defect-tolerance self-healing - weizmann institute of science
TRANSCRIPT
Yevgeny Rakita1, Davide R. Ceratti1, Gary Hodes1, David Cahen1
Department of Materials and Interfaces Weizmann Institute of Science, 7610001, Rehovot, Israel ;
Halide Perovskites: a platform for
‘defect-tolerance’ & ‘self-healing’
Halide Perovskites (HaPs) have taken a unique place among functional semiconductors – at the bulk and the nanoscale. Surprisingly, their optoelectronically-relevant
defect/trap density is extremely low compared to traditional semiconductors made with similar methods. HaPs can be fabricated near RT, from solution, both as thin-films, and
as single crystals with ~1010 cm-3 defect/trap densities for single crystals and < ~1016 cm-3 for polycrystalline films. Usually, only very carefully fabricated semiconductors (e.g.,
MBE-grown GaAs) yield such values.
We show here that these results are due to ‘defect-tolerance’ and ‘self-healing’, intrinsic to HaPs, where the latter is basically “anneal” at RT. These properties directly
explain the phenomenal performance of HaP QDs and other HaP architectures. Thus, understanding HaPs can guide us to new materials with such properties.
• Materials with ‘defect-tolerating’ bands (=positive d’), such as HaPs and
other halide/lead-based materials, are also highly polarizable. Stability of
such systems is enhanced by entropy !
• Entropically stabilized systems mean: low enthalpy of: ∆HReaction
(material ⥄ ∑constituents) + low Eact.
• When at RT ∆GReaction + ∆Eact < ∆GForm(defect), RT annealing or ‘self-
healing’ should occur (and does for HaPs!).
• Other ‘soft’ and highly-polarizable (𝜀𝑠 > 2𝜀∞) materials, such as halide-
and Pb-based materials should possess similar properties.
In part, based on YR’s PhD thesis: arxiv.org/abs/1809.10949
(sections 3.3, 3.4 & chapter 4)
MA
Pb
I
CsP
bB
r
MA
Pb
Br
MA
Pb
Cl
TlC
l
TlB
r
Ag
Cl
Ag
Br
Ag
I
Pb
Te
Pb
Se
Pb
S
Cd
Te
Cd
Se
Cd
S
GaS
b
GaA
s
GaP
InS
b
InA
s
InP Si --
0
2
4
6
8
10
12
Chalcogenides
ab
so
lute
valu
e o
f d
', |
d'| ;
[
eV
]Si
III-V
Halides
HaP
Halide-based materials
have relatively low
absolute deformation
potential, |d’|
(≈ low mechanical stiffness)*
* ~scales with Madelung
(electrostatic) potential
MA
Pb
IC
sP
bB
rM
AP
bB
rM
AP
bC
lT
lCl
TlB
rA
gC
lA
gB
rA
gI
Pb
Te
Pb
Se
Pb
SC
dT
eC
dS
eC
dS
GaS
bG
aA
sG
aP
InS
bIn
As
InP Si -- --
-14
-12
-10
-8
-6
-4
-2
0
2
4
6
TlXPb
chalcogenides
defo
rma
tio
n p
ote
nti
al, d
'
; [e
V]
Si
III-V Cd
chalcogonides
AgX
APbX3
(HaP)
𝒅′ ≡ −𝑩 ∙∆𝐸𝑔
∆𝑃
B – bulk modulus∆𝐸𝑔- change in bandgap
∆𝑃 – change in pressure
Algebraic sign of the
‘deformation potential’,
d’, tells us about its
(valence) band-nature
-10 -8 -6 -4 -2 0 2 4 6
0.1
1
10
GaP
GaSb
InAs
GaAsInSb
CdTeCdSe
CdS
MAPbBr3
AgBr
CsPbBr3
AgCl
MAPbI3
TlBr
MAPbCl3
TlCl
PbTe
PbS
PbSe
'soft'
( e s
/ e
) -
1
d' [eV]
'rigid'
‘Defect-tolerant’
materials (halide-
/lead-based) possess
highly polarizable
structural bonds, i.e.,
𝜺𝒔 > 𝟐𝜺∞.
Bonds are easily
deformed !
Better
(configurational)
entropy
stabilization.
Calculated (ab initio) defects transition states – averaged picture of most
energetically-probable defects in Pb-chalcogenides and HaPs
Adopted from: Kovalenko et al., Science 358, 745–750 (2017)
Defect tolerant *(APbX3, PbX)
defect intolerant*GaAs, CdTe
* Shallow or intra-
band states
* mid-gap
states
Adopted from ref. 6
MAPbI3 ; MA=CH3NH3+)
Conduction band
Valence band
Adopted from ref. 7
shallow or intra-band defect transition states
• With low ‘deformation potential’ (~ soft material), strain fields can be tolerated.
For HaPs, |d’| ~ 1 eV, a strain energy of 1kTRT (= 26 meV) corresponds to ~2.5%
(volumetric) distortion.
• Most classical semiconductors will break under such strains.
Defects and their relaxation
1
Strain fields
Charge
1
2
• If defect is an ion (localized charge) ∆GForm(defect) >> kT
• In absence of kinetic stabilization (low ∆Eact) (can be assumed for low bond-energy
and highly polarizable systems) and if in given material
∆GReaction (material ⥄ ∑constituents) + ∆Eact. < ∆GForm(defect)
the material can decompose and reform to a defect-free state! anneal @ RT
2
Defect (can) create:
Based on calorimetric measurements1 and kinetic studies2 of HaP formation/ decomposition
of HaPs (MAPbX3) to their constituents (MAX+PbX2):
*∆GReaction {~0.1-0.2 eV}1,2+ ∆Eact. {~0.05 eV 2 - 0.45 eV} < # ∆GForm(defect) {~ 1.6 eV}
RT annealing / ‘self-healing’
‘defect tolerant’
* Dominant entropic stabilization! 1
‘deformation potential’ > 0 & ‘defect tolerance’ Easy deformation and entropic stabilization
We thank Omer Yaffe and Igor Lubomirsky for fruitfuldiscussions. We thank Weizmann Institute’s Sustainability andEnergy Research Initiative (SAERI)* and the Israel Ministry ofScience for partial support.
See: D. R. Ceratti, et al. Adv. Mater. 2018, 1706273
MAPbBr3 crystal is cleaved just
before experiment
• Similar results for other APbX3 compounds !
• Kinetic study (T-dependent) of ‘self-healing’, suggest
∆Eself-healing ~ 0.5 eV for MAPbBr3 and 1.2 eV for FAPbBr3
Even if defects exist, their influence on (opto)electronic
behavior will be small!
Conclusions # For Br-based HaPs. 1.6 eV is deduced from radioactive Br & Pb tracer diffusion in PbBr23 and comparison to
ionic diffusion Eact(ion) for different APbBr3 HaPs 4. ∆GForm(defect) for I-based HaPs (ab-initio) ~0.3-1.3 eV 6
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4. Kuku, et al., Solid State Ionics 34, (1989) 1417. Yin, W. et al., Appl. Phys.
Lett. 104, (2014) 0639036. Li et al., J. Phys. Condens. Matter 27, (2015) 355801
5. Bi et al, Nat Commun. 8, (2017) 15330