Nadine Wehmeier

Institut für Solarenergieforschung

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Rolf Brendel

Institut für Solarenergieforschung

Robby Peibst

Institut für Solarenergieforschung

Thorsten Dullweber

Institut für Solarenergieforschung

Till Brendemühl

Institut für Solarenergieforschung

Felix Haase

Institut für Solarenergieforschung

Henning Schulte‐Huxel

Institut für Solarenergieforschung

Byungsul Min

Institut für Solarenergieforschung

Anja Nowack

Institut für Solarenergieforschung

Yevgeniya Larionova

Institut für Solarenergieforschung

Sarah Kajari‐Schröder

Institut für Solarenergieforschung

Cooperative Institutions

Institut für Solarenergieforschung

Leibniz University Hannover

Karlsruhe Institute of Technology

Evonik (Germany)

Carl Zeiss (Germany)

Fraunhofer Institute for Photonic Microsystems

Helmholtz-Zentrum Berlin für Materialien und Energie

University of Münster

University of Göttingen

University of California, Berkeley

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Reduced thermal conductivity of isotopically modulated silicon multilayer structures

Applied Physics Letters (2012)

H. Bracht Nadine Wehmeier S. Eon Anton Plech Daniel Issenmann

We report measurements of the thermal conductivity of isotopically modulated silicon that consists of alternating layers of highly enriched silicon-28 and silicon-29. A reduced thermal conductivity of the isotopically modulated silicon compared to natural silicon was measured by means of time-resolved x-ray scattering. Comparison of the experimental results to numerical solutions of the corresponding heat diffusion equations reveals a factor of three lower thermal conductivity of the isotope structure compared to natural Si. Our results demonstrate that the thermal conductivity of silicon can be effectively reduced with isotopically modulated structures. This offers a promising approach to optimize silicon for thermoelectric applications.

10.1063/1.4742922

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Citations (18)

PECVD BSG Diffusion Sources for Simplified High-Efficiency N-PERT BJ and BJBC Solar Cells

IEEE Journal of Photovoltaics (2015)

Nadine Wehmeier Bianca Lim Agnes Merkle A. Tempez S. Legendre

We investigate boron silicate glasses (BSG) deposited by plasma-enhanced chemical vapor deposition (PECVD) as a boron diffusion source on n-type wafers for the simplified fabrication of crystalline Si solar cells by the codiffusion processes. By varying the SiH ₄ /B ₂ H ₆ gas flow ratio and the layer thickness of the PECVD BSG layers, we obtain sheet resistivities in a wide range from 30 to 500 Ω/□ after thermal B drive-in. Emitter saturation current densities as low as J _0e = 4 fA/cm ² (for R _sheet = 236 Ω/□) are demonstrated using PECVD BSG layers as diffusion sources. A boron concentration in the PECVD BSG of up to 6.4 × 10 ²¹ cm ^-3 is measured by plasma profiling time-of-flight mass spectrometry (PP-TOFMS). A process simulation model of the B diffusion from the PECVD BSG into the Si substrate reproduces the experimental B concentration profile in the Si, measured both by PP-TOFMS and electrochemical capacitance- voltage (ECV) measurements. We fabricate industrial-type passivated emitter and rear totally diffused back-junction (PERT BJ) solar cells, as well as back-junction back-contact cells on n-type wafers. Applying codiffusion from PECVD BSG layers and thus a lean process flow including only one high-temperature step, we demonstrate n-PERT BJ cells with conversion efficiencies of up to 19.85%.

