12
Bull. Soc. Photogr. Imag. Japan. (2015) Vol. 25 No. 1: 12–18

Original Paper

A Study on Electronic Structure of Interfaces between Substrate andDyes for Electron Transfer Mechanism of Dye Sensitization
Tadaaki Tani*

Abstract:

Dye sensitization is now understood as the electron transfer process from excited dye molecules to the conduction band of a­substrate (AgX in photography and TiO2 in dye-sensitized solar cell). The electronic structure that is compatible with the­electron transfer is characterized by the vacuum level shift at the interface between AgX and dyes. It was found that the­a lignment of permanent dipoles in organic dye molecules to the surface of AgX dominantly contributed to the vacuum level
shift, while the formation of coordinate bonds between sulfur atoms with lone-paired electrons in dye molecules and silver ionson the surface of AgX gave the minor and opposite contribution to the vacuum level shift.

Key words:

Dye sensitization, Electron transfer mechanism, Vacuum level shift, Alignment of dipole moments, Chemical adsorption

1. Introduction
Dye sensitization was discovered by H. Vogel for AgX photographic materials in 1873 1), and is still attracting attentions ofmany scientists and engineers, being expected to play essential
roles in new developing technologies 2) such as dye-sensitized s­ olar
cells 3) 4) and organic thin film solar cells 5). It is now accepted thatdye sensitization takes place through the electron transfer mechanism, according to which an optically excited electron in a sensitizing dye molecule is transferred to the conduction band of asemiconductor such as AgX in photography and TiO2 in dyesensitized­solar cell, as illustrated in Figure 1(A) 2)–4) 6) 7). How­ever, severe debates were made between the electron transfer
mechanism and the energy transfer one in photographic science
for many years in the past 2) 6) 7).
The electron transfer mechanism was proposed by Gurney andMott in 1938 under the assumption that an optically excited­electron in a sensitizing dye on an AgX grain is situated above
the bottom of the conduction band of the grain 8). By means ofmolecular orbital theory, Coulson estimated that an electron in
the highest occupied molecular orbital (HOMO) in a dye molecule was situated by ~7 eV below the vacuum level, and that anelectron excited to the lowest unoccupied molecular orbital(LUMO) from the HOMO was therefore 4.5–5.0 eV below thevacuum level, being much lower than the bottom of the conduction band of an AgX grain, (i.e., ~3.5 eV below the vacuum level).
Following Coulson’s estimation, Mott 9) dismissed the electrontransfer mechanism, and instead proposed the energy transfer
one, according to which an excited sensitizing dye molecule trans-

fers its excitation energy to the grain according to the Förster-type
energy transfer 10)–12), as illustrated in Figure 1(B). It is assumed
that the transferred energy excites an electron in the forbidden
band to the conduction band of the grain.
Terenin and Akimov experimentally proved Coulson’s estimate,confirming by UPS that the ionization energy of sensitizing dyes
in isolated state in vacuum were ~7 eV 13), and supported the­energy transfer mechanism, taking into account the fact that evensensitizing dye molecules in isolated state on the surface of AgX
grains could bring about dye sensitization. On the other hand, it
was observed by means of UPS that the difference in the ionization energy between an organic molecule in crystal and the corresponding molecule in vacuum was 1.5–2 eV and ascribed to thestabilizing effect of the polarization of molecules at lattice sites on
the ionized molecule in the crystal 14), However, this result was
not applied to the dye sensitization of AgX grains in photography.
It seems that the electronic energy levels of a sensitizing dye inisolated state on an AgX grain was considered to correspond tothat of the dye in isolated state in vacuum. Using LangmuirBlodgett (LB) membrane method, Kuhn and others 15) proved theoccurrence of the energy transfer mechanism, observing the dye
sensitization of the photographic process on a single crystal of
AgBr by a monomolecular layer of a sensitizing dye, which was
separated from the crystal with such a distance as to prevent theelectron transfer and to allow the energy transfer from a dye layer
in excited state to the crystal.
On the other hand, UPS measurements revealed that the ionization energies of most dye crystals were considerably smaller
than those of the corresponding dyes in vacuum, and that theReceived 24th, February 2015; Accepted 7th, May 2015
*Fellow, The Society of Photography and Imaging of Japan, 2933-2, Yoshidajima, Kaisei-machi, Ashigarakami-gun, Kanagawa, 258-0021, Japan

Tadaaki Tani   A Study on Electronic Structure of Interfaces between Substrate and Dyes for Electron Transfer Mechanism of Dye Sensitization

Fig. 1.

