@article{gu_tao_fu_guo_jiang_guan_li_li_lü_fu_2023, title={Correlating Photophysical Properties with Stereochemical Expression of 6s2 Lone Pairs in Two‐dimensional Lead Halide Perovskites}, volume={62}, ISSN={1433-7851 1521-3773}, url={http://dx.doi.org/10.1002/anie.202304515}, DOI={10.1002/anie.202304515}, abstractNote={Abstract}, number={30}, journal={Angewandte Chemie International Edition}, publisher={Wiley}, author={Gu, Jiazhen and Tao, Yu and Fu, Tonghuan and Guo, Songhao and Jiang, Xiaofan and Guan, Yan and Li, Xiaotong and Li, Chen and Lü, Xujie and Fu, Yongping}, year={2023}, month={Jun} } @article{chu_jia_liu_zhang_li_feng_pi_yang_zhao_liu_et al._2023, title={Lattice engineering for stabilized black FAPbI 3 perovskite single crystals for high-resolution x-ray imaging at the lowest dose}, volume={9}, ISSN={2375-2548}, url={http://dx.doi.org/10.1126/sciadv.adh2255}, DOI={10.1126/sciadv.adh2255}, abstractNote={ Preliminary theoretical analyses indicate that lattice relaxation may be used to release lattice strain in the FAPbI 3 perovskite to warrant both high x-ray detection performance and improved stability. Herein, we demonstrate stable black α-phase FAPbI 3 single crystals (SCs) realized by lattice engineering via annealing in the ambient atmosphere. The engineered α-FAPbI 3 SC detector shows almost all the best figures of merit including a high sensitivity of 4.15 × 10 5 μC Gy air −1 cm −2 , a low detection limit of 1.1 nGy air s −1 , a high resolution of 15.9 lp mm −1 , and a short response time of 214 μs. We further demonstrate high-definition x-ray imaging at a dose rate below 10 nGy air s −1 on the FAPbI 3 SC, indicating a minimal dose-area product of 0.048 mGy air cm 2 to the patient for one-time posteroanterior chest diagnosis, which is more than 3000 times lower than the international reference level of 150 mGy air cm 2 . In addition, the robust long-term stability enables the FAPbI 3 SC x-ray detector to work steadily for more than 40 years. }, number={35}, journal={Science Advances}, publisher={American Association for the Advancement of Science (AAAS)}, author={Chu, Depeng and Jia, Binxia and Liu, Naiming and Zhang, Yunxia and Li, Xiaotong and Feng, Jiangshan and Pi, Jiacheng and Yang, Zhou and Zhao, Guangtao and Liu, Yucheng and et al.}, year={2023}, month={Sep} } @article{li_tao_jiang_cai_gu_zheng_guan_zhang_li_su_et al._2023, title={Optical bandgap anomaly with tuning dimensionality in germanium perovskites: Interplay between quantum confinement and lone pair expression}, volume={12}, ISSN={2451-9294}, url={http://dx.doi.org/10.1016/j.chempr.2023.11.011}, DOI={10.1016/j.chempr.2023.11.011}, abstractNote={

Summary

The optical bandgaps of 2D halide perovskites (HPs) generally decrease with increasing inorganic layer thickness because of the diminishing quantum confinement (QC) effect. Here, we report a reverse bandgap trend in a class of Ge-based HPs, achieved through a judicious selection of organic cations, which exhibits an enhanced stereochemical lone pair expression (LPE) on the Ge2+ cation with increasing inorganic layer thickness. We find the enhanced LPE arises from cooperative interlayer compression and intralayer expansion on the inorganic lattice induced by a small spacer cation and a large cage A-cation, respectively. Experimental and theoretical studies show that such enhanced LPE can shift up the bandgap, which is significant enough to offset the QC effect, leading to the bandgap anomaly with tuning dimensionality. Our results reveal a previously underexplored avenue to tune the optical and electronic properties of 2D HPs by rationally controlling the LPE through interlayer and intralayer engineering.}, journal={Chem}, publisher={Elsevier BV}, author={Li, Xinyu and Tao, Yu and Jiang, Xiaofan and Cai, Guanqun and Gu, Jiazhen and Zheng, Nanlong and Guan, Yan and Zhang, Wenkai and Li, Xiaotong and Su, Jie and et al.}, year={2023}, month={Dec} } @article{ma_gao_xu_li_song_liu_yang_li_du_zhao_et al._2022, title={Centimeter-Sized 2D Perovskitoid Single Crystals for Efficient X-ray Photoresponsivity}, volume={34}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.1c03832}, DOI={10.1021/acs.chemmater.1c03832}, abstractNote={Metal-halide perovskitoids with corner-, edge-, and face-sharing octahedra provide a fertile “playground” for structure modulation. With low defect density, low ion migration, and high intrinsic stability, two-dimensional (2D) perovskitoid single crystals are expected to be ideal materials for room-temperature semiconductor detectors (RTSDs) as high-energy radiation. However, there is no report yet on the use of 2D perovskitoid single crystals for X-ray detection, as well as on how the halide-modulated molecular assembly would affect their structure and properties. Herein, based on an amidino-based organic spacer, we successfully synthesized a novel family of centimeter-sized 2D perovskitoid single crystals, (3AP)PbX4 (3AP = 3-amidinopyridine, X = Cl, Br, and I). This is the first time that centimeter-sized 2D perovskitoid single crystals are demonstrated for X-ray photoresponse. Detailed investigations reveal a unique crystal packing with corner-sharing and edge-sharing octahedra of inorganic frameworks and 3AP cations lying between adjacent inorganic layers in a parallel and antisymmetric manner. Changing the halide from I to Br and Cl results in greater Pb–X–Pb angles and stronger hydrogen bonding in perovskitoids and therefore consequently a better elastic recovery under stress, a more efficient charge transport in the inorganic layer, and a lower ionic migration. By varying halide substitution, an efficient X-ray photoresponse is achieved with a sensitivity up to 791.8 μC Gyair–1 cm–2 for (3AP)PbCl4 and a low detection limit of 1.54 μGyair s–1. These results reveal that the large 2D perovskitoid single crystals provide a promising platform for high performance optoelectronics.}, number={4}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Ma, Chuang and Gao, Lili and Xu, Zhuo and Li, Xiaotong and Song, Xin and Liu, Yucheng and Yang, Tinghuan and Li, Haojin and Du, Yachao and Zhao, Guangtao and et al.