Design of OLED display driver based on SSD1303 and AT89C51

1. Introduction

Organic electroluminescent display (OLED) technology is the next generation of the most competitive flat panel display technology. At present, the research focus of OLED is to improve the stability of the device, the luminous efficiency and the driving technology of high-quality dynamic display to meet the requirements of practical application. From a practical point of view, this paper first discusses the development of stable green organic thin film electroluminescent devices, explains the production of 96×64 dot matrix PM-OLED display screen, and focuses on the method of driving OLED display screen using Solomon's new product, SSD1303, which integrates controller, row driver and column driver, and single chip microcomputer AT89C51. The result of this work is an attempt from laboratory to application, which provides a feasible method for the practical application of OLED.

2. Preparation of Matrix Display Panel

2.1 Structure and materials used in OLED

The structure of OLED adopts the currently more mature multi-layer structure, that is, a stable green organic thin film electroluminescent device composed of multiple layers of organic thin films sandwiched between the anode and the cathode. The structure is ITO/CuPc/NPB/Alq3:QA/Mg:Ag.

In order to effectively inject holes from the anode, the work function of the anode is required to be as high as possible, and ITO (indium tin oxide) is used as the anode.

In order to effectively inject electrons into organic materials, the cathode material should have a low work function, such as Mg, Li, etc. However, since they are easily oxidized in the air and are unstable, they can be alloyed with other stable metals. We use Mg:Ag as the cathode, which can not only improve the quantum efficiency and stability of the device, but also form a stable and strong metal film on the organic film.

The addition of CuPc (copper phthalocyanine) buffer layer is to improve the stability and life of the device. NPB (diamine derivative: N,N'-di(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine) is the hole transport layer. The electron transport layer and luminescent layer is Alq3 (8-hydroxyquinoline aluminum), which is both an electroluminescent material and an electron transport material. The introduction of the transport layer is to improve the injection balance of electrons and holes to improve the luminous efficiency of the device. QA (quinacridone) is a dopant for the device to produce green light, and the device doped with QA has an advantage in stability.

2.2 Display screen preparation

PM-OLED uses a common matrix cross screen, and the OLED is located between the cross-arranged anode and cathode. By selecting the anode and cathode combination, the lighting of each OLED can be controlled.

The matrix display screen is prepared by using a photolithography process on the ITO conductive glass to form a strip electrode (anode) in the X direction, with a line width of 0.4mm and a line spacing of 0.1mm. Then, CuPc, NPB, Alq3 and dopant QA are evaporated on it successively, and finally a metal Mg:Ag alloy electrode (cathode) in the Y direction is made, with a line width of 0.4mm and a line spacing of 0.1mm. Each pixel size is 0.4×0.4 (mm2), the matrix display screen has a resolution of 96×64, and an effective area of ​​48×32 mm2. Since the cleanliness of the ITO surface has a great influence on the performance of the device, the ITO substrate must be cleaned before evaporation, including ultrasonic cleaning, organic solvent vapor degreasing, multiple rinses with deionized water and oxygen plasma treatment.

We used PR650, Keithley 2400 Source Meter and other testers to measure the pixel characteristics. When the applied voltage is >15V, the device can achieve a brightness of >10000cd/m2. When the current density is 20mA/cm2, the brightness of the device is 1900 cd/m2. Since OLED is a current-type device, the brightness of the display can be controlled by current. We use the relationship curve between normalized brightness (brightness/initial brightness) and working time (hours) to represent the device decay curve. The initial brightness is 420 cd/m2, and the half-brightness life is 3300 hours. When the initial brightness is 100 cd/m2, the half-brightness life is 13860 hours. Therefore, the display fully meets the practical requirements.

3. Selection of OLED matrix display driver IC

At present, there are many companies in the world that are engaged in the design of OLED-specific ICs. There is no company in mainland my country that can produce OLED-specific ICs. The relatively strong companies in the international market include Solomon Company in Hong Kong, China and Clare Company in the United States.

Clare's driver ICs for OLED displays include the MXED102 row driver and MXED202 column driver ICs, which are considered comprehensive, finished OLED display drivers.

The SSD1301 launched by Solomon is recognized as the first integrated circuit dedicated to OLED display control and drive that integrates controller, row driver and column driver. The SSD1303 is a new product launched by Solomon. Through experiments, we successfully drove a 96×64 dot matrix OLED display with SSD1303.

The internal circuit block diagram of the SSD1303 chip is shown in Figure 1: It is mainly composed of MCU interface, command decoder, oscillator, display timing generator, voltage control and current control, zone color decoder, and graphic display data memory (GDDRAM), row driver and column driver. The dedicated OLED drive solution of this IC optimizes the OLED display performance and reduces power consumption. The device adopts TCP/TAB package. It has the function of driving a maximum 132×64 dot matrix graphic display, providing a logic power supply of 2.4~3.5V, a power supply of 7.0~16V for the OLED screen, a maximum current of 320μA for the column output, a maximum current of 45mA for the row input, a low current sleep mode of less than 5μA, 256-level contrast control, programmable frame rate, several MCU interfaces, such as 68/80 parallel bus and serial peripheral interface, 132×65bit display buffer, vertical scrolling, support for partial display, and operating temperature: -40 oC~85 oC. The use of such chips can further improve the reliability and competitiveness of products and will be the mainstream products in the future.

4. System hardware and software design:

The whole system consists of three parts: single-chip microcomputer, control drive circuit SSD1303 and OLED display screen. The pins of SSD1303 interface with single-chip microcomputer are: DO~D7 are the data bus of single-chip microcomputer interface, R/W (RW#) is the read/write selection signal, D/C is the data/command selection signal, CS# is the chip selection signal, low level is effective, E (RD#) is the enable signal, and RES# is the reset signal. The single-chip microcomputer adopts the low-power and high-performance AT89C51 produced by ATMEL. The hardware connection of AT89C51, SSD1303 and display screen is shown in Figure 2. P1.0, P1.1, P1.2, P1.3, P1.4 are respectively connected to R/W (RW#), D/C, CS#, E (RD#), RES# of SSD1303, and P0 port is connected to the data bus of SSD1303. The connection of other pins VCC is connected to 12V, VDD is connected to 2.7V, VSS is grounded, etc. Next, these pins are controlled by the program so that the OLED displays the required Chinese characters or graphics. The main program software flow chart is shown in Figure 3.

Figure 2 Hardware connection between MCU AT89C51, SSD1303 and display screen [page]

Figure 3 Main program software flow chart

5. Conclusion

According to the above scheme, we have produced a 96×64 dot matrix OLED display screen based on the research of stable green organic thin film electroluminescent devices, and successfully driven the display screen with SSD1303 and AT89C51. The display effect is good, but during the experiment, we found that there are still some defects in the display. The main reason is some deficiencies in the current OLED production process. These need to be further solved in future work. We believe that with the increasing demand for flat panel displays, OLED display technology will be further developed. As one of the most promising display devices, OLED will become a mainstream technology in the field of flat panel display applications.

The innovation of this paper is that it improves the stability of green organic thin film electroluminescent devices and meets the practical requirements, and designs the control circuit of the OLED matrix display screen, which provides an effective way for OLED to move from laboratory research to practical application, and has positive significance for promoting the industrialization process of OLED displays.