Applied Science and Convergence Technology 2022; 31(5): 116-119
Published online September 30, 2022
Copyright © The Korean Vacuum Society.
aDepartment of Physics, Kunsan National University, Gunsan 54150, Republic of Korea
bCenter for Composite Materials and Concurrent Design, Sungkyunkwan University, Suwon 16419, Republic of Korea
TiO2 thin film typically used as an electron transport layer (ETL) in perovskite solar cells has many advantages such as high refractive index, good photocatalytic properties, excellent chemical stability, and low cost. TiO2 thin films are typically formed by a solution process, but it is difficult to achieve large area coating, accurate thickness control, and mass production with this process. Here, we demonstrated that radiofrequency (RF)-sputtered TiO2 is an effective replacement for use as the ETL for the perovskite solar cells in order to overcome the disadvantages of the solution process. The TiO2 layer was deposited on the substrate by reactive sputtering (RS) with a high-purity Ti metal target. The RFRS TiO2 thin film was systematically characterized and compared with spin-coated TiO2 by means of various analytical techniques. The transmittance of 20-nm-thick as-deposited TiO2 on an indium tin oxide (ITO)-coated glass substrate was 75−80 % in the visible range. After annealing, the amorphous phase was converted into an anatase structure in RFRS TiO2. In the case of the spin-coated TiO2 layer, indium diffusion from the ITO layer and an increase in sheet resistance with the annealing temperature were observed. On the other hand, the RFRS TiO2 had a denser and more uniform surface, and it could be annealed at a lower temperature.Therefore, it was able to block indium diffusion and increase the
Keywords: Perovskite solar cells, TiO2 thin film, RF reactive sputtering, Electron transport layer, Large-area thin-film coating
Organic-inorganic hybrid perovskite solar cells (PSCs) are thirdgeneration solar cells that are proving to be promising replacements for silicon solar cells. Despite their simple manufacturing process at a low temperature, PSCs have recently achieved their best efficiency of 25.7 %, which is comparable to that of silicon solar cells . Titanium dioxide (TiO2) films have been widely used as electron transport layers (ETLs) for sensitized solar cells because of their thermal and chemical stability, as well as a band alignment suitable for solar cells . Several methods are available for TiO2 thin film fabrication, such as solgel, sputtering, chemical vapor deposition, and spray coating [3–7]. While the sol-gel method is inexpensive and can produce a uniform thin film , it is difficult to control the thickness and apply it to a large-area application because the sol-gel method is based on the spincoating method . Furthermore, the heat treatment process involved can lead to crystallization  as well as the formation of cracks or pinholes due to thermal stress [10,11]. Therefore, a magnetron sputtering method has been proposed as a solution [12–15]. In this method, TiO2 is formed through sputtering by reacting a TiO2 ceramic target or a Ti pure metal target with oxygen . Generally, ceramic targets are expensive and fragile, and direct current (DC) power cannot not be used due to the movement restriction of electrons in non-metallic materials . However, for reactive sputtering (RS) using the Ti metal target, both DC power and radiofrequency (RF) power can be utilized. The RF sputtering method facilitates the adjustment of the film thickness through a slow deposition process , and the energy of the sputtered ions reaching the substrate helps the formation of a uniform thin film [18,19]. Moreover, sputtering is performed at a lower temperature than that in other methods, so that it can be applied when using a substrate that is sensitive to heat .
In this work, we propose the application of RFRS TiO2 to multiple cation/anion CsFAMAPb(IBr)3 (CFM)-based PSCs. Although RS with a metal target generally uses DC power, we used RF power in this experiment. This is because a TiO2 phase is formed on the surface of the Ti target during DC sputtering, making it difficult to deposit a uniform and reproducible TiO2 thin film. The RFRS TiO2 layer was optically and electrically analyzed, and then compared with general spin-coated TiO2. Structural characteristics were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The optical transmittance of the TiO2 layers was measured by UV-Vis spectroscopy. Since RFRS produces a thin and uniform TiO2 layer, it led to better device performance depending on the post-annealing treatment. In addition, RFRS TiO2 successfully blocked indium diffusion from the indium tin oxide (ITO) layer during high-temperature annealing. Finally, an improved power conversion efficiency (PCE) of 17.46 % was achieved by using RFRS TiO2 in the CFM-based PSCs.
