Dark injection of electrons and holes and radiative recombination in anthracene with metallic contacts

Volume 25A, number 5

DARK INJECTION RECOMBINATION

PHYSICS LETTERS

OF ELECTRONS IN ANTHRACENE

11 September 1967

AND HOLES AND RADIATIVE WITH METALLIC CONTACTS

W. MEHL a n d B. FUNK C.vattamid E~ropeax Research lRstittde. Cologny, Geneva, Switzerlaml

Received 3 August 1967

A sodium-potassium alloy makes an electron injecting contact to anthracene, evaporated gold forms a hole injecting contact. The simultaneous injection of electrons and holes leads to radiative recombination at room temperatare.

C a r r i e r i n j e c t i o n into a n t h r a c e n e f r o m e l e c t r o l y t i c c o n t a c t s i s p o s s i b l e if the F e r m i e n e r g y of the e l e c t r o l y t e i s roughly equal to the e l e c t r o n affinity ( e l e c t r o n injection) o r the imfization e n e r g y (hole injection) of c r y s t a l l i n e a n t h r a c e n e so that a t the contact the conduction band o r the v a l e n c e band a r e b e n t into the v i c i n i t y of the F e r m i l e v e l of the solid [1,2]. E l e c t r o l y t i c c o n t a c t s have s e v e r a l obvious d i s a d v a n t a g e s so that it s e e m e d d e s i r a b l e to t r y to find solid c o n t a c t s f r o m which s i m i l a r band b e n d i n g and thus i n j e c t i o n of c a r r i e r s into a n t h r a cene can be obtained. We a s s u m e that the s i m p l e "band b e n d i n g w m o d e l which holds f o r e l e c t r o l y t i c c o n t a c t s i s a l s o v a l i d f o r m e t a l l i c c o n t a c t s and we d e s i g n a t e E C the l o w e r edge of the conductiou band, E V the u p p e r edge of the v a l e n c e band, E F the F e r m i l e v e l and 2A the width of the forbidden gap of a n t h r a c e n e . E l e c t r o n i n j e c t i o n should then be p o s s i b l e for m e t a l s with an e l e c t r o n i c w o r k f u n c tion • so that E F -& ] >/A, a n d hole i n j e c t i o n for m e t a l s with [~ - E F >/A. F o r a n t h r a c e n e the F e r m i e n e r g y i s E F = -4.17 eV [3] a n d A ~ 1.9 eV [4] so that e l e c t r o n i n j e c t i o n should be p o s s i ble with m e t a l s for which ~ ~<2.27 eV and hole i n j e c t i o n for m e t a l s with ~ >/6.07 eV. ~< 2.27 eV i s o b t a i n a b l e with ~lkali m e t a l s [5]. R was indeed found that e l e c t r e n s could be i n j e c t e d f r o m a contact c o n s i s t i n g of a n alloy of about 50% by weight sodium a n d p o t a s s i u m . U n expectedly it was a l s o found that e v a p o r a t e d gold (~ ~ 5.3 eV [6]) f o r m s a hole i n j e c t i n g c o n t a c t to a n t h r a c e n e . We have at this t i m e no definite e x p l a n a t i o n for t h i s r e s u l t . D u r i n g s i m u l t a n e o u s i n j e c t i e n of h o l e s and e l e c t r o n s f r o m opposite s i d e s of a n a n t h r a c e n e c r y s t a l u n i f o r m light e m i s s i o n f r o m the c r y s t a l 364

-i

e lo t-.

!

~s

L4 lo

L3 lo

t

lo-2

amp/ cm2 Fig. 1. Light intensity as functio~ of current density. was o b s e r v e d . F r o m a c r y s t a l with a s e m i t r a n s p a r e n t gold contact the i n t e n s i t y of the e m i t t e d light was m e a s u r e d with a c a l i b r a t e d P h i l i p s 150 AV p h o t o m u l t i p H e r . It was found that the light i n t e n s i t y i n c r e a s e d l i n e a r l y with the c u r r e n t d e n sity (fig. 1), and that the efficiency of light g e n e r a t i o n was at l e a s t 0.2 phot(ms p e r e l e c t r o n . T h e s p e c t r u m of the e m i t t e d light was m e a s u r e d through the gold contact and it was foend to have an a d s o r p t i o n edge a t 410/~m and to be o t h e r w i s e i d e n t i c a l with the optically s t i m u l a t e d l u m i n e s c e n c e s p e c t r u m of a s i m i l a r a n t h r a c e n e c r y s t a l [7]. We thus conclude that the i n j e c t e d holes and electrons recombine via a first order

Volume 25A, number5

PHYSICS LETTERS

r e a c t i o n to the f i r s t e x c i t e d s i n g l e t s ta te of a n t h r a c e n e which then d e c a y s r a d i a t i v e l y to the ground s t at e in the s a m e m a n n e r as the r a d i a t i v e r e c o m b i n a t i o n o c c u r s a f t e r double i n j e c t i o n f r o m e l e c t r o l y t i c c o n t a c t s [8,9].

