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학습트랜스크립트
00:00Hello everyone, welcome to this class. I am Yohan Goh. I am today's lecturer.
00:07In this class, I would like to deal with knowledge for photovoltaic systems from
00:12banked-liberal practical applications. I believe this will help you
00:20understand better for research life in photovoltaic systems.
00:26I will do my best for it.
00:31I prefer my learning materials in three parts.
00:37In the first part, I would like to define the thermos used in a portable thermos system,
00:45especially related with electric energy.
00:51Some guys think that it is a very basic concept,
00:56but someone confused some kind of terminology like current, voltage, carrier, and so on.
01:13I am going to deal with those kind of terminology in this part.
01:22Okay, first of all, do you remember the first law of thermodynamics?
01:27That is the conservation of energy.
01:32You know, the energy cannot be destroyed or eliminated.
01:36It just transforms from one to another form.
01:40So, this means that the total energy of a system remain constant in any isolated system of object.
01:49So, accordingly, portable type devices do not create new energy.
01:55It just uses energy from the sun by transforming into an electric city.
02:08So, okay.
02:10I have a question.
02:12What is electric city?
02:15Let's think about it.
02:16I will give you a step.
02:18One, two, three.
02:21You got it?
02:23So, if you can say any answer within three fake currents,
02:31which means that you don't know exactly what it is.
02:35So, electricity is a form of energy.
02:40More exactly, this is the set of the physical phenomena associated with a charge.
02:46So, what is charge?
02:57What is charge?
02:59Have you ever experienced charge?
03:04Every people live with charge.
03:08In real life, we usually experience a charge situation like it.
03:18In, you know, these days is very dry season.
03:21So, people easily experience an electrostatic situation like if you try to use a comb to straighten your hair.
03:35Here, sometimes the hair stick to a comb.
03:39This is because of the charge.
03:42Charge is a source of electrostatic force.
03:47So, this means that without charge, no exists electricity.
03:54So, the charge can be expressed as a cube.
04:03Which means that it equals 1.6 times 10 to minus 19 comb.
04:13It is a quantity involved in one electron or one proton with different polarity.
04:28So, to express charge with time, people like to use the current.
04:42By using current, we can express charge as a numerical way.
04:53So, by dividing time.
04:57So, which calls ampere.
04:59To count the amount of charge per unit time.
05:04So, we adopt current density by dividing the ampere per area.
05:11So, let's think about the, there are two material here.
05:19One is, has a, has an area of 100 square centimeter.
05:29And it generates 10 ampere.
05:34And there is one other.
05:37It has a, one square centimeter with one ampere.
05:48So, which one has better condition?
05:52Of course, material B has a, better, better condition.
05:58So, by using current density, we can compare current, current exactly.
06:07So, in most, in the most case, when it comes to current, people like to use the current density.
06:15And also, this applies in portable type system.
06:28More than anything else, what to keep in mind, we need carriers to convey electric charge.
06:39So, this means, charge, this means a charge never flow alone without any kind of media.
06:54So, the media for carrying charge hold as carrier.
07:04So, carrier includes electrons and holes.
07:14So, electrons can carry minus charge and hold for plus charge.
07:29So, okay.
07:36In one word, electricity is originated from charge movement.
07:59And more exactly, we can, we can, we can, we can express, in order to express charge movement
08:11numerically, we use current.
08:16And to compare their quantity, we use current density.
08:23And also, in order to carry charge, we need carrier, which are electron and hole.
08:38Very simple, isn't it?
08:42Now, we are turned to voltage.
08:46So, a form of energy consists of potential energy and kinetic energy.
08:56So, a position energy, a potential energy is a position relative and kinetic energy is a motion relative.
09:06So, if there is like, something is located at higher position than the other one.
09:21Like, if there is, okay, we can, I'm going name it as V1.
09:36So, V1 is located higher percent than V2.
09:42We can say V1 has a higher potential energy.
09:50This is the same for electric energy.
09:54Electric energy has a position relative.
09:57So, if there are two charges on different electrical potentials, the difference in electrical position gives us voltage.
10:19So, voltage explicitly explains the difference in two points of electrical potential.
