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Impact of Defects on Solar Cell Performance in Selenium. Selenium solar cell

Impact of Defects on Solar Cell Performance in Selenium. Selenium solar cell

    Impact of Defects on Solar Cell Performance in Selenium

    Slides for my talk on modelling defects in selenium and their impact on solar cell performance at MRS Spring 2023, San Francisco.

    Hopefully a preprint on this work will be out soon!

    References: ShakeNBreak website: https://shakenbreak.readthedocs.io/en/latest/ Our general defect calculation package doped is available here: https://github.com/SMTG-UCL/doped

    See the referenced paper from Rasmus Nielsen on Se solar cells here: https://doi.org/10.1039/D2TA07729A

    impact, defects, solar, cell

    Questions welcome! For other computational photovoltaics, defects and disorder talks, have a look at my YouTube channel! https://www.YouTube.com/SeanRKavanagh For other research articles see: https://bit.ly/3pBMxOG

    Decks by Seán R. Kavanagh

    Cation Disorder in ABZ₂ Chalcogenide Photovoltaics (NaBiS₂ AgBiS₂) and Symmetry Breaking at Defects

    Metastable Defect Structures and their Effect on Carrier Recombination (ETH Zürich Defects Workshop Poster 2022)

    Other Decks in Research

    PAKDD2023 Tutorial 2: A Gentle Introduction to Technologies Behind Language Models and Recent Achievement in ChatGPT (Parts 3 and 4)

    Transcript

    102/05/2023Impact of Defects on Solar CellPerformance in SeleniumSeán KavanaghProfs: David O. Scanlon Aron Walsh[email protected](University College London Imperial College London) Y.-T. Huang, S. R. Kavanagh, D.O. Scanlon, A. Walsh,R.L.Z. Hoye, Nanotechnology 32, 132004 (2021)

    202/05/2023Impact of Defects on Solar CellPerformance in SeleniumSeán KavanaghProfs: David O. Scanlon Aron Walsh[email protected](University College London Imperial College London) Y.-T. Huang, S. R. Kavanagh, D.O. Scanlon, A. Walsh,R.L.Z. Hoye, Nanotechnology 32, 132004 (2021)

    Why Selenium? First material used for photovoltaic (PV) solar cells, in 1883 (η 1%). Researchers then moved on to c-Si, CdTe, CZTS/CIGS, perovskites. Recent surge in interest after decades of neglect, following Todorovet al. record efficiency η = 6.5% in 2017. Low-temperature solution-growth processing, “simple” chemistry,chain-like structure could yield benign grain boundaries. Wide-bandgap (~1.9 eV) suitable for single-junction or tandem PV.3

    Se PV Efficiency: Limiting Factor?42Nielsen et al. J Mater Chem A 2022Record Efficiency:1η = 6.5% with Voc= 969 mVTheoretical max η ~ 24% (detailed balance limit)Record Voc2 = 991 mVEg(Se, direct) = 1.95 eV➡ Vocdeficit 600 mV, suggesting non-radiativerecombination at defects to be the keycontributing factor.1Todorov et al. Nat. Commun. 2017

    Selenium: Bulk Properties5Strong inter-chain vdWdispersion interactions~10% volume decrease whenincluding vdW effectsTheory (Hybrid DFT vdW)ExperimentDirection a,b c a,b cLattice Parameter (Å) 4.34 4.96 4.37 4.95Volume (Å3) 81.1 81.9εionic0.63 0.94 0.461 0.621ε∞ (optical)6.71 10.28 6.971 11.621Esurface(J/m2) 0.18 0.17521Danielewicz Coleman Appl. Opt, 1974210.1016/0022-3093(71)90004-4

    Selenium: Bulk Properties6Se pDirect Band gap Eg, dir= 1.83 eV (Expt: 1.95 eV)1Indirect gap Eg, indir= 1.71 eV (Expt: 1.85 eV)2➡ Partial contribution to Vocdeficit from indirect gap1Nielsen et al. J Mater Chem A 20222th Phys Rev Lett 1979CBM:VBM:Theory = HSE06vdWSOC

