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The role of TiO2 in the air-stable hybrid organic–inorganic light-emitting diodes
Synthetic Metals 159 (2009) 2312–2314
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
The role of TiO2 in the air-stable hybrid organic–inorganic light-emitting diodes Katsuyuki Morii ∗ Department of Chemistry, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan
a r t i c l e
i n f o
Article history: Received 28 July 2008 Received in revised form 27 September 2009 Accepted 30 September 2009 Available online 31 October 2009
a b s t r a c t The role of the TiO2 layer in hybrid organic–inorganic light-emitting diodes (HOILEDs) was discussed. A TiO2 layer was fabricated using a process that formed an “oxygen-rich” and “porous” TiO2 layer in an HOILED. The importance of the surface state on the TiO2 and the hole injection scheme in the HOILED was confirmed. All the data support the idea that HOILEDs are strongly hole-dominated LEDs, and that the TiO2 in HOILEDs plays a supporting role. The TiO2 seems to act as a hole-blocking layer. © 2009 Elsevier B.V. All rights reserved.
Keywords: OLED Metal oxide Hybrid Titanium oxide
Due to the impetus to develop organic electronic devices, the interface between organic and inorganic materials has been the focus of much research. The first target of this research is the interface of a metal in contact with the top layer of an organic material [1,2], which plays an important role in electronic devices. Many experiments have resulted in improved organic electronic devices. Recently, different types of interface between organic and inorganic materials have been investigated. Some hybrid electronic devices with an interface between organic and metal oxide materials, for example, field-effect transistors [3], solar cells [4–6], and lightemitting diodes [7–11], have been reported that have excellent physical properties due to their use of metal oxides. We have already published results on a hybrid organic– inorganic light-emitting diode (HOILED), in which an emissive conductive polymer (poly(dioctylfluorene-alt-benzothiadiazole, F8BT) is sandwiched between metal oxide layers (titanium oxide, TiO2 , and molybdenum oxide, MoO3 ) [7]. It has been reported that the key issue for HOILEDs is the interface of the MoO3 , which is located on the top of the F8BT layer, because it is from this layer that many hole carriers are injected into the F8BT layer [12]. It is suggested that HOILEDs are hole-dominated light-emitting diodes. It has been suggested that hole carriers accumulate at the interface between a TiO2 layer and an F8BT layer, and electrons can be extracted by a large number of injected holes. Moreover, the EL characteristics have been markedly improved recently by modify-
∗ Tel.: +81 92 642 25 71; fax: +81 92 642 25 71. E-mail address: [email protected]. 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.09.036
ing the interface at the TiO2 layer using a layer consisting of cesium compounds, to a level that is almost the same as the top-rated organic light-emitting diode fabricated using F8BT [11]. Unfortunately, the operating mechanism of HOILEDs is still not fully understood. In particular, a better understanding of molecular-level phenomena at both interfaces between the metal oxides and the organic materials is required. For example, the characteristics of the interface at the TiO2 layer, and the role of TiO2 are important issues. Here, we, first of all, discuss the EL characteristics of HOILEDs with three different TiO2 layers, and discuss the role of a thin TiO2 layer. According to previous reports that have discussed hybrid organic electronic devices with a TiO2 layer, there are two key issues that need to be addressed: the “doping level”, dependent on the density of oxygen vacancies, and the “size of interface area”. The “doping level” is a difficult and complicated factor. From the viewpoint of the energy diagram, the Fermi level and the surface state will change. Furthermore, it also changes with the chemical structure of the surface. Bolink et al. [9] have already reported that the replacement TiO2 with zinc oxide leads to an improvement in the EL characteristics. This result shows that the physical properties of the metal oxide layer on the cathode side are effective in obtaining a high efficiency. Fortunately, the electronic properties of the metal oxide layer can be easily modified by the fabrication process. Two different doping levels for TiO2 were evaluated in this paper. The “size of the interface area” is important for organic solar cells, and Haque et al. [8] have reported that the effect of the size of the interface area is also a key issue in hybrid LEDs. Here, we have evaluated the effect of the size of the interface area in a second, different TiO2 layer has been evaluated.
