Degradation in next-generation passivating contact solar cells

The solar photovoltaics industry is quickly transitioning from the p-type passivated emitter with rear contact (PERC) cells to n-type-based cell architectures that incorporate various forms of passivating contacts. By reducing surface recombination, these passivating contacts allow for much higher open-circuit voltages and efficiencies. However, recent studies have shown that various forms of stability and reliability may limit their field performance. This thesis aims to advance the current understanding of these instabilities by investigating the roles of passivation layers and processing conditions on these structures. In this thesis, by employing advanced hydrogenation techniques, heterojunction solar cells are investigated to understand their long-term stability. Using a combination of high-intensity illumination and temperature, the transient nature of defects present in commercial heterojunction cells is studied. An observation of a light-induced degradation not previously reported is evaluated, with an analysis of the influences of illumination and temperature on the defect kinetics carried out. The techniques previously established in the literature for identifying defects such as light and elevated temperature-induced degradation (LeTID) and boron-oxygen light-induced degradation (BO-LID) are adapted to study passivating oxide contacts on n- and p-type silicon wafers. The roles of processing conditions such as firing temperature, and passivation layers on the front, rear, and interface layers are studied. An assessment of the long-term stability of p- and n-type silicon wafers with polysilicon layers is made, with some techniques to improve the kinetics of degradation and recovery presented

Ozone (O3) Process Technology (OPT): An Exploratory Brief of Minimal Ozone Discharge applied to Shrimp Product

AbstractGlobally, the demand for energy resources is currently on the rise, which directly leads to increase in the pursuit to identify as well as utilize both enhanced and environmental-friendly technologies underscoring direct challenges associated with energy conservation. Principally, ozone treatment stands among emerging innovative technologies of great potential for the food industry. After its declaration as 'Generally Recognized As Safe' (GRAS) it is but only a few decades ago that the promises of ozone treatment became more evident, in fact, more noticeable in recent times particularly for domestic and home use. These promises of ozone treatment are largely attributable to chemical and physical properties of the ozone (O3) molecule. Contrariwise, the unstable nature of the O3 molecule has been the underwriting factor that frequently impedes the commercialization of this technology. It is in line with these arguments that this paper is underpinned, by presenting ozone process technology (OPT) through an exploratory brief with respect to ozone discharge that has been minimally applied to shrimp product. This exploratory brief is tersely performed using an analytical appraisal, which attempts to reveal some energy aspects that might have ample relevance. In addition, the justification/rationale 'why minimal ozone treatment' is succinctly debated. Anyways, considering that a number of alternative domestic type sanitizers that safely generate ozone are currently penetrating the market worldwide, the concentrations of ozone discharge of almost if not all (of these domestic types) apparently seem not well defined – hence, not consistent. This paucity of definition of concentrations of ozone discharge therefore necessitates supplemental investigations to provide informative and robust data particularly identifying with the domestic-type of ozone-generating facilities. Largely, the motivation for this is that the procurement of domestic-types by many homes around the world is, not only currently on the rise, but more particularly that these facilities are largely believed to effectively sanitize fresh foods. Equally, considering the great energy potential associated with the ozonation process itself, we opine that the ozone process as a technology either in part or whole may well lend itself as a novel, renewable and valuable energy material for future use, a feasible exploration for renewable energy industries to undertake – very appropriate and credible motivation for future investigations

Assessing the stability of p+ and n+ polysilicon passivating contacts with various capping layers on p-type wafers

Polysilicon (poly-Si)-on-oxide passivating contact structures (POLO/TOPCon) enable high-efficiency solar cells as they simultaneously provide a very high level of surface passivation and a high conductance for either electrons or holes. The ease of incorporation with existing manufacturing lines and their tolerance for high-temperature processing has increased the wide acceptance of this structure in the PV industry. In this report, we explore the effects of short high-temperature annealing required for effective hydrogenation and formation of ohmic screen-printed contacts across a wide temperature range (636 °C–846 °C) on the stability of passivating contact structures. We study this on p-type c-Si substrates with phosphorus-doped (n-type) or boron-doped (p-type) polysilicon contacts capped with either an AlOx or SiNx coating. Our experimental results show that irrespective of the poly-Si doping type, AlOx-capped samples suffer a loss in surface passivation across the investigated temperature range, while SiNx-capped samples show an improvement at lower annealing temperatures. Above 744 °C, severely ruptured blisters occur for the samples coated with a SiNx layer, leading to lift-off of the poly layer in extreme cases, and in all cases, significant surface passivation losses, up to 99%. A study of the long-term stability of these fired samples under 1-sun illumination @ 140 °C shows that they suffer from both bulk and surface-like instabilities. Two degradation cycles were observed: the first, a boron-oxygen light-induced degradation (BO-LID) observed after 5 min, with capture cross-section ratios of 15.8–19.2, and a slower secondary degradation, similar to light and elevated temperature-induced degradation (LeTID), with maximum degradation reached after ∼ 14 days. The presence of a silicon nitride layer does not appear to influence the kinetics of post-degradation recovery. Our results suggest that the effect of firing may be influenced by the polarity of the bulk c-Si or perhaps the chemistry of the SiNx film and highlight that passivating contact structures based on p-type c-Si may offer better long-term stability than those based on n-type c-Si

Firing stability of tube furnace-annealed n-type poly-Si on oxide junctions

Stability of the passivation quality of poly-Si on oxide junctions against the conventional mainstream high-temperature screen-print firing processes is highly desirable and also expected since the poly-Si on oxide preparation occurs at higher temperatures and for longer durations than firing. We measure recombination current densities (J0) and interface state densities (Dit) of symmetrical samples with n-type poly-Si contacts before and after firing. Samples without a capping dielectric layer show a significant deterioration of the passivation quality during firing. The Dit values are (3 ± 0.2) x 1011 and (8 ± 2) x 1011 eV/cm2 when fired at 620°C and 900°C, respectively. The activation energy in an Arrhenius fit of Dit versus the firing temperature is 0.30 ± 0.03 eV. This indicates that thermally induced desorption of hydrogen from Si-H bonds at the poly-Si/SiOx interface is not the root cause of depassivation. Postfiring annealing at 425°C can improve the passivation again. Samples with SiNx capping layers show an increase in J0 up to about 100 fA/cm2 by firing, which can be attributed to blistering and is not reversed by annealing at 425°C. On the other hand, blistering does not occur in poly-Si samples capped with AlOx layers or AlOx/SiNy stacks, and J0 values of 2–5 fA/cm2 can be achieved after firing. Those findings suggest that a combination of two effects might be the root cause of the increase in J0 and Dit: thermal stress at the SiOz interface during firing and blistering. Blistering is presumed to occur when the hydrogen concentration in the capping layers exceeds a certain level