In our most recent blog post, we focused on explaining the properties of silicon and why silicon is the most commonly used material for photovoltaic cells. However, silicon is only one of millions of materials that can be used. In fact, Harvard recently ranked approximately2.3 million possible replacements, and the list continues to grow as we speak. As you can see, solar cells come in all different materials, shapes, and sizes.
Contemporary solar panels exhibit a relatively low average efficiency of 15-25%. Modern research in the area of photovoltaic cells is driven by the desire to increase efficiency. Solar cell efficiency is generally defined as the percent of solar energy that is converted into useful electrical energy. Here, we will discuss other materials that can exhibit the photovoltaic effect in relation to the factors that affect the efficiency of these photovoltaic cells. There are three basic aspects that influence the performance of different materials: structure, absorption, and bandgap.
The crystallinity of a particular material corresponds to how orderly the atoms are packed in the crystal structure. There are several structures that semiconductor materials can come in including single-crystalline, multicrystalline, polycrystalline, and amorphous. At one end of the spectrum, you have single-crystalline materials that are packed in a very orderly manner with a regular repeating pattern from layer to layer. On the other end, you have a material that is composed of several smaller crystals causing an irregular pattern due to disrupted arrangements between atoms.
Let’s take a look at Copper Indium Gallium Selenide Solar Cells (CIGS) as an example to further understand the role that structure plays in solar cell efficiency. CIGS is a tetrahedrally bonded semiconductor and has a polycrystalline chalcopyrite structure. The uniformity of its structure and its stacked multilayer structure produces a smoother absorbing surface which results in higher efficiency.
Part of the reason solar panels have low efficiency is that the particles collected from the sun enter the solar cell and spread out in all directions. Getting them all to flow one direction typically requires layers of different channeling material. Each time the particles pass between these layers some get lost, decreasing the energy efficiency of the solar cell. This brings us to absorption.
The absorption coefficient of a particular material corresponds to how far a photon of a specific energy can penetrate the material before it is fully absorbed. The higher the absorption coefficient, the more readily a material absorbs light, and hence, the higher the solar cell’s efficiency. This property is influenced by two factors: the energy of the photon and the material of the cell. Since photovoltaic cells are made of semiconductors, there is an abrupt jump in absorptivity. Photons with energies below the bandgap will have low absorptivity, but as soon as you surpass the bandgap, the absorptivity jumps up. In essence, a higher absorption constant correlates with increased efficiency because more of the light is available to be converted into electrical energy. Structure, as discussed previously, is essential in determining absorptivity since geometry can affect factors such as the angular variability of the light absorbed.
In addition to increasing absorptivity of light through structure, researchers have also developed a thermovoltaic cell(TPV) made of nickel photonic crystals in order to reduce the energy losses associated with the absorption of light in a conventional cell. TPVs convert energy from the photons into thermal energy as well by heating up a receiver and transferring this energy to another absorber that matches the cell’s absorption characteristics. Conventional TPV designs include a filter between the emitter and the cell to reflect back the photons with energy lower than the cell bandgap. This way, their energy is recycled to maintain the temperature of the intermediate. TPVs are becoming increasingly popular due to their maximal usage of the solar energy.
Last, but certainly not least, we have bandgap. Bandgap refers to the amount of energy needed to excite an electron of a particular semiconductor material from its ground state to a free state where the electron can be involved in conduction. Basically, bandgap is the difference between the valence band (the lower energy level) and the conduction band (the higher energy level) where the electron is free to roam. Temperature can also influence bandgap width. A higher temperature increases the energy of the electrons in the material by adding kinetic energy. Therefore, a lower amount of energy is needed to excite the electron from the material. A lower bond energy results in a lower bandgap width. The exact mechanics behind this can be understood with lots of crazy looking equations and a good amount of calculus (which you can see here) but for our purposes, it is only important to understand that a lower bandgap corresponds to a lower solar cell efficiency.
A great example of a solar cell with an ideal bandgap is the Gallium Arsenide orGaAs cell (unit cell shown to the left). The GaAs band gap is 1.43 eV, nearly ideal for single-junction solar cells. Unlike traditional cells, GaAs cells are relatively insensitive to heat. Gallium arsenide also has a very high absorptivity and it only requires a cell of a few microns thick to absorb sunlight. (Crystalline silicon requires a layer 100 microns or more in thickness.) These properties make GaAs especially desirable for applications in extreme climates such as space.
There are several factors that influence the efficiency of solar cells. Professor Bermel of Purdue University (some of his interesting photovoltaic research can be seen here) responded to our question of which factor was the most important by saying “Both optical and electronic structure is absolutely essential. A poorly designed structure in either category can result in very low performance.”Clearly, an efficient solar cell requires a good balance of the three major factors we discussed in this post: structure, absorption, and bandgap.
The question of an ideal solar cell remains. When we asked Professor Bermel what the properties of the ideal material for solar cells would be, he gave us seven musts:
1. Bandgap well-matched to the solar spectrum
2. Strong optical absorption for energies above the bandgap
3. High electronic mobility
4. High minority charge carrier lifetime
5. Durable, reliable (e.g., stable under UV light)
6. Inexpensive, and made from widely abundant elements
7. Eco-friendly and non-toxic
Researchers continue to search for an ideal solar cell material that can satisfy these criteria in order to create a future where solar panels can become the mainstream source of energy.