Symposium ES19—Excitonic Materials and Quantum Dots for Energy Conversion

Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) have recently emerged as an alternative to conventional quantum dots, in particular, as versatile photonic sources and light-harvesting materials, due to their easily tunable phtoluminescence spectra, unique exciton fine structure and facile synthesis [1-5]. A For warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions, all three archetypal A-site monocationic perovskites—CH₃NH₃PbI₃, CH(NH₂)₂PbI₃, and CsPbI₃—suffer from either chemical or thermodynamic instabilities in their bulk form. A promising approach toward the mitigation of these challenges lies in the formation of multinary compositions (mixed cation and mixed anion). In the case of multinary colloidal NCs, such as quinary Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs, the outcome of the synthesis is defined by a complex interplay between the bulk thermodynamics of the solid solutions, crystal surface energies, energetics, dynamics of capping ligands, and the multiple effects of the reagents in solution. Accordingly, the rational synthesis of such NCs is a formidable challenge. We show that droplet-based microfluidics can successfully tackle this problem and synthesize Cs_xFA_1–xPbI₃ and Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs in both a time- and cost-efficient manner. In this showcase study [6], we fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission line widths (to below 40 nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. The excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices, are demonstrated as well. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting a high external quantum efficiency of 5.9% and a very narrow electroluminescence spectral bandwidth of 27 nm.


References:

1. M. V. Kovalenko et al. Science 2017, 358, 745-750
2. Q.A. Akkerman et al. Nature Materials, 2018, 17, 394–405
3. M. A. Becker et al, Nature, 2018, 553, 189-193
4. L. Protesescu et al. Nano Letters 2015, 15, 3692–3696
5.L. Protesescu et al. ACS Nano 2017, 11, 3119–3134
6. I. Lignos et al. ACS Nano 2018, 12, 5504–5517

The bright emission observed in cesium lead halide perovskite nanocrystals has recently been explained in terms of a ground bright exciton state [1], a claim which would make these materials the first known examples where the ground exciton state is not an optically forbidden dark exciton. This unprecedented claim has been the subject of intense experimental investigation which has so far failed to detect the “dark” ground exciton which is expected in electron-hole exchange models. [2,3] Here, we explore other tell-tale properties of the exciton fine structure which do not require direct measurement of the dark exciton energy in order to clarify the exciton fine structure. We review the effective mass/electron-hole exchange theory for the exciton fine structure in tetragonal and orthorhombic CsPbBr₃ nanocrystals. The model requires several input parameters, including crystal field terms to parameterize the effect of the lattice distortion from the cubic to the tetragonal [4] or orthorhombic [2] perovskite phases, as well as the electron-hole exchange energy in these materials. We have employed density functional theory (DFT), together with hybrid functionals and spin orbit coupling in order to determine the crystal field parameters and the electron-hole exchange energy for the tetragonal and orthorhombic CsPbBr₃ phases. With these inputs, we determine the exciton fine structure level order within the context of the exchange model. As expected, we find an optically inactive ground exciton level. However, the calculated short range exchange energy is more than an order of magnitude smaller than what would be required to explain the measured fine structure splitting within a short-range exchange model. The possible impact of long-range exchange on the fine structure splitting is discussed. Importantly, we find that even if we add the long-range exchange interaction to explain the magnitude of the splitting, the level order for the optically active excitons in tetragonal CsPbBr₃ nanocrystals calculated in the exchange model is opposite to what has been observed experimentally. [2,3] An alternate explanation for the exciton fine structure splitting in CsPbBr₃ nanocrystals is offered in terms of the effect of Rashba splitting on the exciton fine structure which supports the existence of the bright ground exciton in these nanocrystals.[1]
[1] M. A. Becker et al., Nature 553, 189 (2018).
[2] M. Fu et al., Nano Lett. 17, 2895 (2017).
[3] J. Ramade et al. Nanoscale,10, 6393 (2018).
[4] Z. G. Yu, Science Reports 6, 28576 (2016).

Materials that can convert absorbed photons to emitted photons with quantum efficiencies exceeding 100% are rare, but of great interest for potentially increasing the overall efficiencies of existing solar-energy conversion technologies beyond their usual thermodynamic limits. This talk will describe some of our group's recent research into understanding, controlling, and exploiting new quantum-cutting doped halide perovskite semiconductor nanocrystals and bulk materials. These unique materials combine broad absorption profiles with photoluminescence quantum yields approaching 200%, making them very attractive for solar spectral conversion in applications such as photovoltaics or luminescent solar concentration. Fundamental spectroscopic and electronic-structure properties will be emphasized as they pertain to understanding the unusual photophysics displayed by these highly luminescent materials. Specific solar applications will also be described.

"Quantum-Cutting Ytterbium-Doped CsPb(Cl_1-xBr_x)₃ Perovskite Thin Films with Photoluminescence Quantum Yields over 190%." Kroupa, D. M.; Roh, J. Y.; Milstein, T. J.; Creutz, S. E.; Gamelin, D. R., ACS Energy Lett., 2018, 3, 2390.
"Picosecond Quantum Cutting Generates Photoluminescence Quantum Yields Over 100% in Ytterbium-Doped CsPbCl₃ Nanocrystals." Milstein, T. J.; Kroupa, D. M.; Gamelin, D. R., Nano Lett., 2018, 18, 3792.

Colloid quantum dots (CQDs) solar cells received great attention in recent years due to its solution processability and strong light harvesting capability in infrared region. The efficiency of CQD solar cells has been improving quickly in recent years, with certified power conversion efficiency up to 11.3%. The use of quantum dots surface engineering effectively reduced defects and improve carriers transport. In addition, device structure engineering significantly prompted the carriers transporting and inhibited interfacial recombination.¹ Unlike most other solar cells, both normal structure and inverted structure exhibit similar high efficiency, the efficiency of CQD solar cells with inverted structure is generally poor (below 5%). In this work, we fabricated lead sulfide CQD solar cells with inverted structure by using nickel oxide as bottom hole transporting layer. The best device show short circuit current density of 27.6 mA/cm², open circuit voltage of 0.53 V, fill factor of 65.7% and an overall power conversion efficiency (PCE) of 9.70% under standard 1.5G solar illumination². This indicate that inverted structure, with appropriate material and interface design, could be another opportunity for more efficient CQD-based photovoltaics.

Reference:
1. Ruili Wang, Xun Wu, Kaimin Xu, Wenjia Zhou, Yuequn Shang, Haoying Tang, Hao Chen, and Zhijun Ning*, Adv. Mater. 2018, 1704882.
2. Ruili Wang, Yuequn Shang, Pongsakorn Kanjanaboos, Wenjia Zhou, Zhijun Ning*, and Edward H. Sargent*, Energy Environ. Sci., 2016, 9, 1130-1143.

Contemporary designs for luminescent solar concentrators (LSCs) employ high radiative efficiency quantum dots in order to achieve high concentration: gain ratios. However, the quantum dot LSC community suffers from a lack of consensus about geometric gain and concentration factor conventions in luminescent solar concentrators (LSCs) to validate and measure device performance. In this talk, we explore inconsistencies in the definition of thermodynamic limits for LSCs in the literature, and re-evaluate their theoretical performance in a unified common framework. Using Monte-Carlo ray tracing simulations, we have modeled 6.25cm² LSCs featuring quantum dot luminophore concentrators and photovoltaics oriented in various conventional and unconventional geometries (e.g., coplanar horizontal, edge-lined, vertical, slanted). We have performed experimental inter-comparison of concentration for LSCs in these configurations that employ highly efficient CdSe/CdS core/shell quantum dots (QDs), which absorb light in the 300-500nm wavelength range and re-emit wavelength luminescence at 635nm, a wavelength that couples to our embedded photovoltaics. The QDs are dispersed throughout a 3.2 mm thick poly(laurylmethacrylate) (PLMA) waveguide layerand the photovoltaic is oriented within the waveguide, according to each target geometry. We standardized each device configuration to conform to a geometric gain of 120. Through our simulations, we have explored the radiative limit for each geometry. These simulations reveal significant discrepancies in measured power output and power conversion efficiency between the different cell geometries tested, and are compared to photovoltaics measurements of fabricated LSC prototypes for each cell geometry. During this talk, we will discuss how performance metrics of LSCs go beyond concentration factors and towards efficiency. We will further propose a new measurement for LSC device performance, which aligns with theoretical thermodynamic limits and can be measured experimentally. Our analysis has the potential to standardize performance measurements in the LSC community, independent of device geometry, luminophore, photovoltaic material, or waveguide form factor.

In developing low-cost and ultra-high efficiency solar cells, the utilization of the photon energy in a wide range of the solar spectrum is crucial. However, the theoretical maximum of the power conversion efficiency of single-junction solar cells under one-sun illumination remains 31 % at best. To surpass the single-junction limit, several concepts have been proposed, including multiple exciton generation, hot-carrier, and intermediate band. Among these concepts, the multi-junction concept has only been proven to overcome the single-junction limit, and multi-junction solar cells are commercially available. However, multi-junction solar cells that can surpass the limit are expensive to fabricate, which hampers the large-scale implementation of solar cells. Thus, development of multi-junction solar cells with low-cost technologies such as solution-based fabrication processes would help to expand their implementation.
Although there have been several repots on multi-junction solar cells including solution-based subcells such as perovskite solar cells, most of the solution-processed solar cells were used as top subcells [3, 4], but little has been reported for solution-processed bottom cells. The highest power conversion efficiency reported so far on solution-processed multi-junction solar cells (Perovskite top subcell/Si bottom subcell) stands at 25.2% at best. We then focused on infrared PbS CQDs and ZnO nanowires (NWs) approximately 1 μm long, and constructed heterojunction hybrid structures. The PbS QD/ZnO NW hybrid structures forming a bulk-heterojuntion allow almost all the photo-generated carriers to reach the PbS QD/ZnO NW interface even when the active layer is thicker than the carrier diffusion lengths. Our recent study revealed that PbS QD/ZnO NW solar cells could be used to convert a wide range of solar energy to electricity [1, 2].
In the presentation, we report high efficiency infrared PbS QD/ZnO NW solar cells that produce sufficiently large photocurrent in the infrared region, and the results of a proof-of-concept study for multi-junction solar cells.
We constructed infrared a PbS colloidal quantum dots/ZnO nanowire solar cell, with the aim of developing solution-processed multi-junction solar cells. Morphology of 1-μm-long ZnO nanowires was optimized, which allowed us to construct otherwise difficult, spatially separated carrier pathways and thick PbS QD layers for high infrared light harvesting. The optical management together with highly infrared-transparent conductive oxide (Ta-doped SnO2; TTO) electrodes were successfully used to enhance the spectral sensitivity in the infrared region. The external quantum efficiency of the solar cell reached 47 % at the wavelength corresponding to the first exciton peak (1560 nm): this is the highest value ever reported on the solution-processed solar cells.
We fabricated series-connected triple-junction solar cells by combining the PbS QD/ZnO NW solar cell (as a bottom subcell) with III-V 2J solar cells (InGaP top subcell/GaAs middle subcell). The series-connected triple-junction solar cell achieved approximately 30% under one-sun illumination, verifying that the PbS QD/ZnO NW.solars cell have a great potential for solution-processed bottom subcells.

