Influence of Monitor Current on Electrical Performance of Photovoltaic Module (PERC/HJT/Topcon) in Thermal Cycle Test

: IEC 61215-2: 2016 is an international standard about testing photovoltaic (PV) module reliability, in which the thermal cycle (TC) test item mainly has focused on thermal stress interaction of PV modules between different materials. At TC testing different stages, except for high working current (Imp), low current (Max. 1 %*Imp) has been turned on continuously as monitor current in order to evaluate the field operation situation of photovoltaic modules. In this paper, the test programme are designed meticulously and I-V data&EL pictures test results are discussed according to the influence level of monitor current on PERC, HJT and TOPCon modules. Three types of PV modules have performed the respective I-V characteristics and thus the values of Isc, Voc and Pmp have appeared different trends.


Introduction
Following with China putting forward the goal of Double Carbon in 2020, the application scale of renewable energy is gradually expanded, especially in the photovoltaic industry. The field working environment of photovoltaic (PV) modules is various and the harsh outdoor environment will lead to the electrical performance deterioration of PV modules after some operation years. Therefore, in order to assess the long-term reliability of PV module, the International Electrotechnical Commission(IEC) has published IEC61215-2: 2016 standard [1] to simulate the reliability of PV module employed in different outdoor environments. A paper [2] studied the monitor current how to influence the PERC PV module during DH250&HF10 tests and concluded that under humidity conditions the monitor current can cause 1.78% and 1.56% degradation at maximum point at power (Pmp), respectively.
Previous researchers have been focused on the reliability of PV module [3,4] , encapsulation material [5,6] and manufacturing process [7,8] about TC aging, but omitted the monitor current impact on PV module. This paper studies the influence of monitor current in the TC test on three types of PV modules (PERC, HJT and TOPCon).

Experimental methods
The thermal cycle (TC) test parameters in the IEC61215-2: 2016 are shown in Figure 1. During the rising stage from -40 ℃ to 80 ℃, the current at maximum point(Impp) is applied as working current and less than 1%*Impp as monitor current in other temperature process that can record whether there is a break in the test process.  Table 1 is the test schemes. Three types of PV module (210 mm half-cell, 132 cells) were prepared with PERC, HJT and TOPCon cell modules (6 pcs. per each type, totals of 18 pcs.). Each type of PV module was divided into three groups, each group sub-divided 2 pcs PV module employed with the same test parameters. In the TC test process, except for working current, we regulated three test parameter modes, in which there were no monitor current, 0.5%*Impp monitor current and 1 %*Impp monitor current, respectively. All of PV modules were manufactured by the laminating machine at the same time and subsequently the pre/post-tests of the electrical performance were also synchronously carried out under controlled environment conditions by utilizing the I-V flashing simulator. For the EL test, the DC power supply was set to DC current as 8 A. During the thermal cycle test, 50 cycles (TC50), 150 cycles (TC150) and 200 cycles (TC200) were tested intermittently for the I-V and EL. Finally, electrical performance data and EL pictures were summarized to observe the influence of the monitor current on the three types of PV modules.

Results and discussion
TC test is mainly to investigate the stress mismatch of different materials of PV module. Different coefficient of thermal expansion determines the interface stress of materials. Table 2 show The material coefficient of thermal expansion involved typical PV modules. Due to the absence of humidity during the experimental process, the oxidation reaction generated by water vapor or the destruction of interface passivation effect is not considered. The test results have mainly reflected the changes of PV module electrical performance or cell visualization.  Figure 2 (Original data) and Figure   3 (normalized) with three monitor current modes (zero urrent/0.5%*Imp/1%*Imp). In order to better display the variation, Figure 4 (normalized) is noted to show the changing trend of Isc, Voc and Pmp. show the upward trend of varying degrees, as Peng [10] and Hallam [11] explained through hydrogen-atom excitation status. The Pmp vibration of the three types of PV module are more significant with 0.5%*Imp monitor current situation. For Pm values, PERC PV module has shown a degradation after T50/T150/T200 series with or without monitor current. In the case of HJT and TOPCon PV modules without monitoring current, Pmp have shown a degradation trend (the normalized value is less than 1), while with monitor current Pmp is greater than the initial value (normalized value is greater than 1), which indicate HJT and TOPCon PV modules are more sensitive to current than PERC PV module. The calculated Pmp fluctuating range is about +1.11% ~ +1.69% and +0.28%~+1.86%, respectively with HJT and TOPCon, comparing with PERC -0.43%~-0.89%.
In a summary, the Pmp of HJT and TOPCon PV modules have performed a rising status after T50/T150/T200 series, which lead to confusion about how to evaluate the influence of TC on HJT and TOPCon modules. However, there is no such phenomena in PERC PV module.  Figure 5 summarizes the typical EL pictures at different TC stages. No matter whether the current is turned on or not, the brightness of PERC PV module have no significant change. With the conditions of 0.5%*Imp, the brightness of HJT PV module is significantly higher than that of the initial state, while with 1%*Imp, the brightness is decreased, but higher than that of the initial status. TOPCon PV modules are similar to HJT, but with slightly lower brightness. The above phenomena is consistent with the changed trend of Pmp.

Conclusions
The electrical properties of PERC and HJT photovoltaic modules have shown different trends with separately monitor current modes. Isc/Voc/Pmp of TOPCon PV modules have expressed a consistent trend. The new PV cell HJT and TOPCon are sensitive to current and different monitor current values will affect the fluctuation of Isc and Voc, especially the value of Pmp is larger than the initial value. This will trigger additional problem for PV module laboratory whether the TC test can truly evaluate the deterioration of PV module encapsulation materials and how to define the qualified criteria of PV module after the TC testing.