Resolving rotational stacking disorder and electronic level alignment in a 2d oligothiophene-based lead iodide perovskite

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) represent diverse quantum well heterostructures composed of alternating inorganic and organic layers. While 2D HOIPs are nominally periodic in three dimensions for X-ray scattering, the inorganic layers can orient quasi-randomly, leading to rotational stacking disorder (RSD). RSD manifests as poorly resolved, diffuse X-ray scattering along the stacking direction, limiting the structural description to an apparently disordered subcell. However, local ordering preferences can still exist between adjacent unit cells and can considerably impact the properties, particularly the electronic structure. Here, we elucidate RSD and determine the preferred local ordering in the 2D [AE2T]PbI4 HOIP (AE2T: 5,5′-bis(ethylammonium)-[2,2′-bithiophene]). We use first-principles calculations to determine energy differences between a set of systematically generated supercells with different order patterns. We show that interlayer ordering tendencies are weak, explaining the observed RSD in terms of differing in-plane rotation of PbI6 octahedra in neighboring inorganic planes. In contrast, the ordering preference within a given organic layer is strong, favoring a herringbone-type arrangement of adjacent AE2T cations. The calculated electronic level alignments of proximal organic and inorganic frontier orbitals in the valence band vary significantly with the local arrangement of AE2T cations; only the most stable AE2T configuration leads to an interfacial type-Ib band alignment consistent with observed optical properties. The present study underscores the importance of resolving local structure arrangements in 2D HOIPs for reliable structure-property prediction

Solution-based synthesis of kesterite thin film semiconductors

Large-scale deployment of photovoltaic modules is required to power our renewable energy future. Kesterite, Cu2ZnSn(S, Se)4, is a p-type semiconductor absorber layer with a tunable bandgap consisting of earth abundant elements, and is seen as a potential 'drop-in' replacement to Cu(In,Ga)Se2 in thin film solar cells. Currently, the record light-to-electrical power conversion efficiency (PCE) of kesterite-based devices is 12.6%, for which the absorber layer has been solution-processed. This efficiency must be increased if kesterite technology is to help power the future. Therefore two questions arise: what is the best way to synthesize the film? And how to improve the device efficiency? Here, we focus on the first question from a solution-based synthesis perspective. The main strategy is to mix all the elements together initially and coat them on a surface, followed by annealing in a reactive chalcogen atmosphere to react, grow grains and sinter the film. The main difference between the methods presented here is how easily the solvent, ligands, and anions are removed. Impurities impair the ability to achieve high performance (>∼10% PCE) in kesterite devices. Hydrazine routes offer the least impurities, but have environmental and safety concerns associated with hydrazine. Aprotic and protic based molecular inks are environmentally friendlier and less toxic, but they require the removal of organic and halogen species associated with the solvent and precursors, which is challenging but possible. Nanoparticle routes consisting of kesterite (or binary chalcogenides) particles require the removal of stabilizing ligands from their surfaces. Electrodeposited layers contain few impurities but are sometimes difficult to make compositionally uniform over large areas, and for metal deposited layers, they have to go through several solid-state reaction steps to form kesterite. Hence, each method has distinct advantages and disadvantages. We review the state-of-the art of each and provide perspective on the different strategies.Fil: Todorov, I. T.. IBM Research. Thomas J. Watson Research Center; Estados UnidosFil: Hillhouse, H. W.. University of Washington; Estados UnidosFil: Aazou, S.. Mohammed V University; MarruecosFil: Sekkat, Z.. Mohammed V University; MarruecosFil: Vigil Galán, O.. National Polytechnic Institute; MéxicoFil: Deshmukh, S. D.. Purdue University; Estados UnidosFil: Agrawal, R.. Purdue University; Estados UnidosFil: Bourdais, S.. No especifíca;Fil: Valdes, Matias Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Arnou, P.. University Of Luxembourg; LuxemburgoFil: Mitzi, D.B.. University of Duke; Estados UnidosFil: Dale, P.. University Of Luxembourg; Luxemburg

