The document discusses lessons that can be learned from CdTe photovoltaics and applied to CZTS solar cells. Specifically, it examines how to engineer defect-tolerant heterojunction interfaces through three key factors: 1) the conduction band offset between materials, 2) emitter doping and thickness, and 3) the type of interface defects. Interface recombination can be reduced by creating a spike in the conduction band for type I heterojunctions or avoiding a cliff for type II heterojunctions. Defect-tolerant interfaces also depend on absorber inversion induced by emitter doping. However, comparisons between materials require consideration of interface mixing and surface reconstruction challenges.
3. MOTIVATION…THE USUAL!
Underperformance of CZTS solar cells!
Low open circuit voltage (Voc) relative to the band gap
+ An excuse to learn what various terminologies related to interfaces
actually mean!
4. MOTIVATION
The usual excuses for CZTS:
1. non-ohmic back contact (normally with Mo)
2. poorly optimized interface with the n-type
(buffer/window) layer
3. defects and disorder in the bulk
Enhanced e-
- h+
recombinatio
n
5. MOTIVATION
The usual excuses for CZTS :
1. non-ohmic back contact (normally with Mo)
2. poorly optimized interface with the n-type
(buffer/window) layer
3. defects and disorder in the bulk
Enhanced e-
- h+
recombinatio
n
Measurements for high-
performance devices generally
suggest this is okay …if we trust
the measurements!
6. MOTIVATION
The usual excuses for CZTS :
1. non-ohmic back contact (normally with Mo)
2. poorly optimized interface with the n-type
(buffer/window) layer
3. defects and disorder in the bulk
Enhanced e-
- h+
recombinatio
n
What we normally care about:
• point defect calculations
• Eris MC large-scale antisite disorder
simulations
• Usually interface recombination only
becomes a significant limitation for
devices with good bulk properties
7. MOTIVATION
The usual excuses for CZTS :
1. non-ohmic back contact (normally with Mo)
2. poorly optimized interface with the n-type
(buffer/window) layer
3. defects and disorder in the bulk
Enhanced e-
- h+
recombinatio
n
But…could we make significant
improvements with CZTS by improving the
CdS-CZTS interface?
Can we learn useful lessons from studies on
other PV technologies?
Plus… learning what ‘spikes’, ‘cliffs’ etc.
actually mean!
8. ALTERNATIVE N-TYPE LAYERS FOR
CZTS?
CdTe CZTS CIGS
• Largely borrowed architecture from CIGS
• Also many similarities with CdTe CZTS, CdTe, CIGS all p-type in contact with n-type CdS
10. LESSONS TO BE LEARNT FROM
CDTE?
Emitter here = n-type material in contact with p-type absorber layer
(also referred to as buffer layer, window later, etc. …a little confusing!)
Choosing contact materials for defect-
tolerant p-n junction interfaces
Reduce interface recombination in PV
devices
12. HETEROJUNCTIONS
Generally an interface between two semiconductors with different band
gaps
Type I: straddling gap
Type II: staggered gap
Type III: broken gap
CB
VB
CB
VB
CB
VB
Clearly a bit of a mess!
Looks more like your
typical p-n junction
band diagram
Can be beneficial for PV
when defects are
present at the interface?
13. HETEROJUNCTIONS
Generally an interface between two semiconductors with different band
gaps
Type I: straddling gap
Type II: staggered gap
Type III: broken gap
CB
VB
CB
VB
CB
VB
Clearly a bit of a mess!
Looks more like your
typical p-n junction
band diagram
Can be beneficial for PV
when defects are
present at the interface?
14. HETEROJUNCTIONS
Generally an interface between two semiconductors with different band
gaps
Type I: straddling gap
p n
Type II: staggered gap
Type III: broken gap
CB
VB
CB
VB
CB
VB
Clearly a bit of a mess!
Looks more like your
typical p-n junction
band diagram
arrows for conduction of
photoexcited minority carriers
(swept across junction by in-
built E-field)
Can be beneficial for PV
when defects are
present at the interface?
e-
- - -
h+ +++
15. HETEROJUNCTIONS
Generally an interface between two semiconductors with different band
gaps
Type I: straddling gap
Type II: staggered gap
Type III: broken gap
CB
VB
CB
VB
CB
VB
Clearly a bit of a mess!
