OLED Photoelectric Performance Comprehensive Test System Based on Microcontroller

　　Organic light emitting diode (OLED) is a new type of flat panel display device and two-dimensional light source. By testing the current-voltage characteristics, luminous brightness-voltage characteristics, temperature-current characteristics, luminous efficiency-voltage characteristics, color coordinates and electroluminescence spectrum of OLED devices, the luminous performance and electrical performance of the device can be judged, and its external quantum efficiency and energy conversion efficiency can be calculated. At present, it is more common to use DC voltmeter and ammeter to measure the current and voltage characteristics of OLED devices. Use a weak light photometer to measure the luminous brightness of the sample, and use a temperature acquisition instrument to measure the temperature of the sample.

　　The data is then input into a computer for subsequent analysis and processing to obtain the photoelectric properties of the sample. This measurement method cannot achieve simultaneous measurement of physical quantities such as voltage, current, brightness, and temperature, and has large errors and low efficiency. The amount and density of data collection are also greatly limited, and the photoelectric properties of the sample cannot be accurately reflected.

　　In response to the above problems, this paper designs a comprehensive test system for OLED optoelectronic performance based on microcontroller. The system is centered on the microcontroller and is a measurement system that organically combines precision constant voltage (constant current) power supply, voltage and current measurement, temperature measurement, and brightness measurement. It can realize test condition setting, data storage, analysis, graphical display, print output and other functions, and realizes high-precision, rapid and automatic measurement of the optoelectronic performance of OLED devices, making the optoelectronic performance test of OLED devices convenient, fast and scientific.

　　2 Overall system structure

　　The overall block diagram of the system is shown in Figure 1. The system consists of a host computer and a slave computer. The slave computer is based on the MSP430F149 microcontroller, including a programmable DC power supply IPD-3303SL, a photoelectric conversion circuit, a temperature measurement circuit, a range switching circuit, a display circuit, and an RS232 interface circuit; the host computer is a general-purpose PC equipped with dedicated data processing software to complete system data processing and photoelectric performance analysis functions.

　　The system works as follows: The programmable DC power supply IPD-3303SL drives the OLED device to emit light according to the voltage initial value, voltage end value, step value and sampling period set by the host computer. The light emitted by the sample is incident on the photodiode and converted into a current signal. After passing through the I/V conversion circuit, the current signal is converted into a voltage signal. The microcontroller controls the range switching circuit to switch the feedback resistor of the I/V conversion circuit to obtain voltage signals of different intensities at the output end. After the output voltage is amplified twice, it enters the A/D channel of the microcontroller. The temperature of the device is collected by the infrared thermopile sensor and input into the A/D conversion channel of the microcontroller. The microcontroller converts the voltage signals of brightness and temperature into digital quantities for processing and displays them through the LCD. The data is transmitted to the host computer through the RS-232 interface for data analysis and processing, and the voltage-brightness, current-temperature, and voltage-current curves are drawn.

　　3 Hardware Design

　　3.1　MCU Unit

　　The MSP430 microcontroller produced by TI in the United States is a highly integrated, high-precision single-chip system. It uses a reduced instruction set and is a 16-bit high-speed processing microcontroller. In addition, it has a wealth of peripheral modules, which reduces the peripheral space when used. Here, the MSP430F149 microcontroller is selected as the MCU. The operating voltage of the microcontroller is 1.8~3.6V, and it has two 16-bit timers, two serial communication interfaces, an on-chip watchdog and a 12-bit A/D. The comprehensive functions and costs are more suitable for this system.

　　The system uses the ADC12 module built into the microcontroller to directly implement A/D conversion. The MSP430F149 has an 8-way conversion interface, and the ADC12 module controls the register to achieve A/D conversion of the brightness and temperature analog signals. Using the 2.5V reference source inside the microcontroller, the analog multiplexer of ADC12 can convert the brightness and temperature signals in a time-sharing manner, and has a sampling and holding function. The ADC12 hardware can automatically store the conversion results in the corresponding registers through settings.

　　3.2 Voltage and current measurement

　　The system selects the programmable DC power supply IPD-3303SL as the instrument to measure the voltage-current characteristics of OLED. IPD-3303SL is a programmable control power supply that works in constant current or constant voltage mode and can achieve step-by-step voltage (current) output. It can be controlled by serial port input commands and the output current (voltage) value can be obtained through the serial port. It also has over-current and over-voltage protection functions, which is suitable for the output and measurement of current and voltage in this system.

　　The IPD-3303SL voltage output range is 0~25V, the measurement voltage accuracy is 1mV, and the current accuracy is 1μA.

　　3.3 Brightness Measurement

　　The brightness measurement circuit consists of an I/V conversion circuit and a secondary amplifier circuit. The light emitted by the OLED device is incident on the photodiode GT101, which outputs a current signal of only 0.002 to 200μA. The I/V conversion circuit device uses a monolithic electrometer type operational amplifier OP07 with extremely low input bias current. OP07 has low input current and low input compensation voltage, so it is suitable as a preamplifier for very sensitive photodiodes. As shown in Figure 2, the offset of the op amp can be zeroed by adjusting R12, and R2 compensates the bias current of the integrated op amp.

