Double Free: A Promising Route toward Moisture-Stable Hypotoxic Hybrid Perovskites

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022Double Free: A Promising Route toward Moisture-Stable Hypotoxic Hybrid Perovskites Guang-Ning Liu, Ruo-Yu Zhao, Rang-Dong Xu, Cuncheng Li and Guo-Cong Guo Guang-Ning Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong 250022 Google Scholar More articles by this author , Ruo-Yu Zhao School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong 250022 Google Scholar More articles by this author , Rang-Dong Xu School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong 250022 Google Scholar More articles by this author , Cuncheng Li School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong 250022 Google Scholar More articles by this author and Guo-Cong Guo *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100873 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The moisture instability and toxicity of lead have hindered the development of lead organometal halide perovskites (Pb-OHPs). Similar to Pb-OHPs, OHPs based on Pb-free elements, including Group VA metals Bi and Sb (denoted as M), exhibit optoelectronic properties due to their isoelectronic configuration and close chemistry of the lone-pair ns2 state. However, M-OHPs are mostly based on hydrophilic protonated organic countercations and possess low structural dimensionalities, which affect the moisture stability and disrupt continuous carrier transport, respectively. Herein, we demonstrate a “double-free” strategy to realize the rational synthesis of moisture-stable hypotoxic hybrid perovskites. A hydrogen-bond-free alkylated countercation was assembled with Pb-free OHP bearing an extended structure in one molecule. As a proof of concept, two double-free M-OHPs are synthesized, which show greatly improved moisture and photostability compared to their corresponding hydrogen-bond OHPs and the extensively studied MA3M2X9 (MA = CH3NH3+). The photoconduction behaviors of both double-free OHPs display short rise and decay recovery time and exhibit excellent photocurrent reproducibility. Importantly, (1-methyl-4-phenylpyridin-1-ium)BiI4 ( Me4ppi-BiI4) displays a comparable switching on/off ratio with MA3Bi2I9, and can endure 75% relative humidity (RH) for at least 243 days. The photophysical property measurements and theoretical calculations show that this remarkable photoconversion performance results from the relatively low exciton binding energy and greatly improved carrier mobility and concentration. Download figure Download PowerPoint Introduction Over the last decade, lead organometal halide perovskites (Pb-OHPs) represented by APbX3 (A = organic cation and X = halogen) have been widely researched in the field of synthesis chemistry and material science.1–5 These compounds exhibit promising application potential in photodetectors, solar cells, and light-emitting diodes.6–14 Their excellent performance is attributed to the physical properties of Pb-OHPs, including good optical absorption, moderate band gap, low exciton binding energy (Eb), and high carrier mobility. However, the moisture instability and toxicity of lead have hindered the practical application of Pb-OHPs.15–17 Therefore, alternatives that not only preserve the advantages demonstrated by Pb-OHPs but also possess hypotoxicity and ambient stability need to be explored.18–21 Similar to Pb-OHPs, OHPs based on Group VA metal ions such as Bi3+ and Sb3+ (denoted as M3+) exhibit optoelectronic properties due to their isoelectronic configuration, ambient stability, and close chemistry of the lone-pair ns2 state.22–26 Different from Pb2+, M3+ features low inclination to form polymeric species, especially those with high dimensionality.27,28 Typically, the desired M-OHPs with structures such as APbX3, which are formed by corner-sharing (MX6), cannot be harvested because of the high +3 oxidation state. Instead, the zero-dimensional (0D) perovskites including A3Bi2X929–32 and A3Sb2X922,33,34 with two face-shared MX6 octahedra are formed steadily. Although (M2X9)-type materials achieve a power conversion efficiency of 2–3% in solar cells,22,32,33 they still suffer from intrinsic issues such as indirect band gaps, wide gap values, and inferior carrier transport.22,35 This predicament is mainly caused by their low structural and electronic dimensionalities, which disrupt continuous carrier transport.36–38 To make progress, extended M3+ halide networks have been obtained in terms of 1D chain-like structures, including (MI4)−22,39 and (MI5)2−.24,40,41 However, the organic countercations employed therein are hydrophilic protonated amine-type