Showing papers by "Stefan W. Glunz published in 2022"

How to make PERC suitable for perovskite–silicon tandem solar cells: A simulation study

Abstract: Low‐cost and high‐efficiency tandem solar cells are promising candidates for a future industrial mass production. Nowadays, the passivated emitter and rear cell (PERC) technology makes up the major market share; therefore, it is an attractive option to use the PERC technology as bottom cell concept for a perovskite–silicon tandem device. Long‐term optimization of the PERC technology led to highly efficient, low‐cost, and mature devices. For PERC‐like bottom cells, mainly an adapted front‐side design is needed: Design constrains of a PERC single junction are relaxed to some extent, because such an updated PERC bottom cell only needs to absorb long wavelength photons, transports about half the current and, in a monolithic tandem, lateral transport, and the use of firing‐through local silver contacts is not a mandatory prerequisite. Consequently, to make PERC suitable for tandem application, a systematic reevaluation of the current PERC technology is performed here considering five different front‐side concepts. We investigate locally contacted and full‐area transparent conducting oxide (TCO)‐based interconnection concepts for PERC, as well as the full‐area tunnel oxide passivating contact (TOPCon). Our simulation work elaborates on the advantages and physical constrains of each concept and gives guidelines for the optimization of the phosphorus diffused emitter and front interconnection layers. We conclude that both full‐area and locally contacted front‐side concepts are promising candidates for achieving tandem cell efficiencies of about 30%.

A Monolithic Silicon‐Mesoporous Carbon Photosupercapacitor with High Overall Photoconversion Efficiency

Abstract: Off‐grid devices require autonomous power supply systems that can be achieved via coupling a solar cell with a supercapacitor in one integrated multifunctional device that is able to harvest energy from the environment, store, and release it on demand. Herein, a straight‐forward approach is proposed to realize a monolithic photosupercapacitor rechargeable under illumination by integrating a silicon (Si) solar cell with an electrochemical double‐layer capacitor (EDLC) based on mesoporous N‐doped carbon nanospheres (MPNC) in a three‐electrode configuration, i.e., via a shared electrode. The optimized porous structure of the MPNC‐EDLC electrodes results in a high storage efficiency of 95% and a superior performance compared to an activated carbon based EDLC. When monolithically integrated with a highly compatible and technologically robust Si solar cell in a photosupercapacitor, fast charging up to 0.63 V (2.5 V for a module of four photosupercapacitors connected in series) in less than 5 s is attained even under weak illumination conditions, with an outstanding peak overall efficiency of 11.8%. These results show high potential toward the development of photorechargeable and decentralized power sources for deployed smart electronic devices.

(Digital Presentation) A Highly Efficient Photosupercapacitor By Integration of a Mesoporous N-Doped Carbon Double Layer Capacitor with a Perovskite Solar Cell

