https://www.unsworks.unsw.edu.au/permalink/f/5gm2j3/unsworks_73647
The boron-oxygen-related light-induced degradation (BO-LID) and light- and elevated temperature-induced degradation (LeTID) are two common types of light-induced degradation (LID) in silicon solar cells that significantly impact performance. Despite years of research for LeTID and decades for BO-LID, the fundamental mechanisms are still poorly understood. This thesis advances the fundamental understanding of BO-LID and LeTID. A new universal metric for the relative defect concentration, beta, is introduced to study LID effects and overcome limitations of the conventional normalised defect density metric. Importantly, beta does not depend on the doping density or excess carrier concentration and can, therefore, be used to compare defect concentrations in different studies. The formulation of beta can also be used to identify the capture cross-section ratio of defects.This thesis addresses the controversy surrounding BO-related defect theories including the conventional widely accepted two-defect theory beta with a fast recombination centre (FRC) and slow recombination centre (SRC) with different recombination properties beta and a recently proposed single-defect theory. A modulation in the extent of fast and slow time-scale BO-related degradation is demonstrated with dark annealing, without modulating recombination properties, which, demonstrates a key link between degradation in both time scales. Moreover, the presence of iron is shown to account for several behaviours previously attributed to the FRC. These observations strongly support the single defect theory.The impact of injection-level dependent reaction rates for LeTID is investigated to explain the stretched exponential degradation and compressed exponential recovery behaviours in some studies. It is concluded that this behaviour is primarily due to changes in excess carrier concentration over time. Moreover, a model is developed to show that light-soaking with higher illumination intensities can minimise such effects. Subsequently, a new high-intensity laser-based light-soaking approach with in-situ photoluminescence measurements is developed and used to understand LeTID, particularly the response to dark annealing prior to illuminated annealing. Finally, a revised LeTID model is presented to account for the response to dark annealing and the likely involvement of hydrogen, with degradation caused by binding of hydrogen with a currently unidentified species.
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... This initial rise in τ observed within the first hour of illumination can be attributed to the activity of iron-boron (Fe-B) complex [35][36][37][38]. The dissociation of these complexes effectively eliminates their related recombination centers, consequently enhancing the minority carrier lifetime [39][40][41]. Further, the highest carrier lifetime values occur at about 680 °C (Fig. 2). This phenomenon can be attributed to the intricate interplay of hydrogen diffusion and passivation produced by the SiNx:H layer onto the silicon substrate [42]. ...
... To elucidate the variation of fast and slow boron-oxygen atoms combination in the silicon wafers, main recombination active defects source of light induced degradation during the illumination test and known as BO-LID [5,15,[26][27][28]34]. A kinetic model was developed employing Python code using ordinary differential equation (ODE) [39]. This model is based on two distinct BO defect theories [34]. ...
In this work, the effect of heat treatment on the minority carrier lifetime (τ) in boron-doped crystalline silicon wafers coated with a silicon nitride (SiNx:H) layer has been investigated. The results showed an initial increase in τ during the early phase of light exposure of the samples, which was attributed to the presence of iron–boron complexes in the c-Si wafers. However, this enhancement was followed by a decrease associated with the formation of boron-oxygen complexes, known as light-induced degradation. Moreover, kinetic models were used to analyze defect interactions in the wafers, showing a correlation between τ behavior and hydrogen-boron complex concentrations, and related by analytical techniques. In addition, the samples were subjected to a dark annealing step, resulting in further degradation due to the firing temperature process and the presence of hydrogen atoms in the silicon nitride layer. Finally, this study provides valuable insights into defect formation mechanisms in c-Si wafers that could improve the stability and efficiency optimization of silicon-based solar cells under operating conditions.
... The adjustment period of the solar panels can last up to 1000 h of exploration, while their efficiency is reduced by 1-3% [13]. After this period their energy conversion from these points of view remains relatively stable. ...
Using solar panels is one of the cleanest ways to generate electricity ever created by mankind. The efficiency of rapidly expanding solar panels decreases during their lifetime for several rea-sons, such as photodegradation, hot spots, potentially induced degradation, etc. Dirt and debris accumulation on the surface of the solar panels can also significantly contribute to their perfor-mance degradation due to the diminishing of the solar radiation reaching their active surfaces. Numerous degradation mitigation methods are cited in the literature. This article briefly out-lines these basic measures.
... (2), the results show great agreement with experimental data (R 2 = 0.99), as presented in Fig. 4. Furthermore, we found that if use an exponential function with two terms, the fitting quality would deteriorate (R 2 = 0.94), indicating it is more appropriate to divide degradation into two processes in this study. However, one limitation should be pointed out: we ignored the dependence of degradation rate on the injection level Δn, which would be affected by the defect concentration N t (Kim, 2020). Fig. 5 shows the Arrhenius plots of the measured rate constants R deg, fast , R deg,slow and R reg as a function of inverse temperature. ...
