Precision engineering chiral interfaces for efficient spin injection in metal halide heterostructures

Domain: chemistry
Published: 2026-02-20
Editorial outcome: major_revision

Abstract
Precise control of interfaces is crucial for spin generation, transport, and detection in opto-spintronics. However, the interface engineering for efficient spin injection remains a significant challenge. Here, we synthesized a helical structure of PbI2 (R-PbI2) via an interfacial chirality-induced growth approach at the heterostructure interface. This few-nanometer-thick R-PbI2 layer shows a lower lattice mismatch with both the adjacent R-NEAPbI3 (R-NEA refers to R-1-(1-naphtyl)ethylamine) and PbI2 layers, and leads to an optimal chiral interface in the chiral heterostructure with minimized residual strain and defect density. Combined with circularly polarized pump-probe spectroscopic and spin-photovoltaic measurements, our chiral heterostructure interface contributes a spin-injection efficiency up to 68%, thus leading to a degree of polarization of 29% in photocurrent. The precise synthesis of a chiral interface offers a promising route to manipulate spin dynamics and achieve a high degree of spin polarization required for advanced opto-spintronic applications.

Introduction
The generation of spin-polarized charge carriers in semiconductors is the fundamental prerequisite for opto-spintronics1–3. Major approaches for generating spin-polarized carriers rely on spin injection by ferromagnetic contact or circularly polarized light4,5, but typically require cryogenic temperatures, external magnetic fields, or precise optical alignment. Recently, chiral metal halide semiconductors (MHSs) have emerged as promising materials in opto-spintronics due to their strong spin-orbit coupling and chirality-induced spin selectivity (CISS)6–9, enabling room-temperature spin polarization without external magnetic fields. However, insulating chiral organic components lead to relatively poor charge transport and electron-phonon spin depolarization, which limits their performance10,11. Heterostructures that consist of chiral MHSs and achiral semiconductors have been demonstrated to enhance the charge transport and the degree of polarization12–16. This approach synergistically combines the spin selectivity of chiral MHSs with the optoelectronic properties of achiral semiconductors17–19.

The interface plays a critical role in both optoelectronic and spintronic applications. In optoelectronics, its precise characteristics control charge separation and recombination, directly impacting performance. For spintronics, the interface is crucial for the effective injection, manipulation, and detection of spin-polarized carriers. Therefore, the selection of a robust fabrication method for the heterostructure is essential. Previous heterostructure fabrication methods include dry-transfer stacking18, hot-casting14, solution-based spin-coating20, and self-assembly21. The interfacial characteristics in terms of morphology, defect density, bonding, and chemical interaction in chiral MHS heterostructures can govern their functional properties, particularly charge transfer, spin dynamics, and chiroptical responses. Dry-transfer method generally aims for chemically inert interfaces to preserve the integrity of the constituent layers18. Hot-casting method relies on specific and designed chemical bonds14. Spin-coating and self-assembly methods can involve interfacial chemical transformations, which might be leveraged for in-situ tuning of properties but also pose challenges in predictability20,21.

Here, we synthesize chiral MHS heterostructures by an interfacial chirality-induced growth approach with PbI2 and R-NEAI (R-NEA denotes R-1-(1-naphtyl)ethylamine) solutions. The chiral interface is engineered with a high-crystallinity and helical R-PbI2 layer, which effectively suppresses interfacial spin depolarization and leads to a spin-injection efficiency of up to 68%. Our results demonstrate that lattice mismatch, residual strain, and defect density at the interface are critical factors for spin-injection efficiency. A spin-photovoltaic device of R-NEAPbI3/R-PbI2/PbI2 heterostructure achieves 29% degree of polarization in photocurrent, twice that of previously reported chiral MHS heterostructures15,16. This chiral MHS heterostructure growth approach paves the way for efficient spin injection and a high degree of spin polarization at room temperature, advancing the development of opto-spintronic applications.

Results
Synthesis of the chiral interface by an interfacial chirality-induced growth method
We spin-coated a precursor solution containing equimolar amounts of PbI2 and R-NEAI onto a glass substrate, followed by annealing at 100 °C for 5 min to grow the R-NEAPbI3 film. Subsequently, the film was post-annealed at 100 °C in air to in-situ grow the PbI2 layer on the top of R-NEAPbI3 (details are provided in the Method). The delocalized R-NEA+ cations between R-NEAPbI3 and PbI2 induced a chiral lattice deformation of PbI2 by interfacial chirality transfer22, shown in Fig. 1a. In the post-annealing process, the R-NEAPbI3 is mainly converted into PbI2, with only a small amount of conversion to a few-nanometer-thickness R-PbI2 between R-NEAPbI3 and PbI2. The R-NEAPbI3 phase content decreased with increasing annealing time, while the PbI2 content correspondingly increased. From the in-situ X-ray diffraction spectra in Fig. 1b, the peak at 12.7° refers to the (001) peak of the PbI2 phase and becomes prominent with increasing annealing time. The full width at half maximum (FWHM) of the (002) peak of the R-NEAPbI3 phase slightly increases from 0.16° to 0.18° with the annealing time. The increase in FWHM of the (002) peak of R-NEAPbI3 is mainly due to the precipitation of PbI2, which reduces the grain size of the remaining R-NEAPbI3 domains, as estimated from the Scherrer equation23. The in-situ X-ray diffraction spectra show that there is no by-product (R-NEA)2PbI4, whose diffraction peak of the (002) plane is at 5.8° (Figure S1). The PbI2 content can be estimated by the energy-dispersive X-ray spectra shown in Figure S2. We achieved 6% PbI2 after 30 min of post-annealing.

In Fig. 1c, as the post-annealing time increases, R-NEAPbI3 is partially converted into PbI2, evidenced by the weakening of the R-NEAPbI3 absorption peak24 at 375 nm and the strengthening of the PbI2 absorption peak25 at 500 nm (detailed analysis in Figure S3). In Fig. 1d, a pair of reverse circular dichroism (CD) peaks at 376 nm and 408 nm corresponds to the excitonic absorption of R-NEAPbI3. Additionally, shoulders near 500 nm in CD spectra are caused by the excitonic absorption of PbI2 rather than (R-NEA)2PbI4. These shoulders become more pronounced with increasing PbI2 content, suggesting that the PbI2 lattice undergoes chiral deformation. The absorption dissymmetry factor, gCD, at 500 nm reaches a maximum value of 0.02 at a PbI2 concentration of 6% (Figure S4). Consequently, CHS-I at this concentration was selected for subsequent research, as it yielded R-PbI2 with the highest relative proportion of PbI2.

A comparable CD spectral feature appears in the S-configuration samples, though with reversed polarity (Figure S5). In contrast, the racemic sample exhibits no CD response (Figure S5). To distinguish the true CD signal from artifacts arising from linear circular dichroism and linear birefringence, we extracted the genuine CD signal following established methods26 (detailed analysis in Supplementary Information). This genuine signal persists at 500 nm corresponds to the exciton absorption peak of PbI2, indicating that the shoulder peak at this wavelength originates from the intrinsic chirality of R-PbI2 rather than experimental artifacts. To assess the universality of this interfacial chirality-induced growth method, we performed post-annealing treatments on 1D R-MBAPbI3, 2D (R-MBA)2PbI4, and 2D (R-4-Br-MBA)2PbI4 (where R-MBA and R-4-Br-MBA denote R-1-phenylethanamine and R-1-(4-bromophenyl)ethanamine, respectively). In these films, the CD signal near 500 nm increases significantly with extended post-annealing time, a trend attributed to the formation of R-PbI2 (Figure S6).

This lattice distortion is further supported by our DFT calculations (details in Supplementary Information and Figure S7). The deformation is attributed to asymmetric hydrogen bonding (H2N–H···I) between the iodine atoms of PbI2 and R-NEA+ at the interface, which induces structure distortion in PbI2 and leads to the formation of a chiral R-PbI2 layer. The chiral axis of R-PbI2 is parallel to the in-plane direction of R-NEAPbI3. From the atomic force microscopic image in Fig. 1e, PbI2 grown on top of the R-NEAPbI3 shows a flake-like structure and inserted vertically into the R-NEAPbI3 layer. The PbI2 flakes have a length of 1.2±0.3 μm, a width of 39±8 nm, and a thickness of 145±0.5 nm. The size and distribution of PbI2 flakes can be tuned by changing the post-annealing time (see Figure S8 for more details).

We characterized the lattice structure using high-resolution transmission electron microscopy (HR-TEM) in Fig. 1f. The image reveals two distinct lattice structures in the heterostructure, a lattice spacing of 0.362 nm, corresponding to the (013) plane of R-NEAPbI3, and a lattice spacing of 0.338 nm and 0.348 nm, associated with the (002) plane of PbI2 and R-PbI2, respectively. Using Fourier and inverse Fourier transforms, we isolated the (002) plane of PbI2 at the interface by applying a mask. Within a 5 nm-thick region at the interface, detailed analysis reveals a helical lattice distortion featuring a spiral radius of 0.02 nm and a pitch of 0.84 nm. These observations agree with our DFT calculations. The interface exhibits chirality because the helically arranged R-NEA+ cations transfer their chiral information to the adjacent PbI2, inducing a helical structure in the PbI2.

Chiral heterostructure interface characterizations
Two different chiral MHS heterostructures are synthesized to demonstrate the critical role of the R-PbI2 layer at the interface. CHS-I refers to the heterostructure with R-NEAPbI3/R-PbI2 and R-PbI2/PbI2 interfaces, and CHS-II refers to the heterostructure with only R-NEAPbI3/PbI2 interface. CHS-II is synthesized without interfacial chirality-induced growth, resulting in PbI2 forming nanoparticles with a diameter of 150±20 nm. The PbI2 nanoparticles is embedded within R-NEAPbI3 layer, as shown in Figure S9. CD response and gCD are zero at 500 nm regardless of the PbI2 content27 (Figure S10), indicating the absence of helical lattice distortion in PbI2 of CHS-II.

In Fig. 2a, we plot the schematic lattice structures of both CHS-I and CHS-II. In CHS-II, the lattice mismatch is mainly located at the R-NEAPbI3/PbI2 interface, whereas in CHS-I, it is primarily at the R-NEAPbI3/R-PbI2 interface. HR-TEM analysis in Fig. 1f reveals that the R-NEAPbI3 (103) plane contacts the PbI2 or R-PbI2 (002) plane, with lattice fringes intersecting at approximately 65°. We calculated the interfacial lattice mismatch28 using the measured interplanar spacings, d(103)=0.362 nm for R-NEAPbI3, d(002)=0.338 nm for PbI2, and d'(002)=0.348 nm for R-PbI2. This yields a lattice mismatch of 3.9% for CHS-I and 6.6% for CHS-II. The lower lattice mismatch in CHS-I shows that the R-PbI2 interlayer effectively relieves the interfacial lattice mismatch. Then we used grazing incident X-ray diffraction spectra shown in Figure S11 and related 2θ vs sin2φ plot in Fig. 2b to characterize the residual strain distribution in CHS-Ⅰ and CHS-Ⅱ. The tensile strain is determined from the slope of 2θ vs sin2φ plot29,30, yielding 0.42% for CHS-I and 1.48% for CHS-II. This reduced tensile strain in CHS-I is attributed to the flexibility of the R-NEAPbI3/R-PbI2 interface31.