K. Morii / Synthetic Metals 159 (2009) 2312–2314
2313
Fig. 1. Schematic structures of HOILEDs with some different TiO2 layers.
We evaluated and controlled the “doping level” by fabricating different compact TiO2 layers, which were employed as a holeblocking layer in dye-synthesized solar cells (DSSCs), through the choice of carrier gas during the deposition procedure. A commonly used carrier gas is air. In this work, this compact TiO2 layers were fabricated using spray pyrolysis in either air or oxygen. Oxygen was employed as the carrier gas to form an “oxygen-rich” compact TiO2 layer. [13] To investigate the “size of interface area”, HOILEDs with porous TiO2 layers were fabricated using a wellestablished deposition route for DSSCs [14]. Only a porous TiO2 layer was employed in this work to study the size of the interface area, and our HOILED samples had no compact TiO2 layer. The fabrication process reported in [7] was used, except for the fabrication of the TiO2 layer. These device structures are shown in Fig. 1. In addition, the three HOILEDs samples were fabricated using the same batch fabrication scheme. Therefore, the other layers were the same completely, except for the TiO2 layer. The initial EL characteristics (current density–voltage, J–V, and brightness–voltage, L–V) for two HOILED samples with different doping levels in the TiO2 layer are shown in Fig. 2. It can be seen that there was no marked difference, current turn-on voltage particularly, between these JV curves. This feature is different from the results observed in solar cells [13]. This suggests that the operating mechanism in an HOILED strongly depends on the behavior of the hole carriers. However, some differences can be observed. In the J–V curves, the current density of the HOILED sample with
Fig. 2. J, L–V characteristics of HOILEDs with “oxygen-rich” (open circles) and conventional TiO2 (closed circles).
Fig. 3. J, L–V characteristics of HOILED with porous TiO2 .
an “oxygen-rich” TiO2 layer is lower than that of a conventional HOILED sample over the entire voltage range, including the reverse bias (not shown). The lower current density below the current turnon voltage and in the reverse bias is the result of a difference in the number of carriers generated from the oxygen vacancies. On the other hand, the reason for the trend above the current turnon voltage is not so easy to formulate, therefore, we will discuss it including the L–V curves. In an HOILED sample, a large gap is observed between the current and the light turn-on voltage. We can explain this by the fact [12] that the gap comes from the difference between the injection voltages of the holes and electrons. This assumes that hole carriers are injected at the lower threshold voltage (current turn-on voltage), and that the electron injection occurs at another higher voltage (light turn-on voltage). This is a feature of operating mechanism for HOILED. The unique interaction between MoO3 and F8BT causes the enhancement of hole injection in the HOILED under current turn-on voltage. In an identical device, the injected holes were transported and accumulated at the interface between TiO2 and F8BT. Electrons were extracted by a large number of injected holes under light turn-on voltage. In the case of the HOILED sample with an “oxygen-rich” TiO2 layer, the surface states were decreased contrary to the “UV effect” [15], thus, it is expected that the light turn-on voltage would be shifted to lower voltages due to the effective electron accumulation on the surface of TiO2 . In fact, we observed a little lower shift. This is a feature of the HOILED samples, and shows that a primary consideration that is important in the operating mechanism of an HOILED sample is not at the cathode side, but at the anode side, which is the interface concerned with hole injection. In the earlier paper [7], we reported on a large shift in the light turn-on voltage using UV illumination. This result is speculated that it is due to insufficient hole blocking by a lot of surface states formed by UV treatment. Interestingly, we observed a feature in the L–V curves. As mentioned above, the J–V curves were similar, but the L–V curves were different. The brightness of the HOILED sample with an “oxygenrich” TiO2 layer was higher than that of a conventional HOILED sample. This was also observed in all the fabricated devices. This interesting result would be explained by a reduction in the number of oxygen vacancy sites, which seem to play a role as an exciton quenching site. This is a second role of TiO2 . Initial EL characteristics for the HOILED sample with a porous TiO2 layer are shown in Fig. 3. Haque et al. [8] have reported that
