Elsevier

Applied Surface Science

Volume 581, 15 April 2022, 151570
Applied Surface Science

Room-Temperature-Grown amorphous Indium-Tin-Silicon-Oxide thin film as a new electron transporting layer for perovskite solar cells

https://doi.org/10.1016/j.apsusc.2021.151570Get rights and content

Highlights

Abstract

We report the amorphous quaternary oxide, indium-tin-silicon-oxide (ITSO), thin film as a new electron-transport layer (ETL) for perovskite solar cells (PSCs). ITSO thin films are grown by magnetron co-sputtering indium-tin-oxide (ITO) and silicon oxide (SiO2) on commercial transparent conducting oxide (TCO) thin films at room temperature. As Si content increases (0–53.8 at%) the optical bandgap increases by approximately 1.3 eV and the electrical resistivity increases by six orders mainly because of the carrier concentration decrease. Consequently, the ITSO electronic structure depends largely on Si content. PSCs employing ITSO thin films as ETLs were fabricated to evaluate the effect of Si content on device performances. Si content influenced the shunt and series resistance. The optimized device was obtained using an ITSO film with 33.0 at% Si content, exhibiting 14.50% power-conversion efficiency. These results demonstrate that ITSO films are promising for developing efficient PSCs by optimizing the growing process and/or In/Sn/Si/O compositions. This approach can reduce PSC manufacturing process time and costs if ITO and ITSO are grown together by continuous sequential sputtering in a dual gun (ITO and SiO2) chamber.

Introduction

Perovskite solar cells (PSCs) based on organometal trihalide perovskites as light-absorbing materials have attracted considerable attention due to the rapid increase in its power conversion efficiency (PCE) up to 25.5% during the last decades [1], [2], [3], [4], [5]. A typical perovskite solar cell is composed of a transparent oxide electrode (TCO), an electron transport layer (ETL), a light absorption layer (LAL), a hole transport layer (HTL), and a counter metal electrode [6]. The advantages of PSCs in a perspective of fabrication are easy solution-based, low-cost coating processes for the ETL/LAL/HTL, and low-temperature (<150 °C) annealing, which can achieve a high-quality LAL for a high-efficiency device [7], [8], [9].

The most commonly used ETL material for PSCs is TiO2, which has suitable electronic energy levels with perovskites and TCOs [10], and satisfactory charge-transport properties [11], [12]. In this context, the best PSCs have been achieved with TiO2 ETLs in the form of a mesoscopic-type device, in which a mesoporous TiO2 layer is required for efficient charge-transport, loading LAL materials, and electron-hole separators [13]. To manifest such good characteristics of TiO2, however, a high-temperature annealing process over 450 °C is needed, which debases advantages of PSCs [14]. Moreover, for flexible PSCs, a planar-type device using a compact thin film as the ETL, rather than a mesoscopic-type, is required to secure high durability for mechanical bending [14]. Alternatively, organic ETL materials, such as fullerene and [6], [6]-phenyl-C61-butyric acid methyl ester, have been widely studied [15], [16]. Organic ETLs have evident advantages such as a straightforward low-temperature solution process and relatively small hysteresis in JV measurements. However, organic ETLs are generally expensive, and have relatively low stability against humidity, oxygen, and heat, thus it is hard to replace inorganic TiO2 ETL. Therefore, intensive studies have also been focused on the development of new inorganic ETL materials [17]. Representatively, SnO2 [18], [19] and BaSnO3 [20] ETLs have been used to produce PSCs presenting high PCE, small hysteresis, and high stability.

Recently, amorphous inorganic materials have been considered strong ETL candidates because of the following reasons. First, amorphous inorganic thin films can be fabricated at low temperatures using vapor deposition [21] or a solution coating method [22], [23]. Second, an amorphous thin film has a smoother surface than a polycrystalline film, which reinforces the bending stress durability by preventing the initiation of cracks from its surface [24]. Finally, the composition of amorphous thin films can be tuned in a wide range [25], [26] compared to crystalline films, which increases the probability of obtaining a suitable energy band structure for a PSC. Recently, therefore, various amorphous inorganic materials such as TiOx [27], [28], [29], WOx [30], [31], [32], Bi2S3,[24] SnOx [33], [34], SnO2 [35], Sn-In-O [36], and Nb2O5 [37], [38], [39] have been evaluated as ETLs of PSCs.

There have been numerous reports studying the electrical and optical properties of indium-tin-oxide (ITO) TCO, with doping of transition metals such as Ti [40], Mn [41] and Zr [42] as well as semimetals such as Si, Ge, and As [43]. In general, however, the ITO properties were investigated as a TCO, in a narrow range of variation with a small amount of doping (≤5 at%). Recently, it was reported that electrical resistivity and optical refractive index of amorphous ITO film can be significantly tuned by incorporation of SiO2, making In-Sn-Si-O (ITSO) [44]. Increased Si content reduced incident light reflection, improving the performance of Si solar cells. Especially, the resistivity was increased by > 105 times with 30% SiO2 content (volume ratio to ITO), implying that ITSO is a good candidate to investigate the feasibility for PSCs by tuning electronic structure. Another study of ITSO-based thin-film transistors also confirms that the Si content in amorphous ITSO can change the electronic structure significantly [45].

