CONJUGATED polymers are organic semiconductors, the semiconducting behaviour being associated with the π molecular orbitals delocalized along the polymer chain. Their main advantage over non-polymeric organic semiconductors is the possibility of processing the polymer to form useful and robust structures. The response of the system to electronic excitation is nonlinear—the injection of an electron and a hole on the conjugated chain can lead to a self-localized excited state which can then decay radiatively, suggesting the possibility of using these materials in electroluminescent devices. We demonstrate here that poly(p-phenylene vinylene), prepared by way of a solution-processable precursor, can be used as the active element in a large-area light-emitting diode. The combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green–yellow part of the spectrum with reasonably high efficiency, suggest that the polymer can be used for the development of large-area light-emitting displays.

Light-emitting diodes based on conjugated polymers

The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This "meso-superstructured solar cell" exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite.

https://science.sciencemag.org/content/sci/early/2012/10/03/science.1228604.full.pdf

Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites

The efficiency of electroluminescent organic light-emitting devices1,2 can be improved by the introduction3 of a fluorescent dye. Energy transfer from the host to the dye occurs via excitons, but only the singlet spin states induce fluorescent emission; these represent a small fraction (about 25%) of the total excited-state population (the remainder are triplet states). Phosphorescent dyes, however, offer a means of achieving improved light-emission efficiencies, as emission may result from both singlet and triplet states. Here we report high-efficiency (≳90%) energy transfer from both singlet and triplet states, in a host material doped with the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP). Our doped electroluminescent devices generate saturated red emission with peak external and internal quantum efficiencies of 4% and 23%, respectively. The luminescent efficiencies attainable with phosphorescent dyes may lead to new applications for organic materials. Moreover, our work establishes the utility of PtOEP as a probe of triplet behaviour and energy transfer in organic solid-state systems.

Highly efficient phosphorescent emission from organic electroluminescent devices

Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

/pdf/highly-efficient-organic-light-emitting-diodes-from-delayed-31otvjgv66.pdf

Highly efficient organic light-emitting diodes from delayed fluorescence

Organic electroluminescent (EL) devices with multilayered thin‐film structures which emitted bright blue light were constructed. Two empirical guides for the selection of blue‐emitting materials were established. The keys to obtain the EL cells with high EL efficiency were excellent film‐forming capability of an emitter layer and the appropriate combinations of emitter and carrier transport materials for avoiding the formation of exciplexes. In one of our organic electroluminescent devices, blue emission with a luminance of 700 cd/m2 was achieved at a current density of 100 mA/cm2 and a dc drive voltage of 10 V.

/pdf/blue-light-emitting-organic-electroluminescent-devices-15vtjoyiw8.pdf

Blue light‐emitting organic electroluminescent devices

We have succeeded in fabricating a novel thin‐film electroluminescent device with a luminescent hole transport layer as an emitter. The cell structure is composed of an indium‐tin‐oxide substrate, a luminescent hole transport layer (emitter), an electron transport layer, and a MgAg electrode. The most essential feature of our device owes for adoption of an oxadiazole derivative as an electron transport layer. The emission intensity of 1000 cd/m2 was achieved at a current of 100 mA/cm2.

/pdf/organic-electroluminescent-device-having-a-hole-conductor-as-hbu6l1sa6a.pdf

Organic electroluminescent device having a hole conductor as an emitting layer

Using the combination of a layered perovskite compound (C6H5C2H4NH3)2PbI4 (PAPI), which forms a stable exciton with a large binding energy owing to its low‐dimensional semiconductor nature and exhibits sharp and strong photoluminescence from the exciton band, and an electron‐transporting oxadiazole derivative, we fabricated an organic–inorganic heterostructure electroluminescent (EL) device. The EL spectrum of the device corresponded well to the photoluminescence spectrum of the PAPI film; the emission was peaking at 520 nm and half‐width of the emission was about 10 nm at liquid‐nitrogen temperature. Further, highly intense EL of more than 10 000 cd m−2 was performed at 2 A cm−2 at liquid‐nitrogen temperature in the device.

Organic‐inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH3)2PbI4

Multilayer organic light-emitting devices with phosphorescent guest emitter, tris(2-phenylpyridine) iridium doped in a host 4,4'-N,N'-dicarbazol-biphenyl layer were prepared. The device with the 6.5 wt% guest emitter exhibited external quantum efficiency and power luminous efficiency of 13.7% and 38.3 lm/W, respectively at the luminance of 105 cd/m2 driven at the voltage of 4.0 V and current density of 0.215 mA/cm2. The half decay lifetime under continuous constant-current driving for the initial luminance of 500 cd/m2 was 170 h.

https://iopscience.iop.org/article/10.1143/JJAP.38.L1502/pdf

High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center

We have succeeded in the fabrication of a stable organic electroluminescent (EL) device with a three-layer structure; hole transport layer/emitting layer/electron transport layer. The EL device was prepared by vacuum evaporation. Efficient carrier double injection into the emitting layer was realized by the use of separate hole and electron transport layers. Bright EL emission was observed in a darkened room at the dc bias voltage of 50 V. Stable emission lasted for more than five hours at this condition. The emission spectrum could be changed with variation of the organic material for the emitting layer.

https://iopscience.iop.org/article/10.1143/JJAP.27.L269/pdf

Tetsuo Tsutsui

Papers

Blue light‐emitting organic electroluminescent devices

Organic electroluminescent device having a hole conductor as an emitting layer

Organic‐inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH3)2PbI4

High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center

Electroluminescence in Organic Films with Three-Layer Structure