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K. Morii / Synthetic Metals 159 (2009) 2312–2314
the brightness increases on increasing the interface area. However, we observed no marked increase or decrease in the J, L–V curves. As a result, the J, L–V curves shown in Fig. 3 were similar to those shown in Fig. 2. These results also suggest that the operating mechanism of the HOILEDs was mainly controlled by the behavior of the hole carriers. In the published case, the number of injected holes is not so much due to no MoO3 layer. This strongly supports the notion that the current turn-on voltage is almost same as that of other HOILEDs. In summary, the EL characteristics of HOILEDs with some different TiO2 layers have been studied. The importance of a hole injection scheme in the HOILED samples was confirmed. It was observed that the current turn-on voltage does not change with the density of oxygen vacancies, and the brightness and light turnon voltage are affected. The reason for these are that the oxygen vacancies on the surface act as exciton quenching sites and surface states in an energy gap. In addition, we observed similar EL characteristics in HOILED samples with a porous TiO2 layer. All the data supported the notion that HOILEDs are strongly hole-dominated LEDs, and that TiO2 in HOILEDs plays a supporting role. As a primary role, TiO2 seems to act as a hole-blocking layer. In addition, it is clear that the surface state is important. This result is important for applications relying on the interface between inorganic and organic materials.
We would like to thank Pascal Comte and Prof. Michael Graetzel in EPFL for stimulating discussions and for providing support for device fabrication. References [1] M.-K. Fung, C.-S. Lee, S.-T. Lee, Organic Light Emitting Devices Synthesis. Properties and Applications, Wilet-vch Verlag GmbH & Co. KGaA, Weinheim, 2006, p. 181 (Chapter 5). [2] C. Shen, A. Kahn, J. Schwartz, J. Appl. Phys. 89 (2001) 449. [3] H. Nakanotani, M. Yahiro, K. Yano, C. Adachi, Appl. Phys. Lett. 90 (2007) 262104. [4] A.J. Breeze, Z. Schlesinger, P.J. Brock, S.A. Carter, Phys. Rev. B 64 (2001) 125205. [5] P. Ravirajan, S.A. Haque, J.R. Durrant, D.D.C. Bradley, J. Nelson, Adv. Funct. Mater. 15 (2005) 609. [6] K. Lee, J.Y. Kim, S.H. Park, S.H. Kim, S. Cho, A.J. Heeger, Adv. Mater. 19 (2007) 2445. [7] K. Morii, M. Ishida, T. Takashima, T. Shimoda, Q. Wang, Md.K. Nazeeruddin, M. Grätzel, Appl. Phys. Lett. 89 (2006) 183510. [8] S.A. Haque, S. Koops, N. Tokmoldin, J.R. Durrant, J. Huang, D.D.C. Bradley, E. Palomares, Adv. Mater. 19 (2007) 683. [9] H.J. Bolink, E. Coronado, D. Repetto, M. Sessolo, Appl. Phys. Lett. 91 (2007) 223501. [10] H.J. Bolink, E. Coronado, D. Repetto, M. Sessolo, E.M. Barea, J. Bisquert, G. GarciaBelmonte, J. Prochazka, L. Kavan, Adv. Funct. Mater. 18 (2008) 145. [11] K. Morii, T. Kawase, S. Inoue, Appl. Phys. Lett. 92 (2008) 213304. [12] K. Morii, M. Omoto, M. Ishida, M. Grätzel, Jpn. J. Appl. Phys. 47 (2008) 7366. [13] H.J. Snaith, M. Graetzel, Adv. Mater. 18 (2006) 1910. [14] H.J. Snaith, L.S. Mende, M. Graetzel, M. Chiesa, Phys. Rev. B 74 (2006) 045306. [15] S. Ferrere, B.A. Gregg, J. Phys. Chem. B 105 (2001) 7602.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
The role of TiO2 in the air-stable hybrid organic–inorganic light-emitting diodes Katsuyuki Morii ∗ Department of Chemistry, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan
a r t i c l e
i n f o
Article history: Received 28 July 2008 Received in revised form 27 September 2009 Accepted 30 September 2009 Available online 31 October 2009
a b s t r a c t The role of the TiO2 layer in hybrid organic–inorganic light-emitting diodes (HOILEDs) was discussed. A TiO2 layer was fabricated using a process that formed an “oxygen-rich” and “porous” TiO2 layer in an HOILED. The importance of the surface state on the TiO2 and the hole injection scheme in the HOILED was confirmed. All the data support the idea that HOILEDs are strongly hole-dominated LEDs, and that the TiO2 in HOILEDs plays a supporting role. The TiO2 seems to act as a hole-blocking layer. © 2009 Elsevier B.V. All rights reserved.
