九州大学学術情報リポジトリ
Kyushu University Institutional Repository
低分子有機半導体薄膜のアモルファス構造制御と電 気・光物性
江﨑, 有
https://doi.org/10.15017/4060121
出版情報：Kyushu University, 2019, 博士（工学）, 課程博士 バージョン：
権利関係：
（様式２）
氏 名 ： 江﨑 有
論 文 名 ：
Structural control of amorphous films of organic semiconductors and its effect on electrical and luminescence properties
（低分子有機半導体薄膜のアモルファス構造制御と電気・光物性）
区 分 ： 甲
論 文 内 容 の 要 旨
In Chapter 1, I briefly explained the outline of this thesis. A structural control of amorphous films is known to be effective for further enhancement of performance of organic light-emitting diodes (OLEDs).
While the effect of molecular orientation on OLED performance has been investigated well, film density has not been actively controlled although the microscopic voids in films (attributed to film density) are known to deteriorate the electrical properties. In this thesis, density and molecular orientation of amorphous films were controlled using substrate temperature (Tsub) during vacuum deposition which dominates kinetic mobility of molecules during film formation. I aimed at improving the electrical properties by increasing film density and maximizing the functionalities of organic materials by tuning molecular orientation.
Through this thesis, I performed a comprehensive study on the relationship between amorphous film structures, electrical properties, and OLED performance for establishing fundamental science and developing OLED industry
In Chapter 2, I discussed the structure and electrical properties of amorphous films of a typical hole-transporting material, N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD), controlled by Tsub during vacuum deposition. Tsub had different effects on the film density and molecular orientation of α-NPD. Film density was a convex function of Tsub; maximum density was attained at Tsub = 270–300 K. α-NPD molecules were randomly oriented at Tsub = 342 K, and their horizontal orientation on the substrate became dominant as Tsub decreased. Hole current and air stability were clearly raised by increasing the film density by 1%–2%; these effects were respectively attributed to enhanced carrier hopping between neighboring α-NPD molecules and suppressed penetration of oxygen and water. These results imply that increasing film density is more effective to enhance the electrical performance of organic thin -film devices with α-NPD films than control of molecular orientation.
In Chapter 3, I evaluated carrier traps in α-NPD films using a thermally stimulated current (TSC) method to clarify the origins of the current enhancement in the high -density films discussed in Chapter 2.
The TSC results revealed that hole traps were uniformly distributed throughout the films and that hole traps were the shallowest for films fabricated at Tsub of around 275 K. Thus, the shallowest hole traps at this Tsub is believed to be one reason for the highest hole current for α-NPD films. This is the demonstration of how Tsub
affects carrier traps, contributing to a better understanding of the underlying physics in organic amorphous
films.
In Chapter 4, I investigated the structure and photoluminescence properties of amorphous films of a typical electron-transporting and emitting material, tris(8-hydroxyquinolinato)aluminum (Alq3), controlled by Tsub and deposition rates. The molecular orientation and density of the Alq3 films exhibited clear dependence on Tsub. We found that photoluminescence quenching was stronger in Alq3 films fabricated at lower Tsub and higher deposition rates. This photoluminescence quenching was not related to molecular orientation and film density and likely originated from the inclusion of impurities in the films. These results indicate that decreasing the amount of impurities in films by increasing Tsub or decreasing deposition rate is crucial to achieve the maximum photoluminescence performance from films.
In Chapter 5, the comprehensive effect of the structural control of amorphous films was investigated by evaluating performance of OLEDs with α-NPD and Alq3 layers deposited at different Tsub. I succeeded in controlling spontaneous orientation polarization (SOP) of Alq3 films by changing Tsub. Although amorphous structures did not affect the electron transporting properties of Alq3, SOP probably affected electron injection and influenced the current density–voltage properties of OLEDs. External quantum efficiency (EQE) was dramatically affected by Tsub and enhanced by 42% at most. Not only the fluorescence quantum yield and light outcoupling efficiency, but also the carrier balance and singlet-polaron annihilation influenced by the SOP and accumulated interfacial charge were probably related to this EQE enhancement. Operational stability was also affected by Tsub.
In Chapter 6, I summarized main results of this thesis and mentioned future perspectives.
