Deep Level Transient Spectral System Construction And Defect Detection Based On Lock-in Amplifier

Solar thin-film cells have wide application prospects and great development potential in the field of new energy,but solar thin-film cells will inevitably produce defects in the process of fabrication.The detection of defects is great important to improve the efficiency of thin film cells.Deep Level Transient Spectroscopy（DLTS）technology is one of the most effective means to detect defects in solar thin film cells.However,the commercially available DLTS measurement systems are expensive,highly dependent on imports,and have low selectivity.Most of the measurement systems have a fixed test pulse frequency at 1 MHz,which is not friendly to devices with good low-frequency signal response.The complicated system composition is not conducive to maintenance and upgrading,as well.In this paper,based on MFLI lock-in amplifier,a DLTS measurement system with low cost,accurate test,and flexible adjustment of test parameters is built independently.It is practically applied to the defect detection of solar thin-film cells,and a variety of defects are successfully detected to verify the reliability of the system.The main research is as follows.:（1）According to the principle and functional requirements of DLTS test system,MFLI lock-in amplifier is selected to achieve the functions of pulse generation（frequency of 1 k Hz～1 MHz,amplitude of-10～10 V）and data acquisition（857 k Sa/s）simultaneously,combine with CCS-150 helium cooling system to achieve temperature control（10～500 K）to build a test DLTS test system.（2）With the DLTS measurement system built,the system was verified and improved.First,the test accuracy of the system was verified by scanning the C-f and C-V curves of the same light-emitting diode with the LCR,the results showed that the scanning error of both instruments was less than 1%.Then,the DLTS test was performed on the light-emitting diode and solar thin-film cell at room temperature to verify the test function,including the flexible adjustment of the pulse frequency（10k Hz～1 MHz）,pulse amplitude（-10～10 V）,voltage width（100μs～10 ms）and data acquisition period（～50 ms）.Last,make improvements of temperature control through PID algorithm（temperature fluctuation less than 0.1 K）,fixture compensation and data processing to improve the accuracy and efficiency of the measurement system.（3）DLTS tests were performed on thin-film cells of different materials for various test conditions.Two defects were detected at E_V+330 me V,E_C-210 me V in the CIGS thin film cells.Three defects were detected at E_V+0.55 e V,E_C-0.25 e V and E_C-0.64e V in the Sb₂Se₃ thin-film cells,respectively.Similar defect characteristics were detected for two Sb₂（S,Se）₃ thin-film cells with different efficiencies.The defects were located at E_C-0.70 e V and E_V+0.52 e V in the lower efficiency sample,at E_C-0.69 e V and E_V+0.42 in the higher efficiency sample.Three Sb₂S₃ thin-film cells were detected,the defects located at E_C-0.70 e V（and E_V+0.52 e V）,E_C-0.69 e V and E_C-0.42 e V,respectively.The experimental results show that the DLTS measurement system built in this study can perform DLTS tests on thin-film cells of different materials under different test conditions,and the test results are reasonable and reliable,basically achieving the expected functions and objectives.