The 69th Japan Society of Applied Physics (JSAP) Spring Meeting 2022
Effective proposals in OLED and flexible devices were reported

March 22-26, The 69th Japan Society of Applied Physics (JSAP) Spring Meeting 2022 was held by online method. Main topics of electric devices are closed up based on the proceeding.

Outcoupling-efficiency of OLED is improved by use of low-index porous hole-transport layer

As concerns OLED, the research group of Yamagata University reported that extraction quantum efficiency (EQE) and outcoupling efficiency were further improved by making use of low-index porous hole transport layer (HTL).

In the past, the research group has reduced index of OLED to 1.56 by formation of pillared Nano size phase separation periodic structure, concretely, by co-evaporation of HTL material 2-TNATA (n=1.81@550nm) and low-index fluorine resin perfluoro butenyl vinyl ether (PBVE, n=1.34), while keeping conductive property of film. As a result, EQE and outcoupling efficiency of device have been increased at 1.22 times compared to that of the conventional OLED. In this time, the research group tried to form porous organic semiconductor film by selectively solving and eliminating PBVE only in mixed film, in order to decreasing index further.

In this experiment, TPT1 (HTL) and PBVE were co-evaporated on silicon wafer and COP substrates at 100nm thickness. The next, the substrates were dipped into fluorine series solvent for 5 minutes, as a result, PBVE only was solved and eliminated, and then annealed in vacuum environment. In short, porous HTL inclusive of air (n=1) is formed.

Reference device A with conventional TPT1 as HTL (ITO/TPT1(60 nm)/α-NPD(10 nm)/CBP:Ir(ppy)2(acac)(15 nm)/TPBi(25 nm)/TPBi:Cs2CO3(40 nm)/LiF(1 nm)/Al(100 nm), and this new device with porous HTL (ITO/porous TPT1(80 nm)/α-NPD(10 nm)/CBP:Ir(ppy)2(acac)(15 nm)/TPBi(60 nm)/LiF(1 nm)/Al(100 nm) were pilot-produced.

Figure 1 shows index of TPT1 film, TPT1:PBVE mixed film, and porous TPT1 film. Index of HTL was reduced to n=1.42 by use of porous film. And also, change of thickness before and after porous process was mere 1 nm and under. In short, film thickness was not almost changed. Figure 2 shows SAXS profile of mixed film and porous film. As figure, Nano size periodic structure has been basically kept in porous film.

Fig.1 Anisotropic refractive indices of neat, 50-vol%-PBVE co-deposited, and porous TPT1 films. Inset:Chemical structure of TPT1.1)

Fig.2 SAXS profiles of 50-vol%-PBVE co-deposited and porous TPT1 films. 1)

Fig.3 EQEs of the devices A and B. Inset: Angular distributions of electro luminescence of these devices.1)

Figure 3 shows EQE and angular distributions of electro luminescence. EQE of device B with porous HTL was 32 % at the maximum. As a result, outcoulpring-effieint was increased to approximate 1.4 times by use of porous HTL.

Trade-off problem in release process of flexible device is solved

With respect to flexible device, the research group of The University of Tokyo reported to optimize release process of flexible device with ultra-thin substrate, which was manufactured by the laser transfer method.

Fig.4 Adhesion between parylene and supporting substrate depending on annealing temperature2)

As you know, the laser transfer method is mainly used as manufacturing method of flexible at the moment. Concretely, first of all, a ultra-thin device was manufactured on rigid substrate, such as glass and thick plastic film, and then, it was released from the supporting substrate. However, if adhesion between manufactured device and supporting substrate is higher, device is broken in release process, on the other hand, if adhesion is lower, substrate is released in manufacturing process of device. For this reason, the research group developed highly reliable release process by optimization of process condition of fluorine polymer, which was a release layer.

In this experiment, first of all, a fluorine polymer was spin-coated as release layer due to weakening adhesion between glass and palyrene, which became to be a final substrate. The next, palyrene was deposited at 1.5 μm thickness by the CVD deposition method.

