Multidentate chelation via pyridine-based molecules for high-efficiency perovskite solar cells

Abstract

The non-radiative recombination loss at the interface between a perovskite layer and hole transport layer is one of the main sources of energy loss in perovskite solar cells (PSCs). Non-radiative recombination caused by surface defects greatly limits further improvement of power conversion efficiency (PCE). Interfacial passivation is one of the commonly used methods to improve PCE, but traditional passivation methods often involve single-point coordination, and their passivation effect is limited. In this study, we used a multi-site binding passivation method, introducing diethyl 2,6-pyridinedicarboxylate (DIP) as a passivation layer at the interface between the perovskite and the hole transport layer. The ester and pyridinium groups can interact effectively with uncoordinated Pb²⁺, and this multidentate chelation effect can repair various defects, improve crystallization, and promote interfacial carrier transfer. As a result, the optimized device achieves an efficiency of 23.68%. Additionally, due to the hydrophobicity of DIP, the device exhibits excellent humidity stability, maintaining 89.40% of its initial PCE after 550 hours at a relative humidity of 65 ± 5%.

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1. Introduction

Over the past few decades, metal halide perovskites have demonstrated remarkable performance in photovoltaics.^1–5 The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has rapidly increased from the initial 3.8% to 26.7%.⁶ However, the PCE of PSCs is still far from the theoretical Shockley–Queisser limit.⁷ Polycrystalline perovskite films prepared by the solution method are loaded with large numbers of defects in the surface and bulk phases, which lead to non-radiative recombination, affect the carrier transport, and result in a reduction of the PCE.^8–14 The passivation of these surface defects is the most direct and effective strategy to improve the PCE.^15,16 To passivate surface defects, many functional molecules are used by researchers. You et al. reported the use of an organic halide salt, phenethylammonium iodide (PEAI) on HC(NH₂)₂–CH₃NH₃ mixed perovskite films for surface defect passivation. They found that the PEAI layer forms on the perovskite surface and this results in higher-efficiency cells by reducing the defects and suppressing non-radiative recombination.¹⁷ Furthermore, other compounds such as iodopentafluorobenzene¹⁸ and potassium halides¹⁹ are also used to passivate surface defects. Among many molecules, pyridine derivatives have attracted our attention; they are mostly formed by combining pyridine groups with carbonyl, carboxyl or amine groups as well as halogen groups, and have demonstrated good passivation effects on defects. At the same time, the various sites on the pyridine ring can be linked to multiple functional groups, which can passivate defects to a greater extent.^20–25 However, there has been relatively little research on the multidentate chelation passivation of perovskites using pyridine derivatives. Multidentate chelation can utilize various functional groups to passivate different defects of perovskites. By attaching specific functional groups to the target sites, a synergistic passivation effect can be achieved. The multi-site binding makes the connection between functional molecules and perovskite more stable, so as to achieve a more obvious optimization effect.

Herein, we introduce a pyridine-based molecule with a rich ester group as a passivator, diethyl 2,6-pyridinedicarboxylate (DIP), which can be anchored to the surface of a perovskite. The ester and pyridyl groups interact effectively with the Pb²⁺ defect sites, to reduce the surface recombination and minimize the formation of defects. Moreover, due to the excellent hydrophobic properties of the DIP molecule, its use significantly improves the stability of PSC devices. Consequently, the PSCs achieved a superior PCE value of 23.68%. These unencapsulated devices demonstrate excellent stability, and can maintain 92.96% of their initial efficiency after 1500 hours at room temperature under a nitrogen (N₂) atmosphere. Under ambient conditions with a relative humidity of 65 ± 5%, the DIP-modified device retains 89.40% of its initial PCE after 550 hours.

