A touch screen with a touch function has increasingly become a mainstream technology in the display field due to its simple, light and thin structure as well as low production cost.
The touch screens mainly include capacitive touch screen, resistive touch screen, optical touch screen, and so on. Among them, the capacitive touch screen is a relatively common and universal one.
The working principle of the capacitive touch screen will be briefly described hereinafter in conjunction with the structure thereof.
FIG. 1 is a top view of a capacitive touch screen. Referring to FIG. 1, the capacitive touch screen comprises a plurality of touch-driving electrodes 800 arranged along a first direction and a plurality of touch-sensing electrodes 900 arranged along a second direction. The first and second directions are perpendicular to each other. The adjacent touch-driving electrode 800 and the touch-sensing electrode 900 are coupled to each other so as to generate mutual capacitance Cm. When a finger touches the screen, an equivalent electric field of the finger will change a value of the mutual capacitance Cm. A touch detecting unit for detecting a touch point detects a variation of the current corresponding to the mutual capacitance Cm at the touch point before and after it is touched by the finger, and thereby detects a position of the touch point.
FIG. 2 is a sectional view of the touch-driving electrode 800 and the touch-sensing electrode 900 as shown in FIG. 1. Referring to FIG. 2, a voltage is applied to the touch-driving electrode 800 and the touch-sensing electrode 900 when the touch screen is touched, so as to form an electric field therebetween. The line with an arrow in FIG. 2 represents an electric field line. The electric field formed between the touch-driving electrode 800 and the touch-sensing electrode 900 consists of two portions, i.e., a forward electric field formed between the opposite, overlapping surfaces of the touch-driving electrode 800 and the touch-sensing electrode 900, and a projection electric field formed between the non-overlapping surfaces. The capacitance between the touch-driving electrode and the touch-sensing electrode corresponding to the forward electric field is parasitic capacitance C1 that has no beneficial effect on the touch, and the capacitance C0 between the touch-driving electrode and the touch-sensing electrode corresponding to the projection electric field is capacitance that can be changed by the finger. Cm is approximately equal to C0+C1. The finger can change the value of the mutual capacitance Cm merely by changing the electric field projected to outside of the touch-driving electrode 800 and the touch-sensing electrode 900.
In terms of structure, the touch screens at least include Add-on touch screen and In-Cell/On-Cell touch screen.
Regardless of the Add-on touch screen or the In-Cell/On-Cell touch screen, when a ratio of the forward electric field to the projection electric field is big, the variation of the current or voltage, corresponding to the capacitance Cm detected by the touch detecting unit at the touch point before and after it is touched by the finger, is not obvious, and the touch effect is poor. In order to ensure a relatively big variation of the current or voltage corresponding to the capacitance Cm at the touch point before and after the touching and to obtain a better touch effect, it is required to increase a surface area of at least one of the touch-sensing electrode and the touch-driving electrode. Increasing the surface area of the touch-sensing electrode and/or the touch-driving electrode will result in an increase in an area of a vertically overlapping surface between the touch-sensing electrode and the touch-driving electrode as well as an increase in the parasitic capacitance therebetween, thereby it is difficult to reduce the ratio of the forward electric field to the projection electric field and improve the touch effect. When the touch-sensing electrode and the touch-driving electrode are built in the display screen, there is also big parasitic capacitance between the touch-sensing electrode and the touch-driving electrode with large areas, and the conductive, functional film layers in the display screen such as a gate line, a data line and a common electrode. In addition, the quality of the image displayed on the display screen will be lowered.