Transient response of passive matrix polymer LED displays
D. Brauna,*, K. Ericksona, G. Yub
Electrical Engineering Department, Cal Poly State University, San Luis Obispo,
b UNIAX Corporation, Santa Barbara, California, U.S.A.
Video displays based on polymer and organic displays require high resolution and high data rates. However, the transient response of passive matrix displays based on polymer and organic light-emitting diodes limits display resolution, image uniformity, and image contrast. We use circuit simulation in order to quantify these consequences. This work analyzes how the transient response depends on display resolution, display geometry, pixel position, and row and column electrode design. We conclude that advanced display designs can achieve uniform images at high resolution and contrast.
Keywords: Polymer light-emitting diode; Transient display response
Much interest in polymer and organic light-emitting diodes derives from their potential to deliver the next generation of flat and flexible displays [1-9]. High information content and full motion video displays must display images with sufficient spatial resolution, speed and contrast to please the human eye. Previous work has explored techniques to improve display uniformity and resolution based on DC display characteristics . This study explores how electrode resistance influences the transient display characteristics.
2. Simulation Results
Fig. 2 compares the transient response as pixel resolution increases along one row. The approach used is to insert the electrical data for a LED from Fig. 1 into the PSpice circuit simulation tool. The display contains pixels with the circuit shown in the inset to Fig. 1. Pixel area is 300 mm x 300 mm = 0.09 mm2 with 50 mm spaces between pixels. Each pixel contains one LED, a row resistance of RCathode = 0.10 mW , a column resistance of RAnode = 11.6 W , and a capacitance C = 40 pF. Given a metallic cathode with sufficiently low resistance, additional columns have little influence on the transient response. Column resistance limits performance.
Fig. 1. Current as a function of voltage recorded for a polymer LED made from alkoxy-PPV. The inset shows the equivalent circuit of each pixel.
Row Electrode Resistance
The simplified circuit diagram above for a 100 pixel row emphasizes how resistance in the row electrode can influence the transient response of pixels along one row.
Fig. 2 compares the transient response of one row to a 5V pulse as pixel resolution increases. Given a metallic cathode with sufficiently low resistance, 0.1 mW per pixel in this case, additional columns have little influence on the transient response. Column resistance limits speed .
Fig. 2. Transient response along one row with one column (circles), 100 columns (squares), and 1000 columns (diamonds).
Column Electrode Resistance
The simplified circuit diagram above for a 100 pixel column emphasizes how resistance in the column electrode can influence the transient response of pixels along one column. The approach is to insert the equivalent circuit for one pixel directly into the PSpice circuit simulation tool along with a circuit net list for the 100 or 1000 pixel column circuit. The top pixel in the column sees a larger series resistance than do lower pixels. Therefore, it responds more slowly to a voltage pulse applied at the bottom of the column than do lower pixels in the column. The extra series resistance also causes the top pixel to saturate to a smaller current density (lower brightness) at the same applied bias.
Passive matrix array of LEDs. Rows 1 through 4 form the cathode electrodes, and columns A through D form the anode electrodes.
Equivalent circuit of each pixel. In addition to the DC current vs. voltage characteristics of the polymer LED, the circuit model includes elements to account for row and column resistance (RCathode and RAnode), pixel capacitance (C) and pixel leakage (R).
Fig. 3 compares the transient response along one column for various values of RStrap, the resistance of an auxiliary electrode used to decrease anode resistance [5,10]. As column resistance limits performance, low RStrap is required to preserve display uniformity  and transient response. Fig. 4 confirms that low RStrap also improves the contrast of the display by allowing the display to discern more levels of brightness. The contrast calculation assumes a 100 Hz frame rate. The lowest gray level occurs if the pulse width equals the 90% point on the rising edge of the transient.
Fig. 3. Transient response along a column with 100 or 1000 rows for various values of RStrap
Much interest in polymer and organic light-emitting diodes
derives from their potential to deliver the next generation of flat panel displays.
The devices are relatively easy to make, because they consist of an electroluminescent
layer sandwiched between an anode, usually transparent, and a cathode. A passive
matrix display results by patterning the anode into columns and the cathode
into rows to form an array of pixels from the intersections between the cathode
and anode electrodes. High information content and full motion video displays
must display images with sufficient spatial resolution, speed and contrast to
please the human eye.
Fig. 4. Gray levels (solid lines) and bit resolution (dashed lines) vs. RStrap.
Tradeoffs exist between display resolution and display contrast. This work has analyzed how the transient response of passive matrix polymer displays depends on:
Reducing anode electrode resistance with a parallel conduction path (RStrap) provides one key to transient response, assuming that the cathode electrode resistance is sufficiently low. Because anode-to-cathode capacitance influences display speed, thicker pixels can reduce pixel capacitance. Display response slows as row and column lengths increase, so lower resolution displays offer speedier response and greater contrast.
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