Spin-photogalvanic effect in chiral lead halide perovskites

Abstract

Low-temperature solution-made chiral lead halide perovskites (LHPs) have spontaneous Bychkov–Rashba spin orbit coupling (SOC) and chiral-induced spin selectivity (CISS) qualities. Their coexistence may give rise to considerable spin and charge conversion capabilities for spin-orbitronic applications. In this study, we demonstrate the spin-photogalvanic effect for (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ polycrystalline film-based lateral devices (100 μm channel length). The light helicity dependence of the short-circuit photocurrent exhibits the circular photogalvanic effect (CPGE) and linear photogalvanic effect (LPGE) with decent two-fold symmetry for a complete cycle in a wide temperature range from 4 K to 300 K. Because of the Rashba SOC and the material helicity, the effect is converse for the two chiral LHPs. In addition, its magnitude and sign can be effectively tuned by constant magnetic fields. The Rashba effect, CISS-generated unbalanced spin transport, and chiral-induced magnetization are mutually responsible for it. Our study evidently proves the future prospect of using chiral LHPs for spin-orbitronics.

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1. Introduction

Low-temperature solution processable lead halide perovskites (LHPs) have received unprecedented attention because of their potential applications in highly efficient solar cells and light emitting diodes (LEDs).^1–9 In addition to desirable optoelectronic properties such as high mobilities, strong photo-absorption strengths and high color purities for light emissions, LHPs are known to exhibit strong spin–orbit coupling (SOC) due to the existence of heavy chemical constituents such as lead (Pb) and iodine(I).^10–13 This results in degenerated energy band splitting in the momentum vector space. In addition, most of them possess the crystalline structure inversion asymmetry (SIA). As a result, the so-called Bychkov–Rashba SOC can be made possible.^14,15 All of these factors ensure that LHPs are promising to the development of spin-orbitronics,¹⁶ in which electronic spins act as information carriers and quantum bits for data transmission and storage. This has led to the studies of magnetic spin-valves,¹⁷ magneto-optics,^18,19 and spin-LEDs.²⁰ Recently, some experiments and theories have pointed out that their spin-optoelectronic properties can be extensively modified by introducing chiral organic ligands into the inorganic crystalline frameworks. This yields Ruddlesden–Popper (RP) two-dimensional (2D) chiral LHPs.^21–23 The implementation of the chiral organic ligands may also bring in switchable ferroelectric property, which offers an additional degree of freedom for the manipulation of Rashba bands through externally applied electrical fields.^24,25 To date, they have been studied for applications in circularly polarized luminescence (CPL),²⁶ nonlinear optics,²⁷ magneto-optics,²⁸ spin-transport²⁹ and spin-LEDs.^30–32 CPL cannot be easily achieved in achiral LHPs unless ultrahigh magnetic fields are involved.³³

With this regard, we have successively synthesized the chiral LHPs based on (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ (MBA = C₆H₅C₂H₄NH₃), while demonstrating the magnetic field manipulation of CPL and magneto-photoluminescence (magneto-PL) from both experimental and theoretical aspects in 2021.²⁸ We found that the connection among the exciton's wavevector (K), material helicity (τ) and effective spin–orbit coupling strength (α) are mutually responsible for the exciton's dispersion relation.^34,35 Despite the photo-induced CPL, we think it is of equal importance to study the photo-induced electrical transport in term of the short-circuit photocurrent for the chiral LHP system. Because of the naturally crystalline property, SIA and chiral-induced spin selectivity (CISS) are expected to contribute for the spin-photogalvanic effect.^36–38 Moreover, the temperature and field dependences remain unknown for the chiral LHP system.