cations or heteroaromatic cations with strong hydrophilic groups (typically –NH3+, =NH2+, or NH+), which could form strong N–H···O hydrogen bonds with external water molecules, thereby greatly reducing the moisture stability of OHPs.23,42–46 This inference is supported by the computational simulation of the decomposition mechanism, which demonstrates that the hydrogen-bond interactions between external water molecules and organic countercations trigger the decomposition reaction of OHPs.47,48 To address the toxicity and moisture instability of Pb-OHPs, we demonstrate herein a “double-free” strategy to realize the rational synthesis of moisture-stable hypotoxic OHPs. Hydrogen-bond-free organic cations formed via in situ alkylation reactions were employed to direct the self-assembly of Pb-free OHPs with extended inorganic structures. The strategy was proposed taking into account the following three features. First, the potential hydrogen-bond donor atoms (e.g., N) of organic cation were capped by hydrophobic alkyl groups to prevent external water from eroding the hybrid perovskites via forming hydrogen bonds. Second, the hydrogen-bond-free cations were well designed and modulated via facile in situ alkylation reactions on heteroaromatic molecules. Finally, M-OHPs with extended [e.g., one-dimensional (1D) and two-dimensional (2D))] inorganic structures are expected to address the discontinuous carrier transport issue of cluster-based (M2X9)-type materials. As a proof of concept application, the in situ alkylated 4-phenylpyridine (4pp) species were employed as hydrogen-bond-free cations to direct the formation of two M-OHPs, namely (1-methyl-4-phenylpyridin-1-ium)BiI4 ( Me4ppi-BiI4) and (1-ethyl-4-phenylpyridin-1-ium)SbI4 ( Et4ppi-SbI4). Both double-free OHPs adopt extended 1D stair-like chains, which are charge- compensated by typical hydrogen-bond-free counterions. They could endure harsh moisture and photostability assessments in both powder and polycrystalline film forms, exhibiting greatly improved stability compared with MA3M2X9 (MA = CH3NH3+) and corresponding hydrogen-bond compounds (4-phenylpyridin-1-ium)BiI4 ( 4ppi-BiI4) and (4-phenylpyridin-1-ium)SbiI4 ( 4ppi-SbI4). The photoconductivity tests showed that both double-free OHPs exhibited good photoconduction behavior with short rise and decay recovery time and excellent photocurrent repeatability. Experimental Methods Single crystals of double-free and corresponding hydrogen-bond OHPs were prepared by one-pot solvothermal methods. Crystalline MA3M2I9 used as a reference compound Supporting Information Figure S1, was prepared according to reported methods with only slight modification.49 Their crystal structures were determined by a charge-coupled device (CCD) diffractometer. The photoelectric properties of the polycrystalline thin films of OHPs were evaluated on a Solartron electrochemical system. Hall effect and theoretical calculations were used to determine the photophysical properties of the double-free materials. More experimental and theoretical details are available in the Supporting Information. Results and Discussion The crystal structures of Me4ppi-BiI4 and Et4ppi-SbI4 were determined by single-crystal X-ray diffraction studies. The results showed that both materials are crystallized in the monoclinic crystal system with the space group of P21/n. The crystal and structure refinement data of both compounds are presented in Supporting Information Table S1, and the selected important bond angles and bond distances are presented in Supporting Information Table S6. Both compounds contain stair-like 1D (MI4)− anionic chains that are charge compensated by positive-charged 4pp-derived hydrogen-bond-free cations (Figures 1a and 1b). In their structures, each Group VA metal M is coordinated by six I to form a (MI6) octahedron. Two octahedra first shared one edge in the equatorial plane to achieve a binuclear (M2I10) unit, which further shared edges with adjacent units along the meridian of the octahedra to generate the stair-like (MI4)− chains. Such chains extended along the a axis with a Bi⋯Bi distance of 4.622–4.736 Å and an Sb⋯Sb distance of 4.593–4.633 Å. The bond distances of Bi–I (2.931–3.310 Å) and Sb–I (2.854–3.223 Å) were in their normal ranges.39,50 As illustrated in Supporting Information Scheme S1, the hydrogen-bond-free cations Me4ppi and Et4ppi were formed via in situ N-alkylation reactions, which endow one positive charge per molecule.46 This result is confirmed by the infrared spectral analyses ( Supporting Information Figure S2 and Table S2). They piled up with each other along the a direction in their crystal