Lattice anchoring between colloidal crystal and perovskite semiconductor stabilizes can be a useful strategy for a stable semiconductor interface for solar cell application

1. Lattice anchoring stabilizes
solution-processed semiconductors
Debabrata Bagchi
13/02/2020
Supervisor: Prof. Sebastian C. Peter
Solid State and Inorganic Chemistry Lab

2. Edward H. Sargent Shana O. Kelley
2
Mengxia Liu1, Yuelang Chen2, Chih-Shan tan1, Rafael Quintero-Bermudez1, Andrew H. Proppe1,2, Rahim Munir3,4,
Hairen Tan1,5, Oleksandr Voznyy1, Benjamin Scheffel1, Grant Walters1, Andrew Pak tao Kam1, Bin Sun1, Min-Jae
Choi1, Sjoerd Hoogland1, Aram Amassian3,6, Shana O. Kelley2,7, F. Pelayo García de Arquer1 & Edward H. Sargent1*
Aram Amassian
1,2
3 4
5
6
https://doi.org/10.1038/s41586-019-1239-7
Nature 2019, 570, 96-101

3. Outline
 Introduction
 Solution Processed Semiconductor
 Where to use?
 Problem associated with Perovskite & Quantum Dot
 Novelty of the Paper
 Design of Lattice anchored hybrid materials
 Spectroscopic & Microscopic study
 Optoelectronic Properties
 Conclusion & Future aspects
 Solar Cell Application
3

4. Solution-processed semiconductors
García de Arquer, F. P. et al. Nat. Rev. Mater. 2017, 2, 16100. 4
Solution-processed semiconductors are the class of photoactive
materials that can be processed in ink form through wet chemistry
Spray coating Spin coating Inkjet printing Roll-to-roll printing
Solution phase synthesis
Organic Semiconductors Perovskite Quantum dots
 Building blocks are pi-
bonded molecules or polymers
 molecular crystals or
amorphous thin films
 ABX3 crystal structure
 hybrid organic-inorganic or-
inorganic caesium lead halide
Bandgap:
1.55-2.3 eV
Bandgap:
Variable, depend
on size and shape
 Semiconductor particles a
few nanometres in size
 Optoelectronic properties
change as a function of both
size and shape
Bandgap:
1.0-2.0 eV

5. Organic semiconductor, organic dyes & perovskite
Quantum dots
5
Where to use?
Light Sensing Light Emitting Diode
Photovoltaic Solar Cell
Photovoltaic Solar Cell
FF =
IMMP . VMMP
ISC . VOC
h =
VOC . ISC . FF
Pin
 Fill Factor (FF)
 Power Conversion
Efficiency (h)
García de Arquer, F. P. et al. Nat. Rev. Mater. 2017, 2, 16100.

6. 6
Instability of Perovskite materials
Ju. M. G. et al. Joule 2018, 2, 1231-1241.
 ABX3, where A = CH3NH3
+ (MA+) or
HC(NH2)2
+ (FA+); B = Pb2+; & X = halides
Organic-inorganic halide perovskites Power conversion efficiency (PCE): 27.3%
Stability Issues
All-inorganic perovskites
strong covalent/ionic bonding in the crystal structure
 Caesium lead halide perovskite,
CsPbX3 (X = halide)
 Cubic-phase (α-phase) CsPbI3 has a
bandgap (1.73 eV) suited to solar cells
PCE: 18.56%
Liu. C. et al. NPG Asia Mat. 2018, 10, 552-561.

7. 7
Novelty of the Paper
PbS-CQD
O2
O2
O2
O2
Oxide Shell
Aggregation
a-CsPbI3 d-CsPbI3
Perovskite Colloidal Quantum Dot
x
x
CsPbX3-PbS Hybrid materials
CQDs promote the epitaxial growth of α-phase perovskite and
anchor the atoms of the perovskite to the CQD surfaces

8. 8
Epitaxial alignment between CsPbX3 and CQDs
 Atomistic model of a CQD:perovskite
lattice-anchored hybrid materials system
 The lattice constant of CQDs and
perovskites of different stoichiometry
By tuning the ratio of Br to I near-zero lattice mismatch (ε) for PbS CQDs at a Br
66% (ε < 0.2%), enabling the strain-free epitaxial growth of perovskite.
 Pb–Pb distance: PbS PbSe α-CsPbBrxI3−x
5.94 Å 6.12 Å 5.85 Å to 6.21 Å
Beal, R. E. et al. J. Phys. Chem. Lett. 2016, 7, 746-751.
*
*

9. 9
CQD synthesis and ligand exchange
ODE (14.2 g, 56.2 mmol)
Oleic acid (1.34 g, 4.8 mmol)
PbO (0.45 g, 2.0 mmol)
At 95 °C under
vacuum for 16 h
At 120 °C under
Argon atm
Bis(trimethylsilyl) sulphide
0.18 g, 1 mol
1-octadecene (ODE)
10 ml
At 80 °C under
vacuum for 24 h
+
 PbS-CQD synthesis
Iodide precursor (0.6 ml)
(TBAI dissolved in oleylamine)
 Solution-phase iodide treatment
6 ml
15 min
Stirring at RT
Precipitated in
ethanol medium
Dispersed
in octane
Cooling down
to RT
Nanocrystal was
precipitated using acetone
Dispersed in toluene
And kept in Glove Box

10. 10
Surface engineering of CQD
 The fabrication of conductive CQD films relies on replacing the long alkyl
ligands used in CQD synthesis with short ligands.
 Protic solvents are typically used to aid in desorbing the original ligands and
thus present sites for binding of the final ligand.
 Iodide ligand improves the stability of CQD because of its high kinetic energy of
desorption

