The 81th JSAP Autumn Meeting, 2020
Proposes of new manufacturing process became active for flexible TFT

September 8 - 11, The 81th JSAP Autumn Meeting, 2020 was held by online method. Topics of OLED, organic-TFT, oxide-TFT, perovskite solar cell, and manufacturing technology are closed up based on the proceeding.

Luminous efficiency and lifetime of deep red OLED are enhanced by use of exciplex host

As regards OLED, the research group of Yamagata University reported high luminance efficiency Red OLED, had deep red emission such as neighborhood 670 nm.

In the pilot device, phenanthoroline derivative and arylamine derivative were used as n-type exciplex host and p-type host respectively. In short, exiplex host was applied as host material due to luminous efficiency and lifetime characteristic. As figure 1, NBphen, DPB, and pDPB were used as phenanthoroline derivative material. These n-type hosts and p-type host material NPD were co-evaporated. As a result, co-evaporated layer was formed.

Fig.1 Molecule structure of host material1)

The next, a deep-red OLED was pilot-manufactured by using this co-evaporated emission layer. (DPQ)2Ir(dpm) with emission wavelength 670 nm was used as dopant material. This device is composed of ITO anode (130 nm)/polymer buffer layer (20 nm)/NPD hole transport layer (20 nm)/NPD:phenanthoroline derivative :(DPQ)2Ir(dpm)=50:50:1wt.% (40 nm) emitting layer/NBphen electron transport layer (50nm)/NBphen:20wt.%:Liq (20nm)/Liq buffer layer (1nm)/Al cathode (80nm).

Mw(g/mol) Ip/Eg/Ea(eV) V1,100,1000(V) EQEmax,100,1000(%)  LT80(hs) NBphen 584.2  -6.3/3.0/-3.3 2.41/3.48/6.40 16.7/15.3/11.7  640 DPB 434.2 -6.2/3.1/-3.1 2.48/4.35/7.27 17.4/15.0/11.7  105  pDPB 434.2  -6.2/3.1/-3.1 2.41/4.23/7.58 16.0/13.5/10.3  3 Table.1 Property of host material and device characteristics1)

Optical characteristic was estimated, as a result, in all phenanthoroline derivatives and NPD, exiplex was observed by PL spectrum. On the other hand, maximum external quantum efficiency and emission starting potential of the device with NBphen n-type host were 16.7 %, and 2.4 V respectively. And also, in fixed current density same as 25 mA/cm², lifetime property was greatly improved.

Soluble type electrode with high work function was formed damage-free against organic semiconductor

With respect to organic-TFT, the research group of University of Tokyo, OPERANDO-OIL, JST and NIMS reported a new transfer process. In this process, an another substrate with formed metal electrode in advance by solution process is transferred to an original substrate with organic semiconductor layer due to damage-free against organic semiconductor.

Fig.2 (a)Schematic illustration of the electroless-plated gold contacts fabrication. (b)Transistor characteristic of the monolayer OTFT in the saturation regime. (c)TLM plots for monolayer OTFTs.2)

In this experiment, first of all, a fluorine series polymer (CYTOP) was coated on the glass substrate. And then, vacuum-ultraviolet light (VUV) was irradiated to the substrate by the intermediary of photo mask. The next, a fluorine series F-SAM was deposited on the substrate; as a result, it was selectively deposited on irradiated area only. Subsequently, Nano size Ag grain was coated, as a result, it was selectively deposited on F-SAM film. Furthermore, Au was non-electrolytic plated on Ag grain selectively by work of Ag catalyst. As a result, a metal electrode with high work function was formed in the substrate.

Poly-para-xylene and soluble polymer (PVA) were coated on the substrate and dried, and then, the substrate was released. As a result, Au plated electrode film was completed. The next, p-type organic semiconductor C9-DNBDT-NW was stacked on monolayer crystal film, and then, PVA layer was dissolved in water. As a result, a source/drain electrode was transferred to the substrate.

As figure 2-(b), high carrier mobility same as 13 cm²/V･s was obtained. And also, its contact resistance was relatively lower same as 120Ω･cm. In short, excellent carrier injection property was gained by this soluble electrode (figure 2-(c)).

Anodically oxidized Al2O3 is effective for gate insulator of flexible IGZO-TFT

As concerns oxide-TFT, Kochi University of Technology announced an IGZO (In-Ga-ZnO) -TFT with anodically oxidized Al2O3 gate insulator for flexible devices. As you know, the conventional SiOx gate insulator is not suitable for low thermal resistance substrate because of necessity of high temperature CVD deposition process (400 ℃ order). For this reason, anode oxidation was adopted in this time due to low temperature process.

Fig.4 The results of a positive bias temperature stress test3)

Fig.3 TFT transfer characteristics3)

In this expreriment,Al:NdTi was deposited on a 4-inch glass substrate as a gate electrode, and then, Al2O gate insulation film was formed by anode oxidation method. The next, the substrate was annealed for 1 hour and treated by O2 plasma. Subsequently, IGZO:H (In:Ga:Zn = 6:2:1 at %) was deposited at 20 nm thickness as a channel, and then, the substrate was annealed for 1 hour. The next, SU-8 was coated as a protection film, and then, Mo/Al/Mo source/drain was formed, finally the substrate was annealed for 150 ℃. All patterns were formed by the photolithography method. Channel width and length are 66μm and 22μm respectively.

Figure 3 shows transfer characteristics of IGZO:H-TFT. Excellent properties were obtained, for example, carrier mobility = 24 cm²/V･s, Vth ＝ -0.84V, S factor = 0.19 V/dec, and hysteresis = 0.22V.