11 September 1967

3. F. Lohmann, Z. Naturforsch. 22a (1967) 843. 4. N. Geacintov and M. Pope, J. Chem. Phys. 45 (1966) 3884. 5. Landolt-B~rnstein, eds. K . H . Hellwege and A. M. Hellwege, (Springer Verlag, Berlin, 1959)II. Band, 6. Tell, p. 913. 6. W.M.H. Sachtler, G.J.H. Dorgelo and A. A. Holscher, Surface Sci. 5 (1966) 221. 7. J. L. Brebner and W. Mehl, to be published. 8. W. Mehl and W. Btichner, Z. physik. Chem. N.F. 47 (1965) 76. 9. W. Helfrich and W.G. Schneider, Phys. Rev. Letters 14 (1965) 229.

References 1. W.Mehl, Ber. Bunsenges. Physik. Chem. 69 (1965) 583. 2. W. Mehl, J. M. Hale and F. Lohmann, J. Electrochem. Soc. 113 (1966) 1166. * * * * *

MAGNETOSTATIC

RESONANCES

IN AXIALLY

MAGNETISED

YIG

RODS

D. E. LACKLISON and M. F. LEWIS

The General Electric Company Limited, Central Research Laboratories, Hirst Research Centre. Wembley, England Received 4 August 1967

Magnetostatic-resonance measurements have been performed on axially magnetised YIG rods. It is shown that the results can be represented by a parameter, having values for short rods, significantly larger than the theoretical value for an infinitely long rod.

It has been shown [1] that the d e v i a t io n b e tween t h e o r y and e x p e r i m e n t , f o r the d e l a y - f i e l d c h a r a c t e r i s t i c s of m a g n e t o s t a t i c w a v e s in s h o r t , a x i a l l y m a g n e t i s e d , YIG r o d s , was due to n e g l e c t of the r a d i a l v a r i a t i o n of the d e m a g n e t i s i n g f i e l d in the v i c i n i t y of the r o d c e n t r e . T h i s c a u s e s the o u t e r p a r t of the r o d to be c u t - o f f , and m o d i f i e s the e f f e c t i v e r a d i u s u s e d in d e t e r m i n i n g a p a r a m e t e r , A ( d e f i n e d l a t e r ) . We have performed magnetostatic resonance measurem e n t s which show that the e f f e c t i v e v a l u e s of A a r e l a r g e r than t h o s e s u g g e s t e d in r e f . 1. The m e a s u r e m e n t s c o n s i s t e d of X - Y r e c o r d e r t r a c i n g s of the change in coupling between two c r o s s e d f i n e - w i r e c o u p l e r s , s i t u a t e d c l o s e to t h e end of a rod, when the d.c. m a g n e t i c field, Ho, was swept through the r a n g e f o r which r o u n d t r i p p r o p a g a t i o n o c c u r s . T h i s does not o c c u r u n ti l the i n t e r n a l field, Hi , at the c e n t r e of the r o d i s l e s s than co/~, (notation a s in r e f . 4). F o r continuous ex c it a t io n , the nth o r d e r m a g n e t o s t a t i c r e s o n a n c e o c c u r s when

lO.6mm

.

4mm

0

4 3

•4

i

I

I

J

I 5

I

I

I

I

I ~0

I

I

I

i

, IS "n,

t

I

i

i

i 20

i

I

I

I

I

I

I

=

2n~r.

(1)

I

I

25

I 30

Fig. 1. Graphs of AHo versus n for a signal frequency of 2.04 GHz. Th e d i s p e r s i o n r e l a t i o n f o r an infinitely long, a x i a l l y m a g n e t i s e d , r o d is [2] (co/~, - H i ) = A / k 2

½L 2 ~ kdz -L½

2C

(2)

w h e r e A = 2~M(2.405/R) 2, M is the s a t u r a t i o n m a g n e t t s a t i o n , and R i s the r a d i u s of the rod.

365