Nanoimprinted diffraction gratings for crystalline silicon solar cells: implementation, characterization and simulation

Light trapping is becoming of increasing importance in crystalline silicon solar cells as thinner wafers are used to reduce costs. In this work, we report on light trapping by rear-side diffraction gratings produced by nano-imprint lithography using interference lithography as the mastering technology. Gratings fabricated on crystalline silicon wafers are shown to provide significant absorption enhancements. Through a combination of optical measurement and simulation, it is shown that the crossed grating provides better absorption enhancement than the linear grating, and that the parasitic reflector absorption is reduced by planarizing the rear reflector, leading to an increase in the useful absorption in the silicon. Finally, electro-optical simulations are performed of solar cells employing the fabricated grating structures to estimate efficiency enhancement potential

Passivation of black silicon boron emitters with atomic layer deposited aluminum oxide

The nanostructured surface – also called black silicon (b-Si) – is a promising texture for solar cells because of its extremely low reflectance combined with low surface recombination obtained with atomic layer deposited (ALD) thin films. However, the challenges in keeping the excellent optical properties and passivation in further processing have not been addressed before. Here we study especially the applicability of the ALD passivation on highly boron doped emitters that is present in crystalline silicon solar cells. The results show that the nanostructured boron emitters can be passivated efficiently using ALD Al2O3 reaching emitter saturation current densities as low as 51 fA/cm2. Furthermore, reflectance values less than 0.5% after processing show that the different process steps are not detrimental for the low reflectance of b-Si.Peer reviewe

Full-Wafer Roller-NIL Processes for Silicon Solar Cell Texturisation

The highest solar cell efficiencies both for c-Si and mc-Si were reached using template based texturing processes. Especially for mc-Si the benefit of a defined texture, the so called honeycomb texture, was demonstrated impressively. However, up until now, no industrially feasible process has been available to pattern the necessary etching masks with the sufficient resolution. Roller-Nanoimprint Lithography (Roller-NIL) has the potential to overcome these limitations and to allow high quality pattern transfers, even in the sub-micron regime, in continuous in-line processes. Therefore, this etch-mask patterning technique is a suitable solution to bring such elaborate features like the honeycomb texture to an industrial realization. Beyond that, this fast printing-like technology opens up new possibilities to introduce promising concepts like photonic structures into solar cells

Promoting self-regulated learning in preschoolers

Self-regulated learning (SRL) is important for a person's school career and their later academic success, and it should therefore be fostered as early as possible. Nevertheless, research focusing on the promotion of SRL in preschoolers is limited. The present study aims to examine the efficacy of an SRL intervention based on a longitudinal control-group-design for preschoolers (direct-level intervention) and their kindergarten teachers (indirect-level intervention). The SRL intervention took place in either a) an autonomous learning environment, where SRL learning strategies were practiced with no special focus on the stimulation of communicative abilities or b) in a social-interactive learning environment, where SRL learning strategies were practiced while communicative abilities were stimulated. The sample consisted of 189 preschoolers (49.5% ♀, 50.5% ♂, mean age: 5.6 years, SD = .47 years) and 30 kindergarten teachers. SRL and general self-regulation ability (gSR) served as performance measures. The results of the paired t-tests revealed an increase in SRL and gSR for preschoolers irrespective of the condition, while a group-differential intervention benefit for preschoolers (i.e. direct-autonomous or direct-interactive intervention) could not be confirmed by the applied repeated measures ANOVA and contrast analyses. Further, we did not find any substantial benefit from teacher intervention (i.e. indirect intervention) analysed by non-parametric Wilcoxon test. This unexpected result is discussed in light of methodical considerations. Nevertheless, the study provides important implications for future intervention studies.&nbsp

The evaluation of an educational game to promote pre-service teachers’ self-regulated learning

The present study describes the evaluation of Regulatia, an immersive web-based educational game for preservice teachers to promote self-regulated learning (SRL). Based on Zimmerman’s model of SRL, learners immerse themselves in the underwater kingdom Regulatia and must find a way back home. Regulatia fosters the use of SRL-specific strategies and combines game elements with learning content. In this paper, the goal is to evaluate the first functional prototype of the game, examining its usability as well as users’ game experience to create a basis for an effective game in the future. The findings based on a sample of N = 31 pre-service teachers from a Southwestern German university indicate great usability and a good feedback system, high perceived knowledge improvement, and pleasant visual aesthetics. Potential for optimization was revealed for the scope and the level progression of the game

