Liquid crystal (LC) is a kind of functional soft material that can form an orderly arrangement through self-organization at the molecular and supramolecular level. The factors that affect the formation of LC mainly depend on the molecular structure and the interaction between molecules. LC materials and semiconductor materials are intersecting fields. LC materials formed by molecules with aromatic π-conjugated centers will have a dominant position in intrinsic electronic conductance and higher carrier mobility, forming a LC material with semiconductor properties. In this type of material, the highly arranged order of LC molecules makes it have a relatively high electron and hole mobility that does not change with the external electric field and temperature. At the same time, due to the self-alignment ability of LC molecules, it is easy to fabricate a thin film device with a high degree of molecular order in a large area.
Figure 1. The formation of liquid crystal materials (from disorder to order, P and L represent molecules)
- Organic photovoltaic devices: Utilizing the high orderliness of LC materials, applications in organic photovoltaic devices can be divided into two aspects. On the one hand, the LC itself acts as a semiconductor material, that is, an electron donor or acceptor. On the other hand, the doping of LC molecules improves the crystalline phase of the organic thin film, thereby increasing the efficiency of exciton generation and dissociation, and at the same time facilitates the transport of electrons and holes, and ultimately achieves the goal of improving the performance of organic photovoltaic devices. Discotic LC materials (It is composed of disk-shaped molecules stacked in large rings and planar disks, with the axis perpendicular to the molecular plane) are used as electron donors or acceptors to facilitate the separate transmission of electrons and holes, forming a higher open circuit voltage, and improving energy conversion efficiency. As additives, liquid crystalline substances have extremely strong self-assembly capabilities and can form ordered LC phase microstructures under certain conditions. Use the good morphology control ability of LC materials to control the heterojunction interface and microstructure of organic photovoltaic devices. Not only can the phase separation of the donor-acceptor material on the nanoscale be achieved, but also the order of the entire active layer structure can be improved, thereby facilitating exciton separation, charge transfer and transport, and thereby improving the photoelectric conversion efficiency of the device.
- Display device: The LC material display uses the arrangement direction of the LC molecules to change under the action of an electric field, thereby changing the transmittance to complete the color reproduction in the time domain and the space domain. The LC display doped with semiconductor nano-particles uses nano-level semiconductor particles, which are uniformly dispersed in the LC material and interact with the LC molecules to achieve the purpose of improving the characteristics of the LC display device. Nanoparticles can help the LC molecules to form a vertical alignment effect on the surface of the untreated glass substrate, and can make the LC device avoid the use of an alignment layer, so that it can be applied to a flexible display based on a plastic substrate prepared by a normal temperature process.
- Ma Heng, Jiang Lu-lu, Li Meng, Li Chen-xi. Research Progress of Liquid Crystal Material Applied in Organic Photovoltaic Devices [J]. Chinese Journal of Liquid Crystals and Displays, 2013, 28(5), 653.