Photovoltaic Metallization Enables Efficient Solar Cells

DuPont says its Solamet PV17x photovoltaic metallization paste has become the leading frontside silver paste series on the market today due to its ability to raise efficiency in standard solar cell constructions. Its unique properties enable Lightly Doped Emitter (LDE) cell designs. LDE is a strong differentiator for solar cell producers as it can boost efficiency by up to 0.4 percent. DuPont Microcircuit Materials is taking steps to increase its supply capability for Solamet PV17x to help meet the increased demand for higher efficiency solar cells.

“Solamet® PV17x series continue to advance technology to help meet the industry’s goal of 20 percent efficiency by 2012,” said Peter Brenner, photovoltaics global marketing manager, DuPont Microcircuit Materials. “This product has a two-stage advantage, since on its own it can raise efficiency by up to 0.2 percent, and when used to enable LDE, can raise efficiency by up to 0.4 percent. We’re very pleased to see such strong demand and are working to quickly increase supply capability for the growing customer base as it becomes qualified in stages through the next three months.”

Solamet® PV17x allows for contact to be made to the most lightly doped junctions. Doping diffusion optimization is a key area of experimental study in the photovoltaic industry for the design of high efficiency cells. Diffusion optimization has been significantly limited by the inability of traditional frontside photovoltaic silver pastes to contact lightly surface doped emitters. Dupont found the industry had no real commercially available option for making a screen printed frontside metallization that could economically and practically enable an LDE. However, the excellent silicon to silver contact of Solamet has demonstrated its capability to enable a wider range of diffusion optimization and higher cell efficiency.

Extensive testing is underway within DuPont and in collaboration with several research organizations as well as in customer trials to fully characterize and continue to advance this technology. For example, RWTH-Aachen University recently published a comparative study involving Solamet and four competing metallization pastes. Solamet outperformed four competing products, demonstrating its ability to contact 100 Ohm/sq emitters on multicrystalline cells – the first time this had been achieved – with lightly doped phosphorous surface concentration. This enabled an efficiency increase of one full percent versus the homogenous emitter base line and 0.4 percent higher efficiency was confirmed versus laser doped selective emitter technologies.

“We presented a characterization of POCl3 parameters influencing the electrically active phosphorus concentration profiles by electrochemical capacitance voltage measurements,” said Ali Safiei, (PhD researcher), Institute of Semiconductor Electronics at RWTH Aachen University. “For the first time we could demonstrate a successful direct contacting of an optimized high sheet resistance emitter at 100 Ω/sq by increasing the n++ layer and at the same time reducing the dead layer. Multicrystalline silicon solar cells were fabricated using five different silver pastes resulting in an absolute efficiency gain of Δη = 1 percent in comparison to a standard 55 Ω/sq emitter. Based on these investigations we evaluated a 160 Ω/sq emitter and could successfully demonstrate by laser doping that a n++ layer of up to 25 nm depth (a Lightly Doped Emitter) leads to high FF and an absolute efficiency gain of Δη > 0.6 percent.”

The breakthrough formulation of Solamet also enables cell makers to use up to 15 percent less material, in line with the company’s intent to accelerate product developments that help the photovoltaic industry reduce its dependence on silver metals and offset some of the impact that rising silver prices have on the cost of producing solar cells and solar modules.

Continued development on metallizations is ongoing with the aim of continuing to improve efficiency and further integrating Solamet pastes with complementary processes such as LDE and local back surface field (LBSF) cell architectures.