Keywords: OLED Metal oxide Hybrid Titanium oxide
Due to the impetus to develop organic electronic devices, the interface between organic and inorganic materials has been the focus of much research. The first target of this research is the interface of a metal in contact with the top layer of an organic material [1,2], which plays an important role in electronic devices. Many experiments have resulted in improved organic electronic devices. Recently, different types of interface between organic and inorganic materials have been investigated. Some hybrid electronic devices with an interface between organic and metal oxide materials, for example, field-effect transistors [3], solar cells [4–6], and lightemitting diodes [7–11], have been reported that have excellent physical properties due to their use of metal oxides. We have already published results on a hybrid organic– inorganic light-emitting diode (HOILED), in which an emissive conductive polymer (poly(dioctylfluorene-alt-benzothiadiazole, F8BT) is sandwiched between metal oxide layers (titanium oxide, TiO2 , and molybdenum oxide, MoO3 ) [7]. It has been reported that the key issue for HOILEDs is the interface of the MoO3 , which is located on the top of the F8BT layer, because it is from this layer that many hole carriers are injected into the F8BT layer [12]. It is suggested that HOILEDs are hole-dominated light-emitting diodes. It has been suggested that hole carriers accumulate at the interface between a TiO2 layer and an F8BT layer, and electrons can be extracted by a large number of injected holes. Moreover, the EL characteristics have been markedly improved recently by modify-
∗ Tel.: +81 92 642 25 71; fax: +81 92 642 25 71. E-mail address: [email protected]. 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.09.036
ing the interface at the TiO2 layer using a layer consisting of cesium compounds, to a level that is almost the same as the top-rated organic light-emitting diode fabricated using F8BT [11]. Unfortunately, the operating mechanism of HOILEDs is still not fully understood. In particular, a better understanding of molecular-level phenomena at both interfaces between the metal oxides and the organic materials is required. For example, the characteristics of the interface at the TiO2 layer, and the role of TiO2 are important issues. Here, we, first of all, discuss the EL characteristics of HOILEDs with three different TiO2 layers, and discuss the role of a thin TiO2 layer. According to previous reports that have discussed hybrid organic electronic devices with a TiO2 layer, there are two key issues that need to be addressed: the “doping level”, dependent on the density of oxygen vacancies, and the “size of interface area”. The “doping level” is a difficult and complicated factor. From the viewpoint of the energy diagram, the Fermi level and the surface state will change. Furthermore, it also changes with the chemical structure of the surface. Bolink et al. [9] have already reported that the replacement TiO2 with zinc oxide leads to an improvement in the EL characteristics. This result shows that the physical properties of the metal oxide layer on the cathode side are effective in obtaining a high efficiency. Fortunately, the electronic properties of the metal oxide layer can be easily modified by the fabrication process. Two different doping levels for TiO2 were evaluated in this paper. The “size of the interface area” is important for organic solar cells, and Haque et al. [8] have reported that the effect of the size of the interface area is also a key issue in hybrid LEDs. Here, we have evaluated the effect of the size of the interface area in a second, different TiO2 layer has been evaluated.
