Photoelectric behaviours of covalently linked porphyrin derivatives

Solar Enen~ Mate~ls and ,So~arCells

ELSEVIER

Solar Energy Materials and Solar Cells 37 (1995) 25-31

Photoelectric behaviours of covalently linked porphyrin derivatives J i a n - g u a n g Jia *, X u - r u i X i a o , J i n - m e i Xu, W e n - y u a n Q i a n , B a o - w e n Z h a n g , Yi C a o The Institute of Photographic Chemistry, Academia Sinica, Beijing 100101, China Received 10 November 1993; revised 23 September 1994

Abstract

The photoelectric behaviours of covalently linked porphyrin-donor-acceptor compounds: diad, triad, tetrad and porphyrin with Cl6 alkyl substitute incorporated into molecular laver assemblies by LB film technique have been investigated. The photovoltages and photocurrents were measured and compared for monolayer of their LB films. The increase of photovoltages and photocurrents with the order tetrad > triad > diad > TPPOC16H33 indicates that the photoelectric behaviours can be improved successfully in the well designed porphyrin linked donor-acceptor compound systems.

I. Introduction

Considerable effort has to be paid to the studies of organic materials for light energy conversion [1,2]. Porphyrin and its derivatives have been widely used as photosensitizers either in solid state or in photoelectrochemical cells due to their chemical stabilities and absorption in the visible region [3-5]. Recently, particular interest has been payed to porphyrin for the imitation of nature photosynthesis because of their structural analogy to chlorophyll. Porphyrin may construct an artificial photosynthetic reaction-center, which simply functions as a photovoltaic device by linking a donor and an acceptor with a covalent band. Various porphyrin linked d o n o r - a c c e p t o r compounds which have been synthesized for the imitation

* Corresponding author. 0927-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 7 - 0 2 4 8 ( 9 4 ) 0 0 1 9 4 - 4

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J.-g. Jia et al. / Solar Energy Materials and Solar Cells 37 (1995) 25-31

of natural processes have attracted our interest in the studies of their photoelectric behaviours in relation to the design of biometric photovoltaic devices. In this paper, we report the photoelectric behaviours of porphyrin and its linked donor-acceptor compounds which were incorporated in well organized monomolecular layer assemblies deposited on a SnO 2 electrode in order to develop new organic molecular photosensitized systems for light-to-electric conversion.

P,

TTP-OC~-I~

R= - o ¢16H~3

=OC~H6 - - N x / ~ ~ - C , , $ H 3 3 Br-

Diad

I"

-OCsH 6 -

Triad

-CloH~2 8r-

z"

T et rad 0

o

Br-

l-

Fig. l. Structure formulaeof porphyrin compounds.

J.-g. Jia et al. / Solar Energy Materials and Solar Cells 37 (1995) 25-31

27

2. Experimental details 2.1. Materials

Porphyrin TPPOC16H33 and porphyrin linked donor-acceptor compounds such as diad(S-A1), triad(S-A1-D) and tetrad(S-A1-A2-D), where S, A 1, A 2, and D correspond to porphyrin, viologen, terephthalic acid ester, and carbazole, respectively, were synthesized in our laboratory. Detailed procedures of synthesis were described elsewhere [6,7]. All involved products were purified and characterized by UV, IR and ~H-NMR spectroscopies. The structural formula of the synthesized compounds are illustrated in Fig. 1. 2.2. Preparation o f L B films

A chloroform solution of amphiphilic porphyrin compounds including diad, triad, tetrad, and porphyrin TPPOCI6H33 mixed with stearic acid (1:3.12) were spread onto the distilled water subphase to form a stable monomolecular layer. The depositions of the monolayer onto the hydrophilic SnO 2 conducting glass surface were carried out under a surface pressure of 20 dyn cm-1. 2.3. Instruments and measurements

The photoelectric behaviours were measured in 0.1 M H2SO 4 solution u n d e r irradiation of focused white light from a 250 W tungsten halide lamp equipped with a 7 cm water filter to separate heat. The intensity of the incident light was 75 m W / c m 2. A saturated calomel electrode (SEC) and a Pt-foil were used as reference and counter electrode, respectively, for measuring the open-circuit voltages (Voc) and short-circuit currents (Isc) by GDM-8045 digital multimeter. T h e photocurrent action spectra were obtained from short-circuit currents under monochromatic irradiation and absorption spectra were measured on a Shimadzu UV120-02 spectrometer.

