Application of poly-Si on oxide junctions as one or both polarities of high-efficiency IBC solar cells

This work deals with the application of passivating contacts, specifically POLO contacts, which can increase the selectivity of the charge carriers at the metal contact and thus the efficiency due to their excellent passivating effect. These poly-Si based contacts, also called TOPCon, are just establishing themselves in industrial solar cell manufacturing. However, there are still some open questions regarding their operation and optimal fabrication. The first part of the paper is focused on the so-called POLO2-IBC cell, with which Felix Hasse’s team, with the collaboration of the author of this thesis, was able to achieve a record efficiency of 26.1 % for p-type material in 2018. With an area of 4 cm² and very complex patterning processes, this back-contacted record cell is not an industrially relevant cell, but it demonstrates the great potential of POLO contacts. A distinctive feature of this cell is the continuous layers of thin oxide and overlying poly-Si that form the so-called „poly-Si on oxide“ (POLO) junctions. The electron-collecting and hole-collecting contacts, fabricated by doping via ion implantation, are separated only by narrow intrinsic POLO regions. Extensive monitoring of the fabrication process and a simulation study show that the potential of this cell type with 26.1 % has not yet been fully exploited. Furthermore, the comparison with a high resistivity base material shows the significantly higher susceptibility of the passivation quality with decreasing doping concentration, so that despite higher intrinsic recombination, the used 1.3 Ohm cm material is identified as most suitable. Separating the p+ and n+ contact through the undoped region proves to be an elegant solution, which, however, only works under certain conditions. Diffusion of dopants from the n+ and p+ regions into the intrinsic region improves the otherwise poor passivation quality there. At the same time, however, this increases the unwanted recombination current between the contacts, so the choice of the appropriate width is of immense importance. With the understanding of the working principle gained, less complex structuring is conceivable in the future. However, in order to make the leap from the laboratory cell to industrial application, stability against the firing step for contact formation with common screen printing pastes is essential. Although the POLO contacts prove by their manufacturing process alone that they can withstand high temperatures with very good passivation quality, firing without a capping layer leads to deterioration of the passivation quality. In the second experimental part of this work, it is shown that this behavior is probably due to the two orders of magnitude higher heating and cooling rates in combination with thermal stresses. However, the degradation can be counteracted with the help of hydrogen rich dielectric layers. Nevertheless, it is shown, that too much hydrogen can also have a negative effect here. For typical firing temperatures of around 800 °C, a stack of Al2O3/SiNy layers yields the best results. In the last section of this work, the use of a n+POLO contact in a screen-printed POLO-IBC cell can be successfully demonstrated with an efficiency of 23.92 %. The excellent passivation quality of 0.2 fA/cm², which can be obtained using Al2O3/SiNy stack to cap the n+POLO junction, plays an important role here. Overall, this work has helped to transfer passivating contacts from high-efficiency POLO2-IBC laboratory cells to the promising POLO-IBC cell concept suitable for industrial applications

Projective spacetime-geometry of smectic A textures

The aim of this work is to get a relation between smectics (SmA) and relativity theory. Through whose spacetime symmetries one obtain new insights about SmA. The properties of SmA are decisive determined by “focal conic domains“ (FCD) respectively the “focal lines“. With FCD one attain to Minkowskispacetime and identified SmA to projections of intersecting light cones. Then generalised that to curved spacetimes: de-Sitter-spacetime and anti-de-Sitter-spacetime. Finally one transform the free energy density between SmA’s by using spacetime symmetries

On the chances and challenges of combining electron-collecting nPOLO and hole-collecting Al-p+ contacts in highly efficient p-type c-Si solar cells

ISFH is following a distinct cell development roadmap, which comprises—as a short-term concept—the combination of an n-type doped electron-collecting poly-Si on oxide (POLO) junction with an Al-alloyed p+ junction for hole collection. This combination can be integrated either in front- and back-contacted back junction cells (POLO-BJ) or in interdigitated back-contacted cells (POLO-IBC). Here, we present recent progress with these two cell concepts. We report on a certified M2-sized 22.9% efficient POLO-BJ cell with a temperature coefficient TCη of only −(0.3 ± 0.02) %rel/K and a certified 23.7% (4 cm2 d.a.) efficient POLO-IBC cell. We discuss various specific conceptual aspects of this technology and present a simulation-based sensitivity analysis for quantities related to the quality of the hole-collecting alloyed Al-p+ junction which are subject to continuous improvement and thus hard to predict exactly. We report that the measured pseudo fill factor values decrease more due to metallization than would be expected from recombination in the metallized regions with an ideality factor of one only. The gap to pseudo fill factor values that are theoretically achievable at the respective open-circuit voltages is 1.1%abs (Ga-doped wafer) for POLO-IBC and 1.4%abs (B-doped wafer) to 2%abs (Ga-doped wafer) for POLO-BJ. With an embedded blocking layer for Ag crystallites in the poly-Si, we present a concept to reduce this gap

