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Sol-gel processed niobium oxide thin-film for a scaffold layer in perovskite solar cells
Thin Solid Films 674 (2019) 7–11
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Sol-gel processed niobium oxide thin-film for a scaffold layer in perovskite solar cells
T
Eiichi Inamia,b, , Takamasa Ishigakib,c, Hironori Ogatab,c ⁎
a
School of Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan Research Center for Micro-Nano Technology, Hosei University, 3-11-15 Midori-cho, Koganei, Tokyo 184-0003, Japan c Faculty of Bioscience and Applied Chemistry, Department of Chemical Science and Technology, Hosei University, 3-7-2, kajino-chou, koganei, Tokyo, 184-8584, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Perovskite solar cell Niobium oxide Scaffold layer Charge transport Interface engineering
Sol-gel processed niobium oxide (Nb2O5) thin-film was synthesized for a scaffold layer (SL) of methylammonium lead iodide-based perovskite film. Complementary characterizations, including scanning electron microscopy, Xray diffraction, optical absorption spectroscopy have revealed that the Nb2O5 SL facilitates efficient crystallization of the perovskite film compared to the conventional mesoporous titanium oxide (m-TiO2) SL. Also, photoluminescence spectroscopy revealed the higher electron-extraction capability at the perovskite/Nb2O5 interface than the perovskite/m-TiO2 interface. The present sol-gel processed Nb2O5 SL and the superior optoelectronic properties of the overlying perovskite film suggest a promising avenue for high-performance and cost-effective perovskite solar cells.
1. Introduction
scaffold layer (SL) of perovskite film plays vital roles not only in selective extraction of the photo-excited electrons as an electron transport layer (ETL), but also enhanced crystallizations and morphological evolutions of perovskite film [24]. Several metal oxides, such as titanium oxide (TiO2) [12,20,25–29], aluminum oxide (Al2O3) [11,30,31], and zinc oxide (ZnO) [19,32] have been applied for the SLs. The most efficient cell reported thus far was based on a mesoporous (m)-TiO2 SL [12,20,25–29]. As the other candidate, niobium oxide (Nb2O5) is expected to exhibit greater blocking effect than TiO2 [33]. So far, Nb2O5 has been applied mainly in dye-sensitized solar cells, where the shortcircuit photocurrent (Jsc) and open-circuit voltage (Voc) were improved by effective blocking of the recombination [33–42]. More recently, Nb2O5 film has started to be applied as a blocking layer in a PSC [31,43,44]. They found that, the cell with Nb2O5 blocking layer exhibits larger Voc than that with TiO2 blocking layer, while Jsc is reduced due to its high resistance and capacitance[31]. Further, Nb2O5 film has also been revealed to benefit as a ETL [45]. It was demonstrated that the Nb2O5 films fabricated by magnetron sputtering show high electronextraction-property and electron mobility regardless of the crystallinity. This suggests that Nb2O5 SL with high performance is realizable even with room-temperature treatment, that is great advantageous in lowcost PSC processing. In this study, we investigated the performance of the Nb2O5 SL fabricated by sol-gel methods, that is further low-cost processing than the magnetron sputtering technique. For comparison, three kinds of
In the past few years, organic-inorganic halide perovskite solar cells (PSC) have attracted widespread interest as a low-cost and solution processable energy devices. Following the first application of the PSC in 2009 [1], the power conversion efficiency of 3.81% has rapidly improved to over 20% at present [2]. Such developments are owing to their excellent optoelectronic properties, such as strong optical absorption in visible and ultraviolet light range [3], tunable band gap [4,5], small exciton binding energy [6,7], and high mobility charge carrier with a lifetime of a few hundreds nanosecond [8–10]. Therefore, fabricating highly crystalline, homogeneous and planar perovskite thinfilms has been a successful strategy for utilizing full potential of perovskite materials and improvements of the device performance. Nowadays, several sophisticated processes for synthesizing perovskite materials have been reported, e.g., varying the compositions [11–13], adding additives [14,15], employing different solvents [16]. Independently, the following deposition processes have also been developed, e.g., fast deposition-crystallization (FDC) method [17,18], two step sequential deposition method [19–21], vacuum evaporation deposition method [11,22], and vapor assisted solution process [23]. These studies focused on optimizing crystallizations and thin-film formation kinetics of perovskite materials itself. Alternatively, the substrate surface modification is also a possible approach for morphological control of perovskite films. It is known that ⁎
Corresponding author at: School of Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan. E-mail address: [email protected] (E. Inami).
https://doi.org/10.1016/j.tsf.2019.01.043 Received 2 August 2018; Received in revised form 26 December 2018; Accepted 22 January 2019 Available online 29 January 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Top-view scanning electron microscopy images of fluorine doped tin oxide substrate. (b-d) Top-view scanning electron microscopy images of three scaffold layers on fluorine doped tin oxide substrates: (b) compact titanium oxide, (c) mesoporous-titanium oxide/compact titanium oxide, (d) niobium oxide/compact titanium oxide. Inset in (c) is a close-up view of the mesoporous-titanium oxide scaffold. (e-g) Top-view scanning electron microscopy images of perovskite films on (b-d), respectively.