In the system, the light helicity (i.e., circular and linear polarization) dependent photocurrent with the absence of a bias voltage can produce the circularly photogalvanic effect (CPGE) and linear photogalvanic effect (LPGE). The magnitude of the photocurrent varies periodically with respect to the changing polarization. The CPGE can be thought of as a transfer from the light angular moment to a directional free charge carrier. The effect is recognized as one of the crucial methods to validate the Rashba band splitting.^12,39,40 It appears in polar semiconductors,⁴¹ topological insulators,^42,43 and 2D electron gas-based semiconductor quantum well structures.^36,44 Recently, the CPGE was also demonstrated in 2D layered hybrid perovskite single crystals and chiral 2D organic–inorganic hybrid perovskites.^39,45 Two prerequisites are required to realize it. One is the photoelectrical response capability, while another criterion is an unbalanced spin-dependent electronic transport due to the presence of degenerated Rashba bands. The former one can be satisfied by choosing an appropriate photoexcitation source. Moreover, the latter one can be made possible by the chiral LHPs since they possess inherently crystalline inversion symmetry breaking. Additionally, the chiral-induced spin selectivity (CISS) may facilitate the spin-dependent electronic transport because of the inherent chiral-induced SOC.

In this work, we synthesized and fabricated lateral spin-orbitronic devices. For concreteness, we chose (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ as the chiral LHPs. Both can form high quality polycrystalline films with certain growth modes on soda-lime glass substrates. A pronounced spin-photogalvanic effect can be detected in a wide temperature range from 4 K to 300 K. The effect is attributed to the interplay of the perovskite Rashba band and CISS.

2. Experiments

2.1 Materials and device fabrication

In our experiments, all chemicals were commercially available products and could be used without further purifications. Both prototypical chiral perovskites, for instance (R)-(+)-α-methylbenzylamine (R-MBA, 98%, optical purity >97%) and (S)-(−)-α-methylbenzylamine (S-MBA, 98%, optical purity >97%), were chosen. More details about the single crystal syntheses can be read from our early papers.³⁵ In short, 200 mg of lead oxide (PbO), 200 μl R- or S-MBA, and 6 ml of aqueous hydriodic acid (HI) were mixed in a glass vial. The mixture naturally led to some yellow precipitates. Then, it was placed in an oil bath. The solution experienced a thermal treatment at 100 °C in order to dissolve the precipitates. After this, it was naturally cooled down to room temperature. We observed many orange needle-like monocrystals. They were filtrated and rinsed with toluene several times. Finally, they were all dried in an oven for several hours.

Prior to the device fabrication, transparent glass substrates were cleaned by a standard chemical method using acetone, isopropanol and ethanol in an ultrasonic bath. After drying, they were treated by plasma oxidation for approximately 1 min. The precursor solutions of chiral LHPs were prepared by dissolving the pre-prepared single crystals (20 wt%) in N,N-anhydrous dimethylformamide (DMF) organic solvent. Thin films were fabricated by the conventional spin-coating method on top of the substrates with a spinning speed and time of 4000 rpm and 30 s, respectively. All of the samples were subsequently annealed at 100 °C for 8 min. After this, they were transferred into an integrated thermal evaporation system. Then, 80 nm thick gold (Au) electrodes were deposited onto the top surfaces of the films. The channel length for charge transport was approximately 100 μm.

2.2 Material characterization and transport measurements

The morphologies of the chiral HOIPs thin films were examined by a field emission scanning electron microscope (SEM, JSM-6700F) system. Ultraviolet–visible photo-absorption spectra were conducted by a spectrometer (UV-2600). The measurement of the transmission circular dichroism (CD) was performed by a Jasco CD spectrometer (J-1500). The crystalline phases of (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ pristine films were analyzed by an X-ray diffraction (XRD) system (Bruker D-8 Advance). During the transport measurements, the devices were mounted onto a self-designed sample holder. This holder was placed inside a cryogenic closed-cycle system. The complete system was located in-between a pair of electromagnets. A semiconductor laser diode (405 nm), combined with a set of linear and circular polarizers were used as the photoexcitation source. The device could be illuminated by the laser beam through a viewing port, and electrical signals could be recorded via a source-meter unit.

