Performance degradation analysis of high-voltage biased thin-film PV modules in hot and humid conditions

Abstract

Considerable degradation has been observed in negatively-biased, framed a-Si:H thin-film photovoltaic (PV) modules fabricated with earlier generation fluorinedoped tin oxide (Sn0z:F) transparent conducting oxide (TCO) layers on superstrate glass. These modules were tested under high-voltage (HV) bias in a hot and humid climate and leakage currents (LC) from module frames to ground were monitored. Modules negatively-biased at -600 V, -300 V and -150 V showed clear signs of delamination after 8, 15 and 27 months respectively. The adhesional strength was completely lost in the damaged area. An a-Si:H cell in the degraded region delaminated entirely from the superstrate glass and transferred itself on to the bottom glasslEVA surface, then cracked and curled. LC values from support to ground in new, unframed laminates fabricated with an improved SnOz:F TCO layer were -100 times lower under a -600 V bias in the same hot and humid environment. INTRODU C I O NUtility power applications involving grid-connected PV modules are being deployed widely in developed nations.Therefore, in addition to the possible harsh environmental conditions such as intermittent solar irradiance, high humidity, heat and wind, grid-connected PV modules must withstand HV bias. High LC generated in biased PV modules can lead to electromigration and degradation and thus can become an important issue for reliability. It can also become a safety issue because part of the HV potential on the encapsulated PV circuit can appear on the body of the modules.Efforts are being made not only to reduce the cost of thin-film PV modules but also to enhance their performance, reliability and durability. As the use of these modules becomes more widespread, their continued success will depend on the rdiability of modules under long-term exposure to widely varying environmental conditions. Temperature and humidity are two of the most severe environmental conditions [1,2]. While there are several possible causes for thin-film module failure, one area of recent concem is the magnitude of LC generated in biased PV modules, in both field-deployed modules and modules subjected to accelerated tests [3,4]. Delamination of Sn02:F TCO thinfilm from glass substrate has been observed in earlier studies [MI. This is commonly caused by a combination of moisture ingress through the module encapsulation and an electrochemical reaction at the Sn02:Flglass interface. This has been attributed to migration of sodium which is a major component of soda lime glass (56.7. Experiments conducted directly on glassnCO samples showed that the sodium migration is enhanced by heat and operational or applied voltage, and that delamination requires both sodium migration and the presence of humidity [5,8].Earfier analysis of samples from delaminated regions in PV modules deployed in harsh coastaI climates showed high surface concentrations of impurities such as sodium and phosphorous [9]. While there has been research to develop robust packaging schemes that will enhance HV isolation of the cell f...

Potential-induced degradation: a challenge in the commercialization of perovskite solar cells

“…Researchers found a different PID mechanism in many studies when moisture enters the modules, especially under humid conditions. 16,17,42,51,114,117,118 In addition to the surrounding environment, the packaging quality of the module can affect how much moisture gets inside. For example, a module with high-quality packaging will be protected from moisture damage even in wet environments.…”

Degradation of photovoltaic modules under high voltage stress in the field

“…The service lifetime and reliability of PV modules deployed in such a HV configuration can be compromised by leakage-current-induced electrochemical corrosion. The HV leakage currents (LC) that occur continuously during operation can degrade electrical contacts or PV absorber material itself and/or lead to delamination of the layers [3][4][5], typically leading to high series resistance, lower performance, or even failure. High levels of ambient moisture and temperature exacerbate the HV leakage at any voltage, especially for thin-film modules [6].…”

“…In that study, we ignored the contribution to the LC through the back sheet or glass, but have now considered and depicted this pathway as I 4 in Fig. 1.…”