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Relationship between structure and optoelectrical properties of organic–inorganic hybrid materials containing fullerene derivatives
Synthetic Metals 159 (2009) 776–779
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Relationship between structure and optoelectrical properties of organic–inorganic hybrid materials containing fullerene derivatives Y. Kawabata, M. Yoshizawa-Fujita, Y. Takeoka, M. Rikukawa ∗ Department of Materials and Life Sciences, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
a r t i c l e
i n f o
Article history: Received 26 August 2008 Received in revised form 28 December 2008 Accepted 5 January 2009 Available online 4 February 2009 Keywords: Fullerene derivative Perovskite Optical property
a b s t r a c t Novel lead iodide-based layered perovskite compounds, which contain fullerene derivatives, N-methyl2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (AmPF) and N-(n-dodecyl)-2-(4-ammoniumphenyl)fulleropyrrolidine iodide (C12AmPF), in their organic layers were fabricated as thin solid films by spin-coating. The XRD profiles showed that (AmPF)PbI4 molecules were arranged in a closer-packing form, compared with (C12AmPF)PbI4 . The photoluminescence spectra of thin films suggested the presence of energy transfer between C60 moiety and lead(II) iodide layers, which led to the disappearance of fluorescence peak at 517 nm and the appearance of a new fluorescence peak at 780 nm. (AmPF)PbI4 and (C12AmPF)PbI4 films exhibited photoconductivity when ultraviolet light was irradiated, and the photocurrent values with applying 1.0 V bias voltage were 1.77 A and 1.48 × 10−2 A, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic–inorganic perovskites have recently attracted great attention because these compounds may combine useful properties of both inorganic and organic moieties as molecular scale crystalline composites. In addition, hybrid perovskites have the great advantage of being processed into thin films by room-temperature techniques such as spin-coating. By changing molecular structures of the inorganic and organic components, a variety of readily processable hybrid perovskites have been fabricated, some of which have been found to exhibit interesting physical properties such as tunable exciton absorption, and efficient light emission [1]. Many of the interesting properties of hybrid perovskites arise predominantly from the inorganic component. In these cases, the organic layers play a secondary role by making the two-dimensional structure and by providing a lower dielectric constant layer around the inorganic sheets. A recent trend in this field of hybrid materials is to incorporate molecules with lowered HOMO–LUMO gap such as extended -conjugated systems [2,3], which can combine semiconducting properties, luminescent, or nonlinear optical properties, into the organic layers [4–6]. Incorporation of fullerene derivatives as a -conjugated system to the organic–inorganic perovskites is also expected to provide interesting optical and electrical properties because of their three-dimensional -electron conjugation [6]. In this study, novel
∗ Corresponding author. E-mail address: [email protected] (M. Rikukawa). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.01.001
layered perovskites, which contain fullerene derivatives, N-methyl2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (AmPF-I2 ) and N-(n-dodecyl)-2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (C12AmPF-I2 ) (Fig. 1), in their organic layers were fabricated as thin solid films by a spin-coating method. We also prepared layered perovskite compounds consisting of dodecylammonium iodide (C12-I) for comparison. In order to explore the optical and electrical properties of these thin solid films, the fullerene based perovskites were characterized by X-ray diffraction, UV–vis absorption, and photoluminescence measurements. Photocurrent measurements with the thin films were also carried out. 2. Experimental 2.1. Synthesis of fullerene derivatives AmPF-I2 (1) was prepared according to the reference [7]. C12AmPF-I2 (2) was prepared by the following procedure. 2-(4Nitrophenyl)-fulleropyrrolidine (3) was prepared by Prato reaction. N-(n-dodecyl)-2-(4-nitrophenyl)-fulleropyrrolidine (4) was prepared from 3 and dodecanal [8]. To a suspension of 3 (100 mg) in CH2 Cl2 (60 ml) dodecanal (2.28 mmol), triacetoxyborohydride (2.28 mmol), and acetic acid wear added, and the mixture was stirred for 24 h at room temperature. The remaining borohydride was eliminated by an addition of saturated sodium hydrogen carbonate aqueous solution. The solution was evaporated under reduced pressure. The compound 4 was purified over a silica gel column chromatography using toluene/petroleum ether (1/4, v/v). A brown powdery solid was obtained (84% yield) (Scheme 1). 1 H
Y. Kawabata et al. / Synthetic Metals 159 (2009) 776–779
777
Fig. 1. Structures of AmPF-I2 and C12AmPF-I2 .
