1. Work function recovery of air exposed
MoOx
thin films
Irfan
Department of Physics and Astronomy
University of Rochester, New York 14627
Alexander Turinske: University of Wisconsin
Zhenan Bao: Stanford University
Yongli Gao: University of Rochester

2. Transition
Metal Oxides,
MoOx
, VOx
, ..
Metal
Production
CdTe/CdS based
Solar Cells
Organic
Photovoltaic Cells
Organic Thin Film
Transistors
Strong Interface
p Doping
Organic Light
Emitting Diodes
Efficient Spin
Injection
Photochromic
Lenses
Applications:

3. Exposure and Effect
Solution ??

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
le v e l
( = 0 )
Io n iz a t io n
p o t e n t ia 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 fin ity

5. Vacuum Annealing of MoOx
WF saturation ~ 460 º
C; value >6.3 eV

6. Angle Resolved XPS:
Shallow probing
depth with the
angle θ

7. O/ Mo Ratio Vs Angle

8. UPS Data: Interface with CuPc
WF recovery
after annealing.
Shift of CuPc
HOMO levels
with increasing
thickness.

9. Energy Level Alignment:
C. Wang Y.49_9 ; Room 162A

10. Discussion:
One hour of air exposure to thermally evaporated MoOx
films
(WF ~6.8 eV) substantially reduces WF (~5.4 eV).
The reduction is due to adsorb oxygen rich over layer.
The recovery is due to the detachment of the adsorbed over layer.
The recovery was saturated around 460 ºC, with WF ~6.3 eV.
The work function of air exposed MoOx
film gradually recovers
with increasing temperature.

11. Air exposure of MoOx
thin film creates an unfavorable
energy level alignment for charge injection at anode,
and so it should either be avoided or work function
must be recovered.
Work function of air exposed MoOx
films can be
substantially recovered with the annealing around 460
ºC in side vacuum.
Summary:
Vacuum annealed MoOx
film has similar energy level
alignment as the pristine MoOx
film.
Irfan et al., Appl. Phys. Lett. (Ready for Submission)

12. Acknowledgment:
NSF Grant No. DMR-1006098.
Prof. Ching W Tang: University of Rochester
Prof. Franky So : University of Florida