lattice, which are further stacked parallel with the inorganic (MI4)− chain to form the 3D supramolecular packing diagrams (Figures 1c and 1d). Careful assessment of the two crystal structures suggested that Coulomb interactions and weak C–H⋯I hydrogen bonds exist between inorganic (MI4)− chains and organic hydrogen-bond-free cations, which enhanced the structural stability of the two double-free OHPs ( Supporting Information Figures S3 and S4). Importantly, the pyridine N atoms were covalently bonded with the hydrophobic alkyl groups. Thus, no strong N–H⋯O or O–H⋯N hydrogen bonds could be formed with water molecules from moist air or bulk water. The corresponding hydrogen-bond compounds 4ppi-BiI4 and 4ppi-SbI4 contained similar (MI4)− chains but were charge compensated by hydrophilic protonated organic counterions ( Supporting Information Scheme S2), possessing characteristic inclination to form N–H⋯O hydrogen bonds with external water (Table 1). Their detailed crystallographic structures are shown in Supporting Information Figures S5 and S6. Figure 1 | (a) Polyhedral view of the Pb-free perovskite-derivative chain; Corners: (SbI6) and (BiI6) octahedral units labeled with M–I bond distances. (b) View of the hydrogen-bond-free countercations Me4ppi and Et4ppi employed in this work. (c and d) Three-dimensional (3D) packing diagrams of the double-free OHPs Me4ppi-BiI4 and Et4ppi-SbI4 viewed along the a direction, respectively. Download figure Download PowerPoint Table 1 | Crystallographic Data of the OHPs Involved in This Work Compound Space Group Unit Cell Parameters (Å, deg) Character Me4ppi-BiI4 P21/n 7.687, 14.160, 12.925, 90, 91.73, 90 Double-free OHP Et4ppi-SbI4 P21/n 7.689, 19.939, 13.044, 90, 100.59, 90 Double-free OHP 4ppi-BiI4 P–1 7.700, 11.140, 11.446, 80.58, 72.66, 70.58 Hydrogen-bond OHP 4ppi-SbI4 C2/c 12.590, 18.042, 7.784, 90, 92.54, 90 Hydrogen-bond OHP Fine-milled polycrystalline powders for the double-free OHPs on the submicron scale were first employed to evaluate their moisture stability ( Supporting Information Figure S7). The powders were exposed to a 75% relative humidity (RH) environment for a long time. Two extensively studied Pb-free light absorbers (namely, MA3Bi2I9 and MA3Sb2I9) were selected as the references, which underwent similar stability evaluations. Powder X-ray diffraction (PXRD) measurements were used to evaluate the phases of the samples after the stability treatments (Figures 2a and 2b and Supporting Information Figure S8). The colors of the powdery samples were also monitored, which could be used as a supplementary evidence for their structural changes ( Supporting Information Figure S9). After 3 days of 75% RH treatment, the structures of Me4ppi-BiI4 and Et4ppi-SbI4 remained unchanged as their PXRD patterns after the humidity tests matched well with those of the as-synthesized ones, and their colors were retained. This result is distinctly better than that of MA3M2I9, which remained stable no more than 2 days under similar moisture conditions. After extending the duration time, obvious color and phase changes were observed for Et4ppi-SbI4 4 days later. However, Me4ppi-BiI4 remained extremely stable even after 243 days. The stability of both double-free OHPs in polycrystalline films was further evaluated to characterize the stability of the materials in practical application ( Supporting Information Figure S10). The preliminary PXRD patterns under different conditions are shown in Supporting Information Figure S11. From the case of Et4ppi-SbI4, the polycrystalline film possesses superior stability compared to its corresponding powdery sample because the stability of the film was retained for at least 92 days compared with that of the powder (3 days). The results strongly indicate the enhanced moisture stabilities of Me4ppi-BiI4 and Et4ppi-SbI4 and the effectiveness of the “double-free” strategy. However, Me4ppi-BiI4 was much more stable than Et4ppi-SbI4 in humid environments. This is because of the increased Lewis alkalinity from Sb(III) to Bi(III), which makes the Bi(III) compound more difficult to hydrolyze. Figure 2 | PXRD patterns of (a) Me4ppi-BiI4 and (b) Et4ppi-SbI4 powders in different conditions. The enhanced relative intensities of the diffraction peaks at 28° and 37° in panel (a) are probably due to the preferred orientation of powdery crystalline particles. The moisture resistance experiment was performed under a 75% RH environment at room temperature. The photoresistance experiments were conducted under sunlight (June–July, Jinan, Shandong, China; ∼2.60 mw·cm−2 at 2:00 PM) and 254-nm UV light (3.40 mw·cm−2). Download figure Download PowerPoint To