3. Results and discussion

We began with the fundamental characterization of the material property. The polycrystalline film surface morphologies and roughness of (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ were characterized by SEM, as shown in Fig. 1(a) and (b), respectively. They have similar morphologies. In order to validate the successful implementation of chirality into the LHPs, the CD property was examined and the spectra are given in Fig. 1(c) for (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄. The CD spectra manifest the mirror symmetry with the common photo-absorption band-edge at about 518 nm (i.e., 2.39 eV). In Fig. 1(d), the UV-visible photo-absorption peaks that occur at 334 nm and 370 nm are ascribed to bound exciton transitions.⁴⁶ Such absorption band-edge is consistent with the CD spectra. Fig. 1(e) shows the CPL spectra for the (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ polycrystalline films with the dominant emissive peaks at approximately 515 nm. The spectra are symmetric with respect to each other, indicating their chiral character. The crystallinities of the films are revealed by the out-of-plane XRD spectra in Fig. 1(e). By indexing, the spectra contain successively diffracted bands at 2θ = 6.18°, 12.32°, 18.50°, 24.74°, and 31.06°, which correspond to the (002), (004), (006), (008) and (0010) crystalline phases, respectively.^46,47

Fig. 1 (a) and (b) are SEM surface morphologies for (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ polycrystalline films, respectively. (c)–(f) are their CD spectra, UV-visible absorption spectra, CPL spectra and XRD spectra, respectively.

Fig. 2(a) shows the lateral device structure on top of the glass substrate. It consists of Au/(R-MBA)₂PbI₄ (or, (S-MBA)₂PbI₄)/Au. Judging from the inset SEM image, the channel length was designed and fabricated to be approximately 100 μm. Such channel length guarantees that the photogenerated spin-transport takes place within its coherent length. Owing to the existence of the material chirality, we expected to observe remarkable responses of polarized light-induced photoluminescence. Fig. 2(b) and (c) show the experimental results of the light polarization-dependent PL intensities for (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄, respectively. During the measurements, a quarter-wave plate (QWP) could be rotated in-plane. The PL intensity varies according to PL (σ⁺) > PL (linear) > PL (σ⁻) for (R-MBA)₂PbI₄ (Fig. 2(a)). It is vice versa for (S-MBA)₂PbI₄ due to their chirality (Fig. 2(c)). It should be noted that the linearly polarized photoexcitation acts as the superposition for an equal amount of the left- and right-handed circularly polarized photoexcitation.

Fig. 2 (a) The schematic drawing of the lateral device configuration Au/chiral LHP/Au. The inset image shows the channel length of approximately 100 μm. (b) and (c) are the variations of the PL intensities due to the alternative switches among the left-, linear and right-circularly polarized photoexcitation for (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄, respectively. The photoexcitation wavelength and the laser power were 405 nm and 90 mW, respectively.

With the same devices, we carried on with the CPGE measurement. In this case, QWP was used to generate the polarization state for the incident light. The rotational angle (α) was defined by the angular rotation of QWP with respect to its fast axis. The photocurrent was continuously recorded at different angles. Fig. 3(a) and (b) are the experimental results of the in-plane photocurrent as a function of α for (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ at 4 K, respectively. The purple solid lines are the corresponding fitting curves. The schematic illustrations for the light helicity are listed on top of the figures. In both devices, the variations of the photocurrent through the complete cycle are mainly determined by the helicity of the incident light. The switches among the left- (σ⁺), linear (σ⁺ + σ⁻), and right- (σ⁻) circularly polarized light lead to the periodic change of the photocurrent within the full revolution. Notably, the photocurrent generation due to the linearly polarized light is indeed the linear photogalvanic effect (LPGE). We confirmed that the magnitude of CPGE mainly relies on the helicity of the chiral LHPs since their left and right handedness are reflected from the signals. Fig. 3(c) provides the temperature dependence of the effect from 4 K to 300 K for (R-MBA)₂PbI₄. It remains considerably large at 300 K, and the effect is greatest at 4 K. The increase of the temperature smears out the spin-related effect due to the thermal disturbance of photoexcited polarized electrons. Such temperature dependence can be equally measured for (S-MBA)₂PbI₄. Even for a crystalline inversion asymmetric material, the Rashba effect may be overruled at higher temperatures. The complete lineshape of the spin-photogalvanic effect can be well fitted by the analytical expression:

