Investigating Effects of P-N Junction Geometry in Silicon Solar Cells

Abstract

This thesis presents an investigation into the effects of p-n junction geometry on output characteristics, especially Voc and efficiency of high efficiency silicon solar cells. Test structures and fully contacted cells with limited junction area (LJA) are fabricated. With the assistance of simulation tools such as PC1D and Quokka, relationships between Jo, Voc, efficiency and emitter parameters including its area, spacing and perimeter are established. Reduced Jo can be achieved and hence lead to an enhanced Voc provided all recombination sources are minimised except from the emitter. Through characterisation of fabricated test structures and simulation, it is revealed that a well-calculated emitter spacing can help maintain a high level of Jsc comparable to that of a full area solar cell. Recombination losses from different parts of an LJA cell structure are separately quantified with Quokka models, enabling a breakdown of total Jo into various cell regions. It is demonstrated that in a thin silicon cell structure with well passivated surfaces, an increase of over 30 mV in Voc can be achieved by reducing the emitter area to 3% of the total cell area. As a consequence, maximum efficiency begins to shift from full area emitter toward lower emitter fraction, and finally for a simulated LJA cell, over 25% efficiency occurs at 3% emitter area with closely spaced stripe emitters or high lifetime silicon substrates.