Conjugated Nanohoops with Dibenzo[a,e]pentalenes as Non-alternant and Antiaromatic π-Systems

Conjugated nanohoops are excellent candidates to study structure-property relationships, as optoelectronic materials and as hosts for supramolecular chemistry. While carbon nanohoops containing aromatics are well studied, antiaromatic units had not been incorporated until recently by our group using dibenzo[a,e]pentalene (DBP). The non-alternant electronic character of the DBP units significantly influences the optoelectronic properties of such nanohoops. We herein summarize our synthetic strategies to DBP-containing nanohoops, their structural and electronic properties, chirality and host-guest chemistry. We demonstrate how incorporating antiaromatic units leads to unique properties and opens new synthetic avenues, making such nanohoops attractive as potential electronic materials.

Exciton transport in molecular organic semiconductors boosted by transient quantum delocalization

Designing molecular materials with very large exciton diffusion lengths would remove some of the intrinsic limitations of present-day organic optoelectronic devices. Yet, the nature of excitons in these materials is still not sufficiently well understood. Here we present Frenkel exciton surface hopping, a highly efficient method to propagate excitons through truly nano-scale materials by solving the time-dependent Schrödinger equation coupled to nuclear motion. We find a clear correlation between diffusion constant and quantum delocalization of the exciton. In materials featuring some of the highest diffusion lengths to date, e.g. the non-fullerene acceptor Y6, the exciton propagates via a transient delocalization mechanism, reminiscent to what was recently proposed for charge transport. Yet, the extent of delocalization is rather modest, even in Y6, and found to be limited by the relatively large exciton reorganization energy. On this basis we chart out a path for rationally improving exciton transport in organic optoelectronic materials.

On the energy gap determination of organic optoelectronic materials: the case of porphyrin derivatives

A correct determination of the ionization potential (IP) and electron affinity (EA) as wells as the energy gap is essential to properly characterize a series of key phenomena related to...

Defect tolerance in CsPbI3: Reconstruction of potential energy landscape and band degeneracy in spin-orbit coupling

Lead halide perovskites have been intensively developed to be high-performance photovoltaic and optoelectronic materials, where the unique defect tolerance property is believed to contribute to their excellence. However, the defect...

Access tetracoordinate boron-doped polycyclic aromatic hydrocarbons with aggregation-induced emission under mild conditions

The boron-doped polycyclic aromatic hydrocarbons (PAHs) have attracted ongoing attention in the field of optoelectronic materials due to their unique optical and redox properties. To investigate the effect of tetracoordinate boron in PAHs bearing N-heterocycles (indole and carbazole), a facile approach to four-coordinate boron-doped PAHs was developed, which does not require elevated temperature and pre-synthesized functionalized boron reactants. Five tetracoordinate boron-doped PAHs (NBNN-1 – NBNN-5) were synthesized with different functional groups. Two of them (NBNN-1 and NBNN-2) could further undergo oxidative coupling reactions to form fused off-plane tetracoordinate boron-doped PAHs NBNN-1f and NBNN-2f. Compared to the three-coordinate boron-doped counterparts, the UV/Vis absorption and fluorescent emission are significantly red-shift. Unlike the distinct impact of coordination number of boron on optoelectronic properties, the difference of functional groups on the boron atom has negligible impact on their optical and electrochemical properties. The compounds NBNN-1f and NBNN-2f show aggregation-induced emission.

Study on chemical synthesis of SnSSe nanosheets and nanocrystals

Two kinds of raw material combination for the hot injection method were investigated for the chemical synthesis of SnSSe nanosheets and nanocrystals, which are low-toxic optoelectronic materials. When SnSe quantum dots were synthesized by mainly using oleic acid as Se precursor solvent, the quantum dots changed from spherical to cubic as the size increased. The growth condition dependence of the nanocrystal formation process was discussed. When SnSSe nanocrystals were synthesized by mainly using trioctylphosphine as Se precursor solvent, it was found that the nanocrystal shape changed from dot to rod or sheet by reducing the proportion of S. The bandgap energy did not simply depend on the composition ratio of S but was affected by the change in the nanocrystal shape depending on the quantum confinement effect.