}, year={2022}, month={Feb}, pages={1699–1709} } @article{li_cuthriell_bergonzoni_dong_traoré_stoumpos_guo_even_katan_schaller_et al._2022, title={Expanding the Cage of 2D Bromide Perovskites by Large A-Site Cations}, volume={34}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.1c03605}, DOI={10.1021/acs.chemmater.1c03605}, abstractNote={Two-dimensional (2D) halide perovskites have outstanding optoelectronic properties, and they feature a variety of organic cation spacers and cage A-site cations that can be incorporated into their structures. It has recently been reported that the Goldschmidt tolerance factor can be relaxed and expanded in iodide 2D perovskites. Bromide 2D perovskites, whose multilayer structures and optical properties are much less studied, provide a great platform for studying structure–property relationships for 2D perovskites with large A-site cations. Herein, we report the synthesis and structure of three new 2D bromide perovskites─(BA)2(MHy)2Pb3Br10 (BA, butylammonium; MHy, methylhydrazinium), (BA)2(EA)2Pb3Br10 (EA is ethylammonium), and (BA)2(DMA)Pb2Br7 (DMA is dimethylammonium). We compared them with other 2D perovskites with different A-site cations but with the same spacer and layer thickness. Single-crystal structures show that the Pb–Br bonds are elongated to accommodate the large A-site cations. Additionally, the octahedra in (BA)2(MHy)2Pb3Br10 and (BA)2(EA)2Pb3Br10 are highly distorted, and their different stacking patterns of the inner and outer layers lead to the formation of the n = 3 phases. Density functional theory calculations show that 2D perovskites with larger A-site cations (e.g., DMA, MHy, and EA) have smaller band dispersions and larger effective masses than those with Cs+ and MA. (BA)2(MHy)2Pb3Br10 also exhibits one of the largest Rashba splittings in the literature. Structures with large cage cations also exhibit high band gaps within the same n number and short photoluminescence (PL) lifetimes. Temperature- and power-dependent PL measurements reveal that the broad shoulder in the PL peak originates from the trap states.}, number={3}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Cuthriell, Shelby A. and Bergonzoni, Ashanti and Dong, Hao and Traoré, Boubacar and Stoumpos, Constantinos C. and Guo, Peijun and Even, Jacky and Katan, Claudine and Schaller, Richard D. and et al.}, year={2022}, month={Jan}, pages={1132–1142} } @article{hoffman_hadar_li_ke_vasileiadou_strzalka_chen_kanatzidis_2022, title={Film formation mechanisms in mixed-dimensional 2D/3D halide perovskite films revealed by in situ grazing-incidence wide-angle X-ray scattering}, volume={8}, ISSN={2451-9294}, url={http://dx.doi.org/10.1016/j.chempr.2021.12.022}, DOI={10.1016/j.chempr.2021.12.022}, abstractNote={Mixed-dimensional 2D/3D hybrid halide perovskites retain the stability of 2D perovskites (formula (A′)2(A)n−1PbnI3n+1) and long diffusion lengths of the 3D materials (AMX3), thereby affording devices with extended stability as well as state-of-the art efficiencies approaching those of the 3D materials. These films are made by spin-coating precursor solutions with an arbitrarily large average layer thickness n (⟨n⟩ > 7) to give films with both 2D and 3D phases. Although the 2D and 3D perovskite film formation mechanisms have been studied, little is understood about composite 2D/3D film formation. We used in-situ grazing-incidence wide-angle scattering with synchrotron radiation to characterize the films fabricated from precursor solutions with stoichiometries of (BA)2(MA)n−1PbnI3n+1 (⟨n⟩ = 3, 4, 5, 7, 12, 50, and ∞ (MAPbI3)). Four different mechanisms are seen depending on the stoichiometry in the precursor solution. Kinetic analysis shows faster and earlier growth of the solvate with increasing ⟨n⟩.}, number={4}, journal={Chem}, publisher={Elsevier BV}, author={Hoffman, Justin M. and Hadar, Ido and Li, Xiaotong and Ke, Weijun and Vasileiadou, Eugenia S. and Strzalka, Joseph and Chen, Lin X. and Kanatzidis, Mercouri G.}, year={2022}, month={Apr}, pages={1067–1082} } @article{vishnoi_zuo_li_binwal_wyckoff_mao_kautzsch_wu_wilson_kanatzidis_et al._2022, title={Hybrid Layered Double Perovskite Halides of Transition Metals}, volume={144}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.1c12760}, DOI={10.1021/jacs.1c12760}, abstractNote={Hybrid layered double perovskite (HLDP) halides comprise hexacoordinated 1+ and 3+ metals in the octahedral sites within a perovskite layer and organic amine cations between the layers. Progress on such materials has hitherto been limited to compounds containing main group 3+ ions isoelectronic with PbII (such as SbIII and BiIII). Here, we report eight HLDP halides from the A2MIMIIIX8 family, where A = para-phenylenediammonium (PPDA), 1,4-butanediammonium (1,4-BDA), or 1,3-propanediammonium (1,3-PDA); MI = Cu or Ag; MIII = Ru or Mo; X = Cl or Br. The optical band gaps, which lie in the range 1.55 to 2.05 eV, are tunable according to the layer composition, but are largely independent of the spacer. Magnetic measurements carried out for (PPDA)2AgIRuIIICl8 and (PPDA)2AgIMoIIICl8 show no obvious evidence of a magnetic ordering transition. While the t2g3 MoIII compound displays Curie-Weiss behavior for a spin-only d3 ion, the t2g5 RuIII compound displays marked deviations from the Kotani theory.}, number={15}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Vishnoi, Pratap and Zuo, Julia L. and Li, Xiaotong and Binwal, Devesh Chandra and Wyckoff, Kira E. and Mao, Lingling and Kautzsch, Linus and Wu, Guang and Wilson, Stephen D. and Kanatzidis, Mercouri G. and et al.}, year={2022}, month={Apr}, pages={6661–6666} } @article{li_dong_volonakis_stoumpos_even_katan_guo_kanatzidis_2022, title={Ordered Mixed-Spacer 2D Bromide Perovskites and the Dual Role of 1,2,4-Triazolium Cation}, volume={34}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.2c01432}, DOI={10.1021/acs.chemmater.2c01432}, abstractNote={Two-dimensional (2D) halide perovskites exhibit extraordinary stability and structural diversity because they can incorporate different organic cations (both cage A-site cation and the spacers). Here, we report on the behavior of Tz = 1,2,4-triazolium as an A-site cation as well as a spacer cation in new perovskite compounds. We describe the synthesis and structure–property relationships of a new family of hydrogen-bonding stabilized 2D perovskites (IPA)2(Tz)n−1PbnBr3n+1 (n = 1–3) (IPA = isopropylammonium, Tz = 1,2,4-triazolium), which represents the rare example of 2D perovskites incorporating the large A-site cation (Tz) that can form both the n = 2 and 3 phases. However, excess of Tz cations can split the n = 2 layers to act as a spacer and combine with another spacer PA (PA = propylammonium) or BA (BA = butylammonium) to form n = 1 structures (PA)(Tz)PbBr4 and (BA)(Tz)PbBr4. These ordered mixed-spacer perovskites with different interlayer distances are unusual because the different spacers tend to stay in the same interlayer. (IPA)2(Tz)n−1PbnBr3n+1 all exhibit broad photoluminescence emission but for different reasons. For the n = 1 phase, the large octahedral tilting (small Pb–Br–Pb angle) gives rise to the broad emission, while for the n = 2 and 3 phases, the individual octahedral distortion (large distortion index) plays a more important role. For the mixed-spacer (PA)(Tz)PbBr4 and (BA)(Tz)PbBr4, their band gaps are in between the parent n = 1 compounds [(BA)2PbBr4 or (PA)2PbBr4, and (Tz)2PbBr4]. Density functional theory calculations suggest that the band structures of (PA)(Tz)PbBr4 and (BA)(Tz)PbBr4 combine features from both parent compounds yet differ from a simple superposition. This work demonstrates the dual role of Tz molecules as the A-site cation in multilayer 2D perovskites and as the spacer in ordered mixed-spacer 2D phases.