The defect densities of CHS-Ⅰ and CHS-Ⅱ were evaluated using the space-charge-limited current method (device configurations are in Figure S12), as shown in Fig. 2c,d. The current (I) initially increases linearly with voltage (V), which is the ohmic response. Then the current rises exponentially (I/Vn, n>2) at 0.14 V for CHS-I and 0.4 V for CHS-II, indicating a transition to the trap-filled limit (TFL), in which all the available trap (defect) states are filled by the injected carriers32. The onset voltage is determined by:

VTFL = en d L2 / 2εrε0 (1)

where L is film thickness, εr is the permittivity, and ε0 is the vacuum permittivity, and nd is the defect density33,34. Notably, nd in CHS-I is 0.39×1016 cm-3, approximately one-third that of CHS-II (1.10×1016 cm-3). To exclude sample-to-sample variability as the cause of the reduced defect density, we fabricated multiple devices for both CHS-I and CHS-II under identical synthesis and processing conditions (Figure S13). The nd of CHS-I is (0.38±0.04)×1016 cm-3, which is lower than that of CHS-II [(1.26±0.59)×1016 cm-3], indicating that CHS-I possesses a more efficient passivation mechanism than CHS-II. For CHS-I, post-annealing partially decomposes R-NEAPbI3 into PbI2 and R-NEAI, enriching the interface with excess R-NEA+ and I-. The R-NEA+ cations act as Lewis bases to passivate undercoordinated Pb and Pb vacancies, while I- compensates iodide vacancies, thereby reducing the interfacial defect density35.

To investigate the exciton dynamics in the heterostructure, we performed transient absorption (TA) spectroscopy using a linearly polarized pump pulse of 365 nm wavelength and a broadband linearly polarized probe. Figure 2e,f show the absorption difference between pump and probe (ΔA = Apump on - Apump off) from TA spectra. Figure 2e exhibits distinct ground state bleaching (GSB) signals at 410 nm and 500 nm for R-NEAPbI3 and PbI2, respectively. There are two photo-induced absorption signals at 450 nm and 520 nm for R-NEAPbI3 and PbI2, respectively. It indicates that excitons are formed after photo-excitation of R-NEAPbI3 and PbI2 in the heterostructure. Figure 2f shows the time evolution of excitons probed in the GSB region at 500 nm. The TA signals exhibit mono-exponential decay kinetics, yielding an exciton lifetime of 16.9±0.3 ps for CHS-Ⅰ, which is longer than the 8.3±0.3 ps measured for CHS-Ⅱ. The exciton lifetime in CHS-I or CHS-II is shorter compared to that of pristine PbI2 (26.2±0.9 ps, Figure S14) due to promoted exciton dissociation in the type-II heterojunction formed by R-NEAPbI3/PbI2 heterostructure21. The shorter exciton lifetime in CHS-II (Fig. 2f) indicates more efficient exciton dissociation and charge transfer, consistent with the bulk heterostructure formed by PbI2 nanoparticles distributed throughout the R-NEAPbI3 matrix36. In contrast, the PbI2 nanosheets, vertically inserted R-NEAPbI3 matrix in CHS-I, provide a smaller interfacial area, leading to slower exciton dissociation and a longer lifetime.

Modulation of exciton spin dynamics at the chiral interface
We investigated the exciton spin polarization dynamics in the heterostructures at room temperature using circularly polarized TA (CTA) spectroscopy (schematic in Fig. 3a). A right-circularly polarized (σ+) pump pulse at 365 nm selectively generated excitons with angular momentum J=+1 in both PbI2 phase and R-NEAPbI3 phase. The exciton spin relaxation of this initial +1 state can lead to the formation of -1 exciton via spin-flip37. The subsequent dynamics were monitored by probing the TA spectra with broadband right- or left-circularly polarized light (σ+ or σ-). The time evolution of TA spectra collected at 500 nm with co-circular (σ+σ+) and cross-circular (σ+σ-) pump-probe configurations are shown in Fig. 3b,c. The relaxation of exciton spin polarization is evaluated by the difference between the TA signals obtained with co-circular and cross-circular configurations (ΔAσ+σ- - ΔAσ+σ+). Exponential fitting of the decay curves yields exciton spin relaxation times of 19.2±1.9 ps for the PbI2 phase in CHS-Ⅰ and 9.2±1.5 ps in CHS-Ⅱ, respectively. Based on temperature-dependent and pump fluence-dependent spin relaxation time, we determine that the spin relaxation in PbI2 is dominated by the Elliott-Yafet mechanism (details in Figure S15 and Figure S16). The spin relaxation time and bandgap of PbI2 are positively correlated38,

τE ∝ 1 / (E - Δso)2 E T2 τp (2)

where Δso is the spin-orbit splitting of the valence band, Eg is bandgap, T is temperature, τp is momentum relaxation time. A 0.06 eV bandgap increase in CHS-I (Figure S17) arises mainly from the PbI2 morphology rather than the few-nanometer R-PbI2 layer. The thickness of PbI2 in CHS-I (39±8 nm) is much thinner than that of CHS-II (150±20 nm), leading to stronger quantum confinement and thus a larger bandgap39, which in turn prolongs the exciton spin relaxation time in the PbI2 phase of CHS-I. The exciton lifetime τ and spin relaxation time τs determine the fraction of spin-polarized electrons before the spin injection at the interface, that is, a renormalization factor12:

F = 1 / (1 + τ / τs) (3)

The renormalization factors F for CHS-Ⅰ and CHS-Ⅱ are 53.2±2.5% and 52.6±4.2%, respectively, indicating that the initial spin polarization created by the light within the PbI2 layer is comparable for both samples. In contrast to typical TA spectra, where circularly polarized excitation directly induces spin polarization, the observed spin splitting between σ+ and σ- probe spectra under linearly polarized excitation must arise from the spin-selective electron transfer at the heterojunction (schematic in Fig. 3d). The excitation wavelength of linearly polarized pump pulse is fixed at 465 nm rather than 500 nm to selectively excite PbI2 phase and avoid the interference between the R-NEAPbI3 and PbI2 excitons in the GSB region (500 nm), respectively. We measured the TA spectra with linear pump and σ+ or σ- probe, which are plotted in Fig. 3e,f. The degree of CTA polarization in PbI2 represents the degree of spin polarization between excitons with J=+1 and J=−1, which can be calculated by9,40,41 (ΔAσ+ - ΔAσ-)/(ΔAσ+ + ΔAσ-) × 100%. In CHS-Ⅰ, the degree of CTA polarization increases to -40% within 0–20 ps, corresponding to the spin-selective transfer from PbI2 to R-NEAPbI3 for generating spin polarization in PbI2. Subsequently, within 20–100 ps, the degree of CTA polarization gradually decays to 0% due to the spin relaxation. We also measured CHS-I samples with 2% and 20% PbI2 (Figure S18), the degree of CTA polarization is below than that of the CHS-I sample with 6% PbI2. At low loading (2%), the chiral R-PbI2/R-NEAPbI3 interfacial area is too small for efficient spin injection, whereas at high loading (20%), excess PbI2 introduces additional defect states as spin-relaxation channels. An intermediate loading of 6% maximizes the effective interfacial area while limiting defect-assisted depolarization, yielding the highest CTA polarization. These demonstrate an inherently long exciton spin relaxation time in CHS-I without circularly polarized light excitation. In contrast, the CTA polarization stabilizes near 0% in CHS-II, indicating that there is no significant spin-selective transfer from PbI2 to R-NEAPbI3 in the CHS-II sample. This comparison indicates that the R-NEAPbI3/R-PbI2 and R-PbI2/PbI2 interfaces in CHS-I effectively promote the spin-polarized electron injection from PbI2 to R-NEAPbI3, resulting in an increased and sustained degree of CTA polarization of -40%. For comparison, the degree of CTA polarization of pristine R-NEAPbI3 achieves 5% in Figure S19 due to the chirality-induced spin splitting9.

Spin-injection efficiency of a spin-photovoltaic device based on the R-NEAPbI3/R-PbI2/PbI2 heterostructure
To quantify the spin-injection efficiency at the interface, a spin-photovoltaic device is fabricated and shown in Figure S20. A key practical parameter of a spin-photovoltaic device is its sensitivity to circularly polarized light, which can be quantified by the degree of photocurrent polarization (DOPcurrent),

DOPcurrent = (Iσ+ - Iσ-)/(Iσ+ + Iσ-) × 100% (4)

where Iσ+ and Iσ- are photocurrents under σ+ and σ- excitations. As shown in Fig. 4a, DOPcurrent is determined by four steps. In the first step, the circularly polarized light excitation at 505 nm selectively excites the PbI2. The σ+ or σ- selectively generates ms=+1/2 or ms=−1/2 electrons and corresponding holes according to the spin-dependent optical selection rule42. The initial degree of spin polarization (P0) generated42 by σ+ or σ- excitation in PbI2 is nearly 100%. Spin depolarization in the second step is due to spin relaxation mechanisms inherent to PbI2, as accounted for by a renormalization factor. The subsequent loss in spin polarization during the third step is determined by the interfacial spin-injection efficiency (Pinj), defined as the differential spin polarization before and after spin injection. Finally, the fourth step’s reduction in spin polarization depends on the spin filtering efficiency (Pfilter) of R-NEAPbI3 due to the CISS effect.

The I–V responses at room temperature for spin-photovoltaic devices based on CHS-I and CHS-II are shown in Fig. 4b,c. The CHS-I device shows distinctly different photocurrent (Iσ+ and Iσ-) in Fig. 4b between σ+ and σ- excitations, despite identical intensities of 1 mW/cm2. Iσ+ is higher than Iσ- because the CISS effect in R-NEAPbI3 selectively favors the transport of electrons with ms=+1/2. In contrast, the CHS-II device exhibits a negligible difference in photocurrent between σ+ and σ- excitations in Fig. 4c. These observations indicate that the difference in photocurrent response under circularly polarized light is due to differences in the loss of spin polarization within the heterostructure. The CHS-I device shows a DOPcurrent of 29%, which is twice as high as that of CHS-II and previously reported chiral MHS heterostructures15,16. To exclude the possibility of sample-to-sample variations, we conducted a statistical analysis of DOPcurrent for CHS-I and CHS-II. The DOPcurrent of CHS-I is 27.9±1.1%, which is higher than the DOPcurrent of CHS-II (9.4±1.8%) (Figure S21). The DOPcurrent of CHS-I device is also higher than the device with only R-NEAPbI3 as the absorber layer (DOPcurrent = 10% as shown in Figure S22). Finally, interfacial spin-injection efficiency can be calculated by12:

Pinj = DOPcurrent / (P0 F Pfilter) (5)

To calculate the Pinj of the device, we assumed a Pfilter of approximately 80% for R-NEAPbI3. This value is derived from established literature on structurally analogous chiral metal-halide systems (containing R-NEA or R-MBA) that share the same spin-filtering geometry, specifically transport along the chiral axis7,8,13,43–45. Figure 4d reveals that Pinj for CHS-Ⅰ increases from 54% at 0.2 V to 68% at −0.4 V. In contrast, Pinj for CHS-Ⅱ remains relatively constant at about 26% over the voltage range studied. The high spin-injection efficiency in CHS-I is attributed to the engineered optimal chiral interface, which leads to the suppression of the interfacial spin scattering46. We therefore conclude that the spin-photovoltaic device based on the R-NEAPbI3/R-PbI2/PbI2 heterostructure shows high interfacial spin-injection efficiency, leading to improved CTA and photocurrent polarization, which is correlated with the presence of a helical R-PbI2 layer at the interface. The R-PbI2 layer provides a structurally and chirally matched interface that is associated with efficient spin injection governed by the R-NEAPbI3 layer. Although the spin-injection efficiency of CHS-I reaches up to 68%, DOPcurrent is limited to 29%, primarily due to the renormalization factor (F) of approximately 50%, indicating that nearly half of the spin polarization is lost during spin transport in the PbI2 phase. To further increase F, strategies such as promoting exciton dissociation and extending the spin relaxation time in the PbI2 phase can be considered. Enhancing exciton dissociation may be achieved by decreasing the thickness of the PbI2 layer while maintaining the thickness of the R-PbI2 layer. Additionally, prolonging the spin relaxation time, which is dominated by the Elliott-Yafet mechanism, could be accomplished by operating the device at lower temperatures.

