ペロブスカイト型の電荷ダイナミクス

電気通信大学大学院理工学研究科 先進理工学専攻

沈 青

Organic-Inorganic Hybrid Solar Cells

Hole

transport

material

(HTM)

Electron

transport

material

(ETM)

h+ e-

Absorber

Absorber

(dye, quantum dots (QDs), perovskite)

Schematic illustration of structure of an

organic-inorganic hybrid solar cell (OIHSC)

Solar light

Dye: 1% （1998）1) → 7.2% (2011）2)

Quantum dots (QDs): 5% (2010) 3) → 6.3% (2012) 4)

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

Efficiency of OIHSCs with different absorbers:

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.

Improvements in absorption of dye and conductivity of HTM

Sb2S3 and improvement in HTM

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/

Halogen anion

CH3NH3+

Pb2+

Organohalide Lead Perovskites

Organic-Inorganic Hybrid Solar Cells

Dye: 1% （1998）1) → 7.2% (2011）2)

Quantum dots (QDs): 5% (2010) 3) → 6.3% (2012) 4)

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

Efficiency of OIHSCs with different absorbers:

Improvements in absorption of dye and conductivity of HTM

Sb2S3 and improvement in HTM

Halogen anion

CH3NH3+

Pb2+

Organohalide Lead Perovskites

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

Hole

transport

material

(HTM)

Electron

transport

material

(ETM)

h+ e-

Absorber

Absorber

(dye, quantum dots (QDs), perovskite)

Schematic illustration of structure of an

organic-inorganic hybrid solar cell (OIHSC)

Solar light

（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 longer lifetimes of photoexcited carriers in Pb perovskie

S. D. Stranks et al., Sicence 342, 341 (2013); G. C. Xing et al., Science 342, 344 (2013).

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 longer lifetimes of photoexcited carriers in Pb perovskie

S. D. Stranks et al., Sicence 342, 341 (2013); G. C. Xing et al., Science 342, 344 (2013).

Unique properties of Pb perovskite for solid state 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) influence

charge separation and collection efficiency?

Some open questions for perovskite-based solar cells

Our studies on Chaｒge Separation and Recombination in Perovskite Solar cells

Ag/Au

FTO glass

Dense TiO2

Spiro-OMeTAD

PorousTiO2/Perovskite

(1) TiO2/CH3NH3PbIxCl3-xl /Spiro-OmeTAD (one step)

◇ surface passivation of TiO2 surface with Y2O3 ChemPhysChem, Vol. 15, 1062 (2014).

◇ Dependence on TiO2 morphology Phys. Chem. Chem. Phys., Vol. 16, 19984 (2014).

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

◇ Interface engineering by inserting HOCO-RNH3 +I- anchor group at Perovskite/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

τei: electron injection time;

τreg: hole transfer time τ1: life time of photoexcited

electron-hole pairs τrec1: recombination time of electrons

in TiO2 with holes in sensitizer

τrec2: recombination time of electrons

in TiO2 with holes in HTM

τet: electron transport time in ETM

τht: hole transport time in HTM

The charge separation and recombination dynamics from sub-picoseconds

to milliseconds can be detected with a transient absorption (TA) method.

Charge Separation and Recombination in OIHSC One Key for Improving the Efficiency

Absorber

Recombination in CH3NH3PbIxCl3-x on Y2O3

0 500 1000 1500 2000 2500 3000

-1.0

-0.5

0.0 0.9 J/cm2

1.8 J/cm2

Norm

aliz

ed

A

Time (ps)

Without Spiro

Probe at 775 nm

Probe at 785 nm

34 ps (9%)

>> ns (91%)

ChemPhysChem, Vol. 15, 1062 (2014).

