基于长波发射的有机电致发光器件及其相关性能的研究
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摘要
有机电致发光二极管(Organic light emitting diode,OLED),凭借其高亮度、高效率、全固态自主发光、视角大、轻薄、响应速度快以及可实现柔性显示等诸多优势,极有可能成为继液晶显示器和等离子显示器件的下一代主流显示,同时,鉴于全球面临能源危机,白光OLED作为一种新型的节能照明方式,引起了人们的广泛关注。但有机电致发光器件的研究与应用仍然存在许多瓶颈,例如在OLED的制备中,经常会出现波长比有机材料本征发射更长的长波发射,严重影响了器件的色纯度。因此,研究有机电致发光器件中的长波发射,对控制和提高器件的色纯度有着重要意义,本论文针对此问题展开了以下工作:
(1)针对聚乙烯咔唑(PVK)的电致发光(EL)光谱中经常出现长波发射的现象,系统地研究了紫外光照射对不同溶剂中PVK光致发光(PL)光谱的影响,重点考察了紫外光照射对PVK氯仿溶液及薄膜的PL光谱和吸收谱的影响,并对其EL光谱进行表征。结果表明,紫外光照射对PVK氯仿溶液及薄膜影响较大,随着照射时间的增加PVK的PL谱发生红移(且溶液中的红移现象更为明显),其EL光谱中红光区域的长波发射增强。此外,以PVK为发光层,比较了BCP、TPBi作为电子传输兼空穴阻挡层,对器件光学性能以及电学性能的影响,结果表明基于TPBi的器件亮度更高、启亮电压更低,器件的性能更好。
(2)详细地分析了TPD掺杂对PVK的EL光谱中长波发射的影响,以PVK:TPD掺杂体系为发光层,比较了以BCP、TPBi作为电子传输兼空穴阻挡层的器件性能,在结构为ITO/PVK:TPD (70 nm)/TPBi (10 nm)/ Mg:Ag的器件中得到了性能稳定的蓝光和红光发射。在此双层器件基础上,设计了结构为ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Alq3 (20 nm)/Mg:Ag的白光器件,通过优化PVK与TPD的掺杂比例,调整器件的EL光谱,当PVK与TPD的掺杂比例为1:4时,器件在11 V~15 V电压范围得到(0.33,0.33)附近的稳定的白光发射。同时,研究掺杂比例对白光器件电学性能的影响,结果发现随着TPD掺杂比例的增加,器件的亮度、电流升高,启亮电压降低,器件的性能更理想。
(3)鉴于芴类材料中经常出现长波发射的现象,以芴类小分子材料BFLBBFLYQ为核心材料,首先分析BFLBBFLYQ:TPD掺杂体系在溶液中及薄膜状态的吸收光谱和PL光谱,表征了以掺杂体系为发光层的有机电致发光器件的EL光谱,系统地研究掺杂体系中激基复合物产生的长波发射;同时,讨论了TPD的掺入对器件电流及亮度的影响。制备了结构为ITO/BFLBBFLYQ:TPD (50 nm)/Alq3 (40 nm)/Mg:Ag的双层器件,鉴于掺杂体系与Alq3的发光光谱重合,以红色荧光染料为探针,通过将其掺杂于器件的不同位置中,推断双层器件的载流子复合区域位于靠Alq3层的BFLBBFLYQ:TPD/Alq3界面附近。制备基于BFLBBFLYQ的单层器件与双层器件,发现结构为BFLBBFLYQ/BCP双层器件的EL光谱中存在一个异于单层器件的位于590 nm的发射峰,其可能是BFLBBFLYQ与BCP形成的电致激基复合物的产物,另外,研究表明在BFLBBFLYQ中掺入PVK可以消除此长波发射。
综上所述,本工作对基于咔唑和芴类功能团的PVK和芴类材料的光电特性进行了深入细致的研究,为制作高性能的、颜色稳定的蓝光及白光OLED器件打下了基础。
Organic light emitting diode (OLED) has potentailly to be the candidate for next-generation display along with the current liquid crystal displays (LCDs) and plasma display panel (PDP) due to its high luminance and efficiency, full solid self-emission, wide view-angle and temperature range, ultrathin thickness, fast response and the possibility to realize flexible display. As a novel energy-efficient lighting method, white organic light emitting diode (WOLED) also attracts wide attention because of the global energy crisis. However, there are still several problems for the application of the OLED. During the fabrication of blue OLED, low energy emission located at green or red area was always detected in the electroluminescence (EL) spectra, which affects the color purity of the device seriously. So study on the low energy emission in OLED has great significance to control and enhance the color purity of the devices. Consequently, this work is dedicated to study the following issues.