Figure 4 shows adhesion between parylene and supporting substrate depending on annealing temperature. Adhesion of device without annealing was 0.05 MPa, on the other hand, it was increased to 0.40 MPa by 150 ℃ annealing. In short, trade-off problem in release process could be solved by adhesion control using simple anneal process.

ITO electrode of solution-processed oxide-TFT is directly patterned without photoresist, too

As regard oxide-TFT, NHK announced new solution-processed electrode using precursor liquid of metallic oxide.

Fig.5 Positive direct patterning process of ITO electrode3)

In the experiment, first of all, In(NO3)3･xH2O and SnCl2・2H2O were solved into mixed solvent of 2-Methoxyethanol and ethylene glycol, as a result, a precursor liquid was prepared. This liquid was spin-coated on glass substrate, and then, annealed at 250 ℃. The substrate is irradiated with UV light (wavelength 185 nm and 254 nm) by the intermediary of photo mask. Finally, it was wet-etched by use of citric acid. As a result, ITO electrode is formed.

Precursor liquid becomes to be insoluble against etchant by formation of M-O-M bond. If UV light is irradiated, remain solvent is disported, as a result, film density is decreased. In fact, film density was decreased from 3.39 g/cm³ to 3.16g/cm² after irradiation. For this reason, low dense ITO film can be easily etched. In short, ITO electrode is patterned by making use of difference of solubility between irradiated area and non-irradiated area.

The research group developed oxide-TFT with this ITO electrode and sputtering deposited ITZO semiconductor. Its carrier mobility was relatively high same as 15 cm²/Vs at the maximum.

Durability of perovskite solar cell is enhanced by O2 plasma treatment　

Relating to perovskite solar cell, National Institute of Advanced Industrial Science and Technology (AIST) reported that device characteristics were enhanced and manufacturing process time was speeded up by plasma treatment of SnO2 film.

Fig.7 Test result of light resistance4)

Fig.6 J-V curve and parameter of PSCs4)

Pilot-produce device is composed of Au/Spiro-OMeTAD/Cs0.05 (FA0.89MA0.11)0.95Pb (I0.89Br0.11)3/SnO2/FTO. Before deposition of perovskite material, SnO2 film was treated by (a)O2 plasma treatment (30 sec) and UV ozone treatment（20 minutes).

Figure 6 shows J-V curve and parameter. Voc was improved mainly by O2 plasma treatment. Fig.7 shows test result of light resistance. Characteristic of UV ozone treated device was decreased to 80 % compared to initial value after 24h, on the other hand, that of O2 plasma treated device was kept to 90 % after 3000h. In short, light durability is enhanced, and also, treatment process time is speeded up to 40 times by O2 plasma treatment.

Characteristics of perovskite solar cell is improved by surface treatment of electron transport layer

On the other hand, the research group of University of Electro-Communications and Toin University of Yokohama reported about Ag3BiI6 (Silver Bismuth Halide) perovskite solar cell, an ecological lead-free perovskite solar cell.

Fig.8 Change of J-V curve by CsCl treatment5)

In this experiment, liquid inclusive of Titanium diisopropoxide bis(acetylacetonate) and ethanol was spin-coated on the glass with FTO film as dense film, and　then, Ti paste was spin-coated as porous electron transport layer (ETL). The next, CsCl liquid was spin-coated due to surface modification of ETL. Subsequently, Ag3BiI6 liquid inclusive DMSO was spin-coated and annealed as active layer. Finally, P3HT film and Au film were stacked as HTL and electrode respectively.

Figure 8 shows change of J-V curve by CsCl treatment. Jsc, Voc, and FF of CsCl treated device were higher these values of non-treated device, and incident photon-to-current efficiency (IPCE) was 2.33 % at the maximum. In particular, Voc of treated device was greatly increased such as approximate 80 mV among 20 test devices.