10:28So, the point is that, but the point is that, the higher potential does not mean a force to move
10:43charge better.
10:44The force to move charge is related with electric field, not by just the voltage.
10:56So, electric field is a force field that illustrates the direction of electric force.
11:12From plus to minus.
11:17This error means the direction.
11:22So, let's guess something situation.
11:35There are two plates.
11:41There are two plates.
11:44And here.
11:50And the distance is one centimeter.
12:00And the voltage between two electrodes, one volt.
12:11And the other case, you are to plate at a distance of 100 centimeter.
12:25And the voltage between two electrodes is 10 quarts.
12:33So, although, I'm going to the first case, call as a situation, a case, case A, and the other one
12:51case B.
12:57Although, the low voltage is applied in the case of A, but the distance is smaller than the case B.
13:14So, because of the smaller distance, a particle located at here experiences more electric field.
13:32So, for this reason, we consider the distance.
13:41So, electric field, the particle experience, the force is related with distance.
13:53So, the voltage is not the force to move a particle apart, but the electric field is a force field
14:07that force a particle to move apart.
14:13So, this is simple, but sometimes people easily confused to use this kind of concept.
14:29And as for kinetic energy, kinetic energy are the result of potential energy.
14:39So, the kinetic energy acquired by an electron acted upon by a potential difference of one volt.
14:54More simply, kinetic energy is the work to work for one electron or a proton upon one voltage potential.
15:18So far, we just have learned about both T's and current I.
15:25So, now I'm going to describe the correlation of those words using a metaphor of the waterfall.
15:40There are two waterfalls in this slide.
15:44One, the first one, the height of the waterfall is high.
15:57But the other one, the height is relatively low than the first one.
16:13So, the height of the waterfall can be expressed as a voltage.
16:22So, in this sense, the left waterfall has a higher voltage than the right waterfall.
16:36So, but about their amount of the water stream, the first, the left, the left waterfall has a low amount
16:54of the water stream.
16:56So, which means that the first waterfall has low current.
17:09And on the other hand, the right waterfall has a very big water stream.
17:21And on the other hand, the right waterfall has a low height.
17:27So, high, we can say like high current density, high current.
17:47And on the other hand, we can say like high current vehicle.
17:51And on the other hand, the total power involved in each waterfall, we can calculate it by multiplying their height
18:02and the amount of the water.
18:04This is the same, like power equals voltage times current.
18:21So by comparing like this, we can compare the power of each device.
18:33A scientist wants to make a device with big voltage and current, like mixing those waterfalls.
18:55Like it, you know, the height of the waterfall is very high and lots of water streams.
19:14This is the same for portable type device.
19:28I recommend when you confuse the voltage and current, just think about these figures.
19:42So these figures, I believe, make you understand better about the voltage and current.
19:53Okay, next one.
19:57In this Bantheor chapter, we will learn about base considerable atom semiconductor for Bantheory.
20:10In the last chapter, I told you the electron works to deliver charge.
20:16We really put it as carrier.
20:20So in order to understand the electron movement, we have to know the atomic model.
20:30Modern physics have been developed with history of atomic model.
20:34Except the case of Schrodinger model.
20:42The proposed atomic model is based on empirical legions.
20:52Those R came from empirical legions.
21:01And so, and a ball model is only one to be used as an atomic model these days because it
21:14is simple and straightforward.
21:16Also, the Schrodinger model describes it perfectly in atomic model.
21:25The Bohr model is very easy to understand.
21:30So, except the specific case, scientists like to use the Bohr model.
21:42In these four modules, electrons go around in a circle of atoms with a different energy state, which is called
21:56a shell.
21:57The electron participating in bonding with other atoms is going around the outer circle here, which is a balanced shell.
22:11So, if the electron at the outer shell becomes free, like one of the electrons here, if this becomes free,
22:29then this contributes electricity.
22:35That is, free electrons.
22:37So, as I told you, Schrodinger model explains atomic model very exactly.
22:48And if you solve the Schrodinger equation here, you can get a quantum number.
22:56Four kinds of quantum number.
22:58So, and also, you can get orbital shape from this equation.