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

Contacting a single nanometer-sized pinhole in the interfacial oxide of a poly-silicon on oxide (POLO) solar cell junction

The electrical current through poly-Si on oxide (POLO) solar cells is mediated by tunneling and by nanometer-sized pinholes in the interfacial oxide. To distinguish the two processes, a POLO junction with a measured pinhole density of 1 × 107 cm−2 is contacted by different contact areas ranging from 1 μm2 to 2.5 × 105 μm2, and the temperature-dependent current–voltage curves are measured for the different devices. Model regressions to the measured curves, their temperature dependence, and the quantized value of contact resistances indicate average numbers of pinholes per device corresponding to the expected pinhole density. For the small contacts, the different transport processes can be studied separately, which facilitates further improvements in respect to the present-day POLO junctions. Single-pinhole transport is found for one of the contacts with an area of 1 μm2. Random telegraph noise observed for this device in the current–voltage characteristics shows a high sensitivity to single charges

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

csal1 Is Controlled by a Combination of FGF and Wnt Signals in Developing Limb Buds

While some of the signaling molecules that govern establishment of the limb axis have been characterized, little is known about the downstream effector genes that interpret these signals. In Drosophila, the spalt gene is involved in cell fate determination and pattern formation in different tissues. We have cloned a chick homologue of Drosophila spalt, which we have termed csal1, and this study focuses on the regulation of csal1 expression in the limb bud. csal1 is expressed in limb buds from HH 17 to 26, in both the apical ectodermal ridge and the distal mesenchyme. Signals from the apical ridge are essential for csal1 expression, while the dorsal ectoderm is required for csal1 expression at a distance from the ridge. Our data indicate that both FGF and Wnt signals are required for the regulation of csal1 expression in the limb. Mutations in the human homologue of csal1, termed Hsal1/SALL1, result in a condition known as Townes–Brocks syndrome (TBS), which is characterized by preaxial polydactyly. The developmental expression of csal1 together with the digit phenotype in TBS patients suggests that csal1 may play a role in some aspects of distal patterning

The C. elegans tailless/Tlx homolog nhr-67 regulates a stage-specific program of linker cell migration in male gonadogenesis

Cell migration is a common event during organogenesis, yet little is known about how migration is temporally coordinated with organ development. We are investigating stage-specific programs of cell migration using the linker cell (LC), a migratory cell crucial for male gonadogenesis of C. elegans. During the L3 and L4 larval stages of wild-type males, the LC undergoes changes in its position along the migratory route, in transcriptional regulation of the unc-5 netrin receptor and zmp-1 zinc matrix metalloprotease, and in cell morphology. We have identified the tailless homolog nhr-67 as a cell-autonomous, stage-specific regulator of timing in LC migration programs. In nhr-67-deficient animals, each of the L3 and L4 stage changes is either severely delayed or never occurs, yet LC development before the early L3 stage or after the mid-L4 stage occurs with normal timing. We propose that there is a basal migration program utilized throughout LC migration that is modified by stage-specific regulators such as nhr-67

For none, one, or two polarities—How do POLO junctions fit best into industrial Si solar cells?

We present a systematic study on the benefit of the implementation of poly-Si on oxide (POLO) or related junctions into p-type industrial Si solar cells as compared with the benchmark of Passivated Emitter and Rear Cell (PERC). We assess three aspects: (a) the simulated efficiency potential of representative structures with POLO junctions for none (=PERC+), one, and for two polarities; (b) possible lean process flows for their fabrication; and (c) experimental results on major building blocks. Synergistic efficiency gain analysis reveals that the exclusive suppression of the contact recombination for one polarity by POLO only yields moderate efficiency improvements between 0.23%abs and 0.41%abs as compared with PERC+ because of the remaining recombination paths. This problem is solved in a structure that includes POLO junctions for both polarities (POLO2), for whose realization we propose a lean process flow, and for which we experimentally demonstrate the most important building blocks. However, two experimental challenges—alignment tolerances and screen-print metallization of p+ poly-Si—are unsolved so far and reduced the efficiency of the “real” POLO2 cell as compared with an idealized scenario. As an intermediate step, we therefore work on a POLO IBC cell with POLO junctions for one polarity. It avoids the abovementioned challenges of the POLO2 structure, can be realized within a lean process flow, and has an efficiency benefit of 1.59%abs as compared with PERC—because not only contact recombination is suppressed but also the entire phosphorus emitter is replaced by an n+ POLO junction