Perovskite solar cells (PSCs) as a representative of third-generation photovoltaic technology have achieved remarkable progress. Their power conversion efficiency (PCE) is significantly enhanced from the initial 3.8% reported in 2009 to the 27% in 2025. Compared to conventional designs that employ expensive hole transport layer (HTL) materials and noble metal electrodes, carbon is considered as an ideal substitute for noble metal electrodes due to its unique structural diversity, chemical stability, and rich surface chemistry. Also, a similarity between the Fermi level of carbon materials and that of metals gives carbon electrodes an advantageous edge in practical applications. HTL-Free carbon-based perovskite solar cells (C-PSCs) typically adopt an HTL-free structure, which simplifies the fabrication process and positions them as promising candidates for single-junction solar cells. However, a direct contact between carbon electrodes and perovskite surfaces inevitably exacerbates a non-radiative recombination that may occur during thin-film preparation. This study was thus to introduce guanidinium iodide (GAI) into perovskite films based on the FAMACsPbI₃ system. GAI could effectively passivate iodine defects and promotes crystal growth and enhance crystal stability through a robust hydrogen-bond network, thereby significantly improving the efficiency and stability of C-PSCs.

Perovskite precursor solutions were prepared via dissolving lead iodide (PbI₂), formamidinium iodide (FAI), methylammonium chloride (MACl), cesium iodide (CsI), and methylammonium iodide (MAI) in a mixed solvent of N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), with different concentrations of guanidinium iodide (GAI) (i.e., 0, 1, 2 mg·mL^-1 and 3 mg·mL^-1) as dopants. The perovskite layer was then deposited on a substrate pre-coated with a TiO₂ layer by a one-step spin-coating method, followed by annealing on a hotplate at 150 ℃ for 13 min. A conductive carbon paste was uniformly blade-coated onto the surface of the perovskite active layer and annealed at 100 ℃ for 10 min to complete the fabrication of the perovskite solar cell.

The crystal phase and morphology of the films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The surface composition, chemical states, and elemental information of the perovskite films were determined by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The optical absorption properties and energy band variations were analyzed by ultraviolet-visible (UV-vis) spectroscopy and steady-state/time-resolved photoluminescence (PL) measurements. The carrier separation/transport dynamics and defect state density in the devices were evaluated by electrochemical impedance spectroscopy (EIS), Mott-Schottky (M-S) analysis, space-charge-limited current (SCLC) measurements, and dark current-voltage (I-V) curves. The performance enhancement of GAI-doped perovskite devices was further investigated via measuring current density-voltage (J-V) characteristics under simulated sunlight.

The XRD patterns of samples doped with varying concentrations of GAI (i.e., 0, 1, 2 mg·mL^-1 and 3 mg·mL^-1) reveal that the perovskite lattice expands with increasing GAI concentration, as evidenced by a gradual shift of diffraction peaks toward lower angles. At a doping concentration of 2 mg·mL^-1, the perovskite exhibits an optimal crystallinity. The AFM and SEM characterization further demonstrates a reduced surface roughness, an enlarged grain size, and an improved film smoothness. The PL measurements show a significantly enhanced peak intensity at the GAI doping level of 2 mg·mL^-1, corroborating a positive role of GAI in promoting perovskite crystallization. The XPS and FTIR analysis elucidates the impact of GAI doping on the internal lattice structure, confirming the presence of hydrogen bonding (N—H...I) and clarifying the mechanistic role of GAI during perovskite crystallization. The effect of GAI doping on the carrier separation and transport is systematically investigated through EIS, Mott-Schottky (MS), SCLC, and dark current measurements. The results indicate that GAI incorporation increases a recombination resistance, suppresses a non-radiative recombination, facilitates a photogenerated carrier separation, and reduces a defect state density from 1.58×10¹⁶ cm^-3 to 1.17×10¹⁶ cm^-3. Consequently, the GAI-doped device achieves a champion power conversion efficiency (PCE) of 17.33%, outperforming the control device (15.7%). In addition, the results of stability tests demonstrate a remarkable improvement (i.e., unencapsulated devices retain approximately 95% of their initial efficiency after 600 h of ambient storage, whereas the control group degrades to below 90%).

This study proposed a low-concentration GAI-doped bulk passivation technique for perovskite films. The research demonstrated that moderate GAI doping could promote a perovskite crystal growth and significantly mitigate both bulk and interfacial defects. Consequently, GAI-doped perovskite films could effectively suppress a non-radiative recombination, enhance carrier transport, and substantially improve the efficiency and stability of HTL-free carbon-based perovskite solar cells (C-PSCs). A high power conversion efficiency of 17.33% was achieved through this strategy. The unencapsulated devices retained approximately 95% of their initial efficiency after 600 hours of exposure to ambient air, exceeding the performance of the control group. These findings could provide a novel research direction for developing highly efficient and stable C-PSCs.