Solar Cells

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Quantum Efficiency

In the previous posts, we talked about the working of a solar cell and the several parameters that can be used to describe its efficiency. Now, we talk about one more of those parameters called as the quantum efficiency.  

Quantum efficiency -- or to be more precise, the external quantum efficiency is the ratio of the charge carriers collected by the solar to the amount of photons that strike it at any given time. This can be expressed both in terms of energy and wavelength. In order to understand quantum efficiency, let us at first look at two ideal situations. First, the entire wavelength of photons are absorbed by the solar cell, resulting in collection of all the charge carriers created. Or, no photon is absorbed because their energy is lesser than the bandgap. The quantum efficiency vs the wavelength graph looks like the figure  below, retrieved from here.

If you notice in the graph above, the real quantum efficiency varies from the idealized situations. This is because the efficiency depends a lot on the recombination [link] effects. This means, that the same factors that affect the collection probability also affect the quantum efficiency. For example, if carrier recombination on the front surface passivation is high, the blue light that is absorbed in the front surface will be affected and the quantum efficiency at that wavelength is low. Similarly, if a carrier is generated in the bulk, then low diffusion length [link] affects the green portion of the quantum efficiency. The mathematical way to view quantum efficiency is to think of it as an integral of the collection probability over the device thickness, normalized by the incident number of protons.  

The quantum efficiency mentioned here is the external quantum efficiency and it includes the effect of optical losses such as transmission and reflection. Now, we will talk about internal quantum efficiency that only deals with the light left after reflected and transmitted light have been lost.  

The internal quantum efficiency of a solar cell refers to the efficiency with which the electrons that are not reflected or transmitted, out of the cell can generate collectable carriers. This can be calculated by correcting the external quantum efficiency curve after measuring the reflected and transmitted light from the device.  

The quantum efficiency is dependent on the same things as collection probability as it is essentially just a ratio of the sum of collection probability over the entire device divided by the number of protons. Diffusion length is defined as the average distance the carrier can cover within a material before they recombine. If the emitter diffusion length is low, this means that the carriers generated at the surface of the emitter will not be collected, making the quantum efficiency zero. Similarly, front surface recombination causes a loss in the quantum efficiency. If the base diffusion length is larger than the actual size of the base, then the rear surface recombination velocity has a large effect on it. These are shown in the graph below, which is retrieved from here. That link has a wonderful tool that you can use to scroll the various parameters and see its effects on the quantum efficiency. I have taken three such examples and shown them below, with the value of the parameters. 

So far, we have learnt a lot about functioning of solar cells, some of the factors that can affects its efficiency, and ways to quantify it. There are still more things to consider, but I feel like we know enough to start digging into the current research that is happening in this field. Of course, as new concepts are required to be understood for analyzing a paper, I will try to post about that before posting about the paper.  

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Light Generated Current

As mentioned in the previous posts, the light generated current is the current generated due to the creation of electron hole pairs due to the light. This will depend on two factors, one of them is the collection probability, and the other is the generation rate. In the last post, we talked about collection probability. We will talk about generation rate in this one and talk of the spectral dependence of the current.

The generation rate is the rate at which light can create charge carriers in the semiconductor material. This is usually the highest at the surface and reduces as you go further into the semiconductor as shown in the figure below.

The light generated current depends on the generation rate and the collection probability. Therefore, for any given point in the semiconductor, the generation current is the integration of collection probability and the generation rate as given by the equation below. The variation of both with as we go deeper into the cell is given below. 

The non-uniform nature of the collection probability gives rise to interesting consequences. As we know, light is electro-magnetic radiation made of multiple components. Each of them has a different wavelength and energy. Because of this, different wavelengths are absorbed at different depths inside the solar cell. For example, blue light is absorbed within the first few tenths of a micrometer inside silicon. However, the collection probability there is low. This means that blue light does not generate any current inside a silicon solar cell, and it causes a loss in efficiency. Variation of generation rate with the cell depth for red and blue light is given below. 

In the next post, we will talk about efficiency of a solar cell and the ways to quantify it.

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Collection Probability

In the last few posts, we talked about the mode of operation of a solar cell. Now, it is necessary to understand the factors that affect the efficiency of the solar cell and the parameters that affect it. First of these parameters is the collection probability. In this post, I will explain about it.  

What will make a solar cell efficient? The simplest answer would be that it should create as much current as possible. In order for that to happen, as much as the light generated carriers need to complete their circuit and recombine. There are several steps necessary for this to happen. First, the solar cell has to absorb most of the light that falls on it. Next, the light should be energetic enough to create electron-hole pairs. The internal electric field of the PN junction has to influence these electron hole pairs and complete the circuit, without recombining with each other, even though that is the more energy efficient process. Collection probability is a parameter that quantifies the last aspect that I mentioned.  