films [Nb2O5 film, m-TiO2, and compact (c)-TiO2 film] were fabricated, and compared the morphological and optoelectronic properties of their overlying perovskite films. Complementary characterizations, including scanning electron microscopy (SEM), X-ray diffraction (XRD), optical absorption spectroscopy, and photoluminescence (PL) spectroscopy have revealed that Nb2O5 scaffold shows the most prominent electron extraction property. Interestingly, such feature was revealed to be assisted by the highly crystalline perovskite thin-film, that is, the Nb2O5 film functions assisting crystallization of the perovskite film as well as efficient ETL. Our findings suggest that the Nb2O5 SL is the one possible candidate toward fully solution-processable PSC devices.
on the top of the c-TiO2 layer in an argon-filled glovebox. Then, the substrates were dried at 120∘C for 5 min and subsequently sintered at 500∘C for 1 h in air [31]. On the top of these three kinds of SLs, methylammonium lead iodide (CH3NH3PbI3) perovskite films were prepared by FDC method [17,18] in the glove box. A N, N-dimethylformamide (DMF) solution of CH3NH3PbI3 with concentration of 45 wt% were dropped onto the SLs and the substrates were spun at 5000 rpm. Just after the change in the substrate color from yellow to white, chlorobenzene (500μL) were immediately dropped onto the substrates. Dropping the second solvents further changed the substrate color from white to light brown. The obtained films were then dried at 100∘C for 10 min, resulting in the black color perovskite films.
2. Experimental details
2.2. Characterization details
2.1. Sample preparations
The local morphologies of the fabricated films were characterized by SEM (Hitachi SU-8020). XRD patterns were obtained using Rigaku Smartlab system with Copper K-alpha radiation. Optical absorption spectra of the perovskite films were measured using UV–vis spectrometer (Jasco V-770). PL spectra were measured using Spex Fluorolog 3–21 spectrofluorometer.
To investigate the scaffold-dependent optoelectronic properties of the perovskite materials, we first fabricated three c-TiO2 blocking layers, on two of which m-TiO2 and Nb2O5 SLs were further fabricated, preparing bare c-TiO2, m-TiO2/c-TiO2, and Nb2O5/c-TiO2 (i.e., preparations of bare blocking layer and different SLs on same blocking layer). Then, morphological/optoelectronic properties of perovskite films on the three scaffolds were compared. The preparation of each layer was carried out according to the conditions considered to exhibit the most excellent electron transporting properties which has been reported so far. Fluorine doped tin oxide (FTO) films on glass substrates (16 cm × 16 cm2) were ultrasonic-cleaned sequentially in acetone, ethanol and distilled water for 15 min, and then treated by a ultraviolet (UV)-ozone cleaner (Bio Force Nanosciences Inc.) for 20 min. c-TiO2 film was prepared on the FTO substrate by spin-coating (2000 rpm for 30 s) a mixture solution of 0.2 M titanium (IV) isopropoxide (Aldrich) and 0.4 M acetyl acetone (Aldrich) dissolved in ethanol [46], and then dying at 80∘C for 5 min. The process was repeated three times to avoid leakage current due to the pinholes. The films were finally sintered at 450∘C for 30 min to form 90-nm-thick c-TiO2 film [46]. m-TiO2 film was prepared by spin-coating (2000 rpm for 30 s) TiO2 paste (PST-18NR, Nikki Syokubai Kasei) diluted in ethanol (paste: ethanol = 1: 3.5 wt ratio) on the top of the c-TiO2 layer, and subsequent sintering at 550∘C for 30 min in air [46–48]. Nb2O5 film was prepared by spin-coating (4000 rpm for 90 s) 0.3 M niobium ethoxide (Wako) diluted in ethanol
3. Results and discussions 3.1. Scaffold-dependent properties of perovskite films Fig. 1 (a)–(d) show the SEM images of FTO substrate and three kinds of SLs. The c-TiO2 layer [Fig. 1 (b)] exhibits a roughly corrugated surface. This is because that the small thickness (around 90 nm) of the c-TiO2 film cannot buffer the roughness of the underlying FTO surface [Fig. 1(a)]. The roughness were apparently buffered by further depositions of m-TiO2 or Nb2O5 layer [Fig. 1 (c) and (d)]. m-TiO2 layer shows well-organized porosity, which size is in the order of few tens nm [inset of Fig. 1(c)]. Nb2O5 layer also shows the porous-like structure, while the pore size is relatively large (in the order of few hundreds nm). By covering these three kinds of SLs with perovskite films, drastic morphological differences have been observed [Fig. 1 (e)–(g)]. First, it can be clearly seen that the perovskite grain size strongly depends on the SLs; the average diameters were roughly estimated to be 400 nm for the c-TiO2 SL, 150 nm for the m-TiO2 SL, and 500 nm for the Nb2O5 SL. 8
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Fig. 2. X-ray diffraction spectra of the perovskite films on niobium oxide (blue), mesoporous-titanium oxide (red), and compact titanium oxide (black) scaffold layers. The baselines are offset for ease of comparison. Solid and open circles denote the peaks of perovskite crystal and fluorine doped tin oxide substrate, respectively. Small peak of lead iodide is indicated by an arrow. Inset shows the normalized peak at around 14.2∘. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Optical absorption spectra of the perovskite films on niobium oxide (blue), mesoporous-titanium oxide (red), and compact titanium oxide (black) scaffold layers. Inset shows the logarithmic scaled data as a function of excitation photon energy. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