}, number={14}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Dong, Hao and Volonakis, George and Stoumpos, Constantinos C. and Even, Jacky and Katan, Claudine and Guo, Peijun and Kanatzidis, Mercouri G.}, year={2022}, month={Jul}, pages={6541–6552} } @article{fu_quintero_welton_li_cucco_de siena_even_volonakis_kepenekian_liu_et al._2022, title={Short Aromatic Diammonium Ions Modulate Distortions in 2D Lead Bromide Perovskites for Tunable White-Light Emission}, volume={34}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.2c02471}, DOI={10.1021/acs.chemmater.2c02471}, abstractNote={White-light broadband emission in the visible range from the low-dimensional halide perovskites is commonly attributed to structural distortions in lead bromide octahedra. In this paper, we report Dion–Jacobson-phase two-dimensional (2D) lead bromide perovskites based on short aromatic diammonium cations, p-phenylene diammonium (pPDA), m-phenylene diammonium (mPDA), and two 1D compounds templated by o-phenylene diammonium (oPDA). All of the compounds exhibit white-light emission. Single-crystal X-ray diffraction analysis reveals that the distortion of the Pb octahedra is influenced by the stereochemistry of the cations and their interactions with the perovskite layers. Solid-state 1H and 207Pb NMR spectroscopy analysis further confirms this trend, whereby different 1H and 207Pb chemical shifts are observed for the pPDA and mPDA spacer cations, indicating different hydrogen-bonding interactions and octahedral distortions. Owing to the octahedral distortion, 2D (mPDA)PbBr4 compounds exhibit broader white-light emission than 2D (pPDA)PbBr4. Density functional theory calculations suggest that (pPDA)PbBr4 and (mPDA)PbBr4 are direct-band-gap semiconductors, and they exhibit larger electronic band gaps and effective masses than the Ruddlesden–Popper-phase (BA)2PbBr4. Among the films of these compounds, 2D (mPDA)PbBr4 shows the best stability, which is attributed to stronger hydrogen-bonding interactions in the material.}, number={21}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Fu, Ping and Quintero, Michael A. and Welton, Claire and Li, Xiaotong and Cucco, Bruno and De Siena, Michael C. and Even, Jacky and Volonakis, George and Kepenekian, Mikaël and Liu, Runze and et al.}, year={2022}, month={Oct}, pages={9685–9698} } @article{li_guan_li_fu_2022, title={Stereochemically Active Lone Pairs and Nonlinear Optical Properties of Two-Dimensional Multilayered Tin and Germanium Iodide Perovskites}, volume={144}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.2c07535}, DOI={10.1021/jacs.2c07535}, abstractNote={Two-dimensional (2D) metal halide perovskites are promising tunable semiconductors. Previous studies have focused on Pb-based structures, whereas the multilayered Sn- and Ge-based analogues are largely unexplored, even though they potentially exhibit more diverse structural chemistry and properties associated with the more polarizable ns2 lone-pair electrons. Herein, we report the synthesis and structures of 2D tin iodide perovskites (BA)2(A)Sn2I7, where BA = n-butylammonium and A = methylammonium, formamidinium, dimethylammonium, guanidinium, or acetamidinium, and those of 2D germanium iodide perovskites (BA)2(A)Ge2I7, where A = methylammonium or formamidinium. By comparing these structures along with their Pb counterparts, we establish correlations between the effect of group IV-cation's lone-pair stereochemical activity on the perovskite crystal structures and the resulting semiconducting properties such as bandgaps and carrier-phonon interactions and nonlinear optical properties. We find that the strength of carrier-phonon interaction increases with increasing lone-pair activity, leading to a more prominent photoluminescence tail on the low-energy side. Moreover, (BA)2(A)Ge2I7 exhibit strong second harmonic generation with second-order nonlinear coefficients of ∼10 pm V-1 that are at least 10 times those of Sn counterparts and 100 times those of Pb counterparts. We also report the third-order two-photon absorption coefficients of (BA)2(A)Sn2I7 to be ∼10 cm MW-1, which are one order of magnitude larger than those of the Pb counterparts and traditional inorganic semiconductors. These results not only highlight the role of lone-pair activity in linking the compositions and physical properties of 2D halide perovskites but also demonstrate 2D tin and germanium iodide perovskites as promising lead-free alternatives for nonlinear optoelectronic devices.}, number={39}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Xinyu and Guan, Yan and Li, Xiaotong and Fu, Yongping}, year={2022}, month={Sep}, pages={18030–18042} } @article{li_kepenekian_li_dong_stoumpos_seshadri_katan_guo_even_kanatzidis_2022, title={Tolerance Factor for Stabilizing 3D Hybrid Halide Perovskitoids Using Linear Diammonium Cations}, volume={144}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.1c11803}, DOI={10.1021/jacs.1c11803}, abstractNote={Three-dimensional (3D) halide perovskites have attracted enormous research interest, but the choice of the A-site cations is limited by the Goldschmidt tolerance factor. In order to accommodate cations that lie outside the acceptable range of the tolerance factor, low-dimensional structures usually form. To maintain the favorable 3D connection, the links among the metal-halide octahedra need to be rearranged to fit the large cations. This can result in a departure from the proper corner-sharing perovskite architectures and lead to distinctly different perovskitoid motifs with edge- and face-sharing. In this work, we report four new 3D bromide perovskitoids incorporating linear organic diammonium cations, A'Pb2Br6 (A' is a +2 cation). We propose a rule that can guide the further expansion of this class of compounds, analogous to the notion of Goldschmidt tolerance factor widely adopted for 3D AMX3 perovskites. The fundamental building blocks in A'Pb2Br6 consist of two edge-shared octahedra, which are then connected by corner-sharing to form a 3D network. Different compounds adopt different structural motifs, which can be transformed from one to another by symmetry operations. Electronic structure calculations suggest that they are direct bandgap semiconductors, with relatively large band dispersions created by octahedra connected by corner-sharing. They exhibit similar electronic band structures and dynamic lattice characteristics to the regular 3D AMX3 perovskites. Structures with smaller Pb-Br-Pb angles and larger octahedra distortion exhibit broad photoluminescence at room temperature. The emerging structure-property relationships in these 3D perovskitoids set the foundation for designing and investigating these compounds for a variety of optoelectronic applications.}, number={9}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Kepenekian, Mikaël and Li, Linda and Dong, Hao and Stoumpos, Constantinos C. and Seshadri, Ram and Katan, Claudine and Guo, Peijun and Even, Jacky and Kanatzidis, Mercouri G.