EE 237: Solar Energy Conversion Week 7: Thin film technologies ...
EE 237: Solar Energy Conversion Week 7: Thin film technologies CdTe CIGS a-Si OPV May 13th 2013 tanford University
1
Quiz: Who made this ?
tanford University
2
Quiz: Who made this ?
1
2 Suntech
tanford University
3 Yingli
Trina
3
Quiz: Who made this ?
tanford University
4
Quiz: Who made this ?
IBC
tanford University
5
Quiz: Who made this ?
tanford University
6
Quiz: Who made this ?
Metal Wrap Through tanford University
7
Quiz: Who made this ?
KGO station, Dumbarton bridge
tanford University
8
Quiz: Who made this ? SolFocus
Non imaging optics 650X
tanford University
9
Quiz: Who made this ?
tanford University
10
Quiz: Who made this ?
Thin Film Based First Solar Miasole etc.
tanford University
11
Thin Film USP
• No c-Si • Thinner / Flexible -> Low BOS • Less Layers -> Less Steps
tanford University
12
How thin is thin film PV ?
tanford University
13
Thin Film PV
1 CdTe
2 CIGS
3
α-Si 4 OPV
tanford University
14
Lecture Today Thin film basics
2
1
CIGS
CdTe
4
3
α-Si
OPV
Common Issues 1
2 Module
tanford University
TCO
15
Quiz: Where are the cells ?
Thin Film Based First Solar Miasole etc.
tanford University
16
Thin film solar modules: monolithic integration transparent electrode glass
absorber transparent electrode glass
1 Module tanford University
17
Thin film solar modules: monolithic integration
metal absorber transparent electrode
glass
Hard to make interconnect gap (“dead area”) < 0.05cm…
…so need cells >1cm wide
1 Module tanford University
18
1 Module
tanford University
19
1 Module
• •
Shaded cells can get reverse biased Shading stresses the device
tanford University
EE 237 : Aneesh Nainani
20
Bypass diodes
1 Module
G24i Dye Sensitized Solar Module tanford University
21
1 Module
tanford University
Nanosolar’s Design
22
1 Module Solyndra’s cylindrical modules • Pros: 1) less angular dependence – so more power delivered over the course of the day 2) dirt and snow fall off/ around tubes and don’t need to be cleared 3) the modules sit right on top of a flat roof • The racks are built in to the module • Can be lifted by 2 people • Do not require permanent installation – no holes in the roof • Cons: 1) missing a lot of active area 2) module efficiency ~10% (my personal estimate based on module power output) 3) requires customized fabrication equipment to accommodate cylindrical substrates
tanford University
23
A comparison of Solyndra’s models to their 1 competitors Module
tanford University
24
Wind Performance
Ability to Avoid Heating
tanford University
25
2
Trasmittivity
TCO
Resistivity
tanford University
26
2 TCO
tanford University
27
Other Issues common to thin film
tanford University
28
Different thin film options Thin film basics
2
1 CdTe
4
3 CIGS
α-Si
OPV
Common Issues tanford University
29
CdTe: Industrial Status First Solar is the leader. It takes them 2.5 hours to make an 11 % module.
