Solar cell

• 1. Introduction

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  A solar cell begins with light and ends with electricity, and that transformation happens in silence, with no moving parts. In 1839, a nineteen-year-old French physicist named Edmond Becquerel stood in his father's laboratory and built the world's first photovoltaic cell. He had stumbled onto something that would take more than a century to become practical. What is it about certain materials that lets them pull electricity from sunlight? How did a technology born in a Paris lab end up on the International Space Station, on rooftops in Germany, and in the Tengger desert in China? And why did the price of a solar module fall more than 99% in less than half a century? Those are the questions this documentary will answer.

• 2. The Physics of Light Into Power

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  Edmond Becquerel's 1839 discovery rested on a phenomenon called the photovoltaic effect: when light strikes certain materials, it knocks electrons loose and sends them moving as electrical current. The physical heart of a modern solar cell is a p-n junction, a boundary between two differently treated layers of semiconductor. On one side, boron is introduced into the silicon crystal. On the other, phosphorus is used instead. That doping lowers the activation energy required to free an electron, shrinking it from 1.12 electronvolts down to just 0.05 electronvolts, a twenty-fold reduction. When photons arrive and strike the semiconductor, they excite electrons from the valence band up into the conduction band, creating pairs of electrons and holes. The local electric field at the junction sweeps those pairs apart toward opposite electrodes. If an external circuit is connected with a low enough resistance, those electrons flow through it, doing useful work. If nothing is connected, electrons and holes drift back together and release heat instead. A single silicon p-n junction cell can produce an open-circuit voltage of roughly 0.5 to 0.6 volts under sunlight.

• 3. From Vanguard to the Cosmos

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  Solar cells found their first major application not on any rooftop but in orbit. In 1958, engineers added photovoltaic cells to the outside of the Vanguard satellite as a secondary power source alongside its primary batteries. The addition extended the mission with almost no changes to the spacecraft. A year later, in 1959, the United States launched Explorer 6, which carried large wing-shaped solar arrays made up of 9,600 Hoffman solar cells. That distinctive wing design became standard for satellites. By the 1960s, solar cells were the primary power source for most Earth-orbiting satellites, chosen for the best power-to-weight ratio available. Space users were willing to pay premium prices for the most efficient cells, which paradoxically kept the cost of terrestrial cells high. There was no commercial incentive to develop cheaper, less-efficient alternatives when the only customers with deep pockets wanted only the best. In the early 1990s, spacecraft technology diverged sharply from ground-based panels. Space vehicles shifted to gallium arsenide-based III-V semiconductor materials, which eventually produced the multi-junction photovoltaic cells now standard on satellites. Those modern cells are lightweight, compact, flexible, and highly efficient, capturing a wider slice of the solar spectrum than silicon can manage. In 2020, the US Naval Research Laboratory conducted its first test of space-based solar power generation aboard the Boeing X-37, an experiment called the Photovoltaic Radio-frequency Antenna Module.

• 4. The Long Fall in Price

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  In 1971, the cost of a solar cell was approximately $100,000 per watt. That figure was not a typo. Space users paid it because they had no choice, and that captive market left no reason to invest in cheaper manufacturing. The push to change this came from an unlikely direction. In late 1969, Elliot Berman joined an Exxon task force examining projects thirty years out. He concluded that a price of around $20 per watt would create significant commercial demand, and in April 1973 he founded Solar Power Corporation as a wholly owned Exxon subsidiary. His team cut costs by eliminating the polishing and anti-reflective coating steps, relying on rough-sawn wafer surfaces instead. They replaced expensive hand-wired space-grade components with printed circuit boards, acrylic plastic, and silicone adhesive. They sourced silicon from cast-off material in the electronics industry. By 1973 they had a product, and they persuaded Tideland Signal to use their panels to power navigational buoys for the US Coast Guard. The semiconductor industry's shift to integrated circuits in the 1960s had also created an unplanned benefit: larger silicon ingots became cheaper, and solar cell costs fell alongside them. Adjusted for inflation, a solar module cost $96 per watt in the mid-1970s. Process improvements and expanding production brought that figure below 30 cents per watt in 2018 and as low as 20 cents per watt in 2020. An observation called Swanson's law, noted in The Economist in late 2012, describes this pattern: solar cell prices fall roughly 20% for every doubling of industry capacity. High subsidies in Germany briefly reversed the trend around 2004, when soaring demand pushed up the price of purified silicon. But the Great Recession and the rise of Chinese manufacturing resumed the decline. Prices for solar modules in Germany fell from 3 euros per peak watt in January 2008 to 1 euro per peak watt just four years later. By the end of 2016, spot prices for assembled panels had fallen to a record low of $0.36 per watt peak. Costs of solar photovoltaic electricity fell by roughly 85% between 2010 and 2021, during which period solar and wind together grew from 1.7% to 8.7% of global electricity generation.

