A Tour of Solar Cell Manufacturing

Estimated Reading Time: 8 minutes

TL;DR

Solar cells have become an increasingly important part of green energy. Plummeting prices over the last decade have driven mainstream adoption of solar cell technology. In today’s issue, we review some of the science and economics of modern solar cells.

Accountability for George Floyd

The jury in the murder trial for Derek Chauvin has ruled guilty on all charges (source) (but sentencing has yet to occur). The original information the Minneapolis police department released about George Floyd’s death clashes with the reality of George Floyd’s murder. How many other atrocities are obscured by bland language in police reports?

Chris Vanderveen @chrisvanderveen

Reminder to all journalists... This is how Minneapolis initially reported the death of #GeorgeFloyd

A Very Brief History of the Solar Cell

The photovoltaic effect itself was discovered in the late 19th century and explained theoretically in the early 20th century by Albert Einstein. The first practical usage of the photovoltaic effect came much later in the space age, when satellite engineers realized that solar cells could provide a practical power source for objects in space. Early solar cell research was driven by the mandates of the space age, pushing a focus on low weight and high efficiency, with cost not a big factor.

The Explorer 6 satellite launched in 1959 was powered by solar cell panels. Solar cells continue to be a major power source for satellites (source).

Solar cells slowly started to find other use cases on earth; for example, solar cells were used to power buoys on the ocean in the 1970s (source). Following the 1973 oil crisis, solar cells started to find increasing usage across industries. Increasing usage helped drive down costs, sparking yet more usage. By recent measures, solar cells have now started to become competitive with other sources of energy such as natural gas for major energy deployments (source).

Solar cells costs per watt of energy have dropped sharply over the last several decades (source).

Swanson’s Law

Swanson’s law is the empirical observation that the cost of solar cell modules drop by 20% as cumulative volumes of shipped solar cells doubles. As the chart below shows, costs have dropped by 75% every 10 years. In a general sense, Swanson’s law is an instance of general economies of scale. Unfortunately, as we saw in our subscriber post, some of these cost reductions may be driven by forced labor in Xinjiang. Sustaining Swanson’s law in an ethical fashion may require bringing scaled solar cell manufacturing back to the US and allied democracies subject to the rule of law.

Swanson’s Law observes that the cost of solar cell modules drops by about 20% for every doubling of cumulative volumes shipped (source).

Solar Cell Physics

Solar cells make use of the properties of semiconductors but in a different manner than modern chips. As we learned in our issue on transistors, p-type semiconductors are doped to induce positive “electron holes.” Light from the sun jolts free electrons which are drawn towards the n-type region of the solar cell (which has an abundance of negative charge).

A diagrammatic representation of the process by which a photon of light jolts an electron out of the p-doped region of the solar cell to the n-doped region of the solar cell. Recall that p-doped regions have an excess of positive charges (electron holes) and n-doped regions an excess of negative charges (source).

The physics of semiconductors are driven by the presence of a region of forbidden energies typically called the band gap. As the diagram below shows, most electrons occupy the valence band of energies. A photon of the correct wavelength can promote an electron from the valence band across the band gap to the conduction band where it can start moving across the semiconductor.

The band gap is a region of forbidden energies. A photon of the right wavelength can promote electrons from the occupied valence band, through the forbidden band gap to the conduction band where it can conduct charge (source).

Efficiency vs Cost Tradeoff

Research into improving solar cells has proceeded on two primary axes. One is improving the efficiency of the solar cell (the efficiency measures the percentage of solar energy that can be converted into current and can never exceed 100%). Higher efficiency solar cells are critical for higher-end applications of solar cells such as satellites where cost isn’t a primary factor. The other primary driver of research is price. Solar cell costs will need to continue to drop for major utilities to consider swapping over to solar installations over traditional natural gas and coal. As we just learned, costs have dropped exponentially, but as the diagram below shows, efficiencies have risen much more gradually. There has been a slow, but steady increase in solar cell efficiencies, but there is still considerable room for improvement.

There is a tradeoff between the efficiency and cost of solar cells. Crystalline silicon cells, the ones most deployed commercially, have lower efficiency than multijunction cells (27% vs 47% for top performing cells from either category), but are much cheaper. Applications such as solar cells for satellites use expensive multijunction cells while solar cells for other installations tend to use silicon (source).

Discussion

Solar cell technology has improved dramatically over the last several decades with significantly increased efficiencies and dramatic exponential decreases in cost. However, as we saw in our subscriber post for the week, some proportion of price decreases are being driven by unethical manufacturing practices by PRC solar cell manufacturers using forced labor from Xinjiang. The PRC has built a dominant position in clean energy (source), leading to concerns that increasing a focus on solar cells (and other clean energy) could play into the PRC’s hands and strip the US of energy independence. The US cannot legitimize the ongoing genocide in Xinjiang, but neither can it ignore climate change. Considerable investment will be needed to bootstrap domestic and allied clean energy supply chains for the US to ethically grow its solar footprint.

Highlights for the Week

• https://www.wsj.com/articles/chinas-fishing-fleet-the-worlds-largest-drives-beijings-global-ambitions-11619015507: The PRC is using commercial fishing fleets to expand its global footprint. As we’ve seen in our issue on CSSC, naval power is a mainstay of PRC plans. The US needs a better plan to counter PRC naval efforts.

• https://spectrum.ieee.org/tech-talk/semiconductors/processors/cerebras-giant-ai-chip-now-has-a-trillions-more-transistors: Cerebras’s new AI chips have 2.6 trillion transistors! It’s not clear what the killer use case for these systems is. More benchmarking work will be needed considering Cerebras systems cost millions of dollars.

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About

Deep Into the Forest is a newsletter by Deep Forest Sciences, Inc. We’re a deep tech R&D company specializing in the use of AI for deep tech development. We do technical consulting and joint development partnerships with deep tech firms. Get in touch with us at partnerships@deepforestsci.com! We’re always welcome to new ideas!

Credits

Author: Bharath Ramsundar, Ph.D.
Editor: Sandya Subramanian