Stable reverse bias or integrated bypass diode in HIP‑MWT+ solar cells

Tadeo Schweigstill,Stefan W Glunz,Florian Clement,Alma Spribille,Jonas D Huyeng

doi:10.1051/epjpv/2021016

Abstract

The Metal Wrap Through+ (HIP-MWT+) solar cell is based on the PERC concept but features two additional electrical contacts, namely the Schottky contact between p-type Si bulk and Ag n-contact and the metal-insulator-semiconductor (MIS) contact on the rear side of the cell below the n-contact pads. To prevent thermal hotspots under reverse bias, both contacts shall either restrict current flow or allow a homogenous current flow at low voltage. In this work we present both options. First the stable reverse bias characteristics up to −15 V with a MIS contact using industrially manufactured SiON passivation and second, an integrated by-pass diode using AlOX as insulator in the passivation stack allowing current flows at approximately Vrev = –3.5 V depending on the chosen screen-print paste. The examined Schottky contacts break down at around Vrev = –2.5 V. Reverse bias testing of the cells reveals a solid performance of the cells under reverse bias and an average conversion efficiency of η = 21.2% (AlOX) and η = 20.7% (SiON), respectively.

Highlights

The passivated emitter and rear cell (PERC) [1] technology is the standard technology in industrial production [2] and it can be combined with the metal wrapped through approach (MWT) [3] resulting in the MWT-PERC concept [4]
The findings were converted into fully functional solar cells and the performance of the contacts is demonstrated on cell level determining the long-term stability of the contacts under reverse bias
This breakdown behavior of the AlOX based MIS contact is similar to the breakdown of the Schottky contacts, which is needed for a controlled breakdown across a large area that would be required for a HIP-MWT+ cell with an integrated bypass diode

Summary

Introduction

The passivated emitter and rear cell (PERC) [1] technology is the standard technology in industrial production [2] and it can be combined with the metal wrapped through approach (MWT) [3] resulting in the MWT-PERC concept [4]. A simplified version of the MWT-PERC concept is the High-Performance Metal Wrap Through+ (HIP-MWT+) cell where only one additional process step (drilling of vias) is required to create a PERC with p- and n-contact located on the rear side of the cell [5] This back-contact configuration results in several obvious benefits such as a rear side module interconnection and reduced front side shading. – one can exploit the contacts by designing dielectric layers which insulate under forward bias but allow a current flow through the cell under reverse bias across a large area This way an integrated bypass diode is built into the cell with only little additional production effort. A passivation based on the electrical field effect using AlOX seems appropriate to function as a rectifying contact

Experimental

Current–voltage characteristics of Schottky contact and MIS contact

Results and discussion

Spatially resolved current flows of Schottky contact and MIS contact

Cell results and long-term stability of Schottky and MIS contacts

Conclusion