New research training group of the state of Lower Saxony with the participation of MBE

Since the beginning of December 2023, the Ministry of Science and Culture of Lower Saxony has been funding a new Research Training Group entitled "Photovoltaic Technologies for the Future - FuturePV". A total of 12 positions for doctoral students will be funded for 4 years as part of this Research Training Group, which is lead by the ISFH. A corresponding job advertisement cen be found here (in german only).

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.