External busbars for improving current generation in multijunction solar cells

TL;DR

This paper introduces an external busbar design for multijunction solar cells that reduces shading losses, allows for smaller cell sizes, and improves overall efficiency demonstrated through fabrication and testing of GaInP/GaAs/GaInNAsSb cells.

Contribution

The paper presents a novel external busbar grid design and a fabrication process that enhances solar cell performance and scalability, especially for micro-concentrator applications.

Findings

Enhanced solar cell efficiency with external busbars.

Reduced shading and voltage losses due to design.

Improved fill factor of 85% at one-sun illumination.

Abstract

We report an improved device fabrication process employed in the development of an advanced front contact grid design employing external busbars. The advanced fabrication process results in enhanced solar cell performance measured at one-sun illumination. In this grid configuration the busbar area is located outside the active solar cell and the grid fingers travel across the mesa sidewalls. With this design the solar cell size can be scaled down without limitations as the area of the busbar is not restricting the component size. Thus, the demonstrated design is beneficial especially for micro-concentrator solar cells. In general, this approach minimizes the power losses originating from the grid shadowing and dark area related voltage losses. The performance of the proposed design is validated by fabricating GaInP/GaAs/GaInNAsSb triple-junction solar cells employing the grid design with external busbars. Light-biased current-voltage and electroluminescence characteristics of the cells reveal that an additional contact GaAs etching step prior to front contact metal deposition is needed to ensure good photovoltaic performance with a fill factor of 85% at one-sun illumination. The improvement is attributed to removing the plasma-damaged material layer that can extend to a depth beyond 100 nm, leading to resistive losses.