• Energy Research

The recombination of photogenerated charge carriers at metal-semiconductor interfaces remains a major source of efficiency loss in photovoltaic cells. Here, we present SiN x and AlO x nanolayers as promising interface dielectrics to enable high efficiency hole selective passivating contacts. It is demonstrated that SiN x deposited via direct plasma enhanced chemical vapour deposition can be grown controllably at thicknesses of 2 nm. The valence band offsets between crystalline silicon and ultrathin AlO x and SiN x nanolayers are measured as 3.5 and 1.4 eV, respectively. This predicts a larger tunnelling current for holes, compared to SiO x used typically. Resistivity measurements show that SiN x and AlO x nanolayers have lower contact resistivities compared to SiO x , with values as low as 100 mΩ·cm 2 . Analysis of the current transport mechanisms confirmed that tunnelling dominates the conduction through SiN x , while a mixture of tunnelling and pinholes are present in the AlO x structure. Lifetime measurements gave initial indications of the passivation quality of the films, with just 10 cycles of AlO x achieving 260 μ s after annealing and 1.9 ms with extrinsic field effect passivation added. Finally, the intrinsic built-in charge in the dielectrics was determined using surface photovoltage measurements and simulations are used to estimate the influence of nanolayer built-in charge in both poly-Si and dopant-free passivating contacts to enable future high efficiency solar cells.

Abstract Recent progress in the electronic quality of high-performance (HP) multicrystalline silicon material is reported with measurements and modeling performed at various institutions and research groups. It is shown that recent progress has been made in the fabrication at Trina Solar mainly by improving the high excess carrier lifetimes τ due to a considerable reduction of mid-gap states. However, the high lifetimes in the wafers are still reduced by interstitial iron by a factor of about 10 at maximum power point (mpp) conditions compared to mono-crystalline Cz wafers of equivalent resistivity. The low lifetime areas of the wafers seem to be limited by precipitates, most likely Cu. Through simulations, it appears that dislocations reduce cell efficiency by about 0.25% absolute. The best predictors for PERC cell efficiency from ingot metrology are a combination of mean lifetime and dislocation density because dislocations cannot be improved considerably by gettering during cell processing, while lifetime-limiting impurities are gettered well. In future, the material may limit cell efficiency above about 22.5% if the concentrations of Fe and Cu remain above 1010 and 1013 cm−3, respectively, and if dislocations are not reduced further.

Publiziert als Diskussionsbeiträge der Scientists for Future 5 (43 pp). Die Erstveröffentlichung (nur in Deutsch) erfolgte am 16. Dez. 2020, diese geringfügige Revision 1.1 (Deutsch und Englisch) am 16. Jan. 2021. GERMAN SUMMARY (English Summary further below): Die Zeit drängt. Ohne schnell wirksame Gegenmaßnahmen werden Erderhitzung und Biodiversitätsverlust Ausmaße annehmen, welche die Lebensweise von Menschen nicht abschätzbaren Risiken aussetzen. Obwohl die Herausforderungen weiten Teilen der Bevölkerung bewusst sind, werden dringend nötige Entscheidungen aufgeschoben oder nur teilweise umgesetzt. Eine Ursache hierfür sind fehlende Foren, in denen sich Bürger:innen mit Expert:innen austauschen und gemeinsam mögliche Szenarien und Lösungen erörtern können. Scientists for Future empfiehlt deshalb, mit geeigneten Formen von Bürger:innenversammlungen eine breite und demokratisch partizipative Beteiligung an Zukunftsgestaltung und -sicherung zu ermöglichen. Diese sollten auch unabhängig von einem Auftrag von Regierung oder Parlament initiiert werden. Wir rufen daher zu einem Gründungstreffen auf, um Planung und Durchführung einer Bürger:innenversammlung zum Thema Klima im Jahr 2021 zu ermöglichen. Eine sorgfältige Planung ist nötig, damit die Durchführung neutral und offen geschieht. Hierfür werden einige zentrale Kriterien beschrieben. ENGLISH SUMMARY: Time is pressing. Without quick and effective countermeasures, global warming and loss of biodiversity will assume proportions that expose people’s way of life to incalculable risks. Although large parts of the population are aware of the challenges, urgently needed decisions are postponed or only partially implemented. One reason for this is the lack of forums where citizens can discuss possible scenarios and solutions with experts. Therefore, Scientists for Future recommends enabling a broad and democratic participatory involvement in shaping and securing the future by appropriate forms of citizens’ assemblies. These should also be initiated independently of a mandate from government or parliament. We therefore call for a founding meeting to enable the planning and implementation of a citizens’ assembly on the topic of climate in 2021. A careful planning is necessary, so that the execution happens neutrally and openly. For this purpose, some central criteria are described. ___ Suggested citation: Hagedorn, G.; Baasch, S., Blöbaum, A., Brendel, H., Hardt, J.N., Heiland, S. Klins­mann, M., Matthies, E., Pfennig, A., West, C., Wipfler, B., et al., (2021). Scientists for Future empfiehlt eine reprä­sen­ta­tive Klima-Bürger:innen­ver­sammlung im Jahr 2021 / Scientists for Future recommends a representative Climate Citizens’ Assem­bly in 2021 (Version 1.1, in Deutsch/German & Englisch/English). Diskussionsbeiträge der Scientists for Future, 5, 23 pp. doi:10.5281/zenodo.4417265. Volume 5 of "Diskussionsbeiträge der Scientists for Future". This publication contains both the German original text and an English translation.

Crystalline silicon solar cells with the passivated emitter and rear cell (PERC) design are currently the mainstream cell architecture in industry. Due to the rather complicated device structure, it has been challenging to understand how variations in manufacturing tools cause the observed scattering of the cells’ performances. This also makes it difficult to optimize PERC cells in fabrication lines. In this article, we report on a method where we use numerical device modeling, machine learning, and statistics for getting a deeper understanding of how process variations influence device performance. For this, we use seven model input parameters that affect PERC device performance the most and perform about 400 numerical device simulations in an expected range of these parameters. We then trained and validated a machine learning model on these 400 simulations, which serve to describe about 15 000 fabricated PERC cells. Because the IV parameters of each cell can be described with different sets of the seven model input parameters, we define Euclidean surroundings of the IV -parameters of each manufactured cell and analyze the behavior of the input parameters in these surroundings. The proposed method is applied to commercial production of PERC cells and requires only the four IV parameters of each cell, measured at the end of production (no dummy wafers or lifetime samples are needed). Still the method gives concrete information for improving PERC cells with a modest amount of numerical modeling and hence in a very short time. The method is generally applicable also to other solar cells than PERC cells.

In this paper, the physical mechanisms involved in electron-beam-induced current (EBIC) imaging of semiconductor pn-junctions are reviewed to propose a model and optimize the acquisition of experimental data. Insights are drawn on the dependence of the EBIC signal with electron accelerating voltage and surface conditions. It is concluded that improvements in the resolution of EBIC are possible when the surface conditions of the specimens are carefully considered and optimized. A lower accelerating voltage and an increase of the surface recombination velocities are quantitatively shown to maximize the EBIC lateral resolution in locating the pn-junction. The effect of surface band bending is included in the model, and it is seen to primarily affect the surface recombination. Introducing controlled surface damage is shown as a potential method for resolution enhancement via focused ion beam milling with Ga+ ions. These findings contribute to the understanding of this technique and can produce further improvements to its application in semiconductor device technology.