NMR(CDCl3 ) ı: 8.30 (d, 2H); 8.03 (br s, 2H); 5.18 (s, 1H); 5.13 (d, 1H); 4.19 (d, 1H); 3.13(m, 1H); 2.60 (m, 1H); 1.98 (m, 1H); 1.90 (m, 1H); 1.29 (m, 18H); 0.89 (t, 3H) ppm. TOF-MS: calc. 1052.8, obs. 1053.1.N(n-dodecyl)-2-(4-aminophenyl)-fulleropyrrolidine (5) was synthesized in chloroform by simple reduction of 4 with powdered Sn and HCl. The compound 5 was purified over a silica gel column chromatography using toluene (53% yield). 1 H NMR(CDCl3 ) ı: 7.56 (br s, 2H); 6.71 (d, 2H); 5.07 (d, 1H); 4.95 (s, 1H); 4.08 (d, 1H); 3.68(br s, 1H); 3.22 (m, 1H); 2.50 (m, 1H);1.88 (m, 2H); 1.29 (m, 18H); 0.89 (t, 3H). TOF-MS: calc. 1022.8, obs. 1023.1. C12AmPF-I2 (2) was synthesized from 5 and equimolar amount of hydroiodic acid (57% aq.) in chloroform for 24 h under nitrogen atmosphere. After the reaction, the compound 2 was filtered and dried for 48 h under vacuum at 60 ◦ C. FAB-MS(+): calc. 1024.8, obs. 1025.3.
2.2. Fabrication and characterization of perovskite films Perovskite films, (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12)2 PbI4 , were prepared by mixing stoichiometric amounts of corresponding ammonium derivatives (AmPF-I2 , C12AmPF-I2 , and dodecylammonium iodide, CH3 (CH2 )11 NH3 I, abbreviated as C12-I) and lead iodide in N,N-dimethylformamide (DMF) and spin-coating on hydrophilic quartz substrates. X-ray diffraction patterns of the obtained perovskite thin films were collected on a Rigaku RINT 2100 diffractometer with Ni-filtered Cu K␣ radiation at 40 kV and 30 mA. UV–vis absorption spectra of the perovskite films were measured using a SHIMADZU UV-3100PC. Photoluminescence spectra were taken with a HITACHI F-4500 spectrometer with a monochromated Xe lamp as an excitation source at an incident angle of 45◦ . The spin-coated films were excited at 520 nm for (AmPF)PbI4 and (C12AmPF)PbI4 , and at 494 nm for (C12)2 PbI4 . The excitation wavelengths were selected to show the maximum photoluminescence intensity. All optical measurements were carried out at room temperature.
Fig. 2. XRD profiles of the spin-coated films.
3. Results and discussion 3.1. Characterization of perovskite films The XRD patterns showed that all the spin-coated films form a layered structure, as shown in Fig. 2. The interlayer d-spacing values of (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12)2 PbI4 were calculated to be 25.2 Å, 31.8 Å, and 23.9 Å, respectively. These results suggest that (AmPF)PbI4 molecules are arranged in a closer packing form than (C12AmPF)PbI4 . In addition, the XRD profiles exhibited that (AmPF)PbI4 has higher crystallinity than (C12AmPF)PbI4 . Although higher solubility of C12AmPF with common organic solvents is one of the advantages, the results of X-ray diffraction indicate that the long alkyl chains of C12AmPF impede the self-organization of perovskites. 3.2. Optical properties of perovskite films The UV–vis absorption and photoluminescence spectra of the perovskite films are shown in Fig. 3. For the conventional PbI2 based layered perovskite compound, (C12)2 PbI2 , a quantum well effect
2.3. Optoelectrical measurements Photocurrent measurements of (AmPF)PbI4 and (C12AmPF)PbI4 films on Au electrodes were carried out by exposing the films to a mercury lamp with an intensity of 2700 mW at 546 nm (high power).