1. “PbS-Quantum-Dot-Based Heterojunction Solar Cells Utilizing ZnO Nanowires for High External Quantum Efficiency in the Near-Infrared Region”, H. Wang et al. J. Phys. Chem. Lett., 4, 2455 (2013).
2. “Solution-Processed Short-wave Infrared PbS Colloidal Quantum Dot / ZnO Nanowire Solar Cells Giving High Open Circuit Voltage”, H. Wang et al., ACS Energy Letters, 2, 2110 (2017).
3. “Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency”, F. Sahli et al., Nat. Maters, 17, 820 (2018).
4. “High-performance perovskite/Cu(In,Ga)Se₂ monolithic tandem solar cells”, Han et al., Science, 361, 904 (2018).

This talk will describe why. I will begin by describing the type of structural dynamics that occurs in quantum dot solids and the methods that we can use to measure structural dynamics, such as inelastic neutron scattering and inelastic x-ray scattering, or simulate it, such as ab initio molecular dynamics. Then I will describe the theory of how electrons and vibrations interact, and the challenges we face in calculating non-radiative electronic transitions rates. Finally, I will explain why we should care about structure dynamics, specifically the impact the type of dynamics has electronic properties such as mobility and Shockley Read Hall non-radiative recombination. In my talk, I will rely on the example of PbS NCs and present a predictive model for electronic and thermal transport. Furthermore, I will describe how changing the structural dynamics (e.g., by changing the surface of NCs) can by used to systematically change the transport by reducing both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations.

Luminophores with sharp emissions coupled to broad absorptions have many applications including
solar energy harvesting. Lanthanide ions have very sharp, typically ~10 meV FWHM, emissions that
are largely insensitive to temperature or surrounding environment. However, lanthanides do not absorb
well outside of these transitions, so their ability to act as luminophores is limited. Doping semiconductors
with lanthanides has been pursued in thin films, but these systems typically only show luminescence at
cryogenic temperatures. As such, we seek to place lanthanides onto quantum dots to combine the
broad absorption of nanomaterials with the narrow emission of the lanthanides.
We use lanthanide trifluoroacetates and standard hot injection processes to grow an optically active Yb
doped shell around InP quantum dots. The particles are then characterized via a variety of optical and
structural methods. The structural methods, which include high resolution electron microscopy,
elemental mapping, and extended x-ray absorption fine structure measurements, show that the
nanoparticles take on a core/shell structure. Optically, we are able to see that there is a near infra-red
emission upon excitation with UV/visible light. The emission is traced back to the quantum dot
absorption via a photoluminescence excitation measurement, while the lifetimes of the near infra-red
emission show that the Yb ions are somewhat passivated from the surface of the nanoparticle.
This work shows the synthesis, optical, and structural characterization of Yb decorated InP quantum
dots. These materials could be used as stable and sharp luminescence centres in the near infra-red
and beyond.

Epitaxy is at the basis of many fabrication protocols in the semiconductor industry, allowing the development of complex architectures for optoelectronic devices. Epitaxial growth critically depends on the interaction between adsorbing particles, and between the particles and the substrate. While this process has been demonstrated for both atoms and microparticles,^1,2 the intermediate case of nanocrystals has remained elusive due to our limited understanding of interparticle interactions at the nanoscale.^3,4
Here, we demonstrate the epitaxial growth of semiconductor nanocrystals, quantum dots (QDs), on a flat, unfunctionalized and unpatterned silicon substrate via critical Casimir forces. By tuning the interplay of attractive critical Casimir and repulsive electrostatic interactions, we show that the epitaxial process can be biased towards either 2D layer or 3D island growth, consisting of crystalline or amorphous superstructures. These results demonstrate the potential of the critical Casimir interaction to direct the growth of future artificial solids based on QDs as fundamental building blocks.

(1) Amano, H.; Sawaki, N.; Akasaki, I.; Toyoda, Y. Applied Physics Letters 1986, 48, 353.
(2) Van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321.
(3) Rupich, S. M.; Castro, F. C.; Irvine, W. T.; Talapin, D. V. Nature communications 2014, 5, 5045.
(4) Wang, M. X.; Seo, S. E.; Gabrys, P. A.; Fleischman, D.; Lee, B.; Kim, Y.; Atwater, H. A.; Macfarlane, R. J.; Mirkin, C. A. ACS nano 2016, 11, 180.

Perovskite nanocrystals (NCs) have unique properties that allow researchers to expand on the abilities of polycrystalline perovskite materials. For example, the black perovskite phase of CsPbI₃ is stabilized in the NC material compared to the polycrystalline CsPbI₃ film and NC materials with high PLQY are used to fabricate efficient LEDs and PV devices with high open-circuit-voltages (V_OC). In this work, we first study the interactions of ligands with the surface of CsPbX₃ NCs in order to determine molecules that could be used to passivate polycrystalline film surfaces. Most surface passivation on bulk CsPbX₃ films has been focused on molecules containing the ammonium moiety, but we show NCs capped solely with oleate molecules that have high PLQY. We also show the effects of other anionic passivation molecules, specifically those with phosphonate and thiocyanate moieties. It is interesting to note that the oleate-only ligand shell slows halide exchange between particles when CsPbX₃ NCs of different halide compositions are mixed in solution, suggesting that oleylammonium plays a role in the mechanism of halide exchange between the NCs. Secondly, we use the NCs in combination with the polycrystalline films in order to capitalize on the high V_OC from NC PV devices and the strong light absorption from the thick polycrystalline layer.

Colloidal quantum dots are of interest in both single-junction and infrared (back-cell) hybrid applications. I will update on joint progress in the synthesis, materials processing, photophysics, and device physics of colloidal quantum dot solids and solar cells.

Quantum dot (QD) technology has received a lot of attention for the advantages of high quantum yield (QY), narrow full-width at half-maximum (FWHM) and tunable wavelength. Recently, photoluminescence-based QD LCD display equipped with QD film as a light converter has been commercialized. In the near future, electroluminescence QLED is expected to be a next generation display competing with other potential flexible displays. With a similar device structure of OLED, QLED display based on inorganic quantum dots would possess better stability, more color purity and wider color gamut than other displays. Moreover, QD layers in the device are compatible with solution processing, which makes QLED display can be easily scaled up to a large area, mass productive and cost effective. In this research, a solution-processed QLED device with a structure of ITO/PEDOT:PSS/PVK/QD/ZnO/Al was prepared with spin-coating and inkjet-printing. A QD ink with mixed solvent has been developed to reduce coffee-ring effect by Marangoni convection flow. Moreover, a designed printing route can avoid the nonuniformity of the QD film caused by droplet spreading. Combining mixed solvent with optimized printing route, the brightness of inkjet-printed QLED already can reach 89% than spin-coated QLED. Additionally, by inkjet printing process, a 2 * 2 pixel RGB QLED device has been demonstrated that exhibits a promising application in future.

Colloidal quantum dots (QDs) present a promising solution to the problem of attaining cheap, size-tunable light harvesters for third generation solar cells. Cadmium Selenide (CdSe) QDs were synthesized using a one-pot microwave irradiation technique to meet this need. Synthesized quantum dots showed excellent luminescent behavior under UV lamp, which was further confirmed by other spectroscopic techniques. Photoluminescence (PL) and ultraviolet-visible spectroscopy (UV-VIS) spectra revealed that changing the time and temperature of the microwave during synthesis changed the size of the QDs from a few nanometers to tens of nanometers, causing their fluorescence intensity to red shift and enabling light absorptions at higher wavelength regions. QD size manipulation via the microwave technique was further confirmed via transmission electron microscopy (TEM). The properties of CdSe QDs make them suitable candidates for tunable band gap light harvesting, and plans are in place to incorporate them in P3HT/PCBM hetero-junction solar cells as a means of increasing their solar energy power conversion efficiency (PCE).

Funder Acknowledgement(s): This work is supported by the NSF-CREST Grant number HRD 1547771 and NSF-CREST Grant number HRD 1036494.

Few methods currently exist to produce UV light from lower energy photons. UV light is useful in photocatalysis and many applications of light in the biological disciplines. The highest photon up-conversion quantum yield for the production of UV light from visible light is 5.2% ± 0.5 using CdS nanocrystals (NC) light absorbers. Here the goal is to increase the energy transfer efficiency with higher quality CdS NCs. Specifically, we investigate a variety of sulfur containing precursors to kinetically controlled nucleation to yield different sizes of CdS NCs with peak absorbances ranging from 398nm to 453nm. The CdS NCs made were characterized using optical absorption and photoluminescence spectroscopy. The goal is to eliminate the current need for the ZnS shell with CdS NCs that have minimal surface trap states. The next step will be to bind ligands to the NC and optimize upconversion parameters. We will report the photon upconversion QYs with 2-naphthoic acid transmitter ligands that enhance triplet energy transfer from CdS NCs to 2,5-diphenyloxazole emitter.

Copper Indium Sulfide (CIS) colloidal quantum dots (QDs) are promising candidates for commercially-viable QD-based optical applications, for example as colloidal photocatalysts or in luminescent solar concentrators (LSCs). CIS QDs with good photoluminescence quantum yields (PLQYs) and tunable emission wavelength via size and composition control have been previously reported. However, developing an understanding and control over the growth of electronically-passivating inorganic shells would enable further improvements of the photophysical properties of CIS QDs. To improve the optical properties of CIS QDs, we focus on the growth of inorganic shells via the popular metal-carboxylate/alkane thiol decomposition reaction. We (1) study the role of Zn-carboxylate and Zn-thiolate on the formation of ZnS shells on Cu-Deficient CIS (CDCIS) QDs, (2) leverage this knowledge to yield >90% PLQY CDCIS/ZnS core/shell QDs, (3) demonstrate control over interfacial alloying, and (4) propose a mechanism for ZnS shells grown from zinc-carboxylate/alkane thiol decomposition.