    Defect Calculation Workflow15➡ Energy➡ Concentration➡ Transition Level➡ Deep/Shallow➡ Doping➡ Carrier capture➡ Diffusion➡ …

    Computational Defect ThermodynamicsVBM CBMD1D0D-1Fermi Level EF(eV)Formation Energy ΔHX,q(eV)ε(1/0) ε(0/-1)EFVBMCBMε(1/0)ε(0/-1)Kavanagh, Scanlon, Walsh, Freysoldt Faraday Discussions 2022

    Intrinsic Defect Formation Energy Diagram17Sei0 = Dominant, lowest energy native defect Electrically benign Neutral across most of the bandgap, with anegative-U (0/-2) level just below the CBM.0-1-20-21Calculated with doped (GitHub.com/SMTG-UCL/doped) ShakeNBreak (shakenbreak.readthedocs.io), using VASP

    Why is Sei0 so low energy?Split-interstitial motif:Sei0 joins Se chain whichtwists to accommodate.➡ Low-energy nodangling bonds- stays neutral.18

    Intrinsic Defect Formation Energy Diagram190-1-20-21Calculated with doped (GitHub.com/SMTG-UCL/doped) ShakeNBreak (shakenbreak.readthedocs.io), using VASPVSe(0/-1)VSe(1/0)VSe(-1/-2)VSe- Moderate formation energy, expectlow but non-negligible concentrations. Multiple in-gap defect levels, which couldbe active for carrier recombination.

    Vacancy Defects (VSe)1:Terminated Bridging Chain0 (ΔE = 27 meV):‘Self-healed’Chain0 and.1:Two TerminatedChains-2:Elongated Bonds; ‘Self-healed’ Bridging

    Intrinsic Defect Formation Energy Diagram210-1-20-21Calculated with py-sc-fermi= EFAnneal Temperature = 300KFermi level (EF) determined by charge neutrality condition: p ND = n NA–

    220-1-20-21= EFIntrinsic Defect Formation Energy DiagramAnneal Temperature = 463 K (190℃)Calculated with py-sc-fermiFermi level (EF) determined by charge neutrality condition: p ND = n NA–

    230-1-20-21= EFIntrinsic Defect Formation Energy DiagramAnneal Temperature = 1000KCalculated with py-sc-fermiFermi level (EF) determined by charge neutrality condition: p ND = n NA–

    24 Se without impurities. insulating/weakly p-type Large p-type doping window High sensitivity to p-type impurities n-type doping not possible But experiment1 sees ~1016 holes/cm-3…1Nielsen et al. J Mater Chem A 20220-1-20-21Calculated with doped (GitHub.com/SMTG-UCL/doped) ShakeNBreak (shakenbreak.readthedocs.io), using VASP= EFSeiVSeAnneal Temperature = 463 K (190℃)

    What Else is In There?Potential impurities: Halogens are common impurities in chalcogenide materials, oftenpresent in supplier precursors ➡ F, Cl, Br Typically grown on Tellurium substrates ➡ Te Typically annealed in air to aid crystallization ➡ O Hydrogen is a very common impurity in materials ➡ H Sulfur present in precursors, can bond easily with Se ➡ S25 Experiment1 sees ~1016 holes/cm-3… Not from intrinsic defects

    26Substitutions: XSeInterstitials: XiFClBrHOSTeCalculated with doped (GitHub.com/SMTG-UCL/doped) ShakeNBreak (shakenbreak.readthedocs.io) Using dopant-rich chemical potential limitsHalogensChalcogensHydrogen

    VSe1:Terminated Bridging ChainStable Positive Halogen Interstitials?Fi1:Two Bridging Chains Se-F BondFi-1:Intercalated F-1 ion

