ペロブスカイト型の電荷ダイナミクス

沈 青

電気通信大学大学院理工学研究科 先進理工学専攻

Organic-Inorganic Hybrid Solar Cells
Efficiency of OIHSCs with different absorbers:

Solar light

Dye: 1% （1998）1)

e-7.2% (2011）2)

Improvements in absorption ofdye and conductivity of HTM

Absorberh+→Quantum dots (QDs): 5% (2010) 3) → 6.3% (2012) 4)
Sb2S3 and improvement in HTM

Holetransportmaterial(HTM)

Electrontransportmaterial(ETM)

Perovskite lead iodide: 9.7%（2012）5) → 20.1%(2014）6)

Schematic illustration of structure of anorganic-inorganic hybrid solar cell (OIHSC)
Absorber(dye, quantum dots (QDs), perovskite)1) M. Gratzel et al., Nature 1998, 395, 583−585.
2) M. Gratzel et al., J. Am. Chem. Soc. 2011, 133, 18042−18045.
3) S. I. Seok et al., Nano Lett. 2010, 10, 2609−2612.

Organohalide Lead Perovskites
Halogen anion

CH3NH3+Pb2+
4) S. I. Seok et al., Nano Lett. 2012, 12, 1863−1867.
5) N. G. Park et al., Sci. Rep. 2012, 2, 591.
6) http://www.nrel.gov/ncpv/

Organic-Inorganic Hybrid Solar Cells
Solar light

Efficiency of OIHSCs with different absorbers:

Dye: 1% （1998）1)

e-7.2% (2011）2)

Improvements in absorption ofdye and conductivity of HTM

Absorberh+→Quantum dots (QDs): 5% (2010) 3) → 6.3% (2012) 4)
Sb2S3 and improvement in HTM

Holetransportmaterial(HTM)

Electrontransportmaterial(ETM)

Perovskite lead iodide: 9.7%（2012）5) → 20.1%(2014）6)

Schematic illustration of structure of anorganic-inorganic hybrid solar cell (OIHSC)
Absorber(dye, quantum dots (QDs), perovskite)

Organohalide Lead Perovskites
Halogen anion

CH3NH3+
Pb2+

Why the efficiency of perovskite-based OIHSCs can be so high?

Unique properties of Pb perovskite for solid state solar cells（１）high optical absorption coefficient
N719: α = 1500 cm-1 at 540 nm; CH3NH3PbI3: α = 15000 cm-1 at 550 nm（２）long electron and hole diffusion length in the perovskite
CH3NH3PbI3: 100 nm; CH3NH3PbI3-xClx: > 1000 nm
S. D. Stranks et al., Sicence 342, 341 (2013); G. C. Xing et al., Science 342, 344 (2013).

longer lifetimes of photoexcited carriers in Pb perovskie

Unique properties of Pb perovskite for solid state solar cells（１）high optical absorption coefficient
N719: α = 1500 cm-1 at 540 nm; CH3NH3PbI3: α = 15000 cm-1 at 550 nm（２）long electron and hole diffusion length in the perovskite
CH3NH3PbI3: 100 nm; CH3NH3PbI3-xClx: > 1000 nm
S. D. Stranks et al., Sicence 342, 341 (2013); G. C. Xing et al., Science 342, 344 (2013).

longer lifetimes of photoexcited carriers in Pb perovskie

Some open questions for perovskite-based solar cells
1. What is the time scale of charge separation and recombination
and which interface do they occur?
2. How about charge separation and charge collection efficiency?
3. How about surface engineering (surface passivation) influencecharge separation and collection efficiency?

Our studies on Chaｒge Separation andRecombination in Perovskite Solar cells(1) TiO2/CH3NH3PbIxCl3-xl /Spiro-OmeTAD (one step)◇ surface passivation of TiO2 surface with Y2O3
ChemPhysChem, Vol. 15, 1062 (2014).

◇ Dependence on TiO2 morphology

Ag/Au
Spiro-OMeTAD

PorousTiO2/Perovskite

Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

Dense TiO2

(2) TiO2/CH3NH3PbI3/Spiro-OmeTAD (two step)

FTO glass◇ Interface engineering by inserting HOCO-RNH3 +I- anchor group atPerovskite/TiO2 interfaces
J. Phys. Chem. C, Vol. 118, 16651 (2014).

(3) TiO2/CH3NH3SnxPb1-xI3 /Spiro-OmeTAD (one step)◇ Carrier dynamics depends greatly on the ratio x of Sn and Pb
Submitted for publication

Charge Separation and Recombination in OIHSCOne Key for Improving the Efficiency

τei: electron injection time;τreg: hole transfer timeτ1: life time of photoexcitedelectron-hole pairsτrec1: recombination time of electronsin TiO2 with holes in sensitizerτrec2: recombination time of electronsin TiO2 with holes in HTM
τet: electron transport time in ETM
τht: hole transport time in HTM

Absorber

The charge separation and recombination dynamics from sub-picoseconds
to milliseconds can be detected with a transient absorption (TA) method.