Recently, research on an organic light emitting diode (OLED) based on thermally active delayed fluorescence (TADF) having a high internal quantum efficiency is being actively conducted. In particular, it is recognized that OLED by solution process can be applied to large display devices and flexible display manufacturing because manufacturing process is simple and large area device can be manufactured.
In this presentation, we have demonstrated a series of new dendritic TADF emitters applicable in simple structured or all soluble OLEDs. All these new materials exhibit excellent solubility in common organic solvents and apparent TADF characteristics. The TADF emitters used in simple structured OLEDs without any HIL or HTL realized excellent performance with the high EQE. Besides, another series of TADF emitters displaying aggregation induced emission employed in all soluble OLEDs achieved EQE as high as 11%. Therefore, we strongly believe that these results will definitely open the doors to develop “simple structured OLEDs” instead of conventional OLEDs consisting complicated multi-layered device structure and difficult fabrication procedures.

In recent years, quantum dots (QDs) have gained interest as narrowband emitters for displays, but have traditionally suffered from reliability issues, high manufacturing cost, and toxicity concerns, which prevent them from being incorporated into products beyond displays. At UbiQD, we envision making QDs ubiquitous across many industries with a new kind of QD that is intrinsically more stable, lower-cost, and avoids toxic compounds. We are first focusing on bringing to market QD-tinted films and glass for spectrum-optimized greenhouses. This talk will highlight the recent developments including data from several commercial greenhouse pilot projects, and an overview of the first-ever QD greenhouse product.

The Shockley-Queisser limit places an upper bound on solar conversion efficiency for a single p-n junction solar cell at slightly more than 30%. To surpass this limit, multi-exciton generation is being explored in inorganic semiconductors, while singlet fission (SF) is being investigated in arrays of conjugated organic molecules. In an optimal SF process, the lowest singlet excited state of one molecule (S₁) that is positioned next to a second molecule in its ground state (S₀) is down-converted into two triplet excited states (T₁) each residing on one of the two adjacent molecules. The two triplet states initially form a correlated pair state ¹(T₁T₁), which then evolves into two separated triplet states (T₁ + T₁). As such, the energetic requirement for SF is E(S₁) larger than 2 E(T₁).

We have set our focus in recent years on intramolecular SF in molecular materials and their studies in solution rather than on intermolecular SF investigations in crystalline films.

Implicit in intramolecular SF is a resonant, direct excitation of the SF material. In pentacene dimers linked by a myriad of molecular spacers, SF takes place with quantum yields of up to 200%. In addition, all key intermediates in the SF process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons, have been identified. This approach is, however, limited to the part of the solar spectrum, where, for example, the pentacene dimers feature a significant absorption cross-section. To employ the remaining part of the solar spectrum necessitates non-resonant, indirect excitation of the SF materials via either up- or down-conversion. For example, the up-conversion approach is realized with singlet excited states in pentacene dimers, which are accessed by two-photon absorptions (TPA). TPA is then followed in the second step of the sequence by an intramolecular SF – similar to what is seen upon resonant, direct excitation. Quite different is the down-conversion approach, which is based on an intramolecular Förster resonance energy transfer (FRET) and thereby the (photo)activation of the SF material. FRET requires the use of a complementary absorbing chromophore and enables funneling its excited state energy unidirectionally to the SF performing pentacene dimer. Again, SF completes the reaction sequence.

When an ensemble of molecules interacts with the optical modes of a microcavity or an electromagnetic nanostructure, the collective excitations of the strongly coupled light-matter (polariton) system are no longer purely molecular nor electromagnetic, but hybrid superpositions of the two.

In this talk, I will discuss our recently developed theoretical models that explain the mechanisms whereby polaritons can enhance excitonic processes such as singlet-fission and harvesting of triplets in organic materials.

Singlet fission in organic semiconductors provides an efficient means to achieve exciton multiplication. To realise useful applications for efficiency enhancement in photovoltaic devices requires that the energy of each of the triplet excitons is efficiently harvested. I will describe an all-optical approach where triplet excitons from fission are transferred to inorganic semiconductor nanoparticles which can then emit at energies close to the photovoltaic bandgap. The challenge here is to achieve high efficiencies for nanoparticle luminescence whilst still allowing triplets to tunnel into the particles. I will describe measurements where fission takes place in solution and the triplets are efficiently transferred to nanoparticles. The key to achieving this is the use of acene ligands attached to the particles, providing adequate passivation of the particles whilst facilitating the transfer of triplets into the particles.

I will also describe measurements using electron spin resonance and luminescence at high magnetic fields to probe the interactions between triplets generated by fission in acene films. These measurements demonstrate the presence of bound triplet pairs with overall singlet, triplet and quintet characters, and allow their binding energies to be determined. I will present recent results using transient electron spin resonance techniques and magneto-photoluminescence at very low temperatures to study the mechanisms by which quintet pair states are formed from pair states with initial singlet character.

Approaches to utilizing triplet excitons born from singlet fission require consideration of schemes for triplet state lifetime elongation and either exciton dissociation or energy transfer. To isolate triplets after singlet fission, molecular design principles can be used to provide pathways for exciton migration and localization. Subsequently, an interface with a charge or energy acceptor can be introduced. This interface and the associated dynamics have nanoscale components and are often complex structurally and chemically. I will describe our efforts to characterize charge and energy transfer at interfaces involving singlet fission molecules and designed molecular charge acceptors, quantum dots, and oxides. In some cases molecular assembly at the interface naturally alters the intermolecular coupling and thus the intrinsic singlet fission dynamics, which leads to a kinetic competition between various pathways that is challenging to elucidate. Covalently bound chromophores (i.e. dimers and oligomers) can then be attractive alternatives due to their well-defined interchromophore geometries. We use a combination of synthesis, electrochemistry, and various forms of transient spectroscopy to monitor the relationship that molecular structure and electronic state energies have on the processes of singlet fission, exciton migration, energy transfer, and exciton dissociation.

The ability to undergo spin-allowed exciton multiplication makes singlet fission materials promising for photovoltaic applications. The majority of studies to date have emphasized understanding the first step of singlet fission, where the correlated triplet pair is produced. Here we examine the separation of correlated tripletpairs,¹(T…T), in polycrystalline pentacene films via temperature dependent transient absorptionspectroscopy. Single wavelength analysis reveals a profound delay in ¹(T…T) dynamics.Moreover, the dynamics of ¹(T…T) exhibit temperature dependence while other features showno discernable temperature dependence. Previous literature have suggested that correlated tripletseparation is mediated by a thermally activated hopping process. Surprisingly, we found that thetime constants governing triplet pair separation display two distinct temperature-dependentregimes of triplet transport. The high temperature regime follows a thermally activated hoppingmechanism. The experimentally derived reorganization energy and electronic coupling isverified by density matrix renormalization group quantum chemical calculations. In addition, weevaluated the low temperature regime and show that the trend can be modelled by a Miller-Abrahams-type model that incorporates the effects of energetic disorder. We conclude that thecorrelated triplet pair separation is mediated by thermally activated hopping or a disorder drivenMiller-Abrahams-type mechanism at high and low temperature, respectively. We observe thatcrossover between two regimes occurs ~ 226 K. We find the time constant for triplet-tripletenergy transfer to be 1.8 ps at ambient temperature and 21 ps at 77K.

Singlet fission, the conversion from one high-energy singlet exciton into two lower-energy triplet excitons in organic semiconductors can be used to overcome the Shockley-Queisser efficiency limit for single-junction solar cells. However, increasing the photocurrent by singlet fission requires efficient charge generation from triplet excitons. While such charge generation has been shown in purely organic solar cells, and in quantum dot/organic hybrids, it has never been shown in combination with the most important solar cell material, silicon.

Here we first calculate the efficiency potential of such singlet fission solar cells given realistic device parameters. We show how the efficiency depends on the quantum yield of all processes, the optical properties, and the energetics involved. This model reveals that direct charge transfer from the triplet exciton allows for the highest efficiency gain compared to more indirect paths, and, surprisingly, that the highest efficiency can be reached for singlet fission materials with a relatively low (singlet) bandgap of around 1.9 eV.

With optimistic, but realistic device parameters, the efficiency of the record-efficiency silicon solar cell could increase from 27% to 38%, even more than optimistic models predict for tandem solar cells. We also show that singlet fission cells behave differently than tandem cells under real-life operation conditions, and have a different dependence on the silicon base-cell efficiency. Finally, I will present experimental evidence for charge transfer on aromatically passivated silicon surface using a spatially-resolved quenching experiment.

Single junction silicon solar cells are fundamentally limited by the thermalization of high energy photons. In principle, it is possible to surmount this inefficiency by employing optical downconversion schemes. For example, one blue photon could be split into two near infrared photons, thereby doubling the available photocurrent. Unfortunately, however, material systems for implementing such conversions remain elusive.
We propose that singlet exciton fission is particularly suitable to solve this challenge. It can readily be employed as multiple exciton generation process, since it occurs on sub-ps timescales and at very high efficiencies^1,2. In organic solar cells, the generation of multiple carriers led to an external quantum efficiency of > 1.26 electrons per photon, exceeding the unity limit for conventional technologies³. It has also been shown that the efficiency of triplet exciton transfer from tetracene to colloidal inorganic nanocrystals is as high as 90% ⁴.
While the efficiency of exciton fission has been confirmed in a great number of molecular materials, it remains to transfer the energy to low-bandgap semiconductors⁵. Tetracene, a singlet fission active chromophore with a triplet energy of 1.25 eV, seems energetically suitable for the sensitization of silicon solar cells. In this talk, we present photoluminescent and magnetic field studies on direct energy transfer from tetracene to silicon. Moreover, we investigate the impact of different silicon passivation schemes, interlayer thicknesses and silicon doping levels on charge and energy transfer across the tetracene/silicon interface.
Finally, we show our most recent device results on interdigitated back contacted silicon solar cells featuring a tetracene downconversion layer. Various challenges arising from developing a fabrication process employing both organic and inorganic materials will be addressed. The performance of the solar cells is then discussed based on current-voltage characteristics, spectral response curves and magnetic field data.

References.
[1] Smith, M. B. & Michl, J. Recent Advances in Singlet Fission. Annual Review of Physical Chemistry 64, 361–386 (2013).
[2] Yost, S. R. et al. A transferable model for singlet-fission kinetics. Nature Chemistry 6, 492–497 (2014).
[3] Congreve, D. N. et al. External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell. Science 340, 334–337 (2013).
[4] Thompson, N. J. et al. Energy harvesting of non-emissive triplet excitons in tetracene by emissive PbS nanocrystals. Nature Materials 13, 1039–1043 (2014).
[5] Rao, A. & Friend, R. H. Harnessing singlet exciton fission to break the Shockley–Queisser limit. Nature Reviews Materials 2, 17063 (2017).