Optical Analysis of Perovskite/Silicon Tandem Solar Cells: Effect of Rear Side Grating and TOPCon Tunnel Junction

In the focus of the presented work is the analysis of a rear side reflection grating in context of a perovskite/silicon tandem solar cell. The typical configuration of a perovskite/Si tandem device requires a hole transport layer at the rear side of the silicon solar cell (p-i-n bottom cell structure). As such a poly-Si passivating contact like TOPCon is an attractive candidate. Until now, research faces the challenge to deposit p-TOPCon layers with good surface passivation properties on textured surfaces. A planar surface would avoid this issue and due to its smaller surface area intrinsically allowing for a better passivation. The optical disadvantage of the planar rear side can be eliminated by an appropriate optical grating at the rear side which enables ideal light trapping. In this work a new approach is developed to describe the optical properties of a diffraction grating as structured rear side reflector in a silicon bottom cell. The light distribution and the parasitic absorption per grating interaction are fitted to reflectance and absorptance measurements of a III-V on silicon triple-junction device with a rear grating structure. Compared to previous models, this new approach makes it possible to reliably quantify the different loss mechanisms in the spectral region above 1000 nm. An application of the simulation model to a perovskite/silicon tandem device shows the potential of the system with rear side grating. In addition, an integration of a TOPCon tunnel junction is evaluated and a process chain for the integration of a structured rear side reflector into the tandem system is discussed

Bias-voltage photoconductance and photoluminescence for the determination of silicon-dielectric interface properties in SiO2/Al2O3 stacks

This paper presents an advanced measurement method for controlling the surface charge carrier density of passivated silicon wafers during photoconductance and photoluminescence measurements, by employing semitransparent poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) electrodes with an applied bias voltage. This is employed to study and analyze charge carrier dynamics in dielectric layers by measuring their direct influence on effective lifetime. With this method, the carrier population at the surface and the effective carrier lifetimes of n- and p-type samples can be investigated, from which the fixed charge carrier density Qf of the passivation can be extracted. Additionally, the defect density Dit can also be derived from the minimum lifetime values at flatband voltage. In SiO2/Al2O3 stacks with varying SiO2 interlayer thickness, it was shown that by changing the SiO2 thickness, the carrier density Qf can be tuned to a wide range of values, which corresponds to the results obtained in other studies. An increase in interlayer thickness resulted in a decrease in Qf. Varying the SiO2 thickness, the behavior of the respective effective lifetime under bias voltage also changes, exhibiting hysteresis-like effects, which are attributed to additional charges getting trapped at the surface during bias-voltage application. This effect is much more pronounced for samples with a thinner SiO2 layer as well as for the n-type samples. Additionally, the doping type also influences the magnitude of Qf, with p-type samples generally reaching lower absolute values. It was also shown that aging of the samples had a significant effect on the measured Qf, which was increased compared to the initial Qf of the passivation. This effect was more pronounced for the n-type samples. The measurements were realized by a cost-effective and easy-to-use microcontroller-based potentiostat, which can be used as a simple add-on to existing photoconductance or photoluminescence measurement setups

Key Aspects of p-Type TOPCon on Textured Surface for Silicon Bottom Cells in Tandem Devices

Both-sides TOPCon solar cells are an interesting candidate for a highly efficient and thermally robust Silicon (Si) bottom cell for tandem devices, such as Perovskite-Si solar cells. However, preparation of p-type TOPCon on a textured surface is necessary which is particularly challenging. This work aims to gain a deeper understanding of the limiting factors and thereby optimize the SiOx/poly-Si(p) contact to close the gap to its n-type counterpart. Using symmetrical lifetime samples, we first show that a high level of surface passivation can be achieved using thermally grown interfacial oxides of different thicknesses. The samples strongly benefit from an effective thermal activation of hydrogenation by means of fast-firing. Thus, a recombination current density J0s as low as 23.4 fA/cm² was achieved for p-type TOPCon on textured surface featuring an in situ Boron-doped poly-Si layer prepared by plasma enhanced chemical vapor deposition (PECVD). Moreover, we show that the passivation quality strongly depends on surface morphology. Smoothening the random pyramids’ valleys and – what is more – the tips has a positive impact on the surface passivation