Comment by philipkglass | Hacker News Reader

Comment by philipkglass | Hacker News Reader

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The historical account may be accurate. The commentary on present day technology is inaccurate. The article criticizes modern solar technology as toxic and unsustainable.

According to the article, the best iteration of Cove's invention achieved 5% solar-to-electricity conversion using an antimony alloy. Modern silicon solar panels achieve over 20% solar-to-electricity conversion using silicon. Silicon is non-toxic and is more than a million times as abundant in the Earth's crust as antimony [1]. Silicon solar cells do not contain toxic compounds. Modules made with silicon cells may contain toxic compounds if they were older ones manufactured (pre-RoHS) with lead based solder or glass frit containing lead, but the same issue would apply to antimony based solar modules; the cell material is not related to the soldering technology. Every other criticism of silicon given in the article (mining, complex machines, global supply chains, fossil fuels used upstream) would equally apply to a hypothetical antimony-based solar industry of comparable scale.

Low Tech Magazine is a great source for introducing modern audiences to old forgotten technologies. But it has apparently run short of forgotten technologies that may actually be better and has fallen into a rut of exaggerating contemporary-technology drawbacks and minimizing the problems of old technologies.

[1] https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth...

"Silicon solar cells do not contain toxic compounds. "

This is absolutely untrue. The silicon is just the starting base, you still need to dope the cells, often with an arsenic compound (Indium-Gallium Arsenide, Gallium Arsenide, etc.)

In the EU and California, solar panels are exempt from RoHS requirements.

I used to manufacture solar panels for Sunspark Technology in California. You can't get the same kind of conductivity using tin bus ribbon without sacrificing usable cell area, as you need a wider, flat strip of tin, compared to lead. The newest "invisible bus bar" panels are using silver wire. These cells, when I worked there pre-COVID, were dumping ~11A at full light exposure. The inter-string bus bars were lead-coated copper, as were the string to junction box interconnect ribbons. 60/40 was used in the junction box. We manufactured panels for everyone, including Jinko, that behemoth of a solar corporation.

Solar panels very often have toxic stuff everywhere.

Indium gallium arsenide and gallium arsenide are compound semiconductors that are far too expensive for use in mass market panels. They are used as cells or components of multi-junction cells for satellites and a few other high value applications. Commercial silicon cells are doped with phosphorus and boron or gallium.

On further reading I find that you are correct about RoHS exemptions for lead in solar modules:

https://www.pv-magazine.com/2019/12/07/the-weekend-read-taki...

I appreciate the correction.

Commercial cells doped with boron are meant for use in low-light applications due to higher efficiencies in lower/diffuse light, and are not very suited to direct-sunlight applications due to the tradeoff. Indium and Gallium-doped single-crystal silicon still wins in efficiency. Phosphorous I have not seen in any of the roughly 5 million cells I've handled as part of QC. I've heard of nanoparticles of phosphorous helping perovskite-based cells, but that's about it. I've never handled one in commercial production.

This back and forth made my day :)

Not saying you're wrong, but here's some of the claims that the article makes about the advantages of antimony Schottky vs n-p silicon solar cells:

> In the 1970s and 1980s, scientists investigated Zn4Sb3 for use in photovoltaics and concluded that the material’s “obvious advantages are apparent simplicity and relatively low temperature of the preparation procedure.” [23] The melting point for Zn4Sb3 is 570 degrees Celsius, while it’s 1,400 degrees for silicon.

> Silicon modules are sandwiched between two laminate encapsulant layers (usually EVA, an ethylene/vinyl acetate copolymer). These layers are essential to ensure module service lifetime. [1-3] To recycle the silicon – the most valuable component of a solar panel – these layers need to be separated, but burning them also destroys the modules. Silicon cells can only be recycled by a combination of thermal, chemical, and metallurgical steps. That is an expensive process with an impact on the environment. [...] In contrast, the solar cells built by George Cove were entirely recyclable. They required no protective layer and did not even contain solder.

> Schottky cells do not require a high-temperature phosphorus-diffusion step, which ordinarily creates the n-layer of the p-n junction in silicon today. This alone reduces the energy input into the solar cell production process by 35%.

... and a point that maybe that efficiency of Schottky cells (irrespective of composition) hasn't really been explored:

> Scientists also reached 17% experimental efficiency for a graphene/silicon Schottky cell, up from 1.5% ten years earlier.

Schottky barrier cells are a well established idea in photovoltaic research. Here's a 1975 report on an effort to develop low cost Schottky cells:

"Silicon Schottky photovoltaic diodes for solar energy conversion"

https://ntrs.nasa.gov/api/citations/19760005411/downloads/19...

Here's a very early (1953) review of photovoltaic cells that includes Schottky barrier cells:

"Photovoltaic Cells and Their Possible Use as Power Converters for Solar Energy"

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.52...

There are a couple of problems with the implications given by the quote

Schottky cells do not require a high-temperature phosphorus-diffusion step, which ordinarily creates the n-layer of the p-n junction in silicon today. This alone reduces the energy input into the solar cell production process by 35%.

Reducing the solar cell energy inputs by 35% is a clear win only if the cell output does not fall commensurately. A record 17% efficiency for a Schottky cell compares to over 24% record efficiency for PERC cells made with conventional high temperature phosphorus diffusion:

"Trina Solar claims record 24.5% efficiency for 210mm PERC cells"

https://www.pv-tech.org/trina-solar-claims-record-24-5-effic...

That means that the best Schottky cell generates about 69% as much energy as the best PERC cell while taking about 65% as much energy to manufacture. The Schottky cell generates about 7% more output energy per unit of input energy -- a pretty marginal improvement.

There is a more promising alternative to high temperature diffusion cells already in mass production: the heterojunction solar cell. It was originally commercialized by Sanyo (later acquired by Panasonic) but its patents have recently expired so several manufacturers now make heterojunction devices.

Major solar manufacturer Longi just set a cell efficiency record of 26.5% with a heterojunction design:

https://pv-magazine-usa.com/2022/06/24/longis-heterojunction...

Heterojunction cells are processed at low temperature because the delicate amorphous silicon layers cannot withstand high heat:

"Silicon heterojunction (SHJ) solar cells have typically a low process temperature limit (~250°C) because high-temperature annealing processes can degrade the passivation of the hydrogenated amorphous silicon (a-Si:H) due to the hydrogen effusion during the annealing."

(From https://www.intechopen.com/chapters/61900)

Given the combination of low temperature processing and high efficiency found in heterojunction cells, I am personally doubtful that the Schottky barrier cell can offer superior energy return on energy invested for solar power systems.

Just want to reiterate I have no dog in this fight, and thank you for this interesting counterpoint. Probably the one surviving argument of the article is recyclability. I presume multijunction cells are hard to recycle vs something more low tech and maybe hobbyist level approachable (which is the whole shtick of that publication)

You have to wonder if some of these MEG solar designs will ever make it out of the lab. Could they be combined with a Schottky design to increase the receptive wavelength range?

It's clearly trivial (in an industrial sense anyway) to manufacture silicon solar cells, do we care about the melting point, or should we focus on the EROI?

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