12.6% 0.55 - 0.42 €/W
 a-Si:H/mcSi / (Oerlikon,ThinFab140) / EFFICIENCY 2012 ≈ 10.8% (154 W) 0.35 $/W
New Record (January 2013)
Quelle: Alberto Mittiga/ENEA / and H.W. Schock Annu. Rev. Mater. Res. 2011. 41:297–321

6. Introduction: Storage: High capacity, Higher Operating voltage, and Long Cycle
Quelle
http://www.treehugger.com/files/2008/02/lithium-ion_battery_factory.php
LITHIUM BATTERIES:
• high energy density (3 times lead-acid).
• Application spans beyond the electronics market
• Li-ion nanophosphate is inherently safer.
• Safe non-flammable electrolytes.
Structurally stable compounds, such as: LiFePO4
High capacity, Higher Operating voltage, and Long Cycle

7. Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Introduction: Key Task of Photovoltaic
Power [Watt/cm2] = Voltage [Volt ] x Current density [A/cm2]
PV products can be optimized for
location, with lower associated financial
risk based on predictable performance
 Key aim is to generate electricity from solar spectrum
EFFECIENCY INCREASING
LESS AREA
LESS MATERIAL
COST FOR PV REDUCED
LOW €/Wp
Materials with small Band gap
But low voltage
Excess energy lost to heat
 Generating a large current (JSC)
Materials with large band gap
But low current
Sub-band gap light is lost
 Generating a large voltage (VOC)
 Two challenges
Solar cell design
versus
solar spectrum
Voltage [Volt ]
JSC[A/cm2]
Power[Watt/cm2]
VOC
0
JSC
maximum
power
point
Jm
Vm
Vm x Jm
AM1,5

8. Efficiencies beyond the Shockley-Queisser limit
500 1000 1500 2000 2500
0
200
400
600
800
1000
1200
1400
1600
Leistungsdichte[W/m
2
µm]
AM15
GaInP
GaInAs
Ge
Wellenlänge [nm]
(1) Lattice thermalization loss (> 50%)
(2) Transparency to h < Band gap
(3) Recombination Loss
(4) Current flow
(5) Contact voltage loss
Not all the energy of absorbed photon
can be captured for productive use.
(Th. Maxi efficiency ~32% ).
source
1.7 eV
1.1 eV
0.7 eV
R.R. King; Spectrolab Inc., AVS 54th International Symposium, Seattle 2007
Optimistic calculation
Best commercially available cells  37% efficient at 25°C.
75% efficient  0.30 × 0.75 × 850 ≈ 200 W/m2 of electrical power.
At $200/m2 the capital cost would be $1.50/W.

9. Si
14
Si
Ge
32
Ga
31
As
33
Cd
48
Te
52
P
15
In
49
Al
13
Sb
51
Cu
29
Se
34
In
49
31
IIB IIIB IVB VB VIBIB
C
6
B
5
Zn
30
Sn
50
S
16
O
8
N
7
Periodic Table
ZnS
Ge
GaAs
CdTe
InP
AlSb
CdS
Scientific Background
Silicon
IV
Tetrahedrally coordinated
4
...



mn
mqnq MN
n,m atoms/unit cell
Grimm-Sommerfeld rule
Source: Ennaoui Osaka seminar
CuInxGa1-xSe2
Cu2SnZnSe4
Diamond Structure
I-III-VI2
II-IV-V2
AlxGa1-xSe2
Cu(In,Ga)Se2
Cu2(ZnSn)Se4
Zincblende
Structure II-VIIII-V

10. Common Symbol+ -
Si
14
Ge
32
Ga
31
As
33
Cd
48
Te
52
P
15
In
49
Al
13
Sb
51
Cu
29
Se
34
In
49
31
IIB IIIB IVB VB VIBIB
C
6
B
5
Zn
30
Sn
50
S
16
O
8
N
7
Periodic Table
Silicon (IV): Diamond Structure
Doping Technology of Silicon: pn junction of Silicon
PERL: passivated emitter and rear cell ( 25%)
Martin Green, UNSW’s cell concepts PIP 2009; 17:183–189 / http://www.unsw.edu.au/

