OLED Display Symmetrical Driving Circuit Gray Scale Mismatch
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Solution Overview
Problem
Existing organic light emitting diode (OLED) displays face challenges in accurately expressing gray scales due to characteristics discrepancies between programming and driving TFTs, leading to non-uniform brightness and reduced display quality.
Innovation Solution
The OLED display employs a configuration with symmetrical driving devices and switch TFTs to form parallel and series current paths, using a storage capacitor to maintain control voltage, and includes a sub capacitor to cut-off current paths, thereby reducing characteristics mismatch and improving gray scale expression.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional OLED display uses asymmetrical driving TFTs and programming TFTs, then the device complexity is reduced, but the gray scale expression ability deteriorates due to characteristics mismatch
Solution Approach 1:
The patent applies asymmetry principle by introducing a sub-capacitor connected between the gate of the driving TFT and the data line, which is not present in conventional symmetrical designs. This asymmetric configuration allows the sub-capacitor to selectively store voltage during programming period and release it during light emitting period, compensating for characteristics mismatch between driving TFT and programming TFT, thereby improving gray scale expression without significantly increasing overall device complexity
Solution Approach 2:
The sub-capacitor performs preliminary action by pre-storing the voltage difference between driving TFT gate and data line during the programming period. This pre-stored voltage is then released during the light emitting period to compensate for TFT characteristics mismatch, enabling accurate gray scale expression before the actual light emission occurs
2Productivity
If the channel width of programming TFT is increased to improve charging ability, then the charging efficiency improves, but the characteristics mismatch with driving TFT increases
Solution Approach 1:
The sub-capacitor acts as an intermediary element that mediates the characteristics mismatch between programming TFT and driving TFT. It captures the voltage difference caused by different channel widths during programming and releases it during light emitting, allowing the programming TFT to have larger channel width for faster charging while maintaining gray scale accuracy through the compensating action of the sub-capacitor
3Manufacturing precision
If digital type compensation method is used to overcome brightness non-uniformity, then the manufacturing precision requirement is reduced, but picture quality deteriorates due to flickering and false contour
Solution Approach 1:
The sub-capacitor serves as an intermediary that enables analog-type compensation functionality within a digital driving framework. By storing and releasing voltage to compensate for TFT characteristics variations, it eliminates the flickering and false contour issues associated with digital compensation methods while maintaining the manufacturing precision advantages of digital driving
Solution Approach 2:
The invention changes the operating parameters of the driving TFT by utilizing the voltage stored in the sub-capacitor. This voltage adjustment modifies the gate-source voltage of the driving TFT during light emitting period, effectively compensating for characteristics mismatch and improving picture quality without requiring higher manufacturing precision
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly reduces the mismatching ratio between TFTs, enhancing the ability to express gray scales and increasing display quality by improving the charging efficiency and contrast ratio.
Implementation Method 1
a storage capacitor that is connected between the first node and the third node
Implementation Method 2
If a drive voltage is applied to the anode electrode and the cathode electrode, holes in the hole injection layer HTL and electrons in the electron injection layer respectively move to the emission layer EML to excite the emission layer EML. And, as a result, the emission layer EML emits a visible light
Data Source
AI summary
An organic light emitting diode display includes a first driving device that includes a first control electrode supplied with a voltage from a first node, and switches a current path between a second node and a third node in accordance with a voltage of the first node; a second driving device that is connected to be symmetrical with the first driving device through the second node and the third node, and includes a second control electrode supplied with a voltage of the first node; a high-level driving voltage source that supplies a high-level driving voltage via the third node; an organic light emitting diode device that is connected between the second node and a ground voltage source; gate and data lines; first to third switch devices; a driving circuit that drives the first to third switch devices; and a storage capacitor connected between the first node and the third node.