Scheme 1. Synthesis of compound 4.
Fig. 3. UV–vis absorption (solid line) and photoluminescence (dashed line) spectra of the spin-coated films excited at 520 nm for (AmPF)PbI4 and (C12AmPF)PbI4 , and at 494 nm for (C12)2 PbI4 .
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Fig. 4. Schematic energy diagram for the (AmPF)PbI4 and (C12AmPF)PbI4 spincoated films.
was observed. Due to the exciton confined in the inorganic layers, the layered perovskite exhibited sharp peaks in the absorption and photoluminescence spectra at room temperature. Similarly, (C12)2 PbI4 showed a maximum absorption peak at 511 nm and strong exciton emission peak at 517 nm due to the binding exciton. However, similar absorption and photoluminescence peaks were not observed in the spectra of (AmPF)PbI4 and (C12AmPF)PbI4 thin films. These results revealed that stable excitons were not formed in these fullerene based perovskite films. This is probably due to the lower HOMO–LUMO gaps of AmPF and C12AmPF than that of alkylammonium like C12-I. A new photoluminescence peak of (AmPF)PbI4 and (C12AmPF) PbI4 were observed at 780 nm. The PL peak might be derived from the energy transfer from inorganic semiconducting layers to AmPF or C12AmPF organic semiconductor regions. Several studies on perovskites containing -conjugated molecules in organic layer report similar energy transfers between organic and inorganic layers of perovskite [9]. The path of energy flow in the fullerene based perovskite is described in Fig. 4. EX , S1 , and T1 represent exciton state in inorganic layers, single and triplet excited state of C60 , respectively. The fluorescence sharp peaks at 780 nm correspond approximately to energy gap between the triplet excited state and ground state of fullerene. It is proposed the excitons formed in the inorganic layers were trapped in the -conjugated networks of C60 organic layers. The fluorescence quantum yield of C60 is known to be very low, which is due to not only the intersystem crossing from the singlet excited state to the triplet excited state, but also the forbidden electronic transitions between the singlet excited states and ground state. Interestingly, it is found that the photoluminescence of fullerene molecules is enhanced in fullerene-containing perovskite compounds. The higher crystallinity fullerene based perovskite, (AmPF)PbI4 , especially exhibited stronger emission at 780 nm than (C12AmPF)PbI4 thin films. Detailed studies on the lifetime of fluorescence are in progress. 3.3. Optoelectrical properties of perovskite films Photocurrent measurements of (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12)PbI4 thin films on Au electrodes were carried out. As shown in Figs. 5–6, (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12 )2 PbI4 films on Au electrodes exhibited photoconductivity when ultraviolet light was irradiated, and the photocurrents with 1.0 V bias voltage were 1.77 A, 1.48 × 10−2 A, and 1.19 × 10−2 A, respectively. In the case of (C12)2 PbI4 , the possible origin of photocurrent was only the lead(II) iodide sheets. Although (C12AmPF)PbI4 thin films have much lower crystallinity than (C12)2 PbI4 , the photocurrent values of (C12AmPF)PbI4 films were similar to that of (C12)2 PbI4 . On the other hand, (AmPF)PbI4
Fig. 5. I–V curves of (C12)2 PbI4 and (C12AmPF)PbI4 spin-coated films on Au electrodes.
Fig. 6. I–V curves of (AmPF)PbI4 spin-coated film on an Au electrode.