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

E

CH3NH3PbI2Cl

e-

h+

Y2O3

34 ps (9%)

A few μs – 10 μs (91%)

(a) Three recombination processes: (1) 34 ps (9%) (2) 3.7 μs (64%) (3) 60 μs (27%)

10-7 10-6 10-5 10-4

-1.0x10-4

0.0

1.0x10-4

A

Experimental Fitting

Time (s)

3.7±0.1 μs (70%) 60±1 μs (30%)

Charge separation at CH3NH3PbIxCl3-x /Spiro-OMeTAD interface

Spiro

OMeTAD

E

e-

h+

Y2O3

16 ns

0 500 1000 1500 2000 2500 3000

-1.0

-0.5

0.0 without Spiro with Spiro fitting

Nor

mal

ized

A

Time (ps)

With Spiro

Without Spiro

Hole injection time: 16 ns

Probe at 775 nm

CB

CB

VB

VB

τ: 16 ns

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

There is charge separation at CH3NH3PbIxCl3-x/ Spiro-OMeTAD interface

CH3NH3PbIxCl3-x

charge recombination at CH3NH3PbIxCl3-x /Spiro-OMeTAD interface

Spiro

OMeTAD

E

e-

h+

Y2O3

0.37 μs

Recombination time: 0.37 μs

CB

CB

VB

VB

There is charge separation at CH3NH3PbClI2/ Spiro-OMeTAD interface

CH3NH3PbIxCl3-x

τ: 0.37 μs

1.0x10-6 1.5x10-6 2.0x10-6 2.5x10-6

-5.0x10-5

0.0

5.0x10-5

1.0x10-4

A

Experimental Fitting

Time (s)

With Spiro

Probe at 1310 nm

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

Probe light : 1310 nm Monitor holes in Spiro

Charge separation and recombination at CH3NH3PbIxCl3-x /Spiro-OMeTAD interface on Y2O3

E

CH3NH3PbI2Cl

e-

h+

Y2O3

34 ps (9%)

A few μs – 10 μs (91%)

(a)

Spiro

OMeTAD

E

CH3NH3PbClI2

e-

h+

Y2O3

16 ns

0.37 μs

(b)

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

Charge separation at CH3NH3PbIxCl3-x/TiO2 interface

0 200 400 600 800 1000

-1.0

-0.5

0.0 CH

3NH

3PbI

xCl

3-x/TiO

2

CH3NH

3PbI

xCl

3-x/Y

2O

3

fitting

Norm

aliz

ed

A

Delay time (ps)

Y2O3

TiO2

TiO2

e-

h+

1.8 ns

E CB

CB

VB

VB

There is electron injection from CH3NH3PbIxCl3-x to TiO2.

τ: 1.8 ns

ChemPhysChem, Vol. 15, 1062 (2014).

CH3NH3PbIxCl3-x

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

Charge separation at CH3NH3PbIxCl3-x/TiO2 interface

0 200 400 600 800 1000

-1.0

-0.5

0.0 CH

3NH

3PbI

xCl

3-x/TiO

2

CH3NH

3PbI

xCl

3-x/Y

2O

3

fitting

Norm

aliz

ed

A

Delay time (ps)

Y2O3

TiO2

TiO2

e-

h+

1.8 ns

E CB

CB

VB

VB

τ: 1.8 ns

CH3NH3PbIxCl3-x

τei τ1 ≥μs

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

Electron injection efficiency ηET

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

8x10-7 10-6 1.2x10-6 1.4x10-6-2.0x10-4

0.0

2.0x10-4

4.0x10-4

Experimental Fitting

A

Time (s)

Charge recombination at CH3NH3PbIxCl3-x/TiO2 interface

τ: 0.14 μs

E

TiO2

CH3NH3PbIxCl3-x

e-

h+ 0.14 μs

CB

VB

CB

VB

ChemPhysChem, Vol. 15, 1062 (2014).

Probe light wavelength: 658 nm Monitor electrons in TiO2

Phys. Chem. Chem. Phys. (2014). DOI: 10.1039/C4CP03073G.

T. 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.

8x10-7 10-6 1.2x10-6 1.4x10-6-2.0x10-4

0.0

2.0x10-4

4.0x10-4

Experimental Fitting

A

Time (s)

Charge recombination at CH3NH3PbIxCl3-x/TiO2 interface

τ: 0.14 μs

E

TiO2

CH3NH3PbIxCl3-x

e-

h+ 0.14 μs

CB

VB

CB

VB

Probe light wavelength: 658 nm Monitor electrons in TiO2

16 ns τrec1 τhi

Spiro

OMeTAD

ηHT = (1/τhi)/(1/τhi + 1/τrec1) ηHT: about 90%

Hole injection efficiency ηHT

Charge separation efficiency: ηCS = ηET ηHT ≈ 90%

Charge recombination at TiO2/Spiro-OMeTAD interface

1E-6 1E-5 1E-4-1.0x10-3

0.0

1.0x10-3

ΔA

Time (s)

