Comparison of the thermal stability of single Al2O3 layers and Al2O3/SiNx stacks for the surface passiviation of silicon

We measure surface recombination velocities (SRVs) below 10 cm/s on low-resistivity (1.4 Ωcm) p-type crystalline silicon wafers passivated with plasma-assisted and thermal atomic layer deposited (ALD) aluminium oxide (Al2O3) films. Ultrathin Al2O3 films (< 5 nm) are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is very low compared to e.g. plasma-enhanced chemical vapor deposition (PECVD). Hence, we examine the passivation quality of a stack consisting of an ultrathin Al2O 3 passivation layer deposited by ALD and a SiNx capping layer deposited by PECVD. Our experiments show a substantial improvement of the thermal stability during firing at 810°C for the Al2O 3/SiNx stacks compared to a single Al2O 3 layer. We report on a 'regeneration effect' observed for Al 2O3 single layers after firing, where the degraded passivation is significantly improved after annealing at 400°C and also by illumination at room temperature using a halogen lamp. Nevertheless, for Al 2O3/SiNx stacks we measure SRVs < 15 cm/s after firing, whereas for Al2O3 single layers the regenerated SRVs are in the range of 10-30 cm/s. Al2O 3/SiNx stacks are hence ideally suited for the implementation into industrial-type silicon solar cells, although 'regenerated' Al2O3 single layers should result in a comparable cell performance.State of Lower SaxonyGerman Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)/032505

High-rate atomic layer deposition of Al2O3 for the surface passivation of Si solar cells

High-rate spatial atomic layer deposition (ALD) enables an industrially relevant deposition of high-quality aluminum oxide (Al2O3) films for the surface passivation of silicon solar cells. We demonstrate a homogeneous surface passivation at a deposition rate of ∼30 nm/min on 15.6×15.6 cm2 silicon wafers of 10 nm thick Al 2O3 layers deposited in a novel inline spatial ALD system. The effective surface recombination velocity on n-type Czochralski-grown (Cz) silicon wafers is shown to be virtually independent of injection level. Surface recombination velocities below 2.9 cm/s and an extremely low interface state density below 8×1010 eV-1cm-2 are achieved. We demonstrate that the novel inline spatial ALD system provides the means to integrate Al2O3 passivation layers into industrial solar cells.State of Lower SaxonyGerman Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)/032505

Oxidation as Key Mechanism for Efficient Interface Passivation in Cu (In,Ga)Se2 Thin-Film Solar Cells

Copper-indium-gallium-diselenide (CIGS) thin-film solar cells suffer from high recombination losses at the back contact and parasitic absorption in the front-contact layers. Dielectric passivation layers overcome these limitations and enable an efficient control over interface recombination, which becomes increasingly relevant as thin-film solar cells increase in efficiency and become thinner to reduce the consumption of precious resources. We present the optoelectronic and chemical interface properties of oxide-based passivation layers deposited by atomic layer deposition on CIGS. A suitable postdeposition annealing removes detrimental interface defects and leads to restructuring and oxidation of the CIGS surface. The optoelectronic interface properties are very similar for different passivation approaches, demonstrating that an efficient suppression of interface states is possible independent of the metal used in the passivating oxide. If aluminum oxide (Al2O3) is used as the passivation layer we confirm an additional field-effect passivation due to interface charges, resulting in an efficient interface passivation superior to that of a state-of-the-art cadmium-sulfide (CdS) buffer layer. Based on this chemical interface model we present a full-area rear-interface passivation layer without any contact patterning, resulting in a 1% absolute efficiency gain compared to a standard molybdenum back contact. © 2020 authors. Published by the American Physical Society

Comparison of ICP-AlOx and ALD-Al2O3 layers for the rear surface passivation of c-Si solar cells

The deposition rate of the standard (i.e. sequential) atomic layer deposition (ALD) process is very low compared to the plasma-enhanced chemical vapour deposition (PECVD) process. Therefore, as a short- and medium-term perspective, PECVD aluminium oxide (AlOx) films might be better suited for the implementation into industrial-type solar cells than ALD-Al 2O3 films. In this paper, we report results achieved with a newly developed PECVD deposition process for AlOx using an inductively coupled plasma (ICP). We compare the results to high-quality ALDAl2O3 films. We examine a stack consisting of a thin AlOx passivation layer and a PECVD silicon nitride (SiNy) capping layer. Surface recombination velocities below 9 cm/s were measured on low-resistivity (1.4 Ωcm) p-type crystalline silicon wafers passivated either by ICP-PECVD-AlOx films or by ALD-Al2O3 films after annealing at 425°C. Both passivation schemes provide an excellent thermal stability during firing at 910°C with SRVs below 12 cm/s for both, Al2O3/SiNy stacks and single Al 2O3 layers. A fixed negative charge of -4×10 12 cm-2 is measured for ICP-AlOx and ALD-Al2O3, whereas the interface state density is higher for the ICP-AlOx layer with values of 11.0×1011 eV-1cm-2 compared to 1.3×1011 eV -1cm-2 for ALD-Al2O3. Implemented into large-area screen-printed PERC solar cells, an independently confirmed efficiency of 20.1% for ICP-AlOx and an efficiency of 19.6% for ALD-Al2O3 are achieved.BMU/0325296Solland Solar Cells BVSolarWorld Innovations GmbHSCHOTT Solar AGRENA GmbHSINGULUS TECHNOLOGIES A

