It should come as no surprise that the demand for renewable energy is on the rise, along with the cost of fossil fuels, global temperature, and sea levels. Sunlight is a form of renewable energy which is abundant and universally available and thus there is growing interest in its uses for energy applications. Nowadays, everyone knows what a solar panel is, indeed, you may be reading this now below one on the roof of your home or office. Photovoltaic (PV) solar energy offers an economically viable source of energy, paving the way to a sustainable, environmentally friendly, and decarbonised world. To fully utilise solar power, it is imperative to understand the operation of solar cells and the factors which affect their efficiency. At the heart of the solar cell lies a layer of semiconducting material, such as silicon, gallium arsenide (GaAs) or polymer, and is undoubtedly the most important part of the solar cell. With a unique combination of the properties of conductors and insulators, these materials are especially adept at converting sunlight to electricity.
How do Solar Cells Work?
Solar cells are electronic devices, also known as PV cells, which are made of two types of semiconductors (an n-type and a p-type) and convert light into electricity in a process known as the photovoltaic effect. The n- and p-type layers are created by a process called ‘doping’ which changes the electrical and structural properties of the semiconductor through the intentional addition of impurities. Without getting into the specifics of doping, the n-type layer has an overall negative charge due to the addition of electrons and the p-type has an overall positive charge due to the removal of electrons and thus the formation of electron vacancies, referred to as holes.
When these two types of semiconductors are in contact, some of the electrons close to the edge of the n-type semiconductor fill the holes of the p-type semiconductor across what is known as the depletion zone, until equilibrium is reached. Now, the edge of the p-type side contains negatively charged ions and the adjacent edge of the n–type side contains positively charged holes. This causes the formation of an internal electric field.
Light incident on the depletion zone is absorbed as energy. This energy from the photons of light will allow the electrons (that carry charge within the semiconductor) to essentially ‘break free’ from their previous bound state and, due to the electric field that is set up across the depletion zone, creates a flow of electrical current. An array of solar cells can convert solar energy into direct current (DC) electricity, or an inverter can convert the power to an alternating current (AC).
The diagram below shows the operation of the solar cell with a silicon semiconductor.
Types of Solar Cells
When it comes to materials, we must first understand what characteristics are desired for optimum performance. Some cells are designed to handle sunlight that reaches the Earth’s surface, while others are optimized for use in space. Solar cells can be made of only one single layer of light-absorbing material or use multiple physical configurations to take advantage of various absorption and charge separation mechanisms.
Some widely used semiconductors include monocrystalline silicon (mono c-Si), multicrystalline silicon (poly-Si), gallium arsenide multijunction (GaAs) and polymer solar cells (PSCs).
Monocrystalline silicon is the common choice for high-performance PV devices. Since there are less stringent demands on structural imperfections compared to microelectronics applications, a lower-quality solar-grade silicon is often preferred. Mono c-Si is a photovoltaic, light absorbing material in the manufacture of solar cells. It has a crystal lattice of which the entire solid is continuous, unbroken to its edges, and free of any grain boundaries. Mono c-Si can be prepared intrinsic, consisting only of exceedingly pure silicon, or doped, containing very small quantities of other elements added to change its semiconducting properties. This variety of silicone has been described as the most important technological material of the last few decades in what has become known as “the silicone era”. This is a result of its availability at an affordable cost has been essential for the development of the electronic devices on which the present day electronic and informatic revolution is based. Mono c-Si differs from poly-Si, that consists of small crystals, also known as crystallites.
Poly-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry. They are the most common type of solar cells in the fast-growing PV market. However, the material quality of Poly-Si is lower than that of mono c-Si due to the presence of grain boundaries. Grain boundaries introduce high localized regions of recombination due to the introduction of extra defect energy levels into the band gap. This reduces the carrier lifetime from the material. In addition, grain boundaries reduce solar cell performance by blocking carrier flows and impacting current flow. Both materials are selected for solar cells as silicon is an abundant and durable element. However, upon comparison the mono c-Si solar panels are generally thought of as a premium solar product. The main advantages of mono c-Si panels are higher efficiencies. As the cell is composed of a single crystal, the electrons that generate the flow of electricity have more room to manoeuvre. As a result, mono c-Si panels are more efficient than their poly-Si counterparts, whose main selling point is the lower price point.
Gallium arsenide is a semiconductor with a greater saturated electron velocity and electron mobility than that of silicon. It also has the useful characteristic of a direct band gap this means it is a compound which can emit light efficiently. GaAs is a compound of the element’s gallium and arsenic. It is a crystal structure. GaAs thin-film solar cells have reached nearly 30% efficiency in lab environments, but they are very expensive to make. Cost has been a major factor in limiting the market for GaAs solar cells with their main use being for spacecraft and satellites.