23:07So, if you have a chance in the future, I suggest you to solve this equation by hand.
23:19It will very helps you out for your research life, as I really suggest.
23:31But this time, it's beyond the concept, beyond the goal of this class.
23:39So, I'm not going to deal with any more about the Schrodinger equation.
23:49It's just what you know, that is, that by solving a Schrodinger equation, you can get a quantum number, which
24:06describes electron shape, which describes electron status of energy.
24:14This is the thing you have to remember to understand this class.
24:25Now, we think about the real case using the material of silicon.
24:32So, in order to learn semiconductor, people usually use a silicon atom because silicon is the material for semiconductor industry,
24:44as you know.
24:46So, when you learn about the semiconductor theory or band theory, most of the books or articles centered on silicon
25:00material.
25:00So, these figures only illustrate the balanced band energy state to express the bonding type of silicon.
25:18So, in silicon material, in silicon material, it has four electrons at the outer shell, like here.
25:32There are four electrons going around the nuclei.
25:40And so, when they meet each other, like the same silicon material, they form a bond by sharing electrons to
25:58fulfill octet rule.
26:03So, octet rule means the elements tends to bond in a way that each atom has an electron in its
26:15balance shell.
26:16It's a rule.
26:17So, by forming bonding with a shared electron, we can call it as the cobalance bond.
26:31So, silicon has a chemical bond by sharing electron failures between atoms.
26:38The point is, balanced electrons can only participate in formation of chemical bonds.
26:50So, these kind of electrons do not conduct electricity.
26:58So, only electricity is generated by free electrons.
27:08Don't confuse it.
27:12So, free electrons is the electrons that are free from attraction force of a nucleus and does move freely.
27:21So, and also, the absence of an electron in a particle place in an atom, which is generated by absence
27:34of an electron.
27:36So, whole and free electrons conduct electricity.
27:50And also, the electrical properties of solid materials are a consequence of its electron band structure.
28:03So, we can classify the band structure according to the electrical properties.
28:09So, there are three kinds of electrical band structure according to their electrical properties.
28:21The first one is the conductor.
28:23The conductor has no band gap.
28:29So, here.
28:42The field band means that balances take energy.
28:52The involved band bridges, a multi-band conduction band.
29:00So, these bands overlap.
29:04So, those band overlap, so materials.
29:07So, materials that have no band gap usually have a high conductivity because the Sono electrode
29:21based tension inside the electric valve or the electric valve.
29:22여기는 여기에 있음,
29:25쉽지 않게 움직입니다.
29:302. Semiconductor
29:35Semiconductors don't conduct electricity
29:38그리고
29:40이미지와
29:40이 타이밍에
29:48타이밍화가
29:49이 타이밍화가
29:51이 타이밍화의
30:02이즈화가
30:03어떻게 사용할지
30:04하나는 더 잘 모른다.
30:12이 정도의 전천들을 변경할 수 있고,
30:16이 전천들을 변경할 수 있고,
30:23or if you dock the semiconductor material, you can easily increase the conductivity of the semiconductor.
30:34Also, materials that have a big band gap, they cannot
30:44conduct electricity, which is considered non-metal materials.
30:57According to the electrical property, we can classify three kinds of band structures.
31:14Now, I'm going to talk about why bands and band gap occurs.
31:25Okay, let's think of a specific extension. There is a silicon atom in an isolated system.
31:39The elemental number of silicon is 14,
31:46which indicates the material has 14 electrons. If we describe the energy state of electrons in silicon,
31:56which can be written as 1s squared, 2s squared, 2p6, 3s squared, 3p squared.
32:10So, our main concern is the balanced shell, that is 3s and 3p.
32:24Using the Bohm model, we can do atomic figure like here.
32:32Like here.
32:37And as you can see, below the atomic model, I do their energy state according to potential energy.
32:49The 3s and 3s and 3p is the main energy state to participate in chemical bonding.
33:04So, now we are only considering about 3s and 3p.
33:11And now, we extend the situation to multiple atom systems at a long distance and there is no
33:26interruption between each atom here and here.
33:33In this situation, every atom can exist as holding their unique property.
