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학습트랜스크립트
00:06이 말씀드리기 시작할 수 to talk about the new generation from potable
00:12perubscribed solar cells by computing silicon's solar cells.
00:20perovskite solar cell originate from the material used for generating electricity.
00:32And the perovskite means the structure, ABX3 structure.
00:40There are lots of the perovskite material in this world, especially the perovskite material
00:51used for portovoltaic system usually composed of metal halide system, like heavy metal halide,
01:04like lead iodide, lead bromide, lead chloride, which is the most frequently used to make a perovskite material.
01:17And also, the most important material to form perovskite structure is the A-site material.
01:31Usually, in the high-efficiency perovskite solar cell, we usually use organic material like
01:39methyl ammonium iodide, pomaminidium iodide, which is located at the center of the octahedron of heavy metal halide.
01:49Here, this acts as a holder.
01:59By tuning this side, you can control the perovskite material characteristics.
02:06That is a very interesting thing.
02:08Also, changing halide material, you can control the perovskite material.
02:15This is why people have fever on this material.
02:27In this slide, I am going to deal with the physical property of perovskite material as comparing silicon material.
02:36So, the most advantage of perovskite material, which has a high absorption coefficient than silicon material.
02:51Almost 10 times higher absorption.
02:54Absorption means how well absorbed light from sun.
03:00So, and also high absorption coefficient means you don't need thicker films.
03:08So, because of this characteristic, usually perovskite material has a thickness of 500 to 10 nanometers.
03:23In contrast, silicon solar cells' thickness is almost 50 micrometers to 200 micrometers,
03:39much thicker than perovskite material.
03:42This is based on the characteristic of absorption coefficients.
03:49And also, last slide, I told you that perovskite materials can be tuned by changing materials.
04:00The most attracting feature by changing materials, you can control the band gap.
04:09This is an amazing feature because when you use silicon material as a semiconductor, you cannot control,
04:23you cannot adjust the magnitude of band gap.
04:31But when you use perovskite material as a solar cell material, you can control band gap.
04:40This means you can control the voltage generating from what you made.
04:53So, this is a very interesting feature.
04:58Another feature is that perovskite has an ambipolar transportation.
05:04This means the perovskite can generate an electron hole by itself.
05:12But in the case of silicon materials, in order to generate an electron or hole, they need doping.
05:28So, without any kinds of doping in perovskite materials, they can generate an electron and hole.
05:39Which means you can build your material in a very simple process.
05:46And also, perovskite material has a direct band gap.
05:53And in addition, they have a shallow trap.
05:58This means the trap acts as scavenger for an electron.
06:05So, if there are lots of traps in the material, electrons, they preserve,
06:15their energy.
06:18So, shallow traps means the material performs very efficiently.
06:28And also, perovskite material has a high dielectric constant.
06:33The dielectric constant means this property is related to generating voltage.
06:42So, high dielectric constant means they easily generate electrons.
06:51So, this is 2.5 times higher than silicon material.
06:58And also, perovskite materials has a long diffusion length.
07:06According to this part, silicon is almost 30 micrometer.
07:15The diffusion length of silicon material has more than 30 micrometer.
07:19But compared to any other kinds of solar cells, like organic solar cells or die-sense solar cells,
07:27they have a very short diffusion length.
07:30Almost a few tens of nanometers for organic solar cells, die-sense solar cells.
07:39But the perovskite material has a micrometer scale diffusion length.
07:46Those kind of characteristics make the perovskite material become the next generation of photovoltaic material.
07:58And also, a perovskite layer can be formed through a solution process.
08:08This is very amazing.
08:12OK, now let's compare silicon solar cell and perovskite solar cell.
08:19The left slide shows the perovskite solar cell structure.
08:29And the other is silicon solar cell structure.
08:38I told you the perovskite material has a higher absorption coefficients than silicon material.
08:51So, these brown colors, perovskite materials.
09:08The thickness of perovskite material is almost 500 nanometer.
09:17By comparison, silicon's much higher thickness is here, almost 150 micrometer.
09:32And also, you can hear P plus, M plus layer in the silicon solar cells.
09:44This means these layers are dubbed by P-type material.
09:52And these layers are dubbed with N-type materials.
09:58Because in order to generate electron hole and electron.
10:08In contrast, you can see here, there is no any kind of layer.
10:15Just this layer, hole transport layer.
10:18This layer, electron transport layer.
10:21They don't need any kind of doping.
10:23Because they have an ambipolar characteristic.
10:29So, just make interface with different type of material.