The TiO2 films were prepared on 1.5 × 1.5 cm2 ITO-coated glass substrates. The ITO substrates were cleaned prior to coating by using an ultrasonicator with surfactants, deionized water, acetone, and ethanol sequentially, followed by blow-drying with N2 gas. The RFRS TiO2 was deposited on the substrate using a Ti metal target (99.99 %, Materion) with an RF power of 80 W under 2.5 sccm O2 gas flow. The base and working pressures were 5.0 × 10−8 Torr and 3.0 mTorr, respectively. For the spin-coated TiO2, the cleaned ITO substrate was further UV/ozone treated for 30 min. The TiO2 solution was then spin-coated onto the treated ITO substrate at 7,000 rpm for 8 s, 1,000 rpm for 10 s, and 2,000 rpm for 40 s, and then dried at 125 °C for 5 min . The spin-coating procedure was repeated twice to ensure the appropriate thickness, following which the post-annealing treatment was performed at 450 °C for 1 h in the furnace. The TiO2 solution consisted of 1:0.15 mol of 1-butanol (99.8 %, Sigma Aldrich) and titanium diisopropoxide bis(acetylacetonate) (TTIP, 75 wt% in isopropanol, Sigma Aldrich), and used after stirring for 2 h.
The structure of the CFM based PSCs was Au/Spiro-OMeTAD/CFMbased perovskite/TiO2/ITO on glass. A perovskite precursor solution was prepared, which contained the following solvents: lead(II) iodide (PbI2, 99.999 %, Sigma Aldrich), lead(II) bromide (PbBr2, > 98.0 %, TCI), formamidinium iodide (FAI, Greatcellsolar), methylammonium bromide (MABr, Greatcellsolar), and cesium iodide (CsI, > 99.0 %, TCI) in
The transmittance of the TiO2 films was measured using a UV-Vis spectrometer (Hitachi, UH4150). The surface morphology and thickness of the TiO2 layers were monitored by field-emission scanning electron microscopy (FE-SEM, Hitachi, SU8220). The TiO2 structure was confirmed by using a high-resolution X-ray diffractometer (HR-XRD, Malvern Panalytical, Empyrean). The indium concentration and depth profile of the TiO2 layer were analyzed using an Xray photoelectron spectrometer (Ulvac-PHI, Japan). Sheet resistance measurements were carried out using a 4-point probe system (AIT, SR1000N), and the performance of the solar cell was measured using a solar simulator under air mass (AM) 1.5 G solar illumination (class AAA) equipped with a 450 W Xenon Arc Lamp 6280NS.
Figure 1(a) shows an RF reactive magnetron sputtering system using a Ti metal target. Radio waves ionized the inert gas (Ar) through plasma, following which the ionized Ar atoms bombarded the target material. The sputtered atoms were deposited on the ITO substrate, where they reacted with active O2 gas to form a TiO2 layer coating the substrate. Figure 1(b) shows the transmittance spectra of spincoated TiO2 after annealing for 1 h at 450 °C (black solid line) and as-deposited RFRS TiO2 (red solid line). RFRS TiO2 and spin-coated TiO2 showed a similar transmittance (75–80 %) in the range between 350 to 900 nm. As seen in the SEM images in Fig. 2, the thickness of the spin-coated TiO2 was approximately 50 nm [Fig. 2(b)], while the thickness of the RFRS TiO2 was 20 nm, which was less than half of that of the spin-coated TiO2 film [Fig. 2(d)]. Despite this difference in thickness, the similar transmittance values of the two samples implied that the post-annealed spin-coated TiO2 contains more anatase structures than does the RFRS TiO2 . However, as the thickness of TiO2 increased, the series internal resistance increased and short-circuit current density decreased ; thus, we expect that RFRS deposition of a thin TiO2 layer would positively impact device performance. The transmittance curve of RFRS TiO2 annealed under the same condition as spin-coated TiO2 (450 °C for 1 h) is shown in Fig. 1(b). As shown in Fig. 1(b), the transmittance of RFRS TiO2 (red solid line) seems to be slightly improved compared to before annealing due to the change in crystallization from amorphous to anatase structure. Additionally, the surface of the spin-coated TiO2 is more granular and has a multitude of pinholes on the surface as shown in Fig. 2(a). In contrast, the RFRS TiO2 film has a denser and more uniform surface [Fig. 2(c)], which improves the hole blocking capability and shunt resistance [24,25]. Figures 2(e) and 2(f) show the top-view and cross-sectional SEM images of RFRS TiO2 layer annealed at 450 °C for 1h, respectively. Even after annealing at 450 °C for 1h, the morphology was uniform without significant damage and the thickness was maintained at 20 nm.
