Defect detection in thin-film photovoltaics; Towards improved efficiency and longevity

Abstract

Abstract-The Photovoltaic (PV) industry is seeking to increase efficiency and functional lifetime of PV modules manufactured on polymer substrates. High resolution and high speed surface inspection for the quality control of the manufacture of large area flexible PV modules are necessary to guarantee maximum quality, longer lifetime and enhanced product yield. Flexible PV films are the newest development in the renewable energy field and the latest films have efficiencies at or beyond the level of Si-based rigid PV modules. These modules are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll technology. However, they are at present highly susceptible to long term environmental degradation as a result of water vapor transmission through the protective encapsulation to the active layer. To reduce the WVTR the PV encapsulation includes a barrier layer of amorphous Al 2 O 3 on a planarised polymer substrate. This highly conformal barrier layer is produced by atomic layer deposition (ALD). Nevertheless water vapour transmission is still facilitated by the presence of micro and nanoscale defects in these barriers which results in decreased cell efficiency and reduced longevity. Analytical techniques including: White Light ScanningInterferometry (WLSI), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to characterise the water vapor barrier defects. Areal surface texture parameter analysis allows the efficient separation of small insignificant features from significant defects. This parametric analysis is then correlated with the water vapour transmission rate as measured on typical sets of films using standard MOCON test. The paper finishes by drawing conclusions based on analysis of WVTR and defect size, density and distribution, where it is postulated that small numbers of large features have more influence on the deterioration of water vapor transmission rates than large numbers of small features. This result provides the basis for developing roll-to-roll in process metrology devices for quality control.

Effect of Water Adsorption on the Photoluminescence of Silicon Quantum Dots

“…Water is a particularly common solvent in the synthesis process, biosensor, and the electrode of light-emitting and solar cells for Si QDs. Many recent experimental studies have shown that water-induced field effect, which caused the change of electrical conductivity of hydrogen-terminated silicon substrates. − Furthermore, the photoluminescence (PL) spectra of silicon surface affected by water vapor can be used as a humidity sensor, , and Si QDs used as the electrode in light-emitting or solar cells are susceptible to degradation by water vapor. , Therefore, a critical fundamental question for Si QDs is whether and how the electronic structure and hence the PL properties is altered on the nanoscale, in particular, with the presence of water.…”

“…15−18 Furthermore, the photoluminescence (PL) spectra of silicon surface affected by water vapor can be used as a humidity sensor, 19,20 and Si QDs used as the electrode in light-emitting or solar cells are susceptible to degradation by water vapor. 21,22 Therefore, a critical fundamental question for Si QDs is whether and how the electronic structure and hence the PL properties is altered on the nanoscale, in particular, with the presence of water.…”

Imaging methods of detecting defects in photovoltaic solar cells and modules: a survey

“…ELI is an imaging method to record images of radiation emitted from a photovoltaic cell that is powered in the forward direction [22]. The range of cell radiation measured, depending on the detector used, is 300-1250 nm [23][24][25][26]. The recommended exposure time ranges from 120 s [25] to 300 s [24] or 400 s [27].…”

“…The range of cell radiation measured, depending on the detector used, is 300-1250 nm [23][24][25][26]. The recommended exposure time ranges from 120 s [25] to 300 s [24] or 400 s [27]. This method allows characterization of individual photovoltaic modules/cells, as well as whole panels [8,23,28,29].…”

Development of a simple cup method for water vapor transmission rate measurements under high-temperature conditions

“…To realize highperformance hermetic packages, high-barrier films [1][2] and encapsulants [3][4] have been widely developed, and their moisture barrier properties must be studied. For this purpose, a reliable method of measuring water vapor transmission rates (WVTRs) under high-temperature conditions of 60 ˚C and 85 ˚C, such as those utilized for damp heat testing, [5][6] is required [7].…”