Stable perovskite solar cells. Copyright

Stabilizing Buried Interface via Synergistic Effect of Fluorine and Sulfonyl Functional Groups Toward Efficient and Stable Perovskite Solar Cells

The interfacial defects and energy barrier are main reasons for interfacial nonradiative recombination. In addition, poor perovskite crystallization and incomplete conversion of PbI2 to perovskite restrict further enhancement of the photovoltaic performance of the devices using sequential deposition. Herein, a buried interface stabilization strategy that relies on the synergy of fluorine (F) and sulfonyl (S=O) functional groups is proposed. A series of potassium salts containing halide and non-halogen anions are employed to modify SnO2/perovskite buried interface. Multiple chemical bonds including hydrogen bond, coordination bond and ionic bond are realized, which strengthens interfacial contact and defect passivation effect. The chemical interaction between modification molecules and perovskite along with SnO2 heightens incessantly as the number of S=O and F augments. The chemical interaction strength between modifiers and perovskite as well as SnO2 gradually increases with the increase in the number of S=O and F. The defect passivation effect is positively correlated with the chemical interaction strength. The crystallization kinetics is regulated through the compromise between chemical interaction strength and wettability of substrates. Compared with Cl −. all non-halogen anions perform better in crystallization optimization, energy Band regulation and defect passivation. The device with potassium bis (fluorosulfonyl) imide achieves a tempting efficiency of 24.17%.

Introduction

These merits including low cost, solution processing and outstanding power conversion efficiency (PCE) make perovskite solar cells (PSCs) attract mammoth attention in academia and industry [1,2,3,4,5]. The superior properties of perovskites should be responsible for the extremely Rapid PCE increase from 3.8% in 2009 [6] to presently certified 25.7% [7]. However, the further performance improvement is severely limited by the interfacial nonradiative recombination [8]. In regular PSCs, the interface of electron transport layer (ETL) with perovskite layer is usually referred to as buried interface. It is very difficult to perform in situ characterization of the bottom surface of perovskite films [9]. Therefore, it is much more challenging to modify buried interface compared with the perovskite/hole transport layer (HTL) interface. Buried interfacial defects are a main reason of interfacial non-radiative recombination [10]. The charge trap density at grain boundary (GB) and interface was reported to be much larger than that within perovskite grains [11]. over, the donor type defects and acceptor type defects may exist simultaneously at buried interface which are usually deep-level defects [12, 13]. It is well known that considerable quantity of Sn interstitial defects (Sni) and oxygen vacancy defects (Vo) usually distribute at the surface and in the interior of SnO2 ETL [14]. Sni and Vo defects can be formed spontaneously because of their low formation energies which can affect the photoelectric properties and energy levels of SnO2 [15]. The trap carriers at the heterojunction interface are easy to be trapped by interfacial deep-level defects, resulting in interfacial non-radiative recombination losses and accordingly diminishing PCE and stability. Apart from buried interface defects, interfacial energy barrier resulting from imperfect energy Band alignment also could result in interfacial carrier nonradiative recombination [8]. Interface modification by appropriate materials can improve interfacial energy Band alignment and minimize interfacial energy barrier, resulting in enhanced device performance [12, 16, 17]. The quality of perovskite films plays a key role in fabricating stable and efficient PSCs. The perovskite film quality is primarily determined by its crystallization process. One step and two step approaches are usually employed to prepare perovskite films. Nevertheless, the PCEs of the PSCs based on two step method [18,19,20] are still lower than that of the PSCs using one step method [4, 21]. This should be due to more difficult crystallization control for two step method as compared to one step method. It has been extensively demonstrated that it is an effective approach to modulate perovskite crystallization by modifying ETL substrates [22, 23]. In a word, multifunctional molecules are urgently needed to be developed to manage interfacial carrier through simultaneous realization of interfacial defects passivation, interfacial energy Band alignment optimization and perovskite crystallization modulation.

