Chiral Hybrid Perovskite Single‐Crystal Nanowire Arrays for High‐Performance Circularly Polarized Light Detection

Abstract Circularly polarized light (CPL) detection has emerged as a key technology for various optoelectronics. Chiral hybrid perovskites (CHPs) that combine CPL‐sensitive absorption induced by chiral organic ligands and superior photoelectric properties of perovskites are promising candidates for direct CPL detection. To date, most of the CHP detectors are made up of polycrystalline thin‐film, which results in a rather limited discrimination of CPL due to the existence of redundant impurities and intrinsic defect states originating from rapid crystallization process. Here, it is developed a direct CPL detector with high photocurrent and polarization selectivity based on low‐defect CHP single‐crystal nanowire arrays. Large‐scale CHP nanowires are obtained through a micropillar template‐assisted capillary‐bridge rise approach. Thanks to the high crystallinity and ordered crystallographic alignment of these arrays, a CPL photodetector with high light on/off ratio of 1.8 × 104, excellent responsivity of 1.4 A W−1, and an outstanding anisotropy factor of 0.24 for photocurrent has been achieved. These results would provide useful enlightenment for direct CPL detection in high‐performance chiral optoelectronics.


Statistical Analysis
Multiple devices were tested by Keithley 4200 to extract current under CPL illumination (λ = 510 nm, power = 0.32 mW/cm 2 , Figure 4d). The sample size (n) for each statistical analysis was chosen as 5. Origin software was used to calculate the standard deviation and mean value for statistical analysis. Error bars represent the standard deviation of five representative measurements from the same batch.
Multiple light intensities were monitored by a power meter ( Figure S19). The sample size (n) for each statistical analysis was chosen as 50. Origin software was used to calculate the standard deviation and mean value for statistical analysis. Figure S1. Photographs of CHP single crystals taken at different stages of the synthesis process.                      Figure S1. Photographs of CHP single crystals taken at different stages of the synthesis process.

