1.Introduction

Electronic and optoelectronic devices using organic materials such as organic photovoltaic devices (OPVs), organic thin-film transistors (OTFTs), and organic light-emitting diodes (OLEDs) have been drawing tremendous attention from the viewpoints of both practical application as lightweight, flexible and low-cost electronic devices and fundamental scientific research to understand the electronic properties of the organic materials. To achieve a high performance in these devices, the formation of suitable nanostructures in both horizontal and vertical dimensions is of high importance since they largely influence the optical, mechanical and electrical properties of the organic films. Such nanofabrication methods could be generally categorized into two classes: top-down and bottom-up. Conventional top-down methods such as photolithography and patterned vacuum deposition techniques are now used for the fabrication of the structures for microelectronics. Alternative bottom-up approaches like nanoparticle formation, molecular self-organization and surface modification by self-assembly are relatively new and could provide with completely new prospective for the simple and low-cost fabrication techniques. In this thesis, the control of the nanostructure in the organic semiconductors in both horizontal and vertical dimensions are studied by using bottom-up approaches and investigate their application in OPVs and OTFTs.

In the first section, TiO2 nanorod array was synthesized by a bottom-up approach and used as a pre-built nanostructure for the application to OPVs. In the second section, self-organization of molecular materials was utilized to control the vertical nanostructure in OPVs. In the third section, the detailed analysis was conducted to show that this novel surface structure formed by the self-organization during the wet coating process has a well-ordered monolayer structure. In the forth and fifth sections, this type of molecular self-organization at the solvent/air interface was applied for the high performance OTFTs. Finally, I will give the summary of this work.

2. Design and synthesis array of TiO2 nanorods and its application in OPVs

Initial studies focused on the construction of inorganic nanostructures in the active layer of organic photovoltaic devices by using TiO2 as the electron acceptor material and poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as the electron donor polymer. For this purpose, a new method to fabricate an array of TiO2 nanorod assemblies on a flat TiO2 surface was developed. The TiO2 nanostructures with a height of approximately 40 nm were synthesized via a low temperature sol-gel reaction in a reversed micelle system (Figure 1). Devices with this interdigitating structure of MEH-PPV and single crystal TiO2 nanorod assemblies show threefold improvement of short circuit current compared with the devices with the bilayer structure of MEH-PPV and flat TiO2 (Figure 2). This improvement is attributed to the efficient exciton collection of the TiO2 nanorod assemblies in the photovoltaic devices. Furthermore, the devices with the TiO2 nanostructures show a comparable fill factor with that of the flat ones, which could be explained by the effective electron transport within the TiO2 nanorods because of their single crystalline nature. These results demonstrate that the interdigitating structure with pre-built nanorod array has advantages in the exciton collection and charge transport, leading to the high fill factor and improved short circuit current. On the other hand, the interface area between donor/acceptor in this interdigitating structure was hardly comparable with mixture-type bulk heterojunction devices, which limited the overall device performance.

3. Layered nanostructure control by using self-organization

The ordered arrangement of materials at nanoscale is important not only in horizontal dimensions as shown in the previous section, but also in vertical dimensions since the vertical distribution of materials influences the intralayer and the interlayer carrier transports. For organic devices, it is often necessary to make multilayered structures to control the flow of the charge carriers inside the film and to match the energy level of the materials. Although the construction of such multilayer could be conducted by dry processes such as vacuum deposition, the solution-based processes are more attractive since they are scalable, low-cost and compatible with the various mass printing and coating techniques currently used in industry. However, there is an intrinsic difficulty for the solution-based multilayer formations because each layer has to be coated from an orthogonal solvent (i.e. the solvents used for the next layer cannot dissolve the previous layer). To solve this problem, a novel and simple approach to form layered nanostructures through molecular self-organization was developed in this work.