Many inorganic semiconductors of interest for solar energy conversion applications have indirect band gaps, which means that access to the lowest energy excited state via photon absorption requires coupling to a lattice phonon. The most famous example of an indirect semiconductor used in solar energy conversion is, of course, silicon, but many metal oxide semiconductors that have recently emerged as promising photoelectrodes for solar fuels generation, such as BiVO₄ and CuFeO₂, also have indirect band gaps. This talk presents recent data that identify spectral signatures of indirect transitions in transient absorption spectra of solution-processed nanostructured thin films of transition metal oxide semiconductors. Comparison of transient absorption spectra to thermal difference spectra generated by taking the difference between a steady-state absorption spectrum collected at elevated temperature and one collected at room temperature enables distinction of spectral features associated with thermal energy (i.e. phonons) in the semiconductor lattice from features associated with purely electronic transitions of photoexcited carriers. These distinctions not only enable accurate assignment of transient absorption spectra of metal oxide thin films, which is critical to their meaningful interpretation, but they also provide a means to probe directly the generation and evolution of thermal lattice energy in these films. Further characterization of these thermal processes will provide important insights into the function of metal oxide thin film semiconductors in devices for solar energy conversion.

The negligible spin-orbit coupling in many organic molecules creates opportunities to alter the energy of excited electrons by manipulating their spin. In particular, molecules with a large exchange splitting have garnered interest due to their potential to undergo singlet fission (SF), a process where a molecule in a high-energy spin-singlet state shares its energy with a neighbor, placing both in a low-energy spin-triplet state. When incorporated into photovoltaic and photocatalytic systems, SF can offset losses from carrier thermalization, which account for ~50% of the energy lost by these technologies. Likewise, compounds that undergo SF’s inverse, triplet fusion (TF), can be paired with infrared absorbers to create structures that upconvert infrared into visible light. However, designing functional applications based on either of these processes requires organic:inorganic junctions that readily transfer energy from one material to the other. In this presentation, I will describe our group’s recent efforts to produce organic:inorganic junctions that transmit excitons in a particular spin state. Specifically, I will describe work examining perylenediimide (PDI) films grown on Si(111) surfaces. Photoexcitation of the PDI layer produces a spin-singlet exciton that undergoes SF over a few hundred picoseconds to yield a triplet exciton pair. Transient reflectivity measurements indicate these triplets exhibit a reduced lifetime in PDI:Si bilayers relative to PDI films grown on quartz, suggesting triplet energy transfer from PDI to Si. I will also describe work on quantum dot superlattices wherein the quantum dot surface ligands have been replaced with exciton delocalizing ligands, molecules with valence orbitals that strongly hybridize with quantum dot band edge states. We have recently show ligands of this type can significantly speed exciton transport in quantum dot solids, leading to dot-to-dot energy hopping rates as short as 200 fs. Work using exciton delocalizing ligands to produce quantum dot structures that effectively deliver energy to triplet exciton accepting molecules for photon upconversion will be described.

We show exciting research opportunities when the concept of molecular self-assembly meets photon upconversion (UC) based on triplet-triplet annihilation (TTA).^1,2 In dense dye assemblies, triplet excitons can efficiently migrate and annihilate. Highly efficient photon upconversion has been realized in a wide range of chromophore assemblies, such as non-solvent liquids, ionic liquids, amorphous solids, gels, supramolecular assemblies, liquid crystals, and crystals. The control over their assembly structures allows for unexpected air-stability and efficient upconversion at weak excitation intensity.
In addition, we would like to introduce our recent progress in developing new UC mechanisms.^3,4 NIR-to-vis UC is particularly important for various applications, however, it remains challenging mainly due to the energy loss during the S₁-to-T₁ intersystem crossing (ISC) of sensitizers. We circumvent this energy loss by employing a sensitizer with direct S-T absorption in the NIR region. Sensitizer-doped emitter nanoparticles are dispersed into an oxygen-barrier polymer. The obtained composite film shows a stable NIR-to-vis UC even in air, expanding the scope of molecular sensitizers for NIR-to-vis UC. We have also unveiled the potential of three-dimensional (3D) metal-halide perovskites to sensitize organic triplets. Nanocrystals of surface-modified inorganic cesium lead halide perovskite (CsPbX₃, X = Br/I) are found to work as efficient triplet sensitizers for TTA-UC at low excitation intensity.

References
1) N. Yanai, N. Kimizuka et al., J. Am. Chem. Soc., 2013, 135, 19056-19059; J. Am. Chem. Soc., 2015, 137, 1887-1894; Sci. Rep., 2015, 5, 10882; Angew. Chem. Int. Ed., 2015, 54, 7544-7549; Angew. Chem. Int. Ed., 2015, 54, 11550-11554; Chem. Sci., 2016, 7, 5224-5229; Angew. Chem. Int. Ed. 2018, 57, 2806-2810; J. Am. Chem. Soc. 2018, 140, 8788-8796; J. Am. Chem. Soc. 2018, 140, 10848-10855.
2) N. Yanai, N. Kimizuka et al., Chem. Commun., 2016, 52, 5354-5370 (invited review); J. Phys. Chem. Lett. 2018, 9, 4613-4624 (Perspective).
3) N. Yanai, N. Kimizuka et al., J. Am. Chem. Soc., 2016, 138, 8702-8705; Chem.-Eur. J. 2016, 22, 2060-2067; J. Mater. Chem. C, 2016, 4, 6447-6451; J. Mater. Chem. C, 2017, 5, 5063; Chem. Commun. 2017, 53, 8261; ChemistrySelect, 2017, 2, 7597; Dalton Trans. 2018, 47, 8590-8594.
4) N. Yanai, N. Kimizuka, Acc. Chem. Res., 2017, 50, 2487-2495.

Photon upconversion is the process by which low energy photons are converted into one higher energy photon. This process has seen wide advancements in recent years owing to its broad scope of potential applications. However, the number of materials that can be used as upconversion annihilators are quite limited. Here I will discuss our groups’ efforts towards expanding the library of annihilators for upconversion. This includes exploring new families of chromophores, as well as synthetic modifications to existing materials for more efficient upconversion. Finally, I will detail a new application of upconversion: performing photoredox catalysis with infrared radiation.

The ability to efficiently convert low-intensity light between the visible and infrared would be an enabling technology—particularly for applications such as 3rd-generation photovoltaics, biological imaging, and cost-effective sensitized silicon focal plane array detectors with response in the short-wave infrared (SWIR; λ:1–3µm). Accordingly, we are advancing recent approaches that combine two excitonic materials—organic semiconductors and colloidal nanocrystals—to achieve broadband, non-coherent photon up-conversion from the SWIR to the visible.

To achieve upconversion, our general approach is to synthesize lead sulfide nanocrystals that are able absorb SWIR photons (λ>1 µm) and funnel these excitations to sensitize the spin-triplet excited state of an organic semiconductor (e.g. rubrene). In the molecules, strong exchange-splitting allows the excitonic energy to be combined via triplet-triplet annihilation to create visible light. In solid state devices, we have observed excitonic upconversion from λ=1.1μm→612nm, the threshold of the SWIR. Further, by using ligand engineering to optimize Dexter-mediated nanocrystal→organic energy transfer, we have achieved upconversion efficiencies of 7±1% with λ=808 nm excitation. Further, transient spectroscopy shows that there is no fundamental barrier to efficient performance of the annihilator materials at 1/10th the intensity of natural sunlight! Indeed, trials in solution with a novel emitter molecule and an standard organometallic sensitizer have achieved upconversion from λ>700nm with spectrally-integrated irradiance less than 1/3rd of the AM1.5 solar standard, and offer a route to free-floating upconverting fluorophores.

However, although colloidal quantum dots are near-ideal SWIR sensitizers—their photoexcitations are functionally spin-mixed at room temperature, and their optical gap can be tuned during colloidal synthesis—we now must advance our understanding of materials, device architectures, and photophysical dynamics to enable wide application of this hybrid approach to excitonic photon conversion. Indeed, our recent work indicates that process additives offer facile size-tuning of small (excitonic peak: λ<1.1µm) PbS nanocrystals, as well as insight into the underlying growth mechanism

One of the holy grails of the photon upconversion is to convert near infrared photons to high energy photons in the ultraviolet or violet spectrum. The solar spectrum at sea level is around 50%[2] low energy infrared photons that current solar panels do not absorb. By utilizing the strong absorption of inorganic nanocrystals (NC), and the ability of conjugated organic emitters to undergo triplet triplet annihilation (TTA), a hybrid tunable system can be designed with high upconversion Quantum Yields (QYs). A thin layer of this upconversion system may potentially improve solar panel efficiency beyond the Shockley Queisser limit of 30% [1]. Here, the organic emitter of interest is perylene. Perylene has many qualities that make it a promising emitter, such as a high fluorescence QY, a low-lying triplet energy state and fluorescence in the violet wavelengths. However, these high fluorescence QYs are only observed in dilute solution, and perylene needs to be tightly bound to the NC surface for efficient energy transfer. In this poster, we address the first problem with bulky substituents on perylene to increase solubility and minimize excimer formation. As for the second limitation, we investigate the covalent binding of perylene to CdSe NCs of various sizes for experimental evidence of the Marcus inverted regime. Information about the relationship between the driving force for triplet energy transfer and its rate will help design better ligands once the reorganization energy, or the coupled vibrations that participate are identified.

Silicon nanowires (SiNWs) exhibit unique opto-electronic properties originating from 1-D confinement. All opto-electronic properties: bandgap, charge carrier relaxation rates, and electron nonradiative lifetimes all are influenced by continuous sampling of momentum in growth direction. Here, motivated by our previous studies ^1-3, we quantify the influence of momentum dispersion on hot–electron relaxation rates and nonradiative lifetimes for SiNWs with <100> and <111> crystallographic directions. Photoexcited dynamic processes in reference SiNWs are computed via “on–the–fly” nonadiabatic couplings between electronic and nuclear degrees of freedom based on density functional theory (DFT). The dynamics of electronic degrees of freedom is propagated by a Redfield equation of motion for the reduced density matrix⁴. Our results show that transitions allowing change of momentum prompt electron relaxation faster than those not allowing change of momentum. Our study also indicates that the electron relaxation time in <100> SiNW is longer than that in <111> SiNW.