Without Spiro

Probe light wavelength: 1310 nm Monitor lifetime of holes in Spiro

1E-6 1E-5 1E-4-1.0x10-3

0.0

1.0x10-3

ΔA

Time (s)

Experimental fitting

With Spiro τ: 60 μs

E

TiO2

e-

h+

Spiro

OMeTAD

60 μs

CB

VB

CB

VB

CH3NH3PbIxCl3-x

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

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

Charge recombination at TiO2/Spiro-OMeTAD interface

1E-6 1E-5 1E-4-1.0x10-3

0.0

1.0x10-3

ΔA

Time (s)

Without Spiro

Probe light wavelength: 1310 nm Monitor lifetime of holes in Spiro

1E-6 1E-5 1E-4-1.0x10-3

0.0

1.0x10-3

ΔA

Time (s)

Experimental fitting

With Spiro τ: 60 μs

E

TiO2

e-

h+

Spiro

OMeTAD

60 μs

CB

VB

CB

VB

CH3NH3PbIxCl3-x

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

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

0.0 0.2 0.4 0.6 0.8Cu

rre

nt

De

nsi

ty /

mA

cm-2

voltage / V

efficiency: 6.59%

Relationship between Carrier Dynamics and Photovoltaic Performance

E

TiO2

CH3NH3PbIxCl3-x

e-

h+

1.8 ns

0.14 μs Spiro

OMeTAD

60 μs

0.0

0.2

0.4

0.6

0.8

1.0

350 450 550 650 750

IPC

E

Wavelength / nm

at 470 nm: IPCE=59%

IPCE＝ηabsorption ηCS ηCC

ηCS = ηET ηHT ≈ 90%

ηabsorption ≈ 95%

ηCC < 70%

The problem: smaller τrec2 smaller ηCC.

16 ns

Jsc: 11.8 mA/cm2

Voc: 0.79 V

FF: 0.71

CB

VB

CB

VB

τrec2

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

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

0.0 0.2 0.4 0.6 0.8

Cu

rre

nt

De

nsi

ty /

m

Acm

-2

voltage / V

Effects of surface treatment on carrier dynamics and photovoltaic performance

τrec2: 60 μs 180 μs Jsc: 11.8 mA/cm2 16.5 mA/cm2; efficiency： 6.59% 7.53%

After surface treatment of TiO2

E

TiO2

CH3NH3PbIxCl3-x

1.8 ns

0.14 μs

TiO2 Y2O3

CH3NH3PbIxCl3-x

e-

h+

3.3 ns

1 μs

Spiro

OMeTAD

E

60 μs

180 μs

Spiro

OMeTAD

Without surface treatment

16 ns

16 ns

With surface treatment

CB

VB

CB

VB

CB

VB

CB

VB

ChemPhysChem, Vol. 15, 1062 (2014)

e-

h+

h+

h+

0.0

4.0

8.0

12.0

16.0

20.0

0.0 0.2 0.4 0.6 0.8

Cu

rren

t D

en

sity

/ m

Acm

-2

Volatage / V

cell B

cell A

(b)

Effects of TiO2 nanoparticle sizes on carrier dynamics and Photovoltaic Performance

E

TiO2

CH3NH3PbI2Cl

e-

h-

1.8 ns

0.14 μs

CH3NH3PbI2Cl

e-

h-

0.7 ns

0.13 μs

Spiro

OMeTAD

60 μs

600 μs

Spiro

OMeTAD

16 ns

16 ns

TiO2 size: 18 nm 30 nm 18 nm

30 nm

τrec2: 60 μs 600 μs IPCE(at 470nm):59% 85% Jsc: 11.8 mA/cm2 17.6 mA/cm2; efficiency： 6.59% 9.54%

TiO2: 18 nm

TiO2: 30 nm E

TiO2

(ηCC ≈ 100% )

CB

VB

CB

VB

CB

VB

CB

VB

Summary

(1) The carrier lifetime of perovskite CH3NH3PbIxCl3-x is as large 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 program of 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