Diode Factor in Solar Cells with Metastable Defects and Back Contact Recombination

To achieve a high fill factor, a small diode factor close to 1 is essential. The optical diode factor determined by photoluminescence is the diode factor from the neutral zone of the solar cell and thus a lower bound for the diode factor. Due to metastable defects transitions, the optical diode factor is higher than 1 even at low excitation. Here, the influence of the backside recombination and the doping level on the optical diode factor are studied. First, photoluminescence and solar cell capacitance simulator (SCAPS) simulations are used to determine the back surface recombination velocity of Cu(In, Ga)Se2 with various back contacts and different doping levels. Then, experimental results and simulations show that both back surface recombination and high doping density reduce the optical diode factor. The back surface recombination reduces the optical diode factor with undesirable extra nonradiative recombination. The smaller value achieved by higher doping can increase quasi-Fermi level splitting at the same time. The simulations show that the back surface recombination reduces the optical diode factor due to an illumination-dependent recombination rate. In addition, a higher majority carrier doping reduces the influence of majority carrier gain from metastable defect transitions, thus reducing the optical diode factor

Crystalline silicon surface passivation using aluminum oxide : fundamental understanding and application to solar cells

In the present thesis, different aspects of the silicon surface passivation provided by aluminum oxide (Al2O3) were investigated; from the fundamental understanding of the surface passivation especially on n-type silicon wafers to the implementation into "Passivated Emitter and Rear Cell" (PERC) solar cells

Exploring Learning Difficulties in Abstract Algebra: The Case of Group Theory

In an earlier contribution to Education Sciences we presented a new concept inventory to assess students’ conceptual understanding of introductory group theory—the CI2GT. This concept inventory is now leveraged in a pretest-post-test design with N=143 pre-service teachers to enrich this body of work with quantitative results. On the one hand, our findings indicate three recurring learning difficulties which will be discussed in detail. On the other hand, we provide a summative evaluation of the Hildesheim Teaching Concept and discuss students’ learning gain in different sub-domains of group theory. Together, the results allow for an empirical perspective on educational aspects of group theory and thus bridge the gap between qualitative and quantitative research in this field which constitutes a desideratum to date

Towards Describing Student Learning of Abstract Algebra: Insights into Learners’ Cognitive Processes from an Acceptance Survey

In an earlier contribution to Mathematics, we presented a new teaching concept for abstract algebra in secondary school mathematics, and we discussed findings from mathematics education research indicating that our concept could be used as a promising resource to foster students’ algebraic thinking. In accordance with the Design-Based Research framework, the developed teaching concept is now being revised in several iteration steps and optimised towards student learning. This article reports on the results of the formative assessment of our new teaching concept in the laboratory setting with N=9 individual learners leveraging a research method from science education: The acceptance survey. The results of our study indicate that the instructional elements within our new teaching concept were well accepted by the students, but potential learning difficulties were also revealed. On the one hand, we discuss how the insights gained in learners’ cognitive processes when learning about abstract algebra with our new teaching concept can help to refine our teaching–learning sequence in the sense of Design-Based Research. On the other hand, our results may serve as a fruitful starting point for more in-depth theoretical characterization of secondary school students’ learning progression in abstract algebra

Exploring Learning Difficulties in Abstract Algebra: The Case of Group Theory

In an earlier contribution to Education Sciences we presented a new concept inventory to assess students’ conceptual understanding of introductory group theory—the CI2GT. This concept inventory is now leveraged in a pretest-post-test design with N=143 pre-service teachers to enrich this body of work with quantitative results. On the one hand, our findings indicate three recurring learning difficulties which will be discussed in detail. On the other hand, we provide a summative evaluation of the Hildesheim Teaching Concept and discuss students’ learning gain in different sub-domains of group theory. Together, the results allow for an empirical perspective on educational aspects of group theory and thus bridge the gap between qualitative and quantitative research in this field which constitutes a desideratum to date

Assessing Learners’ Conceptual Understanding of Introductory Group Theory Using the CI²GT: Development and Analysis of a Concept Inventory

Prior research has shown how incorporating group theory into upper secondary school or undergraduate mathematics education may positively impact learners’ conceptual understanding of mathematics in general and algebraic concepts in particular. Despite a recently increasing number of empirical research into student learning of introductory group theory, the development of a concept inventory that allows for the valid assessment of a respective conceptual understanding constitutes a desideratum to date. In this article, we contribute to closing this gap: We present the development and evaluation of the Concept Inventory of Introductory Group Theory—the CI2GT. Its development is based on a modern mathematics education research perspective regarding students‘ conceptual mathematics understanding. For the evaluation of the CI2GT, we follow a contemporary conception of validity: We report on results from two consecutive studies to empirically justify that our concept inventory allows for a valid test score interpretation. On the one hand, we present N=9 experts‘ opinions on various aspects of our concept inventory. On the other hand, we administered the CI2GT to N=143 pre-service primary school teachers as a post-test after a two weeks course into introductory group theory. The data allow for a psychometric characterization of the instrument, both from classical and probabilistic test theory perspectives. It is shown that the CI2GT has good to excellent psychometric properties, and the data show a good fit to the Rasch model. This establishes a valuable new concept inventory for assessing students’ conceptual understanding of introductory group theory and, thus, may serve as a fruitful starting point for future research into student learning of abstract algebra