This PDF file contains the front matter associated with SPIE Proceedings Volume 11084, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Devices based on small-gap mercury chalcogenide semiconductor nanocrystal inks have recently demonstrated increasingly high performance photodetection in the short and mid-wave infrared. These new colloidal inks are generating increasing interest because they could provide higher operating temperatures and vastly reduced costs compared to the current epitaxially-grown devices. However, in order to further increase detector operation temperatures and use these materials as infrared light sources, more detailed understandings of the carrier dynamics are required. Described here are picosecond mid-infrared absorption and emission studies of HgTe and HgSe colloidal quantum dots focusing on multicarrier nonradiative relaxation. Comparisons of the interband and intraband transitions in intrinsic and n-type systems reveal phenomena such as suppression of Auger relaxation in nanoparticles vs. bulk materials and slow or absent Auger relaxation in n-type quantum dots. Yet, the measured lifetimes are still limited by other nonradiative pathways that appear unique to small-gap nanomaterials. The deeper understanding of nonradiative relaxation in small-gap nanocrystals afforded by these experiments provides a path towards realizing high performance infrared photodetection near room temperature and robust mid-infrared light emission with colloidal quantum dots.

Among the large set of molecular light harvesters, conjugated polyelectrolytes (CPE) stand apart due to the chain connectivity of ionic subunits, and to the strong coupling between electronic and ionic degrees of freedom. Substantial questions remain about precisely how soft molecular scaffolds can be used to tune the backbone electronic structure. We have used mixed ionic/nonionic micelles as model soft assemblies, wherein by tuning the micelle composition, we have investigated the influence of surface charge density and sign of the surface charge on the CPE/micelle interaction. We find that both the micelle charge density and the precise arrangement of charged sidechains along the CPE conspire to determine the conformational ensemble of accessible CPE states. Further, the sidechain positioning along the backbone determine whether the adsorbed CPE chain is either contorted or highly extended, leading to relatively localized or delocalized excitons, respectively.

The imaging and sensing technology operating in the THz region of the electromagnetic spectrum has a number of applications, with demonstrator products already available on the market for oncology imaging, production monitoring, and non-destructive test [1]. However, the THz sources now at hand are still bulky and too expensive for expanding this technology to other proposed applications, which also include, among other, THz bandwidth photonics and security imaging. A higher level of integration with control electronics, a lower production cost, and a broader wavelength range of emission towards the far-infrared, are all desirable features to expand the fields of application of THz radiation. N-type Ge/SiGe quantum cascade structures grown on top of a Si(001) substrate are particularly promising for realizing a Si based THz source [2]. The low effective mass and long non-radiative relaxation times due to the non-polar nature of the material, are expected to i) provide gain values close to those demonstrated in III-V quantum cascade structures at 4 K, and ii) to potentially enable 300 K operation. In this presentation we will discuss the optical and structural properties of n-type s-Ge/SiGe multi-quantum wells and asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition [3]. Extensive structural characterization obtained by scanning transmission electron microscopy (STEM), atomic probe tomography (APT) and X-ray diffraction shows the high material quality of strain-symmetrized structures (up to 5 micron active region thickness) and heterointerfaces (featuring interface roughness below 0.2 nm), down to the ultrathin barrier limit (about 1 nm). By performing THz absorption spectroscopy measurements combined to theoretical modeling on different asymmetric coupled quantum well systems (with varying large-well width or barrier thickness), we unambiguously demonstrated inter-well coupling and wavefunction tunneling [3]. The agreement between experimental data and simulations allowed us to characterize the tunneling barrier parameters and, in turn, achieve a highly-controlled engineering of the electronic structure in forthcoming unipolar cascade systems based on n-type Ge/SiGe multi quantum-wells. Furthermore, by pump-and-probe, and time domain spectroscopic data with a thorough theoretical modeling, we will show that this material system is indeed promising as active material in quantum cascade lasers (QCL). We found i) narrow intersubband (ISB) absorption lines; ii) relatively long non-radiative ISB relaxation times at high temperature; iii) relaxation times for different ISB transitions favorable to population inversion. Leveraging on the promising results obtained by spectroscopy experiments, we theoretically investigate an electrically-pumped Ge/SiGe THz QCL through a non-Equilibrium Green Function formalism (nextnano.QCL), using as material parameters to model the scattering processes the values estimated from the analysis of the optical experimental data [4]. As expected, due to the weaker interaction with the phonon field with respect to III-V based devices, we find a lower impact of the temperature on the gain spectrum. In addition, simulations show that the interface roughness values measured on our samples allows to achieve gain overcoming the losses of double-metal waveguides at room temperature. We believe that the present results will motivate new experimental efforts aimed at demonstrating room-temperature operation in group IV QCL THz devices. References [1] D. J. Paul, Laser Photon. Rev. 4, 610 (2010). [2] K. Driscoll, and R. Paiella, J. Appl. Phys. 102, 093103 (2007). [3] C. Ciano, M. Virgilio, M. Montanari, L. Persichetti, L. Di Gaspare, M. Ortolani, L. Baldassarre, M. H. Zöllner, O. Skibitzki, G.Scalari, J. Faist, D. J. Paul, M. Scuderi, G. Nicotra, T. Grange, S. Birner, G. Capellini, and M. De Seta, Accepted in Phys. Rev. Appl., 11, 014003 (2019). [4] T. Grange, D. Stark, G. Scalari, J. Faist, L. Persichetti, L. Di Gaspare, M. De Seta, M. Ortolani, D.J. Paul, G. Capellini, S. Birner and M. Virgilio, Submitted to Applied Physics Letters. Preprint available at https://arxiv.org/pdf/1811.12879.pdf.