showed two orders higher photocurrents than (C12AmPF)PbI4 and (C12)PbI4 . These results clearly suggest that the fullerene organic layer contributes to photocurrent generation. Since there is little difference in the fullerene structure between (AmPF)PbI4 and (C12AmPF)PbI4 except the lengths of alkyl side chains, the high photocurrent of (AmPF)PbI4 is probably due to the high self-ordered structure, which was demonstrated by XRD measurements. Consequently, not only the semiconducting organic layers but also the strong self-organized nature is found to be important to activate the optoelectrical properties of these materials. 4. Conclusion Organic–inorganic hybrid perovskites, (AmPF)PbI4 and (C12AmPF)PbI4 , were fabricated by spin-coating. The XRD profiles showed that (AmPF)PbI4 molecules were arranged in a closer packing form than (C12AmPF)PbI4 . UV–vis absorption and photoluminescence spectra exhibited an energy transfer from the inorganic layers to the fullerene layers. The optoelectrical investigation of these compounds on Au electrodes was carried
Y. Kawabata et al. / Synthetic Metals 159 (2009) 776–779
out. The highest photocurrent value was obtained for (AmPF)PbI4 thin films when ultraviolet light was irradiated. It suggested that the layered perovskite compounds that contain fullerene derivatives with higher crystallinity provide high optoelectrical activity. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas “Super-Hierarchical Structures” (No.446) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
779
References [1] Y. Takeoka, K. Asai, M. Rikukawa, K. Sanui, Bull. Chem. Soc. Jpn. 79 (10) (2006) 1607–1613. [2] D.B. Mitzi, K. Chondroudis, C.R. Kagan, Inorg. Chem. 38 (1999) 6246–6256. [3] X. Haipeng, S. Jingzhi, Q.J. Anjun, Phys. Chem. 110 (2006) 21701–21709. [4] (a) C.D. Dimitrakopolous, P. Malenfant, Adv. Mater. 14 (2002) 99–117; (b) H.E. Katz, Z. Bao, S. Gilat, Acc. Chem. Res. 34 (2001) 359–369. [5] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 420–428. [6] Y. Takeoka, K. Asai, M. Rikukawa, K. Sanui, Chem. Commun. 24 (2001) 2592–2593. [7] K. Kikuchi, Y. Takeoka, M. Rikukawa, K. Sanui, Colloid Surf. A 199 (2005) 257–258. [8] S. Xiao, Y. Li, H. Liu, H. Li, Tetrahedron Lett. 45 (2004) 3975–3978. [9] X.H. Zhu, N. Mercier, P. Fre‘re, P. Blanchard, J. Roncali, M. Allain, C. Pasquier, A. Riou, Inorg. Chem. 42 (2003) 5330–5339.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Relationship between structure and optoelectrical properties of organic–inorganic hybrid materials containing fullerene derivatives Y. Kawabata, M. Yoshizawa-Fujita, Y. Takeoka, M. Rikukawa ∗ Department of Materials and Life Sciences, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
a r t i c l e
i n f o
Article history: Received 26 August 2008 Received in revised form 28 December 2008 Accepted 5 January 2009 Available online 4 February 2009 Keywords: Fullerene derivative Perovskite Optical property
a b s t r a c t Novel lead iodide-based layered perovskite compounds, which contain fullerene derivatives, N-methyl2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (AmPF) and N-(n-dodecyl)-2-(4-ammoniumphenyl)fulleropyrrolidine iodide (C12AmPF), in their organic layers were fabricated as thin solid films by spin-coating. The XRD profiles showed that (AmPF)PbI4 molecules were arranged in a closer-packing form, compared with (C12AmPF)PbI4 . The photoluminescence spectra of thin films suggested the presence of energy transfer between C60 moiety and lead(II) iodide layers, which led to the disappearance of fluorescence peak at 517 nm and the appearance of a new fluorescence peak at 780 nm. (AmPF)PbI4 and (C12AmPF)PbI4 films exhibited photoconductivity when ultraviolet light was irradiated, and the photocurrent values with applying 1.0 V bias voltage were 1.77 A and 1.48 × 10−2 A, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic–inorganic perovskites have recently attracted great attention because these compounds may combine useful properties of both inorganic and