Journal o f Bioenergetics and Biomembranes (1976) 8, 1 8 9 - 1 9 8

Photoelectrochemical Effects in the Electrolyte-Pigment-Metal System II. Metal-Free Phthalocyanine Film Action Spectra of Short-Circuit Photocurrents with Increase of the Film Thickness Jean-Guy ViIlar Laboratoire de Photosynth~se, Centre National de Ia Recherche Scientifique, 91190 Gif-sur- Yvette, France Received 26 July 19 75

Abstract The e l e c t r o l y t e - p i g m e n t - m e t a l s y s t e m can be described as a n a l o g o u s to a p h o t o s e n s i t i v e j u n c t i o n region. W h e n t h e t h i c k n e s s of t h e p i g m e n t film is increased, the a c t i o n spectra o f t h e m a x i m u m short-circuit p h o t o c u r r e n t u n d e r c o n t i n u o u s i l l u m i n a t i o n differ f r o m t h e a b s o r p t i o n spectra b o t h in direct i l l u m i n a t i o n ( p i g m e n t - e l e c t r o l y t e ) a n d in b a c k illumination ( m e t a l - p i g m e n t ) . One is led to believe t h a t there exist t w o p h o t o a c t i v e regions in t h e s y s t e m for t h e p r o d u c t i o n of t h e short-circuit p h o t o c u r r e n t ; these t w o active regions are associated respectively w i t h each interface. W h e n t h e metallic s e m i t r a n s p a r e n t electrode is m a d e o f a l u m i n u m the two interfaces have opposite sign c o n t r i b u t i o n s to t h e p h o t o c u r r e n t ; this allows t h e d e t e r m i n a t i o n of c o n d i t i o n s in w h i c h one can observe specifically t h e c o n t r i b u t i o n of t h e p i g m e n t - e l e c t r o l y t e interface, t h a t is t h e i n t e r a c t i o n s b e t w e e n e x c i t e d p i g m e n t m o l e c u l e s a n d the r e d o x s y s t e m in t h e electrolyte.

Introduction The electrolyte-pigment-metal system has a real interest as a model system for the primary light energy conversion in the photosynthetic © 1976 Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission of the publisher.

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JEAN-GUY VILLAR

apparatus as far as the description of processes between excited pigment molecules and the redox system in the electrolyte is possible in this system. Some authors [1-4] consider that under these experimental conditions it is impossible to correlate directly measured electrical transients and kinetic processes for excited pigment molecules. Others [5] conclude that the production of electrical phototransients has its origin only in a photovoltaic process specific of the pigment-metal interface. The description we have given [6], for thin films of pigment, of the short-circuit photocurrents suggests the analogy of functioning with a photosensitive rectifying system. One looks here for the region of the system where the charge separation takes place being the origin of the production of photocurrents.

Results As previously described [6], we measure the maximum short-circuit photocurrent under continuous illumination. 1. On direct illumination (pigment-electrolyte interface) a weakly absorbed wavelength (470 nm) gives a maximum photocurrent iM continuously increasing when film thickness is increased in the range studied. With the use of a strongly absorbed wavelength (620 nm), however, iM decreases after passing through a maximum value (Fig. 1). This corresponds to a "main absorption distance" effect for the illuminating wavelength: That is, the strongly absorbed wavelength is mainly absorbed near the illuminating plane, while the weakly absorbed wavelength is more uniformly absorbed through the whole thickness of the film in the thickness studied here (1 g). 2. The action spectrum is drawn with the value of (diM/dI) f o r / = 0. In a first approximation we have used for the function iM = f ( I ) the expression IM = I/(a + bI), b being independent of wavelength and a dependent on wavelength; thus the action spectrum is drawn with a -1 obtained in the linear representation 1/I = a/I + b (I: light intensity). In the region where iM is increasing with film thickness, under illumination with a wavelength that is strongly absorbed, the action spectrum remains identical to the absorption spectrum. For the film thickness range where iM is decreasing under illumination with a strongly absorbed wavelength the action spectrum is first modified then completely reversed compared with the absorption spectrum. The intermediate forms present a lowering of the action spectra for wavelengths of maximal absorption; this leads to the appearance of "fictitious maxima" in the action spectrum on the sides of the absorption bands, for example at 420 and 530 nm (Fig. 2).

ELECTROLYTE-PIGMENT-METALSYSTEM. II

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Figure 1. Variation of the maximum short-circuit photocurrent produced by a semitransparent platinum electrode covered with a metal-free phthalocyanine film when increasing film thickness (electrolyte 96%; glycerol 4% saturated KC1 ac~ueous solution; reference electrode Ag/AgC1; active surface of electrodes 6.25 cm ). (a) Direct illumination (pigment-electrolyte interface) with a weakly absorbed wavelength: 470 nm. (b) Direct illumination (pigment-electrolyte interface) with a strongly absorbed wavelength: 620 nm. Abcissa: optical density of the films at the maximum of absorption 620 nm. Ordinate: maximum short-circuit photocurrent. This k i n d of e v o l u t i o n has already been observed b o t h on some electrochemical systems and o n dry systems used for p h o t o c o n d u c t i v i t y m e a s u r e m e n t s [ 7 - 1 0 ] . One i n t e r p r e t a t i o n leads to the a s s u m p t i o n of an active region near the p i g m e n t - m e t a l interface. Thus in our case only the p h o t o n s absorbed in a thickness 6 near the plane of the metallic electrode could c o n t r i b u t e to the p r o d u c t i o n of the short-circuit p h o t o c u r r e n t ; the r e m a i n i n g part of the film w o u l d play a screening role. As long as the thickness of the film l is less t h a n 6, the n u m b e r of "useful absorbed p h o t o n s " increases with film thickness and similarly the p h o t o c u r r e n t increases; also the a c t i o n s p e c t r u m stays identical to the a b s o r p t i o n spectrum. For thicknesses l greater t h a n 6, the n u m b e r of

