J. Phys. Chem. B 2000, 104, 8177-8181
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Application of Surface Photovoltage Technique to the Determination of Conduction Types of Azo Pigment Films Xie Tengfeng, Wang Dejun,* Zhu Lianjie, Wang Ce, and Li Tiejin Department of Chemistry, Jilin UniVersity, Changchun, 130023, P. R. China
Zhou Xueqin, and Wang Mang Institute of Polymer Science and Material, Zhejiang UniVersity, Hangzhou, 310027, P.R. China ReceiVed: January 18, 2000; In Final Form: May 8, 2000
Surface photovoltage spectrum (SPS) and field-induced surface photovoltage spectrum (FISPS) are valuable techniques for the determination of pn conduction type of organic semiconductors. Here, conduction characters of azo pigment films composed of pure 4,4′-2-[N-(β-hydroxyl)naphthyl]azodiphenyl (A) and 4,4′-2-{N-[βhydroxyl-γ-(p-nitroanilino)naphthyl]}azodiphenyl (B) as well as A doped with I2 vapor (AI2) and B doped with NH3 gas (BNH3) were explored by means of these techniques. The results of the exploration show that film A has n-type character and film B p-type character. On the basis of the reverse photovoltaic properties, films AI2 and BNH3 were judged to be characteristic of p-type and n-type conduction, respectively. It appears that a group rich in electrons, either an electrophobic substituent (naphthanol) at the end side of the azo pigment molecule (A) or a dopant (NH3), bestows the azo pigments with n-type character, whereas an electrophilic group, such as the substituent (p-nitroanilino) connected to azo pigment molecules (B) and I2, gives them p-type character. The phenomena are fully discussed based on the energy band modes of the Fermi levels of these materials.
Introduction Photoconductive organic pigments have attracted considerable attention in the field of optoelectronics due to their various applications.1-3 Among them, azo pigments have been used to fabricate organic photoreceptors, a kind of necessary charge photogenerating material for electrophotography, static copy, laser print, hologram, and production of organic p-n heterojunction solar energy cells.4-13 This is because they exhibit the some unique advantages: (i) low cost and easy fabrication; (ii) larger absorbing chromophores within the solar spectrum; (iii) high photosensitivity; (iv) good semiconductor behavior,14 and (v) adjustability of Fermi levels and alternation of conduction types brought about by different substituents and dopants. Several studies have indicated that the photoelectric conversion efficiency of the pigments could be improved by changing substituents and dopants, which could increase the lifetime of the photogenerated carriers.15-17 The excellent features of the azo pigments applicable in various fields resulted from reasonable selection of the pn conduction type of the materials. However, few techniques can be used not only to judge pn conduction type of materials and but also to understand the effect of substituents and doping on the behavior of azo pigments in photoacceptor and photovoltaic cells. The photovoltaic properties of organic pigment films depend on their Fermi levels, which seem to be strongly influenced by molecular structures and kinds of dopants. Particularly, no information has been found about the reason that they behave as n-type or p-type semiconductor.18 Surface photovoltage spectrum (SPS) is a useful tool to investigate the photophysics of excited states generated by absorption in the aggregate state.19 Since the sensitivity of the method is about 108 q/cm2, or about one elementary charge per 107 surface atoms, which exceeds that of such conventional spectroscopies as XPS and Auger spectroscopy by many orders of magnitude.20 It has been
successfully employed for the study of the charge transfer in photostimulated surface interactions, dye sensitization processes, and photocatalysis.21-23 On the basis of the principle of SPS, Dr. Zhang et al. developed a field-induced surface photovoltage technique (FISPS),24,25 which can demonstrate the optoelectric properties of organic semiconductors under the effect of an external electric field.26 We expect that by combining SPS with FISPS, the detailed studies on the effect of substituents on the photovoltaic properties of azo pigments can help us reach the origin that determines the conduction type of the azo pigments. We expect as well that the Fermi levels and conduction types of the