Figure 4 shows results of positive bias temperature stress test. It means change of Vth shift (ΔVth) at certain stress temperature (R.T.- 100 ℃) and applied voltage at +12 V for 104 seconds. In R.T.condition, ΔVth was greatly low same as 0.21 V after 104 seconds. By contrast, in high temperature conditions, Vth was shifted negatively until 103 seconds, on the other hand, was shifted positively since then. Furthermore, if stress temperature was increased, this trend became to be noticeable.

Influence of solvent is researched in light absorption layer process of perovskite solar cell using 2 step method

Fig.7 The XRD patterns of (a) PbI2 and (b) Perovskite films.4)

As you know, perovskite solar cell is expected to be a next generation solar cell because of high efficiency. In this category, Aichi Institute of Technolog reported the research result of pilot manufactured device with perovskite light absorption layer by 2 step process.

Deposition method of perovskite layer by solution process is classified into 1 step method using mixing liquid inclusive of all components and 2 step method, which is way to deposit PbI2 film and then treat by MAI. In the former, generation of crystal core and growth of crystal occur in the same time. On the other hand, in the latter, perovskite phenomenon is gradually generated, so that, perovskite grain is enlarged. In short, it's possible to gain excellent perovskite by using this 2 step method. In this research, influence of mixing solvent in precursor liquid using 2 step method was researched.

As table 7 - (a), in case of using mixing solvent of DMSO and DMF, if ratio of DMSO solvent is increased, thickness of PbI2 film was decreased. And also, thickness of perovskite film itself was decreased because of dependence of PbI2 film. As table 7 - (a), if ratio of DMSO solvent is decreased, in spite of increase of thickness, peek strength of PbI2 (001) was decreased. This phenomenon means that amorphous area in the film was increased.

DMSO(vol%) 0 25 50 75 100 Jsc(mA/cm2) 22.4 21.8 21.4 20.3 20.5  Voc(V) 1.05 1.00  0.976 0.930  0.949  FF 0.585  0.583  0.614  0.552  0.551  PCE(%) 13.8 12.7  12.8 10.5  10.7  Table.3 The summary of PV parameters4)

DMSO(vol%) 0 25 50 75 100 PbI2(nm) 261 244 215  154 110 Perovskite(nm) 310 273 261 189  256 Table.2 Comparison of thickness4)

As table 7 - (b), if ratio of DMSO solvent is decreased, peek strength of unreacted PbI2 was decreased. In short, MAI in amorphous area is easy to penetrate compared to crystal area, as a result, unreacted PbI2 is expected to be reduce. And also, as table 3, if DMSO was increased, characteristics of solar cell were degraded because of decrease of Voc and FF.

Grain size of perovskite is enlarged by zone-heating recrystallization

Tokyo Institute of Technology proposed a new manufacturing process (zone-heating recrystallization method：ZHR method) of perovskite layer, in order to enlarge perovskite grain.

This method has been proposed for silicon solar cell primitively. In this method, core growth area is controlled and surface flatness of silicon is enhanced by not only substrate anneal, but also lamp anneal. This technology was adopted for perovskite solar cell due to control of surface flatness and orientation.

Fig.8 Structure image of ZHR system5)

In this experiment, first of all, a glass substrate with ITO transparent conductive film was treated by UV treatment (254 nm, 1 hour). And then, density TiO2 layer was coated as electron transport layer by the spray coating method, and then, annealed at 450 ℃. The next, perovskite (CH3NH3PbI3) was spin-coated on the substrate by 1 step method, and then, annealed at 100 ℃ for 30 minutes. Subsequently, it's treated by ZHR method. Concretely, the substrate was loaded into the ZHR system in Ar environment, and then, annealed at 100 ℃ by use of bottom heater, and then, annealed and scanned by upper lamp. In this time, 3 process conditions such as (i) scanning output of lamp, (ii) scanning speed, and (iii) fixed integrated annealing amount were estimated. Finally, spiro-OMeTAD film and Au film were deposited as hole transport layer and metal electrode respectively.

As a result of observation by SEM, perovskite grain in all samples was enlarged, and its film thickness was kept. In fact, efficiency of all samples was enhanced. However, if treated time is long, device efficiency was decreased. It's reason why decomposition of perovskite is proceeded by uniform annealing of all surface of sample. In short, if the substrate is annealed at high temperature for relatively short time by ZHR method using lamp heating, device quality is enhanced because of enlargement of perovskite grain.

Reference
1)Tsuneyama, et.al.：Stable deep-red phosphorescent OLEDs based on phenanthoroline-based n-type exciplex host, The 81th JSAP Autumn Meeting, 2020, 11-008 (2020.9)
2)Makita, et.al.：High-performance Organic Thin-film Transistors with Electroless-plated Gold Contacts, The 81th JSAP Autumn Meeting, 2020, 11-072 (2020.9)
3)Kouno, et.al.：Low-temperature processed In-Ga-Zn-O thin-film-transistors with an anodize Al2O3 gate insulator, The 81th JSAP Autumn Meeting, 2020, 16-009 (2020.9)
4)Ookawa, et.al.：Effect of mixture solvents for PbI2 film in perovskite layer fabricated using 2-step method, The 81th JSAP Autumn Meeting, 2020, 11-214 (2020.9)
5)Suzuki, et.al.：Relationship between enlarging grain size of perovskite layer and power generation property using zone-heating recrystallization method , The 81th JSAP Autumn Meeting, 2020, 11-215 (2020.9)