Halide-mixing braking strategy for 1.95 eV wide-bandgap perovskites enabling high-efficiency triple-junction tandems

Abstract

Wide-bandgap perovskites are widely used in tandem solar cells due to their tunable bandgaps (1.5–2.3 eV) enabled by mixed halide compositions. However, significant open-circuit voltage losses persist, especially when the bandgap is increased to ∼1.95 eV with high bromine (Br) content. High Br incorporation often leads to heterogeneous halide distributions within the bulk, resulting in severe phase segregation and enhanced carrier recombination. To address these issues, a halide-mixing braking strategy is employed by introducing potassium cyanate as a halide-mixing “brake”. This approach effectively slows the halide exchange rate during annealing, promoting homogeneous halide distribution throughout the films. Additionally, it improves perovskite film quality by reducing defect densities, thereby suppressing non-radiative recombination losses. As a result, single-junction 1.95 eV-bandgap perovskite solar cells achieved a power conversion efficiency of 15.93%, with a high open-circuit voltage of 1.40 V and a fill factor of 0.83. Furthermore, mechanically stacked triple-junction all-perovskite tandem solar cells employing 1.95, 1.60, and 1.25 eV perovskite light absorbers achieved efficiencies exceeding 30%. Therefore, this work provides a simple and effective strategy for optimizing high-Br-content perovskites, enabling the development of high-efficiency wide-bandgap perovskite and multi-junction tandem solar cells.

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Broader context

Tandem solar cells based on perovskites are a promising route to surpass the efficiency limits of single-junction photovoltaics, offering scalable and cost-effective pathways to high-performance solar energy conversion. Wide-bandgap perovskites with bandgaps above 1.9 eV are particularly attractive as the top subcells in both triple-junction all-perovskite and perovskite–silicon tandems, since they enable efficient photon harvesting and voltage generation. However, achieving stable and efficient wide-bandgap devices remains a major challenge. In mixed-halide perovskites, the high bromine content required to widen the bandgap typically induces compositional inhomogeneity, halide segregation, and elevated non-radiative recombination losses, which severely limit open-circuit voltage and overall device performance. Overcoming these intrinsic materials challenges is essential to realize the full potential of perovskite tandem photovoltaics. In this work, we propose a simple yet effective “halide-mixing braking” strategy to regulate halide incorporation during crystallization. This approach delivers homogeneous halide distribution, suppresses phase segregation, and improves film quality, thereby enabling efficient wide-bandgap perovskite solar cells and high-performance multi-terminal triple-junction tandems. Our findings provide new insights into halide management in mixed-halide perovskites and offer a practical strategy for advancing next-generation multi-junction photovoltaic technologies.

1. Introduction

In recent years, single-junction perovskite solar cells (PSCs) have developed rapidly through interfacial and compositional engineering, achieving power conversion efficiencies (PCEs) exceeding 27%.^1,2 However, the performance of single-junction PSCs is constrained by the theoretical Shockley–Queisser (S–Q) limit. Tandem solar cells (TSCs) have emerged as effective candidates for further improving PCE by integrating perovskite layers with different bandgaps, enhancing solar spectrum utilization while mitigating thermalization losses.^3–6 To date, PCEs of 30.1% and 34.85% have been certified for all-perovskite and perovskite/silicon double-junction TSCs, respectively, surpassing the efficiencies of single-junction PSCs.^7–12 Triple-junction PSCs hold the potential for even higher PCEs, with theoretical efficiency limits exceeding 50% and high open-circuit voltages (V_OC).^13,14 However, the practical PCEs of triple-junction devices remain lower than those of double-junction TSCs. A major challenge lies in the wide-bandgap (∼1.95 eV) top cells used in triple-junction PSCs, which exhibit significant V_OC losses, thereby hindering further development.^15,16 Additionally, mixed-halide wide-bandgap perovskites often undergo phase segregation into iodine-rich and bromine-rich domains during crystallization,^17–19 leading to halide heterogeneity in the bulk.^20–23 Uncontrolled growth of mixed-halide compositions also results in disordered grain orientations and various defects, such as halogen vacancies (V_I, V_Br) and A-site cation vacancies,^24–26 which induce significant voltage losses and severe halide phase segregation. These issues are particularly pronounced in high-Br-content perovskites needed to achieve bandgaps above 1.9 eV, substantially degrading device performance.^27–30

