|
有机光电子学是涉及有机半导体理论、材料及其在发光二极管、光伏器件、信息存储和生物传感等方向应用的国际热点研究领域，在新型信息显示、固体照明、新能源和疾病诊疗等方面显示出了广阔的应用前景。对有机半导体来讲，其激发态性质决定光电性能。有机半导体的激发态包括单重激发态和三重激发态，辐射跃迁到基态分别伴随着荧光和磷光发射。然而，通常情况下，单重激发态到三重激发态之间的系间窜越过程是自旋禁阻的，这就使得在有机半导体中三重激发态难以得到利用。因此，如何有效利用和调控三重激发态成为有机光电子领域的重大科学难题。
在最新出版的《半导体学报》2019年第7期上，南京邮电大学有机电子与信息显示国家重点实验室培育基地赵强教授分别从有机电致发光、有机存储、有机光伏、疾病诊断与治疗以及光催化等方面阐释了如何有效利用三重激发态，并对未来工作进行了展望。希望本文能对后续有机半导体三重激发态的有效利用和调控提供一定的借鉴和指导意义。
Organic optoelectronics is an emerging research field, which has attracted extensive interests in the last few decades owing to its practical applications, like organic light-emitting diodes (OLEDs), organic memory devices, organic photovoltaic (OPV), sensors, and organic field-effect transistors[1, 2]. Organic semiconductors play a crucial role in this field. Compared to the traditional inorganic semiconductors, organic semiconductors open a fascinating research direction because of some unique advantages, such as flexible design, low cost, and rich optical and electronic properties. In organic optoelectronics, the excited states greatly determine the photoelectronic properties and application areas as shown in Fig. 1. Based on the electron spin in the molecule, the excited states of organic semiconductors include singlet and triplet states. As we know, the radiative transitions of singlet and triplet excited states are always accompanied by fluorescence and phosphorescence emission, respectively. However, because of the spin-forbidden transitions from the singlet excited states to the triplet excited states, it is often difficult to utilize the triplet excited states directly. Meanwhile, the theoretical basis of the triplet states is also relatively deficient. Thus, most of researches only focused on the regulation of the singlet states. In fact, effective utilization of the triplet excited states will be beneficial to significantly improve the performance of organic optoelectronic devices. To date, it still remains challenges to modulate and utilize the triplet excited states effectively for organic optoelectronic area. Great efforts have been devoted to solving this problem, and a series of functional materials and devices based on the triplet organic semiconductors have been explored.
Figure 1. (Color online) Utilization of triplet excited states in organic semiconductors. Copyright 2019 Chem Rec[5]. Copyright 2014 Nat Commun[6]. Copyright 2018 Angew Chem Int Ed[9]. Copyright 2018 J Am Chem Soc[10]. Copyright 2006 J Am Chem Soc[15].
Organic light-emitting diodes
OLEDs possess several outstanding advantages, like ultrathin and flexible display, self-illumination and rapid response speed, which have been demonstrated to be a promising solid-state lighting and display technology[3–5]. In the process of electroluminescence, both singlet and triplet excitons can be generated with the population ratio of 25% : 75%. However, conventional OLEDs mainly adopted fluorescent materials, which limited the device efficiency. Up to now, several methods have been proposed to utilize the triplet excitons. For example, transitional metal atoms (like Ir, Pt, etc) can be introduced into the molecules to enhance spin-orbit coupling (SOC) constants and boost the ISC, which can utilize both singlet and triplet excitons effectively. Additionally, numerous thermally activated delayed fluorescence (TADF) materials have been successfully explored. These materials have the capacity of reverse intersystem crossing from the triplet excited states to the singlet excited states. Theoretically, 100% excitons can be harvested to realize highly efficient emission in these material systems.
Organic memory devices
Triplet organic semiconductors possess rich charge transfer states. For example, triplet transition-metal complexes have metal-to-ligand charge transfer transition, ligand-to-ligand charge transfer transition, metal-to-ligand-ligand charge-transfer transition, etc. Generally, these triplet charge transfer states are very sensitive to the external environments. Slight stimulus can result in changes in the optoelectronic properties, including absorption spectra, emission spectra, emission lifetimes, and conducting states. Hence, these triplet organic semiconductors can be utilized as multimode information storages or encryption media, which can effectively increase the storage density and security[6–8].
Organic photovoltaic
Solar energy is a kind of renewable and clean energy. The efficient utilization of solar energy has always been a hot subject. OPV is one of the practical devices for the conversion from solar energy to electrical energy. The mechanism of OPV contains the following processes: the generation, diffusion and dissociation of the excitons, the transport and collection of the charge carriers. Triplet organic semiconductors have unique advantages in this field, because it can increase the migration distance of the excitons and is beneficial to improve the performance of the solar cells[9].
Diagnosis and treatment of diseases
With the continuous development of life science, increasing requirements have been put forward for disease detection and diagnosis technology. Optical detection with good selectivity, high sensitivity and short response time has gradually become a convenient detection method in biological area. Triplet exciton transitions in organic semiconductors are accompanied by phosphorescent emission with long luminescence lifetime. Utilizing this characteristic, time-resolved luminescence techniques can be used for the optical detection, which could effectively reduce the disturbance from the short-lived fluorescent signals and enhance the sensitivity and signal-to-noise ratio in complicated system. Additionally, triplet excited states can be easily quenched by O2, which can generate singlet oxygen. Thus, triplet organic semiconductors can also be rationally designed as photosensitizers for photodynamic therapy[10–13].
