When light hits a solar cell, current is generated. How does this happen? In simple words, the sun’s energy is converted into electrical energy. The science behind this conversion process helps us understand the principles of a solar cell. This will further guide us in understanding the factors that can affect the performance of a photovoltaic system. These factors can even be at the atomic level. Thus, we need to look into the atoms of solar cells to have a thorough understanding of how they generate current.
Solar cells are made of semiconductor materials such as silicon. We need to understand how charge is generated in semiconductor materials. Silicon is the most widely-used material for making solar cells. So let us use silicon’s example to help us understand better. A silicon atom has four electrons in its outermost shell (valence shell). These outermost electrons are called valence electrons and they help an atom in bonding with other atoms. When silicon atoms bond with each other, they form a covalent bond. This means that the bond is formed by the sharing of valence electrons between them. So a piece or a strip of silicon is just multiple silicon atoms bonded in a covalent bond. Silicon atoms covalently bond to form uniform structures or ‘lattices’. The material properties of a semiconductor (such as boiling point and conductivity) depend on the way the electrons are bonded in these lattices.
The electrons in a covalent bond are not free to move or gain energy. This means that electrons cannot participate in current flow as they are somewhat stationary. However, this is only true for temperatures equal or close to absolute zero. Hence, at low temperatures, semiconductors act like insulators. At higher temperatures, especially at the temperatures at which solar panels function, electrons can easily break themselves out of the bond. So to summarise, the conductivity of a semiconductor increases with the temperature. What happens when the electrons break themselves out of a bond? When the electrons are in a bond, they are in the valence band of energy. When these electrons gain energy, they move to the conduction band. There is a certain minimum energy that electrons require to move from the valence band to the conduction band. This is the called the Band Gap of a semiconductor. When an electron breaks out of its bond in the valence band, it leaves behind a positively-charged empty space. This empty space is called a ‘hole’. This hole’s positive charge then attracts the next electron in the lattice to move towards the hole. This neutralises the hole but a second hole is created in the next electron’s space. This is called an ‘electron-hole pair’. The cycle continues and this is how charge is carried around in a semiconductor in electron-hole pairs. But how does sunlight gets utilized to generate electron-hole pairs in a semiconductor?
In this article, we’ll be explaining 3 main functions that take place within a solar cell and how they’re significant in its operation. We’ll first be giving you an overview of how sunlight gets absorbed into the solar cell including all parameters that are necessary for light absorption such as absorption coefficient and absorption depth. Then we’ll be diving into how charge carriers are generated and their transport mechanism within the cell using diffusion and drift to generate current, and finally, we will conclude with a description of a process called recombination that affects the efficiency of the solar cell.
Absorption of Light
When sunlight which is comprised of photons, interact with the material, the energy of photons is a key parameter to determine the absorption of light. When the energy of photons is greater than or equal to the bandgap energy of the material, the photon absorbs into the material and excites an electron to the conduction band of the material thereby leading to the operation of absorption of light. There are 3 mechanisms which are dependent on the absorption of light:
•Eph < EG: When the energy of photons (Eph) is lesser than the bandgap energy of the material (Eph), the interaction is very weak and the photons pass through the material without getting absorbed.
•Eph = EG: When the energy of the photon matches with the energy of the material bandgap which results in absorption of photons, electrons and holes are generated and electrons get excited into the conduction band.
•Eph > EG: When the energy of photons is much higher than the bandgap energy of the material, the energy generated is wasted because the electrons that were excited promptly thermalize back to the edges of the conduction band [1, 2].
The semiconductor material acts as a PN junction and facilitates the movement of electrons and holes to form electron-hole pairs from the photon energy.
Apart from the energy of the photons, the absorption coefficient and absorption depth of the material also plays a role in absorption.
1] Absorption coefficient: It’s a property of the material which determines how far light of a particular wavelength can infiltrate into a material before it’s absorbed by the material. Different materials have different values of absorption coefficients and knowing the value of a material’s absorption coefficient is essential in determining the materials to be used for solar cell applications.