11. Device fabrication
1. Surface etch, Texturing
2. Doping: p-n junction formation
3. Edge etch: removes the junction at the edge
4. Oxide Etch: removes oxides formed during diffusion
5. Antireflection coating: Silicon nitride layer reduces reflection
Cells
Purifying the silicon:
STEP 1: Metallurgical Grade Silicon (MG-Silicon is produced from SiO2 melted
and taken through a complex series of reactions in a furnace at T = 1500 to
2000°C.
STEP 2: Trichlorosilane (TCS) is created by heating powdered MG-Si at around
300°C in the reactor, Impurities such as Fe, Al and B are removed.
Si + 3HCl SiHCl3 + H2
STEP 3: TCS is distilled to obtain hyper-pure TCS (<1ppba) and then vaporized,
diluted with high-purity hydrogen, and introduced into a deposition reactor to form
polysilicon: SiHCl3 + H2→Si + 3HCl Electronic grade (EG-Si), 1 ppb Impurities
STEP 1
STEPE 2 and 3
Electronic
Grade Chunks
Source: Wacker Chemie AG, Energieverbrauch: etwa 250kWh/kg im TCS-Process, Herstellungspreis von etwa 40-60 €/kg Reinstsilizium
Ingot sliced
to create wafers
Making single
crystal silicon
Czochralski (CZ) process
crucible
Seed crystal slowly grows
Microelectronic
1G: Crystalline Si PV technology

12. Nanotechnology in Roman Times: The Lycurgus Cup
Plasmons of gold nanoparticles in glass reflect green, transmit red
Because of plasmonic excitation of electrons in the metallic particles suspended within
the glass matrix, the cup absorbs and scatters blue and green light – the relatively short
wavelengths of the visible spectrum. When viewed in reflected light, the plasmonic
scattering gives the cup a greenish hue, but if a white light source is placed within the
goblet, the glass appears red because it transmits only the longer wavelengths and
absorbs the shorter ones.”
Nanosacle: 1m/1000 000 000 Photonic and Plasmonics
Quelle: http://daedalus.caltech.edu/research/plasmonics.php and US Department of Energy
Mesoscale structure
• Defects and interfaces are functional at the mesoscale.
• Control of light is critical for next generation high performance solar cells.
Photonic
(A) SEM image of a nanodot focusing array
(B) SP intensity showing subwavelength focusing
Ekmel Ozbay Sciences 311 (2006) pp. 189-193
1µ
Catalytic reactive surface
Nanotechnology: Photonic/Plasmonic/Solar cell
NanosynthesisModeling and SimulationCharacterization
Plasmonic

13. Glass, Metal Foil, Plastics
CdTe based device
Quelle: Noufi, NREL, Colorado, USA,
Substrate configuration
*CIGS based device
CdTe and CIGS Thin Film Solar cells (2nd. Generation)
Superstrate configuration
Common features
 p-type materials due to intrinsic defects and fast diffusing impurities (Cl in CdTe and Na in CIGS).
 Heterojunction made using an high band gap buffer layer CdS (2.42 eV), ZnS (3.6 eV)
 Efficiency for Polycrystalline Thin-Film Solar cells larger than their single crystal counterpart.
 Excellent outdoor stability (with good lamination) and radiation hardness
Tolerance to wide range of molecularity Cu/(In+Ga)
Yields device efficiency of 17% to 20%
Equilibrium vapor pressure of Cd and Te much
higher than that of CdTe
The pure phases tend to evaporate

14. R&D Directions
In situ optical processing
Optimal range for efficient thin film
solar cells: 22-24 at % of Cu
a-phase highly narrowed at room temperature
Possible at growth T from RT to 550°C
b-phase (CuIn3Se5) defect phase defect pairs (VCu, InCu)
d-phase (high-temperature)  Cu & In sub-lattice
Cu2Se
• built from chalcopyrite structure by
• Cu interstitials Cu-Inanti sites
• Melting point at ca. 530°C surfactant
for recristallization (large grains)
 Reduce the manufacturing cost.
 Through efficiency improvements.
 Reduce the thickness of CIGS.
(0.7 µm thickness and 17% efficiency)
 Interface engineering.
 Band gap adjustment: 1.03eV-1.7 eV.
 Cadmium free buffer layer R&D.
 Low cost processing.

