Over the past two decades, the cost of solar cells, structures that can convert light energy into electricity, has steadily declined. The National Renewable Energy Laboratory, a U.S. government laboratory that studies solar cell technology, estimates the factors contributing to the increasing affordability of solar cells. They estimate that the hard costs, which are the costs of the physical solar cell hardware, and the soft costs, which include the associated labour force or the cost of obtaining necessary government permits, are about the same. With more potential consumers and more professionals installing new solar cells, soft costs have fallen, allowing companies to mass produce solar cells and install them easily. Hard costs are less than half what they were in 2000, mainly due to lower material costs and increased light-trapping ability of cells. In addition to innovative design, the development of cheaper and more efficient solar cells required careful consideration of the physics of solar radiation.

Solar Cells: Costs, Challenges, and Design

Solar cells are used to convert light into electricity, so they must be made of materials that can efficiently capture energy from light. This material can be sandwiched between two metal plates and can carry electricity generated from light energy to wherever it is needed, such as home lighting or factory machinery. Choosing the right material to capture light requires measuring the difference between two energy levels called the valence and conduction bands. The low-energy valence band is filled with many small negatively charged particles called electrons, while the high-energy conduction band is mostly empty. When an electron collides with a light particle called a photon, it can absorb enough energy to jump from the low-energy conduction band to the high-energy valence band. Once in the valence band, the electron's excess energy is harvested as electricity. It's as if an electron is sitting at the bottom of a hill (the conduction band) and hits a photon (the valence band) that gives the electron jumping energy.

The amount of energy required for an electron to jump into the valence band depends on the type of material. Basically, the size of the figurative mound depends on the properties of the particular material. The size of this energy gap is important because it affects the efficiency with which a solar cell converts light into electricity. In particular, if a photon hits an electron with less energy than the energy required for the electron to jump from the valence band to the conduction band, no light energy is captured. Alternatively, if the light has more energy than is needed to fill this gap, the electrons will absorb exactly the energy they need and waste the rest.

Previously, silicon was the most common material for solar cells. One reason for this popularity is the large gap between the conduction and valence bands of silicon. This is because the energy of most photons is very close to the energy required for electrons in silicon to jump over the energy gap.

In addition to falling material costs, clever technical tricks bring silicon solar cell efficiencies closer to their theoretical maximums. In order for a photon to be converted to energy, it must first collide with an electron. One trick to increasing the likelihood of photon/electron collisions is to pattern the silicon of solar cells into tiny pyramidal shapes. As light is absorbed by the pyramids, it travels further and has a greater chance of colliding with electrons in the silicon before it leaves the cell.

electrons in the silicon before it leaves the cell

In a similar tactic, chemists and materials scientists have developed anti-reflection coatings applied to the front surface of solar cells to prevent useful light from reflecting into space without impinging on the solar cell's electrons. Similarly, more light can be collected by attaching a reflector to the back of the solar cell. The light that reaches the solar cell and is transmitted to the back without hitting electrons is reflected back to the front of the cell, giving the cell another chance to collect light.

Currently, the cost of silicon-based solar cells continues to fall, and despite predictions to the contrary, the cost of silicon itself continues to fall. Silicon solar cells may continue to be popular in the future. Alternatives to silicon solar cells are being developed, but not sufficiently developed to be commercially viable.