To date, huge efforts have been devoted to developing various materials to modify buried interface. However, reported most interface molecules have relatively simple functions, either passivating defects, tuning interfacial energy Band alignment or modulating perovskite crystallization [24,25,26]. It is of great importance for simultaneously achieving multiple functions to enrich chemical bonding modes (e.g., ionic bond, coordination bond and hydrogen bond). Among various interface materials, salts containing both cation and anion are the most appropriate candidate for realizing multiple chemical bonds with perovskite and SnO2 layers [16, 17, 27, 28]. The anions and cations in salts can form ionic bonds with charged defects in perovskite films and thus passivate simultaneously positively and negatively charged defects. Fluorine functional groups incorporating into cations and/or anions can not only form hydrogen bond with organic cations in perovskites but also form coordination bond with Pb 2 and Sn 4 [17, 28]. This suggests that fluorination strategy is a feasible and effective approach to accomplish multiple functions induced by multiple chemical bonds. Except for fluorination strategy, introduction of functional groups (e.g., C=O [29, 30], S=O [16], and C=S [31, 32]) in cation and anion is another effective method for enriching chemical interaction modes because these ligand functional groups can effectively passivate defects and control crystallization kinetics. In recent several years, the additive or interface molecules containing non-halogen anions have received considerable attention [33], Non-halogen anions play an important role in defect passivation (HCOO −. CO3 2−. PO4 3− and NO3 − ) [34,35,36,37], crystallization regulation (HCOO −. SCN − and SO4 2− ) [2, 37,38,39,40,41], and energy level alignment modulation (BF4 − and PF6 − ) [17, 28, 42]. However, compared with halide anions, the working mechanism of non-halogen anions is still obscure. In addition, the synergistic effect of non-halogen and halogen functional groups in same anions is still not revealed up to now. Here, it needs to be noted that most researches often adopted single non-halogen anion to modify buried interface [28, 39]. For example, the use of a single non-halogen anion to improve crystallinity and interfacial carrier transport in perovskite films has been reported by Singh et al. and Chen et al. [43, 44]. The specific roles of various functional groups contained in non-halogen anions and the laws of their synergistic effects have not yet been revealed. At the same time, it is difficult for non-halogen anions to be used at the perovskite/HTL interface to exert their maximum potential, because the modified molecules can only interact with the perovskite film and not with the HTL layer. However, this is not beneficial for maximizing the potentials of non-halogen anions. Therefore, it is urgently needed to systematically and deeply uncover the relationships between structures of non-halogen anions, properties of non-halogen anions, defect density, interfacial carrier dynamics and device performance.

Experimental Section

2.1 Materials

The tin (IV) oxide (SnO2, 15% in H2O colloidal dispersion) was bought from Alfa Aesar. Lead (II) iodide (PbI2, 99.99%), Spiro-OMeTAD (99.86%), formamidine hydroiodide (FAI, 99.5%), methylammonium iodide (MAI, 99.9%), and methylamine hydrochloride (MACl, 99.9%) were bought from Advanced Election Technology Co., Ltd. N, N-dimethylformamide (DMF, 99.8%), 2-propanol (IPA, 99.9%), dimethyl sulfoxide (DMSO, 99.9%), chlorobenzene (CB, 99.8%), and acetonitrile (ACN, 99.8%) were obtained from Sigma-Aldrich. Bis(trifluoromethane) sulfonimide lithium salt (Li-TFSI, 99%) and 4-tertbutylpyridine (tBP, 99%) were obtained from Xi’an Polymer Light Technology Corp. Potassium chloride (KCl, 99.8%), potassium methanesulfonate (KMS, 98.0%), potassium bis (trifluoromethanesulfonyl) imide (KTFSI, 97%), potassium bis (fluorosulfonyl) imide (KFSI, 97%) were purchased from Aladdin.

2.2 Device Fabrication

The etched ITO glass was ultrasonically cleaned sequentially by detergent, deionized water, ethanol, acetone and IPA. The SnO2 colloidal solution was spin-coated on the ITO substrates at 4000 rpm for 30 s and then the SnO2-coated ITO substrates were annealed at 150 °C for 30 min. Afterwards, SnO2 films were treated by UV-ozone for 15 min. For modified SnO2 films, different concentrations of KCl solution in water, KMS solution in IPA, KTFSI solution in IPA and KFSI solution in IPA were spin-coated onto the SnO2 films at 5000 rpm for 30 s and annealed at 100 °C for 5 min. The PbI2 solution (691.5 mg, 1.5 mmol mL −1 ) in DMF:DMSO (9:1) was spin-coated onto pristine and modified SnO2 films at 1500 rpm for 30 s and then PbI2 films were annealed at 68 °C for 1 min. After the PbI2 film cooled down to room temperature, 45 μL of organic mixture solution of FAI (90 mg), MAI (6.39 mg) and MACl (9 mg) in 1 mL IPA was spin-coated onto PbI2 films at 2300 rpm for 30 s, and then the films were transferred to ambient air condition (30–40% humidity) and annealed at 150 °C for 15 min. For PEAI modified perovskite films, the 2 mg mL −1 of PEAI solution in IPA was spin-coated onto the perovskite films at 5000 rpm for 30 s without annealing. The hole transport material solution was prepared through dissolving 72.3 mg Spiro-OMeTAD, 35 μL Li-TFSI stock solution (260 mg Li-TFSI in 1 mL acetonitrile), and 30 μL tBP in 1 mL CB. Then the hole transport layer (HTL) was prepared by spin-coating a Spiro-OMeTAD solution on the top of the perovskite layer at 4000 rpm for 30 s. Finally, 100 nm of Ag electrode was thermally evaporated on HTL using a shadow mask.

