High efficiency laser-doped silicon solar cells with advanced hydrogenation

Abstract

Atomic hydrogen is widely used to passivate recombination active defects in silicon solar cells, yet the passivation mechanisms are poorly understood. Consequently, conventional silicon solar cells do not have effective hydrogenation. Crystallographic and impurity related defects can be incorporated into the silicon during crystal growth, subsequent processes (e.g. oxidation-induced stacking faults), or form under illumination (e.g. boron-oxygen complexes). Processes such as laser doping are of significant interest for high efficiency solar cell fabrication due to the localised nature and industrial applicability, but the introduction of laser-induced defects typically degrades electrical performance. This thesis aims to provide an improved understanding of, and develop an advanced hydrogenation process for silicon solar cells. In addition, this thesis will present new and novel contact structures for silicon solar cells fabricated using laser doping.

This thesis begins with an overview of hydrogen passivation, boron-oxygen defect formation and laser doping for silicon solar cells. An advanced hydrogenation process is then developed based on theoretical calculations to enhance minority hydrogen charge state concentrations using minority carrier injection. Open circuit voltages over 680 mV and pseudo-efficiencies of 23 % are demonstrated on standard commercial grade boron-doped Czochralski silicon through hydrogen passivation of boron-oxygen defects. The same hydrogenation process is then used to passivate oxidation-induced stacking faults. Implied open circuit voltages over 700 mV on large area multi-crystalline silicon and improvements in bulk lifetime from 90 microseconds to 3.4 milliseconds on n-type Czochralski silicon are also demonstrated.

The formation of advanced laser doped structures is then presented to allow deep junction formation >10 microns, utilise dopants from aluminium oxide layers, allow contact to buried layers in silicon solar cells and form transistors. After hydrogen passivation of the laser-induced defects, efficiencies of 20.7 % for p-type and 21.2 % for n-type Czochralski silicon are demonstrated for large area devices featuring such contacts.

Whilst substantial efficiency enhancements can be foreseen for solar cells fabricated on a whole array of silicon materials, a significant amount of further research is required to fully understand and exploit the potential of hydrogen passivation and laser doping.