organic moieties as molecular scale crystalline composites. In addition, hybrid perovskites have the great advantage of being processed into thin films by room-temperature techniques such as spin-coating. By changing molecular structures of the inorganic and organic components, a variety of readily processable hybrid perovskites have been fabricated, some of which have been found to exhibit interesting physical properties such as tunable exciton absorption, and efficient light emission [1]. Many of the interesting properties of hybrid perovskites arise predominantly from the inorganic component. In these cases, the organic layers play a secondary role by making the two-dimensional structure and by providing a lower dielectric constant layer around the inorganic sheets. A recent trend in this field of hybrid materials is to incorporate molecules with lowered HOMO–LUMO gap such as extended -conjugated systems [2,3], which can combine semiconducting properties, luminescent, or nonlinear optical properties, into the organic layers [4–6]. Incorporation of fullerene derivatives as a -conjugated system to the organic–inorganic perovskites is also expected to provide interesting optical and electrical properties because of their three-dimensional -electron conjugation [6]. In this study, novel
∗ Corresponding author. E-mail address: [email protected] (M. Rikukawa). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.01.001
layered perovskites, which contain fullerene derivatives, N-methyl2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (AmPF-I2 ) and N-(n-dodecyl)-2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (C12AmPF-I2 ) (Fig. 1), in their organic layers were fabricated as thin solid films by a spin-coating method. We also prepared layered perovskite compounds consisting of dodecylammonium iodide (C12-I) for comparison. In order to explore the optical and electrical properties of these thin solid films, the fullerene based perovskites were characterized by X-ray diffraction, UV–vis absorption, and photoluminescence measurements. Photocurrent measurements with the thin films were also carried out. 2. Experimental 2.1. Synthesis of fullerene derivatives AmPF-I2 (1) was prepared according to the reference [7]. C12AmPF-I2 (2) was prepared by the following procedure. 2-(4Nitrophenyl)-fulleropyrrolidine (3) was prepared by Prato reaction. N-(n-dodecyl)-2-(4-nitrophenyl)-fulleropyrrolidine (4) was prepared from 3 and dodecanal [8]. To a suspension of 3 (100 mg) in CH2 Cl2 (60 ml) dodecanal (2.28 mmol), triacetoxyborohydride (2.28 mmol), and acetic acid wear added, and the mixture was stirred for 24 h at room temperature. The remaining borohydride was eliminated by an addition of saturated sodium hydrogen carbonate aqueous solution. The solution was evaporated under reduced pressure. The compound 4 was purified over a silica gel column chromatography using toluene/petroleum ether (1/4, v/v). A brown powdery solid was obtained (84% yield) (Scheme 1). 1 H
Y. Kawabata et al. / Synthetic Metals 159 (2009) 776–779
777
Fig. 1. Structures of AmPF-I2 and C12AmPF-I2 .
NMR(CDCl3 ) ı: 8.30 (d, 2H); 8.03 (br s, 2H); 5.18 (s, 1H); 5.13 (d, 1H); 4.19 (d, 1H); 3.13(m, 1H); 2.60 (m, 1H); 1.98 (m, 1H); 1.90 (m, 1H); 1.29 (m, 18H); 0.89 (t, 3H) ppm. TOF-MS: calc. 1052.8, obs. 1053.1.N(n-dodecyl)-2-(4-aminophenyl)-fulleropyrrolidine (5) was synthesized in chloroform by simple reduction of 4 with powdered Sn and HCl. The compound 5 was purified over a silica gel column chromatography using toluene (53% yield). 1 H NMR(CDCl3 ) ı: 7.56 (br s, 2H); 6.71 (d, 2H); 5.07 (d, 1H); 4.95 (s, 1H); 4.08 (d, 1H); 3.68(br s, 1H); 3.22 (m, 1H); 2.50 (m, 1H);1.88 (m, 2H); 1.29 (m, 18H); 0.89 (t, 3H). TOF-MS: calc. 1022.8, obs. 1023.1. C12AmPF-I2 (2) was synthesized from 5 and equimolar amount of hydroiodic acid (57% aq.) in chloroform for 24 h under nitrogen atmosphere. After the reaction, the compound 2 was filtered and dried for 48 h under vacuum at 60 ◦ C. FAB-MS(+): calc. 1024.8, obs. 1025.3.