We performed multi-wavelength excitation measurements (405, 450, 488, and 505 nm) on spin-photovoltaic devices based on both CHS-I and CHS-II (details in Figure S23 and Figure S24). For the CHS-I, DOPcurrent is minimized (>10%) when the excitation wavelength approaches the R-NEAPbI3 exciton absorption peak (375 nm) but is maximized (29%) when the PbI2 phase is selectively excited at 505 nm. To extend the spectral response of the spin-photovoltaic devices, bandgap engineering strategies such as halogen doping should be investigated47,48. In contrast, DOPcurrent of the CHS-II device shows minimal variation (±3%) across excitation wavelengths due to its inherently low spin-injection efficiency. Rather than the orientation-sensitive pure R-NEAPbI3 device, where DOPcurrent varies widely (5%–90%) across literature reports24,49–52, our CHS-I device utilizes a chirality-induced grown interface from the same precursor stack without additional process steps. This approach delivers reproducible circularly polarized light detection (29% DOPcurrent) and offers broad applicability to other chiral MHSs.

In this work, we present a synthesis method of interfacial chirality-induced growth that achieves a record-high spin-injection efficiency of 68% in MHSs. Using CTA spectroscopy, we reveal that lattice mismatch, residual strain, and defect density at the interface play a critical role in chiroptical properties. This precisely engineered interface in CHS-I results in a CTA polarization of -40% under linearly polarized light. The spin-photovoltaic device exhibits a maximum photocurrent polarization of 29% at 505 nm, doubling the performance of previous devices. Our findings highlight the important role of the chiral interface for high-performance opto-spintronic applications.

Methods
Material preparation
All reagents and materials were purchased from Meyer and Aladdin without further purification. Purchased reagents included (R)-(+)-1-(1-naphthyl)ethylamine (≥98%), hydroiodic acid (47 wt% solution in water, stabilized with ≤ 1.5% hypophosphorous acid), hypophosphorous acid (50 wt% solution in water), and lead iodide (99.99%).

Synthesis of chiral organic ammonium salts
12 mL of organic amine R-NEA was dissolved in 50 mL of anhydrous ethanol, and the vacuum environment was maintained by evacuating and continuously passing nitrogen gas. 20 mL of hydroiodic acid and 2 mL of hypophosphorous acid were slowly injected, and the reaction was stirred continuously at 0 °C for 2 h. The solution was dried under a vacuum at 80 °C for a dash-yellow solid product, which was dissolved in 25 mL of anhydrous ethanol, and 150 mL of anhydrous ether was added to precipitate the product, and the product obtained by filtration was repeated three times to obtain a pure white powder product, i.e., iodine salt of organic amine R-NEAI.

CHS-Ⅰ and CHS-Ⅱ films fabrication
First, a 1.25×1.25 cm2 soda-lime glass was soaked in detergent, purified water, acetone, and isopropanol for ultrasonic cleaning for 15 min, and ultraviolet-ozone treatment for 30 min. Homogeneous R-NEAPbI3 film was synthesized by dissolving 0.9 M PbI2 and 0.9 M R-NEAI in DMF and heating the precursor solution at 70 °C for 1 h. The precursor solution was dropwise added to the treated glass substrate, 10 s at 1000 rpm and 60 s at 5000 rpm during the spin coating process, while 200 μL of chlorobenzene was evenly dropwise added as an anti-solvent at 30 s. The R-NEAPbI3 film was obtained by annealing at 100 °C for 5 min. For the synthesis of CHS-Ⅰ film, the R-NEAPbI3 film was further post-annealed for 15 min, 30 min, 60 min, and 90 min at an ambient atmosphere for inducing the in-growth of PbI2 nanosheets. For the synthesis of CHS-Ⅱ film, 1.2 M PbI2 and 0.9 M R-NEAI in DMF as precursor solution was used, but without post-annealing treatment.

Device fabrication
A circularly polarized photodetector was fabricated in the device configuration of ITO/SnO2 (ETL)/CHS-Ⅰ or CHS-Ⅱ/PbI2 heterostructure/BCP (HTL)/Ag. A 1.5×1.5 cm2 ITO glass was cleaned sequentially with detergent, purified water, acetone, and isopropanol under ultrasonication for 30 min and dried by blowing N2 gas, followed by ultraviolet-ozone treatment for 30 min before spin-coating. The SnO2 colloidal dispersion, diluted with deionized water to be 2 wt%, was spin-coated onto ITO glass at 3000 rpm for 30 s and annealed at 150 °C for 30 min. The R-NEAPbI3/PbI2 heterostructure film was deposited under an ambient atmosphere as described in the film fabrication section. On top of the CHS-Ⅰ or CHS-Ⅱ film, BCP was deposited by thermal evaporation under a vacuum at a pressure of below 8×10-5 Pa. Then, a Ag layer was deposited on the top as a counter electrode by vacuum evaporation. The active area of the device was 2×2 mm2.

General measurements
XRD was performed using a Rigaku Smartlab with Cu Kα radiation source (0.144 nm) and 9 kW power. CD and absorption spectra were measured using the Chirascan circular dichroism spectrometer from Applied Photophysics Ltd. SEM images were obtained using a JXA-8530F Plus field emission electron probe from JEOL Co. Ltd. AFM image was obtained using a Dimension ICON from Bruker. HR-TEM was performed using a JEOL JEM-2100. The current-voltage (I-V) characteristics were recorded by a digital sourcemeter (Keithley 2450). The light sources of 405 nm (GH0406AA2G, SHARP, Japan) and 505 nm (GH05035A2G, SHARP, Japan) were emitted by laser diodes. Unpolarized light was converted to CPL using a quarter-wave plate (Thorlabs) and a linear polarizer (Thorlabs). The light intensity was measured and calibrated by a Si photodetector. Device performance was demonstrated to be stable over a continuous two-days testing period. Furthermore, thin film samples exposed to ambient air for two weeks exhibited negligible variation in their CD signals.

TA and CTA spectroscopy
Transient absorption (TA) spectroscopy was carried out by using a Helios pump-probe system (Ultrafast Systems) combined with a regenerative-amplified Ti: sapphire laser system (Legend Elite-1K-HE, 800 nm, 25 fs, 4 mJ/pulse, and 1 kHz repetition rate) and an optical parametric amplifier system. The output light of a regenerative-amplified Ti: sapphire laser (800 nm) is split into two beams. The main part of the fundamental beam (800 nm) was sent to the optical parametric amplifiers (TOPAS-C), which generated a pump pulse with a tunable wavelength from 290 nm to 2.6 µm. A small part of the fundamental beam (800 nm) was introduced into the TA spectrometer to generate the probe light. After passing through a motorized optical delay line, the 800-nm light beam was attenuated with a neutral density filter and focused on a CaF2 crystal, which was used to generate the white-light continuum (WLC) probe pulses with a wavelength of from 360 to 780 nm, respectively. The probe beam and the pump beam were focused and overlapped onto the sample. The optical path difference between the pump light and the probe light, which was controlled by the motorized optical delay line, was used to get the relative time delay between the probe light and the pump light. After sampling, the probe beam was collimated and then focused into a fiber-coupled spectrometer and detected at a frequency of 1 kHz. Chopped by a synchronized chopper at the frequency of 500 Hz, the pump pulse with a wavelength of 365 nm was modulated such that the TA spectra with and without the pump pulses could be recorded alternately. The intensity of the pump pulse used in the experiment was controlled by a variable neutral-density filter wheel. For the CTA measurement, the circularly polarized pump pulse and the probe pulse were used in the TA measurement. Before the linearly polarized pump pulse and the linearly polarized probe pulse were overlapped onto the sample, they each passed through a double Glan-Taylor calcite polarizer (350–2300 nm, Thorlabs) and a mounted achromatic quarter-wave plate (350–850 nm, Thorlabs). The fast axis of the quarter wave plate was set at 45 or ‒45 degrees to the polarization of the linear pump beam and the linear probe beam to generate a left or right circularly polarized excitation. The circular polarization characteristics were confirmed by polarimeters (Thorlabs). All experiments were performed at room temperature. The analysis was carried out by using Surface Xplorer software.