Materials which have low absorption coefficients will poorly absorb light as compared to materials with high absorption coefficients. The absorption coefficient not only depends on the material but also the wavelength of incoming light.
The absorption coefficient formula is given below:
Absorption coefficient unit=cm-1.
From the equation, it’s evident that the absorption coefficient (α) is proportional to the extinction coefficient (k) and inversely proportional to the wavelength (λ) of incoming light.
The absorption coefficient of several semiconductor materials having light absorbed at 300K is shown below [3, 4]:
2] Absorption Depth: It’s the inverse of the absorption coefficient given by α-1. This describes how deep into the material the light penetrates before being absorbed. Light which has higher energy will have a shorter wavelength and correspondingly shorter absorption depth. Similarly, light which has lower energy will have longer wavelengths and greater absorption depths. The absorption depth enables us to understand the design parameters of a solar cell such as the semiconductor thickness because it gives the distance at which light drops to around 36% of its original intensity. This is why blue light which has higher energy is absorbed within a short distance and has short absorption depth as compared to the red light which has lower energy and larger absorption depth.
Now that you’ve understood how light is absorbed into a solar cell, we can move onto the next operation that deals with generation of charge carriers and their transport in a solar cell.
The number of electron-hole pairs generated due to the absorption of photons in a semiconductor is given by the Generation Rate (G). The generation rate is the highest at the surface of the material, where most of the light is absorbed. The intensity of light at any point in the semiconductor can be calculated using the following equation:
Here, IO is the light intensity at the top surface, α is the absorption coefficient and x is the distance into the material at which the light intensity is being calculated. This equation can then be used to calculate the generation rate. If it is assumed that the loss in light intensity directly causes the generation of an electron-hole pair, then the generation rate G in a thin slice of the semiconductor material can be determined by finding the change in light intensity across it. Therefore, G is given by:
Here, NO is the photon flux at the top surface, α is the absorption coefficient and x is the distance into the material. The light incident on solar cells contains many different wavelengths when we consider PV applications. Therefore, different generation rates must be taken into account for the different wavelengths when designing a solar cell. The generation rate for a combination of different wavelengths is the sum of the generation rates at each wavelength of light. Now let us consider how the charge is transported in semiconductor materials [5, 6].
Electrons in the conduction band and holes in the valence band are called ‘free carriers’. This is because they move about freely across the semiconductor’s lattice. Simply put, free carriers in semiconductors move in a random direction at a specific velocity. Since the motion of these carriers is random, they are bound to collide. Scattering lengthis the distance a carrier moves in a random direction before it collides with an atom in the lattice. Upon collision, the carrier deflects and then moves in another random direction. The velocity at which the carrier moves depends on the temperature of the lattice. The average velocity of the carrier is the thermal velocity. It is normally distributed in the lattice hence, making the velocity of some carriers lower than that of the others. The random motion of the carriers makes their movement possible in every direction. So the movement of a carrier in one direction will be negated by its movement in the opposite direction. Therefore, the net outcome of the movement of carriers is zero unless there is an electric field or a concentration gradient .
We shall now consider the cases where there is a net outcome of the movement of carriers termed as diffusion and drift which are detailed below:
Carriers are generated at the surface of the solar cell when light is incident on the cell. However, carriers are not generated in the bulk of the solar cell. This leads to a concentration gradient and through random scattering, a uniform distribution of carriers takes place through a net movement of carriers from high concentration regions to low concentration regions known as diffusion. In time, the random motion of the carrier will even out the concentration gradient that’s induced due to generation and even recombination. The rate at which diffusion takes place depends on the velocity of the carriers and the distance between the scattering events termed as diffusivity. Higher the temperature, greater is the thermal velocity of the carriers and faster the diffusion .
It’s the movement of carriers due to the presence of an electric field. In most cases, electrons move in the direction opposite to that of the electric field. Holes move in the direction of that of the electric field. These carriers will move around randomly when an electric field is absent. Once an electric field is applied, electrons and holes move or ‘drift’ in opposite directions. The net movement of carriers in a semiconductor due to the application of an electric field is characterized by ‘mobility’ which varies in different semiconductor materials [9, 10].