The collection probability is the probability that the PN junction will collect the generated electron hole pair and contribute them to the light generated current. This probability will obviously depend on the distance the generated carriers need to travel compared to their diffusion length. It will also depend on the surface properties of the semiconductors.  

The collection probability of carriers generated in the depletion region is one because the electric field sweeps apart the generated carriers very easily. As we move away from the junction, the probability drops and if we move more than one diffusion length away from the junction, the probability is really low. Similarly, if a carrier generated closer to a region of the surface with higher recombination, then the carrier will recombine, rather than contributing to light generated current.  

Therefore, the collection probability is dependent on the following:   1. Emitter Diffusion Length: The diffusion length is the amount of distance the generated carrier has to diffuse through to reach the PN junction. Therefore, if the light generates a carrier away from the diffusion length, the collection probability is lesser, and it shoots up when the generated carrier is within the diffusion length.

2. Surface Recombination Velocity: The semiconductor’s surface is prone to assist in the recombination of charge carriers. I will talk more about this in a future post. However, this recombination will reduce the collection probability at the front surface.

The above-mentioned effects are the same for Base diffusion lengths and recombination velocities. I have illustrated the effects of the parameters in the figure below.  

In the next post, we will use the concept of collection probability and explain the light generated current quantitatively.  

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Structure of a Solar Cell

In the previous post, I had talked of the basic working principle of a solar cell and mentioned that there was more to the structure of a solar cell than just the PN junction and space charge region. In this post, I will delve further into the structural aspect of a solar cell as represented by the figure below.

 The emitter is the N side of the solar cell, and in the case of the figure above, is the side exposed to the atmosphere. In order to ensure that the carriers generated in the emitter move towards the PN junction, the N side's thickness is usually very low. Base is the P side of the solar cell and it is usually bigger than the N side in order to allow more of the holes that come through the PN junction to recombine with the electrons in the external circuit.

This is the semiconductor part of the PN junction but generally, the resistivity part of the semiconductors makes it difficult for conduction of the current through the load. Therefore, both the front and back of the solar cells have metal contacts to connect the solar cell to the external load. However, the presence of metal contact on the top also hinders the electricity projection as it shades the solar cell from the incoming light. Therefore, it is always a compromise between the space occupied by the metal contact and the efficiency of the solar cell.

Apart from the metal contacts, and the PN junction, solar cells also have an anti-reflection coating on the top, which is to make sure most of the light refracts into the material for electricity generation to occur. The charge separation generates a high voltage in the solar cell. The load and parasitic resistances dissipate the voltage as power loss.

In the next posts, we will talk more about the efficiency of the solar cell, the factors which affect it, and the method to calculate it.

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Solar Cell Operation

In the last few posts, we talked about PN junctions and the way in which a current can flow through them. We also spoke of two situations; one is a forward bias, and another being the reverse bias, and talked about their effect on the PN junction. I think all the posts covered in the 'electron movement' subsection of the 'fundamentals section' are enough to start understanding the working of a solar cell.

The figure below represents a solar cell, and if you notice, it is not a simple PN junction connected to a load. It has many more components which I will try to explain in the posts in this section. At first, let us understand the working principle of a solar cell.

Light falls on the solar cell, exciting the electrons in the material to create electron hole pairs. As I mentioned earlier, electron hole pairs have the tendency to recombine with each other in normal cases. However, our solar cell has a PN junction, which will have an internal electric field that will split this ‘electron and hole’ pair.

The figure below explains each step of the process. First, the light falling on the N side of the semi-conductor creates an electron hole pair. Next, the internal electric field splits up this pair, and the electron travels through the external circuit while the hole travels past the PN junction and becomes a majority career. Once that happens, they recombine at the end of the P side, and the circuit is complete.

This is the fundamental explanation of the working of a solar cell. In the next post, I will explain the purpose of the different parts of a solar cell.

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Section Brief

Section Brief

This section is titled ‘Solar Cells’ and as the name suggests, it will talk about everything theoretical concept related to the cell level of a solar cell. I have divided this section into four sub-sections and I will write briefly about the contents of each of the sections in this post.

1. Solar Cell Operation and Efficiency

This section will introduce the fundamentals of the operation of a solar cell and the factors that affect its efficiency. 

2. Solar Cell Parameters

This section will deal with the parameters that are introduced to quantify several aspects of the solar cell. Some of these include, the IV curve, IQE, EQE, efficiency, Collection Probability, etc. 

3. Types of Solar Cells

Silicon is not the only material used to make solar cells and photo-electric effect is not the only way to make use of solar energy. In this section, I will introduce the several types of solar cells that have been reported. It is important to note that only Silicon and Thin Film Solar cells are commercialised as of now. 

4. Solar Cell Manufacturing

The question after introducing the types of solar cells is to identify the manufacturing process behind them. In this section, I will talk about the current industry manufacturing practices of Crystalline Solar Cells and Thin Film solar cells.