}, year={2022}, month={Feb}, pages={3902–3912} } @article{li_li_kocoj_ji_yuan_macropulos_stoumpos_xia_mao_kanatzidis_et al._2022, title={Ultrafast Excitonic Response in Two-Dimensional Hybrid Perovskites Driven by Intense Midinfrared Pulses}, volume={129}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.129.177401}, DOI={10.1103/PhysRevLett.129.177401}, abstractNote={Two-dimensional organic-inorganic hybrid perovskites (2DHPs) are natural quantum-well-like materials, in which strong quantum and dielectric confinement effects due to the organic spacers give rise to tightly bound excitons with large binding energy. To examine the mutual interactions between the organic spacer cations and the inorganic charge-residing octahedral framework in 2DHPs, here we perform femtosecond pump-probe spectroscopy by direct vibrational pumping of the organic spacers, followed by a visible-to-ultraviolet probe covering their excitonic resonances. Measurements on prototypical lead-bromide based 2DHP compounds, (BA)_{2}PbBr_{4} and (BA)_{2}(FA)Pb_{2}Br_{7} (BA^{+}=butylammonium; FA^{+}=formamidinium), reveal two distinct regimes of the temporal response. The first regime is dominated by a pump-induced transient expansion of the organic spacer layers that reduces the exciton oscillator strength, whereas the second regime arises from pump-induced lattice heating effects primarily associated with a spectral shift of the exciton energy. In addition, vibrational excitation enhances the biexciton emission, which we attribute to a stronger intralayer exciton confinement as well as vibrationally induced exciton detrapping from defect states. Our study provides fundamental insights regarding the impact of organic spacers on excitons in 2DHPs, as well as the excited-state dynamics and vibrational energy dissipation in these structurally diverse materials.}, number={17}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Li, Shunran and Li, Xiaotong and Kocoj, Conrad A. and Ji, Xiaoqin and Yuan, Shaofan and Macropulos, Eleni C. and Stoumpos, Constantinos C. and Xia, Fengnian and Mao, Lingling and Kanatzidis, Mercouri G. and et al.}, year={2022}, month={Oct} } @article{huang_li_tao_guo_gu_hong_yao_guan_gao_li_et al._2022, title={Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s2 Lone Pairs}, volume={144}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.2c03443}, DOI={10.1021/jacs.2c03443}, abstractNote={The electron-phonon (e-ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e-ph interaction in LHPs. Herein, we advance a fundamental understanding of the e-ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure-property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e-ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e-ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.}, number={27}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Huang, Xu and Li, Xiaotong and Tao, Yu and Guo, Songhao and Gu, Jiazhen and Hong, Huilong and Yao, Yige and Guan, Yan and Gao, Yunan and Li, Chen and et al.}, year={2022}, month={Jun}, pages={12247–12260} } @article{li_traoré_kepenekian_li_stoumpos_guo_even_katan_kanatzidis_2021, title={Bismuth/Silver-Based Two-Dimensional Iodide Double and One-Dimensional Bi Perovskites: Interplay between Structural and Electronic Dimensions}, volume={33}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.1c01952}, DOI={10.1021/acs.chemmater.1c01952}, abstractNote={New structures with favorable band structure and optical properties are of broad interest to the halide perovskite community. Recently, lead-free two-dimensional (2D) double perovskites have emerged as dimensionally reduced counterparts of their 3D analogues. Besides the structural diversity provided by the organic cation, the achievement of 2D lead-free iodide double perovskites has attached researchers to explore more structures in this new material family. Here, we report the synthesis and structures of a series of 2D iodide double perovskites based on cyclic diammonium cations (aminomethyl)piperidinium (AMP) and (aminomethyl)pyridinium (AMPY), (4AMP)2AgBiI8 and (3AMPY)2AgBiI8, and compare them with 1D structures with Bi only (x-AMP)BiI5 and (x-AMPY)BiI5 (x = 3 and 4). The crystallographic structures of the double perovskite phases are highly distorted, because of the inability of Ag to form regular octahedral coordination with iodine. The experimental bandgaps of the double perovskite phases are surprisingly similar ((4AMP)2AgBiI8) or even larger ((3AMPY)2AgBiI8) than in the 1D structures with the same cations ((4AMP)BiI5 and (3AMPY)BiI5). DFT calculations suggest that the effective electronic dimensionality of the double perovskites is on par or lower than that of 1D structures. The reduced electronic dimension of the 2D compounds originates from the weak electronic coupling between the corner-sharing Ag and Bi octahedra. The band structures for the 1D compounds are dispersive in the chain direction, suggesting that their electronic and structural dimensions are similar. Low frequency Raman spectra exhibit distinct peaks at room temperature for all compounds reported here, suggesting rigid lattices}, number={15}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Traoré, Boubacar and Kepenekian, Mikaël and Li, Linda and Stoumpos, Constantinos C. and Guo, Peijun and Even, Jacky and Katan, Claudine and Kanatzidis, Mercouri G.}, year={2021}, month={Jul}, pages={6206–6216} } @article{guo_sun_puggioni_luo_li_zhou_chung_cheng_li_rondinelli_et al._2021, title={Local Distortions and Metal–Semiconductor–Metal Transition in Quasi-One-Dimensional Nanowire Compounds AV3Q3Oδ (A = K, Rb, Cs and Q = Se, Te)}, volume={33}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.1c00434}, DOI={10.1021/acs.chemmater.1c00434}, abstractNote={Metal cluster compounds have garnered renewed interest in the search for novel superconductors and topological semimetals owing to structural instabilities of metal-cluster geometries and broken sy...}, number={7}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Guo, Zhongnan and Sun, Fan and Puggioni, Danilo and Luo, Yubo and Li, Xiaotong and Zhou, Xiuquan and Chung, Duck Young and Cheng, Erjian and Li, Shiyan and Rondinelli, James M. and et al.