Evolution of c-Si PV Cell Technologies
Inside this Article
Since Bell Labs introduced the crystalline silicon solar cell to the world in 1954, the technology has enabled exploration in space and transformed electrical power systems back on earth.
A solar cell is an electronic device that directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electrical power. This process requires a material in which the absorption of light raises an electron to a higher-energy state so that it can break free from its atomic structure and move around. Certain metals and semiconductors exhibit this trait, known as the photoelectric effect. Once the higher-energy electron is free, it must be able to move from the solar cell into an external circuit to dissipate its energy. It then returns to the solar cell to complete the circuit.
Sunlight is a form of electromagnetic radiation, and the visible light that we see is a small subset of the total incident energy the sun emits. In 1905, while studying the photoelectric effect, Albert Einstein described light as packets or particles of energy, today known as photons. Even after Einstein explained the physics of the photoelectric effect, it took many years for a practical electrical-generation application of the technology to evolve.
In this article, I briefly explore the history, anatomy, physics and lexicon of crystalline silicon (c-Si) PV cells. I then consider the evolution of modern high-efficiency c-Si PV cells, which is a function of important manufacturing advances as well as innovations in solar cell technologies. Finally, I consider some of the most promising paths forward to higher-efficiency and lower-cost c-Si PV modules for terrestrial applications. As this story will tell, it can take a lot of time—and a measure of good luck—for a solar cell technology to journey from the research laboratory to a format that facilitates mass production and a cost structure that enables commercial market opportunities.
The solar industry widely recognizes Bell Labs as the inventor of the modern-day solar cell. As solar historian John Perlin details in From Space to Earth (see Resources), Bell Labs tasked a group of scientists with developing a source of freestanding power as an alternative to traditional dry-cell batteries, and they began experimenting with photosensitive materials in 1952. After their initial attempts to improve the power output of selenium-based solar cells fell short, researcher Gerald Pearson discovered that silicon-based semiconductors, which Bell was developing for use in telephone transistors, provided a much more efficient base material for PV cells.
In 1954, Bells Labs announced its development of the Bell Solar Battery, an n-type, rear-contact silicon solar cell, shown in Figure 1, with a conversion efficiency of 6%. In spite of media praise for the invention, the company struggled to find a serious market for this device outside of novelty items, such as toys or radios run on solar. The success of the semiconductor transistor, which rapidly achieved economies of scale, ultimately made the Bell Solar Battery obsolete for the telecommunications industry at that time.
The space race was a critical turning point for silicon-based solar cells, which would subsequently take off— literally. On March 17, 1958, the US Navy launched the Vanguard 1, the fourth-ever artificial earth orbital satellite and the first to include a PV power source. Whereas earlier satellites relied on battery power only and had a mission duration of days, the solar- and battery-powered Vanguard 1 remained in service for more than 6 years. The ability to extend the useful life of orbiting satellites by allowing the sun’s energy to recharge onboard batteries was critical to success in space-based applications.
Established in late 1958, NASA demonstrated an interest in photovoltaics, spurring technological advancements that would lead to the development of more-powerful and more-reliable PV cells. With further experimentation and refinement, researchers drastically improved solar cell efficiency to around 14% by 1960. Scientists discovered, for example, that antireflective (AR) coatings on the front surface of the PV cell helped improve the absorption of light compared to bare silicon, which otherwise has a surface reflectance of over 30%. Other advancements, such as applying electrical contacts to the front of the cell rather than the rear, improved manufacturing speed and cost.
In his history of c-Si cell technologies (see Resources), Martin Green notes that while the basic cell design for space applications, shown in Figure 2, remained largely unchanged for about a decade, a number of improvements came about in the 1970s. Researchers discovered, for example, that adding a thin aluminum layer to the back of the c-Si PV cell created a back-surface field that delivered a significant boost in performance. Not long afterward, COMSAT Laboratories boosted performance further by chemically etching the surface of the c-Si PV cell to produce pyramidal structures that reduced reflection. By 1974, terrestrial cell performance had achieved conversion efficiencies of more than 17%.
In 1975, Spectrolab pioneered a screen-printing process for applying front metal contacts to the solar cell, using a metallic paste forced through a patterned template with a squeegee, a process similar to applying graphics to T-shirts. As crude as it may sound, this development ultimately led to dramatic manufacturing cost reductions and enabled PV technologies to become practical in terrestrial applications. Interestingly, oil and gas companies were some of the earliest adopters of terrestrial solar, deploying PV technology for offshore drilling wells. The process of screen-printing fingers on the front surface of a solar cell, such as the one shown in Figure 3, proved so effective that it is still in use today in large-scale c-Si PV manufacturing operations.