Looks more like your
typical p-n junction
band diagram
Can be beneficial for PV
when defects are
present at the interface?
16. 3 FACTORS FOR DEFECT-
TOLERANT INTERFACES1. ∆Ec CBM offset (between n-type and p-type)
Create a h+ barrier at the interface
Not enough h+ present for e--h+ recombination at interface defect states
Ensure barrier isn’t too high to inhibit e- transport across interface
2. Emitter doping (and thickness)
Different effect for type I and II
Type II can use to reduce amount of one type of carrier at the interface (via absorber inversion)
Type I just need to ensure n-type is doped enough for e- collection
Thick enough to allow for emitter doping to influence interface
3. Type of prominent (low energy) defects at the interface
Seems to be the hardest one to tune!
Mid-gap acceptors are the worst
Shallow better
n-type enhances absorber inversion
17. 3 FACTORS FOR DEFECT-
TOLERANT INTERFACES1. ∆Ec CBM offset (between n-type and p-type)
• Create a h+ barrier at the interface
• Not enough h+ present for e--h+ recombination at interface defect states
• Ensure barrier isn’t too high to inhibit e- transport across interface
2. Emitter doping (and thickness)
Different effect for type I and II
Type II can use to reduce amount of one type of carrier at the interface (via absorber inversion)
Type I just need to ensure n-type is doped enough for e- collection
Thick enough to allow for emitter doping to influence interface
3. Type of prominent (low energy) defects at the interface
Seems to be the hardest one to tune!
Mid-gap acceptors are the worst
Shallow better
n-type enhances absorber inversion
18. 3 FACTORS FOR DEFECT-
TOLERANT INTERFACES1. ∆Ec CBM offset (between n-type and p-type)
• Create a h+ barrier at the interface (for type I)
• Not enough h+ present for e--h+ recombination at interface defect states
• Ensure barrier isn’t too high to inhibit e- transport across interface
2. Emitter doping (and thickness)
• Different effect for type I and II
• Type II can use to reduce amount of one type of carrier at the interface (via absorber inversion) – similar effect to above?
• Type I just need to ensure n-type is doped enough for e- collection
• Thick enough to allow for emitter doping to influence interface
3. Type of prominent (low energy) defects at the interface
Seems to be the hardest one to tune!
Mid-gap acceptors are the worst
Shallow better
n-type enhances absorber inversion
19. 3 FACTORS FOR DEFECT-
TOLERANT INTERFACES1. ∆Ec CBM offset (between n-type and p-type)
• Create a h+ barrier at the interface (for type I)
• Not enough h+ present for e--h+ recombination at interface defect states
• Ensure barrier isn’t too high to inhibit e- transport across interface
2. Emitter doping (and thickness)
• Different effect for type I and II
• Type II can use to reduce amount of one type of carrier at the interface (via absorber inversion)
• Type I just need to ensure n-type is doped enough for e- collection
• Thick enough to allow for emitter doping to influence interface
3. Type of prominent (low energy) defects at the interface
Seems to be the hardest one to tune!
Mid-gap acceptors are the worst
Shallow better
n-type enhances absorber inversion
Aside: ‘absorber inversion’
*I think* this means h+ become
minority carriers at the surface of
a p-type absorber material at p-
n junction
Caused by:
• Highly doped n-type layer
• n-type interface defects
Often mentioned along with the
‘potential distribution across the
junction’ and ‘amount of band
bending’ at the interface
20. 3 FACTORS FOR DEFECT-
TOLERANT INTERFACES1. ∆Ec CBM offset (between n-type and p-type)
• Create a h+ barrier at the interface
• Not enough h+ present for e--h+ recombination at interface defect states
• Ensure barrier isn’t too high to inhibit e- transport across interface
2. Emitter doping (and thickness)
• Different effect for type I and II
• Type II can use to reduce amount of one type of carrier at the interface (via absorber inversion) – similar effect to
above?
