Operando direct observation of spin-states and charge-trappings of blue light-emitting-diode materials in thin-film devices

Abstract

Spin-states and charge-trappings in blue organic light-emitting diodes (OLEDs) are important issues for developing high-device-performance application such as full-color displays and white illumination. However, they have not yet been completely clarified because of the lack of a study from a microscopic viewpoint. Here, we report operando electron spin resonance (ESR) spectroscopy to investigate the spin-states and charge-trappings in organic semiconductor materials used for blue OLEDs such as a blue light-emitting material 1-bis(2-naphthyl)anthracene (ADN) using metal–insulator–semiconductor (MIS) diodes, hole or electron only devices, and blue OLEDs from the microscopic viewpoint. We have clarified spin-states of electrically accumulated holes and electrons and their charge-trappings in the MIS diodes at the molecular level by directly observing their electrically-induced ESR signals; the spin-states are well reproduced by density functional theory. In contrast to a green light-emitting material, the ADN radical anions largely accumulate in the film, which will cause the large degradation of the molecule and devices. The result will give deeper understanding of blue OLEDs and be useful for developing high-performance and durable devices.

Direct Evidence of the Internal Deterioration Mechanism due to Molecular Chain Ends in Polymer Solar Cells by Operando Spin Detection

“…We have focused on electron spin resonance (ESR) spectroscopy as an analysis method. The ESR spectroscopy is useful because it can conduct nondestructive measurements during device operation (operando) and microscopically observe the charge states in the cells at the molecular level. ,,− In particular, by using operando ESR spectroscopy, we have demonstrated the correlation between the photodeterioration of solar-cell parameters such as J SC and V OC and photogenerated hole accumulation in a typical polymer solar cell with blend films of P3HT and [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM), ,, high-efficiency polymer solar cells with blend films of a donor–acceptor (D–A) conjugated copolymer poly­({4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2- b :4,5- b ′]­dithiophene-2,6-diyl}­{3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4- b ]­thiophenediyl}) (PTB7) or a PTB7 derivative poly­[4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2- b ;4,5- b ′]­dithiophene-2,6-diyl- alt -(4-(2-ethylhexyl)-3-fluorothieno­[3,4- b ]­thiophene)-2-carboxylate-2-6-diyl] (PTB7-Th) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM), and a highly durable polymer solar cell with blend films of poly­[naphtho­[1,2- c :5,6- c ′]­bis­[1,2,5]­thiadiazole-5,10-diyl­[3,4′-bis­(2-butyloctyl)­[2,2′-bithiophene]-5,5′-diyl]­thiazolo­[5,4- d ]­thiazole-2,5-diyl­[3′,4-bis­(2-butyloctyl)­[2,2′-bithiophene]-5,5′-diyl]] (PTzNTz) and PC 71 BM . In this study, OSCs were fabricated using polymers with different end structures, and the accumulated charge states were directly observed by operando ESR spectroscopy to investigate the correlation between the end structures of polymers and the time durability of solar cells with these polymers in detail.…”

Direct operando observation and control of charge states in organic light-emitting diodes with a thermally activated delayed fluorescence material

“…We used a lock-in detection method for the ESR measurements with a modulation frequency of 100 kHz for the external magnetic field. Thus, we could observe long-lived charges (spins) with a lifetime of >10 μs or accumulated charges in the OLEDs, and cannot observe charges with a lifetime of <10 μs that contribute the standard operation of the OLEDs [28][29][30][31][36][37][38][39][40] . We measured operando ESR signals while increasing Vbias from 0 V to 10 V in 0.5 V increment.…”

“…So far, we have been studying microscopic nature such as electric charge states, electron spin states, and molecular orientations in organic devices, etc. using the ESR method [28][29][30][31][32][33][34][35][36][37][38][39] . We have reported an operando ESR study of OLEDs with Alq3, which have shown the correlation between intrinsic luminescence deterioration and electric charge states at the molecular level 28 .…”

Narrowband Deep-Blue Multi-Resonance Induced Thermally Activated Delayed Fluorescence: Insights from the Theoretical Molecular Design

“…In addition, deep blue TADF materials are still a focused-on area. Blue OLED emitters need to satisfy the criteria of wide bandgap generally, between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, making it challenging to achieve highly efficient and pure blue emission [15].…”

“…In addition, deep blue TADF materials are still a focused-on area. Blue OLED emitters need to satisfy the criteria of wide bandgap generally, between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, making it challenging to achieve highly efficient and pure blue emission [15]. Because of this, in 2016, Hatakeyametal et al, first proposed a new structure of BNdoped multi-resonance (MR) TADF molecule, the DABNA-1 (Figure 1a), with blue emission at 462 nm, and the FWHM of 28 nm [16].…”