correlates with the degree of the crystallinity. The loss of crystallinities, e.g., amorphous structures, point defects, and small crystalline grains, leads to the sub-band gap states. Illumination of such an imperfect crystal promotes favoring sub-band gap absorptions, resulting in the faint absorption edge. Therefore, the result is the consequence of the scaffold-dependent perovskite crystallinity. To obtain further insights, PL spectroscopies were performed for the perovskite films on the different SLs [Fig. 4(a)]. Each spectrum was calibrated by use of the corresponding absorbance at the excitation wavelength of 600 nm, from which the excitation-dependent PL intensities were compensated. Since all spectra show peaks at around the band-gap energy of the perovskite (770 nm), the intensities correspond to the densities of the photo-excited carriers which annihilate through radiative band-to-band recombination in perovskite. Here, we note that relatively blue-shifted PL peak measured for the perovskite/m-TiO2 film is caused by a quantum confinement effect [30,52], which becomes predominant for the smaller-sized perovskite crystallites (26 nm). By comparing the spectral intensities, one can clearly see that the employments of the m-TiO2 and Nb2O5 SLs result in the PL quenching. Furthermore, it should be noted that, the perovskite/Nb2O5 film exhibits more remarkable quenching effect than the perovskite/m-TiO2 film. The PL quenching rate relative to the perovskite/c-TiO2 film was evaluated to be 64.2% for the perovskite/m-TiO2 film while it was enhanced up to 90.3% for the perovskite/Nb2O5 film. The PL quenching indicates that some relaxation channels other than the radiative band-to-band recombination reduce the photo-excited carrier density in perovskite. In the present case, such channels are the carrier trapping at defect states [shown by (1) in Fig. 4(b)] and electron-extraction by the SL [shown by (2) in Fig. 4(b)]. To understand the mechanism of the different PL quenching rate between perovskite/ Nb2O5 film and perovskite/m-TiO2 film, here, we qualitatively discuss the efficiencies of the defect-related carrier trapping. As shown by the present SEM (Fig. 1) and XRD measurements (Fig. 2), perovskite on the m-TiO2 SL was revealed to show smaller grains (crystallites) with higher pinhole defect density than on the c-TiO2 SL. The pinhole defects are known to act as an effective carrier trapping center. Moreover, smaller perovskite grains lead to overall large grain boundary area, where abundant defects effectively trap photo-excited carriers [6,10]. These factors mean that carrier lifetime in the m-TiO2 SL-based perovskite is reduced from that of the c-TiO2 SL based perovskite. In contrast, perovskite on the Nb2O5 SL forms slightly larger grains (crystallites) than on the c-TiO2 SL. Furthermore, our measurement also
Therefore, the largest perovskite grains were formed on the Nb2O5 SL. It was also found that the perovskite layer on Nb2O5 shows less pinhole defects, as seen in the perovskite/m-TiO2 film [indicated by arrows in Fig. 4 (f)], while the surface contains a few bright regions with the size of about a few hundreds nm. As indicated later, these bright regions are lead iodide (PbI2), which are less conductive than CH3NH3PbI3, and thus accumulate more charges during SEM imaging [10]. The morphological and compositional differences of the perovskite films were further confirmed by XRD measurement. In Fig. 2, XRD diffraction patterns of perovskite films were compared for different SLs. The main diffraction peaks denoted by solid circles were in identical positions for three perovskite films, indicating that they form tetragonal crystal structures [49]. The difference we note is that the Nb2O5 SLbased perovskite shows more intense and sharper peaks. In inset of Fig. 2, we compared the normalized diffraction peaks at around 14.2∘, where the full widths at half maximum were 0.11∘ for the Nb2O5 SL, 0.32∘ for the m-TiO2 SL, and 0.23∘ for the c-TiO2 SL. The corresponding crystallite sizes estimated from Scherrer equation were 73 nm, 26 nm, and 36 nm, respectively, confirming that the Nb2O5 SL-based perovskite indeed forms the largest crystallites. Moreover, we found a small but definitely detectable peak of PbI2 at around 12.6∘ [50] only for the Nb2O5 SL (indicated by an arrow in Fig. 2), which is consistent with the appearance of the bright regions in the SEM image [Fig. 1 (g)]. On the other hand, here we note that the underlying SLs (c-TiO2, m-TiO2, and Nb2O5) also have crystalline structures. However, due to their small crystallite sizes and low atomic scattering factors, the crystalline peaks in the film forms are too weak to be detected. Actually, we confirmed the crystalline peaks for their powder forms which were synthesized by the same methods as for the SLs. To investigate the observed compositional and morphological differences on the optoelectronic properties, optical absorption spectra were measured for the perovskite films on the different SLs (Fig. 3). One can see that each film exhibits an absorption onset at arround 770 nm, which corresponds to the direct band-gap energy of the CH3NH3PbI3 [51]. It was revealed that, compared to the perovskite/c-TiO2 film, the absorption edge is steep for the perovskite/Nb2O5 film while faint for the perovskite/m-TiO2 film. By exponential fits of the absorption edges (dashed lines in inset of Fig. 3), the Urbach energies (EU) were estimated to be EU = 68 meV for Nb2O5 SL, EU = 81 meV for c-TiO2 SL, and EU = 107 meV for m-TiO2 SL. It is well known that the absorption edge 9
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Fig. 4. (a) Photoluminescence spectra of the perovskite films on niobium oxide (blue), mesoporous-titanium oxide (red), and compact titanium oxide (black) scaffold layers. Excitation wavelength was tuned to 600 nm. All spectra were normalized by the absorbance at wavelength of 600 nm measured for the corresponding films. (b) Schematic illustration, representing relaxation of photo-excited carriers in perovskite film. Possible relaxation channels, labeled (1) and (2), responsible for for the quenching of the photoluminescence intensities are also represent by arrows in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