Finally, we come to our last modus operandi which plays a significant role in the efficiency of solar cells. This process can negatively affect a solar cell if not controlled and is called recombination.
Recombination is the opposite of generation. In recombination, an electron recombines with a hole and gives up the energy to produce either light or heat. A Light Emitting Diode (LED) works on the concept of recombination. In LEDs, recombination is optimized to emit light. There are three types of recombination processes. These are Radiative, Shockley-Read-Hall, and Auger. In Radiative Recombination, an electron from the conduction band combines directly with a hole from the valence band. In this process, there is a photon released. The energy of this released photon is close to that of the band gap. Thus, it is weakly absorbed in the semiconductor and thus, leaves it in the form of heat or light. The second type of recombination is Shockley-Read-Hall (SRH) Recombination. It does not occur in perfectly pure or un-defected materials. In SRH recombination, an electron or a hole is trapped by an energy band in the ‘forbidden gap’ which is introduced through defects in the crystal lattice. These defects can either be introduced unintentionally or deliberately added to the material through doping. Then, if a hole or an electron moves to the same energy state before the electron is re-emitted thermally into the conduction band, it recombines. In Auger Recombination, there are three carriers. An electron and a hole recombine. Instead of releasing light or heat, the energy produced is given to a third carrier, which is an electron in the conduction band. This third carrier electron then thermalizes back to the edge of the conduction band. This type of recombination is most important at high carrier concentrations, caused by heavy levels of doping or high level injection under concentrated sunlight. But how is recombination linked with solar cells?
Recombination is an important concept in photovoltaics. Recombination rate is the rate at which recombination occurs and is a critical parameter in solar cells. This process depends on the number of excess minority carriers in a semiconductor. For instance, if there are no excess minority carriers, then the recombination rate is zero. An important parameter of recombination rate is the minority carrier lifetime of a material. This is the average time which a carrier can spend in an excited state before it recombines, after electron-hole generation. It is called the lifetime of a material. So, if a material has a long lifetime, it means that the minority carriers generated will sustain in the material longer before they can recombine. Although it depends on the structure, solar cells made of wafers of materials with long lifetimes tend to be more efficient. Another important parameter of recombination rate is the minority carrier diffusion length. This refers to the average length a carrier moves between generation and recombination. The minority carrier lifetime and the diffusion length depend a lot on the type and magnitude of recombination processes in the semiconductor. The method used to fabricate and process the semiconductor wafer also has a major impact on the diffusion length. The diffusion length is related to the carrier lifetime by diffusivity according to the following formula:
Here, L is the diffusion length, D is the diffusivity and τ is the lifetime in seconds. Recombination is also promoted by defects or impurities within or at the surface of the semiconductor. The top surface of a solar cell is highly prone to recombination. This is because the surface of a solar cell represents high disruption of the crystal lattice. The high recombination rate around the surface depletes this region of minority carriers. The surface recombination rate depends on the minority carriers moving towards the surface. This is represented by surface recombination velocity.In a surface where there is zero recombination, the movement of carriers towards the surface is zero. Therefore, the surface recombination velocity is zero. Where the recombination is infinitely fast, the movement of carriers towards this surface is limited by the maximum velocity they can attain. The lifetime of a material is subject to the concentration of minority carriers. Limiting surface recombination can reduce the rate at which minority carriers are depleted. If the rate of minority carrier depletion can be limited, the lifetime of the material can be extended. Hence, limiting surface recombination leads to longer cell lifetimes. This is why recombination is an important concept that can be used to make solar cells efficient [11-15].
As we come to the end of this article, we have seen how light absorption generates charge carriers and their implication. We have seen how these charge carriers move within a solar cell to generate the current that we need. However, just like how every rose has its thorn, we have also seen how recombination can impact efficiency in our quest to improve the conductivity of solar cells. Keeping in mind the parameters that we have discussed, it is important to consider that this technology is being improved every day. Performance analyses are being conducted constantly to increase the efficiency of solar technology while decreasing its cost. In a few years, solar panels will be competent in the commercial market and we may all soon be witness to a surge in a one-of-a-kind solution to clean sources of energy!