Tunable internal quantum well alignment in rationally designed oligomer-based perovskite films deposited by resonant infrared matrix-assisted pulsed laser evaporation

Hybrid perovskites incorporating conjugated organic cations enable unusual charge carrier interactions among organic and inorganic structural components, but are difficult to prepare as films due to disparate component chemical/physical characteristics (e.g., solubility, thermal stability). Here we demonstrate that resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) mitigates these challenges, enabling facile deposition of lead-halide-based perovskite films incorporating variable-length oligothiophene cations. Density functional theory (DFT) predicts suitable organic and inorganic moieties that form quantum-well-like structures with targeted luminescence or exciton separation/quenching. RIR-MAPLE-deposited films enable confirmation of these predictions by optical measurements, which further display excited state behavior transcending traditional quantum-well models-i.e., with appropriate selection of specially synthesized organic/inorganic moieties, intercomponent carrier transfer efficiently converts excitons from singlet to triplet states in organics for which intersystem crossing cannot ordinarily compete with recombination. These observations demonstrate the uniquely versatile excited-state behavior in hybrid perovskite quantum wells, and the value of integrating DFT, organic synthesis, RIR-MAPLE and spectroscopy for screening/preparing rationally devised complex structures

Direct-Bandgap 2D Silver-Bismuth Iodide Double Perovskite: The Structure-Directing Influence of an Oligothiophene Spacer Cation

Three-dimensional (3D) hybrid organic-inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from long-standing issues of environmental stability and lead toxicity. Associated two-dimensional (2D) analogues are garnering increasing interest due to superior chemical stability, structural diversity, and broader property tunability. Toward lead-free 2D HOIPs, double perovskites (DPs) with mixed-valent dual metals are attractive. Translation of mixed-metal DPs to iodides, with their prospectively lower bandgaps, represents an important target for semiconducting halide perovskites, but has so far proven inaccessible using traditional spacer cations due to either intrinsic instability or formation of competing non-perovskite phases. Here, we demonstrate the first example of a 2D Ag-Bi iodide DP with a direct bandgap of 2.00(2) eV, templated by a layer of bifunctionalized oligothiophene cations, i.e., (bis-aminoethyl)bithiophene, through a collective influence of aromatic interactions, hydrogen bonding, bidentate tethering, and structural rigidity. Hybrid density functional theory calculations for the new material reveal a direct bandgap, consistent with the experimental value, and relatively flat band edges derived principally from Ag-d/I-p (valence band) and Bi-p/I-p (conduction band) states. This work opens up new avenues for exploring specifically designed organic cations to stabilize otherwise inaccessible 2D HOIPs with potential applications for optoelectronics

High temperature decomposition of Cu 2 BaSnS 4 with Sn loss reveals newly identified compound Cu2Ba3Sn2S8

The earth abundant quaternary compound Cu2BaSnS4 is being currently studied as a candidate for photovoltaics and as a photocathode for water splitting. However, the chemical stability of this phase during synthesis is unclear. The synthesis of other quaternary tin sulphur based absorbers e.g., Cu2ZnSnS4 involves an annealing step at high temperature under sulphur gas atmosphere, which can lead to decomposition into secondary phases involving Sn loss from the sample. As the presence of secondary phases can be detrimental for device performance, it is crucial to identify secondary phase chemical, structural and optoelectronic properties. Here we used a combination of in situ EDXRD XRF and TEM to identify a decomposition pathway for Cu2BaSnS4. Our study reveals that, while Cu2BaSnS4 remains stable at high sulphur partial pressure, the material decomposes at high temperatures into Cu4BaS3 and the hitherto unknown compound Cu2Ba3Sn2S8 if the synthesis is performed under low partial pressure of sulphur. The presence of Cu4BaS3 in devices could be harmful due to its high conductivity and relatively lower band gap compared to Cu2BaSnS4. The analysis of powder diffraction data reveals that the newly identified compound Cu2Ba3Sn2S8 crystallizes in the cubic system space group I 43d with a lattice parameter of a 14.53 1 . A yellow powder of Cu2Ba3Sn2S8 has been synthesized, exhibiting an absorption onset at 2.19 e