MWCNT electrode is directly patterned by use of laser thermal transfer

As concerns carbon nano tube (CNT) electrode, the research group of Tokai University announced laser thermal transfer method, a patterning technology of MW (multi wall) CNT on plastic film directly.

In the experiment, first of all, MNCNT dispersion liquid was sprayed on glass substrate. The next, polypropylene film and cover glass were laminated on this MWCNT film. Subsequently, MWCNT film was transferred to surface of PP film by irradiating CW laser (wavelength 450 nm) from glass side at 1 - 10 mm/s, 0.5 - 3.0 W.

Fig.9 (a)Conductance VS laser power and (b)transfer characteristics of CNT-FET6)

Furthermore, semiconductor type SWCNT was dropped into two MWCNT electrodes, and then, ion liquid was dropped, as a result, electric double layer transistors (EDLT) was manufactured.

As figure 9-(a), conductive property of this device is increased by decrease of scanning speed and increase of laser power. And also, figure 9-(b), current between drain and source was modulated by gate voltage. In short, it's confirmed that this device was normally functioning as p type FET. By the way, carrier mobility and ON/OFF current ratio were 6.5 cm²/Vs and 1.5×10⁴ respectively.

SWCNT film is changed to hydrophilic property by making use of atmosphere plasma treatment

By contrast, the research group of Tokai University reported that adhesion between SWCNT film and solder was improved by surface modification of SWCNT film from hydrophobic property to hydrophilic property.

In this time, SWCNT powder was dispersed into ethanol by ultrasonic sound, as a result, SWCNT dispersion liquid (0.2 wt%) was prepared. SWCNT film was formed by suck and filtration of SWCNT liquid, and then, it's treated by Ar plasma in atmosphere for 3 sec. Finally, melted solder was adhered to SWCNT film.

Picture 1 wettability test of SWCNT films without and with plasma irradiations. Contact angle of water without plasma treatment was 88.4°, on the other hand, it's decreased to 12.8°after plasma treatment, and also, this hydrophilic effect was kept for 4 weeks.

Picture.2 shows wettability test of SWCNT film with solder. Compared to with and without plasma irradiation, adhesion of the former was higher than that of the latter. In short, SWCNT film and solder were confirmed to be sufficiently adhered.

Reference
1)Aoki, et.al.：Super-low-index porous hole-transport layer for high-outcoupling-efficiency OLEDs, The 69th JSAP Spring Meeting, 2022, 11-030 (2022.3)
2)Sumi, et.al.：Adhesion evaluation between an ultrathin substrate and supporting substrate for transfer integration of ultrathin devices, The 69th JSAP Spring Meeting, 2022, 11-077 (2022.3)
3)Tanaka, et.al.：Development of solution-processed electrode by positive direct patterning, The 69th JSAP Spring Meeting, 2022, 16-026 (2022.3)
4)Yamamoto, et.al.：Stability of Perovskite Solar Cells using Plasma Treatment Electron Transport Layer, The 69th JSAP Spring Meeting, 2022, 11-214 (2022.3)

Pic.1 Wettability test of SWCNT films without and with plasma irradiations. (a)Without irradiation, and with irradiation after (b) a minute, (c)1 week, (d)2 weeks, (e)3 weeks, and (f)4 weeks. 7) Pic.2 Wettability test of SWCNT film with solder. Without plasma irradiation:(a)side view and (b)reverse view. With plasma irradiation:(c)side view and (d)reverse view7)

5)Sanehira, et.al.：Surface Modification of Electron Transport Layer for Improving Photovoltaic Properties of Lead-free Perovskite Solar Cells Using Silver Bismuth Halide, The 69th JSAP Spring Meeting, 2022, 11-241 (2022.3)
6)Sugita, et.al.：Nanotube wiring on plastic films by laser thermal transfer, The 69th JSAP Spring Meeting, 2022, 15-037 (2022.3)
7)Miura, et.al.：Improvement and application of wettability in carbon nanotube films using atmospheric pressure plasma jet irradiation, The 69th JSAP Spring Meeting, 2022, 15-141 (2022.3)