    0.0 0.8 1.6Fermi Level (eV)0.00.51.01.5Formation Energy (eV)SeiVSeFi29 Fluorine interstitials could increase holeconcentrations slightly, but still Experiment sees ~1016 holes/cm-3…Calculated with doped (GitHub.com/SMTG-UCL/doped) ShakeNBreak (shakenbreak.readthedocs.io), using VASP= EFAnneal Temperature = 463 K (190℃)0.00.51.01.5 SeiVSeFi

    Conclusions Future Steps Self-interstitials are neutral and benign for recombination, but Sevacancies could capture charge carriers. ➡ Explicitly calculate non-radiative recombination rates of Vse ➡ Devise passivation strategies; out-of-equilibrium Se over-pressures? Fermilevel control during growth/annealing? Hydrogen/halogens/chalcogens do not seem to be the cause of p-type doping in Selenium. Se chains show strong valence alternation re-bonding to self-compensate. ➡ Pnictogens (N, P, As) perhaps? Chalcogen substitutions are low energy,pnictogens could also substitute and be under-valent (- negatively-charged)30

    Other Results:Band AlignmentLower VBM comparedto Sb2Se3as expected(anti-bonding Sb s – Sep interaction in Sb2Se3gives raised VBM)32

    Other results: Interestingly, PBEsol seems to perform terribly for this material, with orwithout D3 vdW dispersion correction. Gives lattice parameter errors~8%, HSE06-on-PBEsol structure gives a bandgap underestimated by50% compared to HSE06-on-HSE06 or experiment, and the ionicdielectric is off by an order of magnitude (again compared to HSE06 orexperiment which match) due to the severely underestimated interchaindistances (which makes a big difference to inter-chain interactions) PBE does better, but still not great (checked against Materials ProjectPBE results). Errors of around 3-4% in the lattice parameters both withand without D3 (overestimates a,b without D3, underestimates with D3). Dispersion-corrected hybrid DFT (HSE06D3) is very accurate for thebulk properties of Se on the other hand; gives lattice parametersmatching experiment to 1%, closely matches the experimental bandgap(ΔEgand electronic contributions) very well…33

    Re-awakening the world’s first solar cells for indoor photovoltaics applications

    The world’s first solid-state photovoltaics were reported in 1883, and were composed of selenium, which eventually led to the development of the present-day photovoltaics, although the wide bandgap of selenium was limiting for applications of sunlight harvesting.

    In their present work published in Science Advances, Bin Yan and a team of researchers in chemistry, nanotechnology and materials science in China, revisited the concept of the world’s oldest photovoltaics material to describe its role in indoor photovoltaics applications. The adsorption spectrum of the material perfectly matched the emission spectra of commonly used indoor light sources. The researchers used selenium modules to produce an output power of 232.6 μW under indoor light illumination to power a radiofrequency identification-based localization tag.

    The field of photovoltaics

    In 1873, electrical engineer Willoughby Smith first discovered the photoconductivity of selenium, and Charles Fritts constructed the first solid-state solar cells thereafter in 1883 by sandwiching selenium between a metal foil and a thin gold layer. The low preliminary power conversion efficiency of these early discoveries, initiated research in the field of photovoltaics and inspired the emergence of solar cells in 1954, to lay the foundation to the modern photovoltaic industry.

    Until recently, scientists had incorporated indoor photovoltaics to convert indoor light into usable electrical power for wireless devices such as sensors, actuators, and communication devices. In this work, Yan et al. showed the unique advantages of using selenium for indoor photovoltaics with its suitably wide bandgap and intrinsic environmental stability. The team also developed selenium modules to produce an output power of 232.6 μW, to power an internet of things wireless device for radiofrequency identification-based localization.

    Indoor photovoltaics

    It is now possible to power the internet of things devices by harvesting indoor light via indoor photovoltaics (IPV). The concept is a growing research field, where a variety of technologies including dye-sensitized solar cells and organic photovoltaics and lead-halide perovskite solar cells are explored for their functionality.

    Indoor light is typically designed to suit human eye sensitivity, so by design its elements differ from conventional outdoor photovoltaics. When the existing features of selenium were combined with its non-toxicity and excellent stability, Yan et al. deemed the material to be ideal for indoor photovoltaic applications.