Computational details
Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP)53. Projector augmented-wave (PAW) pseudopotentials were employed54, along with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functionals55. To match the lattice parameters of R-NEAPbI3, a 3×2 unit cell of PbI2 was used to construct the heterostructure with the R-NEAPbI3 unit cell. The R-NEA-terminated surface of R-NEAPbI3 was in contact with the PbI2 monolayer. To account for van der Waals interactions, we used the dispersion-corrected DFT method56 of Grimme’s D3. The plane-wave cutoff energy was set to 520 eV, and the first Brillouin zone was sampled using a 3×3×1 Monkhorst-Pack k-point grid for the structure optimization of the heterostructure. The ionic positions were fully relaxed until the total energies and forces were less than 10-6 eV and 0.02 eV/Å, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This work presents a study of how an interlayer of R-PbI2 play an important role in spin-photovoltaic device to improve spin injection efficiency. While the concept is novel and the device results demonstrate some functional merit, several claims lack solid experimental substantiation. Certain findings do not conclusively support the authors' proposed conclusions. Additionally, the logical flow of the manuscript is disjointed. I recommend major revision addressing all concerns below before reconsideration for publication.\nHere's my questions and comments:\n1. The author proposed that the XRD peak increases indicates grain size decrease due to partially converted to R-PbI2. But this only happens at the interface between R-NEAPbI3 and PbI2, I don’t think it can result in a notable XRD peak change.\n2. The author mentions there should be no byproduct (R-NEA)2PbI4. However, in Figure 1c, the shoulder around 400nm looks like (R-NEA)2PbI4 exciton absorption.\n3. The CD in Figure 1d only gives R enantiomer’s data, we know that there could be artifact such as LDLB resulting in CD signal. I’d like to see CD responses of S enantiomer and racemic sample.\n4. The AFM in Figure 1e shows that the PbI2 are flakes inserted vertically in R-NEAPbI3. What is the direction of the chiral axis of R-PbI2? Does it allow CISS to filter spin carriers transported across the interface?\n5. The authors discussed two kinds of heterostructures, one with R-PbI2, one without R-PbI2. For the CHS-II, there is no post-annealing process. How do you know it is a heterostructure of R-NEAPbI3 layer and PbI2 layer rather than a mixture of NEAPbI3 and PbI2. In my opinion you can not rule out this possibility.\n6. The authors states:” The R-PbI2 in CHS-I reduces the interfacial lattice mismatch to 5.4%, compared to an 8.4% mismatch observed in CHS-Ⅱ.” What’s the evidence of this statement?\n7. From I-V characteristics, the authors conclude that R-PbI2 passivates interfacial defect states. Can you explain the mechanism of this passivation?\n8. In Figure 2f, CHS-II gives a shorter lifetime than CHS-I, does that mean more efficient exciton dissociation than CHS-I?\n9. In Figure 3, the authors discussed spin dynamics of the heterostructure. However, I don’t think the lifetime measured by CP-TA is spin relaxation time people talked about in the field of spintronics. Therefore, discussion about the EY mechanism is not appropriate. Moreover, EY mechanism refers to spin relaxation by phonon scattering caused by SOC, which should not be related to bandgap. And it’s hard to believe a bandgap different of ~0.1 eV shown in Figure S10 can be resulted from a few nm of R-PbI2.\n10. As the authors calculated the renormalization factors F for CHS-Ⅰ and CHS-Ⅱ to be 53.2±2.5% and 52.6±4.2%. From an experimental science perspective, it cannot be concluded that F for CHS-I is statistically greater than for CHS-II.\n11. In the spin photovoltaic device study shown in Figure 4, 505nm excitation light is used, which only excites PbI2. Therefore, in the devices only PbI2 is the active layer and R-NEAPbI3 only serves as a charge transport layer. CHS-I shows better performance than CHS-II due to the existence of R-PbI2. But this is not the case in most photovoltaic devices. A more rational approach would be using light that can also excite R-NEAPbI3, followed by comparative analysis of the performance of CHS-I and CHS-II.\n12. Is this method of forming R-PbI2 at interface can be used in other chiral perovskite system? Because R-NEAPbI3 and PbI2 are not good choice for photovoltaic application. And the FF and efficiency shown in Figure 4 is quite small. If this R-PbI2 can only be synthesized in this material, I don’t think it is significant enough for the field of spintronics and optoelectronics.", "reviewer_2": "The manuscript by Jin Xiao and co-workers reports the atomic-precision synthesis of chiral interfaces by developing an interfacial chirality-induced growth approach. The authors demonstrate that the few-nanometre-thick chiral lead iodide (R-PbI2) reduced lattice mismatch, residual strain, and defect density at interfaces. More importantly, the precisely engineered chiral interfaces contribute to the interfacial spin injection, which is one of the key challenges for realizing opto-spintronic applications. This manuscript is logically clear and inspiring to the research community. Therefore, I recommend that this paper be accepted by Nature Communications after some minor revisions.\n1. Although the obvious circular dichroism signal at the exciton absorption peak of PbI2 is one of the important pieces of evidence of chiral interfaces. However, the circular dichroism signal may stem from macroscopic anisotropy of thin films. How to determine whether this circular dichroism signal is due to the intrinsic chirality of R-PbI2 rather than the macroscopic anisotropy of thin films?\n2. In the device demonstration, the spin-injection efficiency is as high as 68%, so why is the photocurrent polarization only 40%? What methods are there to improve the photocurrent polarization in the future?\n3. There are some incorrect word usages in the text. For instance, in line 18 of the main text, “synthesis” should be replaced with “synthesize”. I suggest reviewing the manuscript thoroughly and making the necessary adjustments to the wording where appropriate.", "reviewer_3": "This is a review of “Atomic-precision synthesis of chiral interface in the heterostructure for efficient spin injection” by Xiao et. al. In this study the authors measured the circular polarized light detection efficiency of a photo-diode composed of a hybrid metal halide semiconductor coupled to a light absorption PbI2 layer. Their main claim is that by modifying the interface between the PbI2 and MHS to be a helical interface they are able to increase drastically the efficiency of the CPL, indicating that interface effects are very important.\nThe first part of the paper describes the synthesis and characterization of the fabricated layers, and the second part compares between a bilayer without the helical interface and a bilayer with the interface.\nThe premise of the study is of interest to the community and the results could be interesting. But there are a lot of questions that need addressing to ensure the results presented are correct and not only circumstantial.\nBelow I list these concerns and will highlight the most important in my opinion. I am not an expert in the MHS synthesis, so I cannot assess this part of the manuscript, and leave the review of this part to other referees.\nMain issue. In ref. 23 of this paper the authors study the same (R/S-NEA)PbI3 hybrid perovskite and use this layer as the CPL absorber layer. In this paper the authros claim that only the PI2 is the CPL absorbing layer, which requires the important interface. It seems to me that from a practical application perspective they have only complicated the device. From basic physics it is still interesting. But since they put an emphasis on the applicability of the research – they must address the criticism or reframe their study in another way.\nIt is not clear what is the spread and reproducibility of the study results. It is necessary to show in the supplementary an additional device of each kind to see the differences in order to support the claims regarding the percentages stated in this paper.\nThe choice of the 6% sample is unclear to me. In line 102-104 the authors explain their choice due to a maximum in the g_CD factor for this concentration. However, they do not show the analysis. Please add it to the supplementary for all samples. Also the value is e\nI think it is strange not to analyze an additional sample, and show how the results differ. It gives the impression that for some reason only the 6% sample showed a better data for the spin dynamics. I ask the authors to disproof this by adding results of at least pump probe of another sample. I mean, if you prepare a sample then you do the pump probe. You do not choose just one sample for this experiment. This is not good science.\nSmall comment regarding defect analysis line 144. I don’t know if the defect changes may be from sample-to-sample variations or they are really from some passivation effects. Such changes are seen in different samples. Please comment what was your difference between samples.\nLine 181-184 the authors discuss the renormalization factor F . Values from both samples are identical in the uncertainty of the measurement. It is incorrect to claim that was is larger than the other, and that it indicates the interface facilitates spin transport, which is the main claim of this paper.\nIn three different parts of the article the authors use 3 different wavelengths. The choice is unclear. If it is due to instrument limitations just say so. But for the spin injection experiment a wavelength dependent effect is very important. It would be great if the authors could show results not only for 505nm.\nSmall comment - DOP in line 212 was not explicitly defined. Also – why not plot DOP and not just claim a value.\nIn line 224 the comparison of the citation is to different systems, so they have to justify why they can assume that P_filter is the same in their sample.\nIn line 199 and figure 3e the authors show that the relaxation time is 100ps for CPL pump but it is over 5400PS for LPL pump. This is not possible. It indicates there is an error in their experiment, e.g. the background was not calculated correctly, adding an offset for Delta_A, since the data does look just shifted and not relaxing. Or, there was some problem with the pump -probe timing. If this has been reported previously, please quote. If there is no explanation, it must be an offset. The experiment must be repeated or this part removed.\nSmall comment in the next line 201 the authors state “CHS-II exhibits similar initial spin dynamics”. I don’t see any similarity in the initial spin dynamics.\nFinally, the claim in the conclusion regarding record-high spin-injection efficiency: In general spintronics, each interface can cause spin scattering. In metallic and semiconducting spin devices the interface can have nearly no spin scattering (if one avoids the energy mismatch issues), thus following the definition of spin injection efficiency in this paper and ref 12, it would be 100%, much higher than reported. Thus the authors should be specific. Do they refer to MHS? Or interfaces with optically absorbing layers? The claim it is stated is incorrect."}, "author_response": "We thank you for noting the novelty of our results and potential implications in spin-photovoltaic applications.\n1. The author proposed that the XRD peak increase indicates grain size decrease due to partially converted to R-PbI2. But this only happens at the interface between R-NEAPbI3 and PbI2, I don’t think it can result in a notable XRD peak change.\nWe agree that the R-PbI at the interface cannot result in a notable XRD peak change. In the manuscript, we demonstrate that the increase in full width at half maximum (FWHM) of the (002) peak of R-NEAPbI is mainly due to the precipitation of PbI (not the R-PbI ) on the top of R-NEAPbI . To clarify, in the post-annealing process, the R-NEAPbI is mainly converted into PbI , with only a small amount of conversion to a few-nm-thickness R-PbI between R-NEAPbI and PbI . According to the Scherrer equation [Physical review., 1939, 56(10): 978], 𝛽 = 𝐾𝜆/𝐷𝑐𝑜𝑠𝜃 (𝛽 is the FWHM after subtracting the instrumental line broadening, 𝐾 is a dimensionless shape constant, 𝜆 is the X-ray wavelength, 𝜃 is the Bragg angle and D is equal to the grain size), the increase in FWHM of the (002) peak (𝛽 ) of R-NEAPbI in Fig. 1b (002) 3 refers to the decrease of grain size (𝐷) of R-NEAPbI . We have clarified this in the revised manuscript (Lines 80-83 and Lines 88-90).\n2. The author mentions there should be no byproduct (R-NEA)2PbI4. However, in Figure 1c, the shoulder around 400nm looks like (R-NEA)2PbI4 exciton absorption.\nThe XRD spectra in Fig. 1b lack a peak at 5.9°, which is the expected position for the highest-intensity (002) diffraction peak of (R-NEA) PbI , as shown in Fig. R1. The absence of this characteristic peak strongly suggests that (R-NEA) PbI is not present in the material. Furthermore, the absorption spectra (Fig. 1c) show that a shoulder around 400 nm, present in the pure R-NEAPbI before the post-annealing, does not change with increasing annealing time. This confirms that the (R-NEA) PbI byproduct is not formed during the post-annealing process. We have clarified this in the revised manuscript (Lines 91-92) and added Fig. S1 to the Supporting Information.\nFigure R1. The x-ray diffraction spectrum of (R-NEA) PbI .\n3. The CD in Figure 1d only gives R enantiomer’s data, we know that there could be an artifact such as LDLB resulting in CD signal. I’d like to see CD responses of S enantiomer and racemic sample.