Technologies compete as solar cell efficiency race continues

By Puruswottam Aryal, PhD, an R&D project manager in MetaShield’s solar division

The continued quest for improved solar cell efficiency is vital to the growth of solar. Many companies are innovating ways to increase efficiency using a variety of technical approaches, not just because of the improved ROI great efficiency provides, but also because manufacturers need to differentiate themselves from the competition.

Kaneka recently achieved the world’s highest conversion efficiency of 26.33% in a practical size (180 cm²) monocrystalline silicon solar cell, breaking the previous record of 25.6% by ~0.7%. Natcore Technology achieved an efficiency of 19.4% in its most recent solar cell demonstration in November 2016. The laser-formed base contact is critical in the Natcore Foil Cell, which is an all-back-contact cell. Improving this contact has been a main focus of its research program as performance has been limited by higher resistance at this contact, as well as damage from the laser process. Natcore scientists have discovered a new, laser-based contacting process that overcomes these issues. Despite only a few runs with this new structure, the device efficiency increased by nearly 2% in fewer than six months (Natcore first announced an efficiency of 17.5% in June 2016).

MetaShield LLC has taken a different approach, developing a nanoparticle-based light trapping coating for III-V triple junction solar cells that can improve the efficiency of a cell by up to 1.2% (absolute), when coated on top of the already-present anti-reflective coating.

U.S.-based wafer producer 1366 Technologies and Hanwha Q CELLS jointly hit a new record of 19.6% efficiency in December 2016 using 1366’s “direct wafer” process. The record, independently confirmed by the Fraunhofer ISE CalLab, was achieved using 1366’s kerfless, drop-in 156-mm multi-crystalline wafers and Hanwha’s Q.ANTUM PERC cell process. The previous record of 19.1% was achieved by the two firms one month earlier. According to a 1366 Technologies statement, the direct wafer process creates multi-crystalline wafers directly from molten silicon instead of taking several steps that require more energy and expense.

The principles of physics by which solar cells can be improved are limited, and solar cell manufacturers and their partners will be constrained by these principles at some point. However, the process by which they test these principles are closely guarded trade secrets that vary from manufacturer to manufacturer. In general, though, expect these solar cell suppliers and the technology firms working with them to focus on the following to improve efficiency further:

1. Improved defect passivation at material’s surface and bulk to reduce charge carrier recombination.
2. Reduced series resistance at contacts.
3. Developing all back-contact solar cells to avoid light shading.
4. Developing ways of trapping more light, such as better texturing, incorporating nanoparticles.
5. Developing materials for broadband spectral shift to make use of wavelengths that solar cells cannot absorb currently.

Currently, monocrystalline silicon solar cells are close to their theoretical limit, which is about 30%. It has been theoretically predicted that spectral downshift and upshift can increase this limit to 40% and 50%, respectively. However, this technology is still in the early stage. Even though spectral shifts have been successfully shown in research labs, they cannot be used in solar cells yet. The materials discovered with spectral shifting properties so far absorb light in extremely narrow spectral range. In addition, the efficiency with which these materials perform spectral shift is very low.

While solar cell companies need to continuously improve solar cell efficiency, they also need to reduce manufacturing costs to remain profitable. In response, more solar cell manufacturers are increasing investments in research and development. Last year, Canadian Solar laid off 130 employees at its Ontario manufacturing plant in a variety of departments apart from its R&D group, which numbers approximately 350, and stated its intention of focusing more on R&D with a goal of finding new technologies it can bring to the market.

MetaShield is exploring ways to absorb the maximum number of photons available in the solar spectrum, including the study of plasmonic and dielectric nanoparticles because of their light trapping properties. In addition, the company is studying the spectral shift of solar spectrum to absorb the spectrum portion which is lost due to transmission or heat. Although enhancements due to plasmonic effects are minimal when the solar cell absorber layer is thick, it is expected that plasmonics-based technologies will accelerate the PV industry’s increasing use of thin-film solar panels, which have lower manufacturing costs.

MetaShield’s MetaShieldPV product leverages technology to produce more power with less balance-of-system. This increase in power makes marginal PV projects more cost effective.

“The enhancement provided by MetaShieldPV represents a five-year technological leap forward based on recent yearly average increases of around 0.2% for solar PV module efficiency,” said Martin Ben-Dayan, MetaShield’s founder and CEO, referencing a research report on solar cell efficiency improvements published by GTM Research in April 2014.

The installed price of solar energy has declined significantly in recent years as policy and market forces have driven more solar installations. The continued decrease in solar prices is unlikely to slow down anytime soon. Widespread adoption of plasmonics-based technologies can lead to low-cost solar panels that incorporate solar cells with a much thinner absorber material, using nanoparticles for optical enhancement. With solar already achieving record-low prices, the cost decline observed in 2015 indicates that the coming years will likely see utility-scale solar become cost competitive with conventional forms of electricity generation.