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.