    Optimizing the experiments for improved outcomes

    The research team adopted a superstrate configuration of glass/Fluorine-doped tin oxide with titanium oxide/tellurium/selenium and gold to develop the thin-film selenium solar cells. During the process, they used environmentally-friendly titanium oxide to form the buffer layer, and constructed the non-toxic selenium-based devices to facilitate indoor light applications.

    During the experiments, they studied the selenium solar cells under standard one-sun illumination and measured indoor photovoltaic performances of devices under indoor light at 1000 Lux, with a common LED source of light to simulate the environment of illumination. The outcomes also led to the optimization of the tellurium layer to facilitate significantly different light intensities between indoor light and sunlight.

    Indoor light could comparatively only generate a relatively small number of carriers on account of its very weak intensity. The team therefore improved the device to obtain a positive photodoping effect to optimize the selenium solar cells under indoor light conditions. Yan et al. additionally incorporated tellurium at the selenium/titanium oxide interface to provide a strong bond for surface passivation.

    Applications of the devices

    The devices can be used to investigate a range of indoor lighting conditions typically required to light environments such as the living room, the library, or a bright supermarket. The selenium cells outperformed market-dominating silicon-based cells that are presently an industry standard for indoor photovoltaics, relative to both power conversion efficiency and stability.

    Contrastingly, silicon-based cells only exhibited a power conversion efficiency below 10%, with relatively minimal photostability. On account of these observations, the team considered the selenium-based devices to be a more attractive alternative candidate. They also studied the capacity of the selenium device to power the internet of things wireless devices.

    Outlook

    In this way, Bin Yan and colleagues reinterpreted selenium, the oldest existing photovoltaic material with the emergence of indoor photovoltaic devices, due to its unique capacity to offer a suitable wide bandgap for indoor light harvesting. The material is non-toxic and has intrinsic environmental stability as essential features.

    The scientists optimized the material composition to achieve a power conversion efficiency of 15%, suited for 1000 Lux indoor illumination with selenium cells. This outcome surpassed the existing efficiency of commercial silicon cells. The selenium devices performed without degradation, even after 1000 hours of continuous indoor lighting.

    The outcomes of the study highlight the scope of using selenium for indoor photovoltaics with added potential to power the internet of things devices as an attractive element in photovoltaics.

    information: Bin Yan et al, Indoor photovoltaics awaken the world’s first solar cells, Science Advances (2022). DOI: 10.1126/sciadv.adc9923

    Richard Haight et al, Solar-powering the Internet of Things, Science (2016). DOI: 10.1126/science.aag0476

    Journal information: Science Advances. Science

    Selenium Photo Cell

    Sold by Radio Shack from 1972-1978. “Learn by doing with this educational device — ideal for school projects. Produces.5 volts at.6 milliamps under strong light. Complete with leads, plus instructions for light-powered radio, relay, oscillator. 1 1/2 x 1/8″. ”.From the catalog. The original list price was 1.29 USD

    Part No. Selenium Photo Cell
    Consumer ,
    Materials selenium
    Voltage 0.5 V
    Amperage 0.0006 A
    Acquisition Purchased

    Related Cells

    Instrument, Selenium, Instrument, Selenium

    Cell appearance looks like the C.W. Hewlett selenium cell which was manufactured between 1935 to 1937. However, documentation and year is unknown for this.

    Year unknown, Weston Photronic cells have been produced since 1931. Cells are produced by Huvgen presently. Donated by Kyle Hounsell

    Online Solar Cell Archive

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    The History of Solar Energy: Solar Panels Then Now

    Widespread use of solar panels has soared in recent decades, but the idea of harnessing the sun’s energy isn’t new at all. In fact, there is an extensive history of solar energy. Plants have been using solar energy since the beginning of time to create nutrients, and humans started taking advantage of the sun’s power centuries ago. The Ancient Greeks used solar power to heat homes and baths, and Leonardo Da Vinci even designed a solar system to heat water in the 15th century. Though solar is the most basic form of energy, it’s seen tremendous advancements over the years. Understanding the history of solar technology helps us see how it’s bound to stick around.