(1) Aiming at the long wave emission from the EL spectra of poly-(N-vinylcarbazole) (PVK), the influence of ultraviolet (UV) light on the photoluminescence (PL) spectra of PVK in different solvent, especially on the PL spectra and absorption spectra of PVK in chloroform and the following casted film, was investigated, and the EL spectra of PVK spin-coated from chloroform were also characterized. It is found that the influence on the PVK in chloroform and the following casted film is much stonger than other solvent. As the UV light irradiation time increase the PL spectra, especially in the PVK chloroform solution, redshift and the red long wave emission in EL spectra were enhanced. Additively, using BCP and TPBi as the electron transporting layer (ETL), respectively, double-layer devices with PVK as emission layer (EML) were fabricated, and the optical and electrical properties were compared, which indicated that the devices using TPBi as ETL have a better performance.
(2) The influence of doping TPD on the long wave emission from PVK was analyzed, double-layer devices based on PVK:TPD doping system using BCP and TPBi as the electron transporting layer (ETL), respectively, were fabricated. And after comparing the performance of the two devices, it is found that stable blue and red emission with equal intensity could be obtained from the device, whose structure is ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Mg:Ag. Based on this double-layer devices, WOLED with structure of ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Alq3 (20 nm)/Mg:Ag were designed. By changing the doping rate of PVK and TPD, the EL spectra were regulated. Stable white light emission could be obtained from 11 V to 15 V when PVK and TPD were blended at the rate of 1:4. The influence of doping rate on the electrical properties indicated that the performance of the WOLED was improved as the proportion of TPD increased.
(3) Small molecular fluorene derivative BFLBBFLYQ was investigated in this work. The absorption and PL spectra of the BFLBBFLYQ:TPD doping system in solution and film were analyzed, and the EL spectra of the OLED base of the doping system were also investigated. Long wave emission existed in the PL spectra of the doping system film and the EL spectra of the devices, the reason of which was studied in this work too. Additively, the influence of doping TPD on the current density and luminance of the devices was also discussed. Double-layer device with the structure of ITO/BFLBBFLYQ:TPD (50 nm)/Alq3 (40 nm)/Mg:Ag was fabricated. As the doping system and Alq3 has the same emission spectra, the carrier combination zone was studied by doping red fluorescent dye DCJTB into different position of the devices. Single-layer and double-layer devices base on BFLBBFLYQ were studied too, and a long wave emission at 590 nm was detected in the EL spectra of double-layer device but not existed in the single-layer device. It is found that the long wave emission may be the result of electroplex between BFLBBFLYQ and BCP, and it could be removed by doping PVK into BFLBBFLYQ.
In summary, this work focused on the systematic study on the optoelectronic properties of vovel materials based carbazole and fluorene functional groups, which paved the way on the fabrication of color stable high performance blue OLED.
(1)针对聚乙烯咔唑(PVK)的电致发光(EL)光谱中经常出现长波发射的现象,系统地研究了紫外光照射对不同溶剂中PVK光致发光(PL)光谱的影响,重点考察了紫外光照射对PVK氯仿溶液及薄膜的PL光谱和吸收谱的影响,并对其EL光谱进行表征。结果表明,紫外光照射对PVK氯仿溶液及薄膜影响较大,随着照射时间的增加PVK的PL谱发生红移(且溶液中的红移现象更为明显),其EL光谱中红光区域的长波发射增强。此外,以PVK为发光层,比较了BCP、TPBi作为电子传输兼空穴阻挡层,对器件光学性能以及电学性能的影响,结果表明基于TPBi的器件亮度更高、启亮电压更低,器件的性能更好。
(2)详细地分析了TPD掺杂对PVK的EL光谱中长波发射的影响,以PVK:TPD掺杂体系为发光层,比较了以BCP、TPBi作为电子传输兼空穴阻挡层的器件性能,在结构为ITO/PVK:TPD (70 nm)/TPBi (10 nm)/ Mg:Ag的器件中得到了性能稳定的蓝光和红光发射。在此双层器件基础上,设计了结构为ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Alq3 (20 nm)/Mg:Ag的白光器件,通过优化PVK与TPD的掺杂比例,调整器件的EL光谱,当PVK与TPD的掺杂比例为1:4时,器件在11 V~15 V电压范围得到(0.33,0.33)附近的稳定的白光发射。同时,研究掺杂比例对白光器件电学性能的影响,结果发现随着TPD掺杂比例的增加,器件的亮度、电流升高,启亮电压降低,器件的性能更理想。
(3)鉴于芴类材料中经常出现长波发射的现象,以芴类小分子材料BFLBBFLYQ为核心材料,首先分析BFLBBFLYQ:TPD掺杂体系在溶液中及薄膜状态的吸收光谱和PL光谱,表征了以掺杂体系为发光层的有机电致发光器件的EL光谱,系统地研究掺杂体系中激基复合物产生的长波发射;同时,讨论了TPD的掺入对器件电流及亮度的影响。制备了结构为ITO/BFLBBFLYQ:TPD (50 nm)/Alq3 (40 nm)/Mg:Ag的双层器件,鉴于掺杂体系与Alq3的发光光谱重合,以红色荧光染料为探针,通过将其掺杂于器件的不同位置中,推断双层器件的载流子复合区域位于靠Alq3层的BFLBBFLYQ:TPD/Alq3界面附近。制备基于BFLBBFLYQ的单层器件与双层器件,发现结构为BFLBBFLYQ/BCP双层器件的EL光谱中存在一个异于单层器件的位于590 nm的发射峰,其可能是BFLBBFLYQ与BCP形成的电致激基复合物的产物,另外,研究表明在BFLBBFLYQ中掺入PVK可以消除此长波发射。