determine the importance of hydrogen-bond-free countercation in the enhanced moisture stability, the corresponding hydrogen-bond OHPs, 4ppi-BiI4 and 4ppi-SbI4, with similar (MI4)− chains but charge compensated by hydrophilic protonated 4pp cations were synthesized for comparison ( Supporting Information Scheme S2). Considering that both Bi-OHPs have good stability in moist air, their powders were directly soaked in deionized water to save time. The results shown in Supporting Information Figures S12 and S13 indicate that both double-free OHPs have greatly enhanced stabilities compared to the corresponding hydrogen-bond OHPs, further confirming the effectiveness of hydrogen-bond-free countercations in constructing moisture-stable OHPs. The photo- and thermal stabilities of OHPs are also closely related to their performance. Thus, the powders and polycrystalline films of Me4ppi-BiI4 and Et4ppi-SbI4 were exposed to direct sunlight and/or 254-nm UV light irradiation. As shown in Figures 2a and 2b and Supporting Information Figure S11, the PXRD patterns of Me4ppi-BiI4 and Et4ppi-SbI4 after light irradiation for 76 and 92 days are all in accord with their corresponding as-synthesized samples. These results manifest the excellent photostability of the two double-free OHPs. The thermal stability evaluations suggest that Me4ppi-BiI4 and Et4ppi-SbI4 can remain stable up to 552 and 500 K, respectively, which are comparable with MA3M2X9 ( Supporting Information Figure S14, for details, see Supporting Information Table S3). The diffuse reflectance spectra of polycrystalline samples were obtained at room temperature and converted to absorbance spectra by Kubelka−Munk transformation. As shown in Figures 3a and 3b, Me4ppi-BiI4 and Et4ppi-SbI4 showed step-like absorption features with sharp absorption edges at 640 and 593 nm, respectively, implying their excellent visible-light absorption. The deviation between the absorption of the two compounds is ascribed to metal substitution as they have similar crystal structures.51 The band gap values of Me4ppi-BiI4 and Et4ppi-SbI4 at 2.08 and 2.22 eV match their red and orange yellow colors, respectively. Compared with the Sb-OHP, the Bi-OHP Me4ppi-BiI4, which has a reduced band gap, is primarily due to the increased spin–orbit coupling associated with the heavier Bi atom.51 Meanwhile, density functional theory (DFT) calculations were employed to evaluate their band gaps, which were calculated as 2.02 and 1.50 eV, respectively.52,53 These band gaps are close to the well-studied Pb-free light absorbers for Bi-OHPs such as MA3Bi2I9 (1.89–1.96 eV),54,55 (NH4)3Bi2I9 (2.04 eV),56 and MA2AgBiI6 (2.02 eV)57 and for Sb-OHPs such as MA3Sb2I9 (1.92 eV) (for more details, see Supporting Information Table S3).55 They are also comparable with the band gap of MAPbBr3 at 2.3 eV. These results imply their good optical absorption abilities.58 Figure 3 | Solid-state absorption spectra, Kubelka–Munk plots (insert) and photographs of the single crystals (insert) of (a) Me4ppi-BiI4 and (b) Et4ppi-SbI4. Download figure Download PowerPoint Considering the potential applications of the two double-free OHPs as light absorbers in photovoltaic devices, the optical absorptions of their thin films were also investigated. As shown in the inserts of Supporting Information Figure S15, the fabricated films are uniform and have similar colors as those of their polycrystalline powdery samples. Importantly, the samples still maintained strong light absorptions after being formed as thin films. The optical absorption cutoff wavelengths of the two films were determined as 595 and 556 nm, respectively, which are obviously blue-shifted compared with the values obtained from the powdery samples.27 The reduction of the absorption spectra from crystalline sample to thin film was also found in MAPbI3 and was mainly due to the thickness difference between them.59 To evaluate the photoconductive properties of the two double-free OHPs, photoconductive devices (insert of Figure 4a) were fabricated via a drop-coating method on Au interdigital electrodes. A 500 W Xe lamp with a tunable optical power was chosen as the white light source. Figures 4a and 4b represent the current–voltage (I–V) plots of the two devices with and without white light irradiation. The curves are typically linear and symmetrical, suggesting good contact between the powdery samples and gold interdigital electrode with negligible contact barrier.60 Obviously, once both photoconductive devices were exposed to white light, the currents dramatically increased. At a power intensity of 85 mW·cm−2 and a positive bias voltage of 5 V, the currents were