Active Matrix Addressing

2.7.2.1 Pixel Structure

Active matrix (AM) OLED displays stack cathode, organic, and anode layers on top of another layer – or substrate – that contains circuitry. The pixels are defined by the deposition of the organic material in a continuous, discrete “dot” pattern. Each pixel is activated directly: A corresponding circuit delivers voltage to the cathode and anode materials, stimulating the middle organic layer.

AMOLED can be driven in a manner similar to the active matrix TFT-LCD. A conventional pixel is as shown in Fig. 2-11 [2-17]. The pixel structure of the voltage programmed driving scheme contains one switching transistor (TFT M1), one driving transistor (TFT M2) and one storage capacitor ( Cs ),. In this circuit, TFT M2 is a PFET device connected in a common source arrangement, and plays the role of a current source to supply constant current to the OLED. The Vgs of the driving transistor for a specific output current will be programmed in the capacitor through the switching transistor (TFT M1).

However, such voltage programmed pixel structure encounters the non-uniformity problem.

Scan Line

Data Line Supply Line

OLED

M2 Cs

M1

Fig. 2-11 Principle of active matrix addressing of an OLED display..

The saturation current of a TFT is usually approximated using the following equation(2-3), where W is the channel width, L is the channel length, Cox is the capacitance per unit area of the gate dielectric layer, Vth is the threshold voltage, and U is the field-effect mobility:

) 2 UpCox (Vgs Vth

L

Id = W − ²……… (2-3)

From the asymptotic square equation for transistor current, it is obvious that the threshold voltage and mobility variation will result in great variation of the output driving current, and the non-uniformity in grey scales will be induced. Variations in threshold voltage and mobility depending upon implementation may add to luminance variations. With p-Si., initial variations are higher due to grain size and boundary variations[2-18]. With a-Si, time related electrical stress can give large threshold voltage variations[2-19] [2-20]. These luminance variations are tolerable if they occur gradually across a display, because the eye is insensitive to such variation.

However, if they occur randomly from pixel to pixel, the eye can readily detect these variations.

2.7.2.2 Pixel Addressing

Fig. 2-12 show the active matrix addressed OLED(AMOLED). Each pixel in AMOLED has a driving transistor in series with the OLED which supplies a constant current throughout the frame time. As shown in Fig. 2-11, the scan line of the display are sequentially scanned over the frame time. When a line is selected, all the M1 transistors in this line are switched-on and the voltage data are transferred on the gates of the M2 transistors. These data voltages are maintained on the gates of M2 transistors as M1 transistors are switched-off, when the next line is selected. A storage capacitor can be added to compensate the leakage current of M1 transistors. In this addressing mode, the current is supplied to the organic LED during the whole frame time. The necessary current levels are much smaller , but cause the setting times to be much longer. For a 300um x 100um pixel at typical luminance levels of 100 cd/m², the transistor current is only a few micro-amps [2-21] [2-22] [2-23]. As for a typical pixel capacitance of 5 pF, it takes about 8us for a 3uA current to charge 5 pF though 5 volts.

AMOLED

Fig. 2-12 Circuit diagram of an active matrix OLED

2.7.2.3 Pixel Concern

2.7.2.3.1 Constant Current or Source Follower

In order to continuously supply the current to the AMOLED pixel while the other scan lines are addressed. At least two TFTs are required for one pixel. The equivalent pixel circuit for this approach based on the n-channel TFT is shown in Fig. 2-13 and Fig. 2-14 [2-20].

Fig. 2-13 and 2-14 are the constant current arrangement the source follower arrangement, respectively. Due to the constant current circuit can eliminate the slight threshold voltage (Vth) shift of OLED and maintain the same output current level, it is preferred. That is the brightness remains even if a slight Vth shift of OLED is existent. Since OLEDs are extremely sensitive to moistures, wet process like photolithography is usually forbidden after the deposition of the organic layers. Therefore, the pixel circuits for AMOLED needs to be defined before the deposition of the organic layers. This limitation makes it very difficult to achieve the configuration as shown in Fig. 2-13, where the interconnection between the cathode of the OLED and the drain electrode of TFT M2 needs to be established. This implies that the constant current configuration for the n-channel TFT is difficult to realize for the two TFT circuit. For Source follower in Fig. 2-14, the drawback of this circuit is that the current depends upon the OLED voltage. The OLED voltage may vary from pixel to pixel and increase slowly with usage, ~0.1-1mV/hour, providing additional sources of non-uniformity.

Fig. 2-13 Constant current pixel Fig. 2-14 Source follower pixel

2.7.2.3.2 Driving Device Choice

To solve the above pixel issue, NFET in Fig. 2-14 is replaced by PFET. A current source with storage capacitor ( Cs ) is shown in Fig. 2-11. The OLED voltage dependence on current is eliminated with the circuit. The data voltage is written onto the data storage capacitor Cs when the TFT M1 gate line is brought to negative level. TFT M2 is a PFET device connected in a common source arrangement. TFT M2 play the role of a current source to supply constant current to the OLED. The current is proportional to (Vdd-V^data-Vth)² for Vth >Voled-V^data where Vth is the TFT threshold voltage.

2.7.2.3.3 IR-drop

OLED makes a direct current pass and emit light from the organic compound of the fluorescence excited by supplying electric field. The current is proportion to the data voltage which is written onto the data storage capacitor Cs. The period for the written data voltage is called addressing time. Since OLED is a current driver element, there is an IR drop issue on supply line along scan line direction while a turned on OLED is during addressing time, as shown in Fig. 2-15 [2-23]. A bad affect exist. That is, the pixel closed to supply line will be programmed an exact data voltage, but the far pixel will be programmed a wrong data voltage.

The variation of gate-source leads to the divergent color. A variant of this circuit as shown in Fig. 2-16 uses an additional TFT to block any current flowing into the EL element while the scan line is addressed. The purpose is to reduce signal error due to resistance of the supply line.

Fig. 2-15 IR-drop perspective Fig. 2-16 A variant pixel with additional TFT