综上所述,本工作对基于咔唑和芴类功能团的PVK和芴类材料的光电特性进行了深入细致的研究,为制作高性能的、颜色稳定的蓝光及白光OLED器件打下了基础。
Organic light emitting diode (OLED) has potentailly to be the candidate for next-generation display along with the current liquid crystal displays (LCDs) and plasma display panel (PDP) due to its high luminance and efficiency, full solid self-emission, wide view-angle and temperature range, ultrathin thickness, fast response and the possibility to realize flexible display. As a novel energy-efficient lighting method, white organic light emitting diode (WOLED) also attracts wide attention because of the global energy crisis. However, there are still several problems for the application of the OLED. During the fabrication of blue OLED, low energy emission located at green or red area was always detected in the electroluminescence (EL) spectra, which affects the color purity of the device seriously. So study on the low energy emission in OLED has great significance to control and enhance the color purity of the devices. Consequently, this work is dedicated to study the following issues.
(1) Aiming at the long wave emission from the EL spectra of poly-(N-vinylcarbazole) (PVK), the influence of ultraviolet (UV) light on the photoluminescence (PL) spectra of PVK in different solvent, especially on the PL spectra and absorption spectra of PVK in chloroform and the following casted film, was investigated, and the EL spectra of PVK spin-coated from chloroform were also characterized. It is found that the influence on the PVK in chloroform and the following casted film is much stonger than other solvent. As the UV light irradiation time increase the PL spectra, especially in the PVK chloroform solution, redshift and the red long wave emission in EL spectra were enhanced. Additively, using BCP and TPBi as the electron transporting layer (ETL), respectively, double-layer devices with PVK as emission layer (EML) were fabricated, and the optical and electrical properties were compared, which indicated that the devices using TPBi as ETL have a better performance.
(2) The influence of doping TPD on the long wave emission from PVK was analyzed, double-layer devices based on PVK:TPD doping system using BCP and TPBi as the electron transporting layer (ETL), respectively, were fabricated. And after comparing the performance of the two devices, it is found that stable blue and red emission with equal intensity could be obtained from the device, whose structure is ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Mg:Ag. Based on this double-layer devices, WOLED with structure of ITO/PVK:TPD (70 nm)/TPBi (10 nm)/Alq3 (20 nm)/Mg:Ag were designed. By changing the doping rate of PVK and TPD, the EL spectra were regulated. Stable white light emission could be obtained from 11 V to 15 V when PVK and TPD were blended at the rate of 1:4. The influence of doping rate on the electrical properties indicated that the performance of the WOLED was improved as the proportion of TPD increased.
(3) Small molecular fluorene derivative BFLBBFLYQ was investigated in this work. The absorption and PL spectra of the BFLBBFLYQ:TPD doping system in solution and film were analyzed, and the EL spectra of the OLED base of the doping system were also investigated. Long wave emission existed in the PL spectra of the doping system film and the EL spectra of the devices, the reason of which was studied in this work too. Additively, the influence of doping TPD on the current density and luminance of the devices was also discussed. Double-layer device with the structure of ITO/BFLBBFLYQ:TPD (50 nm)/Alq3 (40 nm)/Mg:Ag was fabricated. As the doping system and Alq3 has the same emission spectra, the carrier combination zone was studied by doping red fluorescent dye DCJTB into different position of the devices. Single-layer and double-layer devices base on BFLBBFLYQ were studied too, and a long wave emission at 590 nm was detected in the EL spectra of double-layer device but not existed in the single-layer device. It is found that the long wave emission may be the result of electroplex between BFLBBFLYQ and BCP, and it could be removed by doping PVK into BFLBBFLYQ.
In summary, this work focused on the systematic study on the optoelectronic properties of vovel materials based carbazole and fluorene functional groups, which paved the way on the fabrication of color stable high performance blue OLED.
引文
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