To address these challenges, various strategies have been explored, including compositional engineering, interface engineering, and additive engineering.^31–34 For instance, Sargent and co-workers reduced Br content and suppressed phase segregation by partially substituting cesium (Cs) with rubidium (Rb), achieving PCEs of 13.79% in single-junction 1.97 eV PSCs and 24.31% in all-perovskite triple-junction TSCs.¹⁵ Yuan et al. demonstrated that field-effect passivation using strong dipole moment molecules, such as piperazine-1-carboxamide hydrochloride and 1,3-propane-diammonium iodide, effectively reduces V_OC losses, enabling 1.95 eV PSCs to achieve a V_OC of 1.38 V and PCE of 25.17% in perovskite/perovskite/silicon triple-junction TSCs.³¹ Concurrently, Wu et al. demonstrated synergistic stabilization of halide ions and enhanced Pb²⁺ fixation in high-Br-content (>60%) mixed-halide wide-bandgap perovskites through cationic β-cyclodextrin chelation, enabling PCEs of 8.63% in single-junction 1.99 eV PSCs and 22.42% in six-terminal triple-junction TSCs.³² Despite these advancements, the performance of 1.95 eV PSCs with high Br content still lags significantly behind other bandgap counterparts, posing a key limitation for high-efficiency triple-junction TSCs. Moreover, limited studies have focused on controlling the complex halide exchange dynamics during crystallization, a critical yet underexplored factor for the fabrication of efficient ≥1.95 eV wide-bandgap PSCs.

High Br-content perovskites typically require extended times for uniform halide mixing, which complicates crystallization and degrades film quality. To address this, we introduce potassium cyanate (KOCN) as a halide-mixing “brake” that prolongs the halide mixing window during crystallization. This strategy effectively slows the migration and exchange of I and Br during film formation, enabling homogeneous halide distribution in the perovskite films. The prolonged homogenization time also improves film quality, promoting the formation of larger grain sizes with reduced defect densities. As a result, single-junction 1.95 eV-bandgap PSCs achieved a PCE of 15.93%, with a high V_OC of 1.40 V and a fill factor (FF) of 0.83. Furthermore, mechanically stacked triple-junction all-perovskite TSCs employing 1.95, 1.60, and 1.25 eV perovskite absorbers achieved a remarkable PCE exceeding 30%. This work demonstrates an effective strategy for regulating crystallization kinetics and halide distribution in wide-bandgap (1.95 eV) perovskites, paving the way for the development of high-efficiency triple-junction perovskite solar cells.

2. Results and discussion

PSCs were fabricated using FA_0.85Cs_0.15PbI_1.1Br_1.9 (FA =  formamidinium) perovskites (denoted as “control”) and perovskites processed with the halide-mixing braking strategy (denoted as “target”). To improve the quality of wide-bandgap perovskite films, KOCN was added to the precursor solutions to facilitate homogeneous halide distribution. We selected OCN⁻ ions because their ionic radius closely matches that of Br, making them more suitable than other commonly used ions such as thiocyanate (SCN⁻). The detailed mechanism of how KOCN influences the growth process is illustrated in Fig. 1a. We hypothesize that the introduction of KOCN slows the halide migration rate, enabling more controlled grain growth and uniform halide distribution. Consequently, films prepared with KOCN are expected to exhibit fewer defect states and reduced non-radiative recombination, leading to improved device performance.

Fig. 1 (a) Schematic illustration of the role of KOCN during the film formation process. (b) and (c) In situ UV-vis absorption spectra of the films during annealing for (b) the control and (c) the target. (d) In situ UV-vis absorption intensity changes over time at the absorption edge. (e) and (f) Evolution of the (100) diffraction peak with different annealing times for perovskite films: (e) control and (f) target.

