Chapter 36: External quantum efficiency

　　 Xu Qiu put the sample in the transfer compartment of the glove box and took it out in no hurry.
　　 He followed the senior sister and came to the external quantum efficiency test instrument. There was a computer and a black box.
　　The light shielding performance of the cabinet is very good when it is closed.
　　 After opening the box, there are light sources, lenses, power supplies, detectors, and lifting platform inside it. The light sources can emit monochromatic light of different wavelengths.
　　 Chen Wanqing began to demonstrate.
　　 She put the standard silicon cell on the lifting platform, connected it to the power supply and the detector, and then adjusted the lens so that the light spot completely hit the surface of the silicon cell.
　　The standard silicon cell is 10cm×10cm, and the light spot is small, so there is no difficulty in operation.
　　 Then turn on the computer, open the test software, and set the parameters.
　　 The scan interval is set to 300-800 nanometers, and the step size is set to 10 nanometers,
start testing.
　　 The instrument "Dangdangdang" is running, and the test results are displayed on the computer in real time.
　　 The abscissa is the wavelength, and the ordinate is the EQE value. Starting from 300 nanometers, a data point is generated every 10 nanometers.
　　 After the test is completed, the software will automatically calibrate according to the test result.
　　 "Senior sister, let me change the next operation." Xu Qiu had already seen the operation process in advance through the system video.
　　 "Are you sure? It won't be easy to wait for the sample to focus on the light spots." Chen Wanqing said.
　　 "It's okay, don't worry."
　　 "Okay." Chen Wanqing gave her position to Xu Qiu after she said.
　　 Xu Qiu took out the sample holder from the transfer cabin, placed the quartz glass window side up, placed it on the lifting platform, and connected the probe to the power supply and the detector.
　　 Next, focus the light spot of the light source on the battery to be tested.
　　This step is really difficult, because the area of ​​each battery device is only 0.09 square centimeters.
　　 If the light spot is not completely irradiated in this range, the light energy outside the range will be lost, and the final test result will be low.
　　 However, Xu Qiu is hard to come by. He adjusted the light spot patiently for a while, then started the test.
　　 "You can do this." Chen Wanqing exclaimed.
　　 "It's just the basic operation, not worth mentioning." Xu Qiu casually waved his hand and said, "In other words, why should the area of ​​the battery device be so small? Is it not good to make a 1 square centimeter?"
　　 "Because the larger the battery area, the lower the photoelectric conversion efficiency," Chen Wanqing said:
　　 "At this stage, in order to achieve high efficiency, most of the research groups are small areas. The general international practice is 0.09 or 0.04 square centimeters, but there are also more exaggerated research groups that will make 0.01 square centimeter devices.

　　 "Let me think about it," Xu Qiu thought for a while and said:
　　 "I understand, the effective layer of the spin-coated film looks almost uniform to the naked eye, but in fact, there are subtle differences in each area.
　　 For example, a device A of 0.09 square centimeters is divided into 9 parts A1-A9, each with an area of ​​0.01 square centimeters.
The photoelectric conversion efficiency of 　　A1-A9 may fluctuate between 5.0-5.5%, so the final efficiency of A is about 5.25%.
　　 Finally, the highest efficiency, 0.09 for a large area is about 5.3%, and 0.01 for a small area is 5.5%, which can even fluctuate to a higher value.
　　 Is this the reason, senpai. "
　　 "Yes, it's almost like this. Organic photovoltaics are still in the laboratory stage, so there is no uniform standard," Chen Wanqing said:
　　" I also saw a document before, and the title said that the photoelectric conversion efficiency reached more than 15%. As a result, the journal is in the second SCI area.
　　 I was shocked at the time. Even if I can't publish "Nature" and "Science", I can publish a big journal of them.
　　 Then I read the text carefully and found that it used monochromatic light instead of simulated sunlight. It was just a gimmick in the title. "
　　…………
　　Sample test is over.
　　 The shape of the EQE curve is roughly the same as the light absorption spectrum of the effective layer. The theoretical short-circuit current density can be calculated through the integral function in the software.
　　 The theoretical result of this sample is 11.96 mA per square centimeter, and the measured result of the J-V curve is 12.05 mA per square centimeter.
　　 The error between the two is 5%, which indicates that the test result of the latter is credible.
After 　　, Xu Qiu tested another five sets of devices.
　　 By 5:15 pm, all tests are finished.
　　 "Senior sister predicted quite accurately, I really don't have to work overtime today." Xu Qiu said.
　　 "Practice makes perfect," Chen Wanqing said: "By the way, what to eat at night?"
　　 "Why, do you want a treat?"
　　 "Didn't I promise you last week."
　　 "I was just making a joke at the time. How embarrassed it is, then go and eat Ge Laoguan hot pot."
　　 "Okay."
　　 Because it is Friday night, there are still more people eating hot pot.
　　 Xu Qiu took a number, A65, there are 12 tables in front, it is estimated that it will take about an hour.
　　While waiting, he was not idle and asked Chen Wanqing for questions.
　　Although the answers can be obtained by reading the literature, the effect of real-person guidance is naturally better.
　　 Scientific research is a systematic work. You must figure out all the details you encounter, and you can use it someday.
　　 Because every piece of data in each test must eventually be transformed into an argument for the result, it is impossible to tell others when writing a paper that we tested a XXX and the data obtained was XXX, and then it stopped.
　　 It is necessary to analyze the data obtained and discuss its relationship with the conclusion of the paper.
If it is a rather abnormal result and cannot be explained, it should also be stated that "this phenomenon is temporarily unexplainable", but it is okay if it appears once or twice. If the whole article is "we can't explain", it will be reviewed by the reviewer. Teach to be a man.
　　 "When I tested it just now, I found that the EQE value of most of the wavelengths of the devices we made was below 60%, while the silicon battery could reach more than 80%. Why?" Xu Qiu asked.
　　" Simply put, there are many problems with the P3HT materials used now~EbookFREE.me~ needs to be improved.
　　 If you want to start concretely, first you need to know the process of converting solar energy into electricity:
Sunlight irradiates photons, and the effective layer composed of optoelectronic materials absorbs the photons, and electron-hole pairs are generated inside it. Then the electron-hole pairs are split into free charges, which are transported in the material, and then collected at the electrodes. Eventually an electric current is formed.
　　This process is very complicated. Any problem in any step in the middle will cause current loss. What you see on the surface is low EQE. "Chen Wanqing said.
　　 "I understand," Xu Qiu said: "So people made improvements based on P3HT, and the best donor material to date is PTB7-TH."
　　 "Yes, the organic photovoltaic field has been developed for 20 years, and the materials have been iterated for many generations, including PTB7-TH. Do you know why it is called this name?" Chen Wanqing asked.
　　 Xu Qiu shook his head, he just knew a name.
　　 Chen Wanqing explained:
　　"This is a series of materials, where T represents a thienothiophene unit, B represents a benzothiadiazole unit, and P represents a polymer formed by the copolymerization of these two units, so it was originally called PTB.
　　 Later, researchers based on PTB and modified it to obtain PTB2, PTB3, and PTB7.
　　 Then the benzene ring on a branch of PTB7 was changed to a thiophene ring, so a thiophene abbreviation -TH was added, and the current PTB7-TH was obtained.
　　 Moreover, in the past 20 years, the main improvement direction has been polymer donor materials, and the acceptors have been using PCBM series.
　　 Perhaps one day in the future, the receptor field will also be able to make a breakthrough, which will make a radical change in the organic photovoltaic field. "