enhanced from 42 to 166 nA for Me4ppi-BiI4 and 0.42 to 0.80 nA for Et4ppi-SbI4, which endowed a switch on/off ratio of 3.95 and 1.90, respectively. The on/off ratio for Me4ppi-BiI4 was larger than that of MA3Bi2I9 (3.87) under similar experimental conditions ( Supporting Information Figure S16).49,61,62 Figure 4 | (a and b) I−V curves in the dark and under Xe lamp illumination with incident intensity of 85 mW·cm−2, (c and d) time-dependent photocurrent response curves, and (e and f) response times of the photoconductive film devices based on Me4ppi-BiI4 and Et4ppi-SbI4. Insert in (a): schematic of the fabricated photoconductive film device. The photoresponses were measured at 2 V. (g and h) I–V curves of the thin-film devices measured under white light irradiations with different incident intensities. Download figure Download PowerPoint Figures 4c and 4d plot the dynamic current–time (I–T) curves for the two photoconductive devices under repeated switching of the white light. As can be observed, both devices show a step photoresponse. The results indicate that the devices based on the two double-free OHPs exhibit excellent photocurrent repeatability under photoswitching conditions. We also investigated the moisture and photostabilities of the two film devices by monitoring the I–T curves. After long-term moisture and photoresistance experiments, the dynamic I–T curves of the two devices still exhibited robust responses with comparable signals as before. In the moisture-treated Et4ppi-SbI4 device, slight photocurrent reductions (about ∼6% and ∼15%) occurred after storage in 75% RH condition for 2 and 3 days, respectively ( Supporting Information Figure S17). These findings confirmed the excellent stabilities of Me4ppi-BiI4 and Et4ppi-SbI4 as photoconductive devices. Figures 4e and 4f clearly suggest that both devices show fast response speed with small rise (τ1) and decay time (τ2) of 120 and 180 ms, respectively. The performance of the two photoconductive devices under different light intensities and monochromatic lights was also investigated. The photocurrent intensities of both devices gradually increased with the increase of the incident power (Figures 4g and 4h). The photocurrent of Me4ppi-BiI4 was obviously higher than that of Et4ppi-SbI4, which is in line with its much wider visible-light absorption range and direct-band-gap characteristic. Further measurements suggested that both OHPs showed obvious wavelength-dependent photoresponses ( Supporting Information Figures S18a–S18d). The photocurrents were enhanced with the decrease of the incident wavelength in the red light range. In particular, they were remarkably enhanced when the wavelength ranged from 600 to 350 nm for Me4ppi-BiI4 and from 550 to 350 nm for Et4ppi-SbI4. The tendencies closely matched their absorption spectra and also indicated their wide photoresponse ranges.60 The photoconversion behavior of hybrid perovskites is closely related to their charge transport capabilities. Thus, the photophysical properties of Me4ppi-BiI4 and Et4ppi-SbI4, including Eb, conductivity type, carrier mobility, and carrier concentration were determined by experiments and theoretical calculations (Figures 5a and 5b as well as Supporting Information Figure S19 and Tables S4 and S5). The value of Eb, which measures the strength of Coulombic attraction in the photogenerated electron-hole pair, was determined by theoretical calculation and temperature-dependent photoluminescence. Me4ppi-BiI4 had an Eb value of 138 meV, which was only half of that of MA3Bi2I9. Et4ppi-SbI4 has a lower Eb value of 43 meV, which was close to that of MAPbI3. According to the Hall measurements, the negative Hall coefficients for both double-free perovskites suggested that their carriers should have n-type (hole) conductivity. This characterization was similar to that of the widely studied Pb-free perovskites, including MA3Bi2I9 and Cs3Sb2I9. However, it is in contrast to the MAPbI3 film, which always shows a p-type (electron) conductivity.63 The calculated carrier concentrations of Me4ppi-BiI4 and Et4ppi-SbI4 were 1.95 × 1013 and 2.84 × 1014 cm−3, respectively, which were 4–5 orders of magnitude higher than that of the MAPbI3 film. Finally, the carrier mobilities were estimated to be 1400 and 1810 cm2·V−1·s−1, respectively, which was significantly higher than the values of MAPbI3 and MA3Bi2I9. These values were also remarkably high and were comparable with those of classical semiconductors including Si, GaAs, InP, and so on.63 All the findings imply that the OHPs synthesized by the “double-free” strategy have great potential in photoconductance applications. Figure 5 | Comparison of (a) the Eb values and (b) the carrier mobilities of Me4ppi-BiI4 and Et4ppi-SbI4 with the selected well-known OHPs. The Eb and carrier mobility values are labeled. FA = formamidinium [HC(NH2)2+]. Download figure Download PowerPoint DFT calculations were further performed to obtain a better understanding of the photoelectric properties of the double-free OHPs. As depicted in Figures 6a and 6b, the valence band maximum (VBM) and conduction band minimum (CBM) of Me4ppi-BiI4 were located at the same K point, which is a typical feature of the direct-band-gap semiconductor. However, Et4ppi-SbI4 exhibited an indirect-band-gap feature. A direct-band-gap feature usually indicates higher light-absorbing capability and hence needs relatively thin layers to absorb the desired solar energy.64 In this regard, Me4ppi-BiI4 is a more promising candidate as a light-absorbing material. From the partial density of states (PDOS) of Me4ppi-BiI4 given in Figure 6c, one can recognize that the VBM is mainly occupied by the 5p state of I, and the CBM is mainly composed of the 6p state of Bi. Thus, the photoresponse of Me4ppi-BiI4 was mainly caused by the electron transitions within the inorganic (BiI4)− chains, exactly between the I-5p and the Bi-6p states. An entirely different situation was observed for Et4ppi-SbI4 as shown in Figure 6d, where the I-5p state provided a major contribution to the VBM; however, the p-π* antibonding orbitals of Et4ppi dominated the CBM. Thus, the photoresponse of Et4ppi-SbI4 was probably due to the electron transitions between the inorganic (SbI4)− chain and hydrogen-bond-free cation. Such an electron transition is relatively more difficult than the intrachain transition in Me4ppi-BiI4, which is in accordance with its indirect-band-gap feature. B3LYP/6-31G(d,p) level calculations were further employed to disclose the role of hydrogen-bond-free cations on the different CBM structures in the double-free OHPs. As shown in Figure 6e, due to the stronger electron-donating ability of ethyl than methyl group,46 the lowest unoccupied molecular orbital (LUMO) energy of Et4ppi is calculated as −6.1099 eV, which is 0.0827 eV lower than that of Me4ppi. Accordingly, the lower lying LUMO of Et4ppi was inserted between the VB and CB of the inorganic (SbI4)− skeleton to form the new CBM of Et4ppi-SbI4, generating its indirect-band-gap structure. Figure 6 | (a and b) Electronic band structures and (c and d) PDOS of Me4ppi-BiI4 and Et4ppi-SbI4, respectively. (e) Energies of the frontier molecular orbitals and gap values calculated at the B3LYP/6-31G(d,p) level for the hydrogen-bond-free cations. Download figure Download PowerPoint Conclusion We proposed a “double-free” strategy to tackle the moisture instability and lead toxicity issues of Pb-OHPs, by assembling hydrogen-bond-free cations with Pb-free perovskite-type extended structures in one crystal lattice. To demonstrate its effectiveness, two Bi- and Sb-based double-free OHPs ( Me4ppi-BiI4 and Et4ppi-SbI4) were synthesized, and their structures and photophysical properties were fully characterized. Therein, the N-donor atoms of the hydrogen-bond-free countercations were capped by hydrophobic alkylation groups, which effectively precluded external water molecules from eroding OHPs via strong hydrogen bonds. Accordingly, Me4ppi-BiI4 and Et4ppi-SbI4 showed superior moisture stability, especially for Bi-OHP, which can endure 75% RH for at least 243 days. These data are much better than those of the well-studied MA3M2I9-type materials and their corresponding hydrogen-bond compounds. Both double-free OHPs showed good photoconduction behavior in the UV and visible-light region, with short rise and decay recovery times, and exhibited excellent photocurrent reproducibility. The intriguing photoconversion performances were ascribed to their relatively low Eb values and greatly improved carrier mobilities and carrier concentrations. This work not only contributes two Sb- and Bi-OHPs with excellent stability and photoresponse, but also demonstrates a novel and promising strategy to obtain moisture-stable hypotoxic hybrid perovskites. Supporting Information Supporting Information is available and includes experimental details, computational methods, crystal and structure refinement data, syntheses of MA3M2I9, infrared (IR) analyses, the synthesis routes for hydrogen-bond-free cation, additional structural figures, a band-gap summary for Pb-free OHPs, stability evaluations for MA3Bi2I9 and hydrogen-bond OHPs, photographs of powdery samples after moisture stability evaluations, PXRD patterns and absorption spectra for thin films, thermogravimetric analysi