The field of power harvesting has experienced significant growth over the past few years due to the ever-increasing desire to produce portable and wireless electronics with extended lifespans. Current portable and wireless devices must be designed to include electrochemical batteries as the power source. The use of batteries can be troublesome due to their limited lifespan, thus necessitating their periodic replacement. In the case of wireless sensors that are to be placed in remote locations, the sensor must be easily accessible or of a disposable nature to allow the device to function over extended periods of time. Energy scavenging devices are designed to capture the ambient energy surrounding the electronics and convert it into usable electrical energy. The concept of power harvesting works towards developing self-powered devices that do not require replaceable power supplies. A number of sources of harvestable ambient energy exist, including waste heat, vibration, electromagnetic waves, wind, flowing water, and solar energy. While each of these sources of energy can be effectively used to power remote sensors, the structural and biological communities have placed an emphasis on scavenging vibrational energy with piezoelectric materials. This article will review recent literature in the field of power harvesting and present the current state of power harvesting in its drive to create completely self-powered devices.

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A review of power harvesting using piezoelectric materials (2003–2006)

Polymer based MEMS and microfluidic devices have the advantages of mechanical flexibility, lower fabrication cost and faster processing over silicon based ones. Also, many polymer materials are considered biocompatible and can be used in biological applications. A valuable class of polymers for microfabricated devices is piezoelectric functional polymers. In addition to the normal advantages of polymers, piezoelectric polymers can be directly used as an active material in different transduction applications. This paper gives an overview of piezoelectric polymers based on their operating principle. This includes three main categories: bulk piezoelectric polymers, piezocomposites and voided charged polymers. State-of-the-art piezopolymers of each category are presented with a focus on fabrication techniques and material properties. A comparison between the different piezoelectric polymers and common inorganic piezoelectric materials (PZT, ZnO, AlN and PMN?PT) is also provided in terms of piezoelectric properties. The use of piezopolymers in different electromechanical devices is also presented. This includes tactile sensors, energy harvesters, acoustic transducers and inertial sensors.

https://iopscience.iop.org/article/10.1088/0964-1726/23/3/033001/pdf

A review of piezoelectric polymers as functional materials for electromechanical transducers

This paper presents a state-of-the-art review on a hot topic in the literature, i.e., vibration based energy harvesting techniques, including theory, modelling methods and the realizations of the piezoelectric, electromagnetic and electrostatic approaches. To minimize the requirement of external power source and maintenance for electric devices such as wireless sensor networks, the energy harvesting technique based on vibrations has been a dynamic field of studying interest over past years. One important limitation of existing energy harvesting techniques is that the power output performance is seriously subject to the resonant frequencies of ambient vibrations, which are often random and broadband. To solve this problem, researchers have concentrated on developing efficient energy harvesters by adopting new materials and optimising the harvesting devices. Particularly, among these approaches, different types of energy harvesters have been designed with consideration of nonlinear characteristics so that the frequency bandwidth for effective energy harvesting of energy harvesters can be broadened. This paper reviews three main and important vibration-to-electricity conversion mechanisms, their design theory or methods and potential applications in the literature. As one of important factors to estimate the power output performance, the energy conversion efficiency of different conversion mechanisms is also summarised. Finally, the challenging issues based on the existing methods and future requirement of energy harvesting are discussed.

A comprehensive review on vibration energy harvesting: Modelling and realization

Materials that exhibit thermally activated delayed fluorescence are promising for the realization of efficient organic light-emitting diodes. However, finding suitable deep-blue thermally activated delayed fluorescence materials is still challenging. Here, we report two highly efficient deep-blue thermally activated delayed fluorescence emitters, TDBA–Ac and TDBA–DI, containing oxygen-bridged, symmetric and rigid boron acceptor moieties. Both emitters have been designed to have high photoluminescence quantum yield and narrow-band blue emission. TDBA–Ac and TDBA–DI exhibited deep-blue emission and a small singlet–triplet energy gap of 0.06 eV and 0.11 eV, respectively, in toluene. The 20wt%-doped films of TDBA–Ac and TDBA–DI in DBFPO host exhibited high photoluminescence quantum yields of 93% and 99%, respectively. The fabricated TDBA–DI device showed an extremely high external quantum efficiency of 38.15 ± 0.42% in the blue region with low roll-off characteristics of 25.2% at high luminance of up to 5,000 cd m–2. The TDBA–Ac-doped device exhibited a high external quantum efficiency of 21.50 ± 0.22% with deep-blue colour coordinates of (0.15, 0.06). The discovery of two deep-blue organic emitters of light could aid the development of next-generation organic light-emitting devices.

Highly efficient blue thermally activated delayed fluorescence emitters based on symmetrical and rigid oxygen-bridged boron acceptors

