Monolithic thin-film chalcogenide-silicon tandem solar cells enabled by a diffusion barrier

TL;DR

This study demonstrates the monolithic integration of CZTS thin-film top cells on silicon bottom cells using a diffusion barrier, enabling high-temperature processing without silicon contamination, and achieves a proof-of-concept tandem device with 1.1% efficiency.

Contribution

It introduces a TiN diffusion barrier layer that allows high-temperature synthesis of CZTS on silicon without contamination, enabling monolithic tandem solar cells.

Findings

TiN effectively prevents element diffusion during high-temperature sulfurization.

Post-process silicon lifetime exceeds 1.5 μs, indicating low defect levels.

First proof-of-concept CZTS/Si tandem device with 1.1% efficiency.

Abstract

Following the recent success of monolithically integrated Perovskite/Si tandem solar cells, great interest has been raised in searching for alternative wide bandgap top-cell materials with prospects of a fully earth-abundant, stable and efficient tandem solar cell. Thin film chalcogenides (TFCs) such as the Cu2ZnSnS4 (CZTS) could be suitable top-cell materials. However, TFCs have the disadvantage that generally at least one high temperature step (>500 C) is needed during the synthesis, which could contaminate the Si bottom cell. Here, we systematically investigate the monolithic integration of CZTS on a Si bottom solar cell. A thermally resilient double-sided Tunnel Oxide Passivated Contact (TOPCon) structure is used as bottom cell. A thin (<25 nm) TiN layer between the top and bottom cells, doubles as diffusion barrier and recombination layer. We show that TiN successfully mitigates in-diffusion of CZTS elements into the c-Si bulk during the high temperature sulfurization process, and find no evidence of electrically active deep Si bulk defects in samples protected by just 10 nm TiN. Post-process minority carrier lifetime in Si exceeded 1.5 ,s. i.e., a promising implied open-circuit voltage (i-Voc) of 715 mV after the high temperature sulfurization. Based on these results, we demonstrate a first proof-of-concept two-terminal CZTS/Si tandem device with an efficiency of 1.1% and a Voc of 900 mV. A general implication of this study is that the growth of complex semiconductors on Si using high temperature steps is technically feasible, and can potentially lead to efficient monolithically integrated two-terminal tandem solar cells.