Polymer solar cells have for a long time not been comparable to the traditional solar cells mentioned above on both performance and stability. The fact that PSC’s can behave as semiconductors is a discovery which Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa received the Nobel Prize in Chemistry for in the year 2000. In terms of the application of solar cells the emerging dye-sensitized solar cells, perovskite solar cells, and organic solar cells have all been regarded as being full of promise. This is a due to the diverse properties of polymer which means it can be used to adjust the device components and structures. In dye-sensitised solar cells, they can be used as flexible substrates, for perovskite solar cells, polymers can be used as the additives to adjust the nucleation and crystallization processes whilst in organic solar cells polymers can often be used as donor layers or buffer layers.
Efficiency of Solar Cells
Solar cell efficiency is the amount of energy received from the sun that is converted into usable energy. This is often the most important parameter of a solar cell and the most difficult to improve.
Most solar cells utilize a single p-n junction, as described above, to convert solar energy into electrical energy. This occurs when a photon of energy Eph is incident on the solar cell. If the energy Eph is greater than the energy required to excite an electron, that is the band gap Eg, an electron-hole pair is created. Energy losses arise from temperature increases resulting due to different mechanics:
- If Eph is less than Eg then the photon does not have enough energy to ionise the electron and thus Eph is just converted to heat.
- If Eph is a bit bigger than Eg the electron-hole pair created will move off with kinetic energy KE = (Eph-Eg). This fast-moving pair will resist moving through the circuit until their kinetic energy is dissipated via phonons into the solid and thus are slowed down, generating heat.
- The electrons and holes annihilate to produce photon in a process called recombination. This photon has a probability of being reabsorbed by the atoms in the solar cell giving rise to heat.
The efficiency of an infinite stack of solar cells is bounded above by the efficiency of its equivalent Carnot engine. The Carnot engine efficiency is given by 1 minus the ratio of TC to TH, where TC and TH are the absolute temperatures of the of the working system (solar cell) and the system supplying the energy respectively. For a solar cell, if we take to be the ambient temperature of Earth and to be the surface temperature of the sun then the maximum theoretical efficiency of solar cell is about 95%. This value however assumes an infinite stack of cells, no reflectance and that the stack doesn’t emit radiation. Solar cells will never reach such efficiencies in reality. Assuming the infinite stack of solar cells is instead exposed to blackbody radiation of 6000K from all directions the efficiency drops to 86.8%. If we were to assume that all the radiation comes only from the sun the efficiency is further reduced to 68.7%. These processes increase the temperature of the solar cell above that of the surrounding atmosphere. Since the solar cell and the atmosphere are in
thermal contact the solar cell must lose heat to the atmosphere to establish thermal equilibrium. The efficiency of solar cells is very temperature dependent. One may design a single p-n junction photovoltaic in a way that the band gap is such that the energy loss is minimised. William Schockley and Hans-Joachim Queisser determined the optimum band gap energy for sun light to be 1.34eV. This results in an efficiency of 33.7%, which is also the maximum efficiency for single p-n junction photovoltaics and is known as the Ultimate Efficiency or the Shockley–Queisser limit. For a multiple p-n junction solar cell the maximum energy can be much higher (up to 44%). This is done by choosing semiconducting materials, such as polymer, with different band gaps as to harness the solar spectrum more efficiently.
Quantum Efficiency refers to the percentage of photons that give rise to electric current. A photon (with energy greater than the band gap) incident on the surface of the semiconductor has a certain probability of creating an electron-hole pair. This probability depends on many different aspects: some photons are reflected from the surface of the solar cell, some energise the nucleus of the atom in the solar cell lattice instead of the valence electrons, some electron hole pair created recombine straight away resulting in no net increase of current, etc. Many of these depend on the photon’s wavelength/frequency and thus the quantum efficiency is usually a function of either.
Methods to minimise losses include texturizing the incident surface of solar cells to reduce reflection and the back side of the solar cell can having a mirror finish to increase the distance some photons will travel in the solar cell thus increasing the probability of electron pair production.
Taking information from various sources, an average value of efficiency can be deduced for various types of solar cells. Crystalline silicon PV cells are the most common solar cells used in commercially available solar panels, representing 87 % of world PV cell market sales in 2011. These, however, achieve efficiencies ranging from 18-24%. By far the greatest efficiencies have been attained by GaAs multijunction cells, with a record of 43.5%. Polymer solar cells currently have the lowest efficiencies of only 8.7% but are only in the early stages of research and development.