Deep Eutectic Solvent-Assisted Lithium Immobilization and 4-Tert -butyl pyridine-Less Regulation Strategy for Enhancing the Stability of Conventional Perovskite Solar Cells

doi:10.6023/A26030067

Abstract

Conventional perovskite solar cells (PSCs) exhibit broad commercial prospects in the photovoltaic field due to their high power conversion efficiency (PCE) and low fabrication cost. However, the limited carrier transport efficiency of the hole transport layer (HTL) and insufficient long-term stability remain critical bottlenecks hindering their industrialization. The traditional doping system of 4-tert-butyl pyridine (tBP)/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) has inherent drawbacks: tBP is volatile and Li⁺ tends to migrate, which easily induce interfacial degradation and device performance decay. Herein, we propose a doping strategy using a deep eutectic solvent (DES) composed of choline chloride (ChCl), urea, and ethylene glycol (EG) with a molar ratio of 1∶2∶1 at a concentration of 1 mg•mL⁻¹ into the Spiro-OMeTAD precursor solution to optimize the Spiro-OMeTAD-based HTL, combined with regulating the tBP/LiTFSI molar ratio to 2∶1 for tBP reduction. Driven by hydrogen bonding and coordination interactions, the DES achieves a synergistic mechanism of “lithium immobilization-defect passivation-molecular network enhancement”: specifically, the carbonyl groups (C=O) of urea in DES coordinate with Li⁺ to suppress thermal-induced migration, the amino groups (-NH₂) form hydrogen bonds with TFSI^- to promote LiTFSI dissociation, and the hydroxyl groups (-OH) of EG assist in stabilizing the coordination network; meanwhile, DES molecules passivate uncoordinated Pb²⁺ on the perovskite surface to reduce non-radiative recombination, and enhance the π-π stacking of Spiro-OMeTAD to optimize carrier transport channels and increase its glass transition temperature (T_g) for improved thermal stability. Benefiting from this strategy, the fabricated conventional PSCs achieve a champion PCE of 21.15%. Moreover, the devices exhibit significantly improved long-term stability under various environmental conditions: 97% of the initial PCE is retained after 1000 h of dark storage at 25 ℃ in N₂ atmosphere (vs. 88% for the control device); the T₈₀ lifetime (time to retain 80% of the initial efficiency) reaches 120 h at 85 ℃ in N₂ atmosphere (vs. 48 h for the control device); and 80% of the initial PCE is maintained after 500 h of storage at 25 ℃ and 30%~40% relative humidity in air (vs. 60% for the control device). This work provides an effective “low-dose, high-compatibility, long-stability” strategy for HTL doping engineering, which is of great significance for promoting the industrialization of PSCs.

Key words: perovskite solar cells (PSCs), Spiro-OMeTAD, deep eutectic solvent (DES), Li⁺ migration, stability

Cite this article

Xiangming Xiong, Lin Zhang, Wenjun Xu, Jinghang Tang, Ying Yang. Deep Eutectic Solvent-Assisted Lithium Immobilization and 4-Tert -butyl pyridine-Less Regulation Strategy for Enhancing the Stability of Conventional Perovskite Solar Cells[J]. Acta Chimica Sinica, 2026, 84(6): 888-896.

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Fig. & Tab. 5

Figure 1. Mechanism of HTL doped with 1 mg•mL−1 DES. (a) DES formation and Li+ immobilization mechanism; (b) FT-IR spectra of DES and LiTFSI; (c) XPS spectrum of Li 1s; (d) XPS spectrum of Pb 4f; (e) XRD pattern of PVK/HTL-Control; (f) XRD pattern of PVK/HTL-DES; (g) Intensity ratio of PbI2 peak to (001) FAPbI3 peak (*: PbI2; ▲: (001) FAPbI3)

Figure 2. Carrier transport properties of HTL doped with 1 mg•mL−1 DES. (a) UV absorption spectra of t5Li10-DES, t10Li14-DES, t39Li23-DES and t39Li23 after 18 h oxidation in dry air; (b) UV absorption spectra of t5Li10-DES after 72 h oxidation in dry air; (c) Absorption peak intensity at 520 nm of UV absorption spectra of t5Li10-DES, t39Li23-DES and t39Li23 after 72 h oxidation in air; (d) Conductivity curves of t5Li10-DES, t10Li14-DES, t39Li23-DES and t39Li23 after 18 h oxidation in dry air; (e) Conductivity comparison; (f) Conductivity variation of HTL during oxidation in air; (g) Hole mobility test curves; (h) Hole mobility comparison; (i) PL spectra

Figure 3. Evaluation of thermal aging performance of HTLs with different doping ratios. (a) UV absorption spectra before and after oxidation in air and thermal aging at 85 ℃; (b) Variation in conductivity before and after oxidation and thermal aging at 85 ℃; (c) Variation in hole mobility before and after thermal aging; (d) XRD patterns before and after thermal aging at 85 ℃; (e) FT-IR spectra; (f) FT-IR spectra before and after thermal aging at 85 ℃; (g) DSC curves; (h) PL spectra of PVK/HTL before and after thermal aging at 85 ℃; (i) UV absorption spectra of PVK/HTL before and after oxidation and thermal aging at 85 ℃

Figure 4. Evaluation of photovoltaic performance of devices regulated by doping engineering. (a) Device structure; (b) Device efficiency with different doping concentrations; (c) Efficiency distribution histogram of devices with 1 mg•mL−1 DES doping; (d) Photovoltaic performance of devices with 1 mg•mL−1 DES doping and different tBP/LiTFSI doping ratios; (e) Device J-V curves; (f) Steady-state power output (SPO); (g) Relationship between tBP/LiTFSI molar ratio in HTL and device efficiency variation

Figure 5. Stability evaluation of devices regulated by doping engineering. (a) Dark-state stability of unpackaged devices under N2 atmosphere at 25 ℃; (b) Thermal stability of unpackaged devices under N2 atmosphere at 85 ℃; (c) Humidity stability of unpackaged devices under air atmosphere at 25 ℃ and 30%~40% RH; (d) Comprehensive evaluation of device stability performance

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