Recombination in CH3NH3PbIxCl3-x on Y2O3
0.0

Normalized A

0.9 J/cm2
1.8 J/cm2

Probe at 775 nm
Without Spiro-0.5Ee-34 ps (9%)>> ns (91%)-1.00500

1000

1500

2000

2500

3000

Time (ps)A

1.0x10-4

0.0

ExperimentalProbe at 785 nm
Fitting

Y2O3(a)processes:
Three recombination(1) 34 ps (9%)(2) 3.7 μs (64%)(3) 60 μs (27%)-1.0x10-4

10-5

10-6

Time (s)h+CH3NH3PbI2Cl

3.7±0.1 μs (70%)
60±1 μs (30%)

10-734 ps (9%)
A few μs –10 μs (91%)

10-4

ChemPhysChem, Vol. 15, 1062 (2014).
Phys. Chem. Chem. Phys., Vol. 16, 19984

Charge separation at CH3NH3PbIxCl3-x /Spiro-OMeTAD interface

Ewithout Spirowith Spirofitting

Normalized A

0.0

CB

Probe at 775 nme-τ: 16 ns

With Spiro-0.5

CBh+16 ns
VB

Without Spiro-1.00500

1000

1500

2000

2500

Time (ps)There is charge separation at
CH3NH3PbIxCl3-x/ Spiro-OMeTAD interface

Spiro
OMeTAD3000

VB
Y2O3 CH3NH3PbIxCl3-x

Hole injection time: 16 ns

Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

charge recombination at CH3NH3PbIxCl3-x /Spiro-OMeTAD interface
Probe light : 1310 nm
1.0x10-4

Experimental
Fitting
τ: 0.37 μs

5.0x10-5

A

Monitor holes in Spiro

ECBWith Spiroe-CB

0.37 μsh+Spiro
OMeTAD

0.0VB
Probe at 1310 nm

VB-5.0x10-5
1.0x10-6

1.5x10-6

2.0x10-6

Time (s)There is charge separation at
CH3NH3PbClI2/ Spiro-OMeTAD interface

2.5x10-6

CH3NH3PbIxCl3-x

Y2O3Recombination time: 0.37 μs

Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

Charge separation and recombination at
CH3NH3PbIxCl3-x /Spiro-OMeTAD interface on Y2O3EEe-e-34 ps (9%)
A few μs –10 μs (91%)

16 nsh+h+
CH3NH3PbI2Cl
Y2O30.37 μs

Spiro
OMeTAD

CH3NH3PbClI2

Y2O3(a)Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

(b)Charge separation at CH3NH3PbIxCl3-x/TiO2 interface
CH3NH3PbIxCl3-x/TiO2
CH3NH3PbIxCl3-x/Y2O3
fitting0.0

Normalized A

E-0.5

CBCB

e- 1.8 nsh+ VB

τ: 1.8 ns

TiO2
VB
TiO2

Y2O3-1.0
0200

400

600

800

CH3NH3PbIxCl3-x1000

Delay time (ps)There is electron injection from CH3NH3PbIxCl3-x to TiO2.
ChemPhysChem, Vol. 15, 1062 (2014).
Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

Charge separation at CH3NH3PbIxCl3-x/TiO2 interface
CH3NH3PbIxCl3-x/TiO2
CH3NH3PbIxCl3-x/Y2O3fitting

0.0

Normalized A

E-0.5

CB

τei

200

400

VB
TiO2600≥μsτ1h+ VB

Y2O30e- 1.8 ns

τ: 1.8 ns

TiO2-1.0

CB

800

CH3NH3PbIxCl3-x

1000

Delay time (ps)Electron injection efficiency ηET
Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

ηET= (1/τei)/(1/τei+1/τ1)
ηET: close to 100%

Charge recombination at CH3NH3PbIxCl3-x/TiO2 interface
Probe light wavelength: 658 nm

Monitor electrons in TiO2T. Yoshihara et al., The Journal of Physical Chemistry B, 2004, 108, 3817-3823.
Physical Chemistry Chemical Physics, 2014, 16, 5242; 2013, 15, 11006; 2014, 16, 5774.
4.0x10-4E

τ: 0.14 μs-4CB

0.14 μs
0.0h+

VB

VB

-2.0x10CBe-A

2.0x10Experimental
Fitting

TiO2-48x10-710-6

1.2x10-6

1.4x10-6

Time (s)ChemPhysChem, Vol. 15, 1062 (2014).
Phys. Chem. Chem. Phys. (2014). DOI: 10.1039/C4CP03073G.