It is well known that materials having a low surface energy such as fluorinated or silicone compounds prefer to migrate to the air/liquid interface during coating. Based on the idea to utilize this phenomenon, a novel fullerene derivative with a fluorocarbon chain (FC7) was designed and synthesized. The C60 part works as an electron-accepting and transporting material and the perfluoroalkyl group gives a low surface energy to the material. When a small amount of FC7 is mixed in the solution for spin coating, the FC7 spontaneously migrates to the surface of the organic layer during spin casting due to the low surface energy of the fluorocarbon and forms a very thin layer of FC7 in a single step (Figure 3). After the deposition of the metal electrode, it could act as a buffer layer between the polymer and the metal electrode, which may decrease the energy barrier for electron injection and collection because of the formation of the interfacial dipole layer. As the result, the fill factor of the OPV devices significantly increased to a value of more than 70%, which directly contributed to the improvement of the power conversion efficiency up to 3.79% (Figure 4).

4. Molecular structure of surface-segregated FC7 thin layer

As shown in the previous section, the OPVs with the surface-segregated FC7 thin layer show improved device performance, particularly the fill factor and short-circuit current. To clarify the functions of FC7 thin layer, the molecular structure of the surface-segregated FC7 thin layer was further studied by sum-frequency generation (SFG) spectra in the collaboration with Prof. Shen Ye at Hokkaido University. Strong SFG signal from FC7/air interface suggest FC7 molecules have an ordered and oriented structure on the surface of organic layer (Figure 5a). Surprisingly, along with changing the length of the fluorinated chain, the intensity of SFG signal clearly shows odd-even effect (Figure 5c and 5d). It reveals that the FCn molecules should have very good packing on the surface of organic layer despite the easiness of the preparation. I named this new class of well-ordered monolayer "surface-segregated monolayers" (SSMs), which gives a promising approach to modified the surface of organic semiconductors, and therefore can be applicable to various organic devices.

5. Organic thin film transistors based on surface-segregated thin layer

As in the discussion of the previous section, the surface segregation of the low surface energy materials was driven by minimization of the total energy. Besides fluorocarbon chains, alkyl chains also show relatively low surface energy. It was reported that spin coated films of poly(3-hexylthiophene) (P3HT) oriented with the side chains normal to the substrate at the solution-to-air interface. This orientation could be attributed to the low surface energy of alkyl side chains that prefer to migrate to the air/liquid interface to minimize the total energy of the system. Interestingly, this molecular orientation was favorable for field-effect mobility in thiophene-containing polymers. If the transistors can be constructed in this oriented layer and study the carrier transport, it could provide new information of the electronic properties of these materials.

To investigate the carrier transport in surface-segregated thin layer, a novel film transfer approach based on contacting and floating off process of the organic films was developed in this work (Figure 6). Organic films can be transferred onto other organic film surfaces with the sizes more than several square centimeters without apparent damage by this method. Transistors are fabricated based on three batches of P3HT and other seven types of thiophene-based polymers (Figure 7a). Very interestingly, all the transistors prepared by the contact film transfer achieve mobility higher than 0.02 cm2V-1s-1, and these values are one to two orders magnitude higher than those of the transistors prepared by the conventional spin coating method (Figure 7b). This improvement could be attributed to the better chain orientation and interchain π-interaction at the air/solvent interface induced by the low surface energy and molecular self-organization during the spin-coating process.

6. Multilayered ambipolar transistors and inverters fabricated by contact film transfer

In the previous section, a novel film transfer was developed to study the carrier transport in surface-segregated thin layers. The benefits of this approach can be also demonstrated by the fabrication of the ambipolar OTFTs based on a bilayer structure of P3HT/PCBM. Furthermore, the interfacial properties between these films can be controlled by the combination with SSMs.