The energy payback time is 0.8 years. tanford University
30
CdTe Solar Cell with CdS window layer • Since CdTe has a direct band gap, too many photons are absorbed close to the surface where recombination is fast – A wide band-gap window layer is used to avoid this Back Contact: Cathode Metal Absorber layer
Window Layer
P-type CdTe
3~8 um
N-type CdS
0.1 um
Transparent Conducting Oxide Front Contact: Anode
Glass Superstrate
0.05 um ~1000 um
Incident Light CdS: tends to be n-type, large bandgap(2.42eV) tanford University
31
1 CdTe
glass
CdS/CdTe
tanford University
32
1
Energy Band Diagram
CdTe
Heavily n-doped 1.5 eV
Lightly p-doped (1015 cm-3)
Ec
2.42 eV Ev
tanford University
33
Advantages of CdTe thin film solar cells • Direct bandgap, Eg=1.45eV • High efficiency (Record:17.3%; Industry: 15.3 module record, typical module 11.7%) • High module production speed • Cheap substrate (commercial glass) • Long term stability (20 years)
tanford University
34
Doping and Impurities in CdTe • CdTe tends to be lightly p-type as a result of intrinsic defects (vacancies and interstitials) – It is difficult to make high quality n-type material • The best dopants for p-type CdTe are copper and antimony • It is difficult to heavily p-dope CdTe – Excess dopant is self-compensating because it is interstitial instead of substitutional – As a result, CdTe is more lightly doped than is ideal
tanford University
35
The back contact • There is frequently a small Schottky barrier at the back contact. • High work function metals would eliminate the barrier, but are expensive. • Doping CdTe at a high density to reduce the barrier width is not easy because of dopant selfcompensation. • Back contact problem solved to a large extent by first solar tanford University
36
CdS kills the efficiency below 500 nm
The challenge in industry is to implement thin CdS layers without having a pinhole. tanford University
37
Vapor Transport • CdTe is heated in an ampoule • vapor is mixed with carrier gas and pushed out of a nozzle • vapor condenses on a moving substrate • typical pressure: 10-100 Torr. • typical temperature: 800 ºC • very high deposition rate • well-suited for scale-up of production • First Solar uses this method
tanford University
38
Depositing the CdTe Absorber layer If the ratio of Cd:Te in a film is not 1:1, the excess of one element forms a liquid that evaporates away since the film is grown in a vacuum. Consequently, the film will have the right composition for a wide range of deposition conditions. This is far from true for many other compounds and gives CdTe a big advantage.
tanford University
39
Cell Activation by CdCl2 treatment Annealing CdTe in the presence of cadmium chloride (CdCl2) around 400°C allows part of the film to liquify. Rapid elemental diffusion enables the growth of crystal grains with a size comparable to the film thickness and reduces defect density at interfaces. It is for this reason than many crude low cost film deposition methods can be used. This is one of the most important and underappreciated techniques of the solar industry.
untreated tanford University
CdCl2, 400°C 40
2 CdTe
• Te : rare earth ($$$ ?) • Cd : poisonous (EU RoHS) • CdTe : Not allowed in Japan tanford University
41
CdTe Emissions in Residential Fires • Flame temperatures in roof fires range from 800o900oC • CdTe becomes encapsulated inside molten glass in fires • Several studies have shown that CdTe releases are unlikely to occur during residential fires or during accidental breakage.
tanford University
42
1
Quiz: Who made this ?
tanford University
2
Quiz: Who made this ?
1
2 Suntech
tanford University
3 Yingli
Trina
3
Quiz: Who made this ?
tanford University
4
Quiz: Who made this ?
IBC
tanford University
5
Quiz: Who made this ?
tanford University
6
Quiz: Who made this ?
Metal Wrap Through tanford University
7
Quiz: Who made this ?
KGO station, Dumbarton bridge
tanford University
8
Quiz: Who made this ? SolFocus
Non imaging optics 650X
tanford University
9
Quiz: Who made this ?
tanford University
10
Quiz: Who made this ?
Thin Film Based First Solar Miasole etc.
tanford University
11
Thin Film USP
• No c-Si • Thinner / Flexible -> Low BOS • Less Layers -> Less Steps
tanford University
12
How thin is thin film PV ?
tanford University
13
Thin Film PV
1 CdTe
2 CIGS
3
α-Si 4 OPV
tanford University
14
Lecture Today Thin film basics
2
1
CIGS
CdTe
4
3
α-Si
OPV
Common Issues 1
2 Module
tanford University
TCO
15
Quiz: Where are the cells ?
Thin Film Based First Solar Miasole etc.