I_xy(α) = C sin(2α) + L cos(4α) + D, (1)

where sin (2α) and cos (4α) represent the lineshapes of CPGE and LPGE, respectively. The coefficients C and L denote the photocurrent for CPGE and LPGE, respectively. D is a polarization-independent quality that stems from some factors, including photothermal response and photo-Dember effect.^37,48,49 As we can see from the inset plot of Fig. 3(c), the C-value that reflects CPGE tends to decrease with the increase of temperature. More intriguingly, we observed that the applied magnetic field can tune the magnitude and sign for the spin-photogalvanic effect. As shown in Fig. 3(d) for the field-dependent spin-photogalvanic effect for (R-MBA)₂PbI₄, a constant in-plane large magnetic field (such as +900 mT) can enhance its magnitude. On the other hand, the negative field (such as −900 mT) helps reverse the effect. In fact, with an external stimulus such as light, the photogenerated electrons experience a chiral-induced SOC, leading to a chiral-induced magnetization for the nonmagnetic chiral LHPs. Such magnetization can be further coupled with an adjacent ferromagnet, while both of them can be manipulated by an external field. Previously, the conducting atomic force microscopic technique-based electronic transport measurements demonstrated the hysteresis-shaped magnetoresistance with a single ferromagnet for the chiral LHPs.⁴⁷ In addition, the chiral-induced magnetization was found in optical Kerr measurements.⁵⁰ They experimentally proved that the different resistive states (high and low resistive states) and different magnitudes of Kerr angles could be realized when magnetic fields were applied through the opposite directions. In our experiment, we found that the spin-photogalvanic effect can be effectively tuned by the constant magnetic fields with the same magnitudes, but opposite signs.

Fig. 3 Experimental results of spin-photogalvanic effect for (a) (R-MBA)₂PbI₄ and (b) (S-MBA)₂PbI₄. (c) and (d) are the temperature and magnetic-field dependent spin-photogalvanic effect for (R-MBA)₂PbI₄. The inset of (c) shows the C value at different temperatures for (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄, respectively. The photoexcitation wavelength and the laser power were 405 nm and 90 mW, respectively.

In principle, the spin-photogalvanic effect in the chiral LHPs can be understood by considering their unique electronic structure and the CISS property. We think that both Rashba band-associated exciton states and CISS for the unbalanced charge transport are mutually responsible for the effect. The energy–momentum dispersion of the system is mainly decided by the Pb–I octahedral inorganic crystalline framework. The heavy chemical constituents (i.e., Pb and I) and the breaking of the crystalline inversion symmetry give rise to the parabolic-like degenerate band splitting and nondegenerate Rashba effect (Fig. 4(a)).^14,51–54 With previously theoretical calculations using the effective mass model, the conduction bands (CB) of LHPs are constructed by Pb-6p and I-5s orbitals.^11,54 The basis functions for electronic spin states in CB can be written by,¹¹

where the subscript +(−) denotes the up(down) component of the pseudo spin component. X, Y, and Z represent the Pb-6p_x, -6p_y and -6p_z atomic orbitals, respectively. The parameter ξ is determined by the physical quantities, such as atomic SOC strength (λ) and the crystal-field strength (δ) since 2ξ = 2√2λ/(λ − 3δ). The basis functions of the electronic spin states in the valence bands (VB) are expressed as,

v₊₍₋₎ = S↑(↓), (3)

Fig. 4 (a) The pictorial illustration for the LHP Rashba band. (b) The schematic drawing for the fine structure of exciton states in LHPs. (c) and (d) The band edge photoexcitation via the circularly polarized light, and the spin-photogalvanic measurement. Experimental results of photo-induced EPR spectra for (R-MBA)₂PbI₄ single crystals measured at (e) 300 K and (f) 150 K. The photoexcitation wavelength and the laser power were 405 nm and 90 mW, respectively. (g) The experimental result of the GIWAX spectrum for the (S-MBA)₂PbI₄ polycrystalline thin film. The similar spectrum could be obtained for the (R-MBA)₂PbI₄ polycrystalline thin film.

A photogenerated exciton may be one of the four possible spin configurations; for instance, c₊v̄₊, c₋v̄₋, c₊v̄₋ and c₋v̄₊. The exciton states can be expressed by:

Γ₁, Γ₂, and Γ₅⁺ represent the dark, polarization emission along the z-axis, and doubly degenerated circular polarizations, respectively (Fig. 4(b)).²⁸ A minute description of the exciton fine structure for LHPs has been previously reported in our early work.^35,46 Without magnetic fields, the Rashba field leads to energy degeneracy and the energy level exchange between Γ₁ and Γ₂. With applied fields, the fine structures of Γ₅⁺ can be further tuned based on the Zeeman effect. The relative positions of the energy states Γ₅⁺ and Γ₅⁻ are decided by the chirality. The optical transition-associated exciton generation, recombination and dissociation can be well depicted by Fig. 4(a) and (c) for the chiral LHPs.

