Introduction
Optoelectronic device
These days, the demand for many types of energy is increasing rapidly and there are many sources of energy such as oil, coal, gas and nuclear energy. However, these energy sources are not suitable for future energy harvesting due to environmental problem and hazard. Although these sources have possessed a large share of energy harvesting until now, such a trend is now shifting towards carbon-free, safe and renewable energy sources such as wind power, tidal power, hydroelectric power and solar power .
Solar energy in particular aroused a lot of interest in the research, because it coincides well with future energy. It means that solar energy is a carbon-free clean energy source and does not harm the earth's environment. Thanks to these many advantages, numerous research projects in the field of solar cells have continued since the 1950s.
As a result, silicon solar cells are already commercialized, and to replace silicon solar cells, a lot of research is going on such as organic solar cells and perovskite solar cells. However, there are several ways to use the electrical energy, which is from solar energy through other optoelectronic devices. One of them is LEDs, which can produce the light with target wavelength from the electricity.
Since the efficient use of energy can be the same effect as the production of energy, these optoelectronic devices are the focus of attention for future energy harvesting and application.
LEDs device
History of LEDs device
- History of perovskite LEDs device
Although the performance of the device was low, they proposed a new direction of the LED device. Therefore, the size of the A site cation plays an important role in determining the band gap of the perovskite. Third, the halide anion at the X site, such as Cl, Br, or I, is most directly involved in the decision of the perovskite bandgap.
It is also an important factor for the increased stability and reduced power consumption of the LED device. Next, one of the crucial things about LED devices is the recombination of hole and electron to make exciton. The measurement was consistent with the different volume percentages of DIO to poly-TPD solution.
PL lifetime and the ratio of non-radiative recombination to radiative recombination were measured according to the different volume percentage of DIO to poly-TPD solution, as shown in Figure 3.4. PL lifetime of the PeNCs was remarkably increased almost 3 times when DIO addition was used compared to the untreated film. The structure of the device was ITO/PEDOT:PSS/Poly-TPD (+ x % of DIO)/PeNCs/CBP/MoO3/Ag, as shown in Figure 3.6, and J-V characteristic was shown in Figure 3.7.
They also observed a very narrow FWHM around 16 nm, which is one of the remarkable properties of perovskite.
Theoretical & Mathematical Development
Perovskite
EQE is defined as the ratio of the number of photons emitted from the LED device to the number of electrons injected into the device and is given in the following equation. Apart from the adjustment of the energy band, mobility improvement is also one of the goals of interlayer modification. By measuring the J-V ratio between specific layers, the layer's mobility can therefore be calculated.
In addition, film morphology is one of the most important components for a well-fabricated device. The quality of the perovskite solution is directly related to its photoluminescence quantum yield (PLQY) which means the ratio of the number of emitted photons to the number of absorbed photons. After the improvement of the optical properties was confirmed, the analysis on the electrical property was also investigated.
From the AFM results, the morphology and roughness of the thin film were greatly improved by adding the DIO layer of poly-TPD. To optimize the performance of the EL device, the amount of DIO additive was varied from 0% to 10. Here, the turn-on voltage is defined as the voltage when the luminance of the EL device is displayed as 1 cd m-2.
The turn-on voltage of the reference device was 5 V, but it decreased when DIO was treated. This means that the charge balance has been improved and the electrical property of the device has been enhanced. In addition, this study will suggest one way to increase the efficiency of the PeNCs LED device.
OLEDs to perovskite LEDs
PeLEDs device
- Working mechanism of PeLEDs
- Characteristics of PeLEDs
- Properties of PeLEDs
Inspired by this result, DIO was introduced to the interlayer which is poly-TPD used as HTL in LED device, as shown in Figure 3.1. The PeNCs films were spin coated on poly-TPD layers with the different amount of DIO. Using this phenomenon, PeNCs were coated thinly in Figure 3.2, and change of PL intensity of poly-TPD and PeNCs was observed.
For the first time, this was simply confirmed by measuring I-V characteristics of simple device whose structure is ITO/Poly-TPD (+ x % of DIO)/Ag, as shown in Figure 3.5. To investigate the reason for this enhancement of current injection, hole mobility of poly-TPD layer was calculated by fitting SCLC flow, as shown in Figure 3.8. Based on this, the fact that the improvement of mobility of poly-TPD influences to the increase of charge injection has been proved.
One is the enhancement of the optical properties by passivating the trap sites at the interface, and the other is the enhancement of hole injection caused by the enhancement of the hole mobility of the poly-TPD layer. Based on this, the EL device is fabricated with the ITO/PEDOT:PSS/Poly-TPD (+ x % of DIO)/PeNCs/TPBi/LiF/Al structure. In summary, the LED device of PeNCs with poly-TPD treated by DIO additive was successfully fabricated and the effect of DIO was confirmed.
