Questneers : Nam-Gyu Park (Sungkyunkwan University), Ki Tae Nam (Seoul National University)
Recent abnormal climate is mainly caused by greenhouse gases generated from fossil fuel use, and renewable energy is receiving attention to solve this problem. Currently, the energy conversion efficiency of solar cells reaches up to 47.6% using multi-junction technology, and research is being conducted in various directions to increase this further, including multiple exciton generation, infinite pn junctions, photon separation using metamaterials, and cryogenic electron control. Theoretically, 60% efficiency may be possible, but implementing this realistically is not easy. What are the reasons why it is difficult to develop 60% conversion efficiency technology, and what are the challenges that must be solved to achieve this?
Heat waves, severe droughts, massive wildfires, and other recent frequent abnormal climate events are the result of global warming. Global warming is a phenomenon that occurs as greenhouse gases created from fossil fuel use gradually increase Earth’s temperature. To solve this, we must use renewable energy technologies that do not use fossil fuels and do not emit greenhouse gases. Solar cell technology is known as the most important technology for solving global warming problems by using infinite solar energy to produce electricity.
The most important consideration when producing electricity using solar cells is energy conversion efficiency. When solar cells have high conversion efficiency, large amounts of electricity can be produced even in very small sizes, providing advantages in economic aspects. To achieve the goal of carbon neutrality, it is estimated that 75 terawatts (TW) of solar cells will be needed by 2050. However, the solar cells currently installed on Earth are only at the level of 1TW, requiring significant investment in the future. If the energy conversion efficiency of solar cells can be dramatically increased, the installation amount of solar cells is expected to be dramatically reduced.
The theoretical maximum conversion efficiency that can be obtained with single junction technology made by joining p-type and n-type semiconductors in solar cells is 33% according to the Shockley-Queisser limit, and the currently highest efficiency single junction gallium arsenide (GaAs) solar cell shows 29.1% efficiency. If single junction solar cells are stacked in multiple layers using multi-junction technology, efficiency can be increased, and using this multi-junction technology, a case recording 47.6% conversion efficiency has recently been reported.
Using MEG (multi exciton generation) technology, one photon can be absorbed to generate multiple electrons. If one photon can generate two electrons across all wavelengths absorbed by the semiconductor absorber, the current density doubles, so solar cell efficiency can theoretically increase up to 60%. However, since multi exciton generation technology is only possible with high-energy photons, generating multi excitons across all wavelength ranges that can be absorbed is realistically difficult. To solve this, innovative new technologies that go beyond existing thinking must emerge.
Above all, maximizing light utilization is important. The most representative example of using light with the highest efficiency is photosynthesis, which we commonly see around us. Photosynthesis in plants has separate centers for reactions and centers for collecting light to create energy. It can be seen as an example of perfect light utilization in that the efficiency of collecting light and converting it to energy is close to 100%. In this regard, photosynthesis is sometimes called a real-world example of room-temperature quantum energy coherence phenomena that humanity has not achieved. If solar cells could utilize light like photosynthesis, high-efficiency electricity production with minimized losses would be possible.
Based on this way of thinking, to absorb all photons with different energies at various wavelengths from the sun and generate multiple electrons, solar cell design optimized for each photon is necessary. Using a baseball game batter as an example, it’s like having to immediately hit 10 home runs adapted to each momentum when a pitcher throws 10 balls with different momenta.
One method to utilize all photons with different energies is to consider infinite pn junctions. If infinite junctions are used, theoretically all photoelectrons can be utilized. According to previously published papers, theoretically up to 80% efficiency is possible with such methods. However, stacking pn junctions infinitely is realistically impossible. To implement this in reality, there could be methods using 2D materials like graphene. That is, if 2D materials are stacked in numerous layers and spatially well controlled, photoelectrons can be confined within a few nanometers three-dimensionally like quantum dots and all energy can be utilized. This method discovers and utilizes new physical phenomena or interactions by effectively controlling space without changing the material properties themselves.
The next method to consider is spatiotemporally separating and utilizing photons with different energy ranges. For this, photons falling randomly like raindrops must be quickly separated hierarchically. In addition to this, methods to minimize losses by incorporating physical phenomena from a quantum perspective must be found. This means that optimal currents must be created based on state information of electrons newly created by light, and the role of photons in having optimal states must also be understood. From a materials perspective, metamaterials can be considered for photon separation. Metamaterials can act like lenses, so photons are expected to be easily separated hierarchically in multiple dimensions such as time, space, and energy dimensions.
Another technical alternative is controlling electron states at cryogenic or liquid nitrogen level temperatures. At very low temperatures, excited states of electrons last longer and can be used to excite to higher energies. Furthermore, at cryogenic temperatures, energy losses due to crystal lattice vibrations decrease, so efficiency increases can be expected.
Additionally, upconversion technology that converts the energy of photons with 800 nanometer wavelength to the energy of photons with 400 nanometer wavelength can be an important alternative. Using this, multi exciton generation technology (MEG) that can create two electrons with one photon becomes possible. However, this has limitations due to energy conservation laws, so deeper consideration appears necessary.
Additionally, methods to increase efficiency by using thermal energy generated in solar cells can be considered. Typical solar cell substrates are black, absorbing light well while converting part of the absorbed light to thermal energy according to entropy laws. If light energy converted to thermal energy could be used to create electrical energy, higher efficiency would be possible. If technologies currently being researched that convert thermal energy to electrical energy and materials that create electron flow using heat based on thermoelectric phenomena could be utilized, higher efficiency can be expected.
In conclusion, to create breakthrough solar cell technology with 60% conversion efficiency, challenging research must be conducted from various perspectives.