revealed the presence of a small amount of PbI2. Chen et al. [53] reported that PbI2 in perovskite film effectively passivate the grains locally, resulting in a substantially reduced carrier recombination at the grain boundaries and the perovskite/SL interface. Therefore, the carrier lifetime in the Nb2O5 SL-based perovskite should be prolonged from that of the c-TiO2 SL-based perovskite. Based on the above discussions, we suggest that, in the case of the perovskite/m-TiO2 film, carrier trapping effects partly contribute to the observed PL quenching, and the residual contribution is the electronextraction by the SL, i.e., two relaxation processes contribute to the observed PL quenching in concert [as indicated by red arrows in Fig. 4(a)]. On the other hand, since the carrier lifetime in perovskite on Nb2O5 SL is prolonged from that on c-TiO2, the corresponding PL intensity is not quenched but just enhanced [blue arrow labeled (1) in Fig. 4(a)]. Nevertheless, it should be noted that the net PL quenching was more efficient than that for perovskite/m-TiO2 film [Fig. 4(a)]. This clearly proves that the electron-extracting capability for the perovskite/ Nb2O5 film [blue arrow labeled (2) in Fig. 4(a)] is more efficient than that for the perovskite/m-TiO2 film [red arrow labeled (2) in Fig. 4(a)]. It should be noted that employment of the m-TiO2 SL is known to lead to the larger perovskite/SL interface area, which provide a wealth of pathways for effective electron transports [54]. However, it is also been reported that, the crystal growth on such mesoporous surface is restricted [10], which disturb the smooth carrier transport [55]. On the other hand, the present Nb2O5 SL, which surface is not mesoporous morphology, functions as the more optimal ETL. We suggest that this remarkable electron-extracting capability is realized by the presences of the large perovskite grains (crystallites) and a small amount of PbI2, both of which facilitate the smooth carrier transport without defectrelated carrier trapping at the grain boundaries and the perovskite/SL interface. Besides, there should be another mechanism that explains Nb2O5 function as more efficient ETL than c-TiO2. We infer that the key mechanism is in the microscopic structural/energetic contacts at the Nb2O5/perovskite interface. Understanding of the detailed mechanism requires elucidation of the perovskite crystallization kinetics and the dynamics of the photo-excited carrier at the interface. These topics are deserved for future works.
high performance Nb2O5 scaffold can be fabricated even by convenient cost-effective method. We also suggest that our work on Nb2O5 scaffold constitutes a further step since the Nb2O5 forms a variety of structures, such as nanosheet [56], nanotube [57], and nanochannel [42], etc. Syntheses of these structures and their employments as scaffold layers may include many exciting possibilities for improving the device performance. Acknowledgements This work was supported by Grant-in-Aid for Scientific Research (16K17521, 16K05887, 17H05353) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), and by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013–2017. One of the authors (EI) also acknowledges financial support from the Murata Science Foundation. Author contributions EI, TI, and HO conceived and designed the experiments. EI performed the experiments, and analyzed the data. EI wrote the paper. All authors discussed the results and commented on the manuscript. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [3] J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Parka, N.-G. Park, 6.5% efficient perovskite quantum-dot-sensitized solar cell, Nanoscale 3 (2011) 4088–4093. [4] S.A. Kulkarni, T. Baikie, P.P. Boix, N. Yantara, N. Mathews, Band gap tuning of lead halide perovskites using a sequential deposition process, J. Mater. Chem. A 2 (2014) 9221–9225. [5] Y.-Y. Sun, M.L. Agiorgousis, P. Zhang, S. Zhang, Chalcogenide perovskites for photovoltaics, Nano Lett. 15 (2015) 581–585. [6] V. D'Innocenzo, G. Grancini, M.J. Alcocer, A.R.S. Kandada, S.D. Stranks, M.M. Lee, G. Lanzani, H.J. Snaith, A. Petrozza, Excitons versus free charges in organo‑lead trihalide perovskites, Nat. Commun. 5 (2014) 4586. [7] Y.-C. Hsiao, T. Wu, M. Li, Q. Liu, W. Qina, B. Hu, Fundamental physics behind highefficiency organo-metal halide perovskite solar cells, J. Mater. Chem. A 3 (2015) 15372–15385. [8] C.R. Kagan, D.B. Mitzi, C.D. Dimitrakopoulos, Organic-inorganic hybrid materials as semiconducting channels in thin-film field-effect transistors, Science 286 (1999) 945–947. [9] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Grätzel, S. Mhaisalkar, T.C. Sum, Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3, Science 342 (2013) 344–347. [10] Y. Li, W. Yan, Y. Li, S. Wang, W. Wang, Z. Bian, L. Xiao, Q. Gong, Direct observation of long electron-hole diffusion distance in CH3NH3PbI3 perovskite thin film, Sci. Rep. 5 (2015) 14485. [11] M. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar
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Sol-gel processed niobium oxide thin-film for a scaffold layer in perovskite solar cells
T
Eiichi Inamia,b, , Takamasa Ishigakib,c, Hironori Ogatab,c ⁎
a
School of Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan Research Center for Micro-Nano Technology, Hosei University, 3-11-15 Midori-cho, Koganei, Tokyo 184-0003, Japan c Faculty of Bioscience and Applied Chemistry, Department of Chemical Science and Technology, Hosei University, 3-7-2, kajino-chou, koganei, Tokyo, 184-8584, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Perovskite solar cell Niobium oxide Scaffold layer Charge transport Interface engineering
Sol-gel processed niobium oxide (Nb2O5) thin-film was synthesized for a scaffold layer (SL) of methylammonium lead iodide-based perovskite film. Complementary characterizations, including scanning electron microscopy, Xray diffraction, optical absorption spectroscopy have revealed that the Nb2O5 SL facilitates efficient crystallization of the perovskite film compared to the conventional mesoporous titanium oxide (m-TiO2) SL. Also, photoluminescence spectroscopy revealed the higher electron-extraction capability at the perovskite/Nb2O5 interface than the perovskite/m-TiO2 interface. The present sol-gel processed Nb2O5 SL and the superior optoelectronic properties of the overlying perovskite film suggest a promising avenue for high-performance and cost-effective perovskite solar cells.
