Power Losses in the Front Transparent Conductive Oxide Layer of Silicon Heterojunction Solar Cells: Design Guide for Single-Junction and Four-Terminal Tandem Applications

In silicon heterojunction solar cells, optimization of the front transparent conductive oxide (TCO) layer is required in order to minimize both electrical and optical losses. In this article, design guidelines for this overall power loss minimization are presented - extending previous TCO optimization work that was limited to the maximization of the short-circuit current density alone - and these are used to prescribe the best TCOs for both single-junction and silicon-based four-terminal tandem applications. The employed procedure determines the loss associated with the front TCO layer as a function of the TCO carrier density, mobility, and thickness, as well as the pitch between the front electrode fingers. For a representative indium tin oxide (ITO) film with a mobility of approximately 20 cm²·V^-1·s^-1 and a carrier density of 2.5 × 10²⁰ cm^-3, the loss over the 700-1200 nm infrared wavelength range - the spectrum reaching the silicon bottom cell in a typical tandem structure - is minimized by using a finger pitch of 3 mm and an ITO thickness of 100-110 nm. This compares with an optimal finger pitch of 2 mm and an optimal ITO thickness of 70 nm for the same cell operating as a single-junction device under full spectrum. The methodology presented can also readily be applied to TCO materials other than ITO, to a wide variety of specific four-terminal tandem architectures and, with minor modifications, to rear TCO layers.