To validate this hypothesis, a series of systematic characterizations were conducted. In situ UV-vis absorption spectroscopy during annealing was employed to investigate the impact of KOCN on crystallization kinetics. The spectral evolution shown in Fig. 1b and c reflected the dynamics of halide (I/Br) migration and exchange. It was evident that the target film exhibited a slower halide-mixing rate compared to the control. As shown in Fig. 1d, the delayed absorption edge in the target film's final absorbance spectrum further confirmed that KOCN effectively suppresses halide migration and exchange rates. This result can be attributed to the interaction between OCN⁻ and FAI, which slows the insertion of FAI into the [PbX₆]⁴⁻ octahedra.³⁵ This interaction is evidenced by the shift of the OCN stretching vibration peak from 2168 nm⁻¹ to 2159 nm⁻¹, as shown in Fig. S1, SI. Consistent with these findings, in situ UV-vis results in Fig. S2a and b, SI demonstrated prolonged homogenization time in the target film, promoting a uniform mixed-halide distribution.³⁶ Moreover, energy-dispersive spectroscopy (EDS) mappings of Br and I for the control and target films are presented in Fig. S3, SI. The target film exhibited a more homogeneous distribution of Br and I than the control film, indicating that the KOCN treatment enables a uniform mixed-halide distribution. X-ray diffraction (XRD) analyses of films at various annealing times further confirmed these observations (Fig. 1e and f). Both films ultimately exhibited good crystallinity; however, the control film showed a distinct shift of the (100) plane peak toward lower angles within 5 s of annealing, indicating rapid iodide ion incorporation into the lattice. Additional XRD measurements before and after annealing (Fig. S4a and b, SI) revealed that both films initially displayed a clear α-phase, but after annealing, the (100)/(110) peak intensity ratio increased from 1.47 (control) to 2.28 (target), indicating enhanced preferential (100) orientation, which benefits carrier transport and reduces trap-state density.²⁶ GIWAXS patterns (Fig. S4c and d, SI) supported this observation, showing a stronger (100) scattering ring in the target film. For comparison, XRD of films prepared with lead thiocyanate (Pb(SCN)₂) displayed a distinct PbI₂ peak (Fig. S5, SI), which is detrimental to device performance,^37,38 highlighting KOCN as a superior additive. Collectively, these results demonstrate that KOCN promotes homogeneous halide distribution and preferred crystallographic orientation, thereby improving perovskite film quality—critical for achieving high device performance.^39,40 Finally, X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of KOCN, with a distinct K signal detected in the target film (Fig. S6, SI). The presence of K⁺ ions further contributes to defect passivation at grain boundaries.^41,42

To further investigate the influence of KOCN on perovskite films, scanning electron microscopy (SEM) and Kelvin probe force microscopy (KPFM) were performed to evaluate film improvements achieved through the KOCN. As shown in Fig. 2a, the control film exhibited small grain sizes (<200 nm), resulting in a high grain boundary density. Increasing grain size is an effective strategy to reduce grain boundaries and enhance device performance.⁴³ Notably, the KOCN can precisely promote grain growth by slowing the halide-mixing rate, consistent with previous reports.⁴⁴ SEM images of the target film (Fig. 2b) clearly showed larger grain sizes compared to the control film. Particle size distributions (Fig. S7a and b, SI) further confirmed that the KOCN-treated film exhibited an average grain size increase of approximately 100 nm relative to the control. Large-area SEM images (Fig. S8a and b, SI) also demonstrated this trend. Cross-sectional SEM images (Fig. S8c and d, SI) revealed that the control film exhibited a stacked grain morphology, which induces defects and impedes carrier transport, potentially leading to severe carrier recombination. In contrast, the target film showed improved morphology, which is expected to facilitate carrier transport and reduce recombination losses. KPFM was subsequently used to investigate the surface properties of the films. As shown in Fig. 2c and d, the root-mean-square (RMS) surface roughness of the control film was measured at 11.8 nm, while the roughness of the target film decreased to 10.6 nm. This reduction in roughness can improve interfacial contact between the perovskite and transport layers. Additionally, surface potential measurements (Fig. 2e and f) indicate that the control film exhibited a non-uniform contact potential difference, likely due to non-uniform halide distribution and poor film quality. In contrast, the target film processed via KOCN treatment demonstrated a uniform contact potential, suggesting improved halide distribution and film quality.²⁴ KPFM data (Fig. S9a, SI) visually confirmed the distribution of contact potential in both films. The non-uniform contact potential in the control film can led to broadened interfacial electronic states and increased recombination, while the uniform contact potential in the target film indicated reduced recombination and improved carrier extraction.^45,46 Furthermore, graphene with a work function of 4.50 eV was used as the reference sample (Fig. S9b, SI). Based on this calibration, the work functions of the films were determined to be 4.98 eV for the control and 4.92 eV for the target. Collectively, these results demonstrated that KOCN facilitated grain growth and promoted uniform halide distribution during crystallization, leading to high-quality perovskite films, which are critical for achieving improved device performance.

