Browsing by Author "Vahlman, H."

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Impact of copper on light-induced degradation in Czochralski silicon PERC solar cells
(2018-11-01) Modanese, C.; Wagner, Mt; Wolny, F.; Oehlke, A.; Laine, H. S.; Inglese, A.; Vahlman, H.; Yli-Koski, M.; Savin, H.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Both multicrystalline and Czochralski (Cz) silicon substrates are known to suffer from various mechanisms of light-induced degradation (LID), including copper-related LID (Cu-LID). Past studies on Cu-LID have mostly been performed on unprocessed wafers, omitting the impact of the solar cell process on the copper distribution. Here, we carefully contaminate Cz-substrates of different quality with different amounts of copper and process the substrates into complete industrial Cz-Si PERC solar cells, reaching a comprehensive mapping of the impact of Cu-LID for the PV industry. The results show that both the copper contamination level and Cz crystal quality are critical factors affecting the extent of Cu-LID. Most importantly, we show that copper can result in significant concentrations in the bulk of the finished PERC cells after being exposed to only trace surface contamination. Consequently, even a small local copper contamination area (~ 3–4 cm2) is sufficient to induce strong LID in the full-sized (156 × 156 mm2) cell parameters, resulting e.g. in ~7% relative efficiency loss during light soaking. The corresponding short circuit current density decreases by up to a factor of two in the contaminated areas.
Modeling of light-induced degradation due to Cu precipitation in p-type silicon. I. General theory of precipitation under carrier injection
(2017-05-21) Vahlman, H.; Haarahiltunen, A.; Kwapil, W.; Schön, J.; Inglese, A.; Savin, H.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Copper contamination causes minority carrier lifetime degradation in p-type silicon bulk under illumination, leading to considerable efficiency losses in affected solar cells. Although the existence of this phenomenon has been known for almost two decades, ambiguity prevails about the underlying defect mechanism. In Paper I of this two-part contribution, we propose the first comprehensive mathematical model for Cu-related light-induced degradation in p-type silicon (Cu-LID). The model is based on the precipitation of interstitial Cu ions, which is assumed to be kinetically limited by electrostatic repulsion from the growing Cu precipitates. Hence, growth and dissolution rates of individual Cu precipitates are derived from the drift-diffusion equation of interstitial Cu and used in a kinetic precipitation model that is based on chemical rate equations. The kinetic model is interlinked to a Schottky junction model of metallic precipitates in silicon, enabling accurate calculation of the injection-dependent electric field enclosing the precipitates, as well as the precipitate-limited minority carrier lifetime. It is found that a transition from darkness to illuminated conditions can cause an increase in the kinetics of precipitation by five orders of magnitude. Since our approach enables a direct connection between the time evolution of precipitate size-density distribution and minority carrier lifetime degradation under illumination, a procedure for calculating the Cu-LID-related lifetime as a function of illumination time is included at the end of this article. The model verification with experiments is carried out in Paper II of this contribution along with a discussion of the kinetic and energetic aspects of Cu-LID.
Modeling of light-induced degradation due to Cu precipitation in p-type silicon. II. Comparison of simulations and experiments
(2017-05-21) Vahlman, H.; Haarahiltunen, A.; Kwapil, W.; Schön, J.; Inglese, A.; Savin, H.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The presence of copper impurities is known to deteriorate the bulk minority carrier lifetime of silicon. In p-type silicon, the degradation occurs only under carrier injection (e.g., illumination), but the reason for this phenomenon called copper-related light-induced degradation (Cu-LID) has long remained uncertain. To clarify the physics of this problem, a mathematical model of Cu-LID was introduced in Paper I of this article. Within the model, kinetic precipitation simulations are interlinked with a Schottky junction model for electric behavior of metallic precipitates. As this approach enables simulating precipitation directly at the minority carrier lifetime level, the model is verified in this second part with a direct comparison to the corresponding degradation experiments and literature data. Convincing agreement is found with different doping and Cu concentrations as well as at increased temperature, and in the dark, both simulated degradation and measured degradation are very slow. In addition, modeled final lifetimes after illumination are very close to experimental final lifetimes, and a correlation with the final precipitate size is found. However, the model underestimates experimentally observed differences in the degradation rate at different illumination intensities. Nevertheless, the results of this work support the theory of Cu-LID as a precipitate formation process. Part of the results also imply that heterogeneous nucleation sites play a role during precipitate nucleation. The model reveals fundamental aspects of the physics of Cu-LID including how doping and heterogeneous nucleation site concentrations can considerably influence the final recombination activity.
(poster) Copper-Related Light Induced Degradation Activated by Firing
(2018) Nampalli, N.; Laine, H. S.; Colwell, J.; Vähänissi, V.; Inglese, A.; Modanese, C.; Vahlman, H.; Yli-Koski, M.; Savin, H.
Abstract
(poster) Impact of copper on light-induced degradation in Czochralski silicon PERC solar cells
(2018) Modanese, C.; Wagner, Mt; Wolny, F.; Oehlke, A.; Laine, H. S.; Inglese, A.; Vahlman, H.; Yli-Koski, M.; Savin, H.
Abstract
Rapid thermal anneal activates light induced degradation due to copper redistribution
(2018-07-16) Nampalli, N.; Laine, H. S.; Colwell, J.; Vähänissi, V.; Inglese, A.; Modanese, C.; Vahlman, H.; Yli-Koski, M.; Savin, H.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
While it is well known that copper impurities can be relatively easily gettered from the silicon bulk to the phosphorus or boron-doped surface layers, it has remained unclear how thermally stable the gettering actually is. In this work, we show experimentally that a typical rapid thermal anneal (RTA, a few seconds at 800 °C) used commonly in the semiconductor and photovoltaic industries is sufficient to release a significant amount of Cu species from the phosphorus-doped layer to the wafer bulk. This is enough to activate the so-called copper-related light-induced degradation (Cu-LID) which results in significant minority carrier lifetime degradation. We also show that the occurrence of Cu-LID in the wafer bulk can be eliminated both by reducing the RTA peak temperature from 800 °C to 550 °C and by slowing the following cooling rate from 40-60 °C/s to 4 °C/min. The behavior is similar to what is reported for Light and Elevated Temperature degradation, indicating that the role of Cu cannot be ignored when studying other LID phenomena. Numeric simulations describing the phosphorus diffusion and the gettering process reproduce the experimental trends and elucidate the underlying physical mechanisms.