10.1109/jphotov.2015.2493364

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Citations (14)

21.0%‐efficient co‐diffused screen printed n‐type silicon solar cell with rear‐side boron emitter

physica status solidi (RRL) - Rapid Research Letters (2015)

Nadine Wehmeier Bianca Lim Anja Nowack Jan Schmidt Thorsten Dullweber

Plasma enhanced chemical vapor deposition (PECVD) is applied to deposit boron silicate glasses (BSG) acting as boron diffusion source during the fabrication of n‐type silicon solar cells. We characterize the resulting boron‐diffused emitter after boron drive‐in from PECVD BSG by measuring the sheet resistances R sheet,B and saturation current densities J 0,B . For process optimization, we vary the PECVD deposition parameters such as the gas flows of the precursor gases silane and diborane and the PECVD BSG layer thickness. We find an optimum gas flow ratio of SiH 4 /B 2 H 6 = 8% and layer thickness of 40 nm. After boron drive in from these PECVD BSG diffusion sources, a low J 0,B values of 21 fA/cm 2 is reached for R sheet,B = 70 Ω/□. The optimized PECVD BSG layers together with a co‐diffusion process are implemented into the fabrication process of passivated emitter and rear totally diffused (PERT) back junction (BJ) cells on n‐type silicon. An independently confirmed energy conversion efficiency of 21.0% is achieved on 15.6 × 15.6 cm 2 cell area with a simplified process flow. This is the highest efficiency reported for a co‐diffused n‐type PERT BJ cell using PECVD BSG as diffusion source. A loss analysis shows a small contribution of 0.13 mW/cm 2 of the boron diffusion to the recombination loss proving the high quality of this diffusion source. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Diborane

Saturation current

10.1002/pssr.201510393

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Citations (8)

POLO Back Junction: An Elegant Way to Implement Electron-Collecting Passivating Contacts in p-Type Industrial Silicon Solar Cells

37th European Photovoltaic Solar Energy Conference and Exhibition (2020)

Rolf Brendel Robby Peibst Thorsten Dullweber Henning Schulte‐Huxel Lasse Nasebandt

10.4229/eupvsec20202020-2ao.6.4

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Citations (1)

Boron-Doped PECVD Silicon Oxides as Diffusion Sources for Simplified High-Efficiency Solar Cell Fabrication

World Conference on Photovoltaic Energy Conversion (2013)

Pietro P. Altermatt N.-P. Harder Bianca Lim Hannes Wagner G. Schraps

We develop plasma-deposited boron-doped silicon oxide layers with multifunctional capabilities: Firstly, they are used as local diffusion sources for attaining low saturation current densities of J0 = 84 fA/cm at sheet resistances as low as 67 /□ and surface boron concentrations in the range of 4–710 cm3, suitable for efficient metallization. Secondly, they act as diffusion barriers against other dopants. Therefore a co-diffusion process consisting of a boron drive-in and a phosphorus diffusion in a POCl3 furnace in only one high temperature step potentially allows to simplify the fabrication of high-efficiency n-type PERT cells. Process and device simulations indicate that a co-diffusion temperature near 900C and a co-diffusion time of about 20 minutes are optimum for reaching cell efficiencies near 21%.

Saturation current

BORO

10.4229/28theupvsec2013-2dv.3.52

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Citations (13)

Kinetics of the Light and Elevated Temperature Induced Degradation and Regeneration of Quasi-Monocrystalline Silicon Solar Cells

IEEE Journal of Photovoltaics (2021)

Nadine Wehmeier Gerd Fischer Sandra Herlufsen Franziska Wolny Matthias Wagner

We investigate the degradation and regeneration behavior of quasi-monocrystalline silicon passivated emitter and rear cells under illumination at elevated temperatures. The decrease and increase of the solar cell efficiencies over time is accelerated under increased temperature or illumination intensity. We examine the defect activation kinetics and determine rate constants both for the degradation and regeneration. We apply temperatures in the range of 37-140 °C and illumination intensities in the range of 0.1-1.4 suns. These conditions typically occur when operating solar modules in the field. The rate constants are strongly increased with increasing temperature and increasing illumination intensity. We perform multiple regressions fits of the degradation and regeneration data with different approaches for the illumination intensity dependence. A linear illumination intensity dependence on the rates of degradation and regeneration is found. Activation energies for the degradation and regeneration of (0.89 ± 0.04) eV and (1.07 ± 0.07) eV, respectively, are extracted that allow for identification of the defect activation and deactivation mechanisms.