1. Fatima; Vogel, D. J.; Han, Y.; Inerbaev, T. M.; Oncel, N.; Kilin, D. S., First-Principles Study of Electron Dynamics with Explicit Treatment of Momentum Dispersion on Si Nanowires Along Different Directions. Molecular Physics 2018, 1-10.
2. Vogel, J.; Inerbaev, T.; Oncel, N.; Kilin, D., First-Principles Study of Charge Carrier Dynamics with Explicit Treatment of Momentum Dispersion on Si Nanowires Along< 211> Crystallographic Directions. MRS Advances 2018, 3, 3477-3482.
3. Fatima; Han, Y.; Vogel, D. J.; Inerbaev, T. M.; Oncel, N.; Hobbie, E. K.; Kilin, D. S., Photoexcited Electron Lifetimes Influenced by Momentum Dispersion in Silicon Nanowires. The Journal of Physical Chemistry C 2019, DOI: 10.1021/acs.jpcc.9b00639.
4. Kilin, D. S.; Micha, D. A., Relaxation of Photoexcited Electrons at a Nanostructured Si (111) Surface. The Journal of Physical Chemistry Letters 2010, 1, 1073-1077.

Photosystem I (PSI) is a photo-active protein found in plants, algae, and cyanobacteria. To facilitate photosynthesis, this protein shuttles electrons across the thylakoid membrane when irradiated by light. PSI’s abundance, ease of extraction, and excitonic properties have led to its use in biohybrid photovoltaics, which leverage the photon-induced electron shuttling mechanism to produce electrical current between two electrodes. In a PSI biohybrid photovoltaic, an electrochemical redox mediator is utilized to transfer charge from the protein to the cathode and the anode. The produced photocurrent is therefore limited by both the electrode surface area and the diffusion rate of the mediator. Herein we show that by confining PSI within a porous indium tin oxide (ITO) electrode, the maximum achievable photocurrent is increased, and diffusional losses are decreased. The prepared ITO electrodes are porous, have a low absorbance in the visible wavelength, and are conductive, making them ideal for dye-sensitized solar cell applications. A process for the facile fabrication of 20 to 100 nm pore size electrodes with sheet resistivity comparable to planar ITO is shown and PSI is infiltrated into the pores through vacuum assisted deposition after extraction from spinach. The PSI-porous electrode can efficiently convert redox mediators in solution upon illumination resulting in either a net conversion reaction or the production of photocurrent. This work highlights the potential of PSI as a photo-conversion species, which could lead to advances in biohybrid photovoltaics, solar redox flow batteries, and photo-electrochemistry for fuel production.

Solar cell efficiency could potentially be increased by exceeding the Shockley-Queisser limit through singlet fission. The Shockley-Queisser limit on photovoltaic efficiency is the theoretical maximum efficiency of a p-n junction given our sun. It states that nearly more than half of the light energy incident on a conventional single junction photovoltaic material is not converted to electrical energy. Some of this lost energy could be harvested through singlet fission. Here, inorganic colloidal PbS nanocrystals (NC) are used to harvest the energy from triplet excitons generated by organic 1,6-diphenyl-1,3,5-hexatriene (DPH). In DPH, singlet fission occurs efficiently, whereby an absorbed photon creates a singlet state that splits into two triplet states—one in the original chromophore and one in a neighboring chromophore. The energy transfer is known as the Dexter process, which is a simultaneous, correlated transfer of charge or energy from a donor to an acceptor non-radiatively. In this work, DPH is the donor while PbS is the acceptor, and Dexter transfer is enhanced by covalently attaching both species to promote wavefunction overlap between donor and acceptor. Synthesis of soluble DPH derivatives with specific functional groups that allow DPH to bind to PbS allow the surface density of molecules on nanocrystals to be controlled. Specifically, phosphonic acid and carboxylic acid functional groups are designed to bind the DPH to the surface of the PbS NC. Energy transfer from DPH to PbS characterized by steady-state and ultrafast absorbance and emission measurements will be presented.

Multifunctional phosphor nanoparticles based on lanthanide ion doped pyrochlore have gained significant interests due to their potential applications in various multifunctional devices for solid-state lighting, optoelectronics, and scintillation. Currently, it is a big challenge to synthesize nanophosphors with high emission intensity, radioluminescence (RL) efficiency and excited state lifetime comparable to traditional phosphors. In this work, La₂Hf₂O₇:Sm³⁺ (LHOS) nanoparticles (NPs) prepared by a molten salt synthesis method at relatively low temperature have characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The LHOS NPs show highly efficient, orange-red emitting properties as UV based phosphor, X-ray scintillator and luminescence thermal sensor. Concentration quenching study is carried out for both photoluminescence (PL) and RL. The emission characteristics show different behaviors under UV and X-ray excitations in terms of controlling magnetic dipole transitions. When LHOS NPs are exposed to energetic X-ray beam, Sm³⁺ ions situated at symmetric environment get excited along with those located at asymmetric environment, which results in high asymmetry ratio of Sm³⁺ under RL compared to PL. The PL and RL mechanisms are proposed based on Van Uitert equation. The important parameters such as thermal quenching activation energy and thermal sensitivity are also evaluated to evaluate the performance of the LHOS NPs as a thermal sensor. Our results indicate that these samarium activated La₂Hf₂O₇ NPs can serve as UV, X-ray and thermographic phosphor.

Halide perovskite are receiving a huge attention in the recent few years. Undoubtedly this attention is mainly due to the outstanding power conversion efficiencies, surpassing 23%, reported for photovoltaic devices, fabricated with polycrystalline films from low cost techniques. The great success of halide perovskites boosted also the interest on the nanoparticles (NPs) of these materials. Perovskite NPs are also generating a huge interest as relative easy preparation methods yield a simple core structure, without need for passivating shells, reach photoluminescence quantum yield (PLQY) higher than 90%.This remarkable PLQY points to low non-radiative recombination and consequently shows excellent rationale for the development of solar cells and optoelectronic devices. In this talk I show the interest of perovskite NPs in the development of different optoelectronic systems and analyze the similarities and differences with standard bulk perovskite thin films. Interestingly the use of NPs can help to overcome some limitations of bulk perovskites stabilizing new interesting crystalline phases or avoiding mixed halide ion migration. In addition, the interaction of perovskites with other materials can provide useful synergies and their potentialities commented.

Diluted magnetic semiconductors (DMS), especially those containing Mn dopants, have been the subject of numerous studies [1]. A key property of these materials is strong interactions between electronic states of a semiconductor host and Mn dopants mediated by spin-exchange interactions [2, 3]. An interesting aspect of these interactions is their influence on multicarrier Auger-type phenomena. In the case of Mn-doped II-VI DMSs, the spin-exchange Auger effect has been invoked to explain highly efficient excitation transfer from a semiconductor to a Mn ion leading to characteristic emission via the internal ⁴T₁−⁶A₁ transition of a 3d electron. This effect has been observed in both bulk and quantum dot (QD) forms of II-VI DMS materials [1, 4]. Previous studies also indicate that the reverse Auger process whereby the Mn excitation is transferred to the conduction-band electron is also highly probable [5]. In particular, Auger de-excitation was cited to rationalize Mn-emission quenching by injected electrons [6]. Furthermore, there are several direct observations of a hot Auger electron produced by this process [7, 8].
Despite the initial indications of a considerable strength of exchange Auger interactions in Mn-doped II-VI QDs, the quantitative understanding of this effect is still lacking. The purpose of the present study is to quantify temporal characteristics of Auger-mediated exchange of excitations between the magnetic ion and the semiconductor by conducting femtosecond transient absorption (TA) measurements of Mn:CdSe QDs. Using low-intensity (sub-single exciton) near band-edge excitation (515 nm), we are able to resolve both “forward” transfer of a band-edge exciton to a Mn ion as well as “back” transfer. We find that the direct transfer takes place on a ca. 100-fs timescale. The rate of the back transfer, on the other hand, strongly (exponentially) depends on the energy difference between the Mn ⁴T₁−⁶A₁ transition and the QD band gap, exhibiting a thermally activated behavior. Furthermore, in the case of above-band-gap excitation (343 nm) and high pump intensities (multiexcitonic regime), we detect the excitation of a “hot” electron from the QD to an external “vacuum” state accompanied by de-excitation of the Mn ion. This indicates an extremely strong exchange coupling of the excited Mn d-d transition to the intraband QD transition, which leads to sub-ps Auger-assisted re-excitation of the “hot” electron prior to its relaxation to the band edge. As a result, the energy transferred from the Mn ion adds up with the kinetic energy of the unrelaxed carrier, which allows it to escape from the QD.
This unusually fast intra-band Auger re-excitation has never been observed in bulk DMS materials, suggesting that it might be specific to strongly confined QDs. The above studies only scratch the surface of the fascinating subject of nanoscale exchange-type Auger interactions. Future work in this areas should deliver tremendous amount of new interesting physics and can potentially lead to novel applications such as photon upconversion, optically controlled electron emission, and transient exciton storage and on-demand release.
1. Furdyna, J.K., Diluted Magnetic Semiconductors. J. Appl. Phys. 64, R29 (1988)
2. Nawrocki, M., Y.G. Rubo, J.P. Lascaray, D. Coquillat, Phys. Rev. B 52, R2241 (1995)
3. Abramishvili, V.G., A.V. Komarov, S.M. Ryabchenko, Y.G. Semenov, Sol. St. Comm. 78, 1069 (1991)
4. Beaulac, R., P.I. Archer, and D.R. Gamelin, J. Sol. St. Chem. 181, 1582 (2008)
5. Peng, B., W. Liang, M.A. White, D.R. Gamelin, and X. Li, J. Phys. Chem. C 116, 11223 (2012)
6. White, M.A., A.L. Weaver, R. Beaulac, and D.R. Gamelin, ACS Nano 5, 4158 (2011)
7. Barrows, C.J., J.D. Rinehart, H. Nagaoka, D.W. deQuilettes, M. Salvador, J.I.L. Chen, D.S. Ginger, and D.R. Gamelin, J. Phys. Chem. Lett. 8, 126-130 (2017)
8. Chen, H.Y., T.Y. Chen, E. Berdugo, Y. Park, K. Lovering, D.H. Son, J. Phys. Chem. C 115, 11407- (2011)

Many Group IV semiconductor nanocrystals - such as Si and Ge - can now be synthesized with control over size and there has been progress towards passivasting their surfaces to achieve good light emitting properties. However, processing Group IV semiconductor nanocrystals into conductive films that are suitable for devices has been challenging because of the unique surface chemistry of these compounds. Most existing strategies for passivating Si NCs involve hydrosilylation of the surface, which involves terminating the Si nanocrystal surface with a covalently bound alkyl chain.

I will discuss new strategies for passivating Si nanocrystal surfaces that can enable the processing of ligand-free, all-inorganic assemblies. Additionally, I will share progress towards synthesizing single-atom thick 2D sheets of Si and Si-Ge alloys and discuss their surface-dependent optical properties. Finally, I will share challenges related to assembling Group IV semiconducting nanosheets into layered structures with long-range order.

