by Emily Kerr
figures by Abagail Burrus

The Sun emits enough power onto Earth each second to satisfy the entire human energy demand for over two hours. Given that it is readily available and renewable, solar power is an attractive source of energy. However, as of 2018, less than two percent of the world’s energy came from solar. Historically, solar energy harvesting has been expensive and relatively inefficient. Even this meager solar usage, though, is an improvement over the previous two decades, as the amount of power collected from solar energy worldwide increased over 300-fold from 2000 to 2019. New technological advances over the last twenty years have driven this increased reliance on solar by decreasing costs, and new technological developments promise to augment this solar usage by further decreasing costs and increasing solar panel efficiency.

Solar Cells: Costs, Challenges, and Design

Over the past 20 years, the costs associated with solar cells, the structures capable of converting light energy into electricity, have been steadily decreasing. The National Renewable Energy Laboratory, a US government lab that studies solar cell technology, estimates contributors to the increasing affordability of solar. They estimate that hard costs, the costs of the physical solar cell hardware, and soft costs, which include labor or costs to obtain required government permits, are about equal (Figure 1). Soft costs have decreased because there are more potential consumers and more installation experts for new solar cells, so companies can produce solar cells in bulk and install them easily. Hard costs are less than half of what they were in the year 2000, mostly due to decreasing material costs and an increased ability of cells to capture light. Engineering more cost-effective and efficient solar cells has required careful consideration of the physics involved in solar capture in addition to innovative design.

Figure 1: Costs associated with solar power. Solar cells become less expensive when the cost of the labor and materials use to build them go down, or when they become better at turning incoming light into electricity.

Because solar cells are used to convert light into electricity, they need to be composed of some material that’s good at capturing energy from light. This material can be sandwiched between two metal plates which carry the electricity captured from light energy to where it is needed, like the lights of a home or machines of a factory (Figure 2). Choosing the right material to capture light involves measuring the difference between two energy levels called the valence band and the conduction band. The lower-energy valence band is filled with many small negatively charged particles called electrons, but the higher-energy conduction band is mostly empty. When electrons are hit with particles of light, called photons, they can absorb enough energy to jump from the low-energy conduction band into the high-energy valence band. Once in the valence band, the extra energy in the electron can be harvested as electricity. It’s as if the electrons are sitting at the bottom of a hill (the conduction band) and being hit by a photon that gives them the energy to leap to the top (the valance band).

The amount of energy needed for electrons to jump into the valence band depends on the type of material. Essentially, the size of the metaphorical hill varies based on the properties of a given material. The size of this energy gap matters because it impacts how efficiently solar cells convert light into electricity. Specifically, if photons hit the electrons with less energy than the electron needs to jump from the valence band to the conduction band, none of the light’s energy is captured. Alternatively, If the light has more energy than is needed to overcome that gap, then the electron captures the precise energy it needs and wastes the remainder. Both of these scenarios lead to inefficiencies in solar harvesting, making the choice of solar cell material an important one.

Historically, silicon has been the most popular material for solar cells (Figure 2). One reason for this popularity lies in the size of the gap between silicon’s conduction and valence bands, as the energy of most light particles is very close to the energy needed by silicon’s electrons to jump the energy gap. Theoretically, about 32% of light energy could be converted into electric energy with a silicon solar cell. This may not seem like a lot, but it is significantly more efficient than most other materials. Additionally, silicon is also inexpensive. It is one of the most abundant elements on earth, and the cost of refining it has decreased dramatically since 1980. The solar cell and electronics industries have driven the decrease in purification cost as they have learned better bulk purification techniques to drive the demand of solar cells and consumer electronics.

Figure 2: Light energy capture in solar cells. When light hits a solar cell, it causes it causes electrons to jump into a conduction band, allowing the light energy to be harvested. Here yellow electrons (labeled e) move through the silicon atoms (labeled Si) in the solar cell when hit by a photon.

In addition to decreasing material costs, clever engineering tricks are pushing the efficiency of silicon solar cells closer to their theoretical maximum. In order for photons to be converted into energy, they must first collide with an electron. One trick to increase the likelihood of a photon/electron collision involves patterning the silicon in solar cells in microscopic pyramid shapes. When light is absorbed into a pyramid, it travels further, increasing the probability that the light will collide with the electrons in the silicon before escaping the cell.

