By Kevin Gu
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Imagine if the windows in your home could generate electricity. It seems paradoxical to try to harness solar energy with windows, but that is exactly what some researchers and companies are doing with organic solar cells, one of the next-generation solar cell technologies. These “solar windows” work by absorbing only ultraviolet and infrared light, allowing visible light to pass through. When the term “solar cell” is mentioned, you may imagine the black panels of silicon mounted on a neighbor’s roof. But silicon is only one of the many players in the suite of solar cell technologies. This article will give an overview of the various types, each with their unique attributes and potential.
How a solar cell works
The light-to-electricity conversion process begins when an electron in a semiconductor absorbs a photon of light. This energetic electron breaks free from the atom it was bound to and drifts away from its host atom. An electric field inside the material pushes the electron to the electrode, providing electric current.
Theoretical and practical limitations prevent the power conversion efficiency from being 100%. For example, silicon solar cells are fundamentally limited to 29% efficiency due to two main effects. First, only photons with more than 1.1 electron volts (eV) of energy can excite an electron. This value is known as the semiconductor’s band gap. Roughly a quarter of the light in the solar spectrum has energy below this value and passes through the cell without being absorbed. Secondly, when an electron absorbs a photon with energy greater than the band gap, the electron only gains as much energy as the band gap. The rest is lost as heat due to a process called thermalization. Together, this means a lower band gap will capture more light but produce less power per photon.
Record efficiencies of various solar cell types. Note that these values are laboratory records. Commercial cells are typically 60 – 80% of this value due to additional inefficiencies involved in scaleup. Source: National Renewable Energy Laboratory. Higher resolution image here.
(Moderate efficiency, moderate cost, mainstream commercial)
In 1954, Bell Labs introduced the first practical solar cell, made of silicon and operating at 6% power conversion efficiency. Silicon became the industry favorite very early on because it is abundant – regular beach sand is mostly silicon – and it has a favorable band gap. Since then, decades of research and development have pushed the efficiency to 25% (blue solid square in the figure above). With current cells just 4% shy of silicon’s theoretical efficiency of 29%, most efforts are now aimed at reducing production and installation cost per watt in order to compete with natural gas and fossil fuels. Silicon solar cells comprise ~90% of the current market.
(Moderate efficiency, moderate cost, minority commercial)
Copper indium gallium selenide (CIGS, commonly called “sigs”) and cadmium telluride (CdTe, “cad-tell”) are the two main commercial thin film technologies in direct competition with silicon. CIGS has the potential to be much cheaper and boasts a higher theoretical efficiency than silicon. However, difficulty in controlling composition of this 4-component material has limited the performance to well below the theoretical limit. CdTe is another promising competitor to silicon. Fabrication and composition control are comparatively simple. The concern is that tellurium is a relatively rare element, and uncertainty of the supply of tellurium has casted doubt on the future of CdTe.
CIGS and CdTe are both currently at 22% efficiency, the costs per watt are similar, and combined they comprise about 10% of the market. In years past, several companies had invested heavily in thin film technology with aims to overtake silicon. Prices were comparable and thin film seemed poised to take the lead. However, a massive increase in manufacturing of silicon solar cells in China caused prices to drop precipitously, outpacing thin films. Silicon is currently the market leader, but thin films are by no means out of the race.
(High efficiency, extremely expensive, niche applications)
Multijunction cells improve light capture and reduce thermalization losses by employing multiple layers of different materials with different band gaps. The top layer absorbs high-energy photons, allowing the lower energy photons to pass through to the next layer, and so forth. The result is that more light is captured at a higher energy. With this strategy, multijunction cells have reached 46% efficiency.
The immense complexity of fabricating multiple layers makes costs roughly 250 times higher per watt than silicon. Multijunction cells are used where efficiency and weight are of critical importance, such as on the International Space Station and Mars rovers. They are also used with mirrors for large-scale solar concentrators, where the cost of the actual cell is small compared to the rest of the infrastructure.
(Low efficiency, low cost, flexible, minimal commercial)
Organic solar cells are a fundamentally different beast. They are comprised of organic polymers, colloquially known simply as plastics. These plastics are not your common polyethylene grocery bag, however. The double bond in the molecular structure gives these polymers semiconducting properties. One advantage of organic solar cells is that the band gap can be changed by modifying the chemical structure of the polymer. As mentioned earlier, by tuning the band gap to only absorb non-visible light, several companies are attempting to produce transparent solar cell windows.
Perhaps the most attractive feature of organic solar cells is that they can be printed from ink. This enables very low cost roll-to-roll printing, similar to printing newspaper. Due to the fundamentally different physics however, organic solar cells do not enjoy the benefit of leveraging the decades of knowledge accumulated for inorganic solar cells (Si, CIGS, CdTe). Consequently, their main disadvantage is low efficiency (~10%). So far, a few companies have attempted to market organic solar cells for niche applications due to their physical flexibility. Reaching an efficiency of 20% would open the path for mainstream energy generation and could cause a major paradigm shift in the solar world.
(High efficiency, low cost, research stage)
The unprecedented meteoric rise in perovskite solar cell efficiency from 9.7% to 20.1% in the past 2 years has spurred a tremendous amount of interest in the research community (on the NREL chart, yellow/red markers beginning at 2013). The active material in perovskites is an organic-inorganic hybrid, commonly methylammonium lead halide. The immense potential demonstrated by perovskites combines the best of both worlds: inexpensive printing processing with high efficiency.
Perovskites are still in the early stages of research. The primary issue is currently stability, as the efficiency degrades too quickly for industrial or commercial applications. The mechanism(s) of degradation are not yet fully understood and a flurry of research effort has been launched to stabilize the efficiency. Even so, the lead-containing perovskite material is highly water-soluble and raises toxicity concerns in the event of panel damage. Attempts to produce lead-free perovskites have thus far achieved 6% efficiency.
Many companies have risen and fallen, risking their fortunes on the conviction that their technology would win out. But the narrative of solar energy has been an unpredictable one. Who could have anticipated the massive increase of Chinese production? Perhaps a stable perovskite or a high-efficiency organic solar cell will upturn the industry overnight. Whether one or several cell types will come to dominate remains to be seen. One thing is clear – each technology has a unique hand to play in the future renewable energy landscape.
Kevin Gu is a Ph.D. student in Chemical Engineering at Stanford University. He works in the lab of Professor Zhenan Bao, where his research focuses on solution processing of organic photovoltaics. He received his BS in Chemical Engineering from Caltech (’13), where he worked on solid oxide fuel cells under Professor Sossina Haile. Due to a long-standing interest in energy technologies, he plans to remain primarily involved in the field of sustainable energy in his future career.