2.2. Fabrication and characterization of perovskite films Perovskite films, (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12)2 PbI4 , were prepared by mixing stoichiometric amounts of corresponding ammonium derivatives (AmPF-I2 , C12AmPF-I2 , and dodecylammonium iodide, CH3 (CH2 )11 NH3 I, abbreviated as C12-I) and lead iodide in N,N-dimethylformamide (DMF) and spin-coating on hydrophilic quartz substrates. X-ray diffraction patterns of the obtained perovskite thin films were collected on a Rigaku RINT 2100 diffractometer with Ni-filtered Cu K␣ radiation at 40 kV and 30 mA. UV–vis absorption spectra of the perovskite films were measured using a SHIMADZU UV-3100PC. Photoluminescence spectra were taken with a HITACHI F-4500 spectrometer with a monochromated Xe lamp as an excitation source at an incident angle of 45◦ . The spin-coated films were excited at 520 nm for (AmPF)PbI4 and (C12AmPF)PbI4 , and at 494 nm for (C12)2 PbI4 . The excitation wavelengths were selected to show the maximum photoluminescence intensity. All optical measurements were carried out at room temperature.
Fig. 2. XRD profiles of the spin-coated films.
3. Results and discussion 3.1. Characterization of perovskite films The XRD patterns showed that all the spin-coated films form a layered structure, as shown in Fig. 2. The interlayer d-spacing values of (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12)2 PbI4 were calculated to be 25.2 Å, 31.8 Å, and 23.9 Å, respectively. These results suggest that (AmPF)PbI4 molecules are arranged in a closer packing form than (C12AmPF)PbI4 . In addition, the XRD profiles exhibited that (AmPF)PbI4 has higher crystallinity than (C12AmPF)PbI4 . Although higher solubility of C12AmPF with common organic solvents is one of the advantages, the results of X-ray diffraction indicate that the long alkyl chains of C12AmPF impede the self-organization of perovskites. 3.2. Optical properties of perovskite films The UV–vis absorption and photoluminescence spectra of the perovskite films are shown in Fig. 3. For the conventional PbI2 based layered perovskite compound, (C12)2 PbI2 , a quantum well effect
2.3. Optoelectrical measurements Photocurrent measurements of (AmPF)PbI4 and (C12AmPF)PbI4 films on Au electrodes were carried out by exposing the films to a mercury lamp with an intensity of 2700 mW at 546 nm (high power).
Scheme 1. Synthesis of compound 4.
Fig. 3. UV–vis absorption (solid line) and photoluminescence (dashed line) spectra of the spin-coated films excited at 520 nm for (AmPF)PbI4 and (C12AmPF)PbI4 , and at 494 nm for (C12)2 PbI4 .
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Fig. 4. Schematic energy diagram for the (AmPF)PbI4 and (C12AmPF)PbI4 spincoated films.
was observed. Due to the exciton confined in the inorganic layers, the layered perovskite exhibited sharp peaks in the absorption and photoluminescence spectra at room temperature. Similarly, (C12)2 PbI4 showed a maximum absorption peak at 511 nm and strong exciton emission peak at 517 nm due to the binding exciton. However, similar absorption and photoluminescence peaks were not observed in the spectra of (AmPF)PbI4 and (C12AmPF)PbI4 thin films. These results revealed that stable excitons were not formed in these fullerene based perovskite films. This is probably due to the lower HOMO–LUMO gaps of AmPF and C12AmPF than that of alkylammonium like C12-I. A new photoluminescence peak of (AmPF)PbI4 and (C12AmPF) PbI4 were observed at 780 nm. The PL peak might be derived from the energy transfer from inorganic semiconducting layers to AmPF or C12AmPF organic semiconductor regions. Several studies on perovskites containing -conjugated molecules in organic layer report similar energy transfers between organic and inorganic layers of perovskite [9]. The path of energy flow in the fullerene based perovskite is described in Fig. 4. EX , S1 , and T1 represent exciton state in inorganic layers, single and triplet excited state of C60 , respectively. The fluorescence sharp peaks at 780 nm correspond approximately to energy gap between the