The 72th JSAP Spring Meeting 2025
Valuable reports about perovskite solar cells have been proposed one after the other

March 14 - 17, The 72th JSAP Spring Meeting 2025 was held in Noda Campus, Tokyo City University of Science & Online. Topics of oxide-TFTs and perovskite solar cell are closed up.

AlOx film is stacked on ZnO active layer due to enhancement of characteristics

As respect to oxide-TFTs, Osaka Institute of Technology reported an original AlOx/ZnO-TFT for backplane of flexible OLED.

Fig.1 Fabricated TFTs1)

Reason of stacked AlOx film is to passivate ZnO channel and prevent Al atom diffusion from upper Al source/drain into channel layer. In this experiment, ZnO film (20nm) and AlOx (2nm) film were deposited on silicon wafer with thermal SiO2 film by the pulse laser deposition method.

Figure 2-left shows sheet resistances. Sheet resistance of as-deposited ZnO film was low same as 11.1kΩ because of much oxygen defects, however, it's increased to 70.0MΩ after 200℃ anneal. Furthermore, after deposition of AlOx film, it's decreased to 62.3kΩ again, and also, decreased to a few MΩ by 200℃ anneal. In short, there is a possibility of control oxygen defects by stacking structure of oxide semiconductor.

Figure 1 shows structure of 4 fabricated bottom gate TFTs with or without O2 gas. Figure 2-roght shows transfer characteristics of TFT without O2 gas (most right figure). Single ZnO-TFT had sufficient drain current, however, was not driven as transistor. After 200℃ anneal, high ON/OFF current ratio same as 10⁶ was obtained because of reduction of oxygen defects. On the other hand, in AlOx/ZnO-TFT, relatively much drain current same as a few dozen μA/μm was gained, too.

Among 4 TFTs, highest quality (high current density and low leak current) was TFT with deposited ZnO and AlOx films using O2 gas (most right figure).

Fig.2 Sheet resistances (left) and Transfer characteristics for TFTs (right)1)

The research group fabricated a top gate AlOx/ZnO-TFT on glass substrate, in order to estimate availability of stacking structure. In this TFT, ZnO film and AlOx film were deposited at room temperature. Its carrier mobility was increased from 5.65cm²/V･s (single ZnO-TFT) to 9.67cm²/V･s. And also, high ON/OFF current ratio same as 10⁶ was obtained. In short, its characteristics are matchable with that of existing IGZO-TFT for 4K display.

Next generation display such as 4K&480Hz display is driven by high mobility oxide-TFT

On the other hand, Hokkaido University announced an original oxide-TFT, which was corresponded to next generation display such as 8K&480Hz display.

It's necessary to realize high carrier mobility same as 70cm²/V･s and over for driving of this high-end display. In short, it's impossible to correspond by the existing IGZO-TFTs. Therefore, the research group developed an original poly crystal In2O3:H-TFT using H2 doping by devise of manufacturing process. As a result, high carrier mobility same as 140cm²/V･s was obtained. This value is comparable with that of LTPS-TFTs.

Fig.3 Transfer characteristics of typical In2O3-TFT passivated with Y2O3 films. (left) After positive bias stress, (right) after negative bias stress.2)

Concretely, originally developed In(OH)3 target material was used, due to effective introduce of H2. Oxide film was deposited by the sputtering method using Ar, O2, in addition to slight H2. At as-depositon state, it's amorphous film, and also, uniform without grain boundary. The next, it's annealed at 200℃. After completion of oxide-TFT, finally, it's annealed at 300℃. As a result, its film becomes from amorphous state to polycrystal state with large grain and less grain boundary. In a word, large crystal are uniformly gained.

As above, carrier mobility of pilot-produced bottom gate type TFT (ITO gate/Al2O3 gate insulator/ITO source, drain/In2O3:H channel) is approximate 10 times compared to that of IGZO-TFT. However, bias stress shift was much because of absorption and desorption of gas in boundary of solid at applied voltage by existence of H2.

For this reason, surface channel was passivated by epitaxial Y2O3 film or epitaxial Er2O3 film, which were lattice matched with In2O3. Figure 3, high reliable oxide-TFT without or minimum Vth shift was obtained with maintenance of high carrier mobility same as 70cm²/V･s.

By the way, in case of using popular HfO2 and Al2O3 as passivation material, manufactured oxide-TFT was unstable with much Vth shift.

PCE of perovskite solar cells is improved by stacked SnO2 layers with different surface flatness

As concerns perovskite solar cells, Kyushu University reported that device characteristics was influenced by structure of SnO2 electron transport layer.