In a similar tactic, chemists and material scientists have designed anti-reflective coatings to put on the front of solar cells to prevent useful light from being reflected back into space without ever hitting an electron in the solar cell. Likewise, putting a reflector on the back of the solar cell also allows more light to be harvested. The light that reaches the solar cell and makes it all the way through to the back without hitting an electron gets bounced to the front of the cell, giving the cell another chance of collecting the light.

Currently, the cost of silicon-based solar cells continues to decrease, and, despite predictions to the contrary, the cost of silicon itself continues to decrease. Silicon solar cells are likely to remain popular for the next few years. Alternatives to silicon solar cells have been developed but aren’t far enough along to be commercially viable.

The Future of Solar Cells

 To outpace current solar cells, a new design would need to be able to capture more light, transform light energy to electricity more efficiently, and/or be less expensive to build than current designs. Energy producers and consumers are more likely to adopt solar power if the energy it produces is equally or less expensive than other, often non-renewable, forms of electricity, so any improvement to current solar cell designs must bring down overall costs to become widely used.

The first option, adding hardware that allows the solar cells to capture more light, does not actually require that we abandon current solar cell designs. Electronics can be installed with the solar cell that let the cell track the sun as it moves through the daytime sky. If the solar cell is always pointing at the sun, it will be hit by many more photons than if it was only pointing towards the sun around midday. Currently, designing electronics that can track the position of the sun accurately and consistently for several decades at a reasonable cost is an ongoing challenge, but innovation on this front continues. An alternative to making the solar cell itself move is to use mirrors to focus light on a smaller, and therefore cheaper solar cell.

Another route to improving the performance of solar cells is to target their efficiency so they are better at converting energy in sunlight to electricity. Solar cells with more than one layer of light-capturing material can capture more photons than solar cells with only a single layer. Recently, lab-tested solar cells with four layers can capture 46% of the incoming light energy that hit them. These cells are still mostly too expensive and difficult to make for commercial use, but ongoing research may one day make implementing these super-efficient cells possible.

The alternative to improving the efficiency of solar cells is simply decreasing their cost. Even though processing silicon has become cheaper over the past few decades, it still contributes significantly to the cost of solar cell installation. By using thinner solar cells, material costs decrease. These “thin-film solar cells” use a layer of material to harvest light energy that is only 2 to 8 micrometers thick, only about 1% of what is used to make a traditional solar cell. Much like cells with multiple layers, thin-film solar cells are a bit tricky to manufacture, which limits their application, but research is ongoing.

In the immediate future, silicon solar cells are likely to continue to decrease in cost and be installed in large numbers. In the United States, these cost decreases are anticipated to increase the solar power produced by at least 700% by 2050. Meanwhile, research on alternative designs for more efficient and less expensive solar cells will continue. Years from now, we are likely to see alternatives to silicon appearing on our solar farms and rooftops, helping to provide clean and renewable sources of energy. These improvements have and will continue to be made possible by increasing bulk manufacturing of solar cells and new technologies that make the cells cheaper and more efficient.


Emily Kerr, Graduate Student in Chemistry and Chemical Biology.

Abagail Burrus is a third-year Organismic and Evolutionary Biology PhD student who studies elaiophore development.

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This article is part of our SITN20 series, written to celebrate the 20th anniversary of SITN by commemorating the most notable scientific advances of the last two decades. Check out our other SITN20 pieces!

21 thoughts on “The Future of Solar is Bright

  1. why aren’t solar technologies subsidized? fossil fuels are in that the price they are sold at doesn’t reflect their true cost – the true price which will be paid sooner or later by future generations.

    1. Fossil fuels are not subsidized. Royalties, leasing, income tax etc are paid by oil companies. Having certain tax exemptions is not a subsidy. A subsidy is forcibly taking money from one party and giving it to another. This does not happen. Chevron paid top 3 income tax for all publicly traded companies in America in 2014.

      Stop being misinformed.

