Investigation of Ti-based barrier layers for application in monolithic Silicon-Chalcogenide tandem solar cells

Description

Chalcogenides, such as Cu2ZnSnS4 (CZTS), made of environmentally friendly and earth abundant elements are promising materials to be used as the high-bandgap material for silicon-based tandem solar cells. Although there have been large efforts in developing an efficient top cell chalcogenide material, present understanding on the monolithic integration with a silicon cell is very limited. Therefore, an in-depth study to provide insight into the possible interactions of the two materials and the requirements for any intermediate layer between the two cells is essential. In this work, we have done a systematic study on the possible effects of CZTS growth on the silicon bottom cell. We investigated the performance of different ultrathin (< 25 nm) Ti-based layers, namely TiN, TiN-Al-TiN, and TiO2, as promising barrier materials to protect the silicon cell during the sulfurization process of CZTS. In this respect, we examined in detail the in-diffusion of impurities into Si during the sulfurization, in the presence and absence of the barrier layers. Samples without any barrier layer suffered from delamination after the annealing dissimilar to the protected ones. TOF-SIMS measurements revealed that the TiN layer could effectively block in-diffusion of impurities into the silicon substrate. Minority carrier lifetime measurement was used as a figure of merit to determine the effect of trace impurities on the photovoltaic properties of the bottom cell. Even though a slight decrease in lifetime was observed after the CZTS synthesis on Si, an ultimate lifetime of ~1.6 ms was measured indicating promising implied Voc of 707 mV for the bottom cell. This shows that fabrication of a CZTS cell on a silicon cell does not necessarily compromise the performance of the silicon cell. Finally, the actual effect of these layers on the performance of a single-junction silicon solar cell is tested, and the optical and electrical losses due to the presence of the barrier layers as well as to the CZTS processing are discussed.

Period	30 Nov 2018
Event title	9th European Kesterite Workshop
Event type	Workshop
Location	Ghent, Belgium
Degree of Recognition	International