11. 11
Incorporation of perovskite into CQD
1:1 volume ratio
PbI2 (0.05 M),
PbBr2 (0.05 M),
CsI (0.1 M),
NH4CH3CO2 in DMF
Film fabrication
Exchange solution
(CQD in octane)
vortexed for 3 min to
transfer CQD to DMF
CQDs were precipitated
by adding toluene
 The amount of perovskite matrix, and thus the average dot-to-dot distance,
is tuned through the ratio of CQD to perovskite.
Perovskite Precursor
2000 r.p.m. for 60 s
Perovskite-CQD hybrid ink
Thermal annealing
100 °C for 10 min
Substrate
Multijunction solar cell

12. 12
CsPbBrI2 matrix 1% Lattice
mismatch
XRD was conducted at the 6-ID-D beamline at Argonne National Laboratory, USA.
X-ray diffraction study
v
 CsPbBrI2 shows a 1% lattice
mismatch
 CsPbBr2I and PbS show
complete agreement in
lattice planes
Lattice anchoring
CsPbBr2I matrix

13. 13
Transmission Electron Microscopy
PbS-CsPbBrI2 matrix
 The shell has a lower contrast compared with the
CQDs, as the perovskite has a lower density than PbS
 As the amount of perovskite increases, the shell grows
thicker & forms a continuous matrix with dots
 No spacing differences between core CQD and
perovskite shell were observed, indicating epitaxial
orientational alignment at two dominant facets.
90% CQDs 5% CQDs
PbS-CsPbBr2I matrix

14. 14
TEM Images and Colour Mapping
 PbS quantum dots with a thin CsPbBrI2 perovskite shell
 PbS quantum dots in CsPbBrI2 matrix structure
 At higher concentration of CsPbBrI2, perovskite makes continuous matrix
with PbS CQD embedded in it.

15. 15
Elemental analysis of PbS:CsPbBr2I hybrid films
10 vol% CQD 20 vol% CQD 33 vol% CQD
Pb Pb
Pb
Cs I
Br S
Cs I
Br S
Cs I
Br S
 Elemental mapping from energy dispersive X-ray spectroscopy (EDX) in scanning
electron microscopy indicates a uniform elemental distribution in the hybrid films

16. 16
Stability of CQD-anchored CsPbX3 perovskites
After 5 h of annealing in air (x is the Br content)
6 month in air

17. 17
Morphology of CQD:Perovskite hybrid films
CsPbBr2I
CsPbBrI2
SEM images of CsPbBr2I
 At low CQD loading no significant changes were observed in grain size
When CQD loading is higher a smaller grain size is observed, which is
consistent with the XRD peak broadening
 Used a volume fraction of CQDs below 15% to ensure uniform coverage
and maintain the original grain size of perovskites

18. 18
Effect of temperature on Packing Density
In situ grazing-incidence small-angle X-ray
scattering (GISAXS)
 Before annealing, pure and hybrid films
each present a hexagonal diffraction pattern
 Hexagonal pattern is no longer observable
in pure CQD films, whereas it is sustained
in hybrid films at higher temperature.
 No degradation is observed below 100°C in
the matrix-protected films.
At 70 °C
Room Temperature
v
v
Pristine
Matrix
protected
v
GIAXS was conducted at the D1 beamline, Cornell High Energy Synchrotron Source (CHESS).

19. 19
Photoluminescence (PL) studies
Pure CQD film
High CQD loading
Low CQD loading
 Matrix-protected films maintain 90% of the initial value after annealing whereas pure
CQD film shows a rapid PL quenching and loses half of the intensity after an hour
Theoretical volume fraction: 64%
 30% of film volume occupied by high-barrier vacuum; perovskite matrix can fill
these voids substantially, this could ease transport by lowering the barrier

20. 20
 Doubling in carrier mobility in the matrix-protected CQDs with 15 vol%
CsPbBr2I compared with pristine CQD films
Transient absorption spectroscopy
 Time traces at the exciton bleach peak of CQD donor films with a range of
acceptor CQD concentrations, Population transfer can be monitored directly
by following the decay

21. 21
Photoluminescence (PL) studies
 PL signal from perovskite is completely
quenched, showing an efficient carrier transfer
from the matrix to the CQDs
 Photocarriers transfer from the matrix to the
CQDs contributes to an enhanced near-infrared
PL emission
 Lattice matched system shows maximum PLQE
at infrared wavelength.

22. 22
 For Br content higher than 33%, the perovskite film
could be stabilized in for more than 6 months
 Degradation of MAPbI3 perovskite arises from the
volatility of organic components.
 The CQD:MAPbI3 film does not show any
improvement in thermal stability compared with pure
MAPbI3
Control Study
Stability studies of CQD:Perovskite hybrid films

23. 23
Solar Cell Application
Device architecture
Pure PbS-CQD
15 vol% CsPbBr2I matrix
Liu, M. et al. Nat. Mat. 2017, 16, 258-263.
Hole transporting p-type material
Charge collector electrode
Electron transporting n-type material
Transparent conductive coating

24. 24
Conclusion & Future aspects
Lattice-anchoring strategy provides solution-processed semiconductor
materials with increased stability relative to either constituent phase
Incorporating PbS CQDs into CsPbBrxI3−x perovskites suppressed
the formation of the undesired δ-phase configuration
Increased the lifetime of α-phase Caesium lead halide perovskite,
including multi-hour thermal stability at 200 °C
The epitaxially oriented perovskite matrix provides excellent stabilisation
to CQD surfaces, inhibiting attack from oxygen at elevated temperatures
Perovskite matrix lowers the energetic barrier to
carrier transport, contributing to a doubling in carrier mobility
Perovskite-CQD hybrid shows enhanced solar cell performance and long
term stability compared to pristine materials.

25. 25