    The First Solar Panels

    The first modern breakthrough in solar energy occurred in 1839 when French physicist Edmond Becquerel discovered the photovoltaic (PV) effect. a process that creates a voltage or electric current when exposed to light. Flash forward to 1883, and the next big leap in solar energy is made — New York inventor Charles Fritts created the first solar cell. This solar module consisted of a wide, thin selenium wafer covered by an even thinner sheet of gold. Its energy conversion rate ranged from 1 to 2 percent, which pales in comparison to today’s solar cells, which operate at an efficiency of 15 to 20 percent. Nonetheless, this was still a significant moment for solar energy and demonstrated the potential of clean power.

    While selenium was the primary component of the solar cell Fritts invented, modern solar modules are constructed with silicon. In 1954, researchers at Bell Laboratories realized that semiconductors, such as silicon, were more efficient than selenium, so they began creating silicon solar cells. Energy conversion rates for these cells were 6%, significantly higher than than the previous selenium cells. The issue, however, was the price. The cells were costly to produce for large-scale applications. These photovoltaic cells were used in satellites and consumer electronics like calculators, but it was too expensive for many people to install on their roofs.

    Environmental Activism Spurs Development

    This changed in the 1970s when an energy crisis forced the government to consider ways to promote renewables. Congress then enacted the Solar Energy Research, Development and Demonstration Act of 1974 to promote the research and development of solar technologies. Additionally, the federal government began offering tax incentives to support the transition to solar and other renewable energy sources. At the same time, a major cultural shift occurred in the United States. The modern environmental movement sprung up, raising awareness of pollution and its consequences for people and the planet. This helped people learn about the benefits of solar over fossil fuels, and its popularity skyrocketed thanks to its benefit on the environment.

    Solar Panels Today

    Now, solar is more affordable than ever. Thanks to increased demand, a more efficient supply chain, and better resource allocation, solar is officially the cheapest electricity in history. As the affordability of solar has increased, so has the technology’s popularity. In 2019, the United States reached a total of 2 million solar installations. and as solar continues to evolve, this number is growing exponentially. People often think using solar energy is new but the idea of harnessing the sun’s power has endured for centuries, and solar panel technology has been advancing for decades. As we continue to face the environmental consequences of traditional energy sources, solar will become a necessary component of our future.

    Learn About Solar Power

    Interested in learning more about solar? Our team can answer any question you have about solar energy, so you can see firsthand why it’s become so popular.

    The Invention Of The Solar Cell

    60 years ago this week, the modern solar cell came into being. Here’s how.

    By John Perlin | Published Apr 22, 2014 9:19 PM EDT

    The great Scottish scientist James Clerk Maxwell wrote in 1874 to a colleague: “I saw conductivity of Selenium as affected by light. It is most sudden. Effect of a copper heater insensible. That of the sun great.”

    Maxwell was among many European scientists intrigued by a behavior of selenium that had first been brought to the attention of the scientific community in an article by Willoughby Smith, published in the 1873 Journal of the Society of Telegraph Engineers. Smith, the chief electrician (electrical engineer) of the Gutta Percha Company, used selenium bars during the late 1860s in a device for detecting flaws in the transatlantic cable before submersion. Though the selenium bars worked well at night, they performed dismally when the sun came out. Suspecting that selenium’s peculiar performance had something to do with the amount of light falling on it, Smith placed the bars in a box with a sliding cover. When the box was closed and light excluded, the bars’ resistance — the degree to which they hindered the electrical flow through them — was at its highest and remained constant. But when the cover of the box was removed, their conductivity — the enhancement of electrical flow — immediately “increased according to the intensity of light.”