Fig. 2 Top-view SEM images of the films: (a) control and (b) target. AFM images of the films: (c) control and (d) target. KPFM images of the films: (e) control and (f) target.

Owing to the improved crystalline quality achieved through the KOCN incorporation, the carrier dynamics of the perovskite films were further investigated using steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. As shown in Fig. 3a, the target film exhibited a noticeably increased PL intensity compared to the control film, indicating reduced non-radiative recombination in the perovskite films following KOCN treatment. Additionally, the TRPL results in Fig. 3b showed that the target film exhibited a prolonged carrier lifetime relative to the control film, with detailed lifetimes extracted from the TRPL decay curves summarized in Table S1, SI. Under simulated 1-sun illumination, the PL spectra (Fig. S10, SI) further showed that the control film underwent pronounced phase segregation, whereas the target film displayed only minimal segregation. Furthermore, PL mapping images of the control and target films (Fig. 3c and d) exhibited consistent trends with the steady-state PL results, with the target film displaying enhanced PL intensity across a larger area. The improved PL intensity and extended carrier lifetimes suggested that KOCN treatment effectively reduced non-radiative recombination and facilitated carrier transport, which can be attributed to the decreased trap density and enhanced film quality.

Fig. 3 (a) Steady-state PL and (b) TRPL spectra of control and target perovskite films. PL mapping images of the films: (c) control (d) target.

The improved properties of perovskite films achieved through the KOCN were expected to significantly enhance PSC performance. To evaluate this, we fabricated single-junction WBG perovskite solar cells with an architecture of ITO/NiO_x/SAM/PVK/C60/BCP/Cu, as illustrated in Fig. 4a. The energy levels of the control and target perovskite films were measured using ultraviolet photoelectron spectroscopy (UPS), as shown in Fig. 4b and c. The Fermi level of the target perovskite film upshifted from −4.88 eV (control) to −4.83 eV, indicating weaker n-type characteristics due to KOCN incorporation. This also suggests a change in the work function of the target film, consistent with the KPFM results discussed above,⁴⁷ and contributing to improved energy level alignment. As shown in Fig. 4c, the energy band alignment diagram of each functional layer demonstrates an optimized structure in the target device, favorable for carrier transport. Notably, although both KPFM and UPS measurements showed the same trend, slight differences in the Fermi level values were observed, which may arise from variations in the measurement principles and environments.⁴⁸ Additionally, transient photocurrent (TPC) measurements were used to investigate the improvement in carrier transport, as shown in Fig. S11, SI. The target device exhibits a shorter photocurrent decay time compared to the control device, suggesting accelerated carrier extraction and transport. Moreover, the trap-filled limit voltage (V_TFL) reduced from 0.66 V to 0.23 V after the modification of KOCN (Fig. S12, SI), the defect-state density (n_trap) of perovskite films can be estimated using the following formula.

where n_trap is the trap density, q is the elementary charge, ε is the relative dielectric constant (ε = 24.4), ε₀ is the permittivity of vacuum (ε₀ = 8.854 × 10⁻¹² F m⁻¹) and d is the thickness of the perovskite layer (d = 300 nm). According to formula, the corresponding n_trap was calculated to be 1.96 × 10¹⁶ cm⁻³ for the control film and 0.69 × 10¹⁶ cm⁻³ for the target film, suggesting the reduction of the trap state with KOCN modification.

Fig. 4 (a) Device architecture of single-junction 1.95 eV-bandgap PSCs. (b) UPS spectra of control and target films. (c) Energy level alignment diagram of each layer in devices. (d) J–V curves, (e) EQE spectra, and (f) steady-state power output of control and target devices. (g) Nyquist plot, (h) light intensity-dependent J–V curves, and (i) Mott–Schottky plots of control and target devices.