1. Introduction
scaffold layer (SL) of perovskite film plays vital roles not only in selective extraction of the photo-excited electrons as an electron transport layer (ETL), but also enhanced crystallizations and morphological evolutions of perovskite film [24]. Several metal oxides, such as titanium oxide (TiO2) [12,20,25–29], aluminum oxide (Al2O3) [11,30,31], and zinc oxide (ZnO) [19,32] have been applied for the SLs. The most efficient cell reported thus far was based on a mesoporous (m)-TiO2 SL [12,20,25–29]. As the other candidate, niobium oxide (Nb2O5) is expected to exhibit greater blocking effect than TiO2 [33]. So far, Nb2O5 has been applied mainly in dye-sensitized solar cells, where the shortcircuit photocurrent (Jsc) and open-circuit voltage (Voc) were improved by effective blocking of the recombination [33–42]. More recently, Nb2O5 film has started to be applied as a blocking layer in a PSC [31,43,44]. They found that, the cell with Nb2O5 blocking layer exhibits larger Voc than that with TiO2 blocking layer, while Jsc is reduced due to its high resistance and capacitance[31]. Further, Nb2O5 film has also been revealed to benefit as a ETL [45]. It was demonstrated that the Nb2O5 films fabricated by magnetron sputtering show high electronextraction-property and electron mobility regardless of the crystallinity. This suggests that Nb2O5 SL with high performance is realizable even with room-temperature treatment, that is great advantageous in lowcost PSC processing. In this study, we investigated the performance of the Nb2O5 SL fabricated by sol-gel methods, that is further low-cost processing than the magnetron sputtering technique. For comparison, three kinds of
In the past few years, organic-inorganic halide perovskite solar cells (PSC) have attracted widespread interest as a low-cost and solution processable energy devices. Following the first application of the PSC in 2009 [1], the power conversion efficiency of 3.81% has rapidly improved to over 20% at present [2]. Such developments are owing to their excellent optoelectronic properties, such as strong optical absorption in visible and ultraviolet light range [3], tunable band gap [4,5], small exciton binding energy [6,7], and high mobility charge carrier with a lifetime of a few hundreds nanosecond [8–10]. Therefore, fabricating highly crystalline, homogeneous and planar perovskite thinfilms has been a successful strategy for utilizing full potential of perovskite materials and improvements of the device performance. Nowadays, several sophisticated processes for synthesizing perovskite materials have been reported, e.g., varying the compositions [11–13], adding additives [14,15], employing different solvents [16]. Independently, the following deposition processes have also been developed, e.g., fast deposition-crystallization (FDC) method [17,18], two step sequential deposition method [19–21], vacuum evaporation deposition method [11,22], and vapor assisted solution process [23]. These studies focused on optimizing crystallizations and thin-film formation kinetics of perovskite materials itself. Alternatively, the substrate surface modification is also a possible approach for morphological control of perovskite films. It is known that ⁎
Corresponding author at: School of Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan. E-mail address: [email protected] (E. Inami).
https://doi.org/10.1016/j.tsf.2019.01.043 Received 2 August 2018; Received in revised form 26 December 2018; Accepted 22 January 2019 Available online 29 January 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Top-view scanning electron microscopy images of fluorine doped tin oxide substrate. (b-d) Top-view scanning electron microscopy images of three scaffold layers on fluorine doped tin oxide substrates: (b) compact titanium oxide, (c) mesoporous-titanium oxide/compact titanium oxide, (d) niobium oxide/compact titanium oxide. Inset in (c) is a close-up view of the mesoporous-titanium oxide scaffold. (e-g) Top-view scanning electron microscopy images of perovskite films on (b-d), respectively.