Fig.5 Surface roughness of 2nd SnO2 and anneal temperature3)

Fig.4 Surface roughness of 2nd SnO2 and anneal temperature3)

In the past, the research group increased PCE (Power Conversion Efficiency）of perovskite solar cell by forming flat SnO2 film and rough SnO2 film in different humidity environment as electron transport layer. However, it's difficult to control humidity in practical. Therefore, in this time, the research group tried to control SnO2 structure by annealing temperature instead of humidity.

In this experiment, a test device (ITO/1st SnO2 ETL/2nd SnO2 ETL/FAPbI3 layer/spiro-OMeTAD hole transport layer/Au was pilot-produced on the glass substrate. As electron transport layer, commercial Nano size SnO2 water solution was spin-coated, and then, annealed at 150℃ due to formation of 1st flat SnO2 film, and annealed at 80 - 150℃ due to formation of 2nd rough SnO2 film.

Fig.6 Durability (100mW/cm2 LED+MPPT+25℃)3)

As a result, PCE of reference device with single SnO2 layer at 150℃ annealing was 19.4%. And also, PCE was dramatically changed by anneal temperature of 2nd SnO2 film. and maximum PCE same as 20.3% was obtained by 120℃ anneal of 2nd SnO2 film (figure 4).

Surface of SnO2 film was observed by use of AFM (Atomic Force Microscope), due to research of reason of changed device characteristics. As a result, surface roughness was changed by anneal temperature. As figure 5, surface roughness was highest at 120℃ anneal, and also, PCE was highest at 120℃ anneal, too. This is reason why compared to 150℃ anneal device, vaporization speed of water solvent from SnO2 film becomes slow, as a result, surface roughness is increased by agglomeration SnO2 grains. In short, contact area of SnO2 and perovskite is increased by increase of surface roughness, and then, extraction efficiency is improved.

After anneal at 120℃ of 2nd layer, furthermore, device was additionally annealed, due to remove of residual moisture in SnO2 film. As a result, PCE was increased from 20.45% to 22.75%. Moreover, as figure 6, durability was enhanced, too. These results mean that fusion property of SnO2 grains is progressed.

PVK layer is directly patterned by mask + UV/ozon treatment

Toin University of Yokohama proposed direct coating/patterning method using mask + UV/ozon treatment as patterning method of perovskite layer in serial connection perovskite solar cells, instead of the laser etching method, the mechanical scribing method, and so on.

As you know, it's necessary to pattern perovskite layer due to serial connection. As patterning method, etching type such as the laser etching method and the mechanical scribing method and direct coating/patterning type such as the ink-jet printing and the mask-through evaporation method are known. The former is mainly adopted for the present moment. However, in case of the former, after etching process, etching residual dross remains on the substrate by flying particles. Therefore, the research group developed this method as a kind of the latter.

Fig.7 Film thickness of each line and difference of wettability by UV ozone treatment4)

In UV ozon treatment, organic materiel is removed on surface of the substrate by action of UV light and active oxygen species, and also, wettability is increased by generation of functioning group. In this research, a mask with open area pattern is set on the substrate. And then, it's treated by UV ozon treatment. As a result, wettability is greatly changed by treated area or untreated area. The next, perovskite precursor solution is spin-coated on the substrate, as a result, it's adhered into treated area selectively.

Figure 7 shows film thickness of each line and difference of wettability by UV ozone treatment. By the way, after anneal, patterned shape of perovskite line is kept without change.

In this process, uniformity is influenced by shape of under layer. Concretely, if solution is spread into direction without step, edge bead is generated. On the other hand, if solution is spread into direction with step, edge bead is not generated. As a result, device characteristics and uniformity are greatly different. For this reason, it's necessary to devise direction of movement of solution.

However, there is most problem in the spin-coating process. By contrast, in sweep coating method such as the slit-coating method, the bar coating method, it's not necessary to devise about this problem because of coating unidirectionally.

Perovskite cell without carrier transport layers is obtained by use of SAMs

The research group of Toin University reported research result of perovskite cell without carrier transport layers. Oriented p-type SAM (Self Assembled Monolayers) and n-type SAM were introduced into bonded interface of cathode/perovskite and into bonded interface of anode/perovskite unsymmetrically, due to selective transport control of electron and hole. As figure 9, perovskite solar cells were pilot-produced by use of conventional perovskite materials.

Fig.9 PCE of perovskite solar cells5)

Fig.8 Microscope image of pilot-produced cell without carrier transport layers5)

PCE of p-type SAM only device, n-type SAM only device, and p-type SAM + n-type SAM device were 16.6%, 12.5%, and 19.1% respectively. This device has low hysteresis property and high long term stability, such as hundreds time. This is reason why it's not influenced by change of physical property based on carrier transport layers. In short, high durability is obtained, compared to the conventional device using carrier transport layers.