Before characterizing the device performance, we performed XRD analysis to investigate the structure of TiO2 corresponding to each method. All the TiO2 samples were prepared on an ITO substrate. The spin-coated TiO2 showed an XRD peak at 25.5° [Fig. 3(a)]. Diffraction pattern at 25.5° is related to the anatase (101) TiO2 structure . Therefore, the main peak at 25.5° signifies that the anatase structure is dominant in the spin-coated TiO2, which is consistent with the transmittance results in Fig. 1. In contrast, as-deposited RFRS TiO2 showed no signal initially, but after annealing at 450 °C for 1 h, a new peak appeared at 25.5° [Fig. 3(b)]. Since the anatase TiO2 structure is generally formed at high temperatures, it can be concluded that a transition from the amorphous to the anatase phase occurred during the postannealing process in RFRS TiO2 .
Further, the annealing treatment may cause indium diffusion at the TiO2-ITO interface as well as structural conversion of TiO2 . ITO has been widely used as a solar cell substrate owing to its high transmittance and low roughness , but it is affected by temperature to a greater extent than is fluorine-doped tin oxide (FTO) . Figure 4(a) illustrates the XPS depth profile, which represents the indium atomic concentration according to the depth of the TiO2/ITO structure. Since there is a difference in thickness between the spin-coated TiO2 layer and the RFRS TiO2 layer, the monitoring depth was normalized for the respective thickness of each film. In the case of spin-coated TiO2, the indium atomic concentration increased in the TiO2 region (green dots) post-annealing at 450 °C. In contrast, the RFRS TiO2 displayed a near-zero indium concentration near the ITO region despite the heat treatment at 450 °C; this result indicates that the RFRS TiO2 was sufficiently dense to block indium diffusion. We also measured the sheet resistance of the ITO substrate depending on the annealing temperature, as shown in Fig. 4(b). After annealing at 250, 350, and 450 °C, the sheet resistance of ITO tended to increase to 35 Ωsq−1 as the temperature increased, and the color of ITO became slightly yellow, as shown in the inset of Fig. 4(b). In particular, the resistance change increased steeply when the temperature was increased from 250 to 350 °C. The increase in sheet resistance is known to be mainly due to a reduction in oxygen vacancies . In other words, the increase in sheet resistance with increasing temperature was caused by both indium diffusion as well as the reduction of oxygen vacancies in the TiO2/ITO structure.
Finally, we characterized device performance using the spin-coated TiO2 and RFRS TiO2; the device structure was Au/Spiro-OMeTAD/ CFM based perovskite/TiO2/ITO, as illustrated in the upper inset of Fig. 5(a). In addition, the statistical efficiency data of PSCs are shown in the lower inset of Fig. 5(a). The current density-voltage (
Figure 6 shows the
We applied an RFRS TiO2 ETL to CFM-based PSCs and systematically characterized it in comparison with spin-coated TiO2. The RFRS TiO2 had a thickness of 20 nm, with a transmittance comparable to 75−80 % in the visible range. After annealing, the amorphous phase was converted to an anatase structure in RFRS TiO2. In the case of spin-coated TiO2, indium diffusion and an increase in sheet resistance with the annealing temperature were observed. However, the RFRS TiO2 had a denser and more uniform surface, because of which it could block indium diffusion, thereby increasing the
This research was supported by the Korea Electric Power Corporation (Grant number: R20XO02-8).