K. Morii / Synthetic Metals 159 (2009) 2312–2314
2313
Fig. 1. Schematic structures of HOILEDs with some different TiO2 layers.
We evaluated and controlled the “doping level” by fabricating different compact TiO2 layers, which were employed as a holeblocking layer in dye-synthesized solar cells (DSSCs), through the choice of carrier gas during the deposition procedure. A commonly used carrier gas is air. In this work, this compact TiO2 layers were fabricated using spray pyrolysis in either air or oxygen. Oxygen was employed as the carrier gas to form an “oxygen-rich” compact TiO2 layer. [13] To investigate the “size of interface area”, HOILEDs with porous TiO2 layers were fabricated using a wellestablished deposition route for DSSCs [14]. Only a porous TiO2 layer was employed in this work to study the size of the interface area, and our HOILED samples had no compact TiO2 layer. The fabrication process reported in [7] was used, except for the fabrication of the TiO2 layer. These device structures are shown in Fig. 1. In addition, the three HOILEDs samples were fabricated using the same batch fabrication scheme. Therefore, the other layers were the same completely, except for the TiO2 layer. The initial EL characteristics (current density–voltage, J–V, and brightness–voltage, L–V) for two HOILED samples with different doping levels in the TiO2 layer are shown in Fig. 2. It can be seen that there was no marked difference, current turn-on voltage particularly, between these JV curves. This feature is different from the results observed in solar cells [13]. This suggests that the operating mechanism in an HOILED strongly depends on the behavior of the hole carriers. However, some differences can be observed. In the J–V curves, the current density of the HOILED sample with
Fig. 2. J, L–V characteristics of HOILEDs with “oxygen-rich” (open circles) and conventional TiO2 (closed circles).
Fig. 3. J, L–V characteristics of HOILED with porous TiO2 .
an “oxygen-rich” TiO2 layer is lower than that of a conventional HOILED sample over the entire voltage range, including the reverse bias (not shown). The lower current density below the current turnon voltage and in the reverse bias is the result of a difference in the number of carriers generated from the oxygen vacancies. On the other hand, the reason for the trend above the current turnon voltage is not so easy to formulate, therefore, we will discuss it including the L–V curves. In an HOILED sample, a large gap is observed between the current and the light turn-on voltage. We can explain this by the fact [12] that the gap comes from the difference between the injection voltages of the holes and electrons. This assumes that hole carriers are injected at the lower threshold voltage (current turn-on voltage), and that the electron injection occurs at another higher voltage (light turn-on voltage). This is a feature of operating mechanism for HOILED. The unique interaction between MoO3 and F8BT causes the enhancement of hole injection in the HOILED under current turn-on voltage. In an identical device, the injected holes were transported and accumulated at the interface between TiO2 and F8BT. Electrons were extracted by a large number of injected holes under light turn-on voltage. In the case of the HOILED sample with an “oxygen-rich” TiO2 layer, the surface states were decreased contrary to the “UV effect” [15], thus, it is expected that the light turn-on voltage would be shifted to lower voltages due to the effective electron accumulation on the surface of TiO2 . In fact, we observed a little lower shift. This is a feature of the HOILED samples, and shows that a primary consideration that is important in the operating mechanism of an HOILED sample is not at the cathode side, but at the anode side, which is the interface concerned with hole injection. In the earlier paper [7], we reported on a large shift in the light turn-on voltage