The detailed current density–voltage (J–V) curves and photovoltaic parameters of the devices are presented in Fig. 4d and Table S2, SI. Notably, the incorporation of KOCN led to an increase in V_OC from 1.35 V (control) to 1.40 V (target) and an improvement in the fill factor (FF) from 0.79 to 0.83. As a result, the PCE increased from 14.41% for the control device to 15.93% for the target device, with V_OC losses of 0.60 V and 0.55 V, respectively, representing one of the highest efficiencies reported for ≥1.95 eV PSCs. The corresponding PCE statistics are presented in Table S3, SI. The integrated short-circuit current density (J_SC) values derived from the external quantum efficiency (EQE) spectra (Fig. 4e) were 13.2 mA cm⁻² and 13.3 mA cm⁻² for the control and target devices, respectively, consistent with the J–V measurement results. Additionally, stabilized power output measurements showed PCEs of 13.86% for the control device and 15.41% for the target device after 600 s of operation (Fig. 4f). Operational stability tests under continuous maximum power point (MPP) tracking in an N₂-filled glovebox (Fig. S13, SI) revealed that the target device retained ∼85% of its initial PCE after 300 h, whereas the control device retained less than 60%. This enhanced stability was attributed to the improved film quality of the target device, characterized by reduced defect densities and homogeneous halide distribution, which suppressed phase segregation and ionic migration in the perovskite films.

The device parameters for different KOCN concentrations are summarized in Fig. S14, SI, revealing an optimal concentration of 2 mg mL⁻¹ for achieving the highest performance. To benchmark KOCN against other reported additives, we also fabricated devices with Pb(SCN)₂. As shown in Fig. S15, SI, the Pb(SCN)₂-based device even exhibited a slightly lower PCE than the control, likely due to residual PbI₂ in the film, consistent with the XRD results in Fig. S5. To further investigate the influence of KOCN on device operation, a series of electrical characterizations were conducted. The charge transport behavior of the devices was examined using electrochemical impedance spectroscopy (EIS), with Nyquist plots measured under dark conditions (Fig. 4g). The target device exhibited a significantly higher recombination resistance (R_rec) compared to the control, indicating effective suppression of defect-mediated recombination. The response of the devices under varying light intensities was also investigated to characterize carrier recombination behavior. As shown in Fig. 4h, both control and target devices displayed a linear relationship between V_OC and the logarithm of light intensity. Notably, the slope of the fitted line for the target device was lower than that of the control device, indicating reduced defect-assisted recombination under illumination.⁴⁹ Additionally, dark I–V measurements (Fig. S16, SI) revealed a lower reverse-bias dark current in the target PSCs, suggesting suppressed leakage currents and further confirming that KOCN effectively reduced carrier recombination, contributing to the observed V_OC enhancement. To elucidate the origin of the higher V_OC in the target PSCs, Mott–Schottky measurements were performed (Fig. 4i). The built-in potential (V_bi) increased from 1.15 V in the control device to 1.20 V in the target device after KOCN treatment, providing a stronger internal electric field to facilitate carrier separation and extraction, thereby contributing to the higher V_OC observed in the target PSCs.⁴⁴

