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.

1. OPTIMIZING LAYERS IN A
PV DEVICE
LESSONS TO BE LEARNT
FROM CDTE?
WMD JC 14th March 2017
Suzanne Wallace

2. MOTIVATION…THE USUAL!

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

9. KEITH’S SETUP: INTERFACE
PREDICTION
Slides:
https://www.dropbox.com/s/vv
p4nfq9n54estz/IoP_Advancesin
PV_KTB.pptx.pdf?dl=0#
Python script:
https://github.com/keeeto/Elec
tronicLatticeMatch
Poor
interface:
rapid e--h+
recombinati
N.B. we assume no interface mixing and
also note that IPs and EAs are often
surface-dependent

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

11. LESSONS TO BE LEARNT FROM
CDTE?

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

23. SPIKES AND CLIFFS
Type I Type II

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!