From materials and interfaces to high-efficiency silicon and tandem solar cells

O. Isabella, S. Smits, K. Kovačević, Y. Zhao, Y. Zheng, R. Brondolin, L. Cao, E. Özkol, L. Mazzarella, P. Procel, Y. Zhao

Photovoltaic Materials and Devices Group, Delft University of Technology, The Netherlands

E-mail: O.Isabella@tudelft.nl

Abstract

This contribution reviews recent advancements in high-efficiency crystalline silicon (c-Si) solar cells and tandem devices developed at TU Delft, with a strong focus on silicon heterojunction (SHJ) technologies and their integration into next-generation architectures. The continuous progress in device performance is driven by improved contact engineering, innovative material stacks, plasma treatments, and simulation-guided optimization. In particular, systematic developments in doped nanocrystalline silicon oxide layers, transparent conductive oxides, and metallization schemes such as copper plating have led to substantial improvements in fill factor (FF), reaching values above 83% and enabling SHJ solar cells with efficiencies exceeding 24%.

A key innovation towards SHJ solar cells with ultimate optical response is the implementation of localized front contacts, which reduce shading and recombination losses while maintaining excellent passivation. Our optimized design can achieve power conversion efficiencies close to 24% with competitive short-circuit current densities well above 40 mA/cm².  Beyond single- junction devices, the integration of SHJ bottom cells into perovskite/silicon tandem structures is explored. These tandem devices demonstrate efficiencies up to 32.6%, underlining the flexibility of our bottom cell technology to be integrated in different top cell manufacturing paradigms, such as hybrid or slot die processes.

Parallel efforts on transition metal oxide (TMO)-based contacts, particularly MoO_x, are presented for both front/back-contact (FBC) and interdigitated back-contact (IBC) architectures. These dopant-free approaches offer simplified processing, high selectivity, and competitive efficiencies also in the 24% mark. Finally, hybrid architectures combining MoO_x with poly-SiO_x passivating contacts  are  introduced,  showing  promising  performance  improvements  through  advanced passivation and contact resistivity control.

Our results emphasize that a combination of innovative materials, device architectures, and interface engineering is key to pushing c-Si solar cell efficiencies at the laboratory scale toward their theoretical limits and enabling scalable, high-performance photovoltaic technologies.