Encouraged by the improved performance of the 1.95 eV WBG PSCs, we proceeded to fabricate mechanically stacked triple-junction all-perovskite tandem solar cells (TSCs). These were assembled using semi-transparent WBG PSCs as the top cell, medium-bandgap (MBG) PSCs as the middle cell, and narrow-bandgap (NBG) PSCs as the bottom cell. The device architectures of the semi-transparent WBG PSCs, MBG PSCs, NBG PSCs, and the tandem configuration are illustrated in Fig. 5a. To reduce damage to the perovskite layer during the sputtering of the ITO transparent top electrode, BCP was replaced with ALD-deposited SnO₂ in the semi-transparent WBG PSCs. These semi-transparent WBG PSCs achieved a remarkable PCE of 14.4%, with a V_OC of 1.39 V and an FF of 80.78%, as shown in Fig. 5b and summarized in Table 1. Additionally, the integrated J_SC of the semi-transparent 1.95 eV WBG PSCs, calculated from the EQE spectrum (Fig. 5c), was 12.5 mA cm⁻², consistent with the J_SC obtained from the J–V measurements. The stabilized power output of the semi-transparent WBG PSCs under constant illumination at the MPP for 100 s is shown in Fig. 5d, yielding a stabilized PCE of 14.20%. After light passed through the semi-transparent WBG top cell, the MBG middle cell employing a 1.6 eV bandgap perovskite absorber achieved a PCE of 8.23% with a J_SC of 8.69 mA cm⁻². Subsequently, the NBG bottom cell with a 1.25 eV bandgap mixed Pb-Sn perovskite absorber achieved a PCE of 7.42% with a J_SC of 10.78 mA cm⁻². For reference, the standalone PCEs of the corresponding MBG and NBG PSCs prior to stacking are presented in Fig. S17, SI and Table 1, with the MBG and NBG PSCs achieving PCEs of 21.28% and 23.17%, respectively. The transmittance spectra of the semi-transparent WBG and WBG + MBG PSCs are provided in Fig. S18, SI. Ultimately, the multi-terminal all-perovskite TSC achieved a promising PCE of 30.04% (Fig. 5b and Table S4, SI), with a stabilized power output maintained at 29.07% (Fig. 5d), demonstrating the potential of this architecture for high-efficiency multi-junction all-perovskite TSCs.

Fig. 5 (a) Device architecture of multi-terminal triple-junction all-perovskite TSCs. (b) J–V curves, (c) EQE spectra, and (d) steady-state power outputs of semi-transparent WBG, light-filtered MBG, and light-filtered NBG PSCs.

Table 1 Photovoltaic parameters of single-junction semi-transparent WBG, MBG, and NBG PSCs before and after light filtering

Device	V OC (V)	FF (%)	J SC (mA cm^−2)	PCE (%)
WBG	1.40	80.78	12.77	14.39
MBG	1.16	83.13	22.02	21.28
Filtered	1.13	83.93	8.69	8.23
NBG	0.88	80.43	32.77	23.17
Filtered	0.85	80.77	10.78	7.42
Total				30.04

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3. Conclusions

In summary, we have demonstrated a halide-mixing braking strategy by employing KOCN as a crystal growth modulator to achieve high-quality perovskite films with Br content exceeding 60%. This approach yielded perovskite films with improved crystallinity and reduced defect densities, while the prolonged halide migration and exchange during crystallization effectively promoted homogeneous halide distribution within the films. This strategy minimized voltage losses and suppresses non-radiative recombination, leading to a significant enhancement in PCE and V_OC. Using this method, we achieved a PCE of 15.93% and a V_OC of 1.40 V for single-junction 1.95 eV-bandgap PSCs and a PCE exceeding 30% for mechanically stacked triple-junction all-perovskite tandem solar cells, representing one of the highest reported efficiencies for triple-junction all-perovskite TSCs to date. This work provides an effective pathway to improve crystal quality and reduce voltage losses in WBG PSCs with high Br content, advancing efforts toward higher efficiency and stability in multi-junction all-perovskite TSCs.

4. Experimental section

Experimental details are provided in the SI.

Author contributions

Z. X. and W. K. conceived the idea and designed the experiments. Z. X., G. L. developed tandem solar cells. J. H., G. C., D. P., H. F., L. H., J. W., S. F., S. Z., Z. Y., Z. Y., K. M., Y. G., performed formal analysis. S. Z., W. K., and G. F. reviewed and did editing. G. F., and W. K. supervised the project.

Conflicts of interest

The authors declare no conflict interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: (Experimental Section, Fig. S1–S18, and Table S1–S4). See DOI: https://doi.org/10.1039/d5ee05574a.

Acknowledgements

This work was supported by Key Lab of Artificial Micro-and Nano-Structures of Ministry of Education of China, Wuhan University. The authors thank the Core Facility of Wuhan University for PL mapping and SEM measurements. The authors also acknowledge the financial support from the National Natural Science Foundation of China (Grant Numbers: 12174290, 12134010, 12574078, W. K.), Guangdong Basic and Applied Basic Research Foundation (Grant Number: 2024A1515012483, W. K.), Shenzhen Science and Technology Program (Grant Number: JCYJ20250604122534005, W. K.), and the Fundamental Research Funds for the Central Universities (Grant Number: 2042025kf0057, W. K.).

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