Single Shot Time-Resolved Terahertz Spectroscopy for Optoelectronic Materials

<p>Optoelectronic materials and devices, such as LEDs and solar cells, are ubiquitous in the modern, technologically driven world. Understanding the fundamental physical process in optoelectronic materials is essential for the design and development of new devices which are more efficient, cheaper, printable, as well as environmentally friendly. Two particularly important material properties for device performance are charge mobility and photoconductivity, as they increase charge separation and extraction efficiencies, and thus give specific insight into device efficiency. The best suited technique for measuring mobility and conductivity on ultrafast timescales is Terahertz spectroscopy. Terahertz spectroscopy is a non-invasive, contact-free probe of the mobility of charges in optoelectronic materials. Terahertz time-domain spectroscopy allows for the direct determination of the entire complex-valued conductivity. As a result, important optical properties such as the complex refractive index and dielectric function of a material can be measured directly. The short duration of THz pulses, on the order of 1 ps, also allows for time-resolved studies of the transient photoconductivity in optically-excited materials with sub-picosecond time resolution, i.e. Time-Resolved Teraherz Spectroscopy (TRTS). Traditionally, only the peak of the THz pulse signal is measured with TRTS, due to the time constraints of a two-delay experiment. This does not allow for frequency-resolved THz spectra. As a result, it discards a lot of the information Terahertz-TDS spectroscopy contains, as well as its advantages over other spectroscopic techniques. Frequency-resolved TRTS would allow for the calculation of transient conductivity at each pump-probe delay time and can differentiate between signals of excitons and free charge carriers. This would allow for robust interpretations of charge mobility in novel materials. However, frequency-resolved TRTS is not practically feasible in a dual-delay configuration. We develop in this thesis a novel single-shot method based on angle-to-time mapping of a rotating probe. This method is applied to build a single-shot Terahertz-TDS spectrometer. A transmissive grating applies pulse front tilt which allows for the measurement of the entire THz transient (over a 5.7 ps window) in a single laser shot on a CMOS multichannel detector, thus alleviating the need for delay stage sampling of the THz transient, and leading to a reduction of experimental time by several orders of magnitude. An optical pump excitation is incorporated to allow a time-resolved measurement (TRTS) of the entire terahertz time-domain spectrum, and thus frequency-resolved TRTS. We show qualitative agreement between the THz time domain spectra obtained with the single shot technique and the standard free-space electro-optic (EO) sampling with balanced photodiodes, with an order of magnitude increased signal sensitivity. A proof-of-concept single shot TRTS study of a Si semiconductor sample is also given, showing we are able to resolve the TRTS signal of the entire THz pulse in a single shot, in time. This technique allows us to obtain significantly more information than traditional TRTS methods without any compromise in experiment time. However we find that the implemented single shot technique seems to suffer at higher frequencies (above 2 THz), which must be addressed to confirm the viability of a full spectrum single shot TRTS experiment. Further improvements, such as tighter focusing of the THz radiation, must be made to both the single-shot spectrometer as well as to the optical pump, for a quantitative single shot measurement. However, the proof-of-concept results in this thesis prove frequency-resolved TRTS is viable by using the developed single-shot detection method. As such it directly allows a novel spectroscopic tool which can lead to new insights into charge mobilities in optoelectronic materials, and may encourage wider application of TRTS.</p>

Single Shot Time-Resolved Terahertz Spectroscopy for Optoelectronic Materials

<p>Optoelectronic materials and devices, such as LEDs and solar cells, are ubiquitous in the modern, technologically driven world. Understanding the fundamental physical process in optoelectronic materials is essential for the design and development of new devices which are more efficient, cheaper, printable, as well as environmentally friendly. Two particularly important material properties for device performance are charge mobility and photoconductivity, as they increase charge separation and extraction efficiencies, and thus give specific insight into device efficiency. The best suited technique for measuring mobility and conductivity on ultrafast timescales is Terahertz spectroscopy. Terahertz spectroscopy is a non-invasive, contact-free probe of the mobility of charges in optoelectronic materials. Terahertz time-domain spectroscopy allows for the direct determination of the entire complex-valued conductivity. As a result, important optical properties such as the complex refractive index and dielectric function of a material can be measured directly. The short duration of THz pulses, on the order of 1 ps, also allows for time-resolved studies of the transient photoconductivity in optically-excited materials with sub-picosecond time resolution, i.e. Time-Resolved Teraherz Spectroscopy (TRTS). Traditionally, only the peak of the THz pulse signal is measured with TRTS, due to the time constraints of a two-delay experiment. This does not allow for frequency-resolved THz spectra. As a result, it discards a lot of the information Terahertz-TDS spectroscopy contains, as well as its advantages over other spectroscopic techniques. Frequency-resolved TRTS would allow for the calculation of transient conductivity at each pump-probe delay time and can differentiate between signals of excitons and free charge carriers. This would allow for robust interpretations of charge mobility in novel materials. However, frequency-resolved TRTS is not practically feasible in a dual-delay configuration. We develop in this thesis a novel single-shot method based on angle-to-time mapping of a rotating probe. This method is applied to build a single-shot Terahertz-TDS spectrometer. A transmissive grating applies pulse front tilt which allows for the measurement of the entire THz transient (over a 5.7 ps window) in a single laser shot on a CMOS multichannel detector, thus alleviating the need for delay stage sampling of the THz transient, and leading to a reduction of experimental time by several orders of magnitude. An optical pump excitation is incorporated to allow a time-resolved measurement (TRTS) of the entire terahertz time-domain spectrum, and thus frequency-resolved TRTS. We show qualitative agreement between the THz time domain spectra obtained with the single shot technique and the standard free-space electro-optic (EO) sampling with balanced photodiodes, with an order of magnitude increased signal sensitivity. A proof-of-concept single shot TRTS study of a Si semiconductor sample is also given, showing we are able to resolve the TRTS signal of the entire THz pulse in a single shot, in time. This technique allows us to obtain significantly more information than traditional TRTS methods without any compromise in experiment time. However we find that the implemented single shot technique seems to suffer at higher frequencies (above 2 THz), which must be addressed to confirm the viability of a full spectrum single shot TRTS experiment. Further improvements, such as tighter focusing of the THz radiation, must be made to both the single-shot spectrometer as well as to the optical pump, for a quantitative single shot measurement. However, the proof-of-concept results in this thesis prove frequency-resolved TRTS is viable by using the developed single-shot detection method. As such it directly allows a novel spectroscopic tool which can lead to new insights into charge mobilities in optoelectronic materials, and may encourage wider application of TRTS.</p>