As we can see from Fig. 4(a) for the pictorial illustration of the Rashba bands, the CB minimum and the VB maximum contain the total angular momenta of J_h = 1/2 and J_e = 1/2, respectively.^11,54,55 The optical transitions obey the selection rule, and the change of the projected angular momentum is Δm_J = ± 1 (Fig. 4(c)). When the incident light is continuously changed among the left, linear and right polarization, the photoexcited electron densities and occupations of electronic density of states (DOS) in the energy–momentum vector space are actually anisotropic. Because of the Rashba splitting, the spin-related carrier transports are in the opposite directions (i.e., group velocity ) for ±k. As a result, the photocurrents are unbalanced along the two directions. Because of the chirality, (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ respond distinctly to the circularly polarized light. The spin-photogalvanic effects for the two are inversely related.

The different number of photogenerated paramagnetic species can be further verified by the polarized light-induced EPR spectroscopy. Fig. 4(e) and (f) provide experimental results of the temperature-dependent EPR spectra for the (R-MBA)₂PbI₄ single crystals under circularly polarized photoexcitation at 300 K and 150 K, respectively. The same thing can be applied for (S-MBA)₂PbI₄.³⁵ The resonant peaks that appear at approximately 509 mT and 125 mT are attributed to the Pb³⁺ and Pb⁺ species, respectively.⁵⁶ From the observation of the spectra, the decrease of the thermal energy leads to the increase of the spectral intensity, which is primarily due to the reduction of the spin dephase. More importantly, σ⁺ generates much greater spectral intensities than those from σ⁻. The results clearly indicate that the polarized light-induced paramagnetic species follow the material chirality. In addition, from the observation of the GIWAX spectrum of (S-MBA)₂PbI₄ in Fig. 4(g), it is known that the film growth mode is the face-on configuration (Fig. 4(d)) along the z-direction. The result is consistent with the out-of-plane XRD spectra in Fig. 1(f). Consequently, the crystalline c-axis is expected to be outward in the direction perpendicular to the propagation direction of the polarized light (z-axis). As a matter of fact, the excitation transition is determined by the electric field vector (E_xy) with respect to the c-axis (Fig. 4(d)). When they are perpendicular, the optical transitions due to the Γ₅^± symmetry can be effectively triggered. On the other hand, the Γ₂ symmetry is allowed when both are in parallel. In our case, the Γ₅^± symmetry associated optical transitions can be effectively triggered for the (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄ thin films.^46,57 Despite the spin-dependent optical transitions and crystalline orientation, another critical contribution is CISS for the electronic transport upon the dissociation of photogenerated exciton. Because of the inherent chiral-induced SOC, the conductive charges are spin-dependent. Only one spin channel that is either up or down dominates the conductive transport. This is also known as the Edelstein effect, which is concerned with a macroscopic spin polarization/magnetization due to the presence of chiral-induced SOC.⁵⁸ It connects the carrier spin with its spatial motion for the chiral LHPs in a macroscopic electrical field. Thus, we believe that the co-existence of both Rashba band and CISS are mutually responsible for the generation of considerable spin-photogalvanic effect for the chiral LHPs.

4. Conclusions

In this work, we have studied the spin-photogalvanic effect for typical chiral LHPs, such as (R-MBA)₂PbI₄ and (S-MBA)₂PbI₄. The pronounced CPGE and LPGE were measured in a wide temperate range from 4 K to 300 K. The constant in-plane magnetic fields can effectively manipulate the magnitude and sign for the effect. The spin-photogalvanic effect originates from the Rashba bands in the momentum vector space for the material with SIA. The CISS property of the chiral LHPs can further contribute to the effect.

Data availability statement

The data that support the finding of the study are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The work was supported by the Fundamental Research Funds for the Central Universities (2019RC101), the National Natural Science Foundation of China (Grant No. 61974010), and the Intergovernmental Cooperation Project, National Key Research and Development Program, Ministry of Science and Technology, China (Grant No. 2019YFE0108400).

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