2.3 Film Characterization

The field emission scanning electron microscopy (FE-SEM, JEOL JSM-7800F) was applied to characterize cross-sectional and surface morphology of perovskite films. X-ray diffraction (XRD) patterns were acquired using a PANalytical Empyrean diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were measured by Thermo Fisher Escalab 250Xi spectrometer using a monochromatized Al source. XPS was calibrated using the peak position of C 1s and UPS was calibrated using the work function of Au. In particular, the samples tested for XPS measurements were prepared by spin-coating the perovskite precursor solutions with different modifiers and annealed to form perovskite films. The optical absorption and transmission spectra were measured by Shimadzu UV3600 Spectrophotometer. The steady-state and time-resolved photoluminescence spectra were performed with a fluorescence spectrophotometer (FLS1000, Edinburgh Instruments Ltd.) which was equipped with a pulse laser diode with a wavelength of 450 nm. The conductivity of SnO2 were carried out on Keithley 2400 source meter with a structure of ITO/PCBM/SnO2 without or with KCl, KMS, KFSI and KTFSI/perovskite/PCBM/Ag. Two-dimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) images were collected on BL1W1A at the Beijing Synchrotron Radiation Facility (BSRF) (λ = 1.54 Å). The trap state density (nt) was determined by the onset of the trap filling limit voltage (VTFL) according to Eq. (1) [45]:

where ε0 is the permittivity of free space, εr is the relative permittivity of perovskite, e denotes the elementary charge and L is the thickness of perovskite layer. Space charge limited current (SCLC) measurement was applied to determine the electron trap density and mobility using the electron-only device with a structure of ITO/SnO2/(KCl, KMS, KFSI or KTFSI)/PCBM/Ag. The SCLC method was employed to measure the electron mobility of the pristine SnO2 film and modified SnO2 films. The electron mobility (μe) is calculated by Eq. (2) [46]:

Efficient and stable perovskite solar cells by build-in π-columns and ionic interfaces in covalent organic frameworks

Show Author’s Information Hide Author’s Information Riming Nie 1 ( ). Xiaokai Chen 1. Zhongping Li 2 ( ). Weicun Chu 1. Si Ma 3. Changqing Li 4. Xiaoming Liu 3. Yonghua Chen 5. Zhuhua Zhang 1. Wanlin Guo 1 ( )

1 State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, Institute for Frontier Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

2 Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea

4 Department of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea

5 Key Laboratory of Flexible Electronics (KLoFE), School of Flexible Electronics (Future Technologies) Institute of Advanced Materials (IAM), Nanjing Tech University (Nanjing Tech), Nanjing 211816, China

cationic covalent organic framework (C-COF), π-columnar arrays, ionic interfaces, charge density, perovskite solar cells

Nie R, Chen X, Li Z, et al. Efficient and stable perovskite solar cells by build-in π-columns and ionic interfaces in covalent organic frameworks. Nano Research. 2023, 16(7): 9387-9397. https://doi.org/10.1007/s12274-023-5603-4

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Perovskite solar cells (PSCs) have attracted much attention due to their rapidly increased power conversion efficiencies, however, their inherent poor long-term stability hinders their commercialization. The degradation of PSCs first comes from the degradation of hole transport materials (HTMs). Here, we report the construction of periodic π-columnar arrays and ionic interfaces over the skeletons by introducing cationic covalent organic frameworks (C-COFs) to the HTM. Periodic π-columnar arrays can optimize the charge transport ability and energy levels of the hole transport layer and suppress the degradation of HTM, and ionic interfaces over the skeletons can produce stronger electric dipole and electrostatic interactions, as well as higher charge densities. The C-COFs were designed and synthesized via Schiff base reaction by using 1,3,5-triformylphloroglucinol as a neutral knot and dimidium bromide as cationic linker. The neutral COFs (N-COFs) were also synthesized as a reference by using 3,8-diamino-6-phenylphenanthridine as neutral linker. PSCs with cationic COF exhibit the highest efficiency of 23.4% with excellent humidity and thermal stability. To the best of our knowledge, this is the highest efficiency among the meso-structured PSCs fabricated by a sequential process.

Lukas Simurka

Lukas Simurka received his M.Sc. degree in physical chemistry from the University of Chemistry and Technology, Prague, Czech Republic in 2011. After that he worked as a researcher at Friedrich-Alexander-University Erlangen-Nuremberg, Germany.

In 2013, he joined Şişecam Science and Technology Center in Turkey, focusing on the optimization of glass surfaces and coatings on glass. He combined his work in the industry with doctoral research at Alexander Dubček University of Trenčín, Slovakia, and received his Ph.D. in Inorganic Technology and Materials in 2018.

Since 2019, he works as a research scientist at TNO/Solliance. His research concentrates on upscaling of perovskite solar cells and modules with a special FOCUS on reliability.