1. Strong Interface p-doping and
Band Bending in C60
Irfan, Huanjun Ding and Yongli Gao
Department of Physics and Astronomy
University of Rochester, Rochester, NY 14627, USA
Minlu Zhang and Ching W. Tang
Department of Chemical Engineering
University of Rochester, Rochester, NY 14627, USA

2. 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.

3. Introduction cntd...
The LUMO level of C60
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)

4. 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 io 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 is s io n
S p e c t r o s c o p y
E le c t r o n
D e t e c t o r
P h o t o n
D e te c t o r
In v e r s e
P h o t o e m is s io n
S p e c t r o s c o p y
E h
E 0
E l
E le c t r o n
a f fi n i ty

5. C60
on MoOx

6. WF and HOMO shifts:

7. 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
.

8. C60
On Air Exposed MoOx

9. 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).

10. 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)

12. 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 C60
.
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)

13. Acknowledgment:
NSF Grant No. DMR-1006098.
Iris Ovshinsky Student Travel Award,
APS:Division of Material Physics.

14. Irfan, Department of Physics, University of Rochester: APS March Meeting 2011
WF vs Exposure:MoOx
thin film

15. 
Chua et al. Nature 434, 194 (2005).

16. 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