ANU C ollege ofEng ineering  & C omputer S c ience

World-leading efficiencies in silicon solar cells are being achieved by cell designs that implement precise and highly-localised chemical and physical processing. Fabricating these cells using traditional methods such as photolithography and furnace diffusion is challenging and relatively expensive.

Daniel Walter, Centre for Sustainable Energy Systems, daniel.walter@anu.edu.au Supervisors: Evan Franklin, Andrew Blakers

Left: Highly localised physical and chemical processing (circled regions) enhance cell performance in this high-performance crystalline silicon solar cell design. Figure from [1]. In this application, dielectric layers are removed in precise locations, and the exposed regions chemically doped.

Above Left: Ablation rate for a silicon nitride/oxide dielectric stack. Above Right: 3-dimensional ablation profile measured with a high-resolution optical profilometer

In this way, a single laser pulse of less than 30 nanoseconds in duration can diffuse dopant atoms into the bulk silicon to form the emitter of a p-n junction, or provide the electric potential that screens regions of high recombination activity. In a more traditional furnace diffusion, such a result involves a process lasting minutes to hours with a high, and expensive, thermal budget.

References: [1] Plagwitz H, Brendel, E. Prog. Photovoltaics 2006;14(1)1:12 [2] Daily Telegraph, March 22, 2011

Laser processing, however, is ideally suited to these demands. It is precise, low-temperature and contaminant free. At the Centre for Sustainable Energy Systems, we are developing industrially relevant laser-based processes for the fabrication of advanced cell designs.

Above: Laser processing of a silicon solar cell at the ANU Centre for Sustainable Energy Systems laboratories. Daily Telegraph [2].

So far, results have been extremely promising. In particular, a deep ultraviolet (UV) excimer laser has successfully demonstrated simultaneous ablation and chemical doping from alumina (Al2O3) films in a controlled and repeatable fashion. This result paves the way for a significantly simplified cell fabrication processes, and has exciting applications for entirely low-temperature cell processing.

The 248nm krypton-fluorine excimer laser system has already demonstrated fine-grained control of the ablation depth of dielectric films on silicon wafers. The high absorption of light at this wavelength by typical dielectric materials allows for layers as thin as 10nm to be removed per laser pulse. By carefully selecting appropriate laser powers, these films can be removed with limited damage to the underlying silicon.

The laser doping process a l l o w s t h e a l u m i n i u m concentration to exceed the solubility limit and produce high surface concentrations (> 1020 cm-3), which reduces the Ohmic resistance of the metal contacts.

Above: The doping profile for aluminium in an excimer laser-doped silicon wafer. The high surface concentration exceeds the solubility limit, and is desirable for low contact resistance.

Above Left: Microscopy Image of laser-processed grid. Each pulse can both ablate and locally dope the silicon, making it ideally suited for contact openings in high-efficiency cell concepts. Above Right: The contact resistivity for aluminium doped local contact openings. Contact resistivity as low as 0.1 mOhm.cm2 has been achieved. Simultaneous Laser Ablation and Doping

Alumina films have demonstrated excellent surface passivation of bulk silicon, and are deposited using low-temperature processes. An excimer laser pulse of sufficient energy will locally ablate the aluminium film, while doping the molten silicon with aluminium atoms.

Excimer Contact Openings for High Efficiency Cells Excimer ablation was employed to produced localised contacts on a high-efficiency silicon solar cell manufactured at ANU. The open-circuit voltage of the finished cell was within 2% of the theoretical maximum value predicted by photoluminescence imaging, demonstrating the inherent advantage of localised, low-damage laser processing.

670

675

680

685

690

695

700

Cell Batch 1 Cell Batch 2

Ope

n C

ircui

t Vol

tage

(mV)

Theoretical Maximum

Experimental Performance Left: Open circuit voltage for high-efficiency silicon cell concept using excimer laser fired contact openings compared to the predicted maximum based on photoluminescence imaging.