13

Schematic illustration showing the electron transfer mechanism for spectral sensitization of AgX by dyes (A) and the energy transfer one (B).

Fig. 2. Schematic illustration showing the electronic energy levels of a sensitizing dye in vacuum and on AgBr with respect to the conduction and valencebands (CB and VB, respectively) of AgBr.

electronic energy levels of excited electrons in the crystals werehigh enough (i.e., 3.0–3.5 eV below the vacuum level) 16)–20) forthem to be transferred to the conduction band of AgX. This result
was not in accord with the fact that only limited fraction of dyes
could sensitize photographic process with AgX 2) 6) 7).
The electron transfer mechanism has become to be accepted­a fter it was confirmed that the electronic structure of the interface
between AgX and dyes was consistent with the spectrally sensitizing ability of the corresponding dyes through the following two
steps.
(a) It has been made clear by the present author and others thatthe ionization energy of a dye molecule in solid is smaller
than that of the corresponding dye molecules in vacuum by
1.5–2 eV, not only for the dye molecule in crystal, but also forthe corresponding dye molecules in isolated state on the sur-face of an AgX grain in a photographic emulsion owing to
the stabilizing effect on the ionized dye molecule by the polarization of its surroundings, and that the electron transferfrom an excited sensitizing dye molecule to the conduction
band of AgX is therefore energetically possible, as illustrated
in Fig. 2 2) 6) 21).
(b) It was found as a result of the cooperative study by the laboratories of K. Seki and the present author that all of the­vacuum levels of layers of Dyes A–E in Figure 3 were shiftedand became to be lower than that of AgBr, and that the­reasonable coincidence of the spectrally sensitizing ability ofthese dyes with the electronic structure at their interfaces
with AgBr could be obtained by taking into account the­vacuum level shift, as shown in the same figure 22).
The idea leading to Result (a) is the dependence of the ion-

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Bull. Soc. Photogr. Imag. Japan. Vol. 25 No. 1 (2015)

Fig. 3. The molecular structures of merocyanine dyes studied in this paper (a) and the electronic energy levels of Ag layer as indicated on the ordinate
with respect to its Fermi level, AgBr layer on the Ag layer, and merocyanine dye on the AgBr layer as measured by means of UPS (b), where
HOMO and LUMO are the highest occupied and lowest unoccupied molecular orbitals, respectively, and VL and CB are the vacuum level andconduction band 22). Sensitizing dyes and non-sensitizing ones are designated as ‘good’ and ‘poor’.

ization energy and electron affinity of organic molecules on the
dielectric constants of their surroundings, and has been extended
to the interpretation of electronic structures of organic layers­under various conditions 23) such as the differences in the ionization energy and electron affinity of an organic molecule between
the surface and bulk of an organic layer 24)–26) and between the­interface with a metal electrode and bulk of the layer.
As stated above, the vacuum level shift was observed first for
the interface between AgBr and dyes 22), and then extensively
studied by Seki’s group for the interfaces between metals andorganic layers with a wide variety of ionization energy 27)–29),­­becoming to be inevitable knowledge for organic devices 30)–32).
They have pointed out several mechanisms giving rise to the vacuum level shift, including charge transfer to equalize the Fermi
levels between metals and organic layers, mirror effect, push-backeffect, chemical interaction between metals and organic layers,­effects of interfacial states, and orientation of polar molecules tothe surface of a metal substrate 33). However, the mechanism forthe vacuum level shift at the interface between dyes and substrates
other than metals such as AgBr has scarcely been analyzed.
This study was undertaken to make clear the mechanism of thevacuum level shift at the interfaces between AgBr and dyes on thebasis of the result of the UPS measurement of the interfaces. It is
expected that the result will provides, not only an additional­evidence for the electron transfer mechanism of dye sensitizationin photography by characterizing the electronic structure of theinterface between AgBr and dyes, but also an insight into the­vacuum level shift at the interface between organic layers andsubstrates other than metals.

