Lec36 6-3 太陽能電池結構

We talked about that the principle is photovoltaic effect, the photoelectric effect refers to the metal. Basically, solar photovoltaic panels could be divided into several large blocks. Substrate, p-n diodes, antireflection layer, a metal electrode. Here are the n-type semiconductor and the p-type semiconductor. These two things will generate spatial potential difference. Spatial potential difference would make the electron get photon to rush to the n pole. The electric hole would run to p pole. N-type and P-type will produce the spatial potential. Generally, there will be an anti-reflection film to reflect the last part of energy. Here are some optical things to let it stay a little longer. If there are some optical computation, it also includes how the photons move. Try to make it stay in this place a little longer. There are two electrodes outside. If you connect the external circuit, then it will turn into a circuit like this. Of course, some photovoltaic panels do not necessarily have n-pole and p-pole as long as there is polarity, it also work. What is the p-n type semiconductor? We should know that. This is electrically neutral. Silicon is 4 electrons and 4 electrons, so there are 8 electrons totally. If we add some phosphorus in the silicon, the outermost layer of phosphorus has five electrons. So there is an electron without binding force, it is easy to be inspired. Oh, but remember it, this is just one more electrons. However the whole element is still electrically neutral. Because of one more electron, it would become the n- type semiconductor. If we mix with the boron, the outermost layer of boron has only three electrons. So, there will be a vacancy, there will be an electric hole. This is called p-type semiconductor. So we often talk about mix which transfer the semiconductor into a p-type or n- type semiconductor. There are four kinds of charged particles. Including the electrons with negative charge and the electric holes with positive charge. The acceptor ions with negative charge and the donor ions with positive charge. If the electron runs away, the acceptor ion will become positive charge. If the electronic hole runs away, then it will be negative charge. Next, if there is an n- type and a p- type semiconductor, we combine them together On the interface, the concentration of n- type semiconductor is higher because an electron is without restriction. At this time, the electrons may diffuse from the high concentration region of n- type semiconductor to a low concentration region of p- type semiconductor. Originally, n- type semiconductor and p- type semiconductor are electrically neutral. At this time, because of the equilibrium of concentrations, the electrons may diffuse from the n- type region to p- type region. At this time, the electrically neutral condition may be lost and in the middle part, it would come up with intermediate potential. Although there is one electron more, it is still electrically neutral. This is the n- type semiconductor. There are less electrons in p-type semiconductor, of course, it is neutral. Then we combine n- type and a p- type semiconductor together. At this time, it will spread, because the material has to be in the balance. Therefore, the electrons will diffuse from the high concentration n- type region to a p- type low concentration region. At this time will be from neutral into the electrode resistance out The n-type semiconductor with positive charge and p-type semiconductor with negative charge become the space charge in the balance. By the time, if the sun came in and the received electron can run out to move, Electrons will go to the n- type semiconductor, because the n- type semiconductor is positively charged, this will produce a current. This is the effect of the photovoltaic effect. Basically, because of the electron concentration balance, it will produce a space charge region in junction. At this time, if the photon get in and let the bonded semiconductor break out into a conductive semiconductor. At this time, it will produce drift. For drift, then basically, the drift will be produced in this region. If it is influenced by allocated potential, it will drift to n-type and generate a photocurrent. It is the principle to generate some electricity. This figure is very simple. N-type semiconductor is positively charged and p-type semiconductor is negatively charged. If the photons come in, then electrons may run to n- type semiconductor and it also means that the electric hole will run to the other side. If today it will be a short circuit with an external circuit. If it is not, it is connected to other places. It will generate a potential difference here. This is the open circuit potential difference. Here is full of negative charge and there is full of positive charge. It is open-circuit potential difference. This is a very simple principle. Photocurrent is generated not only by drift but also by diffusion. The place in contact will produce a spatial potential and the place without contact is still electrically neutral. So when the light comes in, it hits a lot of electrons and of course it will still spread. This diffusion will provide a little currents. So there are three mechanisms for photocurrent. The first one is the drift. The electrons and electronic holes of space-charge region generate drift-current. The second one is to say that in the electrically neutral place, if there are many electrons which absorb the light will spread. As long as the electrons move, it is current. Therefore, it will still be current. Then the electronic hole is the same, the electrically neutral place will generate a lot of electronic holes. If the electronic hole diffuse, it will also generate some currents. So basically, these three physical mechanisms are drift current, electronic diffusion current and the electronic hole diffusion current Three necessary conditions for operation of a photovoltaic cell device. Incoming photons are absorbed to generate the pair of electron and electronic hole. The pair of electron and electronic hole would move because of the impact of spatial charge region before the recombination. And then there will be the internal light current. These separated electrons and electronic holes are transported to the load region. It will produce current in external load region.