Transport, trapping, triplet fusion: thermally retarded exciton migration in tetracene single crystals.

Efficient exciton migration is crucial for optoelectronic organic devices. While the transport of triplet excitons is generally slow compared to singlet excitons, triplet exciton migration in certain molecular semiconductors with endothermic singlet fission appears to be enhanced by a time-delayed regeneration of the more mobile singlet species via triplet fusion. This combined transport mechanism could be exploited for devices, but the interplay between singlet fission and triplet fusion, as well as the role of trap states is not yet well understood. Here, we study the spatiotemporal exciton dynamics in the singlet fission material tetracene by means of time resolved photoluminescence micro-spectroscopy on crystalline samples of different quality. Varying the temperature allows us to modify the dynamic equilibrium between singlet, triplet and trapped excitons. Supported by a kinetic model, we find that thermally activated dissociation of triplet pairs into free triplet excitons can account for an increase of the diffusion length below room temperature. Moreover, we demonstrate that trapping competes efficiently with exciton migration.

Tuning the optical properties of 2D monolayer silver-bismuth bromide double perovskite by halide substitution.

Silver-bismuth double perovskites are promising replacement materials for lead-based ones in photovoltaic (PV) devices due to the lower toxicity and enhanced stability to environmental factors. In addition, they might even be more suitable for indoor PV, due to the size of their bandgap better matching white LEDs emission. Unfortunately, their optoelectronic performance does not reach that of the lead-based counterparts, because of the indirect nature of the band gap and the high exciton binding energy. One strategy to improve the electronic properties is the dimensional reduction from the 3D to the 2D perovskite structure, which features a direct band gap, as it has been reported for 2D monolayer derivates of Cs2AgBiBr6obtained by substituting Cs+cations with bulky alkylammonium cations. However, a similar dimensional reduction also brings to a band gap opening, limiting light absorption in the visible. In this work, we report on the achievement of a bathochromic shift in the absorption features of a butylammonium-based silver-bismuth bromide monolayer double perovskite through doping with iodide and study the optical properties and stability of the resulting thin films in environmental conditions. These species might constitute the starting point to design future sustainable materials to implement as active components in indoor photovoltaic devices used to power the IoT.

Regioselective Fluorination of Acenes: Tailoring of Molecular Electronic Levels and Solid-State Properties.

Optoelectronic properties of molecular solids are important for organic electronic devices and are largely determined by the adopted molecular packing motifs. In this study, we analyzed such structure-property relationships for the partially regioselective fluorinated tetracenes 1,2,12-trifluorotetracene, 1,2,10,12-tetrafluorotetracene and 1,2,9,10,11-pentafluorotetracene that were further compared with tetracene and perfluoro-tetracene. Quantum chemical DFT calculations in combination with optical absorption spectroscopy data show that the frontier orbital energies are lowered with the degree of fluorination, while their optical gap is barely affected. However, the crystal structure changes from a herringbone packing motif of tetracene towards a planar stacking motif of the fluorinated tetracene derivatives, which is accompanied by the formation of excimers and leads to strongly red-shifted photoluminescence with larger lifetimes.

Introducing Distinct Structural and Optical Properties into Organotin Sulfide Clusters by the Attachment of Perylenyl and Corannulenyl Groups.

We report the introduction of distinct optical properties into organotin sulfide clusters by the attachment of extended polycyclic aromatic organic molecules. This was realized by the reactions of [(RNSn)4S6] (RN = CMe2CH2CMeNNH2) with 3-perylenecarbaldehyde and corannulenecarbaldehyde, respectively. The reaction with the first reactant leads to the formation of two products [(RperylSn)3S4][SnCl3] [1a; Rperyl = CMe2CH2CMeNNCH(C20H11)] and [(RperylSn)3S4Cl] (1b). Structural differences between these two compounds are reflected in their different optical absorption and luminescence behavior, yet in both cases, the main emission is red-shifted relative to 3-perylenecarbaldehyde. The second organic molecule affords the compound [(RcorSn)4Sn2S10] [2; Rcor = CMe2CH2CMeNNCH(C20H9)] with intriguing optical properties, including a broad emission with essentially no shift in λmax compared to corannulenecarbaldehyde. All compounds were obtained as single crystals, and their structures were determined by means of single-crystal X-ray diffraction. The optical properties of the highly luminescent compounds were investigated by means of emission and time-resolved photoluminescence spectroscopy measurements.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Lensless digital holographic microscopy as an efficient method to monitor enzymatic plastic degradation.