2. Proposed Model for Electronic Structure ofInterface between AgBr and Dyes
One of the most probable causes for the vacuum level shift at
the interface between a substrate and an organic layer is the
charge transfer between them to equalize their Fermi levels. As
seen in Figure 3, the observed vacuum level shifts do not seem toresult from the equalization of Fermi levels between AgBr anddyes when it is assumed that the Fermi level of a dye is situated at
the midpoint between HOMO and LUMO levels of the dye. On
the other hand, the degree of the observed vacuum level shift­increased with increasing the methine chain length of the dyes
studied. They are merocyanine dyes and have resonance structures, one of which has a permanent electric dipole, as exemplified
by Dye B in Figure 4. It is therefore expected that the orientationof the dipoles of dye molecules to the surface of AgBr is respon­sible for the vacuum level shift.
The absence of the infrared absorption band of the >C=O
stretching vibration of a merocyanine dye is the evidence for theidea that the dye molecule has the above-stated permanent dipole,
and was confirmed for the dyes studied in this paper when they
were adsorbed to AgBr 34). It is known that merocyanine dyeswith >C=S have stronger adsorptivity to AgX than those without>C=S 35). The analysis by means of XANES has revealed that the
double bond between C and S in a dye has been converted to thesingle bond on its adsorption to AgX 36), indicating that a dyewith >C=S is adsorbed to AgX by forming a coordinate bond­between a sulfur atom with lone paired electrons in the dye molecule and a silver ion on the surface of AgX.
The above-stated results lead to a model, according to which a

15

Tadaaki Tani   A Study on Electronic Structure of Interfaces between Substrate and Dyes for Electron Transfer Mechanism of Dye Sensitization

Table 1. Evaluation of potential difference (V ) at the interface of AgBr
and adsorbed dye molecules according to the model proposedin this ­paper and vacuum level shift (D) in the electronic structure of the interface as observed by means of UPS 22).
Dye A

Dye C

Dye D

d (Å)

4.3

6.4

8.6

10.7

l (Å)

13.8

15.9

16.6

17.8

w (Å)

s (C/cm 2)
D (eV) 22)

V (eV)

Fig. 4. Resonance between a neutral structure (a) and a polarized one ofa Dye B molecule having a dipole that is indicated with + and –at its ends (b).

Dye B

(3.9)

(2.8 × 10 –5)

3.9

(3.9)

(3.9)

2.6 × 10 –5(2.4 × 10 –5)(2.2 × 10 –5)(0.98)

1.35(1.67)(1.81)

1.70

2.35

2.92

3.32

d; distance normal to AgBr surface between a positive charge and a negative one of a dipole in a dye molecule according to the model in Figs. 5 for
Dye B.
l; length of the projection of a dye molecule onto AgBr surface according
to the model.
w; width of a Dye B molecule as evaluated from occupied area by themolecule and length of its projection onto AgBr surface.
s; density of dipoles in the proposed model as estimated from the saturated
amount of Dye B molecules adsorbed on AgBr surfaces.

Fig. 5. The model proposed in this paper for a Dye B molecule adsorbed
to AgBr surface. The length of a broken arrow is d in Table 1. A
bar attached to the right end of the molecule is a C-H bond andis shown to indicate the length of the molecule.

Fig. 6. Degree of the vacuum level shift (D) as observed by UPS 22) andthe maximum potential difference (V ) as given by the model
proposed in this paper for Dyes A, B, C, and D adsorbed toAgBr.

merocyanine dye as shown in Figures 3 and 4 is adsorbed to thesurface of AgX by forming the coordinate bond of an S atom
­having lone paired electrons in the dye molecule and a silver ionon the surface of AgX, and aligned the positive and negative
charges of its dipole far and near the surface of AgX, respectively.
This model is qualitatively in accord with the facts that the vacuum level of a dye layer was shifted and became to be lower thanthat of AgBr, and that the degree of the vacuum level shift increased with increasing the methine chain length of the dyes
studied 22).

ate the model, which should give the largest shift of the vacuum
level at the interface between AgBr and the dyes studied. Namely,
the proposed model is composed of the molecular structure withall-trans isomer and the resonance structure, to which the degreeof the contribution of Structure B in Figure 4 is 100%. The modelis exemplified and illustrated for Dye B in Figure 5.