Colloidal semiconductor quantum dots (QDs) have been envisioned as promising materials for application in traditional light-emission devices and prospective single-dot light sources. While QD displays have already entered the market place, realization of singe-dot applications still requires overcoming several challenges including elimination of strong spectral fluctuations at the individual-QD level. Recently, there has been considerable progress in suppressing intensity fluctuations by encapsulating an emitting core (usually CdSe) into an extra-thick protective shell (commonly CdS) [J. Am. Chem. Soc. 130, 5026, 2008; Nat. Mater. 7, 659, 2008]. Despite nearly “blinking-free” emission intensity, these dots, however, still show considerable fluctuations in both emission energy and linewidth. Here we demonstrate a new class of QDs that overcome these deficiencies. In these dots, the CdSe core is enclosed into a compositionally graded, asymmetrically-strained Cd_xZn_1-xSe shell [Nat. Mater. 17, 42, 2018]. These structures exhibit a highly stable emission energy (~0.5 meV standard deviation versus ~10 meV in CdSe/CdS QDs) and an unprecedentedly narrow, subthermal room-temperature linewidth (~20 meV). These unusual properties are derived from unique structural features of these QDs, which leads to strong suppression of exciton-phonon coupling and reduction in propensity for random photocharging. The remarkable spectral characteristics along with fast emission rates (~1/15 ns^-1) and high emission quantum yields (up to ~85%) make these novel structures well suited for practical realization of single-dot light sources.

Colloidal semiconductor nanocrystals have played an important role in the search for and development of next-generation technologies, owing largely to their potential for solution processability and size-tunable optical properties. Two-dimensional semiconductor nanocrystals have recently gained much attention for their unique, layer-dependent electronic, optical and magneto-optical properties, making them attractive for myriad applications in catalysis, energy conversion and optoelectronic devices. Although accessing lateral quantum confinement in these materials may be difficult, their properties may be tuned via other strategies including the formation of heterostructures or multinary alloys. Herein we present colloidal syntheses of heterostructures based on binary and ternary two-dimensional semiconductors such as WSe₂ and Cu₂WSe₄. These syntheses take advantage of nanocrystal conversion chemistry to access otherwise difficult compositions and structures.

Photo-induced water splitting using semiconductor photocatalysts has attracted considerable attention for producing H₂ as a clean energy carrier, while the effective utilization of visible light is imperative to achieve the desired efficiency for practical applications.^[1] Recently, mixed-anion compounds such as oxynitrides have been intensively studied as promising candidates since one can expect that higher energy p orbitals of non-oxide anions (e.g., N-2p) elevate their valence band maximum (VBM) values. Unfortunately, most of them are subject to facile self-oxidation by photogenerated holes, while highly dispersed cocatalyst particles certainly improve the stability of some oxynitrides.^[2] We have recently demonstrated that Sillén–Aurivillius type perovskite oxyhalides such as Bi₄NbO₈Cl can stably and efficiently oxidize water to O₂ under visible light without any surface modifications, and also exhibits a stable Z-scheme water splitting when coupled with a H₂-evolving photocatalyst.^[3] It was revealed that the VBMs of these materials consist mainly of O-2p orbitals, instead of Cl-3p (or Br-4p), but their positions are much more negative than those of conventional oxides. ^{[4, 5]} Thus, they possess narrow bandgaps for visible light absorption as well as sufficiently negative CBMs for water reduction. DFT calculation visualized a fairly strong hybridization between the Bi-6s and O-2p orbitals, which can explain why the O-2p orbitals are elevated in energy, combined with the result on Madelung site potential analysis that can rationalize the origin of high energy of O-2p orbital in these materials. Since O^– anions are known to be relatively stable, photogenerated holes populated at the O-2p orbitals will not lead to self-decomposition but to oxidize water. These results could provide new strategies for developing durable photocatalytic materials for water splitting under visible light, by manipulating the interaction between post-transition metal s orbitals and O-2p orbitals.
[1] R. Abe, J. Photochem. Photobiol. C: Photochemistry Reviews, 11 (2011) 179.
[2] R. Abe, M. Higashi, K. Domen, J. Am. Chem. Soc., 132, (2010) 11828.
[3] H. Fujito, H. Kunioku, H. Kageyama, R. Abe et al., J. Am. Chem. Soc., 38 (2016) 2082.
[4] D. Kato, H. Kunioku, R. Abe, H. Kageyama et al. J. Am. Chem. Soc., 139 (2017) 18725.
[5] H. Kunioku, H. Kageyama, R. Abe et al., J. Mater. Chem. A, 6 (2018) 3100.

Defining the optoelectronic features of semiconductors quantum dots (QDs) by introducing a few impurity atoms is a novel way to tailor new functionalities towards the photonic applications. Formation of localized impurity states in the energy gap enables to engineer the emission properties exceeding the range that can be achieved by size and shape control. In fact, attempts to exploit emission properties of QDs by impurity doping have initiated for compound semiconductor such as Cu or Ag-doped CdSe QDs and Mn-doped ZnSe QDs. In contrast to the great advancements in compound semiconductors, impurity-doping in Si QDs is still at the fundamental level despite its importance in optoelectronics and biophotonics applications.
In this work, we present the development of colloidal Si QDs codoped with boron (B) and phosphorus (P).[1,2] The codopants introduce donor and acceptor levels in the band gap of Si QDs and thus enable the optical transition with the energy below bulk Si bandgap (~1.1 eV). The emission energy is tunable in the range of 0.9-1.8 eV[1] which is optimal for carrier multiplication-facilitated solar cell power conversion. In this work, to extract the information on impurity-induced effects quantitatively, we apply the size-purification process for lifting the size inhomogeneity.[2] We have succeeded in preparing almost monodispersed codoped Si QDs. In the size-purified and selected QDs, we determine the degree of doping-induced shrinkage of the optical band gap over a wide size range. From the comparison of the experimental data with recent results on single QD analyses including scanning tunneling spectroscopy,[3] we discuss the size dependence of donor-acceptor (D-A) states in Si QDs. We present the number of D-A pairs in a QD in a wide size range by the comparison with theoretical calculation. In addition, we investigate the decay dynamics of D-A pair recombinations as a function of emission energy and the QD size. The results indicate that around 5.5 nm is a critical dimension, where the behavior of D−A pairs in a Si QD changes drastically. Finally, we demonstrate the potential of codoped Si QDs as near-IR luminescent probes in bio-imaging. These results demonstrate that codoped Si QDs offering efficient below 1.1 eV emission could be leveraged for not only QD-based solar energy conversion but also bio-imaging operated in the transparent window of biological tissue (700-1300 nm).
[1] Y. Hori, et al., Nano Letters, 16, 2615 (2016) [2] H. Sugimoto, et al., Nano Lett., DOI: 10.1021/acs.nanolett.8b03489 (2018) [3] O. Ashkenazy, et al., Nanoscale, 9, 17884 (2017)

Semiconductor quantum dots (QDs) are intriguing harvesters of light and donors of excited charge carriers for solar energy conversion. To exploit this potential requires the localization of QDs at interfaces with appropriate energetic offsets for charge separation. We and others have reported the use of mercaptoalkanoic acids and related thiol-bearing linkers to tether organic-capped CdS and CdSe QDs to nanocrystalline TiO₂ thin films. Electrons can be transferred efficiently from photoexcited QDs to the TiO₂ substrate through mercaptoalkanoic acid linkers. Unfortunately, adsorbed thiolates can accept valence-band holes efficiently from photoexcited QDs, reducing the distance between photogenerated electrons in TiO₂ and holes in the thiolates, and additionally promoting the deleterious oxidative degradation of linking ligands.

To address this issue, we are exploring the use of amine-bearing linkers as alternatives to the thiolated linkers. Amines are attractive because they can coordinate to cadmium chalcogenide QDs without accepting holes, which maximizes the spatial separation of charge carriers following electron transfer to metal oxides. In addition, amines enhance band edge emission, shifting trap-state emission to higher energy and eliminating electron-hole recombination pathways. This presentation will focus on (1) synthesis of the heterostructures via both in situ¹ and ex situ synthetic approaches, which differ in the ordering of surface-functionalization events, and (2) spectroscopic and photoelectrochemical characterization of excited-state charge-transfer within the heterostructures. In ex situ synthesis, CdSe QDs are first functionalized with the linking ligand, and these linker-functionalized CdSe QDs are then attached to the metal oxide. We have developed a two-step ligand-exchange mechanism to obtain para-aminobenzoic acid (PABA)-functionalized CdSe QDs by using oleate-capped CdSe QDs as the starting QDs and pyridine-CdSe QDs as the intermediate QDs. We subsequently optimized the attachment of PABA-CdSe QDs to TiO₂. Time-correlated single-photon-counting of QD-TiO₂ heterostructures reveal charge transfer-induced dynamic quenching of emission from QDs with excited-state electron-transfer rate constants on the order of 10⁷ s^-1. This presentation will emphasize insights into the mechanism and time scale of photoinduced electron transfer at QD-TiO₂ interfaces as a function of interfacial properties and the mechanism (in situ or ex situ) by which heterostructures were prepared. Results of ongoing photoelectrochemical experiments on QD-sensitized solar cells will also be presented.

(1) Rivera-Gonzalez, N.; Chauhan, S; Watson, D.F. “Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO₂” Langmuir 2016, 32, 9206-9215.

