Photovoltaic modules play a vital role in the generation of renewable energy. However, over time, a module may experience performance degradation due to various factors, one of which is Potential Induced Degradation (PID). In order to ensure the life and efficiency of photovoltaic systems, the Potential Induced Degradation Tester from Millennial Solar came into being. This article will delve into the Potential Induced Degradation (PID) performance evaluation of the transparent backsheet of the module, and introduce the PID tester and its importance. 

PID performance evaluation of transparent backsheet

Currently, transparent backsheet is a viable alternative to glass when constructing bifacial modules. Its main advantages are lower cost than glass, significantly lower weight, better transparency and better protection of the module from hail. In addition, because it does not contain sodium, the transparent backsheet can also reduce the occurrence of PID and reduce the risk of PID on the back of the cell. The test is to study the stability of the module by exposing it to extended potential-induced decay PID stress using a module constructed from a transparent backsheet.

The module used in the test is the P-PERC glass backsheet module, and its main properties are listed in the table below.

Test process

Five modules (one set of reference module, one set of 0V polarized module Mod1, one set of +1500V polarized module Mod2, and two sets of -1500V polarized module) were preprocessed within 486 hours by current injection at 75°C. module Mod3&4), conduct current-voltage measurement control every 162 hours, and avoid light-induced degradation (LID) and light and high temperature-induced degradation (LeTID) during the test phase.

PID testing per IEC 61215 standard (MQT 21) at 85°C, 85% RH for up to 576 hours instead of the 96 hours required in the standard (voltage monitoring every 96 hours or 192 hours). Use a cutting wheel to cut two complete samples on the -1500V polarization module, and then use a diamond wire saw to cut one sample into small pieces of 1cm × 1cm.

Finally, the encapsulated samples were cut out and photoluminescence (PL) measurements were performed using BT imaging and finally transmission electron microscopy (TEM) measurements.

Test Results

Current-Voltage (I-V) Figures 1 and 2 plot the performance evolution of the front and back sides of the module through PID testing respectively.

Figure 1. Performance evolution of module front in extended PID test

Figure 2. Performance evolution of the module backsheet in extended PID testing

As shown in Figure 1, the module performance on the front side is not affected by the applied positive or neutral voltage. After the test, all electrical parameters of the module are similar to those of the untested reference module. In contrast, applying a negative voltage, after 576 hours of PID, the power on the front dropped by nearly 8%, the performance on the back dropped three times more severely (25%), and the current varied significantly, as shown in Figure 2.

Electroluminescence and Photoluminescence Characterization

To better understand the reasons for the performance degradation, EL and PL detection was performed on module Mod3. The EL imaging of module Mod3 and the PL imaging of the cut sample are shown in the figure below.

a) EL imaging of Mod3 after pretreatment; b) EL imaging after 576 hours of PID testing; c) Top: EL and PL imaging after magnification; Bottom: EL and PL imaging in dark areas

Since the PL measurement uses cells cut out of the module, glass cracks can be seen in the picture. EL imaging shows a general decrease in signal and the appearance of some dark areas, with some "light trails" also visible on most cells with locally higher EL intensity. These dark areas and light trails were also confirmed by PL imaging and pointed to potential local passivation changes.

A double-sided module with a transparent backsheet was subjected to extended PID stress testing, and current-voltage, EL and PL measurements showed the presence of two types of PID effects in the module: PID potential type (PID-p) and corrosion type (PID-c). The type of shunting of PID-s is generally associated with the migration of sodium ions from the glass into the cell, and the current-voltage changes did not show any significant Voc fill factor drop. Even if no direct evidence is observed using TEM measurements, the observed localized depassivation may be related to PID-c.

Potential Induced Degradation Tester

E-mail: market@millennialsolar.com

Introduction:

Long-term current leakage will cause changes in the state of the cell carriers and depletion layer, corrosion of the contact resistance in the circuit, electrochemical corrosion of the packaging material, etc., resulting in cell power attenuation, series resistance increase, light transmittance decrease, delamination and other phenomena affect the long-term power generation and life of the module.

Fulfill the standard:

MQT21 clauses in the IEC61215 standard and the IEC62804 standard

Features:

•The module frame end is grounded, which not only simulates the actual situation, but also prevents potential dangers caused by high voltage on the frame;

•Using a 2-wire test method, the positive and negative electrodes are short-circuited and connected to the positive electrode, and the negative electrode is connected to the equipotential hole of the frame;

•Each channel is independent of each other, and the voltage size, polarity and time of multiple channels can be set independently;

•Multiple voltages, leakage current, and insulation resistance are displayed simultaneously;

•Real-time monitoring of voltage, leakage current, and insulation resistance curves;

In the rapidly growing solar industry, the reliability and efficiency of photovoltaic systems are critical. Potential Induced Degradation Tester from Millennial Solar follows IEC standards and helps PV module manufacturers maintain high standards of quality and performance in their projects, playing a vital role in ensuring that modules reach their full potential and deliver expected performance over the long term. Contribute to the growth of renewable energy globally.