films [Nb2O5 film, m-TiO2, and compact (c)-TiO2 film] were fabricated, and compared the morphological and optoelectronic properties of their overlying perovskite films. Complementary characterizations, including scanning electron microscopy (SEM), X-ray diffraction (XRD), optical absorption spectroscopy, and photoluminescence (PL) spectroscopy have revealed that Nb2O5 scaffold shows the most prominent electron extraction property. Interestingly, such feature was revealed to be assisted by the highly crystalline perovskite thin-film, that is, the Nb2O5 film functions assisting crystallization of the perovskite film as well as efficient ETL. Our findings suggest that the Nb2O5 SL is the one possible candidate toward fully solution-processable PSC devices.
on the top of the c-TiO2 layer in an argon-filled glovebox. Then, the substrates were dried at 120∘C for 5 min and subsequently sintered at 500∘C for 1 h in air [31]. On the top of these three kinds of SLs, methylammonium lead iodide (CH3NH3PbI3) perovskite films were prepared by FDC method [17,18] in the glove box. A N, N-dimethylformamide (DMF) solution of CH3NH3PbI3 with concentration of 45 wt% were dropped onto the SLs and the substrates were spun at 5000 rpm. Just after the change in the substrate color from yellow to white, chlorobenzene (500μL) were immediately dropped onto the substrates. Dropping the second solvents further changed the substrate color from white to light brown. The obtained films were then dried at 100∘C for 10 min, resulting in the black color perovskite films.
2. Experimental details
2.2. Characterization details
2.1. Sample preparations
The local morphologies of the fabricated films were characterized by SEM (Hitachi SU-8020). XRD patterns were obtained using Rigaku Smartlab system with Copper K-alpha radiation. Optical absorption spectra of the perovskite films were measured using UV–vis spectrometer (Jasco V-770). PL spectra were measured using Spex Fluorolog 3–21 spectrofluorometer.
To investigate the scaffold-dependent optoelectronic properties of the perovskite materials, we first fabricated three c-TiO2 blocking layers, on two of which m-TiO2 and Nb2O5 SLs were further fabricated, preparing bare c-TiO2, m-TiO2/c-TiO2, and Nb2O5/c-TiO2 (i.e., preparations of bare blocking layer and different SLs on same blocking layer). Then, morphological/optoelectronic properties of perovskite films on the three scaffolds were compared. The preparation of each layer was carried out according to the conditions considered to exhibit the most excellent electron transporting properties which has been reported so far. Fluorine doped tin oxide (FTO) films on glass substrates (16 cm × 16 cm2) were ultrasonic-cleaned sequentially in acetone, ethanol and distilled water for 15 min, and then treated by a ultraviolet (UV)-ozone cleaner (Bio Force Nanosciences Inc.) for 20 min. c-TiO2 film was prepared on the FTO substrate by spin-coating (2000 rpm for 30 s) a mixture solution of 0.2 M titanium (IV) isopropoxide (Aldrich) and 0.4 M acetyl acetone (Aldrich) dissolved in ethanol [46], and then dying at 80∘C for 5 min. The process was repeated three times to avoid leakage current due to the pinholes. The films were finally sintered at 450∘C for 30 min to form 90-nm-thick c-TiO2 film [46]. m-TiO2 film was prepared by spin-coating (2000 rpm for 30 s) TiO2 paste (PST-18NR, Nikki Syokubai Kasei) diluted in ethanol (paste: ethanol = 1: 3.5 wt ratio) on the top of the c-TiO2 layer, and subsequent sintering at 550∘C for 30 min in air [46–48]. Nb2O5 film was prepared by spin-coating (4000 rpm for 90 s) 0.3 M niobium ethoxide (Wako) diluted in ethanol
3. Results and discussions 3.1. Scaffold-dependent properties of perovskite films Fig. 1 (a)–(d) show the SEM images of FTO substrate and three kinds of SLs. The c-TiO2 layer [Fig. 1 (b)] exhibits a roughly corrugated surface. This is because that the small thickness (around 90 nm) of the c-TiO2 film cannot buffer the roughness of the underlying FTO surface [Fig. 1(a)]. The roughness were apparently buffered by further depositions of m-TiO2 or Nb2O5 layer [Fig. 1 (c) and (d)]. m-TiO2 layer shows well-organized porosity, which size is in the order of few tens nm [inset of Fig. 1(c)]. Nb2O5 layer also shows the porous-like structure, while the pore size is relatively large (in the order of few hundreds nm). By covering these three kinds of SLs with perovskite films, drastic morphological differences have been observed [Fig. 1 (e)–(g)]. First, it can be clearly seen that the perovskite grain size strongly depends on the SLs; the average diameters were roughly estimated to be 400 nm for the c-TiO2 SL, 150 nm for the m-TiO2 SL, and 500 nm for the Nb2O5 SL. 8
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Fig. 2. X-ray diffraction spectra of the perovskite films on niobium oxide (blue), mesoporous-titanium oxide (red), and compact titanium oxide (black) scaffold layers. The baselines are offset for ease of comparison. Solid and open circles denote the peaks of perovskite crystal and fluorine doped tin oxide substrate, respectively. Small peak of lead iodide is indicated by an arrow. Inset shows the normalized peak at around 14.2∘. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Optical absorption spectra of the perovskite films on niobium oxide (blue), mesoporous-titanium oxide (red), and compact titanium oxide (black) scaffold layers. Inset shows the logarithmic scaled data as a function of excitation photon energy. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