using UV illumination. This result is speculated that it is due to insufficient hole blocking by a lot of surface states formed by UV treatment. Interestingly, we observed a feature in the L–V curves. As mentioned above, the J–V curves were similar, but the L–V curves were different. The brightness of the HOILED sample with an “oxygenrich” TiO2 layer was higher than that of a conventional HOILED sample. This was also observed in all the fabricated devices. This interesting result would be explained by a reduction in the number of oxygen vacancy sites, which seem to play a role as an exciton quenching site. This is a second role of TiO2 . Initial EL characteristics for the HOILED sample with a porous TiO2 layer are shown in Fig. 3. Haque et al. [8] have reported that
2314
K. Morii / Synthetic Metals 159 (2009) 2312–2314
the brightness increases on increasing the interface area. However, we observed no marked increase or decrease in the J, L–V curves. As a result, the J, L–V curves shown in Fig. 3 were similar to those shown in Fig. 2. These results also suggest that the operating mechanism of the HOILEDs was mainly controlled by the behavior of the hole carriers. In the published case, the number of injected holes is not so much due to no MoO3 layer. This strongly supports the notion that the current turn-on voltage is almost same as that of other HOILEDs. In summary, the EL characteristics of HOILEDs with some different TiO2 layers have been studied. The importance of a hole injection scheme in the HOILED samples was confirmed. It was observed that the current turn-on voltage does not change with the density of oxygen vacancies, and the brightness and light turnon voltage are affected. The reason for these are that the oxygen vacancies on the surface act as exciton quenching sites and surface states in an energy gap. In addition, we observed similar EL characteristics in HOILED samples with a porous TiO2 layer. All the data supported the notion that HOILEDs are strongly hole-dominated LEDs, and that TiO2 in HOILEDs plays a supporting role. As a primary role, TiO2 seems to act as a hole-blocking layer. In addition, it is clear that the surface state is important. This result is important for applications relying on the interface between inorganic and organic materials.
We would like to thank Pascal Comte and Prof. Michael Graetzel in EPFL for stimulating discussions and for providing support for device fabrication. References [1] M.-K. Fung, C.-S. Lee, S.-T. Lee, Organic Light Emitting Devices Synthesis. Properties and Applications, Wilet-vch Verlag GmbH & Co. KGaA, Weinheim, 2006, p. 181 (Chapter 5). [2] C. Shen, A. Kahn, J. Schwartz, J. Appl. Phys. 89 (2001) 449. [3] H. Nakanotani, M. Yahiro, K. Yano, C. Adachi, Appl. Phys. Lett. 90 (2007) 262104. [4] A.J. Breeze, Z. Schlesinger, P.J. Brock, S.A. Carter, Phys. Rev. B 64 (2001) 125205. [5] P. Ravirajan, S.A. Haque, J.R. Durrant, D.D.C. Bradley, J. Nelson, Adv. Funct. Mater. 15 (2005) 609. [6] K. Lee, J.Y. Kim, S.H. Park, S.H. Kim, S. Cho, A.J. Heeger, Adv. Mater. 19 (2007) 2445. [7] K. Morii, M. Ishida, T. Takashima, T. Shimoda, Q. Wang, Md.K. Nazeeruddin, M. Grätzel, Appl. Phys. Lett. 89 (2006) 183510. [8] S.A. Haque, S. Koops, N. Tokmoldin, J.R. Durrant, J. Huang, D.D.C. Bradley, E. Palomares, Adv. Mater. 19 (2007) 683. [9] H.J. Bolink, E. Coronado, D. Repetto, M. Sessolo, Appl. Phys. Lett. 91 (2007) 223501. [10] H.J. Bolink, E. Coronado, D. Repetto, M. Sessolo, E.M. Barea, J. Bisquert, G. GarciaBelmonte, J. Prochazka, L. Kavan, Adv. Funct. Mater. 18 (2008) 145. [11] K. Morii, T. Kawase, S. Inoue, Appl. Phys. Lett. 92 (2008) 213304. [12] K. Morii, M. Omoto, M. Ishida, M. Grätzel, Jpn. J. Appl. Phys. 47 (2008) 7366. [13] H.J. Snaith, M. Graetzel, Adv. Mater. 18 (2006) 1910. [14] H.J. Snaith, L.S. Mende, M. Graetzel, M. Chiesa, Phys. Rev. B 74 (2006) 045306. [15] S. Ferrere, B.A. Gregg, J. Phys. Chem. B 105 (2001) 7602.