Microstructural Analysis of Crystal Defects Leading to Potential-Induced Degradation (PID) of Si Solar Cells

Damp heat testing is an integral part of the module qualification test sequence. To develop accelerated stress tests for module wear out mechanisms, laboratories have been extending the damp heat tests beyond the qualification level. This paper examines the failure analysis of modules exposed to extended damp heat testing and concludes that there is no evidence that the dominant observed failure mode occurs in fielded modules. Analysis of moisture ingress shows that the damp heat test exposes modules to much higher levels of moisture than will ever be experienced in the field. Humidity driven field failures instead often appear to occur in combination with delamination of the encapsulant, suggesting interfaces should be pre-stressed using UV and/or humidity-freeze before applying damp heat.

Equating damp heat testing with field failures of PV modules

The first experiences with potential induced degradation (PID) of silicon solar cells have been presented in literature since 1989 (1-4). It has been shown that PID may cause power losses of more than 30% for modules out in the field. Critical issues on the cell level are the SiNx anti-reflective coatings (ARC), their deposition method, the emitter thickness and metallization (thus influencing the electrical series resistance). The leakage current was found as an indicator for the intensity of degradation. Moreover, different test methods to prove the stability or susceptibility of samples have been presented. In this paper we are showing the influence of the anti-reflection coating to provoke PID and the ability of silicon nitrite layers to prevent PID. For each layer structure, at least four different designs have been manufactured and tested for their stability against PID. To do so, it was necessary to find an appropriate test sequence, which would show polarization effects in an adequate time frame, while there is no standard procedure available that addresses this problem for the time being. Furthermore, it has been necessary to develop suitable reference samples to compare the results and to prove the repeatability of the test sequences. The experiment has been divided into two parts: The pre-evaluation has been realized with small one- cell mini-modules to probe the adequate testing parameters and to analyze the leakage currents for various prone samples. During the second part, we also investigated the reversibility of PID - depending on the state of degradation of these samples - to check whether PID is one 100% reversible, or whether the degradation mechanism also shows an irreversible part.

Potential Induced Degradation Effects on Crystalline Silicon Cells with Various Antireflective Coatings

Detailed PV array model for non-uniform irradiance and its validation against experimental data

This work evaluates the influence of temperature and irradiation on the behavior of mono-crystalline silicon, poly-crystalline silicon, and Copper Indium diselenide (CIS), modules which have been exposed to real conditions. An outdoor experimental setup has been installed, at a Mediterranean site in north latitude 38 °, in order to collect results from current-voltage measurements that corresponded at constant radiation level in order to evaluate the effect of temperature and results from measurements realized at about the same temperature in order to study the effect of irradiation. The results present that the daily generated power normalized to the manufacturer's value is positively influenced by the irradiation and not by the negative effect of temperature. The performance of mono and poly Si appears superior to that of CIS early in the morning, while this advantage is diminished during midday, when the temperature and irradiance are highest, as CIS performance becomes comparable to the other two. However, the temperature affects the efficiency and fill factor for the mono and poly-Si modules recording the lower values on higher temperatures, while it does not the same for the CIS modules. This experimental study can provide information for locations with similar climatic conditions because it helps to take into account variation in temperatures together with variation in radiation and to avoid under-designing of photovoltaic systems and system malfunction.

Kempe

Papers

Testing and analysis for lifetime prediction of crystalline silicon PV modules undergoing degradation by system voltage stress