A big challenge of the 21st century is to cope with the huge amounts of plastic waste on Earth. Especially the oceans are heavily polluted with plastics. To counteract this issue, biological (enzymatic) plastic decomposition is increasingly gaining attention. Recently it was shown that polyethylene terephthalate (PET) can be degraded in a saltwater-based environment using bacterial PETase produced by a marine diatom. At moderate temperatures, plastic biodegradation is slow and requires sensitive methods for detection, at least at initial stages. However, conventional methods for verifying the plastic degradation are either complex, expensive, time-consuming or they interfere with the degradation process. Here, we adapt lensless digital holographic microscopy (LDHM) as a new application for efficiently monitoring enzymatic degradation of a PET glycol copolymer (PETG). LDHM is a cost-effective, compact and sensitive optical method. We demonstrate enzymatic PETG degradation over a time course of 43 days employing numerical analysis of LDHM images.

Pyrene-Terminated Tin Sulfide Clusters: Optical Properties and Deposition on a Metal Surface.

Herein, we present the synthesis of two pyrene-functionalized clusters, [(Rpyr Sn)4 S6 ]⋅2 CH2 Cl2 (4) and [(Rpyr Sn)4 Sn2 S10 ]⋅n CH2 Cl2 (n=4, 5 a; n=2, 5 b; Rpyr =CMe2 CH2 C(Me)N-NC(H)C16 H9 ), both of which form in reactions of the organotin sulfide cluster [(RN Sn)4 S6 ] (C; RN =CMe2 CH2 C(Me)N-NH2 ) with the well-known fluorescent dye 1-pyrenecarboxaldehyde (B). In contrast, reactions using an organotin sulfide cluster with another core structure, [(RN Sn)3 S4 Cl] (A), leads to formation of small molecular fragments, [(Rpyr Cl2 Sn)2 S] (1), (pyren-1-ylmethylene)hydrazine (2), and 1,2-bis(pyren-1-ylmethylene)hydrazine (3). Besides synthesis and structures of the new compounds, we report the influence of the inorganic core on the optical properties of the dye, which was analyzed exemplarily for compound 5 a via absorption and fluorescence spectroscopy. This cluster was also used for exploring the potential of such non-volatile clusters for deposition on a metal surface under vacuum conditions.

(HPy)2(Py)CuBi3I12, a low bandgap metal halide photoconductor.

Bismuth halides represent an emergent class of materials that combines semiconductor properties with non-toxic constituents. However, many simple bismuth halide compounds feature bandgaps that are significantly higher than those of the lead halide perovskites, which they are supposed to replace. One way to address this issue is the preparation of multinary metal halide materials that feature an additional metal ion. Here, we report on the synthesis and properties of (HPy)2(Py)CuBi3I12 (1) a new copper iodido bismuthate, a photoconductor, which shows a low band gap of 1.59 eV and good thermal and air stability.

Nonlinear refraction in CH3NH3PbBr3 single crystals.

We measure both nonlinear absorption and nonlinear refraction in a ${{\rm CH}_3}{{\rm NH}_3}{{\rm PbBr}_3}$CH3NH3PbBr3 single crystal using the Z-scan technique with femtosecond laser pulses. At 1000 nm, we obtain values of 5.2 cm/GW and ${+}{9.5} \cdot {{10}^{ - 14}}\;{{\rm cm}^2}/{\rm W}$+9.5⋅10-14cm2/W for nonlinear absorption and nonlinear refraction, respectively. The sign and magnitude of the observed refractive nonlinearity are reproduced well by the two-band model. Our results suggest that the large nonlinear refractive index measured in perovskite nanostructures cannot be explained by an intrinsically high bound-electronic nonlinear refractive index in this emerging material class but is possibly caused by free carriers or quantum confinement effects.

Trapping of ZnCl2 by bipyridyl-functionalized organotin sulfide clusters, and its effect on optical properties.