Degradation

Intensity

Light intensity

Suns in alchemy

Atmospheric temperature range

10.1109/jphotov.2021.3066239

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Citations (7)

Inkjet-Printed In Situ Structured and Doped Polysilicon on Oxide Junctions

IEEE Journal of Photovoltaics (2021)

Nadine Wehmeier Fabian Kiefer Till Brendemühl Larissa Mettner Sascha J. Wolter

We investigate the inkjet printing of liquid silicon ink to form in situ doped and structured passivating contacts. The ink consists of neopentasilane oligomers in solvents and decomposes into amorphous silicon with a short anneal. By printing boron- and phosphorus-doped ink on silicon oxide, polycrystalline silicon on oxide (POLO) junctions for both p-type and n-type polarities (POLO ² ) are formed and the saturation current densities as low as 5 fA/cm ² are achieved for n ⁺ -POLO junctions. We perform a structured printing in interdigitated back contact (IBC) geometry achieving emitter and base fingers with an average finger height of up to 103 nm. The application of inkjet printing allows for a simplification of POLO and POLO ² solar cell processing. In particular, for POLO ² -IBC cells, a lean process flow is facilitated.

Polycrystalline silicon

10.1109/jphotov.2021.3094131

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Citations (1)

A 22.3% Efficient p‐Type Back Junction Solar Cell with an Al‐Printed Front‐Side Grid and a Passivating n⁺‐Type Polysilicon on Oxide Contact at the Rear Side

Solar RRL (2020)

Byungsul Min Nadine Wehmeier Till Brendemuehl Agnes Merkle Felix Haase

The fabrication of a silicon solar cell on 6 in. pseudo‐square p‐type Czochralski grown silicon wafers featuring poly‐Si‐based passivating contacts for electrons at the cell rear side and screen‐printed aluminum fingers at the front side is demonstrated. The undiffused front surface is passivated with an Al 2 O 3 /SiN x stack, and the rear surface is covered with a thin oxide/n + ‐poly‐Si/Al 2 O 3 /SiN x layer system, contacted by screen‐printed silver fingers. A loss analysis shows that the recombination losses at the metal contacts on both cell sides dominate the total energy losses. A voltage of 700 mV as the highest open‐circuit voltage from a batch of seven cells is achieved, and the best cell efficiency is 22.3%, independently confirmed.

Passivation

Open-circuit voltage

10.1002/solr.202000435

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Citations (14)

21.0%-efficient screen-printed n-PERT back-junction silicon solar cell with plasma-deposited boron diffusion source

Solar Energy Materials and Solar Cells (2016)

Nadine Wehmeier Anja Nowack Bianca Lim Till Brendemühl Sarah Kajari‐Schröder

Passivation

Saturation current

Diborane

10.1016/j.solmat.2016.05.054

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Citations (17)

716 mV Open‐Circuit Voltage with Fully Screen‐Printed p‐Type Back Junction Solar Cells Featuring an Aluminum Front Grid and a Passivating Polysilicon on Oxide Contact at the Rear Side

Solar RRL (2020)

Byungsul Min Nadine Wehmeier Till Brendemuehl Felix Haase Yevgeniya Larionova

This article reports the recent progress of p ‐type back junction solar cells featuring an aluminum front grid and an n + ‐type passivating polysilicon on oxide (POLO) contact at the cell rear side. The best cell has an efficiency of 22.6% and an open‐circuit voltage of 716 mV, independently confirmed by Institute for Solar Energy Research Hamelin (ISFH) CalTeC. The cell area is 244.5 cm 2 . The increase in the SiN x capping layer thickness at the cell rear side reduces the deterioration of passivation quality of the POLO contact by screen‐printed silver. This increases the open‐circuit voltage by 22 mV compared with cells with a thinner nitride layer thickness. The investigation with scanning electron microscopy shows that the damage of the POLO contacts underneath the screen‐printed metal contacts is avoided by increasing the SiN x capping layer thickness. A contact resistivity of 2 mΩ cm 2 is measured using the transfer length method.

Passivation

Open-circuit voltage

10.1002/solr.202000703

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Citations (16)