\nWe measured the CD spectra for both racemic and S-enantiomer samples (Fig. R2). As expected, the racemic sample showed no CD response (Fig. R2a). In contrast, the S-enantiomer heterostructures displayed distinct CD signals at 376 nm and 408 nm (Fig. R2b), which were inverted relative to the R-enantiomer samples.\nFigure R2. Circular dichroism (CD) characterization of racemic and chiral CHS-I films. (a) CD spectrum of the racemic CHS-I film after 60 min of post-annealing. (b) Evolution of CD spectra for the S-CHS-I film with varying post-annealing times. (c) CD and linear dichroism/linear birefringence (LDLB) spectra of R-CHS-I at different annealing durations. (d) CD and LDLB spectra of S-CHS-I under corresponding annealing conditions.\nTo distinguish the true chiroptical signal from measurement artifacts, we applied a Mueller matrix analysis [Nat Commun, 2022, 13, 3259]. The measured signal (CD ) can be expressed as the sum of the real CD response (CDᵣₑₐₗ) and contributions from linear dichroism and linear birefringence (LD/LB),\nCD ≈ CD + (LD'∙LB−LD∙LB')\nmeasure real 2\nSince the LD/LB contribution inverts when the sample is flipped 180°, we could isolate the true CDᵣₑₐₗ by recording spectra from two opposite sample orientations. The deconstructed CDᵣₑₐₗ and LDLB responses for both R- and S-enantiomers are plotted in Fig. R2c,d. The results clearly show the real CD response, confirming that they are not experimental artifacts.\nWe have clarified this in the revised manuscript (Lines 106-113) and added Fig. S5 to the Supporting Information.\n4. The AFM in Figure 1e shows that the PbI2 are flakes inserted vertically in R-NEAPbI3. What is the direction of the chiral axis of R-PbI2? Does it allow CISS to filter spin carriers transported across the interface?\nBecause AFM is limited to topography characterization, it cannot elucidate the orientation of R-PbI at the buried interface. However, combining our DFT calculations (Fig. S7 of the SI)—which show the R-PbI chiral axis runs parallel to the R-NEAPbI inorganic chains—with XRD data (Fig. 1b) confirming a preferential (00n) orientation for R-NEAPbI , allows us to determine that the R-PbI chiral axis lies in-plane.\nWhile both materials possess R-type chirality, their axes are orthogonal: R-PbI is oriented in-plane, while the helically arranged R-NEA+ of R-NEAPbI is aligned along the out-of-plane c-axis. Since the CISS effect relies on transmission along the chiral axis [J. Phys. Chem. Lett., 2012, 3, 2178–2187], R-PbI does not contribute to out-of-plane spin filtering. Crucially, however, the R-PbI layer provides a structurally and chirally matched interface that supports the efficient spin injection governed by the R-NEAPbI layer [Science, 2024, 384, 878–884].\nWe have clarified this in the revised manuscript (Lines 123-124 and 304-306).\n5. The authors discussed two kinds of heterostructures, one with R-PbI2, one without R-PbI2. For the CHS-II, there is no post-annealing process. How do you know it is a heterostructure of R-NEAPbI3 layer and PbI2 layer rather than a mixture of NEAPbI3 and PbI2. In my opinion you can not rule out this possibility.\nUnlike CHS-I, where PbI nanosheets are converted from R-NEAPbI matrix during the post-annealing treatment, as shown in Fig. R3a,b. In the CHS-II system, employing a PbI : R-NEAI >1 feed ratio leads to the simultaneous precipitation of the PbI phase and R-NEAPbI phases. As a result, the PbI phase manifests as nanoparticles distributed both on the film surface and embedded within the R-NEAPbI matrix [Nat. Commun, 2018, 9, 3301], as shown in Fig. R3c,d. Consequently, this configuration constitutes a bulk heterostructure rather than a discrete layered heterostructure. Nevertheless, from a microscopic perspective, the local boundaries effectively function as distinct PbI /R-NEAPbI interfaces; therefore, this morphological difference does not compromise the validity of our subsequent analysis.\nWe have clarified this in the revised manuscript (Lines 146-147) and added Fig. S9 to the Supporting Information.\nFigure R3. Morphological characterization of CHS-I and CHS-II films. (a) Surface and (b) cross-sectional SEM images of the CHS-I structure. (c) Surface and (d) cross-sectional SEM images of the CHS-II structure. In the cross-sectional views, the distinct contrast boundary observed in the middle region corresponds to the interface between the active layer and the silicon wafer substrate.\n6. The authors states: “The R-PbI2 in CHS-I reduces the interfacial lattice mismatch to 5.4%, compared to an 8.4% mismatch observed in CHS-II.” What’s the evidence of this statement?\nWe appreciate your request for clarification regarding the lattice mismatch value. The previous lattice mismatch was calculated from the interplanar spacing. We have reviewed the crystallographic analysis to recalculate the lattice mismatch from the lattice constant of the DFT structural optimization.\nAs illustrated in the schematic structures in Fig. 2a, the heterointerface is defined differently in the two systems: CHS-II involves an R-NEAPbI /PbI interface, whereas CHS-I involves an R-NEAPbI /R-PbI interface. To calculate the interfacial lattice mismatch, we used the in-plane lattice parameters that are parallel to the interface. The in-plane biaxial lattice mismatch is defined as 𝛿 = (𝑎 −𝑎 )/𝑎 [Phys. Rev. Lett., 2005, 94, 176101], where 𝑎 is the lattice parameter of PbI or R-PbI , and 𝑎 is the average of the lattice parameters along two in-plane axes in R-NEAPbI . According to DFT structural optimization, the R-NEAPbI is an orthorhombic crystal system, in which 𝑎 77.87 Å, 𝑏 78.81 Å; the PbI and R-PbI are hexagonal crystal systems in which 𝑎 7𝑏 74.4 Å and 𝑎 7𝑏 74.96 Å. Therefore, the interfacial lattice mismatch is calculated to be 68.1% and 89.5% for CHS-I and CHS-II, respectively. Therefore, the R-PbI contributed to reducing the lattice mismatch at interface.\nWe have clarified this in the revised manuscript (Lines 150-159).\n7. From I-V characteristics, the authors conclude that R-PbI2 passivates interfacial defect states. Can you explain the mechanism of this passivation?\nWe used the space-charge-limited current (SCLC) method to measure the electronically active defects, specifically charge traps at the interface. The density of charge traps in CHS-I is approximately one-third that of CHS-II. Charge traps at the interface of heterostructures are common and can significantly impact the performance of electronic and optoelectronic devices.\nFor the CHS-I, post-annealing induces the partial decomposition of the initial R-NEAPbI into PbI and R-NEAI, creating an excess of R-NEA⁺ cations and iodide (I⁻) ions at the interface. The excess R-NEA+ at the interface acts as a Lewis base to interact with Pb undercoordination defects and Pb vacancies, excess I- interacts with I vacancies, leading to lower interfacial defect densities [Adv. Mater., 2020, 32, 2000865].\nFurthermore, the primary origin of defects in heterostructures is lattice mismatch, which creates strain at the interface. The crystal relieves this strain by forming structural imperfections, principally grain boundaries and dislocations. In the case of CHS-I, the formation of the R-PbI phase at the interface improves the structural transition between layers. This reduces the overall lattice mismatch, thereby decreasing the interfacial strain and suppressing the formation of these charge-trapping defects.\nWe have clarified this in the revised manuscript (Lines 180-182).\n8. In Figure 2f, CHS-II gives a shorter lifetime than CHS-I, does that mean more efficient exciton dissociation than CHS-I?\nWe agree. A shortened lifetime is indeed indicative of more efficient exciton dissociation and charge transfer [Nat. Commun., 2023, 14, 5234]. In the case of CHS-II, the PbI phase exists as nanoparticles dispersed throughout both the surface and the interior of the matrix. This forms a bulk heterostructure with a large interfacial area, which significantly accelerates exciton dissociation [Science, 1995, 270, 1789–1791]. In contrast, the PbI in CHS-I forms flake-like structures that are vertically inserted into the R-NEAPbI layer, resulting in a distinct morphological arrangement with different charge carrier dynamics.\nWe have clarified this in the revised manuscript (Lines 196-201).\n9. In Figure 3, the authors discussed the spin dynamics of the heterostructure. However, I don’t think the lifetime measured by CP-TA is the spin relaxation time people talked about in the field of spintronics.\nTo elucidate the exciton spin dynamics in the heterostructures, we employed circularly polarized transient absorption spectroscopy. This technique uses a circularly polarized pump pulse to preferentially generate a spin-polarized population of excitons. The subsequent evolution of this spin polarization is monitored by a probe pulse that selectively measures the populations of distinct total angular momentum states J 7 ±1). Because each J state is composed of a unique combination of electron and hole spins, monitoring their population changes over time allows us to directly observe spin relaxation processes. The resulting pump-probe signal, which is proportional to the exciton population, is then fitted with a kinetic model to quantify the spin relaxation time (T ), as described previously [Nano Lett., 2015, 15, 1553–1558; Nat Commun, 2020, 11, 5665; Adv. Quantum Technol., 2021, 4, 2100052].\nTherefore, discussion about the EY mechanism is not appropriate.\nSpin relaxation in semiconductors is primarily governed by four mechanisms (Rev. Mod. Phys., 2004, 76, 323]: Elliott-Yafet (EY), D’yakonov-Perel’ (DP), Bir-Aronov-Pikus (BAP), and hyperfine interaction. The hyperfine interaction is generally significant only in systems with weak spin-orbit coupling (SOC) and abundant nuclear spins, such as organic semiconductors, rather than metal halide semiconductors. Furthermore, the spin relaxation time dominated by the BAP mechanism is essentially temperature-independent [Nat Commun, 2020, 11, 5665].\nWe investigated the temperature-dependent spin relaxation time (𝜏 ) of the PbI phase in the CHS-I sample (6% PbI ). As the temperature decreased from 303 K to 193 K, 𝜏 gradually increased from 18±2 ps to 58±10 ps (Fig. R4). This strong temperature dependence effectively excludes the BAP mechanism.\nTo distinguish between the remaining DP and EY mechanisms, we analysed the relationship between spin relaxation and momentum relaxation time (𝜏 ). For the DP mechanism, 𝜏 is inversely proportional to 𝜏 , whereas for the EY mechanism, 𝜏 is proportional to 𝜏 . Increasing the pump fluence enhances carrier scattering, thereby shortening 𝜏 . Consequently, with increased fluence, 𝜏 should increase if dominated by the DP mechanism [APL Mater., 2019, 7, 081116], but decrease if dominated by the EY mechanism [J. Phys. Chem. Lett., 2020, 11, 1502−1507].\nIn our measurements of the CHS-I samples, increasing the pump fluence from 1.5 μJ/cm2 to 24 μJ/cm2 caused 𝜏 to decrease from 75±17 ps to 11±0.6 ps (Fig. R5). This dependence on both temperature and pump fluence confirms that the EY mechanism dominates the spin relaxation.\nFigure R4. Temperature dependence of exciton spin dynamics. (a)-(e) Decay kinetics measured at 303 K, 273 K, 253 K, 223 K, and 193 K are probed at 500 nm in circularly polarized TA (CTA) spectra of CHS-I under 365 nm 𝜎+ excitation with a pump fluence of ~6 μJ/cm2. (f) The spin relaxation time determined from the fits of the CTA spectra, plotted as a function of temperature. The data illustrate the temperature dependence of the spin relaxation process.\nFigure R5. Pump fluence dependent on exciton spin dynamics. (a)-(e) Decay kinetics measured at 303 K are probed at 500 nm in circularly polarized TA (CTA) spectra of CHS-I under 365 nm 𝜎+ excitation with a pump fluence of from 24 μJ/cm2 to 1.5 μJ/cm2. (f) The spin relaxation time as a function of pump fluence.\nMoreover, the EY mechanism refers to spin relaxation by phonon scattering caused by SOC, which should not be related to the bandgap.\nThe EY mechanism describes spin relaxation resulting from momentum scattering (via phonons or impurities) mediated by spin-orbit coupling (SOC). However, the strength of the SOC is critically dependent on the bandgap (E ), leading to the following relationship:\n2 2\n1 ∆ 𝑇 1\n𝑠𝑜\n∝ ( ) ( ) \n𝜏𝐸𝑌 𝐸 −∆ 𝐸 𝜏\n𝑠 𝑔 𝑠𝑜 𝑔 𝑝\nIn the framework of k·p perturbation theory, the intensity of band mixing induced by SOC is inversely proportional to the E [Phys. Rev. B., 2008, 77, 033204]. Consequently, a larger bandgap weakens the effective SOC, thereby reducing the degree of spin mixing in the electron wave function. This reduced mixing decreases the probability of a spin flip occurring during momentum scattering events, ultimately manifesting as a longer spin relaxation time (𝜏𝐸𝑌).\n𝑠\nAnd it’s hard to believe a bandgap different of ~0.1 eV shown in Figure S10 can be resulted from a few nm of R-PbI2.\nWe determined the bandgap of PbI from absorption spectra using the Tauc plot method [Phys. Status Solidi, 1966, 15, 627–637], which relies on the equation:\n(𝛼ℎ𝜐)𝑛 = 𝐴(ℎ𝜐−𝐸 )\n𝑔\nWhere α is the absorption coefficient, hv is the photon energy, A is a proportionality constant, and E is the bandgap. The exponent n 7 2 for direct bandgap semiconductors and n 7 1/2 for indirect bandgap semiconductors. By plotting (αhv)n versus hv, the bandgap is estimated from the intercept of the linear fit with the x-axis.\nFigure R6. The bandgap estimation of the PbI phase by the Tauc Plot method.\nWe revisited our calculations, correcting the exponent from n 7 2 to n 7 1/2 to reflect that PbI is an indirect bandgap semiconductor. With this correction, the bandgap difference between CHS-I and CHS-II was revised to 0.06 eV (Fig. R6). We attribute this shift not only to the a few nm of R-PbI phase but, more importantly, to the morphology of the PbI . The PbI in CHS-I is significantly thinner (40 nm) than that of CHS-II (150 nm). Consequently, the larger bandgap likely results from an enhanced quantum confinement effect in the thinner PbI [Phys. Rev. B., 2007, 75, 045409].