10:40They can generate electron at this interface.
10:54So, the ambipolar transportation junction makes the perovskite have a very simple device structure.
11:07And also, last class, I told you, perovskite material has a direct band gap.
11:19But the silicon has an indirect band gap.
11:22So, do you remember, indirect band gap is very vulnerable to D-pack state.
11:31So, because of this reason, silicon solar cells need a passivation layer here.
11:43Two passivate D-pack state exists at the surface.
11:49On the other hand, there is no specific passivation layer in the perovskite layer.
11:56Also, if you want to have a very high-performing solar cell, you need a passivation layer.
12:03But most of the case, perovskite solar cell doesn't need a passivation layer because they have direct band gap.
12:13And one more thing.
12:17Silicon solar cells have a very specific shape.
12:26Can you see a luxury surface?
12:31But perovskite solar cell has a planar surface.
12:37This is originated by the absorption coefficient.
12:43Because the silicon has a very low absorption coefficient.
12:48So, in order to absorb the light more, they extend by generating surface structure.
13:04By generating surface structure, we can increase the light path.
13:10Like, if there is no surface structure, the light just penetrates.
13:23But in this case, light does not directly penetrate into the material.
13:40So, here, the path of light becomes increased because of this specific surface.
13:59So, the material properties have a different configuration and structure and also techniques to be applied to make a good
14:17efficiency.
14:17So, I really suggest you guys take into consideration the physical properties of material that you want to use for
14:40your experiment.
14:41So, how well understand your material?
14:46You can design your device structure and configuration and also a strategy to make your experimental device.
15:00This is not the case only for perovskite or silicon material.
15:07This is not the case only for perovskite or silicon material.
15:08It can be applied for any different kind of device.
15:17If you take a look at the carrier movement in the perovskite solar cells.
15:24Once the light penetrates come into the front glass here and strikes a perovskite layer.
15:37And then occur electrons.
15:43These are the electrons generated and holes are generated.
15:47The generated carrier moves through electron transport layer and whole moves through whole transport layer.
16:01ETL means whole transport layer and ETL is electron transport layer.
16:21FTO is just a substrate. It's not an important thing.
16:32Total thickness of this device is not beyond one micrometer.
16:41Very thin solar cell device comparing the 200 micrometer of silicon solar cells.
16:52Because of this thickness, perovskite solar cell can be applied in flexible substrate.
17:02So, the thickness of the flexibility.
17:18Now, we are taking into account of the manufacturing process for perovskite solar cells.
17:29General management process is made up of five steps.
17:38Availability means the atmosphere to be adapted for specific processes.
17:48And so, as you can see, this is a very simple process compared to silicon solar cells.
18:04So, except the process of electro deposition process,
18:19making perovskite solar cells does not need a vacuum system.
18:25So, it's very, quite cheap process compared to any other silicon, any other solar cell technology.
18:38And also, there are lots of the verification methods developed so far.
18:45So, mainly can be classified in physical approach and chemical approach.
18:59The physical approach is very simple.
19:05The main concept of those fabrication approaches to make perovskite filling as possible, smooth, as thin as possible.
19:27Because, as I told you, the D-pack state usually exists at material grain boundary.
19:40Like, if there is a perovskite material here, the D-pack state exists at these interfaces.
19:48So, the process of the film fabrication, the goal of the film fabrication is to make a perovskite film very
20:06big scale.
20:08So, indeed, for this purpose, many kinds of the fabrication approach developed here.
20:20The most important thing, in my opinion, is the marble ions.
20:30So, a perovskite material composed of the organic and halide materials.
20:42So, because of those characteristics, usually the material can be movable and those movements influence carrier transportation.
21:01So, and also, their movement energy barrier load, as mentioned here.
21:10Iodide vacancy means for halide material.
21:14And MA, MA plus vacancy means for ABX3 A site material.
21:27So, they have very low, low, low energy voltage to move.
21:41So, those mobile ions have the main problem to be handled to make a good perovskite solar cells.
21:57Because, mobile ions are redistributed by electric field, because if you make the solar cells, there is a built-in
22:15voltage generated by the difference between electron transport and whole transport layer.
22:23So, the built-in voltage forces the mobile ions to be moved.
22:34And then, under illumination and short circuit conditions, the generated electron holes also generate electric field, which those kind of
22:52electric fields intervenes the movement of mobile ions.
22:58So, because of mobile ions, when you measure the ivory curves, we need this spatial consideration is needed.
23:15So, this is the flow to measure solar cell efficiency.