2. Results and discussion

The chemical structure of diethyl 2,6-pyridinedicarboxylate (DIP) is shown in Fig. 1a, which reveals that the molecule contains one pyridine group and two ester groups. Considering the presence of several strongly electronegative groups in DIP, the distribution of electrostatic potential was studied using density functional theory. As shown in Fig. 1b, variations in the electronegativity of the molecule are observed through changes in the color of different regions of its electrostatic potential surface. The red areas on the electrostatic potential (ESP) surface of DIP highlight regions of electronegative concentration, indicating potential sites that can bind with a perovskite. We found that the red regions predominantly surround the carbonyl (C=O) and pyridyl (C=N) groups, indicating high electron density that favors their coordination with uncoordinated Pb²⁺. Additionally, DIP undergoes efficient and reversible coordination with iodine atoms, which is expected to regulate the crystallization process and reduce trap density in the perovskite.²⁶ To confirm the strong interaction between DIP and PbI₂, Fourier transform infrared (FTIR) spectroscopy was used (Fig. 1d). The symmetric stretching vibrations of the C=O and C=N groups in DIP were observed at 1717.5 and 1582.8 cm⁻¹, respectively. After mixing with PbI₂, the corresponding peak positions shifted to 1721.3 and 1579.9 cm⁻¹, indicating that both the ester and pyridyl groups interact with the uncoordinated Pb²⁺ defects in the perovskite.^27–30 The interaction of DIP with perovskite was then characterized by X-ray photoelectron spectroscopy (XPS), and the survey spectra are shown in Fig. 1e and f. In the control film, the XPS characterization of Pb 4f shows two dominant peaks at 143.27 and 138.38 eV, which could be assigned to Pb 4f^5/2 and Pb 4f^7/2, while the Pb 4f peaks shifted toward lower binding energy (143.05 eV for Pb 4f^5/2 and 138.18 eV for Pb 4f^7/2) after DIP passivation. The shift of the peaks demonstrated the decrease in the positive charge of the uncoordinated Pb²⁺, resulting from the donation of the electron pair of the N atom of the C=N group and the O atom of the C=O group to the empty 6p orbital of the uncoordinated Pb²⁺ according to the coordination bond.³¹ The characteristic I 3d^3/2 and I 3d^5/2 peaks of the control perovskite film were located at 630.81 and 619.32 eV, respectively, and these signals shifted toward lower binding energy (630.62 eV for I 3d^3/2 and 619.17 eV for I 3d^5/2) after DIP passivation. These shifts of the peaks are similar to the abovementioned shift of the Pb 4f peaks. According to the detections in the FTIR and XPS spectra, we verified strong interaction between the DIP and the perovskite. The C=O and C=N groups could interact with the uncoordinated Pb²⁺ defects and I⁻ defects in the perovskite to repair the uncoordinated ionic defects.³² We then calculated the binding energy of the perovskite in different binding states (Fig. S1†). The calculation results revealed that the binding energy of the bidentate chelation configuration is much higher than the binding energy of the monodentate model and, in particular, the binding energy value of the two ester groups chelated at the same time is the largest (Fig. 1g), which is −1.652 eV.

Fig. 1 (a) Chemical structure of DIP. (b) ESP surface map of the DIP molecule. (c) Structure illustration of FAPbI₃. (d) FTIR spectra of the solution: DIP and DIP/PbI₂ (all dissolved in DMSO). (e and f) detailed Pb 4f and I 3d XPS spectra, respectively. (g) DFT modeling of the binding energy of DIP molecules on the perovskite surface.