192

JEAN--GUY VILLAR

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ELECTROLYTE-PIGMENT-METAL SYSTEM. II

193

"useful a b s o r b e d p h o t o n s " varies for a given wavelength X as the f u n c t i o n z = 1 0 - e a ( l - 8 ) ( 1 - - 1 0 - e a 5 ) , where eX is the e x t i n c t i o n coefficient. Calculated along the a b s o r p t i o n s p e c t r u m of the film this f u n c t i o n z simulates, with increasing l, the e v o l u t i o n of the action s p e c t r u m registered here. But this i n t e r p r e t a t i o n c a n n o t be fully satisfactory in o u r case as it implies t h a t for values of thickness l greater than 6, the n u m b e r of " u s e f u l " a b s o r b e d p h o t o n s is s i m u l t a n e o u s l y decreasing for all wavelengths; thus, if ofie assumes a direct c o r r e l a t i o n b e t w e e n the m a g n i t u d e of the m a x i m u m p h o t o c u r r e n t im and the n u m b e r o f " u s e f u l " a b s o r b e d p h o t o n s in the thickness 6, iM s h o u l d decrease in the same w a y for strongly and w e a k l y a b s o r b e d wavelengths. More s o p h i s t i c a t e d t r e a t m e n t s , t a k i n g into a c c o u n t diffusion of excitons p r o d u c e d b y d i f f e r e n t wavelengths, have b e e n p r o p o s e d . Our p u r p o s e here is to discuss the fact t h a t o n l y a region near the metallic e l e c t r o d e is at the origin o f the p h o t o c u r r e n t p r o d u c t i o n . It should be n o t e d t h a t o t h e r kinds o f i n t e r p r e t a t i o n have b e e n given, p a r t i c u l a r l y b y D e V o r e [ 1 1 ] , G o o d m a n [ 1 2 ] , and Putseiko [7]. T h e y a d m i t a d e p e n d e n c e o f the a c t i o n s p e c t r u m on t h e processes o f r e c o m b i n a t i o n of the charge carriers at the surface o f t h e p i g m e n t sample and in the b u l k of the sample. 3. We have o b t a i n e d f u r t h e r i n f o r m a t i o n b y s t u d y i n g the system u n d e r b a c k i l l u m i n a t i o n ( m e t a l - p i g m e n t interface). F o r the small thickness range the s i t u a t i o n is the same as for direct i l l u m i n a t i o n , taking into a c c o u n t the r e d u c t i o n of light i n t e n s i t y b y the s e m i t r a n s p a r e n t metallic electrode. We here p r e s e n t (Fig. 3) the a c t i o n s p e c t r u m using b a c k i l l u m i n a t i o n for a 1-/l t h i c k film: One can n o t i c e a lowering in the strong a b s o r p t i o n regions and the presence of so called " f i c t i t i o u s " m a x i m a at 550 and 420 n m ; w i t h direct i l l u m i n a t i o n the a c t i o n s p e c t r u m is c o m p l e t e l y reversed (Fig. 2). We c o n c l u d e t h a t with an increase in thickness o f the p i g m e n t film the evolution o f the a c t i o n s p e c t r u m for the m a x i m u m short circuit p h o t o c u r r e n t is o f the same k i n d for b a c k and f r o n t i l l u m i n a t i o n : The only difference seems to be t h a t the same i n t e r m e d i a t e forms are observed on b a c k i l l u m i n a t i o n for t h i c k e r films, since a reversed form o f the a c t i o n s p e c t r u m on direct i l l u m i n a t i o n c o r r e s p o n d s to an interm e d i a t e form on b a c k illumination. Figure 2. Action spectrum of the maximum short-circuit photocurrent produced by a semitransparent platinum electrode covered with a film of metal-free phthalocyanine (electrolyte 96% glycerol 4% saturated KCI aqueous solution; reference electrode Ag/AgC1; active surface of electrode 6.25 cm 2 ). Abcissa: wavelength nm. Ordinate: (diM/dI)i=O, I light intensity, iM maximum photocurrent; arbitrary unit. (Above) Direct illumination (pigment-electrolyte interface); intermediate form of the action spectrum; (film thickness -~ 0.7 t/; optical density at the maximum of absorption 620nm: _~ 3.6). (Below) Direct illumination (pigment-electrolyte interface). Reversed form of the absorption spectrum (film thickness -~ 1 /z; optical density at the maximum of absorption 620 nm: ~- 5).

194

JEAN-GUY VILLAR I/'d ira\

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Figure 3. Action spectrum of the maximum short-circuit photocurrent produced by a semitransparent platinum electrode covered with a metal-free phthalocyanine film (electrolyte 96% glycerol 4% saturated KC1 aqueous solution; reference electrode Ag/AgC1). Back side illumination (metal-pigment interface). Film thickness 1 /~, optical density at the maximum of absorption 620 nm~ 5. Ordinate: (diM/dI)i=O, I light intensity, iM maximum photocurrent: arbitrary units. Abcissa: wavelength: nm. length; nm.

To summarize these data, we propose to admit the existence in the e l e c t r o l y t e - p i g m e n t - m e t a l system of two photoactive regions: One has a thickness 81 near the e l e c t r o l y t e - p i g m e n t interface, the other has a thlckness 52 near the p i g m e n t - m e t a l interface. For small film thickness, where I