correlates with the degree of the crystallinity. The loss of crystallinities, e.g., amorphous structures, point defects, and small crystalline grains, leads to the sub-band gap states. Illumination of such an imperfect crystal promotes favoring sub-band gap absorptions, resulting in the faint absorption edge. Therefore, the result is the consequence of the scaffold-dependent perovskite crystallinity. To obtain further insights, PL spectroscopies were performed for the perovskite films on the different SLs [Fig. 4(a)]. Each spectrum was calibrated by use of the corresponding absorbance at the excitation wavelength of 600 nm, from which the excitation-dependent PL intensities were compensated. Since all spectra show peaks at around the band-gap energy of the perovskite (770 nm), the intensities correspond to the densities of the photo-excited carriers which annihilate through radiative band-to-band recombination in perovskite. Here, we note that relatively blue-shifted PL peak measured for the perovskite/m-TiO2 film is caused by a quantum confinement effect [30,52], which becomes predominant for the smaller-sized perovskite crystallites (26 nm). By comparing the spectral intensities, one can clearly see that the employments of the m-TiO2 and Nb2O5 SLs result in the PL quenching. Furthermore, it should be noted that, the perovskite/Nb2O5 film exhibits more remarkable quenching effect than the perovskite/m-TiO2 film. The PL quenching rate relative to the perovskite/c-TiO2 film was evaluated to be 64.2% for the perovskite/m-TiO2 film while it was enhanced up to 90.3% for the perovskite/Nb2O5 film. The PL quenching indicates that some relaxation channels other than the radiative band-to-band recombination reduce the photo-excited carrier density in perovskite. In the present case, such channels are the carrier trapping at defect states [shown by (1) in Fig. 4(b)] and electron-extraction by the SL [shown by (2) in Fig. 4(b)]. To understand the mechanism of the different PL quenching rate between perovskite/ Nb2O5 film and perovskite/m-TiO2 film, here, we qualitatively discuss the efficiencies of the defect-related carrier trapping. As shown by the present SEM (Fig. 1) and XRD measurements (Fig. 2), perovskite on the m-TiO2 SL was revealed to show smaller grains (crystallites) with higher pinhole defect density than on the c-TiO2 SL. The pinhole defects are known to act as an effective carrier trapping center. Moreover, smaller perovskite grains lead to overall large grain boundary area, where abundant defects effectively trap photo-excited carriers [6,10]. These factors mean that carrier lifetime in the m-TiO2 SL-based perovskite is reduced from that of the c-TiO2 SL based perovskite. In contrast, perovskite on the Nb2O5 SL forms slightly larger grains (crystallites) than on the c-TiO2 SL. Furthermore, our measurement also
Therefore, the largest perovskite grains were formed on the Nb2O5 SL. It was also found that the perovskite layer on Nb2O5 shows less pinhole defects, as seen in the perovskite/m-TiO2 film [indicated by arrows in Fig. 4 (f)], while the surface contains a few bright regions with the size of about a few hundreds nm. As indicated later, these bright regions are lead iodide (PbI2), which are less conductive than CH3NH3PbI3, and thus accumulate more charges during SEM imaging [10]. The morphological and compositional differences of the perovskite films were further confirmed by XRD measurement. In Fig. 2, XRD diffraction patterns of perovskite films were compared for different SLs. The main diffraction peaks denoted by solid circles were in identical positions for three perovskite films, indicating that they form tetragonal crystal structures [49]. The difference we note is that the Nb2O5 SLbased perovskite shows more intense and sharper peaks. In inset of Fig. 2, we compared the normalized diffraction peaks at around 14.2∘, where the full widths at half maximum were 0.11∘ for the Nb2O5 SL, 0.32∘ for the m-TiO2 SL, and 0.23∘ for the c-TiO2 SL. The corresponding crystallite sizes estimated from Scherrer equation were 73 nm, 26 nm, and 36 nm, respectively, confirming that the Nb2O5 SL-based perovskite indeed forms the largest crystallites. Moreover, we found a small but definitely detectable peak of PbI2 at around 12.6∘ [50] only for the Nb2O5 SL (indicated by an arrow in Fig. 2), which is consistent with the appearance of the bright regions in the SEM image [Fig. 1 (g)]. On the other hand, here we note that the underlying SLs (c-TiO2, m-TiO2, and Nb2O5) also have crystalline structures. However, due to their small crystallite sizes and low atomic scattering factors, the crystalline peaks in the film forms are too weak to be detected. Actually, we confirmed the crystalline peaks for their powder forms which were synthesized by the same methods as for the SLs. To investigate the observed compositional and morphological differences on the optoelectronic properties, optical absorption spectra were measured for the perovskite films on the different SLs (Fig. 3). One can see that each film exhibits an absorption onset at arround 770 nm, which corresponds to the direct band-gap energy of the CH3NH3PbI3 [51]. It was revealed that, compared to the perovskite/c-TiO2 film, the absorption edge is steep for the perovskite/Nb2O5 film while faint for the perovskite/m-TiO2 film. By exponential fits of the absorption edges (dashed lines in inset of Fig. 3), the Urbach energies (EU) were estimated to be EU = 68 meV for Nb2O5 SL, EU = 81 meV for c-TiO2 SL, and EU = 107 meV for m-TiO2 SL. It is well known that the absorption edge 9