• 5. Materials: Many Paths, One Goal

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  Crystalline silicon dominates the solar industry with a market share of 95%. Almost everything else competes for the remaining sliver. Cadmium telluride thin-film cells hold the largest alternative share; the compound rivals silicon in cost per watt, though cadmium is highly toxic and tellurium supplies are limited. Within the silicon family, monocrystalline cells produce higher efficiency because electrons travel through a single continuous crystal lattice without encountering grain boundaries. Their distinctive octagonal shape comes from slicing cylindrical ingots grown by the Czochralski process. Polycrystalline cells, cast from square blocks of molten silicon slowly cooled into many small crystals, are less expensive but also less efficient. A hybrid approach introduced commercially around 2009 uses polycrystalline casting chambers seeded with small fragments of monocrystalline material, yielding mono-like efficiency at poly-like prices. On the efficiency frontier, multi-junction cells stack multiple semiconductor layers, each tuned to absorb a different slice of the solar spectrum. Gallium arsenide-based multi-junction devices hold the lead in efficiency. On the 15th of October 2012, triple-junction metamorphic cells reached a record of 44%. In 2022, researchers at the Fraunhofer Institute for Solar Energy Systems in Freiburg demonstrated a four-junction concentrator cell reaching 47.6% efficiency under 665-fold sunlight concentration. Among emerging technologies, perovskite cells have attracted intense research attention. Their efficiency rose from below 5% at first use in 2009 to 25.5% in 2020, an improvement rate faster than any previous solar technology. Commercialisation is still limited by stability and by the presence of toxic lead in the most efficient variants. Bifacial cells take a different approach to gaining output: with a transparent rear surface they can absorb light from both faces. The first bifacial patent was filed by Japanese researcher Hiroshi Mori in 1966. A practical bifacial cell was described in 1977 patents by Prof. Antonio Luque of the Technical University of Madrid, with Luque's PhD student Andrés Cuevas demonstrating a 50% increase in output power under a white background in 1980. The company Isofoton was founded in Málaga in 1981 to produce these cells commercially, with an initial production capacity of 300 kilowatts per year.

• 6. China's Manufacturing Grip

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  As of September 2018, China produced sixty percent of the world's solar photovoltaic modules. By 2022 that figure had risen to 77.8%. The IEA's 2022 Special Report attributed this position partly to investment exceeding US$50 billion and the creation of around 300,000 jobs in China since 2011. China controls over 80% of all manufacturing stages for solar panels and nearly 95% of key solar PV components, including roughly 40% of the world's polysilicon production concentrated in Xinjiang. In 2021 alone, China's solar PV exports exceeded US$30 billion. That dominance has driven costs down sharply for buyers everywhere, but it also creates risks: supply-demand imbalances, polysilicon production constraints, and potential cost surges if supply chains are disrupted. Other significant producers, including Vietnam and Malaysia, developed much of their capacity through Chinese companies targeting export markets, particularly exports to the United States. Reaching the IEA's goal of over 630 gigawatts of annual solar PV manufacturing by 2030 will require rapid expansion, and critical mineral demand, including silver, may exceed 30% of 2020 global production by the end of that decade.

• 7. The Waste Problem Ahead

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  Solar panels are typically rated for a lifespan of 25 to 30 years before decommissioning. The International Renewable Energy Agency estimated that solar panel electronic waste generated in 2016 reached between 43,500 and 250,000 metric tons. That range is wide because tracking methods vary, but both ends of it are already large. Estimates project that figure rising to between 60 and 78 million metric tons by 2050, as the panels installed during the industry's rapid growth decades reach the end of their lives all at roughly the same time. Crystalline silicon cells, which make up the vast majority of installed capacity, contain hazardous materials including lead, cadmium, and cadmium sulfide. Recycling them is technically possible but not yet routine at scale. Under the 2016 Paris Agreement, 195 countries committed to shifting away from fossil fuels toward renewable energy, ensuring that many more panels will be installed in the coming decades. The question of what happens to those panels at end-of-life may be the next inflection point in the solar industry's long history of solving hard problems.