33:43There is no change in their energy state.
33:48My kid.
33:49But, what if...
33:53What if we make them closer?
33:57Just think about
34:01what happens to those atoms.
34:05If we make them closer to affect each other?
34:12This is the key.
34:14If we make them closer,
34:17than something repulsion occurs between atoms, which split energy state, most of the balanced
34:31energy state, like here, like here, like here.
34:37Energy states never cannot be overlapped, because energy state is eigenvalue, so they want to preserve
34:50their energy state even if they come together, so, and then pushing more for atoms to be
35:04closer than band split occurs in eight times and state, because the, because I told you
35:19there are, silicon has 3S23P2 state, actually, P orbital has, has, P orbital have six states,
35:40so, as a, by, by hybrid, by hybrid, hybridization occurs in 3S3P energy states, the 8N states
35:55occurs. If you further make them closer, then repulsion pores getting strong and the
36:10band state, the energy state is split in two parts. Heart becomes, become stable energy state,
36:22heart becomes unstable state, and more stable energy state, which is called as balance band,
36:34band, balance band, and the state has unstable energy state, which is called, we called
36:49conduction, spand band band, so, let's watch one more time, how bands and band gap occurs, okay,
37:09here's the point, okay, there are some atoms and a distance, and if we make them closer,
37:18and they getting closure and some repulsion force occurs in their energy state and because
37:31of the repulsion force energy state becomes 8n state and then if we make them more closure
37:41then balance band and convex band occurs. So if we draw this situation we can have a schematic band
37:59splitting this figure. In these figures x-axis represents atomic spacing that is
38:11the distance between atoms and y-axis for energy state. Higher energy states mean unstable state,
38:22lower energy state means more stable energy state. So as atomic spacing becomes smaller,
38:39each energy state interrupts each other and then there's 3px up down py up down
39:09pg up down 6n state becomes here this point is 8n state and more closure then occurs 4n state, 4n
39:27state,
39:27which this one is for the conduction band here, this one is for balance band here and the gap between
39:36conduction band and balance band, the band gap occurs.
39:42okay now one more time let's take into the evolution of bonding of silicon atoms.
39:53right here. So I told you 3s and 3p and 3p state there are three energy states. Each energy state
40:07can accommodate two states.
40:11up electron moment and down electron moment. So that there are 6n state here and 3s and 2n state here.
40:30If we come closer and those energy states hybridized,
40:43in this case, in this case, you can see here, each energy states accommodate two electron states. So it becomes
40:578n state.
41:02And again, if you force them to be closer more than those energy states split, like here.
41:16bond which pretty strong ou존 cards, and these relaunches bend, the bond state is exact.还有
41:19state with r5 which is called beans balance band and ocurs anti-bonding state. So,
41:35it seems like very simple. So, I really want you guys having an r varsityHi,
41:46멈축 오류의 회의의 변딕의 침략을 찾아낸 수 있습니다.
41:58다른 나무의 변딕이의 변딕으로 변화하는 방법은
42:02위대의 변딕이의 변딕이의 변딕이의 변화하는 방법입니다.
42:08그리고, 두 가지 방법은 변딕이의 변딕이의 변딕입니다.
42:16그리고 다른 비밀대가 범위한 low백 liber Weekly
42:23빨대가 범위한 소득불에서
42:26고구는 섬세가 범위한 소득불에
42:44In the case of silico material,
42:48proton energy generated by ponon,
42:52ponon means lattice vibration energy.
42:57So this case is very vulnerable to material quality because electrons cannot jump into conduction band directly.
43:15So they jumped and their moment should be changed.
43:21While changing this situation, the electrons easily captured by defects exist at the material surface.
43:34On the other hand, in the direct band gap,
43:39the electrons directly come into conduction band without any intervention.
43:47So this kind of material,
43:54they are very strong to material defects,
44:01which is called defect tolerance.
44:07Those kind of characteristic band structure,
44:13it caused the different technique to make photovoltaic devices.
44:22And next slide, I'm going to talk about it.
44:31Next chapter, next chapter.
44:34So, okay.
44:35Okay.
44:37다음 영상에서 만나요.
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