      1. A tax break is 100% a subsidy for any business purpose. I have no dog in this fight, but a tax break is a way of taking money meant for the government and giving it back to a corporation. A subsidy is taking money from the government to offset a corporation’s costs. Same thing. If you need more proof, where do you think a subsidy comes from? Taxes.
        Company A pays $500 in taxes, gets a $250 subsidy, net payment $250
        Company B gets a 50% tax break from what would be a $500 tax bill, pays $250 in taxes.

    2. If we want to be more efficient and less expensive, the first thing we need to do is to make full use of the photons from the sun. That means we have to use motors, sensors, reflectors and racking systems correctly .

  2. Would it not be better to invest in improving solar technology than in installing inefficient solar panels. The long term benefit may be far greater from investing in developing a better technology. How about unleashing the power of capitalism by offering a billion dollar prize for creation of a solar technology at parity with the current actual cost of electricity produced with carbon fuels. This is after all the only sensible and workable real world means of retiring carbon fuels.

    1. They are already pretty efficient at 18-20% and extremely affordable for the average homeowner, thanks to net metering laws and government incentives. My question back would be, if you could take the same money you are spending on utility electricity and use it to finance a solar energy system on your roof, why wouldn’t you?

  3. I like what you said about the cost of solar cells and structures decreasing over the last few years. My sister has been telling me about how she wants to make sure that her family is using clean energy in the future. I’ll share this information with her so that she can look into her options for professionals who can help her with this in the future.

  4. It’s great that people are using solar energy and contributing towards saving the environment. In developing countries community solar panels can be implemented where more number of houses can use the energy from the same solar panel which can save their money, and they won’t find it costly as the cost of solar panels will be divided among the number of houses using it.

  5. Ima just post up some numbers on solar here. Spent two hours running the figures. Make your own damn mind up.

    I= ideal
    R= realistic
    EARTH SURFACE AREA
    510.1 Million Km2
    510.1 Trillion m2
    So, 255.05 Trillion m2 recieve solar energy at any one time, or for year long calculations use this number

    SOLAR ENERGY BUDGET (optimal angle -unrealistic but max future potential)
    1368w/m2 at top of ATMOS
    – 410w for reflection, (UB, MB, LB deal with atmospheric interference)
    LB 598 W/m2 @ surface harnessable = 0.166Wh= 1.454 kWh/year x 255.05 = 370.8×1000= 370,800 tWh (LBI)
    MB778 W/m2 @ surface harnessable =0.216Wh= 1.892 kWh/Year x 255.05 = 482.6×1000= 482,600 tWh (MBI)
    UB 958 W/m2 @ surface harnessable=0.266Wh= 2.330 kWh/Year x 255.05 = 594.3×1000= 594,300 tWh (UBI)

    1Watt = 1 Joule = 0.000278 W/H (divide energy factor by 3600 for watt hour, multiply by 8760 for annual W/Hr
    & divide by 1000 for Annual kWh) Answers in KwH/Year

    NB: Curiously the actual energy average recieved by earth is around half of
    what pro solar energy statistics say, as they use optimal angles of sunlight (682×2 = 1364)

    SOLAR ENERGY BUDGET (average over hemisphere-realistic numbers)
    174 petawatts = solar energy striking earth
    174 pw/255.05 trillion m2
    = 682 watts/m2 – 30% in reflection
    682-204.6= 477.4 w/m2

    LB 351w/m2 @ surface harnessable=0.098Wh= 0.859kWh/year x 255.05 = 219.1 x1000 = 219,100 tWh (LBR)
    MB 414w/m2@ surface harnessable=0.115Wh= 1.007kWh/year x 255.05 = 256.8 x1000 = 256,800 tWh (MBR)
    UB 477w/m2 @ surface hernessable=0.133Wh= 1.165kWh/year x 255.05 = 297.1×1000 = 297,100 tWh (UBR)

    ENERGY USAGE 410 Quintillion Joules p/y (1 quintillion is 1 followed by 18 zeros so
    410,000,000,000,000,000,000 / 3600 = 113,889 tWh )
    = 410000 Petajoules per year
    = 113,889 tWh per year