azo pigments can also be controlled by intentional doping with known molecules. In this paper, we will present the results of the study on the photovoltaic properties of the azo pigment films composed of pure 4,4′-2-[N-(β-hydroxyl)naphthyl]azodiphenyl (A) and 4,4′2-{N-[β-hydroxyl-γ-(p-nitroanilino)naphthyl]}azodiphenyl (B) as well as film A doped with iodine vapor (AI2) and film B doped with ammonia gas (BNH3). The molecular structures of A and B are shown in Scheme 1. SPS and FISPS show that A and B are characteristic n-type and p-type semiconductors, respectively. Being doped with iodine and ammonia, AI2 and BNH3 are endowed with reverse conduction types to A and B, i.e., p-type and n-type, respectively. The phenomena have been fully discussed by the energy band theory. Experimental Section Materials. Azo compounds A and B were obtained from Zhejiang University. Their synthesis and characterizations have been reported in detail elsewhere.27 The thin solid films of the samples were directly prepared on indium tin oxide (ITO) electrode by a spin-coating technique, after the samples were separately dissolved in dimethylformamide (concentration 2 × 10-3 mol/L). The thickness of the films was about 10 µm, as determined by a spectrophotometric method.24 For doping exper-
10.1021/jp0002244 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/09/2000
8178 J. Phys. Chem. B, Vol. 104, No. 34, 2000
Tengfeng et al.
SCHEME 1: Structure Formulas of Azo Pigment Films A and B
iments, the azo pigment films were exposed to iodine vapor from solid iodine (AR grade) and ammonia from NH3‚H2O (AR grade), in the glass vessel at about 25 °C,16 respectively. Instruments. UV-vis absorption spectra of the azo pigment films were obtained on an UV-365 spectrometer. SPS were measured with a solid junction photovoltaic cell (ITO/azo pigments films/ITO) using light source-monochromator-lock-in detection technique. The principle and setup diagram of SPS and FISPS measurement have already been described in detail elsewhere.24,25 The measurement was performed under at the atmospheric pressure and ambient temperature (about 20 ( 2 °C). Results and Discussion A. Determination of Conduction Types of Films A and B. Figure 1 shows the UV-vis absorption spectra of films A and B. It is obvious that each sample gives two pronounced absorption bands in the range of 300-700 nm. The bands P1A at 300-330 nm and P1B at 300-450 nm are assigned to π-π* transition (P1), while the absorption bands P2A at 330-600 nm and P2B at 450-700 nm correspond to n-π* transition bands (P2).28 The SPS of films A and B are shown in Figure 2, a and b, respectively. Referred to the UV-vis absorption spectra, the surface photovoltaic (SPV) responses also show the π-π* transition bands and n-π* transition bands. The profile of action spectra resembles the UV-vis absorption spectra, except a steep drop below 320 nm due to the self-absorption of ITO electrodes. Additionally, the SPV response of P1A is opposite to that of P2A, while the SPV response of P1B and P2B in the same direction takes place. This indicates that the two azo pigments possess completely different transition characters. From the SPV response intensity of P1B and P2B shown in Figure 2b, it can be seen that the n-π* transition takes on higher photogenerated charge separation efficiency. FISPS in Figure 2 show that the changes in SPV responses of films A and B are completely different under the effect of an external electric field. For film A, the SPV response of P1A hardly changes with the external field. Nevertheless, that of P2A takes positive or negative value as a bias of -0.9 or +0.9 V is applied, and the corresponding intensity was greatly enhanced. On the contrary, the SPV responses of P1B and P2B not only are enhanced but also have a completely different behavior from that of P1A and P2A under the effect of the external field; that is, those of P1B and P2B are simultaneously positive (negative) as a bias of +0.9 V (-0.9 V) is applied. According to the FISPS principle,26 it can be known that the SPV response of electron transition related to located state energy level changes greatly with an external field, while that of electron transition related to nonlocated state energy level or intrinsic transition does so only slightly. Hence, the SPS
Figure 1. UV-vis absorption spectra of A film (curve a) and B film (curve b) with a thickness of 10 µm.
