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Donghang Yan ( )State key laboratory of polymer physics and chemistry,

Changchun Institute of Applied Chemistry, CAS,

Renmin Str. 5625, Changchun 130022, China

Heterojunction effects in OPV cells

Categories of heterojunctions OPV cells using WEG films Optimization towards smart cells

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H.B. Wang, D.H. Yan, Science in China B39, 1 (2009).

Categories of heterojunctions

ZnPc/C60

F16 CuPc/SnCl2Pc

CuPc/F16 CuPc, BP2T/F16 CuPc,P3HT/C60

p-6P/VOPc, p-6P/CuPc, 3PTh/VOPc

Typical organic systems

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FET

-50 -40 -30 -20 -10 0

-4.0x10-6

-3.0x10-6

-2.0x10-6

-1.0x10-6

0.0

1.0x10-6

-30V

-10V

-30V

-50V

-50V

0V

Draincurrent(A

)

Drain-source voltage (V)

CuPc/F16

CuPc heterojunction

CuPc single layer

Gate-source

voltage =0,-10V

Normally-on OFET

F16 CuPc

Heavily doped silicon wafer

Ta2O5

CuPc

Au Au

Au

+ + +

Free holes accumulated at CuPc films

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Semiconductor-Device Electronics, 1991 Oxford University Press

PNJunctions

E

Space charge region

Ion residualFree e

n-type Si p-type Sidoping

- Diffusion theoryNo free e or h

Free h

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Organic PNJunctions

- Diffusion theory

- NEW theory ? !

E

E

n-type Si p-type Si

n-type

F16 CuPc

p-type

CuPc

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C. Shen, A. Kahn, JAP90,4549,2001

H. Peisert et al, JAP93, 9683 (2003).

K.M. Lauet al, APL 88, 173513 (2006).

A.Kahn, JAP. 86,4515,1999

n-F16 CuPc

4.6

6.1p-CuPc

3.12

4.82

Energy

(eV)

3.0

4.0

5.0

6.0

5.16

6.66

5.0

3. 5

6.3

4.8

5.2

3. 6

EFp

EFn

D.H. Yan, H.B. Wang, B.X. Du, Introduction to Organic SemiconductorHeterojunctions, 2008

Thermal equilibrium conditions a theoretical view

According to thermal equilibrium

conditions, the electrons prefer to flow

from the high energywork function

position to low energy work function

position when two semiconductors are

brought into contact.

p-Si n-Si

Depletion HJ

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HJ effects in typical OPV systems

Band modelMolecular model

Working mechanism

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OPV cells usingWEG films

Weak epitaxy growth

Mismatch of charge and exiton transport,

is an intrinsic problem for OPV cells ?

(Accumulation HJ)

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Al

Of MPc WEG films

Glass

ITOInducing layer

WEG ZnPc

C60

EBL

WEG films may over come

the mismatch of charge/exiton transport in OPV cells

Depletion heterojunction with C60

Space charge thickness ~ 40 nm

Charge carrier mobility as single crystal- No effective deep traps at RT

- Shallow traps of 0.056 eV at low T

Exciton diffusion length should be longer

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p-6P(2nm) p-6P(2nm)/ZnPc(3nm)

p-6P(2nm)/ZnPc(30nm) SiO2/ZnPc(30nm)

10 mx10 m

RMS=2.4nmRMS=0.8nm

CuPc

N

N

N

N

N

N

N

N

Cu

p-6P

pp

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Quality of WEG films is good as single

crystal.

TFT transfer curves

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2

2TFL

eNdV

=

Density of deep traps is about 2.6 1016/cm3.

No effective

deep traps

observed

Deep traps dominates

electrical behaviors of

organic crystalline films.

High quality of WEG films

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delocalized transport

Hall effectLakeshore 7707, van der Pauw

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10:1

At the room temperature, the ratio of

thermal activated charges to charges

located at conductive band is 10 to 1

for WEG films.

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Cellstructure

Voc(V)

Jsc(mA/cm2)

FF PCE(%)

Saturationfactor

Ref PHJ 0.52 4.16 0.55 1.19 1.27

WEG PHJ 0.56 5.76 0.65 2.10 1.07

Advanced Materials, adma.200903023, in press

OPV cells using WEG films

Planar heterojunction (PHJ) device

Free electrons and holes can be collected efficiently.

AM 1.5G

+

-

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20nm

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Planar-mixed heterojunction (PM-HJ) device

High efficient cells are in optimization.

SF

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Optimization towardsmart cells

(PCE 10%)

Larger VOC cell NIR cell

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Connecting unit

Tunnel junction

Performance Double Voc

WEG

sub cell 1

Connecting unit

WEG

sub cell 2

(Accumulation OHJ)

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Outlooks

OHJ supplies a view angle to understand and develop OPV cells.

We realized WEG ZnPc films, and applied to OPV cells.

- high charge carrier mobility, low deep traps

- exciton diffusion length is comparable to the absorption- moderate efficiency

Potential space for improving efficiency of OPV cells,

- NIR absorbing materials, and

- smart tandem cells