CH3NH3PbIxCl3-x

Charge recombination at CH3NH3PbIxCl3-x/TiO2 interface
Probe light wavelength: 658 nm
4.0x10-4

Monitor electrons in TiO2

Experimental
FittingE
CBe-τ: 0.14 μs

2.0x10-4

A

0.14 μs
τrec1
0.0VB

TiO2

-2.0x10-4

8x10-7

10-6Time (s)1.2x10-6

CB

Spiro
OMeTAD

h+VB

16 ns
τhi

CH3NH3PbIxCl3-x

1.4x10-6

Hole injection efficiency ηHTηHT = (1/τhi)/(1/τhi + 1/τrec1)ηHT: about 90%

Charge separation efficiency: ηCS = ηET ηHT ≈ 90%

Charge recombination at TiO2/Spiro-OMeTAD interface
Probe light wavelength: 1310 nm

Monitor lifetime of holes in Spiro

Without Spiro

R. Plass et al., J. Physical Chemistry B,
2002, 106, 7578.

Δ A

1.0x10-3

0.0-1.0x10E-3

1.0x10-3

1E-6

1E-5

1E-4

CB

Time (s)τ: 60 μs

With Spiro

CBeh+60 μs

Δ A

Spiro
OMeTAD

VB

0.0VB

TiO2

Experimentalfitting-1.0x10-3

1E-6

CH3NH3PbIxCl3-x

1E-5

Time (s)1E-4Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

Charge recombination at TiO2/Spiro-OMeTAD interface
Probe light wavelength: 1310 nm

Δ A

1.0x10-3

Monitor lifetime of holes in Spiro

Without Spiro

0.0-1.0x10E-3

1.0x10-3

1E-6

1E-5

1E-4

CB

Time (s)τ: 60 μs

With Spiro

CBeh+60 μs

Δ A

Spiro
OMeTAD

VB

0.0

VB

TiO2

Experimental
fitting-1.0x10-3

1E-6

CH3NH3PbIxCl3-x

1E-5

Time (s)1E-4

Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

Relationship between Carrier Dynamics and Photovoltaic Performance
1.00.8IPCEE

at 470 nm: IPCE=59%

1.8 ns
CB0.6e-CB

60 μs τrec2h+0.40.14 μs

16 ns0.2

Spiro
OMeTADVB
0.0350450550650750

Wavelength / nm

TiO2

20.0
18.0

Current Density / mAcm-2

CH3NH3PbIxCl3-x

VB

IPCE＝ηabsorption ηCS ηCC

16.0
14.012.0

ηabsorption ≈ 95%

Jsc: 11.8 mA/cm2

10.08.0

ηCS = ηET ηHT ≈ 90%

Voc: 0.79 V
FF: 0.71
efficiency: 6.59%

6.04.02.0

ηCC < 70%

0.00.00.20.4voltage / V

0.6

0.8

The problem: smaller τrec2smaller ηCC.

Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

Effects of surface treatment on carrier dynamics and photovoltaic performance

EAfter surface treatment of TiO2
τrec2: 60 μs

180 μsJsc: 11.8 mA/cm2
efficiency： 6.59%

Current Density /mAcm-2

CBe-CB
60 μs

0.14μs

16.5 mA/cm2;
7.53%

With surface treatment

h+h+
16 ns

CH3NH3PbIxCl3-x

TiO2

E CBe-3.3 ns

Without surface treatment

CB180 μs
1 μs

0.00.20.40.6

Spiro
OMeTAD

VB

VB

20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0

1.8 ns

h+

0.8

16 ns

VB

voltage / VVB
ChemPhysChem, Vol. 15, 1062 (2014)h+

CH3NH3PbIxCl3-x

TiO2 Y2O3

Spiro
OMeTADEffects of TiO2 nanoparticle sizes on carrier dynamics and Photovoltaic PerformanceE
TiO2 size: 18 nm
τrec2: 60 μs

1.8 nsCB

30 nm

IPCE(at 470nm):59%
Jsc: 11.8 mA/cm2
efficiency： 6.59%
Current Density / mAcm-218 nm

e-600 μs

60 μs0.14 μs20.085% (ηCC ≈ 100% )17.6 mA/cm2;
9.54%
TiO2: 30 nm

cell B

16.0

CB

h-VB 16 ns
CH3NH3PbI2Cl

VB

TiO2E CB(b)Spiro
OMeTAD

0.7 nse-CB

30 nm

12.0

cell A
8.0

TiO2: 18 nm

0.13 μs

h-4.0

600 μsSpiro
16 ns OMeTAD

VB

0.0
0.0

0.2

0.4

Volatage / V

0.6

0.8

VB

TiO2

CH3NH3PbI2Cl

Summary(1) The carrier lifetime of perovskite CH3NH3PbIxCl3-x is aslarge as μs.
(2) Charge separation efficiency is as high as 90% for
TiO2/CH3NH3PbIxCl3-x/Spiro OMeTAD.
(3) The key factor for improving the efficiency is to decrease
the recombination at TiO2/Spiro OMeTAD interface, which
resulted in a smaller charge collection efficiency.

How to suppress the recombination is a key and surface
passivation of TiO2 and interface engineering are important.

Acknowledgment
This research was supported by the CREST programof Japan Science and Technology Agency (JST).
九州工業大学：早瀬修二先生、尾込裕平先生
宮崎大学：吉野賢二先生
電通大：豊田太郎先生