technique proves again that P1 (i.e., P1A and P1B) and P2 (i.e., P2A and P2B) correspond to the π-π* transition and n-π* band transition, respectively. The unusual behavior of P1B might be caused by the large substituent (i.e., naphthol derivative) which can reduce the conjugation extent of the azo parent. It is the conjugation that strengthens the nonlocated property of electrons. The change of P2 representing the conduction type of materials may be described by the following energy band modes. A signal detected by the SPS results from the separation of electron-hole pairs under a built-in field, after a semiconductor is illuminated. In other words, a change in surface potential (δVs ) V′s - V0s) or the number of net surface charges takes place, where V0s and V′s are respectively the surface potential heights before and after illumination. Their magnitudes depend on the number of the net charges, i.e., photogenerated carriers, accumulated on a material surface. In general, we can judge the conduction type of a semiconductor by its surface photovoltage sign (positive or negative). For a p-type semiconductor, its surface band bending is usually downward, when the photogenerated electrons move toward its surface and the photogenerated holes diffuse toward its bulk. This leads to a positive SPV response (δVs > 0) (see Figure 3a). On the contrary, the SPV response of n-type semiconductor is negative (δVs < 0) (see Figure 4a). If we apply a positive electric field (the illuminated surface is positive) vertically on the p-type semiconductor surface, whose direction coincides with the direction of the builtin field, the surface band bending increases downward, and thereby the separation efficiency of the photogeneration carriers is increased and the intensity of SPV response increases in original direction (see Figure 3b). If a negative electric field is applied, whose direction is reverse to that of the built-in field, the surface band bending decreases, and thereby the separation efficiency of the photogeneration carriers reduces and the intensity of SPV response is weakened, even in the reverse direction (see Figure 3c). In contrast to the p-type semiconductor, the SPV response intensity of a n-type semiconductor increases as a negative field is applied and reduces as a positive electric field is applied (see Figure 4b,c). Figure 2 shows that the direction of built-in field of film A coincides with that of the negative electric field, and that of film B is opposite to it. The direction of surface band bending formed between film A and the irradiated ITO electrode is upward, and that formed between film B and the irradiated ITO electrode is downward. Hence, film A takes on n-type character, whereas film B takes on p-type character. Yamashita et al.29 reported that the conduction type and Fermi level of organic semiconductors seem to be strongly influenced by different substituents. We attribute this to their different
Conduction Types of Azo Pigment Films
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8179
Figure 2. SPS and FISPS of A film (a) and B film (b) under the biases of 0 and (0.9 V.
Figure 3. Schematic representations of energy band modes of a p-type semiconductor with a Schottky-type barrier: (a) without a bias; (b) with a positive electric field; (c) with a negative electric field. δVs (δVs′, δVs′′), surface potential difference; V0s, the potential height before illumination; Vs′ (Vs′′, Vs′′′), the surface potential height after illumination; CB, conduction band; VB, valence band; EF, Fermi level.
conduction characteristics strongly influenced by their substituents. The two ends (electrophobic group naphthol) of the structure of compound A enhance the electronic cloud density of system of the entire molecule so that a negative shift of the Fermi level occurs. Therefore, film A has the features of a n-type semiconductor. Because of the strong electrophilic group (pnitroanilino) in compound B, the electronic cloud density of system of the molecules reduces, and its electron-filled level is lower in agglomerate state. That is, a positive shift of the Fermi level occurs. Thus, film B possesses the features of a p-type semiconductor. We also investigated optimal phase (OPSPV) of SPS response of the two films. OPSPV is defined to be the largest absolute value of SPV response at certain phase. Selection of OPSPV becomes rather important for the determination of conduction
Figure 4. Schematic representations of energy band modes of an n-type semiconductor with a Schottky-type barrier: (a) without a bias; (b) with a negative electric field, (c) with a positive electric field. δVs (δVs′, δVs′′), potential difference; V0s , potential height before illumination; Vs′ (Vs′′, Vs′′′) surface potential height after illumination; CB, conduction band; VB, valence band; EF, Fermi level.