triplet excited state and ground state of fullerene. It is proposed the excitons formed in the inorganic layers were trapped in the -conjugated networks of C60 organic layers. The fluorescence quantum yield of C60 is known to be very low, which is due to not only the intersystem crossing from the singlet excited state to the triplet excited state, but also the forbidden electronic transitions between the singlet excited states and ground state. Interestingly, it is found that the photoluminescence of fullerene molecules is enhanced in fullerene-containing perovskite compounds. The higher crystallinity fullerene based perovskite, (AmPF)PbI4 , especially exhibited stronger emission at 780 nm than (C12AmPF)PbI4 thin films. Detailed studies on the lifetime of fluorescence are in progress. 3.3. Optoelectrical properties of perovskite films Photocurrent measurements of (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12)PbI4 thin films on Au electrodes were carried out. As shown in Figs. 5–6, (AmPF)PbI4 , (C12AmPF)PbI4 , and (C12 )2 PbI4 films on Au electrodes exhibited photoconductivity when ultraviolet light was irradiated, and the photocurrents with 1.0 V bias voltage were 1.77 A, 1.48 × 10−2 A, and 1.19 × 10−2 A, respectively. In the case of (C12)2 PbI4 , the possible origin of photocurrent was only the lead(II) iodide sheets. Although (C12AmPF)PbI4 thin films have much lower crystallinity than (C12)2 PbI4 , the photocurrent values of (C12AmPF)PbI4 films were similar to that of (C12)2 PbI4 . On the other hand, (AmPF)PbI4
Fig. 5. I–V curves of (C12)2 PbI4 and (C12AmPF)PbI4 spin-coated films on Au electrodes.
Fig. 6. I–V curves of (AmPF)PbI4 spin-coated film on an Au electrode.
showed two orders higher photocurrents than (C12AmPF)PbI4 and (C12)PbI4 . These results clearly suggest that the fullerene organic layer contributes to photocurrent generation. Since there is little difference in the fullerene structure between (AmPF)PbI4 and (C12AmPF)PbI4 except the lengths of alkyl side chains, the high photocurrent of (AmPF)PbI4 is probably due to the high self-ordered structure, which was demonstrated by XRD measurements. Consequently, not only the semiconducting organic layers but also the strong self-organized nature is found to be important to activate the optoelectrical properties of these materials. 4. Conclusion Organic–inorganic hybrid perovskites, (AmPF)PbI4 and (C12AmPF)PbI4 , were fabricated by spin-coating. The XRD profiles showed that (AmPF)PbI4 molecules were arranged in a closer packing form than (C12AmPF)PbI4 . UV–vis absorption and photoluminescence spectra exhibited an energy transfer from the inorganic layers to the fullerene layers. The optoelectrical investigation of these compounds on Au electrodes was carried
Y. Kawabata et al. / Synthetic Metals 159 (2009) 776–779
out. The highest photocurrent value was obtained for (AmPF)PbI4 thin films when ultraviolet light was irradiated. It suggested that the layered perovskite compounds that contain fullerene derivatives with higher crystallinity provide high optoelectrical activity. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas “Super-Hierarchical Structures” (No.446) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
779
References [1] Y. Takeoka, K. Asai, M. Rikukawa, K. Sanui, Bull. Chem. Soc. Jpn. 79 (10) (2006) 1607–1613. [2] D.B. Mitzi, K. Chondroudis, C.R. Kagan, Inorg. Chem. 38 (1999) 6246–6256. [3] X. Haipeng, S. Jingzhi, Q.J. Anjun, Phys. Chem. 110 (2006) 21701–21709. [4] (a) C.D. Dimitrakopolous, P. Malenfant, Adv. Mater. 14 (2002) 99–117; (b) H.E. Katz, Z. Bao, S. Gilat, Acc. Chem. Res. 34 (2001) 359–369. [5] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 420–428. [6] Y. Takeoka, K. Asai, M. Rikukawa, K. Sanui, Chem. Commun. 24 (2001) 2592–2593. [7] K. Kikuchi, Y. Takeoka, M. Rikukawa, K. Sanui, Colloid Surf. A 199 (2005) 257–258. [8] S. Xiao, Y. Li, H. Liu, H. Li, Tetrahedron Lett. 45 (2004) 3975–3978. [9] X.H. Zhu, N. Mercier, P. Fre‘re, P. Blanchard, J. Roncali, M. Allain, C. Pasquier, A. Riou, Inorg. Chem. 42 (2003) 5330–5339.