23:22So, if you, as I told, the mobile ions influence carrier transportation.
23:37So, if a mobile ion affects the carrier transportation, when you measure the efficiency, you can easily detect those phenomena
23:54situation in the ivory curve.
23:59So, this figure displays various patterns of JV curves affected by mobile ions.
24:14So, in silicon solar cells, when you measure the ivory curves, always you can get the same ivory curve.
24:29So, how many times you measure the ivory curves, how many times you measure the ivory curves, how many times
24:38you measure the ivory curves, how to measure the ivory curves at different temperatures, whether or not you can get
24:50always almost the same curve.
25:01So, if we know the other part of Bayward curves, when you measure the leather curves, how many times we
25:10measure the ivory curve, how many times we measure the ivory curves.
25:18So, all these panels originated from the movement of mobile ion and also mobile ions react with charge select layer.
25:34This means the HTL ETL.
25:39So, especially the interface between titanium oxide and perovskite oxide or Hortense material and perovskite materials,
25:57some kind of chemical reaction layer occurs here.
26:02Or, because of this chemical reaction, insulating metal iodide into layer is formed between metal electrodes.
26:15So, our main concern to make good perovskite solar cell, we must suppress mobile ions in perovskite solar cell.
26:45In order to suppress a mobile ion, you have to make a very nice, nice perovskite film.
26:52So, this is the reason why people try to develop new method or new perovskite material to control mobile ion
27:07effects.
27:15Therefore, in order to measure the performance of perovskite solar cell, there are lots of methods exist.
27:25Because, sometimes, a mobile ion exactly like overestimate current and also generate overestimate field factor.
27:41So, these figures show the dependence of scan rate when you measure the perovskite solar cell efficiency.
27:56As controlling the scan direction, or as controlling current receiving time, or as measuring steady-state power conversion efficiency at
28:11the point of max power point,
28:12you can exactly assess the impact of mobile ion in perovskite solar cell.
28:22So, and also, we call the mobile ion impact in the device, which is called a hysteresis impact.
28:45So, the device with hysteresis impact, the JV curves same and very clear shape, like here.
28:57And also, whether or not to control delay time, always JV curves show the same pattern.
29:13But, if there is hysteresis impact by mobile ion in your device, the JV curves show very highly unstable patterns,
29:32like here.
29:37And also, if there is hysteresis impact by mobile ion in your device, if you measure steady-state power convulsion
29:49efficiency at max power point,
29:51you can never get the same current obtained in JV curves, a little bit lower current density.
30:03In contrast, the device with no hysteresis, they generate the same current density obtained in JV curves, like here.
30:18And also, in order to measure the current density exactly, you need to measure current density by using IPC measurement
30:34technique.
30:35So, this means incident photon conversion efficiency.
30:50Also, there is hysteresis impact in your device.
30:54So, this means the IPC pattern shows very enormous pattern, like here.
31:06So, in one word,
31:13in one word,
31:18if you experience those kind of the very awkward situation or awkward JV curves pattern when you measure your device,
31:35which is this explains that you made very bad device.
31:46Okay.
31:48And to this class,
31:50in the last chapter,
31:54in the last chapter,
31:54I mainly talked about the perovskite solar cell as comparing with the silicon solar cell.
32:02Some people kind of feel this device a little bit difficult to understand.
32:14But the point is that if you understand the material property, you can design your device here.
32:27And these designs represented the material property.
32:35That's the very important point to remember.
32:41And also,
32:43this is a lot of people have a lot of people very mad at perovskite materials.
32:53So,
32:56a perovskite material has a good property.
33:00Not only for the solar cell,
33:02perovskite material also applied to photodetector,
33:11and battery,
33:14battery,
33:16LED,
33:17and also
33:18X-ray detection.
33:23In my opinion,
33:27there is still a problem
33:31from a perovskite material according to their stability.
33:38So,
33:39these materials go further
33:44maybe with the X-ray detection.
33:47It's just in my opinion.
33:49Because,
33:51in order to be used in solar cell system,
33:57silicon,
33:58they have their lifetime more than 25 years.
34:08More than.
34:09But,
34:11the perovskite material only just have two years.
34:15Very short lifetime.
34:19So,
34:20this material is very nice to do experiment,
34:25but still exists their limitation to be applied for solar industry.
34:38Okay.
34:41Anyway,
34:43but thank you
34:45to attend this class.
34:49I hope this class helps you to understand the portovoltaic system.
34:55and also band theory.
35:00Okay.
35:02It's very nice for me to have a class for you guys.
35:06Thank you.
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