type of a material when a lock-in amplifier is used for the SPS measurement. If we keep the other measurement conditions constant, the OPSPV is an eigenphysical quantum, which may give much microinformation on photogenerated charges. It is observed that the OPSPV of films A and B is 0° and 180°, respectively; that is, the photovoltage signs of films A and B are just opposite to each other. This means that the conduction types of films A and B are not identical. When the conduction of film A is of n type, that of film B is inevitably p type. This result is consistent with our previous discussion. B. Determination of Conduction Types of Films AI2 and BNH3. We suggest that the Fermi levels and conduction types of the azo pigments can also be controlled by intentional doping with known molecules. Here, we have tried controlling the
8180 J. Phys. Chem. B, Vol. 104, No. 34, 2000
Figure 5. Dependence of SPV response of film A at 540 nm on the time of I2 doping.
Tengfeng et al. region. The magnitude of this photovoltage may be as high as the value of the surface barrier; i.e., the former can readily exceed the Dember-type surface photovoltage by 2 orders of magnitude. In addition, a weak, partially ionized, donoracceptor complex (Dδ+‚‚‚Aδ-) in this case, in Nevin’s opinion,30 is thought to be formed in the ground state, and the activation energy of the dark conductivity is of the order of several tenths of an electronvolt. The photovoltaic response enhancement has been attributed to ionization of the Dδ+‚‚‚Aδ- complex under illumination, or to the dissociation of the excitons at the acceptor sites. We consider that the surface band bending formed between film A and the irradiated ITO electrode has changed due to the doping of I2. As mentioned above, the surface band bending is upward if film A is not exposed to I2 vapor, which has the character of a n-type semiconductor. For the first several minutes of I2 doping, I2 vapor was adsorbed and aggregated on the surface of film A. Since halogens as dopants have a strong oxidizing power, they should act as electron acceptors in film A. This implies that they may draw an electron from an azo pigment molecule in the ground state and, in turn, thermally liberate a hole to the valence band of the azo molecule, since halogens are known to be capable of forming conductive complexes with the azo pigments on surface.31 Thus, according to the Poisson equation32
d2φ F )2 κ dx 0 Figure 6. SPS and FISPS of AI2 film under the biases of 0 and (0.5 V (dotted line is SPS of A film).
conduction types of azo pigments by the adopting of halogens or ammonia, the dopants with strong oxidizing or reducing ability. Figure 5 shows the dependence of SPV response of film A on the time of I2 doping. It is interesting to note that the SPV response first goes up to 60 µV for first 2.5 min, then down to -100 µV at the 10th minute, finally keeps constant after 10th minute whose absolute value is 50 times as large as that of the original signal. To minimize the error in the following steps of the experiments, we take the film AI2 obtained by exposing film A to I2 vapor for 20 min as an example to investigate the conversion of pn conduction type before and after doping. SPS (Figure 6) shows that the SPV response of film AI2 is opposite to that of film A (the dotted line), meaning that the δVs of film AI2 opposes that of film A. The profiles of action spectra were nearly the same as that of film A, which shows that the I2 molecules doped do not damage the sample (film A), and cause a drastic shift of the Fermi level. Moreover, the OPSPV of film AI2 is 10°, which is essentially the same as that of film A. The SPV response of film AI2 changes in contrast to that of film A as an external electric field is applied (Figure 6). These suggest that the direction of the built-in field of film AI2 coincides with that of the positive electric field and opposes that of film A. Thus, film AI2 is supposed to take on the character of a p-type semiconductor. The reason for this is given below. When film A is exposed to I2 vapor for the first several minutes, a surface state, according to Gatos’ suggestion,20 is introduced onto the surface of the film. Introduction of the acceptor-like surface state onto the surface will cause a striking enhancement of the surface photovoltage in the region of intrinsic excitation. The excess photogenerated carriers suppress the surface barrier through a redistribution of the electrons and holes in the space charge
(1)
where F is the charge density (C/m3), κ is the dielectric constant, 0 is the permittivity of free space, and the relationship between Vs and NA, Ns can be deduced:
Vs ) e(ND - NA)W2/2κ0
(2)
Vs ) eNs2/2κ0(ND - NA)