In this study, we performed light and elevated temperature induced degradation (LeTID) experiments in gallium-doped Czochralski silicon (Cz-Si) wafers and investigated the impact of temperature on reaction kinetics. The degradation was found to have a fast and a slow process, and their reaction rates were extracted by using an exponential function. We further determined the activation energies of each degradation as well as the regeneration according to Arrhenius plots. Moreover, we found that the capture cross-section ratio of the generated defect remained in the range of 27 < k < 35 throughout the degradation. Based on these findings, we suggested that both fast and slow degradation might be attributed to the generation of a same recombination-active defect from different precursors, and consequently have different reaction rates.
Light-and elevated temperature-induced degradation (LeTID) has been extensively studied on p-type silicon materials with increasing evidence suggesting the involvement of hydrogen. Recent findings of the identical phenomenon in n-type silicon wafers have further opened up new areas of understanding into the inherent behavior and root cause of the defect. In this work, we compare LeTID observed in both p-and n-type silicon wafers under both dark and illuminated annealing conditions, highlighting previously unobserved similarities in defect formation and recovery kinetics. We report thermal activation energies of the LeTID-related degradation and recovery in n-type silicon to be 0.76 � 0.02 eV and 0.97 � 0.01 eV without illumination, respectively, and 0.70 � 0.05 eV and 0.83 � 0.15 eV under illumination (0.02 kWm À 2), respectively. Furthermore, we present additional experimentation demonstrating the thermal and illumination dependency of surface-related degradation (SRD) in n-type silicon. We report an extracted activation energy of this SRD of 0.38 � 0.10 eV. Through modelling of the hydrogen charge state fractions, we speculate that the behavior of LeTID both in the dark and under illumination may be explained by the migration of and interactions between charged hydrogen species and dopant atoms within the diffused layers and the silicon bulk.
In this work, we integrate defect engineering methods of gettering and hydrogenation into silicon heterojunction solar cells fabricated using low‐lifetime commercial‐grade p‐type Czochralski‐grown monocrystalline and high‐performance multicrystalline wafers. We independently assess the impact of gettering on the removal of bulk impurities such as iron as well as the impact of hydrogenation on the passivation of grain boundaries and B‐O defects. Furthermore, we report for the first time the susceptibility of heterojunction devices to light‐ and elevated temperature–induced degradation and investigate the onset of such degradation during device fabrication. Lastly, we demonstrate solar cells with independently verified 1‐sun open‐circuit voltages of 707 and 702 mV on monocrystalline and multicrystalline silicon wafers, respectively, with a starting bulk minority‐carrier lifetime below 40 microseconds. These remarkably high open‐circuit voltages reveal the potential of inexpensive low‐lifetime p‐type silicon wafers for making devices with efficiencies without needing to shift towards n‐type substrates. In this work, we integrate gettering and hydrogenation into silicon heterojunction solar cells fabricated using low‐lifetime commercial‐grade p‐type Cz and multicrystalline wafers resulting in verified open‐circuit voltages of 707 and 702 mV, respectively. We assess the impact of gettering on the removal of bulk impurities and hydrogenation on the passivation of crystallographic and B‐O–related defects. Lastly, we show the susceptibility of heterojunction devices to light‐ and elevated temperature‐induced degradation and investigate the onset of such degradation during device fabrication.
Photovoltaic (PV) cells manufactured using p-type Czochralski wafers can degrade significantly in the field due to boron–oxygen (BO) defects. Commercial hydrogenation processes can now passivate such defects; however, this passivation can be destabilized under certain conditions. Module operating temperatures are rarely considered in defect studies, and yet are critical to understanding the degradation and passivation destabilization that may occur in the field. Here we show that the module operating temperatures are highly dependent on location and mounting, and the impact this has on BO defects in the field. The System Advisor Model is fed with typical meteorological year data from four locations around the world (Hamburg, Sydney, Tucson, and Wuhan) to predict module operating temperatures. We investigate three PV system mounting types: building integrated (BIPV), rack-mounted rooftop, and rack mounted on flat ground for a centralized system. BO defect reactions are then simulated, using a three-state model based on experimental values published in the literature and the predicted module operating temperatures. The simulation shows that the BIPV module in Tucson reaches 94 °C and stays above 50 °C for over 1600 h per year. These conditions could destabilize over one-third of passivated BO defects, resulting in a 0.4% absolute efficiency loss for the modules in this work. This absolute efficiency loss could be double for higher efficiency solar cell structures, and modules. On the other hand, passivation of BO defects can occur in the field if hydrogen is present and the module is under the right environmental conditions. It is therefore important to consider the specific installation location and type (or predicted operating temperatures) to determine the best way to treat BO defects. Modules that experience such extreme sustained conditions should be manufactured to ensure incorporation of hydrogen to enable passivation of BO defects in the field, thereby enabling a “self-repairing module.”