15. 1. Glass
2. Mo
CIGS
Buffer
ZnO
Technology: Monolithically Integrated PV
P1
Step 1: Deposition of Cu, In,Ga (Se)
(sputtering, codeposition, Electrodeposition)
Step 2: Rapid Thermal Processing (RTP)
Pulsed
Laser
Front ZnO of 1 cell is connected to the back Mo contatc of the next cell
Se Cu
Ga In
Cu(In,Ga)Se2
Monolithic integration for series connection of individual cells
P3
P2 P1
- P1: Series of periodic scribes to defines the width of the cells
- P2: Mechanical scribes after the absorber and buffer layer
- P3: Mechnical scribes after the window deposition
Si
Module
Vmodule= Vcell x Ncell
 24 V for battery charging

16.  CIGS manufactured on low cost glass substrates.
 CIGS manufactured on flexible substrates.
 Enables access to the largest PV markets.
 Short energy pay back time and less energy consuming process.
 Compatible with existing photovoltaic system infrastructure.
 Easy to integrate into Building (BIPV) market.
Strong point of CIGS
PowerFLEX™ Modules
http://www.globalsolar.com
BIPV thin-film CIGS façade
Honda building in Japan
Light weight
3.5 kg/m2
EFFICIENCY
10.5% to 12.6%
50% more efficient than flexible a-Si

17. Evolution and Record efficiency
20.4%
~11.1%
~12%
19.7%
Cadmium free
CIGSS Jan. 2013
Flexible substrate
CIGSe Jan. 2013
CZTSSe
IBM New York 2013
OPV
2013
 EFFICIENCY 20.4% for Cu(In,Ga)Se2 or (CIGS) on polymer foils
(Swiss Federal Laboratories EMPA achieved January 2013)
 EFFICIENCY 12% for PV (OPV)
(Heliatek: German organic January 2013 )
 EFFICIENCY 11.1% for ink-based Cu2ZnSn(S,Se)4 (CZTS)
(IBM’s Materials Science team + Solar Frontier, Tokyo + DelSolar )
Big issue: CIGS and CZTS
on flexible substrate

18. Problem: Materials availability
Modules
(EFF ≈11%)
Metal
Required
(Tonn/m2)
Reserves
1998
Tonn
Productio
n
1997
Tonn/yr
CdTe
(3 µm)
Te
180 t/GWp
20000 290
CIGS
(2 µm)
In 98 t/GWp 2600 290
 2011 Total PV Annual production≈37 GWp/yr (2 GWp/yr due to CdTe and 1GWp/yr due to CIGS)
 Worldwide continuous electricity consumption : 15 Terawatts
Fthenakis, Renewable and Sust. Energy Rev., 13, 2746-2750, 2009 / http://www.compoundsemiconductor.net/csc/news-details.php?id=19735415
 CZTS Thickness 1 μm and an efficiency of 10% needs 10 g/m2 of material
CZTS PV cells could potentially yield up to 500 GW/year.
 CIGS and CdTe contain rare elements that limit their manufacturing < 100 GW /year.
 Recycling issue for In and Te
B. A. Andersson Prog. Photovolt. Res. Appl. 8, 61 (2000) / Wadia et al. Environmental Science and Technology 2009, 43, 2072
U.S. Geological Survey Fact Sheet 087-02

19. Design to high efficiency solar cells
Light trapping
Reflection Loss: ARC
Material
Parameter
absorption
Important cost factor
thikness
€









