Enhancing the stability of n- and p-type TOPCon layers for efficient TOPCon-based bottom cells in tandem integration

Yigit Mert Kaplan¹, Raşit Turan^1,2, Gökhan Altiner¹, Eni Muka¹, Hisham Nasser¹

¹ Center for Solar Energy Research and Applications (ODTÜ-GÜNAM), Turkiye

² Department of Physics, Middle East Technical University, Turkiye

E-mail: turanr@metu.edu.tr

 Abstract

This work aims to develop efficient Si TOPCon-based bottom cells using industrially available fabrication steps and precursors for two-terminal perovskite/Si tandem devices. The front side of n-type Cz absorber features high quality tunnel oxide/phosphorus-doped poly-Si (n-TOPCon) while the rear side consists of tunnel oxide/boron-doped poly-Si (p-TOPCon). Symmetrical n-type Cz wafers coated with n-TOPCon stacks and hydrogenation overlayers exhibit iV_oc >740 mV. Symmetrical n-type Cz wafers coated with p-TOPCon stacks and hydrogenation overlayers exhibit an excellent iV_oc >720 mV. The layers are developed to maintain excellent surface passivation parameters at high temperature fast-firing (FFO); necessary to create rear side ohmic contact, enabling firing stable n-TOPCon and p-TOPCon maintaining identical iV_oc levels. In particular, p-type poly-Si layer was improved using a two-step annealing process giving a better crystallinity and doping control in the poly-Si layer. The stability of this structure during fast firing proses has been studied and improved through process optimization. These developments are integrated into a lean process sequence to fabricate TOPCon² bottom cells shown in Figure 1 below. Process conditions and layer properties as well as Voc results as will be presented in this talk.

FIGURE 1 2D schematics of rear-emitter TOPCon² bottom cell on n-type Cz wafers