In the study of solar cells, improving the photoelectric conversion efficiency of the cells is a crucial goal. The four-point probe method and TLM transmission method play an important role in studying the uniformity of the sheet resistance of crystalline silicon solar cells and the change of contact resistance before and after doping. Millennial Solar In-line Four Point Probe Tester, designed for photovoltaic process monitoring, can obtain the sheet resistance distribution information of different positions of solar cells; TLM contact resistance tester can quickly, flexibly and accurately measure the contact resistance and line resistance of solar cells, reflecting the problems existing in the diffusion, electrode production, sintering and other processes.

Four-point probe measurement

Schematic diagram of thin film square resistance measurement

In the measurement, five different positions on the solar cell are selected. As shown in Figure (b), a fixed current source is connected to the outer probe of the four-probe measurement instrument, and a voltmeter is used to measure the voltage (V) from the inner probe to determine the sample thin film sheet resistance (R).

Thin film sheet resistance analysis of solar cells

In the preparation of crystalline silicon solar cells, measuring the thin film sheet resistance can evaluate whether the doping is uniform.

Thin film sheet resistance at different diffusion times

All phosphorus-doped silicon wafers showed a significant reduction in sheet resistance. Generally speaking, undoped silicon wafers have very high resistance; however, when doped with phosphorus atoms, their sheet resistance decreases dramatically. It can be seen from the table that the sheet resistance of the silicon wafer at different positions with a diffusion time of 20 minutes is very close, indicating that the silicon wafer is uniformly doped with phosphorus atoms during the diffusion process. As the phosphorus doping diffusion time increases, the average sheet resistance decreases significantly.

Changes in the average film sheet resistance of silicon wafers under different doping times

The film sheet resistance measured after different diffusion times for Wafer-1, Wafer-2 and Wafer-3 is presented in the form of a bar graph. As the phosphorus doping diffusion time increases, the average sheet resistance decreases significantly. At diffusion times of 10, 15, and 20 minutes, the average square resistance of Wafer-1 is 89.20Ω/□, that of Wafer-2 is 68.35Ω/□, and that of Wafer-3 is 49.68Ω/□.

Transmission Line Method (TLM) Measurement

Contact resistance is a key parameter to measure the quality of ohmic contact between metal and semiconductor. The research and calculation of contact resistance can reflect the problems in diffusion, electrode production and sintering processes. The current in the metal-semiconductor contact can be transmitted horizontally or vertically. Current transmission horizontally: sheet resistance. Current transmission vertically: contact resistance.

Schematic diagram of TLM test scheme for six different gate line spacings

The figure shows the method of measuring resistance: changing the distance between adjacent grid lines from 0.1mm to 0.7mm, increasing by 0.1mm each time. By changing the electrode spacing, the relationship between the total resistance and the electrode spacing can be obtained, thereby extracting the interface contact resistance Rc and the sheet resistance Rs of the material under study.

Analysis of front contact resistance of solar cells

Contact resistance (Rc) and contact resistivity (ρc) values in TLM mode

Linear regression analysis curves of different gate line spacing and front contact resistance

 The front contact resistance of silicon wafers was measured by the transmission line method (TLM). The results show that as the diffusion time increases, that is, the doping concentration increases, the front contact resistance decreases.

The parameters that affect the front contact resistance include doping characteristics, silver paste type, and firing formula, among which doping characteristics are the main factors affecting the front contact resistance. Increasing the doping time can increase the doping concentration of phosphorus, but over-doping exceeds the solid solubility of silicon, forming a dead layer with inactive resistance. The dead layer acts as a recombination center, increasing the Auger recombination and reducing the short circuit and quantum efficiency of the solar cell.

Analysis of back contact resistance of solar cells

Silicon wafer resistance at different diffusion times

Linear regression analysis curves of different grid line spacing and back contact resistance

The front contact resistance decreases with the increase of doping concentration, while the back contact resistance increases with the increase of doping concentration. The analysis of the data shows that the average back contact resistance increases with the increase of doping time. For example, the average back contact resistance of wafer-1, wafer-2 and wafer-3 is 1.11Ω, 1.36Ω and 1.56Ω respectively. The corresponding contact resistance also increases with the increase of diffusion time.

In summary, the four-point probe method was used to measure the thin film sheet resistance of doped silicon wafers at different doping concentrations to confirm that the thin film sheet resistance decreases with the increase of doping concentration. Silicon wafers at appropriate doping concentrations are conducive to the manufacture of efficient photovoltaic silicon solar cells. The TLM transmission method can accurately measure the front and back contact resistance of silicon-based solar cells, providing an important method and valuable conclusions for studying the contact resistance characteristics of silicon solar cells, which helps to optimize the performance of solar cells.

In-line Four Point Probe Tester

E-mail: market@millennialsolar.com

 In-line Four Point Probe Tester is designed for photovoltaic process monitoring. It can quickly and automatically scan samples up to 230mm×230mm to obtain sheet resistance/resistivity distribution information at different positions of the sample.

·The maximum sample meets 230mm×230mm

·Measurement range: 1mΩ~100MΩ

·The number of measurement points supports 5-point and 9-point measurement. The simultaneous test of 5 points meets ≤5 seconds, and the simultaneous test of 9 points meets ≤10 seconds

·Measurement accuracy: Ensure the accuracy of the measurement of the same model. The test error between different test instruments is ±1%

Contact Resistance Tester

E-mail: market@millennialsolar.com

TLM Contact Resistance Tester can quickly, flexibly and accurately measure the contact resistance and line resistance of solar cells. It reflects the problems in diffusion, electrode production, sintering and other processes.

· TLM/Line resistance two modes can be switched at will, which is convenient and fast

· Contact resistance/line resistance test platform can be switched

· Multiple probes can realize measurement and analysis at will

The four-point probe method is used to measure the sheet resistance of the thin film. Under different doping concentrations, the influence mechanism of doping distribution on the sheet resistance of the thin film is studied, which provides strong support for improving the efficiency and performance of crystalline silicon solar cells. The TLM transmission method helps to deeply understand the characteristics of contact resistance in solar cells and provides a basis for reducing contact resistance and optimizing solar cell performance. Millennial Solar In-line Four Point Probe Tester and Contact Resistance Tester provide important technical support for the research and development and improvement of solar cells through the measurement and analysis of thin film sheet resistance and contact resistance.