We report the syntheses of two new organotin sulfide clusters with heteroaromatic substituents. A phenanthroline-functionalized tin sulfide cluster [(RPhenSn)4S6] (1; RPhen = CMe2CH2C(Me)N-NC(H)C12H7N2) and a bipyridyl-terminated cluster [(R4-bipySn)4S6] (2; R4-bipy = CMe2CH2C(Me)N-NC(H)-4-C10H7N2) were obtained from reactions of the hydrazone-functionalized organotin sulfide cluster [(RNSn)4S6] (RN = CMe2CH2C(Me)N-NH2) with 1,10-phenanthroline-5-carboxaldehyde or 2,2'-bipyridine-4-carbaldehyde. 1 and 2 were tested towards their capability of trapping metal ions by means of the terminal chelating ligands. The reaction of 2 with ZnCl2 afforded the cluster compound [(R4-bipyZnCl2Sn)4S6] (5), in which four ZnCl2 units are coordinated by the heteroaromatic organic groups. We discuss the structures and demonstrate the effect of ZnCl2 trapping on optical absorption and luminescence properties.

Microscopic insight into non-radiative decay in perovskite semiconductors from temperature-dependent luminescence blinking.

Organo-metal halide perovskites are promising solution-processed semiconductors, however, they possess diverse and largely not understood non-radiative mechanisms. Here, we resolve contributions of individual non-radiative recombination centers (quenchers) in nanocrystals of methylammonium lead iodide by studying their photoluminescence blinking caused by random switching of quenchers between active and passive states. We propose a model to describe the observed reduction of blinking upon cooling and determine energetic barriers of 0.2 to 0.8 eV for enabling the switching process, which points to ion migration as the underlying mechanism. Moreover, due to the strong influence of individual quenchers, the crystals show very individually-shaped photoluminescence enhancement upon cooling, suggesting that the high variety of activation energies of the PL enhancement reported in literature is not related to intrinsic properties but rather to the defect chemistry. Stabilizing the fluctuating quenchers in their passive states thus appears to be a promising strategy for improving the material quality.

High Resolution Mapping of Two-Photon Excited Photocurrent in Perovskite Microplate Photodetector.

We fabricate photodetectors based on solution-processed single CH3NH3PbBr3 microcrystals (MCs) and map the two-photon absorption (TPA) excited photocurrent (PC) with spatial resolution of 1 µm. We find that the charge carrier transport length in the MCs depends on the applied electric field, and increases from 5.7 µm for 0.02 V bias (dominated by carrier diffusion) to 23.2 µm for 2 V bias (dominated by carrier drift). Furthermore, PC shows strong spatial variations. Combining the PC mapping results with time-resolved photoluminescence microscopy, we demonstrate that the spatial distribution of PC mainly originates from the inhomogeneous distribution of trap-states across perovskite MCs. This suggests that there is still large margin for improvement of perovskite single crystal devices by better controlling of the traps.

Evidence for Anisotropic Electronic Coupling of Charge Transfer States in Weakly Interacting Organic Semiconductor Mixtures.

We present a comprehensive investigation of the charge-transfer (CT) effect in weakly interacting organic semiconductor mixtures. The donor-acceptor pair diindenoperylene (DIP) and N,N'-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4/9,10-bis(dicarboxyimide) (PDIR-CN2) has been chosen as a model system. A wide range of experimental methods was used in order to characterize the structural, optical, electronic, and device properties of the intermolecular interactions. By detailed analysis, we demonstrate that the partial CT in this weakly interacting mixture does not have a strong effect on the ground state and does not generate a hybrid orbital. We also find a strong CT transition in light absorption as well as in photo- and electroluminescence. By using different layer sequences and compositions, we are able to distinguish electronic coupling in-plane vs out-of-plane and, thus, characterize the anisotropy of the CT state. Finally, we discuss the impact of CT exciton generation on charge-carrier transport and on the efficiency of photovoltaic devices.

THz generation at 1.55 µm excitation: six-fold increase in THz conversion efficiency by separated photoconductive and trapping regions.

We present first results on photoconductive THz emitters for 1.55µm excitation. The emitters are based on MBE grown In0.53Ga0.47As/In0.52Al0.48As multilayer heterostructures (MLHS) with high carrier mobility. The high mobility is achieved by spatial separation of photoconductive and trapping regions. Photoconductive antennas made of these MLHS are evaluated as THz emitters in a THz time domain spectrometer (THz TDS). The high carrier mobility and effective absorption significantly increases the optical-to-THz conversion efficiency with THz bandwidth in excess of 3 THz.

Hybrid continuous wave terahertz spectroscopy.

We propose a hybrid architecture for continuous wave terahertz spectroscopy employing a conventional two color photomixing system combined with a quasi time domain spectrometer, driven by a multimode laser diode. This approach fuses high spectral intensity with broadband frequency information and overcomes the ambiguity of standard continuous wave thickness measurements.