3. Results and discussions 3.1. Surface pressure-area isotherms

The surface pressure-area isotherms of porphyrin TPPOC16H33 and porphyrin linked compounds are shown in Fig. 2. Four isotherms representing porphyrin TPPOC 16H33 (a), diad (b), triad (c), and tetrad (d) showed a steep increase of the surface pressure on reduction of the molecular area, exhibiting the formation of solid densely packed monomolecular layers in the air-water interface with cross! sectional areas of about 22.5,~2, 153.6,&2, 174.8A2, and 192.5,&2 per molecule in thei case of porphyrin TPPOC16H33, diad, triad, and tetrad, respectively. A smaller monomolecular area for porphyrin TPPOC16H33 suggested that the macroringi

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J.-g. Jia et aL / Solar Energy Materials and Solar Celh"37 (1995) 25-31 0

20

40

60

I c9 o

v

O

~D ~9

r]]

0

80

160 240 320 400 0 Aera (A2) Fig. 2. Surface pressure-area isotherm of porphyrin compounds: (a) TTP-OC16H33 mixedwith stearic acid (----), (b) Diad ( ), (c) Triad ( . . . . . ), (d) Tetrad (. . . . . ).

plane of porphyrin molecules was squeezed from the water surface and laid on the top of stearic acid molecules. For diad, triad and tetrad molecules, the hydrophilic moiety, viologen was reasonably located on the hydrophilic SnO 2 electrode surface. The configuration was considered to be arranged spatially in the order shown in Fig. 2. 3.2. Absorption spectra o f porphyrin L B films

The absorption characteristics of porphyrin LB films involved porphyrin linked compounds and C16 alkyl substituted porphyrin were studied in terms of absorption spectra. The absorption spectrum of the porphyrin tetrad LB film, which closely resembled that of diad and triad LB films, and the absorption spectrum of porphyrin TPPOC16H33 LB film are shown in Fig. 3. The absorption spectrum of the tetrad compound in a chloroform solution is also presented for comparison. As seen from Fig. 3, the maximum absorption of the tetrad LB film, which is located at a wavelength of 400 nm, is about 20 nm red shifted compared to that of its chloroform solution. Analogous red shifts of the absorption maxima were also displayed in the absorption spectra of porphyrin TPP-OC16H33 and porphyrin diad and triad LB films. It is suggested, that the red shifts of the absorption maximum might be attributed to interactions of the planar packed porphyrin macrorings. The absorption spectrum of the porphyrin tetrad LB film is the same as that of the porphyrin TPPOC~6H33 LB film in the region of 400-800 nm indicating that there are no appreciable interactions between porphyrin and other moieties in the ground state.

J.-g. Jia et al. / Solar Energy Materials and Solar Cells 37 (1995) 25-31 1.0

6 {}

0.8

48

0.6

o ~ 6 L~

m 04 S: <~

12,I o ,D

o

~,

0.2

400

\

12

.''....'~. _ ~

':ii:b 480

560

29

640

7EO

oo

800

WavelengLh (rim.) Fig. 3. UV-visible absorption spectra of porphyrincompounds:{a) TTP-OC16H33LB films(----), (b) Tetrad LB films ( ), (c) Tetrad chloroformsolution ( . . . . . . ).

3.3. Photoelectric behaviours of porphyrin LB films The open-circuit photovoltages (Vow) and short-circuit photocurrents (Isc) o~ porphyrin TPPOC16H33 and its linked compounds LB films modified SnO 2 electrode in monomolecular layer are listed in Table 1. A remarkable enhancement of photovoltages and photocurrents was observed for porphyrin linked compounds especially for triad and tetrad which generated at rather high Voc (> 0.7 V) in only a monomolecular layer. The photoelectric behaviours improved in the following order: tetrad > triad > diad > TPPOC16H33. No photoresponse was observed for the bare SnO 2 electrode. The results presented above can be interpreted as follows: photoelectric behaviours of organic molecules based on efficient photoinduced charge separation and fast electro n transfer can be greatly improved in porphyrin linked donor-acceptor compound systems owing to the imitation and application of the multistep electron transfer strategy which proceeded in the photosynthetic reaction center. The electron transfer rate constants ket have shown to be 2.24 x 107 s- 1 and 4.72 x 107 s- J for triad and tetrad in chloroform solution, respectively, calculated by fluorescence lifetime studies. The higher ket value in tetrad compared to triad suggests a faster intramolecular electron transfer in the case of tetrad, resulting in the generation of higher photovoltages and photocurrents.

