Research Projects – Institute of Electronic Materials and Devices

Understanding and engineering polysilicon based passivating contacts for photovoltaic applications

In this project, which is being carried out jointly with colleagues from the Australia National University in Canberra (https://www.anu.edu.au/), the aim is to investigate passivating contacts based on polycrystalline silicon. Such contact structures consist of a thin silicon oxide that is produced either chemically or dry thermally on a silicon wafer. A thin layer of polycrystalline silicon is deposited on this oxide. Understanding the function and high-quality production of such contact structures have been the subject of research at MBE for many years. Within the framework of this project, the long-term stability and the ability of the polycrystalline silicon to bind metallic impurities or to deactivate them electrically are to be investigated.

Led by: Dr.-Ing. Jan Krügener

Year: 2023

Funding: DAAD

Duration: 2023 - 2024
SiC & POLO

In the ‘SiC & POLO’ project, centrotherm, ICB, the University of Constance and MBE have joined forces to produce highly selective and passivating contacts based on silicon carbide. The project partners involved bring all the necessary expertise to the project in a complementary manner: centrotherm will concentrate on process development for SiC deposition. ICB will develop additives for an optimised surface texture and etching solutions to remove edge wrap-around with high selectivity and further improve the environmental compatibility of the additives. The University of Konstanz will be responsible for optical layer characterisation, screen printing contact development and solar cell integration, and MBE will carry out electrical layer characterisation and use structural and electronic investigations to clarify limitations and thus guide the developments.

Led by: J. Krügener

Team: S. Börnert

Year: 2024

Funding: Federal Ministry for Economic Affairs and Climate Action (BWMK)

Duration: October 2024 - September 2027
Preparation and characterization of photonic structures for use in future silicon solar cells

Modern silicon solar cells today achieve record efficiencies of up to 26.8 %. The main limitations compared to the theoretical limit for silicon solar cells of approx. 29.5 % are the intrinsic recombination losses in the silicon volume and the recombination on surfaces and contacts. The latter have been drastically reduced in recent years through the introduction of very effective selective contact layers. A reduction in the unavoidable intrinsic volume recombination can only be achieved by using thinner silicon wafers, but this has a direct negative impact on the achievable photocurrent density and therefore on the efficiency of the solar cell, as the volume of the silicon absorber available for photoabsorption is also reduced when its thickness is reduced. For some years now, structures based on photonic crystals have been investigated that make it possible to achieve high photocurrent densities even with thinner silicon wafers. As has been shown theoretically, photonic crystals on the front side of silicon solar cells allow increased absorption of the incident light and thus enable significantly higher photocurrents and thus higher efficiencies than predicted by the classical theoretical limit. The photonic crystals investigated to date against this background consist of regularly arranged inverted pyramids with edge lengths of a few micrometres. The inverted pyramids are produced using selective, highly anisotropic wet chemical etching processes through a mask of silicon oxide. The first solar cells with photonic crystals on the front surfaces have already been produced on a laboratory scale in a co-operation between MBE and ISFH. However, these were still limited by local inhomogeneities in the manufacturing process of the regular inverted pyramids. As part of the project planned here, conditions are initially to be established that enable the defined production of large-area photonic crystals on silicon. Initial preliminary work has already been carried out at MBE for this purpose, based on structure transfer using conventional photolithography. The process developed in this way will then be systematically varied and the structures produced will subsequently be characterised optically (transmission, reflection) and structurally (scanning electron microscope, atomic force microscope). The results achieved in this way will be used to better estimate the realistically achievable efficiency potential of silicon solar cells with photonic crystals. In addition, new sub-processes are to be developed that can improve the production of photonic crystals. This includes, for example, replacing photolithography with laser lithography or the use of dry etching processes instead of the wet-chemical production used to date. In the future, the solar cells with photonic crystal structures produced as part of this project can also be used as bottom cells for tandem cells.

Led by: Dr.-Ing. J. Krügener

Year: 2024

Funding: Niedersächsisches Ministerium für Wissenschaft und Kultur

Duration: 2023 - 2027
Breaking limits Using Record enabling Silicon Technology with photonic management - BURST

Silicon PV technology is moving towards TW-scale production capacities, enabling the transition to a clean energy system and a climate-neutral economy. Among the various silicon-based PV technologies, silicon solar cells with interdigitated backcontacts (IBCs) are expected to achieve the highest energy conversion efficiencies. Nevertheless, the cost efficiency and applicability of this promising technology can still be greatly improved, as the thickness of the absorber material can be reduced while maximising performance. This is a major challenge that can be addressed by increasing the absorption density of the cells using optical strategies and advanced passivation processes. The increasing demand for ultra-thin solar cells, which offer benefits such as lower material consumption leading to improved lifetime, weight reduction and potential mechanical flexibility for extended applications, has fuelled extensive research to address this challenge. As part of the BURST project, a consortium of industry (NinesPV, HOLO/OR and BENKEI) and research partners(ISFH, TU Delft, LUH, ISC Konstanz and CEA) have joined forces to make future IBC solar cells even better and more cost-efficient.

Led by: J. Krügener

Team: A. Haller

Year: 2024

Funding: Horizon Europe

Duration: May 2024 - April 2027

Research Projects - Semiconductor Technologies for Photovoltaics