In this study, the ITSO amorphous thin film was described as a new ETL for PSCs. Different compositions of ITSO thin films were prepared at room temperature through RF magnetron co-sputtering with a dual gun system by varying the gun power of ITO and SiO2 targets. As the Si content in ITSO thin films increased from 0 to 53.8 at%, the electronic energy levels, such as optical band gap energy (Eg), Fermi level (EF), conduction band minimum (CBM), and valence band maximum (VBM) of the ITSO thin films changed gradually. Moreover, the electrical properties of thin films changed from metallic to insulating nature as the Si content in the ITSO thin films increased. The photovoltaic properties of ITSO ETL-based PSCs also changed with the Si content, and optimal performance was obtained with 33.0 at% of Si content. The best PSC exhibited 14.50% of PCE with 19.72 mA/cm2 of short circuit current density (JSC), 1.04 V of open-circuit voltage (VOC), and 0.71 of fill factor (FF). ITSO-based PSCs maintained approximately 97% of the initial PCE after 7 months of storage.

Section snippets

Materials

Titanium diisopropoxide bis(acetylacetonate) (TDIP, 75 wt% in isopropanol), 1-butanol (1-BuOH, 99.0%), dimethyl sulfoxide (DMSO, ≥99.9%), N,N-dimethylformamide (DMF, 99.8%), diethyl ether (DEE, ≥99.7%), 4-tert-butylpyridine (tBP, 96%), acetonitrile (ACN, 99.8%), and chlorobenzene (CB, 99.8%) were purchased from Sigma-Aldrich. Methylammonium iodide (MAI) was purchased from Greatcell Solar. Lead(II) iodide (PbI2, 99.9985%) was purchased from TCI chemicals. Spiro-MeOTAD (≥99.9978%) and FK 209

Results and discussion

The ITSO thin films were formed by a dual-source co-sputtering method at RT. A great advantage of this method is that the composition of thin films with different In, Sn, and Si contents was precisely controlled by the ITO and SiO2 gun power. Fig. 1 shows the variation of the In, Si, and Sn contents in the ITSO thin films confirmed by EDS (Fig. S1). By adjusting the SiO2 and ITO gun powers, the Si and In contents of ITSO thin films were largely varied from 53.8 at% to 0 at% and from ∼ 42.3 to

Conclusion

In summary, amorphous ITSO thin films were fabricated using the magnetron co-sputtering system at room temperature. The Si content in the ITSO thin film was precisely controlled by varying the gun power of ITO and SiO2 targets. XRD analysis confirmed the amorphous phase of the ITSO thin films. The ITSO film with a Si content of 30–40 at% demonstrated excellent electrical and optical properties, including high mobility, transmittance, and suitable energy band structure for the PSC. The ITSO

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (NRF-2020R1A4A2002161, 2019R1A2C1084010, 2019M3D1A2104108). This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20203040010320).

References (65)

  • Y. Gassenbauer et al.

    Surface potential changes of semiconducting oxides monitored by high-pressure photoelectron spectroscopy: importance of electron concentration at the surface

    Solid State Ionics

    (2006)
  • S. Zhong et al.

    Exploring co-sputtering of ZnO: Al and SiO2 for efficient electron-selective contacts on silicon solar cells

    Sol. Energy Mater. Sol. Cells

    (2019)
  • J. Burschka et al.

    Sequential deposition as a route to high-performance perovskite-sensitized solar cells

    Nature

    (2013)
  • M.M. Lee et al.

    Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites

    Science

    (2012)
  • M. Liu et al.

    Efficient planar heterojunction perovskite solar cells by vapour deposition

    Nature

    (2013)
  • J.J. Yoo et al.

    Efficient perovskite solar cells via improved carrier management

    Nature

    (2021)
  • National Renewable Energy Laboratory, Best Research-Cell Efficiency Chart (2021); www.nrel.gov/pv/cell-efficiency.html...
  • A. Kojima et al.

    Organometal halide perovskites as visible-light sensitizers for photovoltaic cells

    J. Am. Chem. Soc.

    (2009)
  • J.M. Ball et al.

    Low-temperature processed meso-superstructured to thin-film perovskite solar cells

    Energy Environ. Sci.

    (2013)
  • D. Liu et al.

    Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques

    Nat. Photon.

    (2014)
  • H.J. Snaith

    Perovskites: the emergence of a New Era for low-cost, high-efficiency solar cells

    J. Phys. Chem.Lett.

    (2013)
  • D. Yang et al.

    Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells

    Energy Environ. Sci.

    (2016)
  • P. Tiwana et al.

    Electron mobility and injection dynamics in mesoporous ZnO, SnO2, and TiO2 films used in dye-sensitized solar cells

    ACS Nano

    (2011)
  • I. Abayev et al.

    Electronic conductivity in nanostructured TiO2 films permeated with electrolyte

    Phys. Status Solidi A

    (2003)
  • H.J. Snaith et al.

    Anomalous hysteresis in perovskite solar cells

    J. Phys. Chem.Lett.

    (2014)
  • B.J. Kim et al.

    Recent progressive efforts in perovskite solar cells toward commercialization

    J. Mater. Chem. A

    (2018)
  • J. Seo et al.

    Benefits of very thin PCBM and LiF layers for solution-processed p–i–n perovskite solar cells

    Energy Environ. Sci.

    (2014)
  • J. You et al.

    Low-temperature solution-processed perovskite solar cells with high efficiency and flexibility

    ACS Nano

    (2014)
  • Q. Jiang et al.

    Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)2PbI3-based perovskite solar cells

    Nat. Energy

    (2017)
  • H.-S. Rao et al.

    Improving the extraction of photogenerated electrons with SnO2 nanocolloids for efficient planar perovskite solar cells

    Adv. Funct. Mater.

    (2015)
  • S.S. Shin et al.

    Colloidally prepared La-doped BaSnO3 electrodes for efficient, photostable perovskite solar cells

    Science

    (2017)
  • S.S. Mali et al.

    Efficient planar n-i-p type heterojunction flexible perovskite solar cells with sputtered TiO2 electron transporting layers

    Nanoscale

    (2017)
  • Cited by (0)

    1

    These authors contributed equally to this work.

    View full text