Furthermore, in order to investigate the grain boundary and defects at the surface of the perovskite film, Kelvin probe force microscopy (KPFM) was conducted to analyze the surface potential of different perovskite films (Fig. 2a). During the linear scan, the tip of the probe travels through the grain interior and grain boundary. By observing the change in the surface potential of the perovskite (Fig. 2b), we can establish the defect state of the grain boundary and film surface. The deep-level defects have much lower formation energies and are more likely to form at grain boundaries and/or the surface of perovskite films, thus leading to significant changes in the surface potentials.³³ According to the KPFM results, the surface potential distribution over the DIP-modified perovskite film was more homogeneous and had less variation, which proved that the defect states at grain boundaries and the film surface were obviously reduced. The more homogeneous surface potential suggests that it would be more beneficial for carrier transfer to take place in the transport layer.^34,35 Moreover, the DIP treatment can also reduce film roughness from ∼33.0 nm (control) to ∼28.4 nm (target), flattening the height differences between the grain surface and grain boundary (GB) (Fig. S2†). The detailed surface morphology of a perovskite film with or without DIP modification was observed by scanning electron microscopy (SEM). As shown in Fig. 2c, the top-view SEM image of the perovskite film without DIP shows some holes and small grains. In contrast, the DIP-modified perovskite film shows larger grain sizes, and the holes have disappeared. This implied that the interaction between DIP and the perovskite promoted larger sized perovskite grains during the formation of perovskite crystals.^36,37 The cross-sectional image shows the same results in Fig. S3.† X-ray diffraction (XRD) measurements were performed to analyze the crystallinity of the perovskite films before and after DIP modification (Fig. 2d). It can be found that the control perovskite film showed a strong PbI₂ signal at 12.8°, while the DIP-modified perovskite film exhibited a weaker PbI₂ signal and stronger peak corresponding to (001), indicating that the perovskite with DIP modification acquires much stronger crystallinity. The ratio of α-phase FAPbI₃ peak intensity to PbI₂ peak intensity is 4.95 for the control device and 8.80 for the DIP-modified device (Fig. S4†). The performance of the perovskite films was evaluated by steady-state photoluminescence (PL) measurements (Fig. 2e). Compared to the pristine perovskite, the stronger PL intensity of the DIP-modified perovskite suggests that non-radiative recombination is inhibited. The PL mappings show that the target film exhibits enhanced luminescence intensity compared to the control. It is also demonstrated that DIP is uniformly distributed on the surface of the film, contributing to the enhancement of its PL intensity (Fig. S5†). Time-resolved photoluminescence (TRPL) decay measurements were conducted to probe the fluorescence lifetime of the perovskite film (Fig. 2f). The fitted decay times of the corresponding perovskite films are summarized in Table S1.† The TRPL profiles of the control and DIP-modified perovskite films were fitted with a biexponential function. We observed a longer carrier lifetime (τ₁ = 172.15 ns; τ₂ = 1004.53 ns) in the DIP-modified perovskite film compared to that in the perovskite film without DIP modification (τ₁ = 42.89 ns; τ₂ = 232.07 ns). The calculated average carrier lifetime of the DIP-modified samples is 739.83 ns, much longer than that of the control samples (143.82 ns), indicating that carrier non-radiative recombination loss is significantly suppressed in DIP-modified perovskite films.³⁸

Fig. 2 (a) The KPFM images of the control and DIP-modified perovskite films (scale bar, 1 μm). (b) The surface potential line profiles of the control and DIP-modified perovskite films. (c) SEM images of perovskite films without and with DIP modification coated on glass (scale bar, 1 μm). (d) XRD patterns of the perovskite film with and without DIP modification. (e) PL spectra of the perovskite films with and without DIP modification. (f) TRPL spectra of the perovskite films with and without DIP modification.

To evaluate the passivation effect of DIP, Fig. 3a shows the formation energy of iodine vacancy (V_I), anti-site lead–iodine (Pb_I), anti-site iodine–lead (I_Pb), FA vacancy (V_FA) and Pb vacancy (V_Pb) defects on the perovskite surface. Clearly, DIP treatment leads to greatly increased formation energy, which means the corresponding defects are harder to form.³⁹ Furthermore, to reveal the effect of DIP on the properties of perovskite films, ultraviolet photo-electron spectroscopy (UPS) was used to study the surface energetics of perovskite films (Fig. S6†). According to the results indicated in Fig. 3b, the conduction band minimum (CBM) and valence band maximum (VBM) of perovskite films without and with DIP modification could be obtained by UPS and ultraviolet–visible (UV–vis) absorption spectroscopy (Fig. S7†). The CBM was adjusted from −3.91 eV to −3.89 eV upon DIP modification. The upward-bending VBM of DIP-processed films more favorably aligns with the HOMO energy levels of spiro-OMeTAD, facilitating hole extraction and alleviating carrier accumulation.⁴⁰ Subsequently, hole-only devices with FTO/PEDOT:PSS/PVK/spiro-OMeTAD/Au structures were prepared to quantitatively assess the trap-state density and hole mobility using the space charge limited current (SCLC) strategy (Fig. 3c). The trap-state density (N_t) of the electron transport layer (ETL) can be estimated through the following equation:

V_TFL = eN_tL²/2εε₀

where e and L are the elementary charge and the thickness of the films, and ε and ε₀ are the dielectric constant and the vacuum permittivity, respectively.^41–43 The V_TFL values of the control and target are 0.439 V and 0.343 V, respectively. This suggests that DIP can effectively reduce the trap states, consequently suppressing non-radiative recombination.⁴⁴ As evidenced by the dark J–V curves shown in Fig. S8,† the dark saturation current of the DIP-modified PSCs is smaller than that of the control PSCs, indicating the decreased non-radiative recombination after DIP passivation at the interface.⁴⁵ To systematically investigate the reasons for the improved device performance, electrochemical impedance spectroscopy (EIS) was performed in the dark to study interfacial charge transfer and recombination (Fig. 3d). The semicircle in the high frequency region and the semicircle in the low frequency region determine the charge transfer resistance (R_ct) and the recombination resistance (R_rec), respectively. It can be seen that the DIP-modified device has larger R_rec and smaller R_ct values compared to those of the control device, indicating that DIP can effectively inhibit charge recombination at the interface.^45,46 The Mott–Schottky curve (Fig. S9†) reveals an increased built-in potential (V_bi) for the DIP-modified device, which shows a higher V_bi response than the control device. The increased V_bi can enhance the driving force and carrier injection, which is also the reason for the increase in V_oc.

Fig. 3 (a) Formation energy values of perovskite surface V_I, V_Pb, V_FA, Pb_I and I_Pb defects with and without DIP modification. (b) Schematic diagram of the energy level structure. (c) The space charge limited current versus voltage of hole-only devices with and without DIP modification. (d) Nyquist plots of PSCs with and without DIP modification. (e) Illustration of the roles of the DIP treatment in the PSCs.

Based on the above results, a schematic diagram illustrating the passivation mechanism of DIP was proposed, as shown in Fig. 3e. The two ester groups and one pyridine group of DIP can readily interact with Pb²⁺ defect sites, anchoring to the surface of the perovskite, which can reduce surface recombination, and reduce the formation of defects. The synergistic impact of these beneficial effects can significantly reduce defects in perovskite films, ultimately facilitating the generation of pinhole-free, large-grained and highly crystalline perovskite films, improving the device efficiency.

Then, the effects of DIP on the device performance of PSCs were systematically studied. PSC devices with and without DIP addition were fabricated to examine their photovoltaic performance. As shown in Fig. 4a, a standard device structure of FTO/SnO₂/PVK/spiro-OMeTAD/Au was used. The optimal photoelectric performance parameters of the control and the DIP-modified devices under AM 1.5G illumination at 100 mW cm⁻² are shown in Table S2 and Fig. S10.† The J–V curves of the best PSCs are shown in Fig. 4b, Whereas the control device gives a PCE of 22.15%, the DIP-modified device delivers a significantly improved PCE of up to 23.68% with an increased J_sc value of 25.83 mA cm⁻², a V_oc of 1.13 V, and an FF of 81.05%. Fig. 4c shows the monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra for these PSCs. The integrated current densities are 24.80 and 25.10 mA cm⁻² for the control and DIP-treated PSCs, respectively. The remarkable PCE enhancement is mainly dominated by the significant increase in V_oc and FF, which is attributed to the more efficient carrier extraction and passivation of defects induced by DIP.