tanford University
16
Thin film solar modules: monolithic integration transparent electrode glass
absorber transparent electrode glass
1 Module tanford University
17
Thin film solar modules: monolithic integration
metal absorber transparent electrode
glass
Hard to make interconnect gap (“dead area”) < 0.05cm…
…so need cells >1cm wide
1 Module tanford University
18
1 Module
tanford University
19
1 Module
• •
Shaded cells can get reverse biased Shading stresses the device
tanford University
EE 237 : Aneesh Nainani
20
Bypass diodes
1 Module
G24i Dye Sensitized Solar Module tanford University
21
1 Module
tanford University
Nanosolar’s Design
22
1 Module Solyndra’s cylindrical modules • Pros: 1) less angular dependence – so more power delivered over the course of the day 2) dirt and snow fall off/ around tubes and don’t need to be cleared 3) the modules sit right on top of a flat roof • The racks are built in to the module • Can be lifted by 2 people • Do not require permanent installation – no holes in the roof • Cons: 1) missing a lot of active area 2) module efficiency ~10% (my personal estimate based on module power output) 3) requires customized fabrication equipment to accommodate cylindrical substrates
tanford University
23
A comparison of Solyndra’s models to their 1 competitors Module
tanford University
24
Wind Performance
Ability to Avoid Heating
tanford University
25
2
Trasmittivity
TCO
Resistivity
tanford University
26
2 TCO
tanford University
27
Other Issues common to thin film
tanford University
28
Different thin film options Thin film basics
2
1 CdTe
4
3 CIGS
α-Si
OPV
Common Issues tanford University
29
CdTe: Industrial Status First Solar is the leader. It takes them 2.5 hours to make an 11 % module.
The energy payback time is 0.8 years. tanford University
30
CdTe Solar Cell with CdS window layer • Since CdTe has a direct band gap, too many photons are absorbed close to the surface where recombination is fast – A wide band-gap window layer is used to avoid this Back Contact: Cathode Metal Absorber layer
Window Layer
P-type CdTe
3~8 um
N-type CdS
0.1 um
Transparent Conducting Oxide Front Contact: Anode
Glass Superstrate
0.05 um ~1000 um
Incident Light CdS: tends to be n-type, large bandgap(2.42eV) tanford University
31
1 CdTe
glass
CdS/CdTe
tanford University
32
1
Energy Band Diagram
CdTe
Heavily n-doped 1.5 eV
Lightly p-doped (1015 cm-3)
Ec
2.42 eV Ev
tanford University
33
Advantages of CdTe thin film solar cells • Direct bandgap, Eg=1.45eV • High efficiency (Record:17.3%; Industry: 15.3 module record, typical module 11.7%) • High module production speed • Cheap substrate (commercial glass) • Long term stability (20 years)
tanford University
34
Doping and Impurities in CdTe • CdTe tends to be lightly p-type as a result of intrinsic defects (vacancies and interstitials) – It is difficult to make high quality n-type material • The best dopants for p-type CdTe are copper and antimony • It is difficult to heavily p-dope CdTe – Excess dopant is self-compensating because it is interstitial instead of substitutional – As a result, CdTe is more lightly doped than is ideal
tanford University
35
The back contact • There is frequently a small Schottky barrier at the back contact. • High work function metals would eliminate the barrier, but are expensive. • Doping CdTe at a high density to reduce the barrier width is not easy because of dopant selfcompensation. • Back contact problem solved to a large extent by first solar tanford University
36
CdS kills the efficiency below 500 nm
The challenge in industry is to implement thin CdS layers without having a pinhole. tanford University
37
Vapor Transport • CdTe is heated in an ampoule • vapor is mixed with carrier gas and pushed out of a nozzle • vapor condenses on a moving substrate • typical pressure: 10-100 Torr. • typical temperature: 800 ºC • very high deposition rate • well-suited for scale-up of production • First Solar uses this method
tanford University
38
Depositing the CdTe Absorber layer If the ratio of Cd:Te in a film is not 1:1, the excess of one element forms a liquid that evaporates away since the film is grown in a vacuum. Consequently, the film will have the right composition for a wide range of deposition conditions. This is far from true for many other compounds and gives CdTe a big advantage.
tanford University
39
Cell Activation by CdCl2 treatment Annealing CdTe in the presence of cadmium chloride (CdCl2) around 400°C allows part of the film to liquify. Rapid elemental diffusion enables the growth of crystal grains with a size comparable to the film thickness and reduces defect density at interfaces. It is for this reason than many crude low cost film deposition methods can be used. This is one of the most important and underappreciated techniques of the solar industry.
untreated tanford University
CdCl2, 400°C 40
2 CdTe
• Te : rare earth ($$$ ?) • Cd : poisonous (EU RoHS) • CdTe : Not allowed in Japan tanford University
41
CdTe Emissions in Residential Fires • Flame temperatures in roof fires range from 800o900oC • CdTe becomes encapsulated inside molten glass in fires • Several studies have shown that CdTe releases are unlikely to occur during residential fires or during accidental breakage.
tanford University
42