Perovskite layer is passivated by deposition of phthalocyanine at thick film

Toin University of Yokohama proposed to passivate perovskite layer by despotion of phthalocyanine at thick film.

As you know, halogen anions such as I- and Br is inclusive of halide perovskite crystal. In this crystal, ion defect is generated by desorption of halogen anion, especially, these defects are generated on surface of perovskite crystal layer in high volume by anneal at deposition. It's effective to introduce passivation layer, due to compensation of these defects. However, if existing materials such as polymer and SiOx are used, it's necessary to deposit at ultra-thin film because of its insulation property.

Fig.10 J-V characteristic6)

Therefore, the research group tried to deposit Ga phthalocyanine hydroxide (OHGaPc) at relatively thick film. Unlike a existing material such as insulation materials, it has carrier transport function because of p-type semiconductor and compensation function of defects on surface of perovskite crystal because of exsistance of hydroxide. In short, it's possible to transport carrier and passivate perovskite layer.

First of all, OH group in phthalocyanine was confirmed to be interact with perovskite. After spin-coating on perovskite layer, it's confirmed to be oriented on the surface.

Figure 10 shows J-V characteristics of test device with OHGaPc. PCE (Power Conversion Efficiency) of reference without OHGaPc was 21.8%, on the other hand, that of test device with OHGaPc was 21.0%. This was reason why serial resistance was increased by use of OHGaPc. However, compared to device with existing passivation materials at same thickness, it's effective for passivation material.

Not reproducibility, but also, light durability and water resistance are enhanced by thick film of passivation film. In experiment of light irradiation test, PCE was lower degraded compared to that of reference without passivation layer. This is reason why movement of halogen is suppressed in perovskite layer.

Perovskite is vertically grown by spin-coating in N2 gas-flow

Fig.11 XRD patterns of (C4H9NH3)2(CH3NH3)Pb2I7 thin films prepared (a) with N2-gas flow and (b) without N2 gas-flow during spin-coating.7)

Gifu University reported research result of spin-coated sheathed (C4H9NH3)2(CH3NH3)n-1PbnI3n+1(n＝1～4) in N2 gas-flow state by the poster publication.

In this experiment, PbI2, C4H11N･HI, and CH5N･HI as solute, and C3H7NO (DMF) as solvent were used. This solution is spin-coated on glass substrate with SnO2 film in N2 gas-flow. This is reason why perovskite is not horizontally but vertically grown. As a result, transport of carrier is not blocked.

Figure 11 shows XRD patterns of (C4H9NH3)2(CH3NH3)Pb2I7 thin films prepared with N2-gas flow (a), and without N2 gas-flow during spin-coating (b). As figure 11-(b), in the latter, Pb-I film was horizontally grown against the substrate. On the other hand, as figure 11-(a), it's coated in N2 gas-flow, Pb-I layer was vertically grown.

Thermal stability of Sn perovskite solar cells is enhanced by original approaches

The University of Electro-Communications reported enhancement result of high thermal stability of Sn perovskite solar cells by three approaches.

As you know, Sn series perovskite solar cells are not sufficient for PCE and durability. Its reason is mainly caused by oxidation of Sn²⁺.

Therefore, the research group devised as the following methods, in order to enhance high heat resistance. First of all, ALD (Atomic Layer Deposition) deposited SnOx film was inserted between C60 and BCP as ion diffusion layer in the reversed structure device (FTO/hole transport layer/perovskite/PCBM/C60/BCP/Ag), due to suppress of diffusion of iodine. As a result, keep ratio of PCE in 8℃ and N2 environment was improved from 7% (ALD SnO2-free) to 61% by suppress of corrosion of electrode.

The next, PEDOT/PSS with toluene was used as hole transport material instead of PEDOT/PSS with water, due to suppress of separated iodine, which was caused by remain moisture. As a result, decrease ratio of iodine was reduced from 12% to 1% and under in durability test.

Furthermore, a new solvent (secret composition) into perovskite precursor solution was used instead of DMSO. This solvent is reacted with SnI2, and then, stable complex is obtained. For this reason, crystallization speed of perovskite becomes slower. In short, oxidation of Sn²⁺ is suppressed. In high temperature environment, amount of Sn²⁺ in device using DMSO was reduced at 5.6%, on the other hand, that in device using new solvent was increased at 2.7%. And also, in high temperature environment for 430h, keep ratio of PCE was greatly enhanced from 40% to 81%.

Reference
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7)Oshima, et.al.：Orientation control of (C4H9NH3)2(CH3NH3)Pb2I7 thin films by gas-flow assisted spin-coating method, The 72th Japan Society of Applied Physics (JSAP) Spring Meeting 2025, 10-042 (2025.3)
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