Fig. 4 (a) Schematic of the complete architecture of a PSC device. (b) J–V curve of the best performing device with or without DIP modification. (c) The IPCE spectra and integrated J_sc values. (d and g) XRD patterns of control and DIP-treated perovskite films after 30 days aged at relative humidity of 65 ± 5%. (e) Normalized PCE of the unencapsulated device with and without DIP modification at the relative humidity of 65 ± 5%. (h) Normalized PCE of the unencapsulated device with and without DIP modification at 25 ± 5 °C under dark ambient conditions in an N₂ glovebox. (f and i) Water contact angles of control and target perovskite films.

To study the effect of DIP on the perovskite stability, we carried out XRD measurements on perovskite films that had been aged for 30 days under relative humidity of 65 ± 5%. As shown in Fig. 4d and g, the initial perovskite phase of the control sample was largely transformed into either PbI₂ (at 12.7°) or a non-perovskite δ-phase (at 11.8°). However, for the DIP-modified perovskite film, the initial perovskite phase shows only partial decomposition or phase change after moisture erosion, affirming its improved stability. The ratio of α-phase peak intensity to PbI₂ peak intensity is shown in Fig. S11.† We measured the water contact angle of the control and target perovskite films (Fig. 4f and i). The control film shows a water contact angle of 62.7°, and the DIP-modified perovskite film shows a water contact angle of 90.3°, indicating that DIP can confer improved stability in a humid environment. Then, the PCEs of the devices were examined under different storage conditions. Under ambient conditions with a relative humidity of 65 ± 5% (Fig. 4e), the control device retained just 55.29% of its initial PCE value after 550 h. In contrast, the DIP-modified device could maintain 89.40% of its initial PCE, which can be attributed to the enhanced hydrophobic properties after DIP treatment. For the aged stability test at 25 ± 5 °C under N₂ (Fig. 4h), after 1500 h, the control device retained just 65.38% of its initial PCE, while the DIP-modified device could maintain 92.96% of its initial PCE. These results fully demonstrate the excellent stability of DIP-modified PSCs, confirming the improved stability induced by DIP treatment. The introduction of DIP will help enhance the comprehensive stability of PSCs, which is critical for the further application of PSC technology.

3. Conclusion

In summary, a multidentate coordination strategy using diethyl 2,6-pyridinedicarboxylate (DIP) as an additive was employed to regulate crystal growth and inhibit defects in perovskite solar cells (PSCs). Systematic theoretical analyses indicate strong intermolecular interactions between FAPbI₃ with defects and DIP via the multidentate coordination effect. DIP can be anchored to the perovskite surface, with its ester and pyridine groups interacting effectively with Pb²⁺ defect sites, which reduces surface recombination and minimizes defect formation. As a result, higher crystallinity was achieved in the subsequent formation of perovskite films. Ultimately, DIP-modified devices exhibited prolonged carrier lifetimes and enhanced built-in voltage, leading to significantly improved power conversion efficiency (PCE) of 23.68%. This work highlights the passivation effect of pyridinyl derivatives in PSCs and demonstrates that the synergistic passivation by a multidentate molecule provides valuable insights for improving PSC performance.

Author contributions

Yongxiang Zheng: investigation, data curation, and writing – original draft. Jinfeng Zhang: investigation and formal analysis. Aiqin Sun: investigation and formal analysis. Zhiqian Yang: investigation and formal analysis. Mengting Han: investigation. Yan Meng: formal analysis. Yan Meng and Mei Lyu: formal analysis. Yang Huang: conceptualization, methodology, and writing – review & editing. Jun Zhu: funding acquisition, methodology, and formal analysis. Linhua Hu: funding acquisition, supervision, and project administration.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2022YFB4200500), the Key Research and Development Plan Project of Anhui Province (2022h11020014), the National Natural Science Foundation of China (52302237) and the Collaborative Innovation Program of Hefei Science Center, CAS (2022HSC-CIP006), the CASHIPS Director's Fund (YZJJ201902 and YZJJZX202018), the Anhui Provincial Natural Science Foundation (2408085MB029), the Anhui Postdoctoral Scientific Research Program Foundation (2024B811) and the Natural Science Foundation of Hebei Province of China (B2024402018). Numerical computations were performed at the Hefei Advanced Computing Center.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi03269a

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