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Fig. 4. (a) Photoluminescence spectra of the perovskite films on niobium oxide (blue), mesoporous-titanium oxide (red), and compact titanium oxide (black) scaffold layers. Excitation wavelength was tuned to 600 nm. All spectra were normalized by the absorbance at wavelength of 600 nm measured for the corresponding films. (b) Schematic illustration, representing relaxation of photo-excited carriers in perovskite film. Possible relaxation channels, labeled (1) and (2), responsible for for the quenching of the photoluminescence intensities are also represent by arrows in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
revealed the presence of a small amount of PbI2. Chen et al. [53] reported that PbI2 in perovskite film effectively passivate the grains locally, resulting in a substantially reduced carrier recombination at the grain boundaries and the perovskite/SL interface. Therefore, the carrier lifetime in the Nb2O5 SL-based perovskite should be prolonged from that of the c-TiO2 SL-based perovskite. Based on the above discussions, we suggest that, in the case of the perovskite/m-TiO2 film, carrier trapping effects partly contribute to the observed PL quenching, and the residual contribution is the electronextraction by the SL, i.e., two relaxation processes contribute to the observed PL quenching in concert [as indicated by red arrows in Fig. 4(a)]. On the other hand, since the carrier lifetime in perovskite on Nb2O5 SL is prolonged from that on c-TiO2, the corresponding PL intensity is not quenched but just enhanced [blue arrow labeled (1) in Fig. 4(a)]. Nevertheless, it should be noted that the net PL quenching was more efficient than that for perovskite/m-TiO2 film [Fig. 4(a)]. This clearly proves that the electron-extracting capability for the perovskite/ Nb2O5 film [blue arrow labeled (2) in Fig. 4(a)] is more efficient than that for the perovskite/m-TiO2 film [red arrow labeled (2) in Fig. 4(a)]. It should be noted that employment of the m-TiO2 SL is known to lead to the larger perovskite/SL interface area, which provide a wealth of pathways for effective electron transports [54]. However, it is also been reported that, the crystal growth on such mesoporous surface is restricted [10], which disturb the smooth carrier transport [55]. On the other hand, the present Nb2O5 SL, which surface is not mesoporous morphology, functions as the more optimal ETL. We suggest that this remarkable electron-extracting capability is realized by the presences of the large perovskite grains (crystallites) and a small amount of PbI2, both of which facilitate the smooth carrier transport without defectrelated carrier trapping at the grain boundaries and the perovskite/SL interface. Besides, there should be another mechanism that explains Nb2O5 function as more efficient ETL than c-TiO2. We infer that the key mechanism is in the microscopic structural/energetic contacts at the Nb2O5/perovskite interface. Understanding of the detailed mechanism requires elucidation of the perovskite crystallization kinetics and the dynamics of the photo-excited carrier at the interface. These topics are deserved for future works.
high performance Nb2O5 scaffold can be fabricated even by convenient cost-effective method. We also suggest that our work on Nb2O5 scaffold constitutes a further step since the Nb2O5 forms a variety of structures, such as nanosheet [56], nanotube [57], and nanochannel [42], etc. Syntheses of these structures and their employments as scaffold layers may include many exciting possibilities for improving the device performance. Acknowledgements This work was supported by Grant-in-Aid for Scientific Research (16K17521, 16K05887, 17H05353) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), and by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013–2017. One of the authors (EI) also acknowledges financial support from the Murata Science Foundation. Author contributions EI, TI, and HO conceived and designed the experiments. EI performed the experiments, and analyzed the data. EI wrote the paper. All authors discussed the results and commented on the manuscript. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [3] J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Parka, N.-G. Park, 6.5% efficient perovskite quantum-dot-sensitized solar cell, Nanoscale 3 (2011) 4088–4093. [4] S.A. Kulkarni, T. Baikie, P.P. Boix, N. Yantara, N. Mathews, Band gap tuning of lead halide perovskites using a sequential deposition process, J. Mater. Chem. A 2 (2014) 9221–9225. [5] Y.-Y. Sun, M.L. Agiorgousis, P. Zhang, S. Zhang, Chalcogenide perovskites for photovoltaics, Nano Lett. 15 (2015) 581–585. [6] V. D'Innocenzo, G. Grancini, M.J. Alcocer, A.R.S. Kandada, S.D. Stranks, M.M. Lee, G. Lanzani, H.J. Snaith, A. Petrozza, Excitons versus free charges in organo‑lead trihalide perovskites, Nat. Commun. 5 (2014) 4586. [7] Y.-C. Hsiao, T. Wu, M. Li, Q. Liu, W. Qina, B. Hu, Fundamental physics behind highefficiency organo-metal halide perovskite solar cells, J. Mater. Chem. A 3 (2015) 15372–15385. [8] C.R. Kagan, D.B. Mitzi, C.D. Dimitrakopoulos, Organic-inorganic hybrid materials as semiconducting channels in thin-film field-effect transistors, Science 286 (1999) 945–947. [9] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Grätzel, S. Mhaisalkar, T.C. Sum, Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3, Science 342 (2013) 344–347. [10] Y. Li, W. Yan, Y. Li, S. Wang, W. Wang, Z. Bian, L. Xiao, Q. Gong, Direct observation of long electron-hole diffusion distance in CH3NH3PbI3 perovskite thin film, Sci. Rep. 5 (2015) 14485. [11] M. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar
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