Abstract: Internet of Things devices – wireless sensors and actuators – require long-term off-grid power sources that would be cheap, and at the same time have a small footprint with low environmental impact throughout their life cycle. Such an off-grid power source could be realized with a photosupercapacitor: A solar cell is integrated with an electrochemical double-layer capacitor (EDLC) into a single monolithic device using a three-electrode design, where the two components share a common electrode.1 The solar cell converts ambient light into an electric current and charges the integrated supercapacitor via the common electrode, which in turn powers the external device. The supercapacitor, on the other hand, acts as a buffer, mediating between an intermittent light source and the load of the device. In terms of footprint and hence energy and power density, integrated monolithic photosupercapacitors have a clear advantage in comparison with simply wiring a solar cell to a supercapacitor as storage unit. When compared to batteries or pseudocapacitors that involve slow solid-state diffusion reactions and/or surface redox processes, EDLCs are particularly suited for the integration with solar cells into photosupercapacitors as they are able to operate efficiently in fast fluctuating environments where rapid on/off cycles are required without the need to supply a fixed voltage onset.2 Owing to their large specific surface area, good electrical conductivity, and electrochemical stability, mesoporous nitrogen-doped carbon nanospheres (MPNCs) show a great promise for being implemented as an electrode material for supercapacitors.3 , 4 To synthesize MPNCs we used a hard-templating strategy based on the polymerization and self-assembly of aniline in the presence of SiO2 nanoparticles (7 nm), which led to the formation of spherical SiO2/Polyaniline composites. Their carbonization and template removal resulted in highly monodisperse, 140 nm-diameter spherical MPNCs with a large specific surface area (825 m2 g-1) and defined mesopores (7 nm). The MPNCs are easily dispersible and could be processed by doctor-blading in homogeneous 3D-percolated electrodes. Benefiting from the well-defined mesoporous network and the highly accessible electrochemical surface area, our MPNC-based gel-electrolyte freestanding EDLC delivered large energy and power densities and a high capacitance (400 F g-1 at 1 A g-1) with high (95 %) coulombic efficiency. To achieve an energy-autonomous self-powered system, we combined the MPNC-based EDLCs with halide-perovskite-based solar cells in a monolithic three-electrode fashion. As the solar cell we used a p-i-n perovskite solar cell with a FA0.75Cs0.25Pb(I0.8Br0.2)3 (high bandgap) absorber and large photosensitive area (1 cm2) delivering a high open-short circuit voltage (VOC ) of 1.08 V and a short-circuit current (JSC ) of 17.9 mA/cm2. To facilitate the assembly process and to address the adverse sensitivity of the perovskite layer to the electrolyte solvent, we optimized the solar cell layer sequence and termination. At the same time, we used a semi-solid gel electrolyte for the EDLC, which minimizes the contact of the perovskite layer with the electrolyte solvent. This allowed us to obtain a free-standing integrated photosupercapacitor without the necessity of any encapsulation. This reduces the overall device footprint and cost, and simplifies the assembly. Benefiting from the high efficiency of the solar cell and the excellent performance of the EDLC, the integrated hybrid photosupercapacitor demonstrated fast (< 5 s) photocharging up to 1 V under 1 sun illumination and discharge through the EDLC terminals, proving that the solar energy was harvested, converted, and stored. The photosupercapacitor delivered 4.27 μWh/cm2 at a power density of 0.29 mW/cm2 and 1.68 μWh/cm2 at 2.2mW/cm2 with an areal capacitance of 31 mF/cm2. This resulted in an unprecedented overall electrochemical energy conversion efficiency of 11.5 %.5 The strategy for photosupercapacitor fabrication presented here was extended to the integration of MPNC-based EDLCs with other types of solar cells, depending on the intended application. The achieved results pave the way towards the development of off-grid powered energy-autonomous devices. 1 Q. Zeng, Y. Lai, L. Jiang, F. Liu, X. Hao, L. Wang and M. A. Green, Adv. Energy Mater., 2020, 10, 1–30. 2 F. Béguin, V. Presser, A. Balducci and E. Frackowiak, Adv. Mater., 2014, 26, 2219–2251. 3 J. Melke, R. Schuster, S. Möbus, T. Jurzinsky, P. Elsässer, A. Heilemann and A. Fischer, Carbon N. Y., 2019, 146, 44–59. 4 J. Melke, J. Martin, M. Bruns, P. Hu, A. Scho, A. Fischer, S. M. Isaza, F. Fink and P. Elsa, ACS Appl. Energy Mater, 2020, 3, 11627. 5 T. Berestok, C. Diestel, N. Ortlieb, J. Buettner, J. Matthews, P. S. C. Schulze, J. C. Goldschmidt, S. W. Glunz and A. Fischer, Sol. RRL, 2021, 5, 1–13.

21.4% efficient SMART mono-Si PERC solar cells with adapted firing process for LeTID mitigation

Abstract: We have developed a fast firing oven (FFO) firing profile that mitigates light and elevated temperature induced degradation (LeTID) in boron doped passivated emitter and rear cells (PERC), made from high-performance multicrystalline silicon (hp mc) and mono-cast material. During LeTID testing, the highest degradation in relative efficiency in the samples fired with this profile is only -2%rel < Δη <-1%rel depending on the cell, compared to -5%rel < Δη < -6%rel in samples fired with the standard profile. To show that the benefit of this technique can be transferred to modules, the influence of the temperature profile during lamination was investigated. The effect of LeTID mitigation persisted after the simulated lamination process. This new firing profile did not significantly influence the initial efficiencies of the cells that have been fired with it compared to samples fired with a standard firing profile (-0.2%abs < Δη < +0.1%abs). Since the new FFO profile is only a slight alteration to a standard profile, it can be easily integrated into existing production lines. The altered firing profile therefore seems to be a promising way to handle the LeTID challenge and ensure highest cell efficiencies throughout the whole lifecycle of modules made from mono-cast, hp mc, and probably other types of silicon solar cells in the field. The mono-cast wafers used in this work were grown with the seed manipulation for artificially controlled defects technique (SMART). It solves the challenge of grain boundaries growing inward from the crucible wall by introducing dislocation clusters near the wall that prevent this growth. We show that the efficiency of cells that were made with this technique (21.4%) is similar to that of magnetically grown Czochralski (mCz) cells (21.6%) which were otherwise produced identically. A third group of hp mc cells which have also been identically produced except for an acidic texture instead of random pyramids, however only achieved 19.5% efficiency. Therefore, using a standard PERC process, SMART mono material is clearly superior to hp mc material and is similar to mCz silicon. We also show that mCz cells do not show LeTID degradation.

