Henning Nagel

Bio: Henning Nagel is an academic researcher from Leibniz University of Hanover. The author has contributed to research in topics: Passivation & Silicon. The author has an hindex of 4, co-authored 4 publications receiving 595 citations.

Generalized analysis of quasi-steady-state and quasi-transient measurements of carrier lifetimes in semiconductors

Abstract: Recently, a simple yet powerful carrier lifetime technique for semiconductor wafers has been introduced that is based on the simultaneous measurement of the light-induced photoconductance of the sample and the corresponding light intensity [Appl. Phys. Lett. 69, 2510 (1996)]. In combination with a light pulse from a flash lamp, this method allows the injection level dependent determination of the effective carrier lifetime in the quasi-steady-state mode as well as the quasi-transient mode. For both cases, approximate solutions (those for steady-state and transient conditions) of the underlying semiconductor equations have been used. However, depending on the actual lifetime value and the time dependence of the flash lamp, specific systematic errors in the effective carrier lifetime arise from the involved approximations. In this work, we present a generalized analysis that avoids these approximations and hence substantially extends the applicability of the quasi-steady-state and quasi-transient methods beyond their previous limits.

Optimised antireflection coatings for planar silicon solar cells using remote PECVD silicon nitride and porous silicon dioxide

Abstract: Silicon nitride (SiN) films fabricated by remote plasma-enhanced chemical vapour deposition (RPECVD) have recently been shown to provide an excellent electronic passivation of silicon surfaces. This property, in combination with its large refractive index, makes RPECVD SiN an ideal candidate for a surface-passivating antireflection coating on silicon solar cells. A major problem of these films, however, is the fact that the extinction coefficient increases with increasing refractive index. Hence, a careful optimisation of RPECVD SiN based antireflection coatings on silicon solar cells must consider the light absorption within the films. Optimal optical performance of silicon solar cells in air is obtained if the RPECVD SiN films are combined with a medium with a refractive index below 1·46, such as porous SiO2. In this study, the dispersion of the refractive indices and the extinction coefficients of RPECVD SiN, porous SiO2, and several other relevant materials (MgF2, TiOx, ZnS, B270 crown glass, soda lime glass, ethylene vinyl acetate and resin as used in commercial photovoltaic modules) are experimentally determined. Based on these data, the short-circuit currents of planar silicon solar cells covered by RPECVD SiN and/or porous SiO2 single- and multi-layer antireflection coatings are numerically maximised for glass-encapsulated as well as non-encapsulated operating conditions. The porous SiO2/RPECVD SiN-based antireflection coatings optimised for these applications are shown to be universally suited for silicon solar cells, regardless of the internal blue or red response of the cells. Copyright © 1999 John Wiley & Sons, Ltd.

19% efficient n-type Czochralski silicon solar cells with screen-printed aluminium-alloyed rear emitter

Abstract: High and stable lifetimes recently reported for n-type silicon materials are an important and promising prerequisite for innovative solar cells. To exploit the advantages of the excellent electrical properties of n-type Si wafers for manufacturing simple and industrially feasible high-efficiency solar cells, we focus on back junction n+np+ solar cells featuring an easy-to-fabricate full-area screen-printed aluminium-alloyed rear p+ emitter. Independently confirmed record-high efficiencies have been achieved on n-type phosphorus-doped Czochralski-grown silicon material: 18·9% for laboratory-type n+np+ solar cells (4 cm2) with shadow-mask evaporated front contact grid and 17·0% for front and rear screen-printed industrial-type cells (100 cm2). The electrical cell parameters were found to be perfectly stable under illumination. Copyright © 2006 John Wiley & Sons, Ltd.

Exceptionally high bulk minority-carrier lifetimes in block-cast multicrystalline silicon

Abstract: A comprehensive experimental study aiming at the improvement of the bulk minority- carrier lifetime τbulk in multicrystalline (mc) Si wafers by means of phosphorus pre-gettering and hydrogen bulk passivation via remote PECVD silicon nitride films in conjunction with a post- deposition anneal is presented. Within the best grains, the effective minority-carrier lifetime τeff is improved to values exceeding 400 μs. The area-averaged (25 cm≤) τeff of the wafers increased by more than a factor of 5 to 140 μs. While the above-mentioned lifetimes are stable under illumination, this work furthermore presents the first experimental evidence that the bulk lifetime in mc silicon shows similar lifetime instability effects as known from single-crystalline CZ-grown silicon. Keywords: Silicon - 1: Multi-Crystalline - 2: Gettering - 3

PERC-like Si bottom solar cells for industrial perovskite-Si tandem solar cells

Abstract: This work aims for the development of an industrially feasible Si bottom cell for two-terminal perovskite/Si tandem solar cells. A tunnel oxide and poly-Si based passivating front contact (TOPCon) as a replacement of the phosphorus diffused emitter is investigated to upgrade the industrial PERC technology to tandem bottom solar cells (TOPerc). It is shown that TOPCon is compatible with the temperature budget needed for alloying of the local Al rear contact and provides excellent passivation of the planar surface (iVoc > 720 mV). Different possible process routes and fundamental design constraints like the necessity of a hydrogenation step and a single-sided oxide removal after contact firing were investigated. In a proof-of-concept device low contact resistivity (ρc) and high passivation quality could be transferred from test structures to solar cells. The high open-circuit voltage (Voc) of 691 mV and the low series resistance (Rs) of 0.7 Ωcm2 of such a TOPerc solar cell with TCO interconnection layer highlight the potential of the proposed cell structure.

Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells

Abstract: The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.

Surface passivation of crystalline silicon solar cells: a review

Abstract: In the 1980s, advances in the passivation of both cell surfaces led to the first crystalline silicon solar cells with conversion efficiencies above 20%. With today's industry trend towards thinner wafers and higher cell efficiency, the passivation of the front and rear surfaces is now also becoming vitally important for commercial silicon cells. This paper presents a review of the surface passivation methods used since the 1970s, both on laboratory-type as well as industrial cells. Given the trend towards lower-cost (but also lower-quality) Si materials such as block-cast multicrystalline Si, ribbon Si or thin-film polycrystalline Si, the most promising surface passivation methods identified to date are the fabrication of a p–n junction and the subsequent passivation of the resulting silicon surface with plasma silicon nitride as this material, besides reducing surface recombination and reflection losses, additionally provides a very efficient passivation of bulk defects. Copyright © 2000 John Wiley & Sons, Ltd.

Semiconductor Nanowire Optical Antenna Solar Absorbers

Abstract: Photovoltaic (PV) cells can serve as a virtually unlimited clean source of energy by converting sunlight into electrical power. Their importance is reflected in the tireless efforts that have been devoted to improving the electrical and structural properties of PV materials. More recently, photon management (PM) has emerged as a powerful additional means to boost energy conversion efficiencies. Here, we demonstrate an entirely new PM strategy that capitalizes on strong broad band optical antenna effects in one-dimensional semiconductor nanostructures to dramatically enhance absorption of sunlight. We show that the absorption of sunlight in Si nanowires (Si NWs) can be significantly enhanced over the bulk. The NW’s optical properties also naturally give rise to an improved angular response. We propose that by patterning the silicon layer in a thin film PV cell into an array of NWs, one can boost the absorption for solar radiation by 25% while utilizing less than half of the semiconductor material (250% inc...