Although it is desirable to make the model quantitative, it isdifficult to exactly determine the degree of the contribution of
Structure B in Figure 4 to the resonance in a dye and the molecular
structure and orientation of the dye molecule with respect to thesurface of AgBr owing to the presence of isomers, number ofwhich increases with increasing the methine chain length of thedyes studied 37) 38). In this study, it has been undertaken to evalu-

3. Examination of Proposed Model
The adsorption isotherm of Dye B to AgBr grains gave the saturated amount of the dye to the grains as 2.7 × 10 –6 mol/m 2 andthe area occupied by a molecule of Dye B as 61.4 Å 2 36). The proposed model for the orientation of Dye B molecule to the grain asshown in Figure 5 indicates that the dye molecule occupies thelength (l) of 15.9 Å on the surface of AgBr to the direction of its
long axis and should have the width (w) of 3.9 Å. This value iscompatible with the thicknesses of various organic semiconductor

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Bull. Soc. Photogr. Imag. Japan. Vol. 25 No. 1 (2015)

molecules 39) 40) together with the distances between adjacent linesof silver ions on the (100), (111), and (110) faces, which are stableon the surface of AgBr 2), and was used as the thickness of molecules of Dyes A, C, and D, since the molecular structures of Dyes
A-D are the same except the length of methine chain. Thus, theproposed models for the orientation of Dyes A, C, and D give thevalue of l, the area occupied by a dye molecule, and the saturated
amount of the dye molecules (i.e., the density of dipoles in the
proposed model s). The obtained values are given in Table 1.
The potential difference V caused by the above-stated aligned
dipoles is given by sd/e ₀ e with the values in Table 1 for s and dtogether with 8.85 × 1012 for e ₀. The value of 8.0 was used for e as

the average of the dielectric constants of AgBr and dyes (i.e., 12.5
and 3.5, respectively). The obtained values of V for Dyes A–D­together with those of D as the degrees of the vacuum level shifts
given by the UPS measurement 22) are also listed in Table 1.
­Figure 6 shows the relation between the values of D and V, in­dicating the deviation of D from V.

4. Discussions
The obtained results as shown in Table 1 and Figure 6 area­ nalyzed from the following viewpoints.
(1) For all the dyes studied, D was smaller than V.
(2) Although the value of D increased with increasing the­methine chain length of the molecule of the dyes studied,the degree of the increase in the value of V decreased, resulting in the increase in the deviation.
(3) Taking into account the viewpoint (2), Figure 6 suggests thatelectric dipoles with sign opposite to that of V were present
at the interface between AgBr and dyes in the absence of V.
It is considered that Viewpoint (1) is consistent with the proposed model, since the model is based on the molecular and electronic structures with orientation of dye molecules, which should
give the largest potential difference at the interface between AgBr
and the dyes studied. It is considered that Viewpoint (2) arises
from the increase in the number of isomers of a dye molecule withincreasing its mechine chain length. The increase in the number
of isomers in cyanine and merocyanine dyes with increasing their
methine chain length is reflected in the relation between theirwavelengths of absorption peaks and methine chain lengths 37) 38).
Viewpoint (3) is then analyzed as an example of chemical interactions between substrates and organic layers (i.e., chemical
adsorption of organic materials to substrates) for the formation of
dipole moments at the interface between them. The chemical
adsorption of organic molecules to metal silver by forming coordinate bond between a sulfur atom with lone-paired electrons inan organic molecule and a silver ion on the surface of a substrateis popular in self-assembled monolayers of organic molecules on
silver substrates, and was reported to give the potential differenceat their interface, which was nearly proportional to permanent
dipole moments in the molecules 41) 42). Similar results were also
observed for self-assembled monolayers of organic molecules ongold substrates 43) 44). Namely, it is considered from the above-stated results that the coordinate bond formation does hardly produce the potential difference at the interfaces between metals
and organic molecules with sulfur atoms having lone-paired­electrons.
It is therefore considered that the coordinate-bonding between
a sulfur atom with lone-paired electrons and a silver ion on a substrate surface brings about the potential difference at the interface,
not for the case of metal silver, but for the case of ionic crystals ofsilver (i.e., AgX). The surface of an ionic crystal differs from thatof a metal in that the rumpled effect brings about dipoles on its
surface, as seen in AgX 45)–47), and that a silver ion or halide ion ata surface kink site is interchangeable with each other and responsible for the formation of dipoles at the surface of AgX 2) 6) 7).
The displacement of halide ions and silver ions on the surface asa result of the rumpled effect of AgX (i.e., the situation of halideions and silver ions above and below the average surface, respectively) brings about the formation of dipoles at the surface andincrease the ionization energy of AgX (i.e., the energy gap­between the vacuum level and the top of the valence band). The
ionization energy of AgBr was about 6.8 eV in the presence of therumpled effect 48) 49). On the other hand, it was 5.6–5.7 eV when
the rumpled effect was relaxed by coating a gelatin membrane on
the surface of an AgBr layer 49) 50). Since the AgBr layers preparedand used in this study have the ionization energy of about 5.6 eV
20), it is considered that the rumpled effect was r­ elaxed in the layer
not responsible for the dipoles at the interface between AgBr anddyes as a result of Viewpoint (3).
On the contrary to silver metal, surface kink sites with a silver
ion and halide ion on AgX bears positive or negative half-charge,
respectively, as illustrated in Figure 7. As illustrated in the centerof Figure 8, there is no dipole on the surface when the fraction of
surface kink sites with silver ions is equal to that with halide ions.
As illustrated in the left-hand and right-hand sides of Figure 8,
the surfaces are positively and negatively charged as the results of
the appearance of dipoles when the fraction of kink sites with­silver ions is larger and smaller than that with halide ions, respectively. Figure 8 thus indicates that the ionization energy of AgBr
decreases with decreasing the fraction of kink sites with silver

