Strong Interface p-doping and Band Bending in C

60

Irfan, Huanjun Ding and Yongli GaoDepartment of Physics and Astronomy

University of Rochester, Rochester, NY 14627, USA

Minlu Zhang and Ching W. TangDepartment of Chemical Engineering

University of Rochester, Rochester, NY 14627, USA

Introduction:

C60

and it's derivatives have been extensively used in OPV, OTFT.

Lof et al, PRL, 68, 3924(1992)

The HOMO-LUMO gap of C60

is ~2.3 eV.

Xue et al., Appl. Phys. Letts. 84, 3013 (2004).

Haddon et al., Appl. Phys. Letts. 67, 121 (1995).

Tanigaki et al., Nature 352, 222 (1991)

C60

was first synthesized in 1985. Nobel prize in 1996 in Chemistry.

Introduction cntd...The LUMO level of C

60 is very close to the Fermi Level (n-type)

There has been a lot of efforts to p-dope C60

.

Haddon et al., Appl. Phys. Letts. 67, 121 (1995)

Liu et al., Appl. Phys. Letts. 95, 093307 (2009).

Maser et al, Syn. Met. 51, 103 (1992)

Photo-emission Studies

• Photoemission for occupied electronic structure

– X-ray and ultraviolet photoemission (XPS & UPS)

• Inverse photoemission (IPES) for unoccupied electronic structure

h v

V a c u u m

F e r m i l e v e l ( = 0 )

I o n i z a t i o n p o t e n t i a l

E = h v - E h

h v = E 0 - E l

P h o t o e m i s s i o n S p e c t r o s c o p y

E l e c t r o n D e t e c t o r

P h o t o n D e t e c t o r

I n v e r s e P h o t o e m i s s i o n S p e c t r o s c o p y

E h

E 0

E l

E l e c t r o n a f f i n i t y

C60

on MoOx

WF and HOMO shifts:

C60

on MoOx:

Onset of the HOMO level of C60

is measured to be 0.21 eV below

the Fermi Level or LUMO level is 2.1 eV above the Fermi Level.

With increasing thickness of the C60

film, the occupied levels

gradually relaxes to the normal values. A large band bending is observed during the relaxation.

The high work function of the MoOx thin film, pulls up the

energy levels of C60

.

C60

On Air Exposed MoOx

Energy Level Alignment:

Ishii et al., Phys. Stat. Sol.:A 201, 1075(2004). Irfan et al., Appl. Phys. Letts. 96, 243307(2010).Meyer et al., Appl. Phys. Letts. 96, 133308(2010).

Discussion:

With the air exposure of MoOx thin film, both the strong p-type

behavior, and the long band bending in a C60

thin film disappear.

With increasing thickness of the C60

film, the occupied levels

gradually relaxes to the normal values. A large band bending is observed during the relaxation.

The High WF of the MoOx film, strongly attracts electrons from

C60

towards the MoOx and thus creating C+

60 in the organic side.

P. Strobel et al., Nature. 95, 093301 (2004).

W. Chen et al., Prog. Surf. Sci., 84, 279 (2009)

At the interface, the HOMO edge of the C60

film is

observed at 0.21 eV, indicating a strong p-type doping and inversion of C

60.

An exceptionally long band bending of 400Å is observed.

Summary:

With the air exposed MoOx film, the strong p-doping

behavior disappears.

Irfan et al., Organic Electronics (Submitted)

Acknowledgment:

NSF Grant No. DMR-1006098.

Iris Ovshinsky Student Travel Award, APS:Division of Material Physics.

Irfan, Department of Physics, University of Rochester: APS March Meeting 2011

WF vs Exposure:MoOx thin film

Chua et al. Nature 434, 194 (2005).

A Typical UPS Spectrum

HOMO energy EHOMO = EFermi – Ev

Work function = hv – Ecutoff

Ionization potential IP = EHOMO +

-20 -15 -10 -5 0 5

Ecutoff

Binding Energy (eV)

Efermi

Evac

EV

EHOMO

hv

IP

Irfan, Department of Physics, University of Rochester

APS March Meeting 2009