OLED Light Emitting Device with TFT Gate-Source Voltage Control
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Solution Overview
Problem
Current optical write-in devices using OLEDs face challenges in maintaining consistent light quantity due to variations in the Vsd-Id characteristic of thin film transistors and forward direction voltage of OLEDs, leading to unevenness in electrostatic and toner images.
Innovation Solution
A light emitting device that includes a forward direction voltage estimating unit, a driving current amount estimating unit, a source-drain voltage calculating unit, and a gate-source voltage determining unit to calculate and apply the necessary gate-source voltage to the thin film transistor, ensuring consistent light emission by accounting for OLED element characteristics and variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If OLEDs are used as light emitting elements to reduce device cost and integrate LED array with driving circuit on the same substrate, then manufacturing cost is reduced and integration is improved, but light quantity uniformity deteriorates due to initial variations in forward direction voltage and Vsd-Id characteristics
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the gate-source voltage Vgs based on measured forward direction voltage Vf and source-drain voltage Vsd. The driving current correction unit calculates corrected gate-source voltage using the relationship between Vf, Vsd, and Id to compensate for initial variations in OLED characteristics and TFT Vsd-Id characteristics, thereby maintaining uniform light quantity across all OLEDs despite manufacturing variations
Solution Approach 2:
The patent implements feedback by measuring the actual forward direction voltage Vf of each OLED and using this information to correct the gate-source voltage Vgs. The driving current correction unit continuously monitors Vf and adjusts the driving conditions accordingly, creating a closed-loop control system that compensates for initial variations and maintains consistent light emission across all OLEDs
2Device complexity
If a constant voltage Vdg is applied to TFT to simplify driving, then device complexity is reduced, but light quantity variation occurs due to threshold voltage Vth variation in TFT
Solution Approach 1:
The patent changes the driving parameter from constant voltage to dynamically adjusted voltage. The driving current correction unit calculates the corrected gate-source voltage Vgs based on the measured forward direction voltage Vf and source-drain voltage Vsd, thereby compensating for TFT threshold voltage variations and maintaining consistent driving current Id across all OLEDs
Solution Approach 2:
The patent performs preliminary measurement of the forward direction voltage Vf for each OLED before normal operation. This preliminary information is stored and used to pre-correct the gate-source voltage Vgs, so that when each OLED is driven, the appropriate corrected voltage is already applied, preventing light quantity variation before it occurs
3Manufacturing precision
If high accuracy light quantity control is required for OLED-PH to achieve uniform electrostatic latent image, then light quantity variation must be corrected to less than 1%, but using light sensors increases device cost
Solution Approach 1:
The patent uses forward direction voltage Vf as an intermediary parameter to indirectly control and correct light quantity. Instead of directly measuring light output with expensive sensors, the system measures electrical parameter Vf and uses it to calculate and adjust the gate-source voltage Vgs, achieving high-precision light quantity control through electrical measurement and correction rather than optical measurement
Solution Approach 2:
The patent replaces the mechanical/optical measurement system (light sensors) with an electrical measurement and control system. By measuring forward direction voltage Vf and calculating corrected gate-source voltage Vgs through electrical circuits, the system achieves the same light quantity control function without requiring expensive light sensors, substituting electrical control for optical measurement
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 solution effectively suppresses light quantity variations in OLEDs, achieving high accuracy in light emission and improving image quality by determining the optimal gate-source voltage based on the Vsd-Id characteristic and temperature factors.
Implementation Method 1
a current digital to analogue converter (DAC) causes a driving current Id for emitting light at a desired light quantity to forcibly flow in a thin film transistor (TFT) 1402 for controlling a driving current amount supplied to a light emitting element 1401
Implementation Method 2
use of an organic LED (OLED) as a light emitting element allows the LED array and the driving circuit to be formed on the same substrate
Data Source
AI summary
A light emitting device configured to cause a series circuit in which an OLED is connected to a thin film transistor to emit light by applying a predetermined voltage to the thin film transistor includes: a forward direction voltage estimating unit configured to estimate a forward direction voltage in a case where the OLED is caused to emit light at a set light quantity; a driving current amount estimating unit configured to estimate a driving current amount required for causing the OLED to emit light at the set light quantity; a source-drain voltage calculating unit configured to calculate a source-drain voltage applied to the thin film transistor from the predetermined voltage and the forward direction voltage; and a gate-source voltage determining unit configured to determine a gate-source voltage of the thin film transistor corresponding to the source-drain voltage in a case where the driving current amount is a drain current.