 αW
p
e
αL1
1
1R)(1η
λ
hc
e
)J(
Φ(λ)
1
N
N
η photons
in
electron
out



Decisive Material
Parameter
The band gap
0.3 0.5 0.7 0.9 1.1
20
0
40
60
80
100
0
1
2
3
4
5
NumberofSunlightPhotons(m-2s-1micron-1)E+19
RExternalQuantumEfficiency,%
c-Si:H junctiona-Si:H junction
AM 1.5 global spectrum
Wavelength, microns
a-Si:H/c-Si:H Cell Spectral Response
Textured TCO
a-Si
Top cell
Back
Reflector
Glass
substrate
Thin film mc-
Si
Bottom cell
  
GE
λ0λsc dλ.dα-exp.)().ΦR(1.η(λ).qJ
Light from the sun
C10x1.6e
][A.mCurrent
N 19
-2
electron
out 


energy[J]photon
][J.mEnergyInput
N
-2
Photon
in 

20. a-Si/μc-Si Tandems: Tandem Cell Design
Source PVComB/Rutger Schlatmann

21. a-Si/μc-Si Tandems: Lab Record Cells (1 cm², stable)
Source PVComB/Rutger Schlatmann

22. Triple Cell Optimization
Source PVComB/Rutger Schlatmann

23. Triple Cell: Improvements of optical and electrical properties
Interfaces, Zeman& Krc, J.Mater. Res. Vol23(4) 889-898 (2008)
Source PVComB/Rutger Schlatmann
Basic research
Optical + Electrical

24. • 3th. Generation: OPV solar cells
Provide Earth abundant and low-energy-production PV solution.
Organic semiconductors: Abundant: ~100,000 tons/year
• Key component
The electron acceptor
Light harvesting material (conjugated polymer)
Organic Photovoltaics (OPV): Molecular Perspective
Aluminum
Absorber
Polymer Anode
ITO
Substrate
Donor polymer (i.e. P3HT)
absorbs light generating an exciton
Exciton must diffuse to the
Donor/Acceptor interface
Status (Dresden/Germany, 16. Januar 2013 / http://www.heliatek.com/)
New word record 12% efficiency by Heliatek GmbH
Polymer-Fullerene Heterojunction Cells
Electrons travel to the back
electrode and Holes travel to the
front electrode
~200nmthick

25. OPV: R&D
Large Scale Printing
Konarka
Important issue:
optimizing the band gap and LUMO-LUMO offset

26. Donor acceptor concept
Quantum size effect
To to varie the band gap
HOMO-LOMO
Quantum Size effect
Nanosynthesis
R. D. Schaller, V. I. Klimov, Physical Review Letters, 2004, Vol. 92.

27. Excellent review on Concept of Inorganic solid-state nanostructured solar cells
T. Dittrich, A. Belaidi, A. Ennaoui Extremely Thin Absorber (ETA)
Solar Energy Materials and Solar Cells, Volume 95, Issue 6, June 2011, Pages 1527-1536
ZnO
nanorodes
Nanocrystalline based Solar cells
 Electron holes photogenerated
 Immediately injected in mesoporous TiO2 (or ZnO NRs)
Photosynthesis
CO2
Sugar
H2O
O2

28. OPV: Research direction
glass or plastic
transparent conductor
organic-inorganic
metal
Organic multijunction architecture ((Including Encapsulation and reduce cell degradation)
NC Nanoparticles
Nanosynthesis,
Nanotechnology
Organic / Polymer
Chemistry Coating
Technoques
Contact
materials
Contact
materials
Glass
Ag
ZnO-NRs
ZnPc:C60 C60
MoO3
ZnO-NR / C60 / ZnPc:C60 / MoO3 / Ag
200 nm
First solar cells with ZnO-NRs and small molecules / Eff. 2.8%
HZB-Patent WO 2008 / 104173 (Rusu et al.)

