LI Kun, CUI Fuqi, LI Pengzhang, TIAN Chuanjin, ZHAO Wenyan, LIU Yumin

(School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended abstract:[Background and purposes] Carbon-based hole-transport-layer (HTL)-free perovskite solar cells (PSCs) adopt a planar three-layer structure design, offering substantial benefits in terms of cost-effectiveness and environmental sustainability. Nevertheless, the widespread practical application of these devices remains constrained due to challenges such as in adequate power conversion efficiency (PCE) and poor long-term operational stability. Previous research has demonstrated that the construction of a buried interface structure can effectively suppress ion migration in between the perovskite layer and the charge transport layer, thereby enhancing device stability. However, significant technological hurdle persists. During the crystallization process, perovskite thin films are susceptible to the formation of grain boundary defects and surface dangling bonds. These structural imperfections act as non-radiative recombination centers, resulting in substantial losses of photo-generated carriers. In this work, the synergistic optimization effects of a KCl-doped SnO₂ electron transport layer (ETL) in carbon-based HTL-free PSCs were systematically studied. By introducing Cl⁻ and K⁺ into SnO₂ via a solution-based method, effective defect passivation and interface regulation are achieved, with the device PCE was improved from 10.9% to 13.9%. Furthermore, the devices exhibit remarkable environmental stability, retaining over 80% of their initial efficiency after 1000 hours of operation at 60% relative humidity. In summary, our findings highlight the potential of KCl-doped SnO₂ ETLs as a promising strategy for enhancing the performance and stability of carbon-based HTL-free PSCs, paving away for their more extensive practical applications.[Methods] A series of SnO₂ solutions with concentration gradients were prepared via bulk doping with potassium chloride (KCl). The Cs_0.046FA_0.8MA_0.154PbI_2.95Br_0.05 perovskite precursor solution was synthesized by using a one-step method, while the carbon electrode was fabricated by using doctor-blading commercial carbon paste. X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer operating at 40 kV with Cu Kα radiation. Morphology of all samples was characterized by using field-emission scanning electron microscopy (FE-SEM, Hitachi SU-8010, Japan). Current density-voltage (J-V) characteristics of the devices were studied under AM 1.5 simulated sunlight (100 mW·cm⁻²) using a Zolix HPS-1.5XA solar simulator. The light intensity was calibrated with a digital source meter (Keithley 2450). A black mask was employed to define the active area of the solar cell as 0.1 cm². The scan rate was 30 mV·s⁻¹ with a delay time of 0.3 s. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Thermo Scientific ESCALab 250 Xi spectrometer. Steady-state photoluminescence (PL) spectra were acquired using a DW-PLE 03 system, while time-resolved PL (TRPL) spectra were recorded usinga FluoTime 300 spectrofluorometer. External quantum efficiency (EQE) curves were measured under monochromatic light (300–900 nm) using a Newport 66984 EQE system.[Results] We systematically studied the impact of fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) substrates on the upper-perovskite layer growth with the aid of scanning electron microscopy (SEM). It was aimed to elucidate substrate-perovskite interfacial interactions for optimizing perovskite film quality. Subsequent research focused on modifying the buried SnO₂ interface to examine how electron transport layers (ETLs) influenceperovskite morphology and device performance. The KCl-doped SnO₂ (KCl-SnO₂) ETL promoted the formation of densely packed perovskite grains with improved crystallinity. This morphological transformation is expected to enhance the charge carrier transport and collection efficiencies within the device. To further optimize device performance, we conducted a parametric study by varying KCl concentrations in the SnO₂ precursor solution. Notably, devices fabricated with 0.2 mg·mL⁻¹ KCl exhibited a power conversion efficiency (PCE) of 13.9%, demonstrating a ~21% improvement, as compared with the pristine SnO₂-based devices. X-ray photoelectron spectroscopy (XPS) confirmed successful KCl incorporation into the SnO₂ matrix. Additionally, SEM analysis revealed significant increase in perovskite grain size from 240 nm (pristine SnO₂) to 400 nm (SnO₂-KCl). The photovoltaic performance and stability of the carbon-based HTL-free PSC devices were comprehensively evaluated. External quantum efficiency (EQE) measurements yielded integrated short-circuit current densities (Jsc) of 21.8 mA·cm⁻² for the KCl-treated devices versus 20.8 mA·cm⁻² for controls, where the values are consistent with current-voltage (J-V) curve analyses. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra indicated effective charge separation at the SnO₂/perovskite interface. The KCl-SnO₂ C-PSC maintained over 80% of its initial efficiency after 1000 hours in ambient conditions. In contrast, the PCE of control device degraded to 50% within 500 hours. This significant improvement in stability highlights the potential of the KCl-SnO₂ ETL for practical applications of carbon-based PSCs.[Conclusions] In summary, the introduction of KCl doping SnO₂ ETL is a highly effective and robust interfacial engineering approach for carbon-based hole-transport-layer-free (HTL-free) perovskite solar cells. This advancement is attributed to the synergistic interplay between chlorine-mediated energy band alignment and potassium-induced defect passivation. Systematic investigations reveal that KCl modification achieves dual functionality: (1) enhancing charge carrier mobility within the tin oxide scaffold via oxygen vacancy suppression and (2) mitigating interfacial defect states at the SnO₂/perovskite junction, thereby improving perovskite film crystallinity and phase stability. These structural and compositional optimizations significantly suppress non-radiative charge carrier recombination and ionic migration phenomena. Consequently, the optimized device architecture delivers a champion power conversion efficiency (PCE) of 13.9%, accompanied by substantial enhancements in Voc and FF. The innovative composite ETL configuration provides a facile yet versatile methodology for performance enhancement of carbon-based PSCs. Notably, the unencapsulated devices exhibit enhanced stability in ambient air. This multifunctional interface design can be adapted to different organic-inorganic hybrid perovskite systems, particularly promising for the scalable fabrication of large-area photovoltaic modules.

Key words: carbon-based perovskite solar cells; buried interface; synergistic passivation; stability