The charge balance mechanism in green fluorescent organic light-emitting diodes is investigated for different electron transport layers (ETLs) and electron mobilities. Carrier accumulation and an increase in the exciton recombination probability are shown to be critical for improving the current and power efficiencies by aligning the bands at the interface between the emitting layer (EML) and ETL. The peak in the electroluminescence (EL) spectra was found to shift slightly in response to changes in the width of the emission zone and reflected the electron mobility of the ETL. Higher electron mobility resulted in a wider recombination zone in the EML that was manifested by a blue-shift of the EL peak.Organic light-emitting diodes (OLEDs) 1-14 have the potential to be employed in applications such as flat panel displays, flexible electronics, and solid-state lighting. Considerable research has been devoted to uncovering exact OLED operation mechanisms and precisely determining their electrical and optical properties. The charge balance 1-14 is considered to be a key factor for controlling device performance, stability, and color coordinates; in a heterojunction device structure, holes in the hole transport layer (HTL) 1 are regarded as being transported too quickly, while electrons in the electron transport layer (ETL) 2 are transported too slowly, and as a result, holes simply pass through without generating excitons with the electrons in the emission layer (EML), which manifests as a low current efficiency.To overcome this discordance between the hole and electrons mobilities and to improve the carrier charge balance, many researchers have investigated novel structures such as a metal oxide buffer layer, 3,4 electron or hole blocking layers, and new materials for the ETL 5,6 that have a high electron mobility. In the early stages of OLED heterostructure development, tris(8-hydroxyquinolinolato)aluminum (Alq 3 ) was used for both the EML and ETL in a single device. 2 Other materials that have been introduced for the ETL include 3-phenyl-4(10-naphthyl)-5-phenyl-1,2,4-triazole (TAZ), 5,7 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BPhen), 1,5,6 and 2-[3,5-bis(1-phenylbenzimidazol-2-yl)phenyl]-1-phenylbenzimidazole (TPBi) 5,7,8 in an effort to enhance the exiton recombination and current efficiencies by producing a higher electron mobility and adequate band alignment. The electron mobility also has the potential to control the exciton recombinationzone width in the EML. 9 Tang and Chen et al. 9 have shown that a higher hole mobility through the EML resulted in a wider recombination zone, which was beneficial for a longer device lifetime, and that the electroluminescence (EL) peak shifts when the width of the recombination zone changes. Both the higher hole mobility in the EML and the larger emission zone generate an additional contribution to the EL peak from adjacent layers such as the HTL and ETL. 9 In other work, Meerholz et al. 12 adjusted the width of the emission zone in the EML by copolymerization of the HT...
Two donor−acceptor (D−A) medium band gap polymers, P1 and P2, alkoxyphenylthiophene (APTh) linked benzodithiophene (BDT) as an electron-rich unit and 1,3-di(2′-bromothien-5′-yl)-5-(2-ethylhexyl)thieno [3,4-c]pyrrole-4,6dione (TPD) (A1) or [5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (BT) (A2) as an electron-deficient unit, have successfully been synthesized via microwave-assisted Stille polymerization and utilized for bulk heterojunction (BHJ) polymer solar cells (PSCs). P1 shows a well-distinguished absorption shoulder between 590 and 620 nm attributed to the π−π stacking of a polymer backbone; such kind of absorption shoulder is not observed in P2, indicating that the P1 has more planar structure than that of P2. This is due to the fact that the sulfur atom of thiophene spacer and the oxygen atom of carbonyl groups in TPD have more pronounced intramolecular noncovalent interactions (INCI) in P1 than that of the sulfur atom of thiophene spacer and the oxygen atom of alkoxy groups of BT in P2. The bulk heterojunction polymer solar cells (BHJ PSCs) were fabricated with the configuration of ITO/PEDOT:PSS/polymer (P1 or P2):PC 71 BM/LiF/Al. The P1 device shows better photovoltaic performance with open-circuit voltage (V oc ) of 0.91 V and the power conversion efficiency (PCE) of 4.19% than the P2 device (V oc : 0.71 V; PCE: 1.88%) in neat blend films under the illumination of AM 1.5G (100 mW/cm 2 ). Upon treating the active layers containing P1 and P2 with methanol, the PCE of the P1 device is increased from 4.19 to 7.14%. In contrast, the PCE of the P2 device is decreased from 1.88 to 1.82%. Space charge limited current mobility, atomic force microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and impedance spectroscopy studies strongly support the enhanced PCE for the P1 device is attributed to the increased mobility, nanoscale morphology, and reduced resistance upon methanol treatment; these favorable properties for the P1 polymer are highly correlated with the planarity of the backbone.
The photoluminescence (PL) efficiency of emitters is a key parameter to accomplish high electroluminescent performance in phosphorescent organic light‐emitting diodes (PhOLEDs). With the aim of enhancing the PL efficiency, this study designs deep‐blue emitting heteroleptic Ir(III) complexes (tBuCN‐FIrpic, tBuCN‐FIrpic‐OXD, and tBuCN‐FIrpic‐mCP) for solution‐processed PhOLEDs by covalently attaching the light‐harvesting functional moieties (mCP‐Me or OXD‐Me) to the control Ir(III) complex, tBuCN‐FIrpic. These Ir(III) complexes show similar deep‐blue emission peaks around 453, 480 nm (298 K) and 447, 477 nm (77 K) in chloroform. tBuCN‐FIrpic‐mCP demonstrates higher light‐harvesting efficiency (142%) than tBuCN‐FIrpic‐OXD (112%), relative to that of tBuCN‐FIrpic (100%), due to an efficient intramolecular energy transfer from the mCP group to the Ir(III) complex. Accordingly, the monochromatic PhOLEDs of tBuCN‐FIrpic‐mCP show higher external quantum efficiency (EQE) of 18.2% with one of the best blue coordinates (0.14, 0.18) in solution‐processing technology. Additionally, the two‐component (deep‐blue:yellow‐orange), single emitting layer, white PhOLED of tBuCN‐FIrpic‐mCP shows a maximum EQE of 20.6% and superior color quality (color rendering index (CRI) = 78, Commission Internationale de L'Eclairage (CIE) coordinates of (0.353, 0.352)) compared with the control device containing sky‐blue:yellow‐orange emitters (CRI = 60, CIE coordinates of (0.293, 0.395)) due to the good spectral coverage by the deep‐blue emitter.
The highly efficient organic electronic devices achieved by PO-TAZ as an interfacial layer.
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