}, year={2021}, month={Apr}, pages={2611–2623} } @article{christodoulides_guo_dai_hoffman_li_zuo_rosenmann_brumberg_kanatzidis_schaller_et al._2021, title={Signatures of Coherent Phonon Transport in Ultralow Thermal Conductivity Two-Dimensional Ruddlesden–Popper Phase Perovskites}, volume={15}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/acsnano.0c03595}, DOI={10.1021/acsnano.0c03595}, abstractNote={An emerging class of methylammonium lead iodide (MAPbI3)-based Ruddlesden-Popper (RP) phase perovskites, BA2MAn-1PbnI3n+1 (n = 1-7), exhibit enhanced stability to environmental conditions relative to MAPbI3, yet still degrade at elevated temperatures. We experimentally determine the thermal conductivities of these layered RP phases for n = 1-6, where n defines the number of repeated perovskite octahedra per layer. We measure thermal conductivities of 0.37 ± 0.13/0.12, 0.17 ± 0.08/0.07, 0.21 ± 0.05/0.04, and 0.19 ± 0.04/0.03 W/m·K in thin films of n = 1-4 and 0.08 ± 0.06/0.04, 0.06 ± 0.04/0.03, 0.06 ± 0.03/0.03, and 0.08 ± 0.07/0.04 W/m·K in single crystals of n = 3-6. With the exception of n = 1, these thermal conductivities are lower than the range of 0.34-0.50 W/m·K reported for single-crystal MAPbI3. Reduced-order lattice dynamics modeling suggests that the initially decreasing trend of thermal conductivity in similarly oriented perovskites with increasing n may result from the transport properties of coherent phonons, emergent from the superstructure, that do not scatter at the interfaces of organic butylammonium chains and perovskite octahedra. Reduced group velocity of coherent phonons in n = 3-6, a consequence of band flattening in the phonon dispersion, is primarily responsible for their ultralow thermal conductivities. Similar effects on thermal conductivity have been experimentally demonstrated in deposited superlattices, but never in naturally defined materials such as RP phases. GIWAXS measurements reveal that higher n RP phase thin films are less orientationally controlled and therefore possess apparently elevated thermal conductivities relative to single crystals of the same n.}, number={3}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={Christodoulides, Alexander D. and Guo, Peijun and Dai, Lingyun and Hoffman, Justin M. and Li, Xiaotong and Zuo, Xiaobing and Rosenmann, Daniel and Brumberg, Alexandra and Kanatzidis, Mercouri G. and Schaller, Richard D. and et al.}, year={2021}, month={Mar}, pages={4165–4172} } @article{li_hoffman_kanatzidis_2021, title={The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency}, volume={121}, ISSN={0009-2665 1520-6890}, url={http://dx.doi.org/10.1021/acs.chemrev.0c01006}, DOI={10.1021/acs.chemrev.0c01006}, abstractNote={Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.}, number={4}, journal={Chemical Reviews}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Hoffman, Justin M. and Kanatzidis, Mercouri G.}, year={2021}, month={Jan}, pages={2230–2291} } @article{gao_li_traoré_zhang_fang_han_even_katan_zhao_liu_et al._2021, title={m-Phenylenediammonium as a New Spacer for Dion–Jacobson Two-Dimensional Perovskites}, volume={143}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.1c03687}, DOI={10.1021/jacs.1c03687}, abstractNote={Two-dimensional (2D) halide perovskites have several distinct structural classes and exhibit great tunability, stability, and high potential for photovoltaic applications. Here, we report a new series of hybrid 2D perovskites in the Dion-Jacobson (DJ) class based on aromatic m-phenylenediammonium (mPDA) dications. The crystal structures of the DJ perovskite materials (mPDA)MAn-1PbnI3n+1 (n = 1-3) were solved and refined using single-crystal X-ray crystallography. The results indicate a short I···I interlayer distance of 4.00-4.04 Å for the (mPDA)MAn-1PbnI3n+1 (n = 2 and 3) structures, which is the shortest among DJ perovskites. However, Pb-I-Pb angles are as small as 158-160°, reflecting the large distortion of the inorganic framework, which results in larger band gaps for these materials than those in other DJ analogues. Density functional theory calculations suggest appreciable dispersion in the stacking direction, unlike the band structures of the Ruddlesden-Popper phases, which exhibit flat bands along the stacking direction. This is a consequence of the short interlayer I···I distances that can lead to interlayer electronic coupling across the layers. The solution-deposited films (nominal (mPDA)MAn-1PbnI3n+1 compositions of n = 1-6) reveal improved surface coverage with increasing nominal n value with the higher n films being composed of a mixture of n = 1 and bulk three-dimensional MAPbI3 perovskites. The films made from solutions of these materials behave differently from those of other 2D iodide perovskites, and their solar cells have a mixture of n = 1 DJ and MAPbI3 as light-absorbing semiconductors.}, number={31}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Gao, Lili and Li, Xiaotong and Traoré, Boubacar and Zhang, Yalan and Fang, Junjie and Han, Yu and Even, Jacky and Katan, Claudine and Zhao, Kui and Liu, Shengzhong and et al.}, year={2021}, month={Aug}, pages={12063–12073} } @article{fu_jiang_li_traore_spanopoulos_katan_even_kanatzidis_harel_2020, title={Cation Engineering in Two-Dimensional Ruddlesden–Popper Lead Iodide Perovskites with Mixed Large A-Site Cations in the Cages}, volume={142}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.9b13587}, DOI={10.1021/jacs.9b13587}, abstractNote={The Goldschmidt tolerance factor in halide perovskites limits the number of cations that can enter their cages without destabilizing their overall structure. Here we have explored the limits of this geometric factor and found that the ethylammonium (EA) cations which lie outside the tolerance factor range can still enter the cages of the 2D halide perovskites by stretching them. The new perovskites allow us to study how these large cations occupying the perovskite cages affect the structural, optical, and electronic properties. We report a series of cation engineered 2D Ruddlesden-Popper lead iodide perovskites (BA)2(EAxMA1-x)2Pb3I10 (x = 0-1, BA is n-butylammonium, MA is methylammonium) by incorporating large EA cation in the cage. Analysis of the single-crystal structures reveals that the incorporation of EA in the cage significantly stretches Pb-I bonds, expands the cage, and induces a large octahedral distortion in the inorganic framework. Spectroscopic and theoretical studies show that such structural deformation leads to a blue-shifted bandgap, sub-bandgap trap states with wider energetic distribution, and stronger photoluminescence quenching. These results enrich the family of 2D perovskites and provide new insights for understanding the structure-property relationship in perovskite materials.}, number={8}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Fu, Yongping and Jiang, Xinyi and Li, Xiaotong and Traore, Boubacar and Spanopoulos, Ioannis and Katan, Claudine and Even, Jacky and Kanatzidis, Mercouri G. and Harel, Elad}, year={2020}, month={Feb}, pages={4008–4021} } @article{guo_xia_gong_cao_li_li_zhang_stoumpos_kirschner_wen_et al._2020, title={Direct Observation of Bandgap Oscillations Induced by Optical Phonons in Hybrid Lead Iodide Perovskites}, volume={30}, ISSN={1616-301X 1616-3028}, url={http://dx.doi.org/10.1002/adfm.201907982}, DOI={10.1002/adfm.201907982}, abstractNote={Abstract}, number={22}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Guo, Peijun and Xia, Yi and Gong, Jue and Cao, Duyen H. and Li, Xiaotong and Li, Xun and Zhang, Qi and Stoumpos, Constantinos C. and Kirschner, Matthew S. and Wen, Haidan and et al.