【公開日：2024.07.25】【最終更新日：2024.06.28】

課題データ / Project Data

課題番号 / Project Issue Number

23NU0228

利用課題名 / Title

逆型ペロブスカイト太陽電池に Cationic Nitrogen-doped Graphene (CNG)を応用する

利用した実施機関 / Support Institute

名古屋大学 / Nagoya Univ.

機関外・機関内の利用 / External or Internal Use

内部利用（ARIM事業参画者以外）/Internal Use (by non ARIM members)

技術領域 / Technology Area

【横断技術領域 / Cross-Technology Area】（主 / Main）計測・分析/Advanced Characterization（副 / Sub）物質・材料合成プロセス/Molecule & Material Synthesis

【重要技術領域 / Important Technology Area】（主 / Main）革新的なエネルギー変換を可能とするマテリアル/Materials enabling innovative energy conversion（副 / Sub）-

キーワード / Keywords

逆型ペロブスカイト太陽電池, グラフェン,太陽電池/ Solar cell,電子顕微鏡/ Electronic microscope,集束イオンビーム/ Focused ion beam,X線回折/ X-ray diffraction,赤外・可視・紫外分光/ Infrared/visible/ultraviolet spectroscopy

利用者と利用形態 / User and Support Type

利用者名（課題申請者）/ User Name (Project Applicant)

松尾 豊

所属名 / Affiliation

名古屋大学大学院工学研究科

共同利用者氏名 / Names of Collaborators in Other Institutes Than Hub and Spoke Institutes

ARIM実施機関支援担当者 / Names of Collaborators in The Hub and Spoke Institutes

利用形態 / Support Type

（主 / Main）機器利用/Equipment Utilization（副 / Sub）-

利用した主な設備 / Equipment Used in This Project

NU-203：薄膜X線回折装置
NU-204：原子間力顕微鏡
NU-230：段差計

報告書データ / Report

概要（目的・用途・実施内容）/ Abstract (Aim, Use Applications and Contents)

Interfacial layers located between the cathode and electron transport layer in inverted perovskite solar cells are commonly required to achieve high-performance devices. Therefore, it is essential to develop excellent interfacial layer materials to improve efficiency and stability. This paper introduces the graphene-based interfacial layer material, namely cationic nitrogen-doped graphene (CNG), and evaluates its performance on a methylammonium lead iodide (MAPbI₃)-type inverted perovskite solar cells. The device with a CNG interfacial layer achieved a power conversion efficiency of 13.5%, which is higher than a state-of-the-art reference using bathocuproine interfacial layer. Mechanistic studies demonstrated that the CNG interfacial layer can 1) efficiently collect electrons from the lowest unoccupied molecular orbital of electron transport layer by lowering the work function of the silver cathode, 2) improve the conductivity of the silver electrode for better electron transfer, and 3) smooth out the interface contact between electron transport layer and cathode to reduce defects in the device. As a result, the CNG interfacial layer enhanced the inverted perovskite solar cells performance by simultaneously increasing the open-circuit voltage, short-circuit current density, and fill factor. Moreover, the unencapsulated CNG interfacial layer-applied device demonstrated good long-term stability, with 96% efficiency retained over 1000 hours in nitrogen atmosphere at room temperature.

実験 / Experimental

In a typical procedure, two 1.0 mm diameter tungsten electrodes were positioned in the center of the glass reactor. The gap between the two tungsten electrodes was set at 1.0 mm. 300 mL ionic liquid/organic solvent mixture (EMIM DCA, 10 wt%/DMF, 90 wt%) was added to the glass reactor. A bipolar high-voltage pulse was applied to the tungsten electrode by using a bipolar-DC pulsed power supply (KURITA Seisakusho). After 10 min of the reaction, the resulting mixture was vacuum filtration to separate the synthesized powder. The final product, named cationic nitrogen-doped graphene (CNG), was washed thoroughly with ethanol and distilled water before being dried in a vacuum oven for one night at 100 °C to completely remove any remaining ionic liquid and DMF that might still be physically attached to the CNG. CNG is considered to be few-layer graphene, where the average number of layers is three, oriented perpendicularly, with a lateral size of about 20–30 μm. Nitrogen, carbon, and oxygen contents are 13.4%, 81.4%, and 5.2%, respectively. A sheet resistance and a resistivity are 16.0 (Ω sq^–1) and 0.25 (Ω cm) respectively.
Device fabricationThe IPSCs fabricated in this study were mainly according to our protocols. Indium-tin oxide (ITO) patterned glass substrates (15 × 15 mm, ~10 Ω/sq.) were washed and sonicated with detergent, deionized water, acetone and isopropanol for 15 min, respectively. Then, the cleaned substrates were dried by N₂ flow and treated by UV/O₃ for 15 min to enhance the wettability and remove any organic contamination. Then the substrates were transferred into a N₂-filled glove box. 30 µL of PTAA solution (2.0 mg in a mixture of 1.0 mL of toluene/DMF = 9/1) was spin-coated on the ITO substrate at 3,000 rpm for 30 s, and annealed at 110 °C for 10 min. The perovskite precursor solution was prepared by dissolving 355 mg CH₃NH₃I, 122 mg PbI₂ in a mixture of 490 µL DMSO and 55 µL DMF. 30 µL of perovskite precursor solution was spin-coated at 4,000 rpm for 20 s. 140 µL of antisolvent (CB/IPA = 95/5) was slowly dripped onto the substrate 5 s after the start of the spin-coating process. The transparent film was subsequently annealed at 100 °C for 10 min. 30 µL of PCBM solution (20 mg/mL in CB) was spin-coated at 3,000 rpm for 30 s. Next, the thin IL was spin-coated at 5,000 rpm for 30 s. Both BCP and CNG were prepared in a similar concentration around 0.5 mg/mL in IPA. Finally, the device was completed by thermal evaporation of a 70-nm-thick silver through a 0.1 cm² mask as cathode. Cross-sectional scanning electron microscopic (SEM) measurements revealed thickness of the perovskite layer, PCBM, and the ILs are ca. 500 nm, 20–30 nm, and 5 nm or less, respectively. The thickness of layer in our fabricated solar cell was calculated by surface profiler (NU-230). The crystallinity of CNG layer was measured by X-ray diffraction (NU-203). The roughness and surface profile of CNG layer were measured by AFM (NU-204).

