Organic Light Emitting Device Driving Circuit with Reverse Bias
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Solution Overview
Problem
Existing image display apparatuses face brightness fluctuations and degraded contrast ratios due to current flowing through light emitting devices even at the threshold voltage of driving transistors, causing unwanted light emission at the initial stage of the light emission period.
Innovation Solution
A method of driving an image display apparatus that involves applying a reverse bias voltage to the light emitting unit and controlling the potential of the power source line to reduce initial current flow, utilizing pixel circuits with an organic light emitting device and a thin film transistor where the capacitance of the light emitting device is greater than the parasitic capacitance of the driving transistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a current-control type organic EL device is used to control brightness by setting current to an appropriate value, then brightness control is achieved, but brightness fluctuations occur due to changes in current flowing through the light emitting device
Solution Approach 1:
The patent implements a feedback mechanism by detecting the threshold voltage of the driving transistor and using this information to control the current flowing through the light emitting device. The pixel circuit includes a threshold voltage detecting transistor that detects the threshold voltage, and this detected value is used to adjust the driving current, thereby stabilizing brightness and compensating for transistor parameter variations.
2Reliability
If off current near the threshold voltage of the driving transistor is sufficiently reduced, then current flows through the light emitting device until the capacitance of the light emitting device and the parasitic capacitance of the pixel circuit are charged, but the light emitting device emits light at the initial stage of the light emission period, degrading the contrast ratio
Solution Approach 1:
The patent applies a reverse bias voltage to the light emitting device in the period between the write period and the light emission period. This preliminary action charges the capacitance of the light emitting device and the parasitic capacitance before the light emission period begins, preventing current flow and light emission at the initial stage of the light emission period, thereby improving the contrast ratio.
3Illumination intensity
If a reverse bias voltage is applied to the light emitting unit by changing potential of the power source line, then current flow at the initial stage is reduced, but additional control complexity is introduced
Solution Approach 1:
The patent utilizes the existing power source line that is already part of the pixel circuit structure to apply the reverse bias voltage. By changing the potential of this existing power source line, the patent achieves the function of charging the capacitance and preventing initial current flow without requiring completely separate additional control circuitry, thus minimizing added complexity.
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 approach effectively reduces current flow at low gray levels, thereby improving the contrast ratio of the image display apparatus by preventing unnecessary light emission at the initial stage of the light emission period.
Implementation Method 1
a current-control type organic EL (Electroluminescent) device having a function of emitting light through the recombination of electrons and positive holes injected into the light emitting layer
Data Source
AI summary
A method of driving an image display apparatus that includes a plurality of pixel circuits each provided with an organic light emitting device and a driving transistor that is electrically connected to the organic light emitting device and controls light emission of the organic light emitting device, includes: feeding the pixel circuits with an image signal corresponding to light emission luminance of the organic light emitting device; applying a reverse bias voltage to the organic light emitting device; and causing the organic light emitting device to emit light based on the image signal.