A Novel SPICE Compatible Current Model for OLED Circuit Simulation
Y. Li, J-W Lee, B-S Lee, C-S Lu and W-H Chen
National Chiao Tung University, TW
OLED model, circuit simulation, RGB colors
high quality, and low power consumption display [1-2]. It is known that the brightness of the OLED is controlled by the current density of the device; therefore, precisely controlling the current density of active matrix array is required. In this work, we propose a new physical-based OLED model for the electrical characteristics of the OLED devices. Our model considers the effect of high built-in voltage existing between the organic materials and the non-ideal ohmic effect occurring at the contact between metal and organic. Comparisons between the model and measurement for the OLED with red (R), green (G) and blue (B) three colors have shown very good accuracy. The significant improvement is mainly caused from the correction of the high built-in voltage and non-linear contact resistor effect. The new SPICE compatible current-voltage model can be incorporated into OLED circuit simulation with numerical difficulties. It is known the conventional OLED model  is based on the single crystalline diode model. However, the model does not consider the effect of the high built-in voltage existing between the organic materials; furthermore, it considers nothing about non-ideal ohmic effect at the metal/organic contacts. Shown in Fig. 1 is an OLED structure. To have physically sound OLED models, these two observations above should be taken into consideration. For the modeling of the built-in voltage, shown in Fig. 2b, we tie a DC voltage source in series with the ideal diode. Similarly, the linear resistor of the idea diode is also changed to a non-linear resistor, which reflects the physical model the so-called Schottky barrier. With adding those two items into the ideal diode model, the new model can be directly incorporated into SPICE circuit simulator without any convergence problems. From the modeling results shown in the figures 3-8, good agreement between the measurement and simulation is found from the proposed OLED model. The superior electrical characteristics are strongly correlated to the improved model’s accuracy for the effects of the Schottky barrier and the high built-in voltage. Figures 3-5 are the comparisons between the measured and the conventional model, shown in Fig. 2a, simulated IV for the OLED with R, G, and B three colors, respectively. It is found that the conventional model has a totally wrong behavior compared with the measured data. Such deviation not only occurs at the cut-in region but also appears in the turnon situation. As a result, the brightness of OLED can not be calculated for the electroluminescence intensity, because it is significantly dominated by the current density. Figures 6-8 exhibit the comparison results for our model with the measured OLED data with R, G, and B colors, respectively. Dissimilar with the modeling results from the conventional diode model, our new model presents good accuracy when describing the OLED physical characteristics in both the cut-in and the on-state regions. With having the accurate simulation results, the current flowing through the OLED circuit can be correctly simulated. Consequently, the brightness of the OLED panel can be exactly controlled without using the area consuming driving circuit which benefits the design and fabrication of OLED panels. This work is supported in part by the National Science Council of TAIWAN under contracts NSC-93-2215-E- 429-008 and NSC 93-2752-E-009-002-PAE, and the grant of the Ministry of Economic Affairs, Taiwan under contract No. 92-EC-17-A-07-S1-0011.
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Nanotech 2005 Conference Program Abstract