Supplementary Figures
As shown in Figure S1, the photographs indicate that the end products via a temperature cooling method are orange needle-like crystals, which are obviously distinguish from as-formed yellow precipitates. The obviously change of the product morphology indicates that the chiral hybrid perovskites have been successfully synthesized. The CHP precursor solution prepared by synthesized CHP crystals could be beneficial to obtain high-quality single-crystlline array.   Normalized extinction (a.u.) Figure S4. Normalized extinction spectra of (S-MBA)2PbI4 and (R-MBA)2PbI4 films.
Comparing to the CD spectra in Figure 1b, extinction spectra of the chiral perovskite films revealed that the CD peaks were located before the extinction band edge (524 nm), which is consistent with the result in previous paper. 1 The value of the absorption was used to calculate the anisotropy factor of CD (gCD).  Figure S5. Schematic illustration of the preparation of CHP single-crystalline NW arrays.
As shown in Figure S5, the CHP single-crystalline NW arrays were fabricated through a universal and facile micropillar template-assisted capillary-bridge rise approach with an asymmetric-wettability mechanism. [2] The as-synthesized (R-, and S-MBA)2PbI4 crystals were firstly dissolved in solvent to serve as precursor solutions for subsequent preparation process. To generate ordered single-crystalline chiral perovskite arrays, an assembly system was constructed by combining an asymmetric-wettability topographical template and a target substrate. The assembly system was contacted with the CHP precursor solution for a few seconds. Afterward, the CHP precursor solution would rise in the gaps between the top of micropillars and substrate driven by capillary force and Laplace pressure, thus forming individual capillary bridges anchored onto the top of the micropillars. CHP single-crystalline NW arrays were finally generated on the predetermined positions through a dewetting process. The solvent used in the fabrication process of CHP single-crystalline NW arrays is a crucial factor that will influence the crystal growth of NWs. As shown in Figure S6a, as-prepared CHP NW arrays with pure DMF solvent show rough surfaces and discontinuous crystallization due to the fast crystallization rate during the drying process. The addition of DMSO solvent could improve the crystallization quality by lowering the crystallization rate because DMSO has a higher coordination affinity, a relatively high boiling point (189 °C) and a low saturated vapor pressure ( Figure S6b). It demonstrates that the DMF/DMSO mixed solvent is beneficial to the preparation of high-quality CHP NW arrays toward high-performance CPL detection. The precursor concentration is essential to the crystal growth of CHP single-crystalline NW arrays. As displayed in Figure S7, discontinuous microbelts are generated at excessively low concentrations, while the incomplete film is fabricated at high concentrations. The optimal precursor concentration was determined to be 0.1 mol/L for the synthesis of high-quality CHP single-crystalline NW array. As shown in the Figure S10a, dispersive diffraction rings that could be assigned to the diffraction from (002l) peaks were observed from the (S-MBA)2PbI4 film, indicating the film has more random orientation and weaker crystallinity than CHP single-crystalline NW arrays.
SEM image and Zoom-in SEM image demonstrate thin film is polycrystalline with ubiquitous grain boundaries and hundred-nanometer crystallites, which will lead to low photodetection performances. In contrast, the crystallinity and orientation of CHP single-crystal arrays have been obviously improved compared to these polycrystalline films, which is favorable for constructing high-performance CPL detectors.   Figure   S11b. A low trap density in the NW arrays was manifested by the narrow PL peak near the bandgap, which benefits the preparation of high-performance CPL optoelectronic devices. The I-V trace and PL lifetime were measured to compare the charge transport efficiency between CHP single-crystalline NW array and the corresponding thin-film. According to the previously mentioned equation, the trap density of the (S-MBA)2PbI4 film was calculated to be 3.4 × 10 16 cm -3 , noticeably higher than that of (S-MBA)2PbI4 NW array in Figure 2a. The PL lifetimes were extracted by bi-exponential fitting with τ1 = 0.65 ns and τ2 = 2.23 ns, which is smaller than that of NW array (τ1 = 0.92 ns and τ2 = 4.42 ns) in Figure 2b, suggesting that the CHP NWs are expected to possess a longer photocarrier lifetime than the corresponding thin-film. Therefore, the charge transport efficiency has been effectively improved in high-crystallinity and crystallographic-ordered CHP array, [3] making them more promising for achieving high-performance CPL detection.   Figure S14b.
The highest responsivity of this film device is 0.16 A W -1 under irradiance of 0.06 mW/cm 2 , which are nearly one order of magnitude smaller than that of the CHP array CPL photodetector, possibly because these single-crystal mircostructures have longer carrier lifetime and lower charge trap density. These results demonstrate CHP NW array devices can achieve higher photoresponse for CPL detection compared to thin-film devices. , where e is the elementary charge. As shown in Figure S14a, the D* decreases with increasing light intensities, reaching up to 3.9 × 10 12 Jones at a low light power density of 0.06 mW/cm 2 , which is higher than the reported value of chiral perovskite film photodetectors. The responsivity of (S-MBA)2PbI4 array-based photodetector also exhibits obvious voltage dependence at various light power density, which is a typical behavior of perovskite photodetectors ( Figure S15b).  Figure S16 shows the normalized response of nanowire devices versus the input frequency.
The -3 dB frequency, which is defined as the frequency where the response dropped to half of the initial value, was approximately 500 Hz for 510 nm response, indicating the fast response speed of single-crystal nanowire devices. As shown in Figure S18, there are no significant differences in detection performances when CHP nanowire with different sizes (widths and heights). This result demonstrates that these devices with various sizes can exhibit excellent photoresponse performance. for RCP) are counted.
Before the characterization of CPL detection performance, the whole system was calibrated to ensure that the intensity of LCP and RCP illumination was the same during all measurements. [4] Firstly, we waited 15 min to ensure the intensity of the light source (510 nm laser) was stable. Then the CPL was switched from LCP to RCP (then from RCP to LCP…) for 50 cycles, and the light intensity was monitored before each switch by a power meter. At last, 50 data were obtained for LCP and the other 50 data for RCP. As shown in the statistical graph (Fig. S19), the light intensity of LCP and RCP districted in 0.3238 ± 0.004 mW/cm 2 .
The relative standard deviation (defined as the ratio of the standard deviation to the mean) of LCP and RCP light intensity are 0.25% and 0.23%, which are about two orders of magnitude smaller than the gIph (0.1~0.3 in this paper). These results verify that the different responsibilities of LCP and RCP light are resulted from the chiral device rather than the light intensity error. To compare the CPL distinguishability between CHP single-crystalline NW array devices and thin-film devices, we measure the I-V curves under 510 nm RCP and LCP illumination for the (S-MBA)2PbI4 film. The gIph of (S-MBA)2PbI4 film device is calculated to be 0.07 at 5 V bias voltage, which is lower than that in the corresponding microarray device (gIph = 0.24, Figure   4b). The enhanced gIph of the CHP NW array-based detector might be attributed to long spin lifetimes originated from their excellent crystalline nature, [5][6] showing that CHP NW array devices can achieve better distinguish ability for CPL detection compared to thin-film devices. The racemic precursor (0.1 mol L −1 ) prepared by mixing (R-and S-MBA)2PbI4 precursor with a molar ratio of 1:1 was used for the fabrication of (rac-MBA)2PbI4 NW arrays. As shown in Figure S21, the (rac-MBA)2PbI4 NW array-based photodetector did not show any photocurrent differences under 510 nm RCP and LCP illumination, which is significantly different from the results of (R-and S-MBA)2PbI4 photodetectors, indicating that the polarization distinguishability stems from the introduction of homochiral organic ligands. The XRD pattern and photoresponse of (S-MBA)2PbI4 array were monitored after being stored in ambient without any encapsulation for 1 week to check its stability. The XRD pattern of (S-MBA)2PbI4 array revealed negligible degradation after the ambient storage, and the corresponding detector can still perform excellent polarization distinguishability, which demonstrates long-term performance of our array-based device.