}, year={2020}, month={Apr} } @article{tu_spanopoulos_vasileiadou_li_kanatzidis_shekhawat_dravid_2020, title={Exploring the Factors Affecting the Mechanical Properties of 2D Hybrid Organic–Inorganic Perovskites}, volume={12}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/acsami.0c02313}, DOI={10.1021/acsami.0c02313}, abstractNote={Mechanical stability of hybrid organic-inorganic perovskites (HOIPs) is essential to achieve long-term durable HOIP-based devices. While HOIPs in two-dimensional (2D) form offer numerous options in the structure and composition to tune their mechanical properties, little is known about the structure/mechanical-properties relationship in this family of materials. Here we investigated a series of 2D lead-halide HOIPs by nano-indentation to explore the impact of critical factors controlling the properties of both the organic and inorganic layers on the materials' out-of-plane mechanical performance. We find that the lead-halide bond in the inorganic framework can significantly influence the mechanical properties of 2D Ruddlesden-Popper (RP) HOIPs with n = 1. Like 3D HOIPs, stronger lead-halide bond strength leads to higher Young's modulus in these 2D HOIPs, i.e., E_⊥^Cl>E_⊥^Br>E_⊥^I. In contrast, the hardness of 2D RP HOIPs follows a trend of H_Br^2D>H_Cl^2D>H_I^2D, which is different from that found in 3D HOIPs, probably due to the combined effects from the Pb-X bond strength and inorganic framework structural change (e.g., symmetry and distortion). We further show that the interface between the organic layers in 2D HOIPs can be an effective route to engineer the materials' mechanical properties. Replacing the weak CH3-CH3 Van der Waals forces by covalent bonds or phenyl-phenyl interactions in the interface can lead to a much stiffer and harder 2D HOIPs. Finally, we discover that the mechanical performance of 2D HOIPs with linear aliphatic diammonium spacer molecules is affected by the two basic structural parameters, i.e., the thicknesses of the organic and inorganic layers, in a similar way compared to that of 2D RP HOIPs with linear aliphatic monoammonium spacer molecules. Thinner organic layer and thicker inorganic layer can result in 2D HOIPs with larger elastic modulus and hardness values. Our results offer intriguing insights into the structure-property relationship of 2D HOIPs from a mechanical perspective, providing guidelines and inspirations to achieve materials design with required mechanical properties for applications.}, number={18}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Tu, Qing and Spanopoulos, Ioannis and Vasileiadou, Eugenia S. and Li, Xiaotong and Kanatzidis, Mercouri G. and Shekhawat, Gajendra S. and Dravid, Vinayak P.}, year={2020}, month={Apr}, pages={20440–20447} } @article{gao_li_liu_fang_huang_spanopoulos_li_wang_chen_yang_et al._2020, title={Incorporated Guanidinium Expands the CH3NH3PbI3 Lattice and Enhances Photovoltaic Performance}, volume={12}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/acsami.0c14925}, DOI={10.1021/acsami.0c14925}, abstractNote={Guanidinium (GA) has been widely used as an additive in solar cells for enhanced performance. However, the size of the guanidinium cation is too large to be incorporated in the cage of the perovskite structure. Instead GA forms a variety of structures with lead iodide, where its role in the perovskite crystal as well as solar cell devices is unclear. In this study, we demonstrate that GA can be incorporated into the structure of MAPbI3 as (GA)x(MA)1-xPbI3. From single-crystal X-ray crystallographic refinement, we observe lattice expansion and Pb-I bond elongation with GA incorporation similar to exerting "negative pressure" which weakens orbital overlap and widens the band gap from 1.49 to 1.53 eV. We find that highest percentage of GA that can be incorporated into the 3D MAPbI3 structure is 5.26%, as confirmed by nuclear magnetic resonance (NMR). The alloyed (GA)x(MA)1-xPbI3 exhibit increased PL lifetimes from 154.4 ns to 266.3 ns after GA incorporation while the (GA)x(MA)1-xPbI3 devices enlarge Voc from 1.05 to 1.11 V. High efficiencies in solar cell devices up to 20.38% with Jsc of 23.55 mA cm-2, Voc of 1.11 V, and FF of 0.78 have been achieved, with stable photovoltaic performance for 900 hours in air.}, number={39}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Gao, Lili and Li, Xiaotong and Liu, Yan and Fang, Junjie and Huang, Sheng and Spanopoulos, Ioannis and Li, Xiaolei and Wang, Yao and Chen, Lin and Yang, Guanjun and et al.}, year={2020}, month={Sep}, pages={43885–43891} } @article{ke_chen_spanopoulos_mao_hadar_li_hoffman_song_yan_kanatzidis_2020, title={Narrow-Bandgap Mixed Lead/Tin-Based 2D Dion–Jacobson Perovskites Boost the Performance of Solar Cells}, volume={142}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.0c06288}, DOI={10.1021/jacs.0c06288}, abstractNote={The advent of the two-dimensional (2D) family of halide perovskites and their demonstration in 2D/three-dimensional (3D) hierarchical film structures broke new ground towards high device performance and good stability. The 2D Dion-Jacobson (DJ) phase halide perovskites are especially attractive in solar cells because of their superior charge transport properties. Here, we report on 2D DJ phase perovskites using a 3-(aminomethyl) piperidinium (3AMP) organic spacer for the fabrication of mixed Pb/Sn-based perovskites, exhibiting a narrow bandgap of 1.27 eV and a long carrier lifetime of 657.7 ns. Consequently, solar cells employing mixed 2D DJ 3AMP-based and 3D MA0.5FA0.5Pb0.5Sn0.5I3 (MA=methylammonium, FA=formamidinium) perovskite composites as light absorbers achieve enhanced efficiency and stability, giving a power conversion efficiency (PCE) of 20.09% with a high open-circuit voltage of 0.88 V, a fill factor of 79.74%, and a short-circuit current density of 28.63 mA cm-2. The results provide an effective strategy to improve the performance of single-junction narrow-bandgap solar cells and, potentially, to give a highly efficient alternative of bottom solar cells in tandem devices.}, number={35}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Ke, Weijun and Chen, Cong and Spanopoulos, Ioannis and Mao, Lingling and Hadar, Ido and Li, Xiaotong and Hoffman, Justin M. and Song, Zhaoning and Yan, Yanfa and Kanatzidis, Mercouri G.