The recent progress of efficiency improvement, emission color tuning, and lifetime elongation of blue organic light-emitting diodes (OLEDs) is reviewed. The latter is one of the most important bottlenecks for OLED development. The current status of blue light-emitting material design with emission mechanisms such as fluorescence (F), phosphorescence (Ph), thermally activated delayed fluorescence (TADF), and hybridized local and charge transfer (HLCT) is introduced in the first part of this review. Compared to red and green devices, the long exciton lifetime of the high energy triplet exciton in a blue OLED is the one of the main issues. To avoid the accumulation of high energy triplet excitons in the emitter for blue OLEDs, assisted triplet–triplet fluorescence (TTF) and Hyperfluorescence™ are employed to harvest the triplet excitons. In the second part of this review, we focus on issues from an application viewpoint: what are the requirements of blue OLEDs for display and lighting technologies in terms of efficiency, color, and lifetime? Key performance metrics of blue OLEDs with different technologies over time are summarized. Independent of technology, the trend is similar: the external quantum efficiency improves for the first stage of research, followed by color tuning, and then finally lifetime elongation. The state-of-the-art device performance of blue OLEDs with various emission mechanisms is illustrated. Although Ph- and TADF-emission based devices show satisfactory efficiency and electroluminescence (EL) spectra, despite having a lower efficiency TTF-emission based devices are the mainstream for real applications due to their relatively long operation lifetime. Blue Ph-OLEDs have the potential for lighting applications with suitable material selection and device design. We collected the published results and tried our best to make a fair comparison of the operation lifetime among different technologies. Finally, we discuss the possible future outlook from different viewpoints including new materials, device designs, and applications of blue OLEDs.

Blue organic light-emitting diodes: current status, challenges, and future outlook

A novel blue thermally activated delayed fluorescence (TADF) organic light-emitting diode with an emitting layer made up of a TADF assistant dopant and a pure blue-emitting TADF emitter was developed to demonstrate a pure blue color, a high external quantum efficiency, suppressed efficiency roll-off, and an improved lifetime. Two fused B–N type blue TADF emitters with a narrow emission spectrum were used as the blue emitters and the narrow blue emission was harvested by a TADF assistant dopant through a reverse intersystem crossing mediated cascade energy transfer process. A high external quantum efficiency of 31.4%, pure blue emission color of (0.13, 0.15), significant reduction of the efficiency roll-off and more than 10 times lifetime extension were simultaneously achieved using the TADF assisted TADF process.

Ideal blue thermally activated delayed fluorescence emission assisted by a thermally activated delayed fluorescence assistant dopant through a fast reverse intersystem crossing mediated cascade energy transfer process

High Efficiency Deep‐Blue Phosphorescent Organic Light‐Emitting Diodes with CIE x, y (≤ 0.15) and Low Efficiency Roll‐Off by Employing a High Triplet Energy Bipolar Host Material

The development of mechanochromic materials by incorporating aggregation induced emission (AIE) luminogens has gained momentum owing to their wide range of applications in optoelectronics In this work, the authors contribute to the development of AIE active mechanochromic phenanthroimidazole derivatives The positional isomers of phenanthroimidazoles 1 and 2 were designed to study the effect of the position of triphenylamine (TPA) and tetraphenylethylene (TPE) units on their AIE and mechanochromism Phenanthroimidazoles 1 and 2 were synthesized using the Suzuki cross-coupling reaction of TPE boronate ester with iodo-phenanthroimidazole of TPA and bromo-phenanthroimidazole of TPA, respectively The single crystal X-ray analysis of 1 reveals a propeller orientation of multiple phenyl rings of the TPA and TPE units, confirming strong AIE characteristics of 1 and 2 Phenanthroimidazoles 1 and 2 exhibit reversible mechanochromism between blue and green colours, which was studied using powder X-ray diffraction (PXRD) The PXRD studies suggest that a phase transition from a crystalline state to an amorphous state is associated with the colour change Moreover, phenanthroimidazoles 1 and 2 worked efficiently as non-doped emitters based on the AIE character and provided a high external quantum efficiency of 28 and 40%, respectively

Stimuli responsive AIE active positional isomers of phenanthroimidazole as non-doped emitters in OLEDs

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Nearly 100% Horizontal Dipole Orientation and Upconversion Efficiency in Blue Thermally Activated Delayed Fluorescent Emitters

This study shows a molecular design strategy for controlling the dihedral angle of two carbazole donors linked to a 2,4-diphenyl-1,3,5-triazine acceptor by a phenyl unit. Using this approach, six thermally activated delayed fluorescence emitters were synthesized with donors placed in various positions around a central phenyl core, and the photophysical relationship between the donor position and its dihedral angle was investigated. We demonstrate that this angle can affect both the strength of the charge transfer state and the conjugation across the entire molecule, effectively changing the singlet–triplet energy gap of the system. We conclude that materials containing two substituted −ortho donors or one −ortho and an adjacent −meta have the smallest energy gaps and the shortest delayed fluorescence lifetimes. On the other hand, emitters with no −ortho substituted donors have larger energy gaps and slow-to-negligible delayed fluorescence. When applying these materials to organic light-emitting diodes, th...

Dihedral Angle Control of Blue Thermally Activated Delayed Fluorescent Emitters through Donor Substitution Position for Efficient Reverse Intersystem Crossing.

Si Hyun Han

Papers

Ideal blue thermally activated delayed fluorescence emission assisted by a thermally activated delayed fluorescence assistant dopant through a fast reverse intersystem crossing mediated cascade energy transfer process

High Efficiency Deep‐Blue Phosphorescent Organic Light‐Emitting Diodes with CIE x, y (≤ 0.15) and Low Efficiency Roll‐Off by Employing a High Triplet Energy Bipolar Host Material

Stimuli responsive AIE active positional isomers of phenanthroimidazole as non-doped emitters in OLEDs

Nearly 100% Horizontal Dipole Orientation and Upconversion Efficiency in Blue Thermally Activated Delayed Fluorescent Emitters

Dihedral Angle Control of Blue Thermally Activated Delayed Fluorescent Emitters through Donor Substitution Position for Efficient Reverse Intersystem Crossing.