Vertical-field-driven blue-phase liquid crystals for higher brightness

Blue-phase liquid crystals (BPLCs) have attracted much attention as a next-generation display technology owing to their substantial advantages in performance and fabrication.^1–6 For example, the gray-to-gray (midtone) response time of BPLCs is much faster than that of nematic (conventional) liquid crystal displays, as is the refresh rate, for enhanced, more natural viewing. These new devices also enable a much wider viewing angle than previously possible as a result of their optical properties. Finally, BPLCs have no alignment layer, which obviates a rubbing process that went with it, reducing the number of manufacturing steps.⁷

In general, BPLC modes are operated by in-plane-switching (IPS) schemes to obtain grayscale properties because the electro-optic Kerr effect causes field-induced retardation to increase along the field direction. However, transmittance is limited by the low aperture ratio of the patterned electrodes. In addition, in conventional IPS BPLC modes, both the intensity of the electric field and field-induced retardation are sharply decreased near the top substrate. Consequently, they suffer from high driving voltage, low brightness, and hysteresis switching (i.e., grayscale variation) due to the locally high electric field as well as the low optical transmittance.^8–10 In the case of the polymer-stabilized BPLC (PS-BPLC) mode, which is used for room-temperature operation, the issues of driving voltage and hysteresis switching worsen due to the polymer network.

Here, we report a vertical-field-driven PS-BPLC (VFD PS-BPLC) mode that solves the problems typical of the IPS BPLC modes.^{11, 12} Figure 1 shows in schematic from our proposed VFD PS-BPLC cell, in which two prism sheets are attached to the bottom and top substrates. In the field-off state, the VFD PS-BPLC cell shows the ideal dark state under the crossed polarizers because the BPLC layer is optically isotropic. In the field-on state, after passing through the bottom prism sheet, the oblique incident beam undergoes phase retardation of the BPLC layer, although the cell is the C-plate (retarder with out-of-plane optical axis) for the normally incident beam. To enhance the brightness at normal viewing, the top prism sheet is attached to the top glass substrate using structures that are identical to those of the bottom prism sheet, which redirects the oblique rays to the normal ones.

Figure 1. Cross-sectional view of the structure of a vertical-field-driven polymer-stabilized blue-phase liquid cell (PS-BPLC) with two prism sheets. Left: Field-off state (the left side). Right: Field-on state. The inset shows a scanning electron microscopy image of the prism sheets. : Groove vector of prism sheets.: Indicates parallel K and x directions. θ_i: Angle of the incident beam after passing through the prism sheet. θ_p: Basic angle of the prism sheet. n_e, n_o: Extraordinary and no refraction index, respectively.

Figure 2 shows the transmittance curves of the VFD PS-BPLC mode (the cell gap between the top and the bottom substrates is 10μm) and the conventional IPS PS-BPLC mode (the spacing between the IPS electrodes is 3μm). Before the applied voltage is increased to 40V, the transmittance of the IPS PS-BPLC cell shows lower transmittance of ∼23%. The reason is that the field-induced birefringence is still low due to weak electric fields near the top substrate and the low aperture ratio (∼43%). However, the VFD PS-BPLC cell with two prism sheets shows higher transmittance of 56% at the same applied voltage, which means that the oblique incident beam experiences sufficient effective retardation due to the uniform vertical field. The high aperture ratio obtained by removing the patterned electrodes is a second reason for the higher transmittance.

Figure 2. Transmittance and hysteresis measurements of vertical-field-driven PS-BPLC cell with two prism sheets (θ_p=64^°), and those of a conventional IPS PS-BPLC cell. ΔV_IPS: Hysteresis voltage of IPS mode (peak is 40V).

When we compare the hysteresis properties of the two samples, the VFD PS-BPLC cell shows hysteresis-free switching grayscale behavior until the applied voltage reaches 40V. However, the IPS PS-BPLC cell shows severe hysteresis despite lower transmittance. To obtain hysteresis-free switching in the IPS PS-BPLC mode, the available maximum voltage must be 12V and the transmittance 3%. High applied voltages are need to exploit the BPLC molecules near the top substrate by the Kerr effect, which inevitably generates extremely high electric fields as well as severe lattice deformation of the BPLC structure near the bottom substrate.

In summary, we have demonstrated a VFD PS-BPLC device with two prism sheets that solves the low-aperture-ratio and high-driving-voltage problems of conventional IPS PS-BPLCs. After optimizing the BPLC material and the prism sheet, we plan to further improve the brightness and operating voltage using the proposed switching scheme.