(3)
where Vs is the built-in potential at the Schottky barrier junction, e is the electronic charge, W is the depletion layer width, NA is the concentration of ionized acceptors, ND is the concentration of donors, and Ns is the density of surface charge. According to eq 3, since ND - NA is an approximate constant, Vs enhances with an increase in the Ns; i.e., the surface band bending increases. Hence, the separation efficiency of the photogeneration carriers has been improved largely, which make SPV response increase largely. With the prolongation of the time of I2 doping, the SPV response gradually decreases till its sign begins to go in the reverse direction. It suggests that film A of n-type has been altered to film AI2 of p-type. I2 molecules diffuse into the azo pigment film A and draw the electrons from the azo pigment molecules A in the ground state. The positive Fermi level of film A shift takes place and reaches the nearby valence band. According to eq 2, ND is an approximate constant, and the Vs decreases in the reverse direction with an increase in NA; i.e., the direction of surface band bending of the system AI2 gets reversed. As a result, the carriers photogenerated near the photoactive ITO/A interface move in the opposite direction. The SPV response of AI2 system is opposite to that of film A. The absolute value of SPV response intensity is approximately 50 times as high as that of the original signal. It can be seen that the absolute value Vs of film AI2 is much larger than that of film A. The acceptor-like surface state and a weak, partially ionized, donor-acceptor complex (Dδ+‚‚‚Aδ-) are thought to be formed in film A. The SPV response of film AI2 is negatively
Conduction Types of Azo Pigment Films
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8181 naphthyl]}azodiphenyl (B) as well as A doped with I2 (AI2) and B doped with NH3 (BNH3) are characterized. The SPV responses of the azo pigments under different external fields show that film A has the character of n-type conduction, and film B the character of p-type conduction. The substituents at the two sites of the different azo molecules, i.e., the electrophobic groups on the molecules A (naphthol) and electrophilic groups on the molecule B (p-nitroanilino), cause the difference in the conduction type. The conduction type can also be controlled by doping with some specific molecules. Doping with iodine leads to change of the conduction type of film A from n-type to p-type, and the doping with NH3 causes a change of the conduction type of film B from p-type to n-type. It appears that the control of pn conduction type by doping is applicable to other azo pigments.
Figure 7. Dependence of SPV response of film B at 540 nm on the time of NH3 doping.
Acknowledgment. This research was supported by the National Science Foundation of China and Jilin Province Science Committee. References and Notes
Figure 8. SPS and FISPS of BNH3 film under the biases of 0 and (0.5 V (dotted line is SPS of B film).
increased under a negative electric field, and is positively increased under a positive electric field. These results clearly show that the film A of n-type changes to film AI2 p-type. Figure 7 shows the effect of NH3 gas on the SPV response of film B. Only several minutes after film B was exposed to NH3 gas, its SPV response is largely reduced, even reversed and then keeps constant after 10th minute. Figure 8 shows the SPS and the FISPS of film BNH3. The negative SPV response of film BNH3 indicates that its δVs is opposite to that of film B. It is observed that the profiles of action spectra are essentially the same as that of film B, and the OPSPV of film BNH3 is 170°, which is nearly the same as that of film B. But the photovoltage property of film BNH3 under the effect of the electric field is opposite to that of film B. These show that the direction of built-in field of film BNH3 coincides with the direction of the negative electric field and opposes that of film B, which shows that film BNH3 takes on the character of a n-type semiconductor. Because NH3 gas as a dopant has a strong reducing power, it is expected to act as a donor in film B. The negative shift of the Fermi level of the latter result from the NH3 doping. The direction of the surface band bending formed after the contact of film BNH3 with the irradiated ITO electrode comes to be reversed. Therefore, the conduction type of film B changes from p-type to n-type by the doping of NH3. Conclusion By SPS and FISPS techniques, the conduction types of azo pigment films composed of pure 4,4′-2-[N-(β-hydroxyl)naphthyl]azodiphenyl (A) and 4,4′-2-{N-[β-hydroxyl-γ-(p-nitroanilino)-
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