　　In this design, the brightness measurement range is 1×10-3～200×103 cd/m2, which is a jump of 6 orders of magnitude. The amplification factor of a single amplifier cannot meet the requirements, so range switching is required.

　　The I/V conversion circuit uses 6 sets of different feedback resistors to connect 6 normally closed analog switches MAX312 for range switching. MAX312 has only 10Ω on-resistance and 0.35Ω on-resistance flatness.

　　Connect the normally closed terminal to the feedback resistor and the common terminal to the amplifier output.

　　When the input terminal is at a high level, the common terminal and the normally closed terminal are turned on, and the feedback resistor connected to them is turned on, otherwise it is turned off.

　　The first stage outputs a voltage of 0 to 200mV, and the A/D conversion uses a 2.5V reference source. Therefore, the second stage uses an op amp LF441CN with an amplification factor of 12 to amplify the 0 to 200mV voltage to 0 to 2.4V.

　　The amplified voltage Vin is input into the A/D channel of the microcontroller and the upper and lower limit voltage comparator composed of LM324 respectively. The signal amplitude is analyzed according to the output of the comparator to see if it is within the specified range. In order to make the circuit work more stably, a certain margin needs to be reserved for the comparison voltage value. As shown in Figure 3, adjust R10 to make the upper limit voltage of the comparison 2.3V, and adjust R17 to make the lower limit voltage of the comparison 0.1V. When Vin is between 0.1 and 2.3V, the range is appropriate, the comparator outputs the logic level 00, and the A/D conversion circuit is started. Otherwise, when Vin>2.3V, the feedback resistor of the I/V conversion circuit is switched through the analog switch MAX312 and the software to switch to a large range; when Vin<0.1V, it is switched to a small range to adapt to light signals of different intensities.

　　

　　3.4 Temperature measurement

　　During the measurement process, the temperature of the OLED device changes with the current passing through the device. The system uses the infrared thermopile sensor A2TPMI334-L5.50AA300 for non-contact temperature measurement. A2TPMI is a multi-purpose infrared thermopile sensor that integrates a dedicated signal processing circuit and an ambient temperature compensation circuit. This integrated infrared sensor module converts the target's thermal radiation into an analog voltage, which is input into the A/D channel of the microcontroller and converted into the corresponding digital quantity.

　　Its main performance is: sensitivity 42mV/mW; response time 35ms; half-power point response frequency less than 4Hz; the range of measured target temperature is -20～300℃. Its temperature-voltage characteristics are as follows:

　　Tobj＝－2.815 56×6+51.719 67×5－386.824 1×4+1 510.241×3－3 267.076×2+3 820.25×－1792.6 (1) Where: Tobj represents the target temperature, x represents the voltage value at the corresponding target temperature.

　　4 System Software Design

　　4.1 Lower computer software design

　　The system software is written in C language and designed with modular ideas under the integrated debugging environment Workbech provided by IAR. The system software program is solidified in the internal Flash memory of the MSP430F149 microcontroller. The program flow of the system is shown in Figure 4. The entire process can be divided into initialization, command parsing, data measurement, data processing and display procedures. The program design draws on the idea of ​​real-time operating system and divides events and targets. According to the working process, the system has several events such as command parsing, online, calibration, measurement judgment, data acquisition, data packaging and sending. The corresponding functions are completed by processing the events and necessary state conversions are performed. When the voltage changes, the measurement command is received, the measurement state is entered, the data is sent and displayed, and the idle state is returned to continue waiting for commands and then the corresponding operations are performed.

　　4.2 Host computer software design

　　The host computer software is written in Visual C++, with a modular overall design from top to bottom. Through the serial communication programming control MSCommm in Visual C++, the event-driven method is used to receive data. Figure 5 shows the overall design block diagram of the system.

　　The upper computer software management system consists of parameter setting, real-time monitoring, information query analysis and display and other auxiliary functions.

　　The system is a comprehensive information platform that integrates automatic acquisition, preprocessing, and data storage of voltage, current, brightness, and temperature. RS-232 communication is used to realize communication between the program-controlled power supply, the lower computer, and the computer, and brightness, temperature, and current data are obtained in real time. The brightness, temperature, and current values ​​corresponding to the current voltage are stored in the database, and real-time curves are drawn. In the historical data query, the measurement results of OLED devices with different structures can be compared and analyzed.

　　5 Conclusion

　　A scheme of OLED photoelectric performance comprehensive test system with microcontroller as the core is proposed. The system can measure various photoelectric characteristics of light-emitting devices on a platform at the same time, realizing the computerization of photoelectric characteristic experiments. The development process and practical application of OLED photoelectric performance comprehensive test system show that the system can realize fast, accurate and reliable automated measurement and improve work efficiency.