This review summarizes the recent research advances of biphenyl derivatives with charge transport properties and high luminescence efficiency. Approximately 20 biphenyl derivatives are introduced, including molecular design strategies and related optoelectronic properties and their application in optoelectronic devices. This review on organic conductive materials based on biphenyl derivatives with high luminous efficiency provides meaningful guidance and a material foundation for the development of organic electrically pumped lasers. This paper also briefly reviews challenges, development directions, and opportunities for the future development of the field.

Progress Organic electroluminescence is a process of directly converting electrical energy into light. It occurs in organic materials where they emit light under the action of an applied electric field or current. Organic electroluminescent devices are mainly divided into two categories: organic light-emitting diodes (OLEDs) and organic light-emitting transistors (OLETs). In 1963, Pope et al. reported the preparation of the first OLED using anthracene single crystals; however, electroluminescence was only observed in anthracene single crystals with a thicknesses of 10-20 μm when the driving voltage was as high as 400 V.

The construction of high-performance devices requires light-emitting materials with high mobility. In numerous literature reports, we have found that biphenyl derivatives serve as classical charge transport layers and have excellent light-emitting properties and large optical gain at the same time. In 2010, Adachi et al. reported a new type of non-heterostructure OLED device, in which BSBCz molecules are used as the emission layer (its HOMO and LUMO energy levels were 3.1 eV and 5.8 eV, respectively). They introduced two buffer layers at the ends of the emission layer with MoO₃ and the metal cesium-doped BSBCz film acts as hole and electron transport interlayers. The addition of the buffer layers effectively reduces the carrier injection barrier, thereby suppressing the roll-off of electroluminescence efficiency at high current density.

OLET is an integrated device that has both the switching function of an organic field effect transistor (OFET) and the light-emitting function of OLED, which can avoid complex OFET gate voltage switching to drive OLED. This can greatly simplify device fabrication and be easily integrated into display circuits. Recently, Yin et al. develop a novel 4,4'-bis (2-dibenzothiophenylvinyl)-bipheny (DBTVB) molecule based on extending the biphenyl skeleton. The introduction of dibenzothiophene groups not only extends the conjugated chain but also provides the space for the rotation of the molecular conformation to ensure the planarity of the molecule. The single crystal of this molecule exhibits a fluorescence quantum yield of up to 85% and excellent bipolar transport properties with hole and electron mobilities of 3.55 and 2.37 cm²‧V^-1‧s^-1. Simultaneously, this single crystal also exhibits excellent laser properties. A DBTVB-based OLET was fabricated with Au/MoO₃-Ca/CsF as an asymmetric electrode. The external quantum efficiency of the device is as high as 4.03%, which is the best OLET device reported so far.

Because of the rod-shaped molecular structure, the molecules tend to form a herringbone arrangement during the self-assembly process, which can effectively reduce the fluorescence quenching caused by the intermolecular π—π interaction and ensure the efficient transport of carriers. Biphenyl derivatives usually have a large Stokes shift, large transition dipole moment, and excited state oscillator strength, which ensure their good luminescence properties. Therefore, biphenyl derivatives have inherent advantages as laser gain materials. As a typical biphenyl derivative, the 4,4'-bis [(N-carbazole) styryl] biphenyl (BSBCz) molecule has very low intersystem crossing efficiency. Therefore, in the presence of oxygen, the triplet excitons generated by optical pumping are completely quenched and the triplet excited state absorption (TA) in the laser gain wavelength range is almost negligible. These advantages make it a strong candidate for realizing a continuous-wave optically pumped laser. Ren et al. reported an exciton-polariton laser based on the optical microcavity consisting of a 4,4'-bis [4-(di-p-tolylamino)styryl] biphenyl (DPAVBi) single-crystal sandwich in two layers of silver films. Such exciton polaritons can form stable macroscopic condensations at room temperature and generate lasing emission with good spatial coherence.

Conclusions and Prospects The pursuit of organic electrically pumped lasers is a long-term goal that requires the joint effort of scientists in the fields of chemistry, materials science, device physics, and engineering. From the perspective of materials science, developing materials with high mobility and high luminescence properties is important for realizing organic electric pumped lasers. In this review, we summarize the reported applications of biphenyl derivatives in photoelectric devices, including OLEDs, OLETs, organic micro/nano-lasers, organic polariton laser, and organic electrically pumped lasers. The excellent photoelectric properties of these materials make them potential candidates for electrically pumped lasers.

We believe that organic electrically pumped laser will be realized soon, following more and more breakthroughs in this field. The organic electrically pumped lasers can bridge the gap between current microelectronic and nanophotonic circuits to revolutionize laser displays, precision medicine, wearable devices, and information and communication technologies.

Organic semiconductors, which combine high-efficiency light-emission and charge-transport properties, are ideal candidates for realizing organic electrically pumped lasers. However, there are still many challenges in the design and synthesis of materials with such properties. This is because of the intrinsic contradiction between high carrier transport and efficient solid-state emission efficiency in organic solids. High carrier transport requires intense molecular packing and strong intermolecular interactions; however, such interactions can significantly reduce solid-state luminescence efficiency.