    Discovering the Photovoltaic Effect in a Solid Material

    To determine whether it was the sun’s heat or its light that affected the selenium, Smith conducted a series of experiments. In one, he placed a bar in a shallow trough of water. The water blocked the sun’s heat, but not its light, from reaching the selenium. When he covered and uncovered the trough, the results obtained were similar to those previously observed, leading him to conclude that “the resistance [of the selenium bars] was altered…according to the intensity of light.”

    Among the researchers examining the effect of light on selenium following Smith’s report were two British scientists, Professor William Grylls Adams and his student Richard Evans Day. During the late 1870s they subjected selenium to many experiments, and in one of these trials they lit a candle an inch away from the same bars of selenium Smith had used. The needle on their measuring device reacted immediately. Screening the selenium from light caused the needle to drop to zero instantaneously. These Rapid responses ruled out the possibility that the heat of the candle flame had produced the current (a phenomenon known as thermal electricity), because when heat is applied or withdrawn in thermoelectric experiments, the needle always rises or falls slowly. “Hence,” the investigators concluded, “it was clear that a current could be started in the selenium by the action of the light alone.”5 They felt confident that they had discovered something completely new: that light caused “a flow of electricity” through a solid material. Adams and Day called current produced by light “photoelectric.”

    The First Module

    A few years later, Charles Fritts of New York moved the technology forward by constructing the world’s first photoelectric module. He spread a wide, thin layer of selenium onto a metal plate and covered it with a thin, semitransparent gold-leaf film. This selenium module, Fritts reported, produced a current “that is continuous, constant, and of considerable force[,]…not only by exposure to sunlight, but also to dim diffused daylight, and even to lamplight.” As to the usefulness of his invention, Fritts optimistically predicted that “we may ere long see the photoelectric plate competing with [coal-fired electrical-generating plants],” the first fossil-fueled power plants, which had been built by Thomas Edison only three years before Fritts announced his intentions.

    Fritts sent one of his solar panels to Werner von Siemens, whose reputation ranked on a par with Edison’s. The panels’ output of electricity when placed under light so impressed Siemens that the renowned German scientist presented Fritts’s panel to the Royal Academy of Prussia. Siemens declared to the scientific world that the American’s modules “presented to us, for the first time, the direct conversion of the energy of light into electrical energy.”

    The blessed vision of the Sun, no longer pouring unrequited into space.

    Siemens judged photoelectricity to be “scientifically of the most far-reaching importance.” James Clerk Maxwell agreed. He praised the study of photoelectricity as “a very valuable contribution to science.” But neither Maxwell nor Siemens had a clue as to how the phenomenon worked. Maxwell wondered, “Is the radiation the immediate cause or does it act by producing some change in the chemical state?” Siemens did not even venture an explanation but urged a “thorough investigation to determine upon what the electromotive light-action of [the] selenium depends.”

    Few scientists heeded Siemens’s call. The discovery seemed to counter all of what science believed at that time. The selenium bars used by Adams and Day, and Fritts’s “magic” plate, did not rely on heat to generate energy as did all other known power devices, including solar motors. So most dismissed them from the realm of further scientific inquiry.

    One brave scientist, however, George M. Minchin, a professor of applied mathematics at the Royal Indian Engineering College, complained that rejecting photoelectricity as scientifically unsound — an action that originated in the “very limited experience” of contemporary science and in “a ‘so far as we know’ [perspective —] is nothing short of madness.” In fact, Minchin came closest among the handful of nineteenth-century experimentalists to explaining what happens when light strikes a selenium solar cell. Perhaps, Minchin wrote, it “simply act[s] as a transformer of the energy it receives from the sun, while its own materials, being the implements used in the process, may be almost wholly unmodified.”

    The scientific community during Minchin’s time also dismissed photoelectricity’s potential as a power source after looking at the results obtained when measuring the sun’s thermal energy in a glass-covered, black-surfaced device, the ideal absorber of solar heat. “But clearly the assumption that all forms of energy of the solar beam are caught up by a blackened surface and transformed into heat is one which may possibly be incorrect,” Minchin argued. In fact, he believed that “there may be some forms of [solar] energy which take no notice of blackened surfaces[, and] perhaps the proper receptive surfaces” to measure them “remain to be discovered.” Minchin intuited that only when science had the ability to quantify “the intensities of light as regards each of [its] individual colours [that is, the different wavelengths] could scientists judge the potential of photoelectricity.”