Photocatalysis
Hydrogen evolution via photocatalytic water splitting under visible light is considered as one of the promising strategies to solve the challenges of energy shortage and environmental crisis. It is an important subject to select appropriate photosensitizers for efficiently achieving the photocatalytic water splitting. Ideal photosensitizers should have the following features: large absorption coefficient in visible range, good stability, proper redox potential, long-lived excited states, etc[14, 15]. Triplet organic semiconductors, particularly transition-metal complexes (Ir(III), Ru(II), Pt(II), Pd(II) and Re(I) complexes), can meet the above requirements well, especially for their long-lived excited states and excellent photochemical stability.
Conclusion
Although numerous organic semiconductors for efficient utilization of triplet excited states have been designed along with the appropriate mechanisms, there still remain several challenges. For example, the deep blue and near-infrared OLEDs based on phosphorescent transition-metal complexes or TADF materials are of poor stability and short device lifetime. To overcome these obstacles, new material systems should be exploited. Meanwhile, the fabrication of devices should be also optimized rationally. The mechanisms of organic memory are not completely understood up to now, and the solar photovoltaic conversion efficiency needs to be improved. In addition, the application fields of these organic semiconductors are limited and new applications should be explored. In summary, great progress has been made in the researches for utilizing the triplet states, and more efforts will be dedicated to exploring the new organic semiconductors based on triplet states, such as purely organic room-temperature phosphorescent materials.
References
[1] stroverkhova O. Organic optoelectronic materials: mechanisms and applications. Chem Rev, 2016, 116(22), 13279
[2] Daskalakis K S, Maier S A, Murray R. Nonlinear interactions in an organic polariton condensate. Nat Mater, 2014, 13(3), 271
[3] Tao P, Li W L, Zhang J, et al. Facile synthesis of highly efficient lepidine-based phosphorescent iridium(III) complexes for yellow and white organic light-emitting diodes. Adv Funct Mater, 2016, 269(6), 881
[4] Miao Y Q, Tao P, Wang K X, et al. Highly efficient red and white organic light-emitting diodes with external quantum efficiency beyond 20% by employing pyridylimidazole-based metallophosphors. ACS Appl Mater Interfaces, 2017, 9(43), 37873
[5] Tao P, Miao Y Q, Wang H, et al. High-performance organic electroluminescence: design from organic light-emitting materials to devices. Chem Rec, 2018, 18, 1
[6] Sun H B, Liu S J, Lin W P, et al. Smart responsive phosphorescent materials for data recording and security protection. Nat Commun, 2014, 5, 3601
[7] Zhang K Y, Chen X J, Sun G L, et al. Utilization of electrochromically luminescent transition-metal complexes for erasable information recording and temperature-related information protection. Adv Mater, 2016, 28(33), 7137
[8] Ma Y, Shen L, She P F, et al. Constructing multi-stimuli-responsive luminescent materials through outer sphere electron transfer in ion pairs. Adv Opt Mater, 2019, 7(8), 1801657
[9] Yang L, Gu W X, Lv L, et al. Triplet tellurophene-based acceptors for organic solar cells. Angew Chem Int Ed, 2018, 57(4), 1096
[10] Zhang K Y, Gao P L, Sun G L, et al. Dual-phosphorescent iridium(III) complexes extending oxygen sensing from hypoxia to hyperoxia. J Am Chem Soc, 2018, 140(25), 7827
[11] Feng Z Y, Tao P, Zou L, et al. Hyperbranched phosphorescent conjugated polymer dots with iridium(III) complex as the core for hypoxia imaging and photodynamic therapy. ACS Appl Mater Interfaces, 2017, 9(34), 28319
[12] Jiang J Y, Qian Y Y, Xu Z H, et al. Enhancing singlet oxygen generation in semiconducting polymer nanoparticles through fluorescence resonance energy transfer for tumor treatment. Chem Sci, 2016, 10(19), 5085
[13] Huang T C, Zhao M L, Yu Q, et al. De novo design of polymeric carrier to photothermally release singlet oxygen for hypoxic tumor treatment. Research, 2019, 2019, 9269081
[14] Brown G M, Brunschwing B S, Creutz C, et al. Homogeneous catalysis of the photoreduction of water by visible light mediation by a tris(2,2’-bipyridine)ruthenium(II)-cobalt(II)macrocycle system. J Am Chem Soc, 1979, 101(5), 1298
[15] Du P W, Schneider J, Jarosz P, et al. Photocatalytic generation of hydrogen from water using a platinum(II) terpyridyl acetylide chromophore. J Am Chem Soc, 2006, 128(24), 7726
点击阅读赵强教授文章：
Utilization of triplet excited states in organic semiconductors
Song Guo, Shujuan Liu, Kenneth Yin Zhang, Wei Huang and Qiang Zhao
J. Semicond. 2019, 40(7), 070402
doi: 10.1088/1674-4926/40/7/070402
扫描二维码关注获得更多信息
Archiver|手机版|科学网 ( 京ICP备07017567号-12 )
GMT+8, 2024-5-29 18:12
Powered by ScienceNet.cn
Copyright © 2007- 中国科学报社