The schematic image of the prepared bilayer transistor is shown in inset of Figure 8a. The output characteristics are shown in Figure 8a. Clear field effect was observed in both p- and n-channel operation. The output curve shows diode-like behavior at low gate voltage (VG) and high drain-to-source voltage (VDS). This is a typical character in ambipolar transistors and observes in neither P3HT nor PCBM unipolar transistors. From the slope of ID1/2 versus VG at VD = +30 V and -30 V, the electron mobility of 2.1×10-2 cm2 V-1 s-1 and the hole mobility of 1.1×10-2 cm2 V-1 s-1 in the ambipolar devices can be extracted. The ambipolar mobilities achieved in this work are the highest values among the two-component transistors based on solution-processes reported to date. Although it still lower than the record value from single-component ambipolar FETs, this approach can be easily applicable to the other organic materials with higher mobilities. In addition, the bilayer ambipolar transistors showed balanced electron and hole mobility of high degrees, therefore it could be a potential candidate for the application in organic complementary inverters. Inverters were fabricated by connect two identical ambipolar P3HT/PCBM transistors on a single substrate (Figure 8b). With VDD = 75 V and -75 V, a high output voltage gain of 14 is achieved which is defined as the steepness of the characteristics at the transitions between high and low states (Figure 8c). The output voltage swing could be controlled by changing supply voltage VDD from -75 V to +75 V. This result demonstrates the usefulness of the contact film transfer approach to fabricate various electronic devices in very simple steps.

7. Summary

In this work, nanostructure control in organic devices by using bottom-up approaches is studied. Novel concepts are proposed to improve the device performance in OPVs and OTFTs through nanostructure control. These concepts and approaches are general and applicable to other types of organic semiconductors and to the various other organic devices.

Figure 1 FESEM images of a) the flat TiO2 substate; b) the nanostructured TiO2 substrate

Figure 2. Current density-voltage curve of the MEH-PPV/TiO2 photovoltaic devices under 100 mW/cm2 AM 1.5 irradiation.

Figure 3 a) Chemical structure of FCn, where n represents the number of fluorocarbons; b) chemical structure of PCBM; c) chemical structure of P3HT; d) schematic representation of the surface-segregated monolayer formation.

Figure 4 I-V characteristics under 100 mw/cm2 AM1.5 illumination of P3HT: PCBM bulk heterojunction devices with FC7 and without FC7.

Figure 5 a) SFG spectra (ssp) of FC7/PCBM, PCBM, and FC7/PCBM films after plasma etching and of FC7 LB film; b) The calculated optimized structure of FC7 by DFT. The arrows represent the dipole derivative vectors of the vibration modes at 1470 cm-1 and 1430 cm-1, respectively; c) SFG spectra (ssp) of PCBM, FC5/PCBM, FC6/PCBM, FC7/PCBM, FC8/PCBM, FC9/PCBM and FC10/PCBM films; (b) plot of the SFG signal intensities vs. the number of fluorocarbons.

Figure 6. A schematic images depict the transfer process applied to the deposition of a Polymer film onto dielectric layer; a) Prepared two substrates with different structures: /BCB/SiO2/Si and Glass/PEDOT:PSS/Polymer film for transfer stamp; b) lightly laminated two substrates together and put a drop of water on the edge; c) Polymer film was detached with glass substrate after the PEDOT:PSS layer was dissolved; d) remove the glass substrate, leaving the desired polymer film on target substrates.

Figure 7. a) Chemical structures of thiophene-containing polymers discussed in this work; b) Plot of field-effect mobility versus different polymers. Transistors fabricated by contact film transfer are represented by black circles, and transistors fabricated by spin coating are represented by red circles.

Figure 8. a) The output characteristics of the P3HT/PCBM bilayer transistor. The gate voltages ranges from 0 to -60 V (-10 V increments) for holes and from 0 to 60 V (+10 V increments) for electrons; b) A schematic representation of the electrical connections in the inverter; c) Transfer characteristics of CMOS-like inverters based on two identical ambipolar transistors.

