Switching Transistor Disconnects OLED From Reverse Bias
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Solution Overview
Problem
In active-matrix-type display apparatuses using organic EL devices, variations in threshold voltage and mobility of thin-film transistors lead to non-uniform brightness across pixels, and existing correction methods can damage the light-emitting devices by applying reverse bias during non-light emission periods.
Innovation Solution
The display apparatus incorporates a switching transistor between the output node and the light-emitting device, disconnecting it during non-light-emission periods to prevent reverse bias, and includes threshold-voltage and mobility correction mechanisms using auxiliary capacitors and transistors to maintain uniform brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If threshold voltage and mobility correction operations are performed by applying reverse bias to the light-emitting device, then screen uniformity is improved, but the light-emitting device may be damaged
Solution Approach 1:
A switching transistor is introduced as an intermediary component between the output node and the light-emitting device. This switching transistor acts as a mediator that controls the connection state: during light-emission periods, it connects the output node to the light-emitting device to supply driving current, while during non-light-emission periods, it disconnects the light-emitting device from the output node to prevent reverse bias damage. This intermediary component enables the correction operations to be performed without directly exposing the light-emitting device to harmful reverse bias voltages.
Solution Approach 2:
The circuit configuration is made dynamic by introducing the switching transistor that can change its state between connected and disconnected based on the operational period. The switching transistor responds to scanning line signals to dynamically adjust the circuit topology: in the light-emission period, the switching transistor is in the connected state to enable current flow to the light-emitting device, while in non-light-emission periods, it transitions to the disconnected state to isolate the light-emitting device from correction operations. This dynamic reconfiguration allows the system to perform both correction operations and light emission without compromising the light-emitting device.
2Reliability
If a switching transistor is added to disconnect the light-emitting device during non-light-emission periods, then reverse bias damage is prevented, but device complexity increases
Solution Approach 1:
The switching transistor is integrated into the existing pixel circuit structure and serves multiple functions: it acts as a protection device during non-light-emission periods to prevent reverse bias damage, and as a control device during light-emission periods to enable current flow to the light-emitting device. By making this single component multi-functional, the patent avoids adding separate dedicated protection circuits, thereby limiting the increase in device complexity while achieving reliable protection of the light-emitting device.
3Device complexity
If correction operations are performed without disconnecting the light-emitting device, then device complexity is reduced, but the light-emitting device may be damaged by reverse bias
Solution Approach 1:
The switching transistor serves as a protective intermediary that allows correction operations to be performed on the pixel circuit without directly exposing the light-emitting device to harmful reverse bias voltages. During non-light-emission periods, the switching transistor is disconnected to isolate the light-emitting device from the correction operations, enabling the system to perform necessary corrections while maintaining light-emitting device safety. This intermediary approach balances the need for correction operations with the need to protect the light-emitting device.
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 ensures high screen uniformity, prevents damage to light-emitting devices, and improves yield by avoiding reverse bias during non-light-emission periods, thus maintaining consistent brightness and extending device lifespan.
Implementation Method 1
the switching transistor is turned off to disconnect the light-emitting device EL from the output node S, so that a potential generated at the output node S due to an operation of the pixel 2 performed in the non-light-emission period is prevented from being applied as a reverse-bias voltage to the light-emitting device EL
Implementation Method 2
The driving transistor T2 receives as a gate voltage Vgs, at the gate thereof, the video signal written to the holding capacitor C1, and causes a drain current Ids to flow to the light-emitting device EL. Accordingly, the light-emitting device EL emits light at a brightness corresponding to the video signal
Implementation Method 3
The sampling transistor T1 is brought into conduction in accordance with a control signal supplied from the write scanner 4, and samples a video signal supplied from the signal line SL to write the video signal to the holding capacitor C1
Data Source
AI summary
A sampling transistor T1 is brought into conduction in accordance with a control signal supplied from a scanning line WS, and writes to a holding capacitor C1 a video signal supplied from a signal line SL. A driving transistor T2 outputs a driving current to an output node S in accordance with a signal potential of the video signal written to the holding capacitor C1. A switching transistor T3 is arranged between the output node S and a light-emitting device EL. In a predetermined light-emission period, the switching transistor T3 is in an on-state, and supplies the driving current to the light-emitting device EL to cause the light-emitting device EL to emit light at a brightness corresponding to the video signal. In contrast, in a non-light-emission period, the switching transistor T3 is turned off to disconnect the light-emitting device EL from the output node S, so that a potential generated at the output node S due to an operation of a pixel 2 performed in the non-light-emission period is prevented from being applied as a reverse-bias voltage to the light-emitting device EL of a diode type.