    Einstein’s Great Discovery

    Albert Einstein shared Minchin’s suspicions that the science of the age failed to account for all the energy streaming from the sun. In a daring paper published in 1905, Einstein showed that light possesses an attribute that earlier scientists had not recognized. Light, he discovered, contains packets of energy, which he called light quanta (now called photons). He argued that the amount of power that light quanta carry varies, as Minchin suspected, according to the wavelength of light — the shorter the wavelength, the more power. The shortest wavelength, for example, contains photons that are about four times as powerful as those of the longest.

    Einstein’s bold and novel description of light, combined with the discovery of the electron and the ensuing rash of research into its behavior — all happening at the turn of the nineteenth century — provided photoelectricity with a scientific framework it had previously lacked and that could now explain the phenomenon in terms understandable to science. In materials like selenium, the more powerful photons carry enough energy to knock poorly linked electrons from their atomic orbits. When wires are attached to the selenium bars, the liberated electrons flow through them in the form of electricity. Nineteenth-century experimenters called the process photoelectric, but by the 1920s scientists referred to the phenomenon as the photovoltaic effect.

    This new legitimacy stimulated further research into photovoltaics and re-vived the dream that the world’s industries could hum along fuel- and pollution-free, powered by the inexhaustible rays of the sun. Dr. Bruno Lange, a German scientist whose 1931 solar panel resembled Fritts’s design, predicted that, “in the not distant future, huge plants will employ thousands of these plates to transform sunlight into electric power…that can compete with hydroelectric and steam-driven generators in running factories and lighting homes.” But Lange’s solar battery worked no better than Fritts’s, converting far less than 1 percent of all incoming sunlight into electricity — hardly enough to justify its use as a power source.

    The pioneers in photoelectricity failed to attain the goals they had hoped to reach, but their efforts were not in vain. One contemporary of Minchin’s credited them for their “telescopic imagination [that] beheld the blessed vision of the Sun, no longer pouring unrequited into space, but by means of photo-electric cells…[its] powers gathered into electric storehouses to the total extinction of steam engines and the utter repression of smoke.” In his 1919 book on solar cells, Thomas Benson complimented these pioneers’ work with selenium as the forerunner of “the inevitable Solar Generator.” Maria Telkes, too, felt encouraged by the selenium legacy, writing, “Personally, I believe that photovoltaic cells will be the most efficient converters of solar energy, if a great deal of further research and development work succeeds in improving their characteristics.”

    With no breakthroughs on the horizon, though, the head of Westinghouse’s photoelectricity division could only conclude, “The photovoltaic cells will not prove interesting to the practical engineer until the efficiency has increased at least fifty times.” The authors of Photoelectricity and Its Applications agreed with the pessimistic prognosis, writing in 1949, “It must be left to the future whether the discovery of materially more efficient cells will reopen the possibility of harnessing solar energy for useful purposes.”

    The First Practical Solar Cell

    Bell executives presented the Bell Solar Battery to the press on April 25, 1954.

    Just five years later the beginning of the silicon revolution spawned the world’s first practical solar cell and its promise for an enduring solar age. Its birth accidentally occurred along with that of the silicon transistor, the principal component of every electronic device in use today. Two scientists, Calvin Fuller and Gerald Pearson of the famous Bell Laboratories, led the pioneering effort that took the silicon transistor from theory to working device. Pearson was described by an admiring colleague as the “experimentalist’s experimentalist.” Fuller, a chemist, learned how to control the introduction of the impurities necessary to transform silicon from a poor to the preeminent conductor of electricity. As part of the research program, Fuller gave Pearson a piece of silicon containing a small concentration of gallium. The introduction of gallium had made the silicon positively charged. When Pearson dipped the rod into a hot lithium bath, according to Fuller’s formula, the portion of the silicon immersed in the lithium became negatively charged. Where the positive and negative silicon met, a permanent electrical field developed. This is the p-n junction, the heart of the transistor and solar cell, where all electronic activity occurs. Silicon prepared this way needs but a certain amount of outside energy for activation, which lamplight provided in one of Pearson’s experiments. The scientist had the specially prepared silicon connected by wires to an ammeter, which, to Pearson’s surprise, recorded a significant electrical current.