}, year={2020}, month={Aug}, pages={15049–15057} } @article{li_fu_pedesseau_guo_cuthriell_hadar_even_katan_stoumpos_schaller_et al._2020, title={Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening}, volume={142}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.0c03860}, DOI={10.1021/jacs.0c03860}, abstractNote={Organic-inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX3 (where M is a metal, and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in two-dimensional (2D) perovskites, providing us an opportunity to understand how various the A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystals (BA)2(A)Pb2I7 where BA = butylammonium, and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or guanidinium (GA), a series of A-site cation varied in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcm space group, while the DMA imposes the Ccmb space group. We observe that as the A-site cation becomes larger, the Pb-I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting "negative pressure" on the perovskite structures. Optical studies and DFT calculations show the Pb-I bond length elongation reduces overlap of the Pb s- and I p-orbitals and increases the optical bandgap, while Pb-I-Pb angles play a secondary role. Raman spectra show lattice softening with increasing size of the A-site cation. These structural changes with enlarged A cations result in significant decreases in photoluminescence intensity and lifetime, consistent with a more pronounced nonradiative decay. Transient absorption microscopy (TAM) results suggest that the PL drop may derive from a higher concentration of traps or phonon-assisted nonradiative recombination. The results highlight that extending the range of Goldschmidt tolerance factors for 2D perovskites is achievable, enabling further tuning of the structure-property relationships in 2D perovskites.}, number={26}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Fu, Yongping and Pedesseau, Laurent and Guo, Peijun and Cuthriell, Shelby and Hadar, Ido and Even, Jacky and Katan, Claudine and Stoumpos, Constantinos C. and Schaller, Richard D. and et al.}, year={2020}, month={Jun}, pages={11486–11496} } @article{li_he_kepenekian_guo_ke_even_katan_stoumpos_schaller_kanatzidis_2020, title={Three-Dimensional Lead Iodide Perovskitoid Hybrids with High X-ray Photoresponse}, volume={142}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.0c00101}, DOI={10.1021/jacs.0c00101}, abstractNote={Large organic A cations cannot stabilize the 3D perovskite AMX3 structure because they cannot be accommodated in the cubo-octhedral cage (do not follow the Goldschmidt tolerance factor rule), and they generally template low-dimensional structures. Here we report that the large di-cation aminomethylpyridinium (AMPY), can template novel 3D structures which resemble conventional perovskites. They have the formula (xAMPY)M2I6 (x = 3 or 4, M = Sn2+ or Pb2+) which is doubled the AMX3 formula. However, because of the steric requirement of the Goldschmidt tolerance factor rule, it is impossible for (xAMPY)M2I6 to form proper perovskite structures. Instead, a combination of corner-sharing and edge-sharing connectivity is adopted in these compounds leading to the new 3D structures. DFT calculations reveal that the compounds are indirect-bandgap semiconductors with direct bandgaps presenting at slightly higher energies and dispersive electronic bands. The bandgaps of the Sn and Pb compounds are ~ 1.7 eV and 2.0 eV, respectively, which is slightly higher than the corresponding AMI3 3D perovskites. The Raman spectra for the compounds are diffuse, with a broad rising central peak at very low frequencies around 0 cm-1, a feature that is characteristic of dynamical lattices, highly anharmonic, and dissipative vibrations very similar to the 3D AMX3 perovskites. Devices of (3AMPY)Pb2I6 crystals exhibit clear photoresponse under ambient light without applied bias, reflecting a high carrier mobility (μ) and long carrier lifetime (τ). The devices also exhibit sizable X-ray generated photocurrent with a high μτ product of ~1.2×10-4 cm2 /V and an X-ray sensitivity of 207 μC Gy-1 cm-2.}, number={14}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and He, Yihui and Kepenekian, Mikaël and Guo, Peijun and Ke, Weijun and Even, Jacky and Katan, Claudine and Stoumpos, Constantinos C. and Schaller, Richard D. and Kanatzidis, Mercouri G.}, year={2020}, month={Mar}, pages={6625–6637} } @article{ke_spanopoulos_tu_hadar_li_shekhawat_dravid_kanatzidis_2019, title={Ethylenediammonium-Based “Hollow” Pb/Sn Perovskites with Ideal Band Gap Yield Solar Cells with Higher Efficiency and Stability}, volume={141}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.9b03662}, DOI={10.1021/jacs.9b03662}, abstractNote={The power conversion efficiency (PCE) of halide perovskite solar cells is now comparable to that of commercial solar cells. These solar cells are generally based on multication mixed-halide perovskite absorbers with nonideal band gaps of 1.5-1.6 eV. The PCE should be able to rise further if the solar cells could use narrower-band gap absorbers (1.2-1.4 eV). Reducing the Pb content of the semiconductors without sacrificing performance is also a significant driver in the perovskite solar cell research. Here, we demonstrate that mixed Pb/Sn-based perovskites containing the oversized ethylenediammonium ( en) dication, { en}FA0.5MA0.5Sn0.5Pb0.5I3 (FA = formamidinium, MA = methylammonium), can exhibit ideal band gaps of 1.27-1.38 eV, suitable for the assembly of single-junction solar cells with higher efficiencies. The use of en dication creates a three-dimensional (3D) hollow inorganic perovskite structure, which was verified through crystal density measurements and single-crystal X-ray diffraction structural analysis as well as nuclear magnetic resonance measurements. The { en}FA0.5MA0.5Sn0.5Pb0.5I3 structure has massive Pb/Sn vacancies and much higher chemical stability than the same structure without en and vacancies. This new property reduces the dark current and carrier trap density and increases the carrier lifetime of the Pb/Sn-based perovskite films. Therefore, solar cells using { en}FA0.5MA0.5Sn0.5Pb0.5I3 light absorbers have substantially enhanced air stability and around 20% improvement in efficiency. After overlaying a thin MABr top layer, we found that the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 material gives an optimized PCE of 17.04%. The results highlight the strong promise of 3D hollow mixed Pb/Sn perovskites in achieving ideal band gap materials with higher chemical stability and lower Pb content for high-performance single-junction solar cells or multijunction solar cells serving as bottom cells.}, number={21}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Ke, Weijun and Spanopoulos, Ioannis and Tu, Qing and Hadar, Ido and Li, Xiaotong and Shekhawat, Gajendra S. and Dravid, Vinayak P. and Kanatzidis, Mercouri G.