Download Direct Measurement of the Exciton Binding Energy and Effective

Direct Measurement of the Exciton Binding Energy and
Effective Masses for Charge carriers in Organic-Inorganic Trihalide Perovskites
P. Plochocka
Laboratoire National des Champs Magnétiques Intenses,
CNRS-UJF-UPS-INSA, France
Introduction
Idea of perovskite solar cell:
http://en.wikipedia.org/wiki/Perovskite_solar_cel
What do we need from THE solar cell?
1.
2.
3.
High optical absorption and quantum conversion efficiency
Low carrier effective mass, long diffusion lengths
Possibly low band gap
low processing costs
http://en.wikipedia.org/wiki/Perovskite_solar_cel
S. D. Stranks et al., Science 342, 2013
What do we know?
Saddle point nature of M
Channel for holes
M->R
What do we know?
Transition E1/2,g E1/2,u
At R – 1st BG, ~1.6 eV
At M – 2nd BG ~2.8 eV
Secondary transitions at R – contribution to 2 BG
What is the binding energy?
There is a lot of controversy in literature about the value of the binding energy: 2-50meV
Motivation
What is the character of the photo created carriers in perovskite?
Electron hole pair bound into exciton?
What is the binding energy than?
Other basic parameters of the system as for example
mass……
How?
Transmission measurements in high magnetic field
Typical results
Transmission of the white light through the sample of CH3NH3PbI3 at 4K:
Minima observed in the spectra
correspond to the absorption
Differential spectra
Analysis
Energy of the absorption as a function of magnetic field:
2.2
Can we use a hydrogen model to fit the
data?
Energy (eV)
2.1
2.0
1.9
1.8
1.7
0
20
40
60
80
Magnetic Field (T)
100
120
140
Analysis
Energy of the absorption as a function of magnetic field: We use theoretical model by Makado
and McGill (numerical solution for the
hydrogen atom in high magnetic field)
2.2
R* = 16 meV
m* = 0.104
2.1
Energy (eV)
R*=m*e4/2ћ2ε∞2
ε∞= 9
2.0
1.9
1.8
2s
1.7
1s
0
20
40
60
80
100
120
140
Magnetic Field (T)
Makado, P.C. and McGill, N.C., Energy levels of a neutral hydrogen-like system in a 322
constant magnetic field of arbitary strength, J. Phys, C: Solid State Physics. 19 873 (1986)
Analysis
Free electron term:
Energy of the absorption as a function of magnetic field:
=
+ (N+1/2)ℏ
± 1/2
2.2
– cyclotron energy
Zeeman energy
Energy (eV)
2.1
2.0
1.9
1.8
2s
1.7
1s
0
20
40
60
80
Magnetic Field (T)
100
120
140
ff
Discussion
The observation of the 2s transition
places strong constraints upon the
exciton binding energy
Slope and separation of the high
field, high quantum number
Landau levels strongly constrain
the reduced effective mass value
The excitonic transitions dominate for the N=0 landau level (1s, 2s) and the N=1 (1,0) level up
to ~50T, and free electrons dominate for N=1 above 50T and for all higher Landau levels
Temperature dependence
High temperature data
Single wavelength transmission vs magnetic field
Broadband transmission vs magnetic field
Landau fan chart
Smaller binding energy at high temperature
Who is working on it……
Magnetic field measurements: broadband
transmission up to 70T
Quantum electronics group
D. K. Maude
P. Plochocka
Magnetic field measurements: single
wavelength measurements up to 150T
MegaGauss
Olivier Portugall
Oleksiy Drachenko
Atsuhiko.Miyata (post doc)
Zhou Yang
phD
A.A. Mitioglu
phD
R J NICHOLAS
K. Galkowski
phD
Samuel D. Stranks
Henry J. Snaith
Jacob Tse-Wei Wang
Clarendon Laboratory
Parks Road
Oxford
OX1 3PU
Perspectives…….
We are currently working on…..
CH3NH3PbI3-xClx
CH(NH2)2PbI3 , CH(NH2)2PbBr3
Continuation of magnetic field measurements with circular polarization…….
K. Galkowski working on spatial mapping as a
function of temperature
Time resolved measurements (pump probe
and time resolved micro PL in ps range
Semiconductor Nanowires, layer materials as
dichalcogenides, graphene…..
Combination of high/moderate magnetic field, low
temp (mK), time resolved micro - spectroscopy
Pulsed magnetic fields
14 MJ, 24 kV, 1 GW, 80 Tesla
Transmission
Single-turn coils
300 T in 5 mm diameter
150 T in 10 mm diameter
5 µs duration
Repetitive experiments
Huge dB/dt makes experiments difficult
‘Perovskite’ structure (CaTiO3)
ABX3
(CH3NH3)PbI3
http://en.wikipedia.org/wiki/Perovskite_solar_cel
http://en.wikipedia.org/wiki/Perovskite_solar_cel
H.-S. Kim et al., Scientific Reports 2, 2012
• High optical absorption and quantum conversion efficiency
• Low carrier effective mass, long diffusion lengths
• Possibly low band gap
• High optical absorption and quantum conversion efficiency
• Low carrier effective mass, long diffusion lengths
• Possibly low band gap
+ low processing costs
hydrolysis
Hydrolisis at ~1,7 eV
Simplification of the system - due to
higher bandgap, less perovskite
SC than Si SC are to be connected
in series
J. Luo et al., Science 346 (2014)
Solid: (CH3NH3)PbBr3
Dashed: (CH3NH3)PbI3
A. Kojima et al., J. Am. Chem. Soc 139 (2009)
ISC
VOC
ISC
PMAX
VOC
ISC
PMAX
VOC
PMAX
!! =
VOC × ISC
N. J. Jeon et al., Nature Materials 13,
2014
Michael D. McGehee, Nature Materials 13
2014
http://en.wikipedia.org/wiki/Perovskite_solar_cel
The role of mesoporous sapphire scaffold in thin-layer solar cell – to reduce the
number of defects during perovskite crystallization
Ch. Wehrenfenning et al., Adv. Mater. 26, 2014
S. D. Stranks et al., Science 342, 2013
Ch. Wehrenfenning et al., Adv. Mater. 26, 2014
A.Amat et al., Nano Letters 14, 2014
BG:
1.73 eV
A3 per unit vol: 222
~1.6eV
248
~1.5 eV
256
The role of symmetry and H-I bonds during substance crystallization
Trigonal FA vs. tetragonal MA –enhanced Pb-I interaction in FA results in more “Pb”
character of CB, thus higher splittings due to SOC –> lowering FA bangap
J. Even et al., J. Phys. Chem. C 118, 2014
J. Even et al., J. Phys. Chem. C 118, 2014
J. Even et al., J. Phys. Chem. C 118, 2014
Transition E1/2,g
E1/2,u
At R – 1st BG, ~1.6 eV
At M – 2nd BG ~2.8 eV
Secondary transitions at R – contribution to 2 BG
J. Even et al., J. Phys. Chem. C 118, 2014
Saddle point nature of M
Channel for holes
M->R
J. Even et al., J. Phys. Chem. C 118, 2014
J. Even et al., J. Phys. Chem. C 118, 2014
Above phase transition temperature – rotations of organic cation contribute to lowfrequency ε
Lattice vibrations contributing to high energy branch unaffected by phase transition
M. Ibrahim Dar et al., Nano Letters, 2014
D.Giovanni et al., Nano Letters, 2014
Elliot-Yafet scattering mechanism
S.N Habisreutinger et al., Nano Letters 14, 2014
Introduction
H.-S. Kim et al., Scientific Reports 2, 2012