In this research, we report deep ultraviolet (DUV) light-assisted low-temperature crystallization of amorphous TiO2 films and their applications to solar energy conversion devices. Amorphous TiO2 films deposited by atomic-layer-deposition were photoactivated with DUV light under nitrogen atmosphere. Various film characterizations confirmed that DUV irradiation substantially accelerates the nucleation rate and reduces the minimum temperature for crystallization (anatase) of amorphous TiO2 film by 100 degrees compared with thermal annealing without DUV irradiation. Furthermore, it was demonstrated that the DUV photoactivation with concurrent thermal annealing induces the low-temperature formation of unconventional brookite phase and preferential facet orientation, which was never reported in the previous literature. Finally, the DUV-assisted low-temperature crystallization was successfully employed to form high-quality anatase films regardless of substrates, for high-performance photoelectrochemical cells on indium tin oxide and perovskite photovoltaics with very narrow performance variation. Our results suggest that DUV photoactivation can provide a low-temperature post-treatment for functional metal oxide thin films with well-defined crystallinity and preferred phase orientation on unconventional substrates.

Light-driven ion pumps can be fabricated from ion-exchange membranes infiltrated with water as the protonic semiconductor. Absorption of visible light and generation of mobile charge carrier protons are accomplished using photoacids that are covalently bonded to the membranes. Prior results from our work suggest that the photoacid excited-state acidity is not large enough to result in significant yields for conversion of light into mobile protons. Herein we compare a series of photoacid-bearing membranes that are even stronger acids in their excited states, and we determine that excited-state acidity does not correlate with photovoltage. By assessing the photoresponse of a series of bipolar membranes fabricated by laminating a photoacid-bearing cation-exchange membrane to an anionexchange membrane, no clear trend was observed between net built-in electric potential and photovoltaic performance. This suggests that other properties dictate the effectiveness of these light-driven proton pumps.