Using DIO additive, the role of DIO additive can be explained in two main ways; (a) one is the enhancement of optical property by the passivation of trap sites at the interface, and (b) the other is the enhancement of hole injection caused by increased hole mobility of poly-TPD layer.
Perovskite Nanocrystals Light-Emitting Diodes Device with DIO Additive to Poly-TPD Layer
Experimental method & materials
To prepare the cesium oleate solution, 0.1 mmol of cesium carbonate (Cs2CO3) was charged into a 100 mL three-necked flask along with 1 mL of octanoic acid, then mixed by magnetic stirring. Then, 0.1 mmol PbBr2, 0.2 mmol tetraoctylammonium bromide (TOAB) were dissolved in 1 mL toluene in a 50 mL three-necked flask. After that, 1 mL of didodecyl dimethyl ammonium chloride (DDAC) of 30 mgmL-1 in CB was added to the precursor solution.
Then, for the purification step, the crude solution with ethyl acetate (volume ratio between raw material and washing solvent 2:1) is centrifuged at 12,000 rpm for 5 minutes, followed by another centrifugation at 10,000 rpm for 5 minutes. After that, the precipitate, which was dissolved in 1 ml of hexane, was centrifuged at 4000 rpm for 5 minutes and the supernatant solution was collected. These ITO substrates were treated with UV-ozone to make the surface hydrophilic, and the PEDOT:PSS solution was spin-coated onto the UV-ozone-treated ITO substrates at 3000 rpm for 40 seconds, followed by annealing at 150 °C for 15 minutes.
The photoluminescence spectra were measured with an nF900 instrument (Edinburgh Photonics) with a xenon lamp for the excitation source. The TCSPC setup (FluoTime 300) was used to measure the PL lifetime and the ratio of nonradiative recombination to radiative recombination. The time-integrated PL spectra were measured using a TCSPC module (PicoHarp) with a photomultiplier tube (PMA-C 182-N-M).
The J−V−L characteristics of the LED devices were measured using a Keithley 2400 source measurement unit and a Konica Minolta spectroradiometer (CS-2000).
Result & discussion
Based on the PL measurement result, the TCSPC measurement was also executed to confirm the effect of DIO on the radiative recombination mechanism. If the current injection enhancement is also applied to the LED device, the DIO has a good impact on the charge balance. Based on the I-V result, the hole-only device is fabricated to control the hole-side charge injection characteristic according to different amount of DIO.
The J-V characteristic result had the same tendency as the previous I-V measurement, and the current injection was the largest when 5% DIO was added. Since the hole mobility of untreated poly-TPD was calculated to be 7 10-5, which is similar to the reported reference hole mobility of poly-TPD, which is 1 10-4, the reliability of the data is assured. Moreover, on the poly-TPD film, the aggregates are in 0% state, but otherwise, the DIO-treated film showed clear film images without aggregates.
The EL emission of PeNCs, which were CsPbBr3, was observed at 514 nm, as shown in Figure 3.10b. The EL emission from PeNC, which is CsPbBrxCl3-x, was observed at 476 nm, as shown in Figure 3.11b. All devices, regardless of the amount of DIO, showed very low leakage current as shown in Figure 3.10a and Figure 3.11a, especially when 3% or 5% DIO was added, an extremely low leakage current was measured.
Conclusion
Improving the performance of planar organo-lead halide perovskite solar cells by using a mixed halide source”. Anion-exchange red perovskite quantum dots with ammonium iodide salts for highly efficient light-emitting devices”. NiOx electrode interlayer and CH3NH2/CH3NH3PbBr3 interfacial treatment to significantly advance hybrid perovskite-based light-emitting diodes”.
All-Solution Processed Quantum Dot Light-Emitting Diodes Based on Hot-Spin Coated Double-Hole Transport Layers with Highly Efficient and Low Turn-On Voltage”. Effects of additive-solvents on the mobility and recombination of a solar cell based on PTB7-Th:PC71BM". Enhanced charge carrier mobility and lifetime suppress hysteresis and improve efficiency in planar perovskite solar cells".
Functionalized PFN-X (X = Cl, Br or I) for balanced charge carriers of highly efficient blue light emitting diodes. Reduced efficiency and increased stability of perovskite light-emitting diodes with multiple quantum wells. Vibrant and fully saturated blue LEDs based on ligand-modified halide perovskite nanocrystals”.
High Efficiency Perovskite Quantum-Dot Light Emitting Devices by Effective Washing Process and Alignment of Interfacial Energy Levels.