\nWe have clarified this in the revised manuscript (Lines 215-217 and 218-225) and added Figs. S15-S17 to the Supporting Information.\n10. As the authors calculated the renormalization factors F for CHS-I and CHS-II to be 53.2±2.5% and 52.6±4.2% respectively. From an experimental science perspective, it cannot be concluded that F for CHS-I is statistically greater than for CHS-II.\nWe thank you for this comment. The renormalization factor, F, represents the fraction of optically generated spin-polarized electrons prior to their injection across the interface.\nTherefore, the similar F values for CHS-I and CHS-II indicate that the initial spin polarization created by the light within the PbI layer is comparable for both samples. The similar F values confirm that the observed differences in performance stem from spin transport efficiency at the interface rather than variations in initial spin generation. We have corrected and clarified this in the revised manuscript (Lines 228-229).\n11. In the spin photovoltaic device study shown in Figure 4, 505nm excitation light is used, which only excites PbI2. Therefore, in the devices, only PbI2 is the active layer and R-NEAPbI3 only serves as a charge transport layer. CHS-I shows better performance than CHS-II due to the existence of R-PbI2. But this is not the case in most photovoltaic devices. A more rational approach would be using light that can also excite R-NEAPbI3, followed by comparative analysis of the performance of CHS-I and CHS-II.\nWe performed multi-wavelength excitation measurements (405, 450, 488, and 505 nm) on spin-photovoltaic devices based on both CHS-I and CHS-II in Fig. R7. The results are summarized in the Supporting Information, with the degree of photocurrent polarization (DOP ) values detailed in Table R1 and Fig. R8.\nTable R1. Comparison of the degree of photocurrent polarization (DOP ) for CHS-I and CHS-II at multiple excitation wavelengths.\nExcitation wavelength DOP of CHS-I DOP of CHS-II\ncurrent current\n405 nm 12.4±1.4% 11.6±0.8%\n450 nm 19.5±1.3% 8.3±1%\n488 nm 24.3±1.8% 9.2±1%\n505 nm 27.9±1.6% 9.7±1.7%\nUnder 405 nm circularly polarized excitation, the R-NEAPbI phase is primarily excited. This results in a low DOP of approximately 10% for both CHS-I and CHS-II, which is comparable to the DOP observed in a pure R-NEAPbI control device (Fig. R9).\nConversely, 505 nm excitation primarily targets the PbI phase. Here, the DOP originates from spin injection from PbI into R-NEAPbI , followed by selective carrier transport. The resulting DOP depends on the renormalization factor (F) and spin-injection efficiency (P ). Both CHS-I and CHS-II exhibit a similar renormalization factor of about 50%; however, the spin-injection efficiency of CHS-I is approximately twice that of CHS-II. Consequently, CHS-I exhibits a DOP of ~30%, whereas CHS-II is limited to ~10%.\nIn summary, for CHS-I, the DOP is minimized (>10%) when the excitation wavelength approaches the R-NEAPbI exciton absorption peak (375 nm) and maximized (~30%) when the PbI phase is selectively excited at 505 nm. In contrast, the CHS-II device shows minimal variation (~3%) across excitation wavelengths due to its inherently low spin-injection efficiency.\nWe have clarified this in the revised manuscript (Lines 315-326) and added Figs. S22-S24 to the Supporting Information.\nFigure R7. Current-voltage characteristics and photocurrent degree of polarization (DOP ) at multiple excitation wavelengths. Data are presented for two CHS-I devices (S-3 and S-5) and two CHS-II devices (S-4 and S-6). The intensity of the excitation light is fixed at 1 mW/cm2.\nFigure R8. Dependence of the degree of photocurrent polarization (DOP ) on excitation wavelength for CHS-I and CHS-II.\nFigure R9. Characterization of a spin-photovoltaic device based on pure R-NEAPbI . (a) Schematic diagram of device structure used R-NEAPbI as a circularly polarized light absorber. (b) Current-voltage responses and DOP of this device.\n12. Is this method of forming R-PbI2 at interface can be used in other chiral perovskite system? Because R-NEAPbI3 and PbI2 are not good choice for photovoltaic application. And the FF and efficiency shown in Figure 4 is quite small. If this R-PbI2 can only be synthesized in this material, I don’t think it is significant enough for the field of spintronics and optoelectronics.\nWe acknowledge that the fill factor (FF) and power conversion efficiency (PCE) of the CHS-I devices are relatively low. As the reviewer noted, these parameters are limited by the intrinsic material properties of the R-NEAPbI . Specifically, the low performance stems from two main factors:\n1. Restricted carrier transport: The one-dimensional (1D) inorganic framework of R-NEAPbI , combined with the strong dielectric confinement imposed by the low-dielectric-constant R-NEA+ cations, results in inherently low carrier mobility [Nano Lett., 2016, 16, 7001–7007].\n2. Recombination losses: The rapid radiative and non-radiative recombination within the PbI phase further hinders effective carrier transport.\nHowever, the primary goal of using R-NEAPbI was to establish a proof-of-concept platform to elucidate the formation mechanism of the chiral interface and the resulting spin injection. To demonstrate that better performance is achievable and that our method is universal, we extended our study to other chiral perovskite systems: 1D R-MBAPbI , 2D (R-MBA) PbI , and 2D (R-4-Br-MBA) PbI (R-MBA and R-4-Br-MBA refer to R-1-phenylethanamine and R-1-(4-bromophenyl)ethanamine, respectively).\nThese alternative materials possess simpler organic cation structures, lower exciton binding energies, and higher intrinsic conductivity compared to R-NEAPbI Therefore, we expect them to yield significantly higher FF and PCE when optimized. To verify the universality of our interfacial chirality-induced growth method, we applied the same post-annealing treatment to these materials (Fig. R10):\n• 1D R-MBAPbI 3 : Post-annealing (30–60 min) induces a shoulder near 500 nm in the absorption spectrum (Fig. R10b) and a corresponding negative CD peak in Fig. R10a, confirming the emergence of chiral R-PbI .\n• 2D (R-MBA) 2 PbI 4 : Although the PbI 2 absorption overlaps with the excitonic transition (Fig. R10d), a new CD signal appears at 510 nm and strengthens with annealing in Fig. R10c, again indicating R-PbI formation.\n• 2D (R-4-Br-MBA) 2 PbI 4 : The CD intensity around 500 nm increases markedly after annealing in Fig. R10e, suggesting the formation of the same chiral R-PbI phase.\nThese results demonstrate that our growth method is applicable to diverse chiral metal halide semiconductors and is not exclusive to R-NEAPbI We have clarified this in the revised manuscript (Lines 114-118) and added Fig. S6 to the Supporting Information.\nFigure R10. Characterization of chiral metal halide semiconductors under post-annealing treatment. Circular dichroism (CD) and absorption spectra for (a,b) R-MBAPbI ; (c,d) (R-MBA) PbI ; and (e,f) (R-4-Br-MBA) PbI with post-annealing times ranging from 5 to 60 mins.\n\nReviewer #2:\nThe manuscript by Jin Xiao and co-workers reports the atomic-precision synthesis of chiral interfaces by developing an interfacial chirality-induced growth approach. The authors demonstrate that the few-nanometre-thick chiral lead iodide (R-PbI2) reduced lattice mismatch, residual strain, and defect density at interfaces. More importantly, the precisely engineered chiral interfaces contribute to the interfacial spin injection, which is one of the key challenges for realizing opto-spintronic applications. This manuscript is logically clear and inspiring to the research community. Therefore, I recommend that this paper be accepted by Nature Communications after some minor revisions.\nWe sincerely appreciate the positive evaluation of our work and their recommendation for acceptance.\n1. Although the obvious circular dichroism signal at the exciton absorption peak of PbI2 is one of the important pieces of evidence of chiral interfaces. However, the circular dichroism signal may stem from macroscopic anisotropy of thin films. How to determine whether this circular dichroism signal is due to the intrinsic chirality of R-PbI2 rather than the macroscopic anisotropy of thin films?\nTo distinguish the real chiroptical signal from measurements, we applied a Mueller matrix analysis [Nat Commun, 2022, 13, 3259]. The measured signal (CD ) can be expressed as the sum of the real CD response (CDᵣₑₐₗ) and contributions from linear dichroism and linear birefringence (LD/LB),\nCD ≈ CD + 1 (LD'∙LB−LD∙LB').\nmeasure real\nWe have clarified this in the revised manuscript (Lines 106-113).\n2. In the device demonstration, the spin-injection efficiency is as high as 68%, so why is the photocurrent polarization only 40%? What methods are there to improve the photocurrent polarization in the future?\nWe appreciate the insightful comments regarding the photocurrent polarization. In our CHS-I device, the primary limiting factor for polarization is the renormalization factor (F753.2±2.5%), which is defined as:\n𝐹 = 1/(1+𝜏/𝜏 )\n𝑠\nwhere 𝜏 is the exciton lifetime and 𝜏 is the electron spin relaxation time. To increase F and thus enhance the photocurrent polarization, we propose two main strategies:\n• Decrease the exciton lifetime (𝜏): We can improve exciton dissociation efficiency by increasing the interfacial contact area between the two phases. Experimentally, this could be achieved by reducing the total thickness of the PbI layer while maintaining the thickness of the chiral R-PbI interface, thereby facilitating faster charge transfer.\n• Prolong the spin relaxation time (𝜏 ): Since spin relaxation is highly sensitive to dimensionality, engineering the material's structure is effective. For instance, utilizing a 0D structure to achieve full-dimensional carrier confinement is a promising strategy to significantly suppress spin-flip mechanisms and extend the spin relaxation time [Phys. Rev. B., 2002, 66, 125307].\nWe have clarified this in the revised manuscript (Lines 306-314).\n3. There are some incorrect word usages in the text. For instance, in line 18 of the main text, “synthesis” should be replaced with “synthesize”. I suggest reviewing the manuscript thoroughly and making the necessary adjustments to the wording where appropriate.\nThe manuscript has been proofread for spelling and grammar, and all revisions have been highlighted.\n\nReviewer #3:\nThis is a review of “Atomic-precision synthesis of chiral interface in the heterostructure for efficient spin injection” by Xiao et. al. In this study the authors measured the circular polarized light detection efficiency of a photo-diode composed of a hybrid metal halide semiconductor coupled to a light absorption PbI2 layer. Their main claim is that by modifying the interface between the PbI2 and MHS to be a helical interface they are able to increase drastically the efficiency of the CPL, indicating that interface effects are very important. The first part of the paper describes the synthesis and characterization of the fabricated layers, and the second part compares between a bilayer without the helical interface and a bilayer with the interface. The premise of the study is of interest to the community, and the results could be interesting. But there are a lot of questions that need addressing to ensure the results presented are correct and not only circumstantial. Below I list these concerns and will highlight the most important in my opinion. I am not an expert in the MHS synthesis, so I cannot assess this part of the manuscript, and leave the review of this part to other referees.\nWe thank you for acknowledging the importance of our results and are confident that our study will be of interest to the wide audience of Nature Communications.\n1. Main issue. In ref. 23 of this paper the authors study the same (R/S-NEA)PbI3 hybrid perovskite and use this layer as the CPL absorber layer. In this paper the authors claim that only the PbI2 is the CPL absorbing layer, which requires the important interface. It seems to me that from a practical application perspective they have only complicated the device. From basic physics it is still interesting.\nWe thank for this important observation. While we agree that ref. [23] successfully demonstrates CPL detection using a single R-NEAPbI layer, we respectfully disagree that our approach complicates the device without practical gain. We address this in three key aspects:\n1. Reproducibility & orientation dependence: Single-phase R-NEAPbI absorbers are highly sensitive to crystallographic orientation. Literature reports show DOP values varying wildly from 5% to 90% depending on processing conditions, often requiring complex uniaxial orientation control [Adv. Funct. Mater., 2024, 34, 2316265; Chem. Mater., 2022, 34, 2955; Nat Commun, 2023, 14, 7179; Adv. Mater., 2024, 36, 2400493; Sci. Adv., 2020, 6, eabd3274]. This makes device consistency a major challenge.\n2. Fabrication simplicity: In our approach, the interfacial R-PbI is generated in situ via a simple post-annealing step of the precursor stack. It adds no deposition steps or material complexity.\n3. Robustness & universality: This \"built-in\" chiral interface provides a stable spin-selective response (30% DOP ) that is less dependent on bulk texturing.\nFurthermore, we have demonstrated that this method is not limited to R-NEAPbI in Reviewer #1 Question 12; it is applicable to other chiral systems (e.g., R-MBAPbI , (R-MBA) PbI , and their quasi-2D derivatives), serving as a universal strategy to enhance spin injection efficiency across the field of chiral metal halide semiconductors.\nWe have clarified this in the revised manuscript (Lines 321-326).\n2. It is not clear what is the spread and reproducibility of the study results. It is necessary to show in the supplementary an additional device of each kind to see the differences in order to support the claims regarding the percentages stated in this paper.