    Final Figures for Ep ( fomat avg current ratio of power of 15% -> maximum thermodynamic ratio of 56%)
    LBI = 55,620 tWh per year -> 207,648 tWh per year
    MBI = 72,390 tWh per year -> 270,256 tWh per year
    UBI = 89,145 tWh per year -> 332,808 tWh per year

    LBR = 32,865 tWh per year -> 122,696 tWh per year
    MBR = 38,520 tWh per year -> 143,808 tWh per year
    UBR = 44,565 tWh per year -> 166,376 tWh per year

    Solar Panels operate at maximum efficency only 10-30% of the time. These numbers are not reflective of this data,
    where instead atmospheric interference was determined via ratio of wattage interference in the atmosphere to show the maximum possible output. Therefore, these numbers represent the range of potential energy that could be harnessed.

    Using the UBI and UBR, simply multiply these by 0.6 to get a more realistic picture of what solar energy would produce, (0.6 because efficency dwindles incredibly when not at maximum, so benefit of doubt is given to solar here)

    89,145 x 0.6 = 53,487 tWh per year
    332,808 x 0.6 = 199,684.8 tWh per year

    44,565 x 0.6 = 26,739 tWh per year
    166,376 x 0.6 = 99,825.6 tWh per year.

    I study Geology and Oceanography at UoA, any questions on my sources are welcome.

  6. Respected Madam /Sir
    I have 10 acres of land I wish to start solar power plant. BUT I want to know that the solar plants have any disadvantages or only beneficial.
    Thanks and regards
    MEGHSHYAM DAYARAM GADEKAR
    9764071921

  7. I like this blog. I agree that silicon solar cells are likely to continue to decrease in cost and be installed in large numbers.

  8. Here is a real world example of a bright solar future:
    I am a high school physics teacher who had 2 arrays (30 + 35=65) of solar panels installed last year on a ground mount system next to my house in upstate NY. My primary reason was to eliminate our use of fossil fuels but my family is also very happy about the money we are saving. Our home now uses electricity for everything including heat pumps for heating, air conditioning, and hot water. We currently drive one electric car but sized our system for two more. The output of these panels more than covers all our energy uses. Before our solar pv system we were spending $10,000 a year on fuel oil for heat & hot water, gasoline, and electric. Now we only pay a grid connection fee of $20 per month. The cost of our system after government incentives was $35,000 (which is less than the cost of many of the vehicles in my school’s parking lot) so the system will have paid for itself in less than 4 years! For the next 26+ years we will be saving at least $10,000 a year – not a bad investment! Even better we will not be burning the 1,200 gallons of fuel oil and 1,800 gallons of gasoline we had been using yearly. First hand experience – switching to solar is a win for the environment as well as for family finances! I urge every homeowner who is serious about a more sustainable future to act on solar as soon as possible!

  9. On point.
    You can greatly see the benefits of switching into solar panels.
    Let’s contribute in saving the environment.
    Kudos to the Author! job well done.

  10. Here is actual numbers and research done by National Renewable Energy Laboratory. I feel this is very important information especially coming from a federal laboratory.

    Research conducted by NREL suggests that every 1 dollar saving on your energy bill due to the installation of a solar energy source adds to approximately 20 dollars to the existing value of your property. This is dependent on a few factors; let us understand them at length.

    The Size: Property value appreciation is directly impacted by the quality of the panels installed.

    Property Value: Large houses normally get higher boosts in property value, with that being said, the boost in the value is often a small percentage of the overall property value.

    The actual number changes for each property and as per installation, but the latest research shows that the increase of resale value on an average is about 5900 dollars for each 1kW installed solar power system. To give an example, a 3.1kW system in California will be valued at approximately 18000 dollars for a medium-sized property.

    The value of your property increases as you move up the ladder, for example, a 5kw installation gets you a value of approximately 30000 dollars for a medium-sized property.

    It is good to remember that these statistics apply as per the property prices of today as well as the utility rates. As electricity prices go up, in all probability the property price will also appreciate.

    Also, a well known solar company in California called http://www.SemperSolaris.com also suggests adding battery storage to your solar panels as you will get a lot more value especially when you have power outages due to fires or wind.

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