Three-dimensional (3D) hybrid organic−inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from longstanding 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.

For a class of 2D hybrid organic-inorganic perovskite semiconductors based on π-conjugated organic cations, we predict quantitatively how varying the organic and inorganic component allows control over the nature, energy and localization of carrier states in a quantum-well-like fashion. Our first-principles predictions, based on large-scale hybrid density-functional theory with spin-orbit coupling, show that the interface between the organic and inorganic parts within a single hybrid can be modulated systematically, enabling us to select between different type-I and type-II energy level alignments. Energy levels, recombination properties and transport behavior of electrons and holes thus become tunable by choosing specific organic functionalizations and juxtaposing them with suitable inorganic components.Hybrid organic-inorganic perovskites (HOIPs), [1, 2] particularly three-dimensional (3D) HOIPs, are currently experiencing a strong revival in interest as economically processable, optically active semiconductor materials with excellent transport characteristics. Their success is showcased most prominently by record performance gains in proof-of-concept photovoltaic [3][4][5][6][7][8][9][10][11][12] and light-emitting devices. [13][14][15][16][17][18][19][20] The electronic function of 3D HOIPs can be tuned to a limited extent by manipulating the inorganic component (from which the frontier orbitals are derived), but the organic cations are confined by the 3D structure and are thus necessarily small (e.g., methylammonium [3][4][5][6][7][8][13][14][15][16][17][18] and formamidinium [9-11, 19, 21, 22]), with electronic levels that do not contribute directly to the electronic functionality. [23][24][25][26][27][28] However, the accessible chemical space of HOIPs extends well beyond the 3D systems. [1] In particular, the layered, socalled two-dimensional (2D) perovskites do not place a strict length constraint on the organic cation. In these materials, a much broader range of functional organic molecules can be incorporated within the inorganic scaffolds, including complex functional molecules such as oligo-acene or -thiophene derivatives. [1,[29][30][31][32][33][34][35][36][37] Fig. 1a shows the atomic structure of a paradigmatic example of such a 2D HOIP with active organic functionality, bis(aminoethyl)-quaterthiophene lead bromide AE4TPbBr 4 .[34] Similar juxtapositions of targeted organic and inorganic components give rise to a vast, yet systematically accessible space of possible semiconductor materials, [1, 2,[38][39][40] including those in which the molecular carrier levels contribute directly to the low-lying excitations and carrier levels. [1, 30-32, 34, 38, 39, 41] This large space of conceivable organic-inorganic combinations thus offers the unique opportunity to tailor (ideally with computational guidance) materials with particularly desirable semiconductor properties, by intentionally controlling the spatial location and character of the electronic carriers and optical excitations throughout the m...

RIR-MAPLE enables thin-film deposition of organic–inorganic materials with tunable synergistic photophysics.

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]PbI 4 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 PbI 6 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.

The prevalence of ultrafast electron-transfer processes in light-harvesting materials has motivated a deeper understanding of coherent reaction mechanisms. Kinetic models based on the traditional (equilibrium) form of Fermi's Golden Rule are commonly employed to understand photoinduced electron-transfer dynamics. These models fail in two ways when the electron-transfer process is fast compared to solvation dynamics and vibrational dephasing. First, electron-transfer dynamics may be accelerated if the photoexcited wavepacket traverses the point of degeneracy between donor and acceptor states in the solvent coordinate. Second, traditional kinetic models fail to describe electron-transfer transitions that yield products which undergo coherent nuclear motions. We address the second point in this work. Transient absorption spectroscopy and a numerical model are used to investigate coherent back-electron-transfer mechanisms in a transition metal complex composed of titanium and catechol, [Ti(cat)3](2-). The transient absorption experiments reveal coherent wavepacket motions initiated by the back-electron-transfer process. Model calculations suggest that the vibrationally coherent product states may originate in either vibrational populations or coherences of the reactant. That is, vibrational coherence may be produced even if the reactant does not undergo coherent nuclear motions. The analysis raises a question of broader significance: can a vibrational population-to-coherence transition (i.e., a nonsecular transition) accelerate electron-transfer reactions even when the rate is slower than vibrational dephasing?

Planar acceptor moieties in FREAs are necessary, as expanding the π–π stacking by only 1 Å disrupts the packing and decreases performance.

Analogues of 2D photon echo methods in which two population times are sampled have recently been used to expose heterogeneity in chemical kinetics. In this work, the two population times sampled for a transition metal complex are transformed into a 2D rate spectrum using the maximum entropy method. The 2D rate spectrum suggests heterogeneity in the vibrational cooling (VC) rate within the ensemble. In addition, a cross peak associated with VC and back electron transfer (BET) dynamics reveals correlation between the two processes. We hypothesize that an increase in the strength of solute-solvent interactions, which accelerates VC, drives the system toward the activationless regime of BET.

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