Organic EL Driving Method for Charge Discharge and Lifetime Extension
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Solution Overview
Problem
Organic EL devices suffer from deterioration due to charge imbalance and stored charge, leading to reduced luminance and increased drive voltage, with existing AC and pulse voltage methods either lowering light efficiency or inadequately discharging stored charge.
Innovation Solution
Applying alternating positive and negative voltages smaller than the built-in-voltage, with a frequency lower than the device's response speed, during the off-state to efficiently discharge stored charge and prevent deterioration, while maintaining high light emission efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If DC voltage is applied to drive the organic EL device, then light emission is achieved, but charge accumulates in the organic layer causing deterioration and reduced lifetime
Solution Approach 1:
The patent applies periodic AC voltage to the organic EL device, alternating between forward bias (for light emission) and reverse bias (for charge discharge). This periodic switching prevents charge accumulation by regularly discharging stored charges during the reverse bias phase, thereby extending device lifetime while maintaining light emission during the forward bias phase.
Solution Approach 2:
The patent employs reverse bias voltage applied to the organic EL device to discharge accumulated charges. By inverting the voltage polarity periodically, the device enters a state where charges are extracted from the organic layer rather than accumulated, counteracting the deterioration caused by charge buildup during forward operation.
2Reliability
If AC voltage is applied to discharge stored charge, then device lifetime is extended, but light emission efficiency decreases
Solution Approach 1:
The patent uses periodic AC voltage with optimized duty cycle, where the forward bias phase (for light emission) occupies a larger portion of the cycle than the reverse bias phase (for charge discharge). This ensures that light emission efficiency is maintained by maximizing the time spent in the light-emitting state while still providing sufficient reverse bias time to discharge charges and extend device lifetime.
3Reliability
If high frequency pulse voltage is applied, then charge discharge is enhanced, but response time requirements increase device complexity
Solution Approach 1:
The patent optimizes the frequency parameter of the applied AC voltage to match the response characteristics of the organic EL device. By selecting a frequency that is high enough to effectively discharge charges but not excessively high, the patent achieves good charge discharge efficiency without imposing unrealistic response time requirements that would increase device 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 method effectively suppresses organic layer deterioration, maintains luminance, and prolongs the lifetime of the organic EL device by ensuring efficient charge discharge without lowering light emission efficiency.
Implementation Method 1
An organic EL device, in general, has the structure in which an organic thin film containing a light emitting layer is sandwiched by an anode and a cathode, and by applying a DC voltage to the organic EL film, holes are injected from the anode and electrons are injected from the cathode thus emitting light.
Implementation Method 2
the present invention focuses on a method of applying an AC voltage as a means to apply the inverse bias to the organic EL device for discharging the stored charge
Data Source
AI summary
According to one aspect of the present invention, it is possible to sufficiently perform the discharging of charge without lowering the light emitting efficiency of an organic EL device and hence, the device can exhibit the light emitting efficiency higher than a conventional organic EL device and, at the same time, can prevent the degradation of the device. As an organic EL device to which the present invention is applied, on a glass transparent substrate, a transparent electrode, a hole injection layer and a hole transport layer which function as a hole transport function layer, a light emitting layer, an electron transport function layer, and a metal electrode are formed sequentially, and a drive power sources are connected to the transparent electrode and the metal electrode. Further, from the drive power source, as an applying voltage, a voltage which is obtained by overlapping any one of a sine wave, a pulse wave, a triangle wave and a sawtooth wave having two cycles or more to a drive signal or a voltage which is obtained by overlapping a sine wave having two cycles or more to the drive signal is supplied.