本論文(Study on Nanostructure Control in Organic Electronic Devices(有機電子デバイスのナノ構造制御に関する研究))は、有機半導体ドナー/アクセプター界面、有機半導体/電極界面及び絶縁体/絶縁体界面をナノスケールで制御し、これらの界面制御と有機薄膜太陽電池及びFETデバイスの性能との相関を明らかにする事を目的とした研究をまとめたもので、以下の7章から構成されている。

第1章では、研究の背景、目的、及び概要が論じられており、近年までの関連論文の成果や問題点などが詳細に記述され、本論文の研究の意義づけが明確にされている。

第2章では、酸化チタンのナノロッドを新規な手法によって作製し、これとドナー性半導体高分子とを組み合わせた有機薄膜太陽電池を作製し、物質の界面制御と太陽電池性能の相関を明らかにしている。また、SEM像から電極上に一様に酸化チタンナノロッドが作製できているのを確認し、断面TEMからはこのロッドが単結晶に近い高い結晶性を有している事を確認している。このロッド中にドナー分子が一様に浸透することで、ドナーとアクセプターの広い界面を形成すると同時に、結晶性の高いロッド中での高い電子移動度を利用することで、電荷分離効率を維持しつつ電荷輸送の効率化が達成され、高効率な太陽電池デバイスが作製できることが明らかとされた。

第3章では、自発的に有機層/電極界面のバッファ層を構築する新規手法を考案し、有機薄膜太陽電池への応用を検討している。本章では、フッ素原子が低表面エネルギーであることを利用してスピンコート中に自発的に界面バッファ層を構築するフラーレン誘導体(F-PCBM)の合成が述べられ、この分子と半導体高分子ならびにPCBMを混合させスピンコートで膜を作製すると、F-PCBMが表面に偏析することを明らかにしている。また、この分子は表面でダイポール層を形成し、電極と有機層間の電荷移動に有利に働くことを述べている。実際にF-PCBMを有機薄膜太陽電池に応用し、光電変換効率を向上させることに成功している。

第4章では、F-PCBMの表面の構造規則性について詳細に検討している。ここでは、様々な測定からF-PCBMが表面で単分子に近い膜を形成すること、表面のSFG測定でフラーレンの構造規則性に由来するピークが観察されフッ化アルキル基の偶奇効果も見られることを述べ、F-PCBMが表面で高い構造規則性を有していることを明らかにしている。従来の界面修飾手法はSAMやLB法などと比べ本手法は自発的かつ簡便に作製可能であり、有機半導体表面を修飾する新規手法として様々な応用が期待される。

第5章では、有機半導体表面の高い構造規則性を有機デバイスへ応用する手法として接触転写法を検討している。本手法は熱や圧力を必要としない非常に簡便な手法であり、表面の構造を維持しながら有機層と有機層の積層構造を構築できるものである。この手法を用いて、P3HTとPCBMの積層構造を構築してアンバイポーラ特性を有するFETの作製に成功し、物理混合系より高い移動度を達成している。

第6章では、第5章の接触転写法を利用して、半導体高分子の構造規則性がFETデバイスの性能に及ぼす影響を考察している。側鎖の異なるポリチオフェン誘導体を用いスピンコート法で作製したFETデバイスでは、側鎖の長さが長くするに従い分子の構造が乱れ移動度が低下するが、これらの分子にあらかじめ高い構造規則性を有する有機層表面作製し、接触転写法を用いてキャリア輸送に応用することでFETデバイスの移動度が1桁上昇し、さらに側鎖の影響を受けず一定の移動度を達成することに成功している。また、同様の手法をポリチオフェン以外の半導体高分子に応用し、同様の性能向上を観察している。この事は、本接触転写法が有機半導体の電荷輸送性能を向上させる一般的手法として有用であることを示している。またこの手法は容易に積層構造を作製でき、界面の構造、電子状態を制御できる可能性があることから、FETデバイスのみならず、有機太陽電池や有機ELなどの他の有機デバイスへの応用も期待される。

第7章では、本研究の総括、及び今後の展望が論じられている。

本論文における分子設計や配向制御は、今後の有機デバイスの高効率化に向けた新規材料の設計やナノ構造制御手法に新たな指針を与える大変優れた成果であるといえる。

よって本論文は博士(工学)の学位請求論文として合格と認められる。