• Type I just need to ensure n-type is doped enough for e- collection
• Thick enough to allow for emitter doping to influence interface
3. Type of prominent (low energy) defects at the interface
• Seems to be the hardest one to tune! – predict surface defects from theory? Treat/ passivate surfaces
before making junction?
• Mid-gap acceptors are the worst
• Shallow defects better
• n-type enhances absorber inversion
21. 3 FACTORS FOR DEFECT-
TOLERANT INTERFACES1. ∆Ec CBM offset (between n-type and p-type)
• Create a h+ barrier at the interface
• Not enough h+ present for e--h+ recombination at interface defect states
• Ensure barrier isn’t too high to inhibit e- transport across interface
2. Emitter doping (and thickness)
• Different effect for type I and II
• Type II can use to reduce amount of one type of carrier at the interface (via absorber inversion) – similar effect to
above?
• Type I just need to ensure n-type is doped enough for e- collection
• Thick enough to allow for emitter doping to influence interface
3. Type of prominent (low energy) defects at the interface
• Seems to be the hardest one to tune! – predict surface defects from theory? Treat/ passivate surfaces
before making junction?
• Mid-gap acceptors are the worst
• Shallow defects better
• n-type enhances absorber inversion
Relates to Keith’s
prediction setup
22. SPIKES AND CLIFFS – BASED ON
CBM OFFSET
(ESSENTIALLY STEP 1 OF KEITH’S SETUP)
Type I interface:
For a good spike: 0.1 eV ≤ ∆EC ≤ 0.3 eV
Creates absorber inversion
Large barrier to h+ adjacent to interface
e--h+ recombination suppressed due to insufficient h+ and interface (even when electron
transport delayed by interface defects)
When the spike gets too big: ∆EC ≥ 0.4 eV
Impedes e- transport reduces photocurrent and FF
Type II interface:
Cliff: ∆EC < 0
Allows h+ in high concentrations at interface, allowing for e--h+ recombination
at defect trap states
24. SPIKES AND CLIFFS
Type I Type II
CB
VB
e-
- - -
h+ +++
p-n junction (other way around!)
e-
- - -
h+
+++
25. SPIKES AND CLIFFS
Type I Type IIe- transport
reduced if spike
is too large
h+ transport hindered?
26. BAND BENDING & HOLE BARRIER
• Position of e- fermi energy w.r.t p-
type CBM makes it easy for e- to go
into p-type?
‘absorber inversion’?
• Related to ‘potential distribution’
which makes it difficult for h+ to
enter interface region?
Easy for e-
to move
into p-type?
closer than what?
27. ADDED COMPLICATIONS
(Possibly what Prof Jim Matthews at York would have bundled into his ‘+c’
parameter…)
• Mixing at interface, evidence of this for CdTe: something we don’t account for
with Keith’s setup
Reduces lattice strain… but could modify band diagram in ways we can’t measure
accurately?
• For CZTS some people think that the surface is actually not CZTS!
28. VERDICT…
+ Explains ‘spikes’, ‘cliffs’ and associated impacts on device
performance well (ish)
- But you have to look elsewhere for type I, II, II explanations and
‘absorber inversion’ … although it wasn’t actually intended as a
tutorial!
+ Interesting to think about how to engineer ‘defect-tolerant’
junctions (going beyond the standard p-n junction diagram!)
+ Lots of principles to apply to other p-type PV absorber materials!
But relating one PV device to another definitely requires some thought
(how they mix, how surfaces reconstruct), careful measurements (if
feasible) …and a lot of trial and error it seems!
29. VERDICT…
+ Explains ‘spikes’, ‘cliffs’ and associated impacts on device
performance well (ish)
- But you have to look elsewhere for type I, II, II explanations and
‘absorber inversion’ … although it wasn’t actually intended as a
tutorial!
+ Interesting to think about how to engineer ‘defect-tolerant’
junctions (going beyond the standard p-n junction diagram!)
+ Lots of principles to apply to other p-type PV absorber materials!
But relating one PV device to another definitely requires some thought
(how they mix, how surfaces reconstruct), careful measurements (if
feasible) …and a lot of trial and error it seems!