This study allowed us to evaluate the efficiency that could be obtained from germanium (Ge) photovoltaic devices with a high rate of carrier loss at the surface (i.e., a high surface recombination velocity) inserted in TPV systems that use photonic crystal filters. Simulations of the optical behaviour of different types of TPV filters, made of multiple layers of dielectric materials, were first performed, together with the definition of appropriate figures of merit useful for their selection. In particular, the simulations allowed us to demonstrate that to optimise the performance of a Ge cell it is not sufficient to maximise the spectral efficiency of the filter, as above all, it is necessary to maximise the radiation absorbed by the device.

The maximum power produced by the photovoltaic device is obtained by using a single stack of dielectric layers combined with a metal reflector mirror placed on the back of the Ge cell. It was verified that to absorb the radiation emitted at 1500 K, the germanium must be thicker than 10 μm. Furthermore, while in the previous three-year System Research period, SiGeSn was identified as a passivating material potentially capable of reducing the surface recombination velocity of the Ge cell, in this study an analysis was carried out to identify suitable Ge cell structures that are less sensitive to inadequately passivated surfaces. Therefore, several simulations were carried out which demonstrated that the Ge cell structure that is less influenced by high surface velocity is the thick emitter one, with P-on-N polarity.
In particular, considering a surface recombination velocity of 107 cm/sec on the back of the cell, this structure allows us to obtain a conversion efficiency of 22.5%, only slightly lower than the ideal theoretical 23% that would be obtained considering a very low surface recombination velocity of 102 cm/sec. The results obtained are a progress towards the realisation of more efficient and economical TPV systems for home micro-generation.