Lead-halide perovskites are currently the highest-performing solution-processable semiconductors for solar energy conversion, with record efficiencies rapidly approaching that of the Shockley-Queisser limit for single-junction solar cells. Further progress in the development of lead-halide perovskite solar cells must overcome this limit, which largely stems from the ultrafast relaxation of high-energy hot carriers above the bandedge. In this contribution, we use a highly-specialized pump-push-probe technique to unravel the key parameters which control hot carrier cooling in bulk and nanocrystal (NC) lead bromide perovskites with different material composition, NC diameter and surface treatment. All samples exhibit slower cooling for higher hot carrier densities, which we assign to a phonon bottleneck mechanism. By comparing this density-dependent cooling behavior in the different samples, we find that the weak quantum confinement of electronic states and the surface defects in the NCs play no observable role in the hot carrier relaxation. Meanwhile, in accordance with our previous observations for bulk perovskites, we show that the cation plays a critical role towards carrier cooling in the perovskite NCs, as evidenced by the faster overall cooling in the hybrid FAPbBr₃ NCs with respect to the all-inorganic CsPbBr₃ NCs. These observations highlight the crucial role of the cations toward the phononic properties of lead-halide perovskites, and further point towards the defect tolerance of these emerging solution-processed semiconductors.

Metal halide perovskite quantum wells (QWs) have been used to fabricate efficient optoelectronic devices, and exhibit stability superior to that of their bulk 3D counterparts. The perovskite QWs are tuned in synthesis so that they possess different bandgaps and exciton binding energies owing to variable quantum confinement as a function of QW thickness. Accordingly, the device performance of these materials depends on the efficiency of various interwell carrier dynamical processes, principally exciton and charge transfer. I will discuss the use of transient absorption and ultrafast two-dimensional electronic spectroscopy to probe interwell exciton transfer on timescales of 100s of femtoseconds, and show that interwell charge transfer occurs on timescales of 10s to 100s of picoseconds. These results, in addition to photoelectron spectroscopy experiments, are used to reconcile conflicting observations of type-I and type-II band alignment amongst perovskite QWs.

Notwithstanding the success of lead-halide perovskites in emerging solar energy conversion technologies, many of the fundamental photophysical and charge transport phenomena in this material remain debated. Debated issues include the role of the organic and inorganic sublattices, and the coupling between them, on charge formation and transport. Here, we use several, primarily terahertz-based spectroscopies to elucidate electron-phonon coupling, electron-dipole coupling, phonon-band gap coupling and phonon-vibron coupling in this important class of materials. Electron-phonon coupling gives rise to polaron formation. We investigate polaron formation dynamics using the rise time of the terahertz photoconductivity, following femtosecond photo-generation of carriers, as a measure for the polaron formation rates. Polarons are formed on the timescale of 400 femtoseconds, independent of temperature or cation type. This timescale corresponds well with the timescale of the inverse of the LO phonon frequency. Terahertz-pump, optical probe experiments reveal that strongly exciting the lattice phonons, the optical bandgap is modulated coherently because of excitation of the 1 THz phonon mode. These results explain the anomalous dependence of the optical bandgap on temperature. Two-dimensional terahertz-infrared Raman spectroscopy reveals remarkably efficient coupling between the vibrations of the molecular cations and the phonon modes of the inorganic sublattice. This result reveals that even though there is no covalent link between the two sublattices, their mutual coupling a strong. Together, our results illustrate the soft nature of hybrid organic-inorganic perovskites, resulting in strong coupling between different degrees of freedom in this fascinating material.

Metal-halide semiconductors are an attractive class of materials for advanced photonic and optoelectronic applications including photovoltaics, photodetection, and photon sources. Much of the focus in this research area has been on solution processing of these materials, but solvent-free methods would be desirable for many applications. Recently, we reported the development of methods to synthesize compositionally complex metal-halide semiconductors and process them into high-quality thin films via single-source flash sublimation for conformal, large-area coatings. Here, we extend this approach to explore the rich compositional space of metal-halide semiconductors, including hybrid perovskites and allinorganic perovskite and elpasolite materials. These results demonstrate a promising approach for advanced materials discovery and a pathway toward realizing high-throughput vapor processing of metal-halide semiconductor coatings for next-generation photonic and optoelectronic applications.