    While Fuller and Pearson worked on improving transistors, another Bell scientist, Daryl Chapin, had begun work on the problem of providing small amounts of intermittent power in remote humid locations. In any other climate, the traditional dry-cell battery would do, but “in the tropics [it] may have too short a life” due to humidity-induced degradation, Chapin explained, “and be gone when fully needed.” Bell Laboratories had Chapin investigate the feasibility of employing alternative sources of freestanding power, including wind machines, thermoelectric generators, and small steam engines. Chapin suggested that the investigation include solar cells, and his supervisors approved.

    In late February 1953, Chapin commenced his photovoltaic research. Placing a commercial selenium cell in sunlight, he recorded that the cell produced 4.9 watts per square meter. Its efficiency, the percentage of sunlight it could convert into electricity, was a little less than 0.5 percent. Word of Chapin’s solar power studies and dismal results got back to Pearson. He told Chapin, “Don’t waste another moment on selenium,” and gave him the silicon solar cell that he had made. Chapin’s tests, conducted in strong sunlight, proved Pearson right. The silicon solar cell had an efficiency of 2.3 percent, about five times greater than the selenium cell’s. Chapin immediately dropped selenium research and dedicated his time to improving the silicon solar cell.

    His theoretical calculations of its potential were encouraging. An ideal unit, Chapin figured, could use 23 percent of the incoming solar energy to produce electricity. However, he set a goal of obtaining an efficiency of nearly 6 percent, the threshold that engineers of the time felt it was necessary to reach if photovoltaic cells were to be seriously regarded as electrical power sources.

    Chapin, doing most of the engineering, had to try new materials, test different configurations, and face times of despair when nothing seemed to work. At several junctures, seemingly insurmountable obstacles arose. One major breakthrough came directly from knowledge of Einstein’s light quanta (photon) work. “It appears necessary to make our p-n [junction] very next to the surface,” Chapin realized, so that the more powerful photons belonging to light of shorter wavelengths could effectively move electrons to where they could be harvested as electricity. To build such a cell required collaboration with Fuller. Chapin also observed that silicon’s shiny surface reflected a good deal of sunlight that could be absorbed and used, so he coated its surface with a dull transparent plastic. Adding boron to the top of the cell permitted better photon harvesting by allowing for good electrical contact on the silicon strips while keeping the p-n junction close to the surface. Chapin finally triumphed, reaching his 6 percent goal. He could now confidently call the cells he built “power photocells…intended to be primary power sources.” Assured of the cells’ reproducibility and sufficient efficiency, the trio built a number of arrays and demonstrated them at a press conference and the annual meeting of the National Academy of Sciences.

    Proud Bell executives presented the Bell Solar Battery to the press on April 25, 1954, displaying a panel of cells that relied solely on light power to run a 21-inch Ferris wheel. The next day the Bell scientists ran a solar-powered radio transmitter, which broadcast voice and music to America’s top scientists gathered at a meeting in Washington, DC. The press took notice. U.S. News World Report speculated excitedly in an article titled “Fuel Unlimited”: “The [silicon] strips may provide more power than all the world’s coal, oil and uranium….Engineers are dreaming of silicon-strip powerhouses.” The New York Times concurred, stating on page one that the work of Chapin, Fuller, and Pearson, which resulted in the first solar cell capable of generating useful amounts of power, “may mark the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams — the harnessing of the almost limitless energy of the sun for the uses of civilization.”

    From the book Let It Shine. Copyright © 2013 by John Perlin. Reprinted with permission from New World Library.

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