29. H2O→2H2+O2 ∆V=1.23V, ∆G=238kJ/mol
R&D: Hydrogen Fuel
Source: Mildred Dresselhaus, Massachusetts Institute of Technology
D
D
D
D
D
D
Heterogenous process
Homogenous process
Thin Film Material Research
 Band gap must be at least 1.8-2.0 eV
 To absorb most sunlight spectrum
 Compatible with Redox potentials
 Fast charge transfer
 Stable in aqueous solution
 Nanoparticle catalysts
 Nanoparticle: Surface-to-volume ratio
Nathan Lewis, Caltech
(1) Two spatially separated electrodes
coated with catalysts placed in
water.
(2) Cathode produced hydrogen,
and anode produces oxygen
D
D
D
D
D

30. R&D: Fuel Cells
O2- and H+ combine Energy
is given off in electron
form and gives off power
to run an engine
The “waste
products” are
water and heat
Catalyst = Pt Very
expensive
Minimize the Pt quantity
Improve the active layer structure
Propose new materials
 Fuel Cell uses a constant flow of
H2 to produce energy.
 Reaction takes place between
H2 and O2  electrical energy.
Platinum for a reaction that ionizes the gas
O2 is ionized to O2-
H2 is ionized to 2H+  2H+ + O2- = H2O
The most common fuel cell uses
• Proton Exchange Membrane, or PEM
• Need of alternative catalyst (Platin is expensive)

31. Advantages
Zero emission
No dependence on foreign oil
Ability to harvest solar and renewable energy
Not many moving part in a car
Hydrogen weighs less than gasoline :
car would not need as much energy to move
R&D
Fuel Cells: Platinum plate is very expensive.
Batteries: Lithium batteries: high energy density (3 times lead-acid).
Safety issue:
Instead of oxygen releasing (LiCoO2)
Structurally stable alternative compounds, e.g. LiFePO4
Chemistry and anode/cathode design
Li-ion nanophosphate
Storage (Fuel Cell, Batteries)

32. Final Remarks
others
0.3 GW
CIS
0.9 GW
CdTe
2.05 GW
Amorphous/microcrystalline silicon
1.26 GW
Monocrystalline silicon
11.5 GW
Multicrystalline silicon
21.2 GW
2011 Total Production : 37 GW
Quelle: http://www.photon-international.com
Weak point of c-Si:
•Indirect bandgap 1 eV
•Low light absorption
•Huge loss
•Production Cost
Strong point of c-Si:
• High module efficiency: up to 20%
• High stability and reliability
• Mature and “modular” technology
Thin Film Solar PV
Inexpensive ways to produce energy, (few cts/kWh)
Thinner, Efficient, Faster, Cheap
Large area deposition
Energy pay back time
Implementation in building
Scarcity of materials
Monolithic integration
Lower production costs
 Cu(In,Ga)(SSe)2 20.4% flexible ; 19,7% Cd free
 Cu2ZnSn(S,Se)4  Printing technology?
 OPV new record 12%  Printing technology?
 DSSC (11%)  Reliability/Degradation, solid electrolyte
 Quantum devices (long term Research topic)
15 %
0.8 - 0.6 €/W
12.6%, 0.8 - 0.6 €/W
12.2 % , 0.67 €/W
10.8% (154 W) 0.35 $/W

33. IRESEN Event for the launch of calls for proposals 2013 Casablanca, January. 30th, 2013
This material is intended for use in lectures, presentations and as handouts to students, it can be provided in Powerpoint format to allow
customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage.
Flexible PV OPV DSSC Nanoparticles
Thin Film Solar Cell Tandem Solar cellSilicon Solar cell
http://www.iresen.org/index.php
Vielen Dank für Ihre Aufmerksamkeit
‫اهتمامكم‬ ‫على‬ ‫لكم‬ ‫شكرا‬Thank you for your attention
Morocco is going to translate from being a net importer of energy and a country facing
water shortage issues, into a producer of clean renewable energy and water in the region.
‫المياه‬ ‫نقص‬ ‫قضايا‬ ‫يواجه‬ ‫وبلد‬ ‫للطاقة‬ ‫مستوردا‬ ‫كونه‬ ‫من‬ ‫يترجم‬ ‫أن‬ ‫المغرب‬ ‫على‬ ‫ينبغي‬
‫النظيفة‬ ‫المتجددة‬ ‫للطاقة‬ ‫منتج‬ ‫إلى‬‫المنطقة‬ ‫في‬ ‫والمياه‬
‫الكبير‬ ‫التحدي‬Big Challenge