\nTo address the concerns regarding the reproducibility and variability of the study results, we have included additional device characterizations for CHS-I and CHS-II in the Supporting Information. The I–V responses and photovoltaic performance of these devices (Fig. R11) are consistent with the results presented in the main manuscript. Notably, DOP of the CHS-I device reach 27.8% (S-3) and 26.8% (S-5) at –0.4 V, which is approximately three times higher than that of the CHS-II device, 9% and 6% for S-4 and S-6 (Fig. R11). Statistical analysis of S1-3 reveals that the DOP of CHS-I (27.9%±1.1%) is higher than that of CHS-II (9.4%±1.8%).\nWe have clarified this in the revised manuscript (Lines 291-295) and added Fig. S21 to the Supporting Information.\nFigure R11. Current-voltage characteristics and degree of photocurrent polarization (DOP ) for additional CHS-I devices (S-3 and S-5) and CHS-II devices (S-4 and S-6) under dark, 505 nm 𝜎+ and 𝜎− circularly polarized excitation.\n3. The choice of the 6% sample is unclear to me. In line 102-104 the authors explain their choice due to a maximum in the g_CD factor for this concentration. However, they do not show the analysis. Please add it to the supplementary for all samples.\nGiven that the excitonic absorption peaks of both R-PbI and PbI are centred at 500 nm, we evaluated the absorption dissymmetry factor (g ) at this wavelength to optimize the chiral response of the R-PbI phase in CHS-I. The g factor was determined using the following equation:\nCD[mdeg]\n𝑔 = \nCD 32980×Absorbance\nThe calculated g values at 500 nm for various PbI concentrations are summarized in Fig. R12. The data reveal a maximum g of 0.02 at a PbI concentration of 6%. Consequently, this 6% composition was selected for all subsequent studies.\nWe have clarified this in the revised manuscript (Lines 104-105) and added Fig. S4 to the Supporting Information.\nFig. R12. g calculated at 500 nm for various PbI concentrations in CHS-I.\n4. I think it is strange not to analyze an additional sample and show how the results differ. It gives the impression that for some reason only the 6% sample showed a better data for the spin dynamics. I ask the authors to disproof this by adding results of at least pump probe of another sample. I mean, if you prepare a sample then you do the pump probe. You do not choose just one sample for this experiment. This is not good science.\nWe appreciate this valuable suggestion and agree that comparing different compositions significantly strengthens our conclusions. Accordingly, we synthesized additional CHS-I samples with 2% and 20% PbI loadings using the same protocol as the optimized 6% sample.\nWe then performed identical pump–probe measurements (linear pumping, circular probing) to evaluate their spin dynamics. The results are presented in Fig. R13:\n• 2% PbI 2 (Fig. R13a): The CTA polarization rises to only ~-10% within the first 1 ps and decays to ~0% by ~30 ps.\n• 20% PbI 2 (Fig. R13b): The CTA polarization reaches ~-13% within 1 ps and decays to ~0% by ~40 ps.\n• 6% PbI 2 (Fig. 3e): In stark contrast, the 6% PbI 2 sample exhibits a peak CTA polarization of ~-40% under identical conditions.\nThese data reveal a clear non-monotonic dependence of the spin signal on PbI content. We attribute this trend to the trade-off between interfacial generation and bulk relaxation:\n1. At low loading (2%): The effective area of the chiral R-PbI interface is insufficient to support efficient spin injection into the perovskite matrix.\n2. At high loading (20%): The excess PbI likely introduces additional non-radiative recombination pathways or defect states that accelerate spin depolarization.\n3. At intermediate loading (6%): The system achieves an optimal balance, maximizing the interfacial spin transfer area while minimizing depolarization losses.\nThese additional experiments confirm that the 6% composition represents a true experimental optimum rather than a randomly selected favourable sample. The observed spin dynamics are systematic and reproducible.\nWe have clarified this in the revised manuscript (Lines 241-251) and added Fig. S18 to the Supporting Information.\nFigure R13. TA kinetics of CHS-I with 2% and 20% PbI contents. The CTA polarization is calculated by(∆A σ+ −∆A σ− )/(∆A σ+ +∆A σ− ). TA spectra were collected under 465 nm linearly polarized light with a pump fluence of ~6 μJ/cm2.\n5. Small comment regarding defect analysis line 144. I don’t know if the defect changes may be from sample-to-sample variations or they are really from some passivation effects. Such changes are seen in different samples. Please comment what was your difference between samples.\nTo exclude sample-to-sample variability as the cause of the reduced defect density, we fabricated additional devices for both CHS-I and CHS-II under identical synthesis and processing conditions. The resulting trap-filled-limit (TFL) analysis is presented in Table R2 and Fig. R14.\nThe data highlight two key findings. First, CHS-I demonstrates high reproducibility with a tight distribution of onset voltages and defect densities, whereas CHS-II exhibits larger dispersion, consistent with a higher trap landscape. Second, the difference between the groups is large and systematic: CHS-I maintains a consistently lower defect density than CHS-II, with non-overlapping ranges across all replicates.\nTable R2. The onset voltage (𝑽 ) and defect density of CHS-I and CHS-II devices.\nTFL\nDevices Onset voltage (𝑽 ) Defect density (n )\nTFL d\nCHS-I (S-1) 0.14 V 3.9×1015 cm-3\nCHS-I (S-3) 0.12 V 3.3×1015 cm-3\nCHS-I (S-5) 0.15 V 4.2×1015 cm-3\nCHS-II (S-2) 0.40 V 11.0×1015 cm-3\nCHS-II (S-4) 0.28 V 7.7×1015 cm-3\nCHS-II (S-6) 0.54 V 19.2×1015 cm-3\nConsequently, the reduced defect density in CHS-I cannot be attributed to random device variation; rather, it indicates a genuine passivation effect inherent to CHS-I. (A mechanistic discussion of this passivation is provided in our response to Reviewer #1, Question 7.)\nWe have clarified this in the revised manuscript (Lines 173-180) and added Fig. S13 to the Supporting Information.\nFigure R14. Statistical summary of defect density values obtained via SCLC analysis for additional batches of CHS-I (S-3 and S-5) and CHS-II (S-4 and S-6) devices. The data confirm the lower defect density and tighter dispersion in CHS-I.\n6. Line 181-184 the authors discuss the renormalization factor F. Values from both samples are identical in the uncertainty of the measurement. It is incorrect to claim that was is larger than the other, and that it indicates the interface facilitates spin transport, which is the main claim of this paper.\nWe thank you for this comment. The renormalization factor, F, represents the fraction of optically generated spin-polarized electrons prior to their injection across the interface.\nTherefore, the similar F values for CHS-I and CHS-II indicate that the initial spin polarization created by the light within the PbI layer is comparable for both samples. The similar F values confirm that the observed differences in performance stem from spin transport efficiency at the interface rather than variations in initial spin generation. We have corrected and clarified this in the revised manuscript (Lines 228-229).\n7. In three different parts of the article the authors use 3 different wavelengths. The choice is unclear. If it is due to instrument limitations just say so. But for the spin injection experiment a wavelength dependent effect is very important. It would be great if the authors could show results not only for 505 nm.\nThe experimental design exploits the distinct excitonic absorptions of the two constituents: R-NEAPbI (~375 nm) and PbI (~505 nm). Our choice of excitation wavelength was tailored to the specific physical process being probed.\nFor estimating the spin relaxation time of PbI using circular pump-circular probe (CP-TA), we used 365 nm excitation. This wavelength excites both R-NEAPbI and PbI , enabling the extraction of PbI spin dynamics. Conversely, to study selective spin injection from PbI into R-NEAPbI , we utilized 465 nm (linear pump-circular probe). This wavelength avoids the R-NEAPbI absorption peak (~375 nm) while effectively populating PbI . Although 505 nm is resonant with PbI , it was not used for CP-TA because our probe beam is centred near 500 nm; a pump source at 505 nm would degrade signal fidelity due to scatter and filter leakage.\nHowever, for device-level spin-injection experiments, we utilized a 505 nm laser diode to resonantly excite the PbI exciton, thereby optimizing the analysis of injection efficiency. We also explored wavelength dependence beyond 505 nm for CHS-I/CHS-II devices, as detailed in the response to Reviewer #1, Question 11, and referenced in the revised manuscript (Line 206, 234-235, 267-268, and 315-321).\n8. Small comment - DOP in line 212 was not explicitly defined. Also – why not plot DOP and not just claim a value.\nWe have now explicitly defined the degree of photocurrent polarization (DOP ) in the revised manuscript as\ncurrent\nDOP 7\n𝐼𝜎+−𝐼𝜎−\n×100%\ncurrent\n𝐼𝜎++𝐼𝜎−\nwhere 𝐼 and 𝐼 are photocurrents under 𝜎+ and 𝜎− excitations, respectively. DOP is a key practical parameter of a spin-photovoltaic device and can quantify its sensitivity to circularly polarized light.\nFurthermore, we agree that a visual representation is more informative than a single value. We have added an applied bias voltage dependence plot of the DOP in Fig. R15. The DOP current current\nexhibits a discontinuity at the open-circuit voltage because the measured photocurrent approaches zero at this point, causing the denominator of the DOP expression to become nearly zero and leading to the observed discontinuity.\nWe have clarified this in the revised manuscript (Lines 263-265 and Fig. 4b,c).\nFig. R15 Current-voltage responses and calculated DOP of (a) CHS-I and (b) CHS-II devices current\nunder 505 nm circularly polarized excitation.\n9. In line 224 the comparison of the citation is to different systems, so they have to justify why they can assume that P_filter is the same in their sample.\nWe appreciate your scrutiny regarding the applicability of the P value. While the cited systems are not identical in exact composition, our assumption of P ≈ 80% is justified by the fact that they share the same chiral ammonium families (R-MBA, R-NEA) and identical spin-filtering geometries (transport along the chiral axis).\nTo validate this transferability, we compiled literature values for related chiral metal halide systems (see Table R3 below). The data reveal that P consistently clusters in the 75–86% range for R-MBA and R-NEA lead-halide perovskites, independent of dimensionality (1D, 2D, or quasi-2D) or specific halide (Br vs. I), as shown in Table R3.\nTable R3. Summary of reported spin filtering efficiencies of chiral metal halide semiconductors.\nMaterials Dimension P Reference\nfilter\n(R‐MBA) FA Pb Br Quasi-2D 83% Adv. Funct. Mater., 2025, 35, 2415433\n2 2 3 10\n(R-MBA) PbI 2D 86% Sci. Adv., 2019, 5, eaay0571\n2 4\n(R-MBA) Sn 2D 94% J. Am. Chem. Soc., 2020, 142, 13030−1304\n2 4\n(R-NEA) PbBr / 2D\n2 4\n86% Adv. Mater., 2024, 36, 2309335\nCsPbBr QDs heterostructure\n3\n(R-NEA) PbBr 2D 75% Nat commun, 2024, 15, 4672\n2 4\nR-MBAPbBr 1D 80% Adv. Funct. Mater., 2021, 31, 2104605\nWe have clarified this in the revised manuscript (Lines 277-280).\n10. In line 199 and figure 3e the authors show that the relaxation time is 100ps for CPL pump but it is over 5400ps for LPL pump. This is not possible. It indicates there is an error in their experiment, e.g. the background was not calculated correctly, adding an offset for Delta_A, since the data does look just shifted and not relaxing. Or there was some problem with the pump -probe timing. If this has been reported previously, please quote. If there is no explanation, it must be an offset. The experiment must be repeated or this part removed.\nWe re-examined the raw data used in Fig. 3e and found that the observed different relaxation time was an artifact caused by a lack of background light subtraction. After repeating the experiment and applying this correction, both the linear pump-circular probe curves in Fig. R16 now essentially overlap at 100 ps, which is consistent with the trend shown in Fig. 3b,c.\nFor the CHS-I sample, the corrected data show the CTA polarization increasing to ~-40% within 0–20 ps; this corresponds to the spin-selective transfer from PbI to R-NEAPbI that generates spin polarization in the PbI . Subsequently, this CTA polarization gradually decays to 0% within the 20–100 ps window due to the spin relaxation process. In contrast, the degree of CTA polarization stabilizes near 0% for the CHS-II sample. This comparison indicates that the R-NEAPbI /R-PbI and R-PbI /PbI interfaces in CHS-I effectively promote the spin-polarized electron injection from PbI to R-NEAPbI , resulting in the enhanced and sustained CTA polarization.\nWe have corrected and clarified this in the revised manuscript (Lines 241-245 and Fig. 3e,f).\nFigure R16 CTA kinetics of CHS-I and CHS-II with linear pump and 𝝈+ or 𝝈− probes at 500 nm showing net spin polarization. The CTA polarization is calculated by (∆A\nσ+\n−∆A σ−)/(∆A\nσ+\n+∆A σ−).\nTA spectra were collected under 465 nm linearly polarized light with a pump fluence of ~6 μJ/cm2.\n11. Small comment in the next line 201 the authors state “CHS-II exhibits similar initial spin dynamics”. I don’t see any similarity in the initial spin dynamics.\nWe thank you for this keen observation. We have re-evaluated the data and repeated the analysis with rigorous background light correction. The revised results, presented in Fig. 3e, reveal that the initial spin dynamics are not similar. Instead, the CTA polarization for CHS-II remains negligible 0%, indicating that there is no significant spin transfer from PbI to R-NEAPbI in the CHS-II sample.