Single-walled carbon nanotubes (SWNTs) are one dimensional (1D) nanomaterials with a diameter of ca. 1 nm and a length of a few hundred nm or ~μm. The tubular structures are composed of rolled-up single graphene, and the rolled-up manner, which is identified by chiral indices, determines their optical and electronic properties, such as metallic and semiconducting features. The semiconducting SWNTs show photoluminescence (PL) in near infrared (NIR) regions through a relaxation process of exciton generated by photo-excitation. Interestingly, the confinement of the exciton in the 1D nanospace results in high binding energy (a few hundred meV), by which the generated exciton is stable even at room temperature and migrates along the nanotubes.
Recently, a local functionalization technique was found to employ such migrating excitons for enhancement of the PL properties of the SWNTs. The functionalization is conducted by chemical modification of the tube walls. Therein, partial defect doping occurs in the sp² carbon network structures. As a result, in the locally functionalized SWNTs (lf-SWNTs), the doped sites have different electronic structures by the modification, and can trap the migrating excitons and then emit PL efficiently, giving PL intensity enhancement with wavelength shifts. Recent studies are revealing that doped site structures of the lf-SWNTs relate to the resultant PL functions. This presentation reports new findings based on our molecular design approach to create structurally-designed doped sites, in which unique PL properties appear based on the functionalized molecules.[1-6]
For example, bisaryldiazonium salts (2Dz) were synthesized and used for the modification of SWNTs with (6,5) chiral index (lf-SWNTs/2Dz), providing new red-shifted PL.[1] Namely, the PL peak appeared at 1256 nm which was significantly red-shifted than those of non-functionalized SWNTs (985 nm) and mono-functionalized SWNTs (lf-SWNTs/1Dz, 1129 nm). In another project, substituted aryl isomers were introduced as a moiety of the doped site structures of lf-SWNTs. The observed PL was varied with strong dependence on the isomeric substituent positions.[3] Thus, these molecular structure-dependent spectral changes are expected to modulate the NIR PL in a wide wavelength range.
As another function of the lf-SWNTs, dynamic wavelength shifting is achieved through creation of the doped sites that selectively bind molecules and ions.[2, 5, 6] The local binding occurred by employing molecular recognition motives for the doped site structures. The first example was phenylboronic acid-modified lf-SWNTs that showed PL wavelength shifts by attachment of saccharide molecules at the doped sites.[2] Furthermore, the modification of azacrown ether groups, which can capture metal cations, provided PL wavelength shifts depending on the difference in the bound cationic species.[5] Therefore, the local events at the doped sites achieve PL shifting based on the molecular systems that are driven by molecular interactions with selectivity and flexibility.
Therefore, our molecular design approach is quite useful for modulation and functionalization of the NIR PL of the lf-SWNTs. The resultant functions are applicable to develop advanced applications such as bio/medical imaging and sensing, and nanodevices for telecommunication.

[1] T. Shiraki, T. Shiraishi, G. Juhász, N. Nakashima, Sci. Rep. 6, 28393 (2016).
[2] T. Shiraki, H. Onitsuka, T. Shiraishi, N. Nakashima, Chem. Commun. 52, 12972-12975 (2016).
[3] T. Shiraki, S. Uchimura, T. Shiraishi, H. Onitsuka, N. Nakashima, Chem. Commun.
53, 12544-12547 (2017).
[4] T. Shiraishi, T. Shiraki, N. Nakashima, Nanoscale 9, 16900-16907 (2017).
[5] H. Onitsuka, T. Fujigaya, N. Nakashima, T. Shiraki, Chem. Eur. J. 24, 9393-9398 (2018).
[6] T. Shiraki, T. Shiga, T. Shiraishi, H. Onitsuka, N. Nakashima, T. Fujigaya, Chem. Eur. J., in press (2018), DOI: 10.1002/chem.201805342.

Colloidal quantum dots are nanoscale building blocks for bottom-up fabrication of semiconducting solids with properties tailorability beyond the possibilities of bulk materials. Achieving ordered macroscopic crystal-like assemblies has been in the focus of researchers for years, since it would allow for exploitation of the quantum confinement-based electronic properties with tunable dimensionality, but was considered by many as a mere academic game as the polidispersity of colloidal systems should have hindered the emergence of collective behaviors. Lead-chalcogenide colloidal quantum dots show especially strong tendencies to self-organize into 2D superlattices with micron-scale order, making the array fabrication fairly simple. However, most works concentrate on the fundamentals of the assembly process, and none have investigated the electronic properties and their dependence on the nanoscale structure induced by different ligands. In my presentation, I will show the formation of large arrays of colloidal quantum dots and the dependence of the nanostructure, the optical and the electronic transport properties of the superlattice on chemical treatment (ligand exchange) performed. Transistors with average two-terminal electron mobilities of 13 cm²/Vs and contactless mobility of 24 cm²/Vs are obtained for small area superlattice FETs [1]. Such mobility values are the highest reported for CQD devices wherein the quantum confinement is substantially still present, and are comparable to those reported for heavy sintering. The next step to make an electronic metamaterial which could be highly relevant for optoelectronics, is to use the surface of the quantum dots to change their stoichiometry and tune their transport properties. I will show that more than two orders of magnitude improvement in the hole mobility, from below 10^-3 to above 0.1 cm²/Vs, by substituting iodide ligands with sulfide, while keeping the electron mobility stable (~1 cm²/Vs) were achieved [2].
The considerable mobility, the possibility of tuning their transport properties with the simultaneous preservation of the optical band gap displays the vast potential of colloidal QD superlattices for optoelectronic applications.
[1] Balazs, D.M.; Matysiak, B. M.; Momand, J.; Shulga, A. G.; Ibáñez, M.; Kovalenko, M.V.; Kooi, B. J.; Loi, M. A.: Electron Mobility of 24 cm2V-1s-1 in PbSe Colloidal-Quantum-Dot Superlattices, Advanced Materials 30, Article#: 1802265 (2018).
[2] Balazs, D.M.; Bijlsma, K.I.; Fang, H.H.; Dirin, D.N.; Döbeli, M.; Kovalenko, M.V.; Loi, M.A.: Stoichiometric control of the density of states in PbS colloidal quantum dot solids. Science Advances 3, eaao1558 (2017).

Colloidally synthesized nanocrystals are attractive ‘designer atoms’ that can be used to engineer band degeneracies and connectivities that may result in fundamentally new solid-state physics. Here, a compact fluorinated ligand, trifluoromethylthiolate, is used to functionalize PbS semiconductor quantum dots (QDs). The electron-withdrawing fluorinated shells allow band energies to be engineered to introduce a depletion layer in a QD thin film and thus improve charge transport in a device. Importantly, the self-assembled superlattices here have strong electronic coupling between constituent QDs arising from the short interdot distances. Thin-film transistor measurements show current staircases, reminiscent of Coulomb blockades recorded at 4K on single particles, despite the fact that our measurements are performed at RT on micron-sized devices over 1000-10000 QDs. This talk will discuss the charge transport in these PbS QD thin films.

Lead-Chalcogenide quantum dots (QDs) are of interest due to the facility of adjustment of their electrical and optical properties. Using a colloidal self-assembly technique, extended arrays of nanocrystal QDs superlattices (SL) can be generated. In the QD superlattice, bulk-like electronic bands with a bandwidth of 100~200 meV are expected to form which yield higher carrier mobility and diffusion lengths compared to weakly-coupled QDs. However, the electronic properties of such highly ordered QD arrays are not fully understood. In this work, the local density of state of a highly ordered monolayer PbSe superlattice was investigated by low-temperature scanning tunneling microscopy/spectroscopy (STM/STS).

A monolayer of PbSe QDs was prepared using a dip coating deposition technique. First, oleate-capped PbSe QDs dispersed in toluene were dip coated onto a mechanically exfoliated highly ordered pyrolytic graphite (HOPG) substrate. The oleate-capped PbSe film was treated with a NH₄SCN solution that initiated a ligand exchange and formed the SL. Afterward, the HOPG substrate was loaded into a commercial UHV STM chamber with a base pressure of 1x10^-11 torr.

STM/STS was performed on two types of samples. Type 1 samples were UHV annealed at 75°C for 30 minutes while type 2 samples were dosed with TMA at room temperature which served to remove loosely bound ligands and enable stable imaging. The band gaps of type 1 QDs were measured as a function of coordination number; and it was found that QDs with 1 nearest neighbor were isolated while those with 2, 3, and 4 nearest neighbors are incorporated into the superlattice and, therefore, have a delocalized electronic structure.

For type 2 QDs, STM imaging before TMA dosing displayed ~100% more noise than after TMA dosing consistent with the presence of loosely bound adsorbates interfering with the STM tip prior to TMA dosing. In addition, after TMA dosing atomic terraces were observed with a height consistent with approximately half the lattice parameter of a PbSe unit cell. It was found that there was a difference in the Fermi level position for QDs on layer 1 vs. those on layer 2 consistent with substrate induced band bending. Before the TMA dosing, QDs on layer 1 displayed a mixture of intrinsic and p-type behavior while those on layer 2 displayed purely p-type behavior. After TMA dosing, QDs on layer 1 were intrinsic while those on layer 2 stayed p-type, consistent with Fermi level unpinning as a result of TMA passivation.

Although it was theorized that the Al in TMA was bonding to the surface of the quantum dots consistent with the “TMA clean-up effect” noted for many compound semiconductors, it was found via XPS that nearly all the surface atoms on the QDs are chemically inert since XPS showed only trace oxygen before and after TMA dosing and no detectable aluminum after TMA. The data is consistent with the passivation of surface states and/or the removal of trace ligands with a density below 10¹³/cm² which is 10x greater than required to pin the Fermi level but still below the limit of XPS detection. In sum, TMA dosing was found to improve image quality as well as unpin the Fermi level, consistent with the passivation of surface states by removing adsorbates and/or surface states below 0.01 ML. It is hypothesized that the type 2 PbSe QDs readily form chemically and electronically passive surfaces after defect removal by TMA. This result provides a mechanism of defect passivation as well as insight into the electronic/chemical properties of extended QD SL arrays which can be used to improve the design of such arrays for eventual integration into electronic devices.

In molecular electronics, photochromic compounds are considered to be promising candidates for optoelectronic molecular electronic devices. In diarylethenes the π-system of the two aryl rings is separated in the open-ring isomer, while the π-system is delocalized throughout the molecule in the closed-ring isomer. The magnetic interaction in the open-ring isomer is inherently weak from the disjoint nature of the π-system. On the other hand, in the closed-ring isomer resonant closed-shell structure brings about strong magnetic interaction. Based on this idea we have demonstrated that the exchange interaction between two nitronyl nitroxide radicals connected by a diarylethene unit was photoswitched reversibly along with photochromism from the measurement of the magnetic susceptibility, ESR signal intensity, and ESR signal splitting. By preparing the diarylethene-gold nanoparticles network made of diarylethene dithiophenol, the completely reversible photoswitching of conductance through the organic molecule has been achieved.
Diarylethenes, which have same core structures but have different positions of thiol groups that are bound to gold nanoparticles, were prepared. In one diarylethene, which has two thiol groups at the positions equivalent to 5,5'-positions of di(3-thienyl)ethene, but in the other diarylethene, which has two thiol groups at 2- and 5-positions of one of the 3-thenyl group. The gold nanoparticle networks of these diarylethenes were prepared and the change in conductance was measured upon alternate irradiation with UV and visible light. For two diarylethenes, the direction of the photoswitching was opposite, reflecting the difference in the π-connectivity. The result suggests that the topology of π-conjugation between electrodes is the decisive factor in the conductance of gold nanoparticle network (J. Phys. Chem. Lett. 2016, 7, 2113).
Optical switching organic field-effect transistors (OFETs) provide a new direction for optoelectronics based on photochromic molecules. However, the patterning of OFETs is difficult because conventional fabrication processes, including lithography and ion etching, inevitably cause severe damage to organic molecules. We demonstrated laser patterning of one-dimensional (1D) channels on an OFET with a photochromic diarylethene (DAE) layer. A number of 1D channels can be repeatedly written and erased in the DAE layer by scanning focused ultraviolet and visible light laser beams and the conductance of the 1D channel can be controlled by the illumination conditions. This result will open new possibilities for realizing various optically reconfigurable, low-dimensional organic transistor circuits (Nano Lett. 2016, 16, 7474).

