Work function recovery of air exposed MoO

x thin films

IrfanDepartment of Physics and Astronomy

University of Rochester, New York 14627

Alexander Turinske: University of Wisconsin

Zhenan Bao: Stanford University

Yongli Gao: University of Rochester

Transition Metal Oxides, MoO

x, VO

x, ..

Metal Production

CdTe/CdS basedSolar Cells

Organic Photovoltaic Cells

Organic Thin Film Transistors

Strong Interface p Doping

Organic Light Emitting Diodes

Efficient Spin Injection

Photochromic Lenses

Applications:

Exposure and Effect

Solution ??

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

Vacuum Annealing of MoOx

WF saturation ~ 460 ºC; value >6.3 eV

Angle Resolved XPS:

Shallow probing depth with the angle θ

O/ Mo Ratio Vs Angle

UPS Data: Interface with CuPc

WF recovery after annealing.

Shift of CuPc HOMO levels with increasing thickness.

Energy Level Alignment:

C. Wang Y.49_9 ; Room 162A

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.

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)

Acknowledgment:

NSF Grant No. DMR-1006098.

Prof. Ching W Tang: University of Rochester

Prof. Franky So : University of Florida