Daniel Skorka

Degradation of Surface Passivation on Crystalline Silicon and Its Impact on Light-Induced Degradation Experiments

A decrease of surface passivation quality is observed in FZ, Cz, and mc-Si lifetime samples during light-induced degradation (LID) treatments. It is shown that this degradation occurs not only in samples with single SiN<italic>_x</italic>:H layers but also when using layer stacks consisting of SiO<italic>_x</italic>/SiN <italic>_x</italic>:H or AlO<italic>_x</italic>:H/SiN<italic>_x</italic>:H. Time-resolved calculation of the surface saturation current density <italic>J</italic>₀<italic>_s </italic> is shown to be a reliable method to separate changes in the bulk and at the surface of samples during LID treatments. The impact of the observed changes in passivation quality on the outcome and interpretation of LID experiments aiming at changes in the bulk of Cz or mc-Si is investigated and discussed.

Degradation and regeneration analysis in mc-Si

The performance of mc-Si PERC solar cells can be significantly affected by LeTID. The underlying mechanism causing LeTID is still unknown. This work compares the degradation and regeneration behavior under illumination and elevated temperature of an industrial mc-Si PERC solar cell to differently processed minority charge carrier lifetime samples. A strong degradation and also regeneration can be observed on lifetime level. Degradation and regeneration are strongly influenced by the applied process steps, like gettering, temperature load and surface passivation method. Therefore, lifetime studies offer a valuable possibility to identify further parameters influencing LeTID.

Influence of dielectric layers and thermal load on LeTID

Light and elevated temperature induced degradation (LeTID) is observed for multicrystalline (mc) Si passivated emitter and rear cell (PERC) solar cells, strongly limiting solar cell parameters under operation conditions. In this contribution, we investigate the effect of surface passivation layer being present during the firing step based on lifetime samples. The LeTID effect is only observed if the surface passivation layer is present during the firing step. Samples without firing step show no LeTID. A re-passivation of the surface significantly changes the LeTID effect, showing that the whole sample treatment, temperature load and hydrogen content of a sample has to be taken into account investigating and evaluating LeTID.Light and elevated temperature induced degradation (LeTID) is observed for multicrystalline (mc) Si passivated emitter and rear cell (PERC) solar cells, strongly limiting solar cell parameters under operation conditions. In this contribution, we investigate the effect of surface passivation layer being present during the firing step based on lifetime samples. The LeTID effect is only observed if the surface passivation layer is present during the firing step. Samples without firing step show no LeTID. A re-passivation of the surface significantly changes the LeTID effect, showing that the whole sample treatment, temperature load and hydrogen content of a sample has to be taken into account investigating and evaluating LeTID.

Spatially resolved degradation and regeneration kinetics in mc-Si

The strong degradation of multicrystalline (mc) Si material upon illumination at moderate temperatures (LeTID) is an issue that might hinder the application of PERC technology to mc-Si. Interestingly, a regeneration effect can also be observed under the same conditions, starting after several hundred hours at 75°C. Studies are mainly based on time resolved investigations, and some investigations also use spatially resolved measurement techniques. Visualization of the collected data is therefore challenging, but important for a better understanding of the underlying mechanisms. In this contribution we show that based on lifetime samples we can study the same effects as reported on solar cell level with a good spatial resolution. In ungettered samples originally good quality areas show a higher relative degradation as compared to originally poor quality areas. This behaviour changes completely for gettered samples. We also show that the regeneration process sets in earlier in good quality areas, which could be caused by the higher injection level present in these regions. The regeneration process therefore might be qualitatively similar to the known BO-related regeneration. A good point-to-point correlation of different sets of lifetime measurements allows to demonstrate the impact of local differences in material quality on degradation and regeneration behaviour.

Firing and gettering dependence of effective defect density in material exhibiting LeTID

The investigation of LeTID effects in mc-Si material requires repeated measurements of the minority charge carrier lifetime τeff. Due to the inhomogeneous nature of mc-Si, measurements of τeff should be carried out in a spatially resolved way. In this work, we show results for the impact of firing and gettering on LeTID in high quality mc-Si material and analyse them in terms of the effective defect density Neff while retaining the spatial information. We show that especially for low peak firing temperature degradation is not homogeneous and that firing and gettering influence the degree of spatial inhomogeneity.The investigation of LeTID effects in mc-Si material requires repeated measurements of the minority charge carrier lifetime τeff. Due to the inhomogeneous nature of mc-Si, measurements of τeff should be carried out in a spatially resolved way. In this work, we show results for the impact of firing and gettering on LeTID in high quality mc-Si material and analyse them in terms of the effective defect density Neff while retaining the spatial information. We show that especially for low peak firing temperature degradation is not homogeneous and that firing and gettering influence the degree of spatial inhomogeneity.