Fig. 7. An illustration of a (100) surface of AgBr with an ion at a kinksite bearing electric charge of ±1/2 and ions on steps bearing
electric charge of ±1/3.

Tadaaki Tani   A Study on Electronic Structure of Interfaces between Substrate and Dyes for Electron Transfer Mechanism of Dye Sensitization

17

Fig. 8. Electronic energy levels showing the vacuum level shifts at the interfaces between AgBr and dyes (upper figures) arising from the formation of
dipoles at the interfaces (lower figures).

Fig. 9.

Molecular structures and formation of silver salts of silver-complexing agents studied in this paper.

ions and thus with increasing the fraction of kink sites with halideions. The fraction of silver ions at surface kink sites is in equi­
librium with the concentration of silver ions in gelatin layer incontact with AgX. In accord with this idea, the increase in theconcentration of silver ions in the membrane of gelatin on an
AgBr layer increased the ionization energy of the AgX layer 49).
One of the most effective ways to increase the fraction of silver
ions at kink sites on AgX surface is to stabilize them by the chemical adsorption to the surface of AgX of agents to form sparingly
soluble silver salts. Some examples of such agents and the formation of their silver salts are shown in Figure 9. It is expected thatthe chemical adsorption of these agents to the surface of AgX
should increase the fraction of silver ions at surface kink sites on it
and increase the ionization energy of the layer, as illustrated in theleft-hand side of Figure 8. As shown in Figure 10, the ionizationenergy of an AgBr layer increased and the top of its conduction
band was lowered when it was in contact with gelatin layers con-taining these agents 49) 50). It is noted that the chemical interactionof PMT and SPMT in Figure 9 with AgBr layers is similar tothat of the dyes studied in this paper with AgBr layers.
The above-stated results support the model proposed in this­paper in that the chemical interaction between sulfur atoms withlone-paired electrons in the dyes studied in this paper and AgX
surface brings about the formation of the dipoles at the interface
between them as illustrated in the left-hand side of Figure 8,­giving an explanation for Viewpoint (3).

5. Conclusive Remarks
The model for the vacuum level shift at the interface between
AgBr layer and merocyanine dyes was proposed and examined inthis paper to support the validity of the electron transfer mechanism for dye sensitization in photography. It was found that thealignment of permanent dipoles in the dye molecules to the sur-18

Bull. Soc. Photogr. Imag. Japan. Vol. 25 No. 1 (2015)

Fig. 10. Position of the top of valence band of AgBr layer with respect to
the Fermi level of underlying Ag layer. The AgBr layer wascovered with a thin gelatin membrane containing one of thesilver-complexing agents that were indicated in Fig. 9.

face of AgBr layers gave the dominant contribution to the vacuum
level shift, and that the chemical interaction between AgBr layers
and the dye molecules gave the minor and opposite contribution
to it. The chemical interaction was composed of the increase in
the fraction of silver ions at surface kink sites owing to the coordinate bonding between them.
Although the vacuum level shift between organic layers andmetals has been extensively studied with widely varied materials,
the shift between organic layers and non-metal substrates has
scarcely been examined, and can now include the result in thispaper as an example of the model for it.

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