We have prepared electrodes for photo-electro-chemical cells which have enabled to pre-charge colloidal quantum dots in well-controlled fashion. Femtosecond transient absorption measurements were carried out revealing clear speed-up of the photo-induced charge carrier dynamics, particularly the recombination. Such studies allow to understand the behavior of light harvesting materials in operational conditions of optoelectronic devices giving new recipes for improvements.

In recent decades, fluorescent conjugated polymers have been widely studied due to their ability to produce low-cost and lightweight organic electronics. However, given that their solid state packing arrangements are difficult to control or predict, maintaining the desired emissive properties of the materials is challenging. As the emission wavelengths and quantum yields of these materials are highly sensitive to their solid state packing arrangements, pre-aggregation in solution can assist in maintaining properties through the transition between solution and solid state. Here we use poly(3- hexylthiophene) (P3HT), a well-studied organic semi-conductor, to explore the impact of aggregation on the photophysical properties of the material. We have employed various bulk and single molecule fluorescence-based methods to better understand the effect of aggregation on the emission properties of the polymer system. The addition of a highly-polar solvents to induce aggregation leads to strong emission quenching, but no change in the lifetime in solution. However, in the solid state the aggregates exhibit enhanced emission intensity combined with shorter lifetimes as demonstrated by fluorescence lifetime imaging microscopy (FLIM). To better understand the differences of the aggregate properties in the solution and solid state, fluorescence anti-bunching was used to probe the degree of electronic coupling of the polymer chains. Surprisingly, fluorescence anti-bunching measurements reveal the highly collective nature of the P3HT aggregate emission, which likely accounts for its strong fluorescence intensity in the solid state. Studying the aggregated system at both the bulk and single-molecule level will yield a better understanding of the aggregate properties which will lead to better control and higher performance of organic semiconductors in device applications.

Plastics are arguably the most ubiquitous structural materials mankind invented, while semiconducting polymers, one kind of special plastics, present their unique optical and electronic characteristics and have attracted wide interest from biomedical to electronic applications. In this talk, we focus on our latest development of semiconducting polymer blends for organic electronics. We will discuss how we came up with the concept of complementary semiconducting polymer blends that render melt-processability to the solvent-free fabrication of thin film organic electronics, a clear departure from widely practiced solution processing which often involves toxic and environmentally-harmful organic solvents. We will also highlight the marriage of semiconducting polymers and insulating plastics that brings stable charge transport across a wide temperature range from room temperature up to 220°C in thin-film transistors. This discovery lifts the limitation by the fact that most electronics will break down at high temperatures and could be a way to change that.

Singlet fission (SF) in two or more electronically coupled organic chromophores converts a high-energy singlet exciton into two low-energy triplet excitons, which can be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability and low triplet state energies. This work summarizes structurally dependent SF dynamics for 9,10-bis(phenylethynyl)anthracene (BPEA) and its derivatives in the solid-state using time-resolved optical spectroscopies, and electronic structure calculations. By modulating the packing structure in thin films, we can effectively tune electronic energy and coupling. The systematic study in BPEA organic semiconductors shows that maximizing the thermodynamic driving force can achieve the highest SF rate and efficiency.

Cesium lead halide perovskite quantum dots (PQD) have potential for development in display backlights due to its high color purity, high luminous efficiency, wavelength tunability, and easily production. However, its poor stability is the biggest obstacle in practical application. In this study, we use post-treated by controlling the ratios between PQD and ligands (oleylamine and oleic acid are mixed in a volume ratio of 1:1) solution to improve quantum yield (QY). The result shows that the emission wavelength and QY of as-prepared CsPbBr₃ QDs is 514 nm and 84 %, respectively. It was found that the QY of QDs increases with increasing the concentrations of ligands. The relative QY of the post-treated PQDs can be increased from 84 to 212 %, which depends on the ligand concentrations. After aging for one month, the QY of the as-prepared PQD reduces from 84 to 73 %, while no obviously QY decay can be observed for post-treated samples. The results show that post treatment by adding a ligand can improve QY and stability of PQDs effectively.