}, year={2019}, month={May}, pages={8627–8637} } @article{hoffman_che_sidhik_li_hadar_blancon_yamaguchi_kepenekian_katan_even_et al._2019, title={From 2D to 1D Electronic Dimensionality in Halide Perovskites with Stepped and Flat Layers Using Propylammonium as a Spacer}, volume={141}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.9b02846}, DOI={10.1021/jacs.9b02846}, abstractNote={Two-dimensional (2D) hybrid halide perovskites are promising in optoelectronic applications, particularly solar cells and light-emitting devices (LEDs), and for their increased stability as compared to 3D perovskites. Here, we report a new series of structures using propylammonium (PA+), which results in a series of Ruddlesden-Popper (RP) structures with the formula (PA)2(MA) n-1Pb nI3 n+1 ( n = 3, 4) and a new homologous series of "step-like" (SL) structures where the PbI6 octahedra connect in a corner- and face-sharing motif with the general formula (PA)2 m+4(MA) m-2Pb2 m+1I7 m+4 ( m = 2, 3, 4). The RP structures show a blue-shift in bandgap for decreasing n (1.90 eV for n = 4 and 2.03 eV for n = 3), while the SL structures have an even greater blue-shift (2.53 eV for m = 4, 2.74 eV for m = 3, and 2.93 eV for m = 2). DFT calculations show that, while the RP structures are electronically 2D quantum wells, the SL structures are electronically 1D quantum wires with chains of corner-sharing octahedra "insulated" by blocks of face-sharing octahedra. Dark measurements for RP crystals show high resistivity perpendicular to the layers (1011 Ω cm) but a lower resistivity parallel to them (107 Ω cm). The SL crystals have varying resistivity in all three directions, confirming both RP and SL crystals' utility as anisotropic electronic materials. The RP structures show strong photoresponse, whereas the SL materials exhibit resistivity trends that are dominated by ionic transport and no photoresponse. Solar cells were made with n = 3 giving an efficiency of 7.04% (average 6.28 ± 0.65%) with negligible hysteresis.}, number={27}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Hoffman, Justin M. and Che, Xiaoyang and Sidhik, Siraj and Li, Xiaotong and Hadar, Ido and Blancon, Jean-Christophe and Yamaguchi, Hisato and Kepenekian, Mikaël and Katan, Claudine and Even, Jacky and et al.}, year={2019}, month={Jun}, pages={10661–10676} } @article{li_guo_kepenekian_hadar_katan_even_stoumpos_schaller_kanatzidis_2019, title={Small Cyclic Diammonium Cation Templated (110)-Oriented 2D Halide (X = I, Br, Cl) Perovskites with White-Light Emission}, volume={31}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/acs.chemmater.9b01511}, DOI={10.1021/acs.chemmater.9b01511}, abstractNote={Two-dimensional (2D) halide perovskites exhibit excellent potential for optoelectronics because of their outstanding physical properties and structural diversity. White-light emission is one proper...}, number={9}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Guo, Peijun and Kepenekian, Mikaël and Hadar, Ido and Katan, Claudine and Even, Jacky and Stoumpos, Constantinos C. and Schaller, Richard D. and Kanatzidis, Mercouri G.}, year={2019}, month={Apr}, pages={3582–3590} } @article{li_ke_traoré_guo_hadar_kepenekian_even_katan_stoumpos_schaller_et al._2019, title={Two-Dimensional Dion–Jacobson Hybrid Lead Iodide Perovskites with Aromatic Diammonium Cations}, volume={141}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.9b06398}, DOI={10.1021/jacs.9b06398}, abstractNote={Two-dimensional (2D) halide perovskites have extraordinary optoelectronic properties and structural tunability. Among them, the Dion-Jacobson phases with the inorganic layers stacking exactly on top of each other are less explored. Herein, we present the new series of 2D Dion-Jacobson halide perovskites, which adopt the general formula of A'An-1PbnI3n+1 (A' = 4-(aminomethyl)pyridinium (4AMPY), A = methylammonium (MA), n = 1-4). By modifying the position of the -CH2NH3+ group from 4AMPY to 3AMPY (3AMPY = 3-(aminomethyl)pyridinium), the stacking of the inorganic layers changes from exactly eclipsed to slightly offset. The perovskite octahedra tilts are also different between the two series, with the 3AMPY series exhibiting smaller bandgaps than the 4AMPY series. Compared to the aliphatic cation of the same size (AMP = (aminomethyl)piperidinium), the aromatic spacers increase the rigidity of the cation, reduce the interlayer spacing and decrease the dielectric mismatch between inorganic layer and the organic spacer, showing the indirect but powerful influence of the organic cations on the structure and consequently on the optical properties of the perovskite materials. All A'An-1PbnI3n+1 compounds exhibit strong photoluminescence (PL) at room temperature. Preliminary solar cell devices based on the n = 4 perovskites as absorbers of both series exhibit promising performances, with a champion power conversion efficiency (PCE) of 9.20 % for (3AMPY)(MA)3Pb4I13 based devices, which is higher than the (4AMPY)(MA)3Pb4I13 and the corresponding aliphatic analogue (3AMP)(MA)3Pb4I13 based ones.}, number={32}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Ke, Weijun and Traoré, Boubacar and Guo, Peijun and Hadar, Ido and Kepenekian, Mikaël and Even, Jacky and Katan, Claudine and Stoumpos, Constantinos C. and Schaller, Richard D. and et al.}, year={2019}, month={Jul}, pages={12880–12890} } @article{li_hoffman_ke_chen_tsai_nie_mohite_kepenekian_katan_even_et al._2018, title={Two-Dimensional Halide Perovskites Incorporating Straight Chain Symmetric Diammonium Ions, (NH3CmH2mNH3)(CH3NH3)n−1PbnI3n+1 (m = 4–9; n = 1–4)}, volume={140}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.8b07712}, DOI={10.1021/jacs.8b07712}, abstractNote={Low-dimensional halide perovskites have recently attracted intense interest as alternatives to the three-dimensional (3D) perovskites because of their greater tunability and higher environmental stability. Herein, we present the new homologous 2D series (NH3C mH2 mNH3)(CH3NH3) n-1Pb nI3 n+1 ( m = 4-9; n = 1-4), where m represents the carbon-chain number and n equals layer-thickness number. Multilayer ( n > 1) 2D perovskites incorporating diammonium cations were successfully synthesized by the solid-state grinding method for m = 4 and 6 and by the solution method for m = 7-9. Structural characterization by single-crystal X-ray diffraction for the m = 8 and m = 9 series ( n = 1-4) reveals that these compounds adopt the Cc space group for even n members and Pc for odd n members. The optical bandgaps are 2.15 eV for two-layer ( n = 2), 2.01 eV for three-layer ( n = 3), and 1.90 eV for four-layer ( n = 4). The materials exhibit excellent solution processability, and casting thin-films of the n = 3 members was successfully accomplished. The films show a clear tendency for the higher- m members to have preferred orientation on the glass substrate, with m = 8 exhibiting almost perfect vertical layer orientation and m = 9 displaying both vertical and parallel layer orientation, as confirmed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The vertical layer orientation for the (NH3C8H16NH3)(CH3NH3)2Pb3I10 member results in the best thermal, light, and air stability within this series, thus showing excellent potential for solar cell applications.}, number={38}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Xiaotong and Hoffman, Justin and Ke, Weijun and Chen, Michelle and Tsai, Hsinhan and Nie, Wanyi and Mohite, Aditya D. and Kepenekian, Mikaël and Katan, Claudine and Even, Jacky and et al.}, year={2018}, month={Aug}, pages={12226–12238} }