Table 1 Photoelectric behaviours of monolayer of porphyrin c o m p o u n d s Compounds

T P P - O C 16H 33

Diad

Triad

Tetrad

Photocurrent Isc (p.A cm "2) Photovoltage Voc (V)

0.10 0.384

0.12 0.404

1.22 0.706

1.85 0.745

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J.-g. Jia et al. / Solar Energy Materials and Solar Cells 37 (1995) 25-31

0 2,5

(3

c:

l ~ ,"

f 0 0

400

015 ~o

),

I \~-"~-~-

480

560

0 O0

640

720

800

Wavelengkh ( n m ) Fig. 4. Photocurrent and UV-visible absorption spectra of tetrad monolayer: (a) Photocurrent spectrum (-), (b) UV-visible absorption spectrum (----).

Fig. 4 illustrates the photocurrent action spectrum of tetrad in the monomolecular layer. A good agreement between the photocurrent action spectrum and the absorption spectrum has been observed. It can be educed that the photocurrents mainly arose from the excitation of the photosensitizer, i.e., the porphyrin moiety. Q u a n t u m efficiency which is defined as the ratio of the number of collected electrons over the number of photons incident on the monolayer of tetrad LB film at the excitation wavelength of 440 nm was calculated to be 1.17 × 10 -3. 3.4. Dependence o f photocurrents on surface pressure o f L B films

The dependence of short-circuit photocurrents on the surface pressure was studied for tetrad by changing the surface pressure from 20 dyn c m - ' to 30 dyn cm-~ during LB film deposition. The photocurrents I~c of the monolayer LB film increased from 1.85 IxA cm 2 at 20 dyn c m - l to 2.79 ixA cm 2 at 30 dyn cm-1 corresponding to an increase by a factor of 1.5. The maximum absorption at 440 nm measuring for the tetrad monolayer also increased by a factor of 2 at the similar change of surface pressure. The increase in surface concentration of tetrad molecules, which was estimated by a factor of 1.3 from the surface area isotherm as the surface pressure changed from 20 dyn c m - ~ to 30 dyn c m - l, was reasonably supposed to be responsible for this behaviours. Another possible contribution was the molecular construction oriented in a more desirable and favourable structure for faster intramoleeular charge transfer at the higher surface pressure deposition.

4. C o n c l u s i o n s

The photoelectric behaviours of porphyrin TPPOC16H33 and diad, triad, tetrad of covalently linked p o r p h y r i n - d o n o r - a c c e p t o r compounds were investigated by

J.-g. Jia et aL / Solar Energy Materials and Solar Cells 37 (1995) 25-31

31

t h e L B film t e c h n i q u e . P o r p h y r i n l i n k e d c o m p o u n d s e x h i b i t e d r e m a r k a b l e high o p e n - c i r c u i t p h o t o v o l t a g e s a n d s h o r t - c i r c u i t p h o t o c u r r e n t s c o m p a r e d with that Of t h e p o r p h y r i n TPPOC16H33. A r a t h e r high Voc o f 0.745 V, I¢c of 1.8 I~a cm -2 a n d q u a n t u m efficiency o f 1.17 × 10 -3 w e r e o b t a i n e d for t e t r a d with only m o n o m o l e ¢ ular layer i n d i c a t i n g t h a t the p h o t o e l e c t r i c b e h a v i o u r s o f o r g a n i c p h o t o s e n s i t i z e r , e.g., p o r p h y r i n , can b e i m p r o v e d successfully in t h e well d e s i g n e d d o n o r - a c c e p t 0 r l i n k e d systems. S t u d i e s o f t h e p h o t o e l e c t r i c b e h a v i o u r s o f m o l e c u l a r a s s e m b l i e s consisting o f synthetic p h o t o s e n s i t i z e r - d o n o r - a c c e p t o r m o l e c u l a r l i n k e d systems a r e in the progress.

Acknowledgements This w o r k was s u p p o r t e d by T h e N a t i o n a l A d v a n c e M a t e r i a l C o m m i t t e e o f China.

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