Cast- or quasi-monocrystalline silicon wafers are a potential source of low-cost material in the production of crystalline silicon solar cells. Variations caused by dislocation clusters across the cast-mono ingots have been a roadblock for the industrial production of cast-mono solar cells. Post-processing hydrogenation steps are presented in this work in order to passivate defects and dislocations across the silicon wafer and improve bulk carrier lifetime and improve spatial uniformity. Improvement in bulk carrier lifetime is presented and inherent defects in cast-mono silicon are modelled using Shockley-Read-Hall (SRH) recombination theory and the change in defect parameters during the hydrogenation process is analyzed. This analysis provides a better understanding of how post processing hydrogenation affects surface and bulk recombination characteristics in dislocated regions of cast-mono silicon wafers.
Due to rapidly reducing costs, photovoltaics has suddenly emerged as a previously underestimated new force in the race to control global warming. The technology now provides one of the lowest cost options for bulk electricity supply, as demonstrated by recently executed power purchase agreements for long-term electricity supply. However, the technology is still evolving with ongoing cost reductions likely to sustain the rapidly increasing uptake seen over the last 20 years. Present market conditions and the state of development of the technology are outlined, as are prospects for improvements and for significantly impacting carbon dioxide emissions.
A failure to recognize the factors behind continued emissions growth could limit the world’s ability to shift to a pathway consistent with 1.5 °C or 2 °C of global warming. Continued support for low-carbon technologies needs to be combined with policies directed at phasing out the use of fossil fuels.
In this work, we report on the design principles of high-power perovskite solar cells (PSCs) for low-intensity indoor light applications, with a particular focus on the electron transport layers (ETLs). It was found that the mechanism of power generation of PSCs under low-intensity LED and halogen lights is surprisingly different compared to the 1 Sun standard test condition (STC). Although a higher power conversion efficiency (PCE) was obtained from the PSC based on mesoporous-TiO2 (m-TiO2) under STC, compared to the compact-TiO2 (c-TiO2) PSC, c-TiO2 PSCs generated higher power than m-TiO2 PSCs under low-intensity (200–1600 Lux) conditions. This result indicates that high PCE at STC cannot guarantee a reliable high-power output of PSCs under low-intensity conditions. Based on the systemic characterization of the ideality factor, charge recombination, trap density, and charge-separation, it was revealed that interfacial charge traps or defects at the electron transport layer/perovskite have a critical impact on the resulting power density of PSC under weak light conditions. Based on Suns-VOC measurements with local ideality factor analyses, it was proved that the trap states cause non-ideal behavior of PSCs under low-intensity light conditions. This is due to the additional trap states that are present at the m-TiO2/perovskite interface, as confirmed by trap-density measurements. Based on Kelvin probe force microscopy (KPFM) measurements, it was confirmed that these traps prohibit efficient charge separation at the perovskite grain boundaries when the light intensity is weak. According to these observations, it is suggested that for the fabrication of high-power PSCs under low-intensity indoor light, the interface trap density should be lower than the excess carrier density to fill the traps at the perovskite's grain boundaries. Finally, using the suggested principle, we succeeded in demonstrating high-performance PSCs by employing an organic ETL, yielding maximum power densities up to 12.36 (56.43), 28.03 (100.97), 63.79 (187.67), and 147.74 (376.85) μW/cm² under 200, 400, 800, and 1600 Lux LED (halogen) illumination which are among the highest values for indoor low-intensity-light solar cells.
The commercial uptake of silicon solar cells with passivating contacts is set to accelerate over the next ten years, due to the potential for very high efficiency modules. Of particular interest are silicon heterojunction (SHJ) solar cells, which achieve high voltages by hydrogenated amorphous silicon (a-Si:H) passivation layers. A key challenge for the commercialization of SHJ solar cells is ensuring perfectly tuned a-Si:H and transparent conductive oxide (TCO) layers for optimal surface passivation and avoiding current-transport issues. Here, we demonstrate a new multifunctional post-cell fabrication process to address these challenges and improve both surface passivation and carrier transport in heterojunction solar cells. N-type silicon heterojunction solar cells with a 5-busbar metallisation scheme were fabricated in an industrial environment by CIE Power and subsequently received a high-throughput, multifunctional treatment at UNSW. This leads to significant improvements in the surface passivation, resulting in an increase in the open circuit voltage of 7 mV, from 730 mV to 737 mV, as well as improved carrier transport in the device, resulting in a significant reduction in series resistance from 0.79 Ω·cm² to 0.37 Ω·cm², and hence, an improvement in the fill factor of almost 2% absolute. This leads to an improvement in efficiency of 0.7 ± 0.16% absolute, from 22.05% to 22.75%, with a peak efficiency of 22.93%.
The basic idea, derivation, and definition of the lifetime-equivalent defect density (alternatively termed as effective, relative, or normalized defect density) in the context of studies on changes of bulk excess charge carrier lifetime in crystalline silicon is presented, and the general dependencies on injection and temperature are discussed. As the concept of lifetime-equivalent defect density is often applied to light-induced phenomena, the application to boron–oxygen-related light-induced degradation and regeneration is demonstrated by means of simulations, as well as the pitfalls, when other phenomena like iron–boron pairing/dissociation, light- and elevated-temperature-induced degradation and regeneration, or surface-related degradation are superimposed. Finally, the concept of lifetime-equivalent defect density is extended to surface phenomena.