Evaluating root cause : The distinct roles of hydrogen and firing in activating light- and elevated temperature-induced degradation

The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Microwave-induced remote hydrogen plasma (MIRHP) is employed to vary the concentration of bulk hydrogen and to separate the effects of hydrogen and firing temperature in LeTID-affected wafers. We find that hydrogen is required for degradation to occur, and that samples fired prior to the introduction of hydrogen do not degrade. Importantly, samples with hydrogen that have not been fired do degrade, implying that the firing time-temperature profile does not cause LeTID. Together, these results suggest that the LeTID defect reaction consists of at least two reactants: hydrogen and one or more defects that can be separately modified by high-temperature firing. We assess the leading hypotheses for LeTID in the context of our new understanding of the necessary reactants.The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Microwave-induced remote hydrogen plasma (MIRHP) is employed to vary the concentration of bulk hydrogen and to separate the effects of hydrogen and firing temperature in LeTID-affected wafers. We find that hydrogen is required for degradation to occur, and that samples fired prior to the introduction of hydrogen do not degrade. Importantly, samples with hydrogen that have not been fired do degrade, implying that the firing time-temperature profile does not cause LeTID. Together, these results suggest that the LeTID defect reaction consists of at least two reactants: hydrogen and one or more defects that can be separately modified by high-temperature firing. We assess the leading hypotheses for LeTID in the context of our new understanding of the necessary reactants.

Impact of Temperature and Doping on LeTID and Regeneration in mc-Si

Light and elevated temperature induced degradation can be observed in multicrystalline Si PERC solar cells and results in efficiency losses of up to 12%rel. This work investigates the impact of temperature as well as doping on the degradation and regeneration behavior of lifetime samples. While ungettered Ga-doped samples did not show any regeneration effect within the duration of the experiment, increasing the treatment temperature sped up the degradation and regeneration where it could be observed. Ga-doped samples showed a slower degradation compared to B-doped samples. For higher temperatures (≥200°C) the lifetimes during regeneration exceeded the initial lifetimes before starting the illumination treatment for the gettered samples. After regeneration at higher temperature, the stability of lifetime at 75°C was tested. When tested after treatment at 150°C and before the lifetime exceeded the initial value, it stayed stable. After treatment at 250°C until the lifetime exceeded the initial lifetime, it decreased again during the stability test. However, the kinetics of this decrease was much slower than during the initial degradation.

Influence of different transition metal contaminations on degradation and regeneration in mc-Si

Light and elevated temperature induced degradation (LeTID) affects significantly the performance of multicrystalline Silicon (mc-Si) solar cells. The underlying mechanisms of LeTID and following regeneration are still unknown and might depend on contaminations. To investigate the influence of different transition metals on the degradation and regeneration behavior, mc-Si ingots were intentionally contaminated in the Si melt with either 20 ppma Fe, Cu, Cr or a combination of Fe and Cu. Minority charge carrier lifetime (τeff) samples were processed and the degradation and regeneration behavior under illumination (0.9 ± 0.1 sun) and elevated temperature (75°C) is measured repetitively by time-resolved photoluminescence imaging (TR-PLI) at room temperature. The resulting minority charge carrier lifetime maps are analyzed and the effective defect concentration within one sample and also for differently contaminated samples were calculated. This approach leads to different defect generation rates for the differently contaminated samples, which might hint to an influence of different transition metals on the LeTID causing defect.

Effect of Oxygen during Thermal Annealing on the Electrical and Optical Properties of Sputter Deposited Al-Doped ZnO Films for Heterojunction Solar Cell Application

The influence of oxygen during short post deposition annealing on Al-doped zinc oxide (AZO) films for heterojunction solar cells grown by sputter deposition is investigated in terms of their electrical, optical and structural properties. AZO films were annealed for 4 min at 300°C and 450°C in ambient air and N2 atmosphere. It has been shown, that at 450°C the elimination of oxygen during annealing allows an increase in carrier concentration and reduction of the sheet resistance of AZO films four times higher than annealing in ambient air. This effect has not been observed for annealing temperatures below 300°C, where annealing in ambient air proved to have superior influence on the electrical properties. The optical bandgap of all annealed samples was enlarged and for an annealing temperature of 450°C the optical transmission weighted by ASTM G173-03 solar spectrum from 300 to 1100 nm was increased to over 80%. The changes observed in all samples were primarily explained by means of oxygen chemisorption and desorption processes. It is concluded, that oxygen plays an important role in the improvement of electrical and optical properties of AZO thin films during thermal annealing.