結果と考察 / Results and Discussion

The CNG applied in this study was synthesized by solution plasma method, and was directly utilized as the IL through facile spin-coating process without necessity of typical annealing treatment. Fig. 1a showed the schematic structure of the IPSCs and the structure of CNG. The utilized device with a configuration of ITO/PTAA/MAPbI₃/PCBM/IL/Ag was further confirmed using cross-sectional SEM as shown in Fig. 1b. The J–V curves and corresponding photovoltaic parameters of the devices using different ILs are summarized in Table 1 and Fig. 1c, respectively. The device with the CNG showed a PCE of 13.5%, a V_OC of 1.03 V, a J_SC of 16.3 mA/cm², and a FF of 0.81. The histograms of the efficiencies for the IPSCs based on different ILs are shown in Fig. 1d indicating a good reproducibility. We conducted photoelectron yield spectroscopy (PYS) measurements on silver electrodes to investigate the role of CNG (Fig. 2(a)). A schematic diagram of energy levels is shown in Fig. 2(b). The work function of CNG/Ag (4.41 eV) was found to be lower than that of BCP/Ag (4.62 eV) and Ag (4.66 eV), suggesting the formation of a stronger interfacial dipole at the CNG/Ag interface. Generally, the efficiency of electron transport is determined by the energy barrier ΔE_e between the LUMO energy level of the electron transport layer (PCBM) and the work function of the electrode (Ag). Thus, reducing ΔE_e is an important factor in reducing carrier recombination losses in IPSCs. In this study, the decrease in the work function of Ag reduced the energy loss during electron transfer from PCBM to the electrode through the efficient electron extraction by decreasing ΔE_e. X-ray photoelectron spectroscopy (XPS) measurements also revealed that the addition of CNG and BCP caused energy level shifts for electrons other than the outermost electrons of Ag. The data in Fig. 2(b) reveals that the ΔE_e induced by Ag/CNG (0.59 eV) is lower than that induced by Ag/BCP (0.78 eV) or Ag (0.88 eV) without an IL, leading to a reduction in carrier recombination potential at the CNG/Ag interface relative to other device interfaces, and resulting in improved device performance. We successfully utilized graphene-based CNG as an IL between PCBM electron transport layer and the silver electrode in the IPSC. CNG lowered the work function of Ag, and improved the energy barrier between PCBM and Ag electrodes, resulting in an enhancement of charge transfer at both interfaces of the perovskite/PCBM and PCBM/Ag. In particular, the enhanced conductivity and suppressed non-radiative recombination by CNG improved the charge collection efficiency. This led to a more significant improvement in J_SC than in V_OC. The efficiency of PSCs using CNG as an IL improved from 12.5% to 13.5%, which is an 8% improvement compared to IPSCs using BCP. More importantly, stability of IPSC also improved using CNG. This study provides a new strategy for interface engineering of metal electrodes and electron transport layers towards the high efficiency and stability of perovskite solar cells.

図・表・数式 / Figures, Tables and Equations

Fig. 1. Device structures and performances. (a) The configuration of devices fabricated in this work using CNG as the IL. (b) J–V curves of IPSCs without ILs (black), with BCP IL (blue), and with CNG IL (red). (c) Cross-sectional SEM observation of the IPSCs with the CNG IL. (d) A box plot chart of the PCE for the IPSCs without ILs (black), with BCP IL (blue), and with CNG IL (red).

Fig. 2. Mechanistic studies. (a) PYS data of IPSCs in the absence of ILs (black), with BCP IL (blue), and with CNG IL (red). (b) Schematic models depicting carrier transfer at the interface between PCBM and Ag. (c) The μ-PCD data of IPSCs without ILs (black), with BCP IL (blue), and with CNG IL (red). (d) Steady-state PL quenching spectra of IPSCs without ILs (black), with BCP IL (blue), and with CNG IL (red).

Table 1 Photovoltaic parameters of IPSCs using different ILs under 1 sun (AM 1.5G, 100 mW cm^–2)

その他・特記事項（参考文献・謝辞等） / Remarks(References and Acknowledgements)

成果発表・成果利用 / Publication and Patents

論文・プロシーディング（DOIのあるもの） / DOI (Publication and Proceedings)

1. Kento Yokoyama, Cationic nitrogen-doped graphene coated silver as low-work function electrode for inverted perovskite solar cells, Applied Physics Express, 16, 081001(2023).
   DOI: 10.35848/1882-0786/acea18
   

口頭発表、ポスター発表および、その他の論文 / Oral Presentations etc.

特許 / Patents

特許出願件数 / Number of Patent Applications：0件
特許登録件数 / Number of Registered Patents：0件