Photoluminescence study of solution deposited Cu2BaSnS4 thin films

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence PL measurements in the temperature range of 15 300 K. The low temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor acceptor pair recombination and the weak broad emission at 1.6 eV to the free to bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonon

Materials interface engineering for solution-processed photovoltaics

Advances in solar photovoltaics are urgently needed to increase the performance and reduce the cost of harvesting solar power. Solution-processed photovoltaics are cost-effective to manufacture and offer the potential for physical flexibility. Rapid progress in their development has increased their solar-power conversion efficiencies. The nanometre (electron) and micrometre (photon) scale interfaces between the crystalline domains that make up solution-processed solar cells are crucial for efficient charge transport. These interfaces include large surface area junctions between photoelectron donors and acceptors, the intralayer grain boundaries within the absorber, and the interfaces between photoactive layers and the top and bottom contacts. Controlling the collection and minimizing the trapping of charge carriers at these boundaries is crucial to efficiency

Optical and related properties of natural one-dimensional semiconductors based on PbI and SnI units

Optical absorption, reflectance, photoluminescence, photoluminescence excitation and photoconductivity spectra of [NH2C(I)=NH2]3MI5 (M=Pb, Sn), (Bu4N)PbI3 and similar 1D semiconductors at room temperature are reported and compared with those of the corresponding 2D and 0D analogues

Spatially resolved vacuum tunneling spectroscopy on Bi₂Sr₂CaCu₂O₈ by STM at 4.8K

We report scanning tunneling spectroscopy investigations on in-situ cleaved superconducting Bi₂Sr₂CaCu₂O₈ single crystals. Although many investigators report reproducible tunneling studies on high temperature superconductors, there nevertheless remains uncertainties about the correct intrinsic shape of the tunneling spectra. We have been able to obtain higly reproducible spectra while scanning single crystal surfaces in many different areas and taking a spectra every 5Å along lines of several hundred Ångstroms. Furthermore, we show that the spectra are independent of modifacations of the barrier obtained by changing the tip/sample distance. The experimental density of states clearly shows some filling of the gap which does not fit with a BCS-like s-wave prediction, even if some scattering in the tunneling process is accounted for

Influence of Copper Composition on Cu2BaSn S,Se 4 Solution Deposited Films and Photovoltaic Devices with Over 5 Efficiency

Cu2BaSn S,Se 4 is currently in the spotlight for prospective environmentally friendly, stable, thin film solar cell application, with demonstrated device power conversion efficiency PCE exceeding 5 for vacuum deposited absorbers. As suggested by first principles calculations, experimental studies involving related Cu2ZnSn S,Se 4 and Cu In,Ga S,Se 2 absorbers prove that the detailed chemical composition typically plays a sensitive role in altering defects and electronic properties of these complicated compound semiconductors. Herein, the copper composition of Cu2BaSn S,Se 4 has been systematically modified, employing a solution based deposition approach, to provide a more complete picture of the phase stability and optoelectronic property sensitivity for this material. X ray diffraction and scanning electron microscopy show that phase purity is preserved over a film Cu content range of nominally 0.94 amp; 8804; [Cu] [Ba Sn] amp; 8804; 1.01. Terahertz spectroscopy and Hall effect measurements reveal that the majority carrier hole density of amp; 8764;1013 cm 3 and mobility amp; 8764;5 cm2 V s , as well as the minority carrier lifetime a bulk lifetime of 180 ps and a surface recombination velocity gt;106 cm s , are nominally independent of Cu content. The champion PCEs exceed 4.7 for all copper compositions in the phase pure region, with a record value of 5.1 , similar to the reported values for record vacuum deposited devices. These results suggest that Cu2BaSn S,Se 4 films and solar cells at the current performance level may be less sensitive to Cu stoichiometry compared to kesterite materials and therefore may provide a more stable material platform to prepare thin film solar cell