Colloidal quantum dots (CQDs) are attractive materials for energy harvesting and sensing applications because they combine flexible, low-cost solution-phase synthesis and processing with infrared responsivity. PbS-based CQDs with band gaps that can be tuned throughout the near and mid-infrared wavelengths via the quantum confinement effect are a promising materials system for photovoltaic devices and photodetectors that harvest non-visible radiation. The ability to achieve true wavelength-selectivity for applications such as transparent and multijunction photovoltaics, narrow-band photodetectors, and tailored emission sources has been lacking to this point, however. Here, we discuss several new strategies for achieving spectral selectivity and color-tuning in CQD thin films. We use thin film interference engineering and multiobjective optimization algorithms to design devices with controlled absorption, reflection, and transmission profiles while simultaneously maintaining high current in CQD solar cells. We also discuss an alternate method for achieving spectral selectivity in optoelectronic thin films: the use of photonic band engineering within the absorbing region of a semiconductor in which resonant states are strongly coupled to the external reflectivity and transmission spectra. Using optical models and proof-of-principle experiments, we show that photonic bands can be maintained in a new regime: within strongly absorbing materials. We further showed that this effect can be used as a tuning mechanism for enabling spectrally-selective absorption and transmission of light. We demonstrate the viability of this method in experiment by making new materials based on photonic crystal structures consisting of PbS CQDs infiltrated into an inverse opal structure provided by self-assembled dielectric nanospheres. This work represents a new spectral tuning mechanism and should additionally enable applications such as current-matching in multi-junction solar cells and miniaturized wavelength-selective photodetectors for sensing applications.

Beyond the conventional core/shell structures of colloidal semiconductor nanocrystals, the incorporation of active heterojunctions in emerging anisotropic structures has brought about new capabilities for optoelectronic applications. Recently, it has been demonstrated with double-heterojunction nanorods to independently control over electron and hole injection/extraction processes, allowing improved electroluminescence and simultaneous photodetection/photovoltaic capabilities. However, most efforts thus far have been made in Cd-based II-VI materials that face severe usage restrictions in consumer products. Hence, expanding the composition space for colloidal nanocrystal heterostructures of varying shapes should facilitate practical implementation. One of the interesting candidates is I-III-VI₂ compound semiconductors, which have similar crystal structures as the II-VI materials. Above all, much of the chemistry developed for II-VI nanocrystal heterostructures can be adapted for these ternary compounds. However, the direct formation of I-III-VI₂ based nanocrystal heterostructures has barely been reported due to the compositional complexity. Even the previous approaches via sequential cation exchange from II-VI based dot-in-rod structures have several limitations including non-selectivity for co-existing cations and formation of undesirable defects and impurities. Here, we examine and develop synthesis of colloidal I-III-VI₂ nanorods and subsequent epitaxial growth of heterostructures. Starting with CuGaS₂ nanorods, sawtooth-shaped CuGaS₂/CuInS₂ and CuGaS₂/CuInSe₂ nanorod heterostructures (NRHs) are achieved with subtle variations in shape caused by differences in lattice mismatch. Growth of ZnS shell on CuGaS₂/CuInSe₂ NRHs leads to enhanced PL and other interesting/useful optical properties, including large pseudo-Stokes shift and charge separation within the NRHs. Changing the final shell composition to ZnSe leads to similar features as those of ZnS shell initially but continued growth results in unusual brush-like heterostructures. Building on our synthetic strategy to extend anisotropic nanocrystal growth to introduce the multiple active heterojunctions should pave the path for developing application-specific design and synthesis of multifunctional optoelectronic materials.

Colloidal quantum dot light-emitting diodes provide a highly promising platform for efficient, color-tunable, and stable solid-state lighting and display technologies. A quantitative understanding of its device physics is required to achieve high efficiency across a large range of brightness and emission wavelength. While studies have demonstrated the detrimental effects of excess charge carriers, large electric field, and leakage current on device efficiency, these internal state variables have never been directly measured. This leads to challenges in validating quantitative physical models that relate device parameters, internal state variables, and device performance.

In this work, we perform differential absorption measurements using a supercontinuum laser to directly measure both charge density and electric field strength in a working QD-LED at high current density (up to 1000mA/cm^2) and optical brightness. We observe that charge density in the quantum dot film drastically increases with bias before eventually reaching a plateau, demonstrating that a single device can exhibit multiple regimes of charge balance characterized by different efficiency loss mechanisms. We show that the amount of injected charge can be controllably adjusted from ~0.5 electrons per QD to ~2 electrons per QD by simply varying the thickness of the quantum dot layer by less than 15nm. This leads to differing degrees of loss through Auger recombination and can offer insights into the dynamics of exciton formation at the interface of the quantum dot film and neighboring semiconducting transport layers. Finally, we use independent measurements of charging and electric field strength to charaterize the transition from interface limited conduction to space-charge limited conduction. These results provide a significant improvement in the quantitative understanding of the operation QD-LEDs and can lead to better rational optimization of device performance.

Colloidal quantum dots (QDs) have gained considerable attention as promising candidates for realizing solution-processible lasing devices including laser diodes. Important milestones on the way towards a QD-based laser technology have been recent demonstrations of low-threshold optically pumped devices operating under pulsed (Nano Lett. 15, 7319, 2015) and continuous-wave (Nature 544, 75, 2017) excitation. However, demonstration of lasing with electrical pumping remains challenging because of fast optical gain decay via intrinsic nonradiative Auger recombination and thermal instabilities in the QD gain medium under high excitation conditions. Recently, we demonstrated a population inversion and optical gain under electrical pumping in a QD light-emitting diode (QD-LED) using continuously graded QDs with suppressed Auger recombination and employing a current focusing architecture which allowed for achieving high current densities (of ~20 A cm^-2) without device overheating (Nat. Mater. 17, 42, 2017). The next step towards an electrically pumped laser diode is integration of an optical cavity into the QD-LED structure.
In this study, we demonstrate optically pumped lasing in a multilayer waveguide structure whose architecture is similar to that of a standard QD-LED. A one-dimensional periodic distributed feedback (DFB) grating was imprinted onto an ITO transparent electrode using laser interferometric lithography. In order to improve optical mode confinement within the gain-active QD layer, we engineered the refractive index of ITO so as to reduce the mode leakage into the underlying electrode. The developed structure exhibited strong lasing performance (λ= 630.9 nm) with optical feedback provided by the in-plane 2^nd order scattering and out-of-plane light outcoupling due to the 1^st order scattering. Despite the presence of a “lossy” conductive electrode, our fully optimized structures exhibited a very low lasing threshold of ~5.5 µJ/cm² which was on-a-par with the lowest thresholds reported for QD lasers of any type (Nano Lett. 15, 7319, 2015). As the next step, we employed a zinc oxide (ZnO) interlayer, which was commonly used in QD-LEDs as an electron-transport layer (ETL), and investigated lasing performance of an ITO-DFB/ZnO/QD multilayer structure. We found that the QD DFB laser with ZnO still exhibited excellent lasing performance with a low threshold (~5.7 µJ/cm²) achieved upon full optimization of all elements of our multilayer stacks. Finally, we deposited 4,4′,4″-tris(carbazol-9-yl)-triphenylamine (TCTA) as a hole-transport layer (HTL), on top of the QD layer to form a nearly complete LED stack comprised of transparent electrode/ETL/QD/HTL. Importantly, the deposition of TCTA did not distort optical properties of the DFB resonator, which retained a well-defined photonic gap. As a result, the developed device stacks showed good lasing performance although with a moderately increased threshold of ca. 17 µJ/cm², which was expected to become lower upon device optimization. The results of this work along with the recent demonstration of optical gain in QD-LEDs reaffirm the feasibility of solution processible QD-based laser diodes.

Colloidal Quantum dots (QDs) are a unique class of emitters with size-tunable emission wavelengths, saturated emission colors, near-unity luminance efficiency, inherent photo- and thermal- stability and excellent solution processability. The superior photoluminescence properties of QDs have already initiated applications as back-lighting for liquid-crystal displays to improve color gamut, which rapidly grows into multi-billion dollar business. Fully exploiting the superior luminescence properties of QDs in electroluminescence devices promises low-cost, large-area, flexible and yet high-performance LEDs, which shall revolutionize display and solid-state lighting. For the past two decades, a lot of efforts concentrated on materials and device structures of quantum dot light emitting diode (QLED) have been paid to improve devices performance. But the efficiency and lifetime of QLEDs are far behind the requirements for practical applications. Clearly, there is a performance gap between photoluminescence and electroluminescence of QDs.
To bridge the photoluminescence-electroluminescence gap of QDs, in-depth understanding of the formation and decay processes of electrically excited states in QDs is urgent and indispensable. In the past few years, we designed a conceptually new device structure containing an ultrathin insulating layer for QLEDs and integrated almost functional layers by solution process. The insulating layer modulate the electron injection to optimize charge balance and maintain superior emissive properties of QDs, forming efficient exciton generation and radiative decay cycle. Finally we realized the state-of-the-art solution-processed red LEDs with sub-bandgap turn-on voltage (1.7 V), record efficiency (EQE > 20%) and outstanding operational stability (>10⁵ hours @100 cd m^-2). The two functions of the insulating layer can be decoupled and addressed separately. Suppressing exciton quenching at electron-transport-layer (ETL)/QDs interface, which is identified as being obligatory for high-performance devices, is achieved by adopting Zn_xMg_1-xO nanocrystals, instead of ZnO nanocrystals, as ETLs. Optimizing charge balance is readily addressed by other device engineering approaches, such as controlling the oxide ETL/cathode interface and adjusting the thickness of the oxide ETL. Insights revealed by device physics studies further help us to bridge the photoluminescence-electroluminescence gap of QDs, realizing much extended QLED operation stability. These devices show a T₉₅ operation lifetime of more than 3,500 h at an initial brightness of 1000 cd m^-2 for red LEDs, representing the most stable QLEDs so far. These achievements bring QLED technologies a step closer towards real-life applications.