\nWe have corrected and clarified this in the revised manuscript (Lines 252-254).\n12. Finally, the claim in the conclusion regarding record-high spin-injection efficiency: In general, spintronics, each interface can cause spin scattering. In metallic and semiconducting spin devices the interface can have nearly no spin scattering (if one avoids the energy mismatch issues), thus following the definition of spin injection efficiency in this paper and ref 12, it would be 100%, much higher than reported. Thus, the authors should be specific. Do they refer to MHS? Or interfaces with optically absorbing layers? The claim it is stated is incorrect.\nWe agree that the original phrasing was overly broad. Metallic and conventional semiconductor interfaces can achieve near-unity spin injection, making a general comparison invalid. Consequently, we have revised the conclusion to explicitly scope our \"record-high\" claim to the field of metal halide semiconductors (MHSs). Achieving efficient spin injection in these solution-processed interfaces remains a significant challenge due to conductivity mismatches and surface defects. Therefore, our result represents a specific breakthrough for MHS materials rather than for spintronics in general.\nWe have clarified this in the revised manuscript (Line 329)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The response letter and the revised manuscript have addressed most of my concerns, except a few points below:\n1. The author mentioned CHS-II is composed R-NEAPbI3 with PbI2 embedded in it. Therefore, this heterostructure is quite different with CHS-I, where R-PbI2 and PbI2 are on top of R-NEAPbI3. Is it fair to compare these two heterostructures?\n2. The calculated lattice mismatch percentage is still questionable. The in-plane lattice parameters are used for both CHS-II and CHS-I. However, as PbI2 is mixed in R-NEAPbI3 for CHS-II, this calculation may be invalid.", "reviewer_2": "The authors have addressed the reviewers' questions. I recommend its publication.", "reviewer_3": "I have re-read this manuscript and the response of the authors to the critiques raised by myself and the other 2 referees. The authors do a good job in addressing most of the questions. They have performed additional experiments to check many of the reviewers’ concerns. These include testing of additional samples, measuring of handedness by CD, life time measurements and others.\nThere are, however, several issues that remain, one main issue and other minor, yet all are important for a correct portray of the conclusions of the paper.\nMain issue: The authors have recalculated lattice mismatch using in-plane parameters from DFT. While in the previous report reasonable lattice mismatch around 5% was reported, they now claim a high mismatch of 89.5% (for CHS-II) reducing to still a high mismatch of 68.1% for the CHS-I. Both of the mismatch are high, and would not result in epitaxial or heterostructure growth. So either they should not claim a heterostructure, or they should search if there is some super-epitaxy matching ratio that can allow reduction of mismatch to something reasonable (i.e. < 5%). This issue should be addressed.\nThis also points to a problem in the title. This cannot be atomic-precision synthesis, with such an interface, and traps and impurities. I would suggest softening the claim to “nano-scale control” or something of this nature.\nAdditional points:\n1. The authors show now data from different wavelengths, which is very important information. It should be addressed also in the summary, that the effect reaches a maximum at some wavelength (505 nm). i.e., this is not a wide band system, so it is relevant to specific applications. Maybe they can assess how to broaden the response?\n2. The authors use spin filtering of 80% from literature. It is ok to use such data, but when concluding the spin injection efficiency they should remined the reader that this is an estimate under the above assumption. It now reads like this is a measured number and not an estimate (that is reasonable).\n3. Metal halides in solar cells are notoriously unstable. Did the authors do any estimate toward the stability of the reported structures and how they degrade over time? This is very important information for device considerations."}, "author_response": "We appreciate for raising this important point regarding the structural distinctions between CHS-I and CHS-II. To clarify the structural details: in CHS-I, the PbI /R-PbI phase exists as flake-like structures inserted vertically into the R-NEAPbI layer (Fig. R1a,c), creating a PbI /R-PbI /R-NEAPbI heterostructure. In contrast, CHS-II contains PbI as nanoparticle-like structures embedded within the layer (Fig. R1b,d), forming a PbI /R-NEAPbI heterostructure. Therefore, comparing these two heterostructures is not only fair but essential for understanding the mechanism at play. By comparing CHS-I (which possesses the chiral R-PbI layer) with CHS-II (which lacks it), we can isolate the specific influence of the chiral interface on spin dynamics.\nOur characterizations of defect density, strain distribution, exciton spin dynamics, and spin-photovoltaic performance confirm that this interfacial difference is the governing factor. Consequently, this comparison allows us to establish exactly how the interface—specifically the presence of R-PbI —modulates spin-injection efficiency.\nFigure R1. Structural comparison of CHS-I and CHS-II. Schematic side views of (a) CHS-I and (b) CHS-II with corresponding SEM top views of (c) CHS-I and (d) CHS-II.\n2. The calculated lattice mismatch percentage is still questionable. The in-plane lattice parameters are used for both CHS-II and CHS-I. However, as PbI is mixed in R-NEAPbI for CHS-II, this calculation may be invalid.\nWe thank you for this comment. Our previous calculations directly used the in-plane lattice constant to calculate the lattice mismatch, but this method is only suitable for cubic crystal systems where the two phases are in contact with the same crystal plane (Appl. Phys. Lett., 1999, 74, 3636–3638). For our heterostructure, R-NEAPbI is orthorhombic crystal system, while the PbI and R-PbI are hexagonal crystal systems, making it unsuitable for calculating lattice mismatch using lattice constants. We recalculated the lattice mismatch using the interfacial crystallographic orientations of the two phases in the interfacial regions (Nat. Chem., 2024, 16, 201–209). The calculation principle of lattice mismatch is shown in Fig. R2a. HR-TEM analysis reveals that the R-NEAPbI (103) plane contacts the PbI or R-PbI (002) plane, with lattice fringes intersecting at approximately 65° (Fig. R2b). Using the measured d-spacings, d = 0.362 nm for R-NEAPbI , d = 0.338 nm for PbI , and d’ = 0.348 nm for R-PbI , we calculated the lattice mismatch. The revised values are 3.9% for CHS-I (calculated via 𝑑(013)−𝑑 ( ′ 002)) and 6.6% for CHS-II (calculated via 𝑑(013)−𝑑(002)).\n𝑑(013) 𝑑(013)\nWe have updated the manuscript to reflect these changes (Page 8-9, Lines 151–158).\nFigure R2. Lattice mismatch analysis in metal halide heterostructures. (a) Schematic illustration showing the calculation of lattice mismatch using interplanar spacings. (b) HR-TEM image of the interface. The values d , d and d’ represent the interplanar spacings of R-NEAPbI (013) plane, PbI (002) plane, and of R-PbI (002) plane. The scale bar is 4 nm.\nReviewer #2:\nThe authors have addressed the reviewers' questions. I recommend its publication.\nThank you for your recommendation.\nReviewer #3:\nI have re-read this manuscript and the response of the authors to the critiques raised by myself and the other 2 referees. The authors do a good job in addressing most of the questions. They have performed additional experiments to check many of the reviewers’ concerns. These include testing of additional samples, measuring of handedness by CD, lifetime measurements and others.\nThere are, however, several issues that remain, one main issue and other minor, yet all are important for a correct portray of the conclusions of the paper.\nMain issue: The authors have recalculated lattice mismatch using in-plane parameters from DFT. While in the previous report reasonable lattice mismatch around 5% was reported, they now claim a high mismatch of 89.5% (for CHS-II) reducing to still a high mismatch of 68.1% for the CHS-I. Both of the mismatch are high and would not result in epitaxial or heterostructure growth. So either they should not claim a heterostructure, or they should search if there is some super-epitaxy matching ratio that can allow reduction of mismatch to something reasonable (i.e. < 5%). This issue should be addressed.\nWe acknowledge that our previous lattice-mismatch estimation, based on in-plane lattice constants, overestimated the mismatch values. While that method is valid for cubic crystal systems where the two phases are in contact with the same crystal plane (Appl. Phys. Lett., 1999, 74, 3636–3638), which is not accurately reflect the crystallographic relationships at the interface in our heterostructures.\nFor our heterostructure, R-NEAPbI is orthorhombic crystal system, while the PbI and R-PbI are hexagonal crystal systems, making it unsuitable for calculating lattice mismatch using in-plane lattice constants. To characterize the lattice mismatch accurately, we use the interfacial crystallographic orientations and interplanar spacings (d-spacing) directly from HR-TEM images (Nat. Chem., 2024, 16, 201–209) to calculate the lattice mismatch (Fig. R2a). As shown in Fig. R2b, HR-TEM image reveals that R-NEAPbI (103) plane is in contact with the PbI or R-PbI (002) plane, with lattice fringes intersecting the interface at approximately 65°. Using the measured interplanar spacings, d = 0.362 nm for R-NEAPbI , d = 0.338 nm for PbI , and d’ = 0.348 nm for R-PbI , we recalculated the mismatch using the equation 𝛿 = (𝑑 −𝑑 )/𝑑 . This yields a lattice mismatch of 3.9% for CHS-I and 6.6% for CHS-II.\nWe have updated the manuscript to reflect these changes (Page 8-9, Lines 151–158).\nThis also points to a problem in the title. This cannot be atomic-precision synthesis, with such an interface, and traps and impurities. I would suggest softening the claim to “nano-scale control” or something of this nature.\nThank you for the suggestion. To avoid any misunderstanding, we have revised the title to more accurately reflect the physical nature of the heterostructures. The title has been updated to “Precision engineering chiral interfaces for efficient spin injection in metal halide heterostructures”.\nAdditional points:\n1. The authors show new data from different wavelengths, which is very important information. It should be addressed also in the summary, that the effect reaches a maximum at some wavelength (505 nm). i.e., this is not a wide band system, so it is relevant to specific applications. Maybe they can assess how to broaden the response?\nWe agree that the wavelength dependence result is crucial for practical applications. First, per your suggestion, we have updated the summary to explicitly state that the maximum DOP response occurs at 505 nm (Page 17, Line 334). Second, regarding the broadening of the response, we have added a discussion on how halogen doping in metal halides can effectively tune the bandgap. We explain that by carefully controlling the halogen composition, the operational wavelength range can be adjusted or expanded, increasing versatility (J. Am. Chem. Soc., 2020, 142, 4206−4212; Nat. Commun., 2025, 16, 2201). This assessment has been added to the revised manuscript (Page 16, Lines 318-320).\n2. The authors use spin filtering of 80% from literature. It is ok to use such data, but when concluding the spin injection efficiency, they should remind the reader that this is an estimate under the above assumption. It now reads like this is a measured number and not an estimate (that is reasonable).\nWe have revised the manuscript to clarify that 80% spin filtering value was derived from established literature. Consequently, we explicitly state that the spin injection efficiency is calculated based on the assumption of this 80% spin filtering capability in the R-NEAPbI layer. These changes can be found on Page 15, Lines 291-295 in the revised manuscript.\n3. Metal halides in solar cells are notoriously unstable. Did the authors do any estimate toward the stability of the reported structures and how they degrade over time? This is very important information for device considerations.\nWe agree that stability is critical for device applications. Crucially, our devices demonstrated stable performance with no observed degradation throughout the relevant experimental timeframe (around 2 weeks), ensuring the reliability of the reported data. Therefore, this work focuses on the fundamental effects of the chiral interface on carrier transport. While metal halides present known long-term stability challenges, effective mitigation strategies (e.g., encapsulation, interface engineering, additives) have been extensively reported (Science, 2020, 368, eaba2412; Nature, 2022, 611, 278–283; Nat. Photon., 2025, 19, 701–708).\nWe have added a note in the Supporting Information Materials and Methods section (Page 4) confirming that device performance remained stable under ambient conditions throughout the testing period, and that our thin film under ambient conditions, showed no significant change in their CD signals for 2 weeks (Fig. R3).\nFigure R3. Stability assessment of CHS-I structure. (a) CD and (b) absorption spectra comparing fresh and two-week-old CHS-I films."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed all the concerns. I recommend publication.", "reviewer_3": "The authors have addressed the concerns I had with the manuscript and have corrected it accordingly.\nFrom a scientific perspective, the current version should be accepted for publication"}, "author_response": ""}]

Decision letter