Potentiostatic Photoluminescence Imaging of Charge Extraction in Perovskite Solar Cells

Abstract: We propose a novel approach for microscopically resolved photocurrent imaging. The method is based on the notion that electrical bias-dependent photoluminescence images reveal fundamental information on charge extraction of photovoltaic devices. The approach is derived from basic physical principles and verified by means of a near-to-ideal III-V solar cell. It is demonstrated that the approach is of special relevance for liquid-processed perovskite solar cells. We outline the potential to investigate the local charge extraction efficiency, which can be related to fundamental charge extraction properties associated with the quality of interfaces and morphological defects from device processing. The method demonstrates that photoluminescence imaging can be a powerful tool for device optimization as well as fundamental studies when carried out at different bias voltages.

Contactless Inline IV Measurement of Solar Cells Using an Empirical Model

Abstract: The current–voltage measurement is the most important measurement in solar cell quality control. As the contacting process of cells results in mechanical stress and consumes a significant amount of measurement time, this work presents an IV characterization based on contactless measurements only. An empirical model is introduced that can derive the full IV curve and IV parameters as the open‐circuit voltage, short‐circuit current density, fill factor, and efficiency. As a basis, a series of photoluminescence and contactless electroluminescence images and spectral reflectance measurements are used. An advantage of the model's convolutional neural network design lies in the semantic compression of local image structures across the input data. Within an ablation study, it is shown that the empirical model is well suited to combine these data sources, which is the optimal input configuration for contactless IV derivation. The accuracy, e.g., with an error in efficiency of 0.035 % abs and correlation of over 99%, is similar to comparing two contacting IV measurement devices. The contactless IV curves also have a close fit to their contacted counterparts. Within simulations on module level, it is demonstrated that contactless binning performs as well as contacting binning and does not result in any additional mismatch loss.

Combining Drift-Diffusion and Equivalent-Circuit Models for Efficient 3D Tandem Solar Cell Simulations

Abstract: For upscaling silicon based tandem solar cells from small laboratory sizes to full size formats compatible with industrial production, two-dimensional (2-D) and 3-D effects like metal grid layout, perimeter design, and lateral inhomogeneities gain importance for tandem cell development. For understanding and quantifying such effects, 3-D tandem modeling is helpful, but the capabilities of existing solar cell simulation tools is limited in this respect. In this article, we describe a numerically efficient 3-D tandem modeling approach implemented in the solar cell simulation software Quokka3. It combines a 1-D equivalent-circuit (EQC) model of the top cell within the front side's boundary condition with either the quasi-neutral 3-D drift-diffusion model or an EQC model for the bottom cell's bulk carrier transport. This way the addition of a top cell to a single-junction silicon bottom cell model in Quokka3 adds little effort in terms of computational time and is thus compatible with large-area 3-D simulations up to full cell geometries. We showcase the usefulness of this approach by investigating various perimeter designs of small-area silicon-perovskite cells.

Stable reverse bias or integrated bypass diode in HIP‑MWT+ solar cells

Abstract: The Metal Wrap Through+ (HIP-MWT+) solar cell is based on the PERC concept but features two additional electrical contacts, namely the Schottky contact between p-type Si bulk and Ag n-contact and the metal-insulator-semiconductor (MIS) contact on the rear side of the cell below the n-contact pads. To prevent thermal hotspots under reverse bias, both contacts shall either restrict current flow or allow a homogenous current flow at low voltage. In this work we present both options. First the stable reverse bias characteristics up to −15 V with a MIS contact using industrially manufactured SiON passivation and second, an integrated by-pass diode using AlOX as insulator in the passivation stack allowing current flows at approximately Vrev = –3.5 V depending on the chosen screen-print paste. The examined Schottky contacts break down at around Vrev = –2.5 V. Reverse bias testing of the cells reveals a solid performance of the cells under reverse bias and an average conversion efficiency of η = 21.2% (AlOX) and η = 20.7% (SiON), respectively.