Diketopyrrolopyrrole-Based π-Bridged Donor–Acceptor Polymer for Photovoltaic Applications

We report the synthesis, properties, and photovoltaic applications of a new conjugated copolymer (C12DPP-π-BT) containing a donor group (bithiophene) and an acceptor group (2,5-didodecylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione), bridged by a phenyl group. Using cyclic voltammetry, we found the energy levels of C12DPP-π-BT are intermediate to common electron donor and acceptor photovoltaic materials, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. Whereas P3HT and PCBM are exclusively electron donating or accepting, we predict C12DPP-π-BT may uniquely serve as either an electron donor or an acceptor when paired with PCBM or P3HT forming junctions with large built-in potentials. We confirmed the ambipolar nature of C12DPP-π-BT in space charge limited current measurements and in C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT bulk heterojunction solar cells, achieving power conversion efficiencies of 1.67% and 0.84%, respectively, under illumination of AM 1.5G (100 mW/cm²). Adding diiodooctane to C12DPP-π-BT:PCBM improved donor–acceptor inter-mixing and film uniformity, and therefore enhanced charge separation and overall device efficiency. Using higher-molecular-weight polymer C12DPP-π-BT in both C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT devices improved charge transport and hence the performance of the solar cells. In addition, we compared the structural and electronic properties of C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT blends, representing the materials classes of polymer:fullerene and polymer:polymer blends. In C12DPP-π-BT:PCBM blends, higher short circuit currents were obtained, consistent with faster charge transfer and balanced electron and hole transport, but lower open circuit voltages may be reduced by trap-assisted recombination and interfacial recombination losses. In contrast, C12DPP-π-BT:P3HT blends exhibit higher open circuit voltage, but short circuit currents were limited by charge transfer between the polymers. In conclusion, C12DPP-π-BT is a promising material with intrinsic ambipolar characteristics for organic photovoltaics and may operate as either a donor or acceptor in the design of bulk heterojunction solar cells

Binary and Ternary Superlattices Self-Assembled from Colloidal Nanodisks and Nanorods

Self-assembly of multicomponent anisotropic nanocrystals with controlled orientation and spatial distribution allows the design of novel metamaterials with unique shape- and orientation-dependent collective properties. Although many phases of binary structures are theoretically proposed, the examples of multicomponent assemblies, which are experimentally realized with colloidal anisotropic nanocrystals, are still limited. In this report, we demonstrate the formation of binary and ternary superlattices from colloidal two-dimensional LaF₃ nanodisks and one-dimensional CdSe/CdS nanorods via liquid interfacial assembly. The colloidal nanodisks and nanorods are coassembled into AB-, AB₂-, and AB₆-type binary arrays determined by their relative size ratio and concentration to maximize their packing density. The position and orientation of anisotropic nanocrystal building blocks are tightly controlled in the self-assembled binary and ternary lattices. The macroscopic orientation of the superlattices is further tuned by changing the liquid subphase used for self-assembly, resulting in the formation of lamellar-type binary liquid crystalline superlattices. In addition, we demonstrate a novel ternary superlattice self-assembled from two different sizes of nanodisks and a nanorod, which offers the unique opportunity to design multifunctional metamaterials

Smectic Nanorod Superlattices Assembled on Liquid Subphases: Structure, Orientation, Defects, and Optical Polarization

Directing the orientation of anisotropic nanocrystal assemblies is important for harnessing the shape-dependent properties of nanocrystal solids in devices. We control the orientation of smectic B superlattices of CdSe/CdS dot-in-rod nanocrystals through assembly on different polar interfaces and quantify the superlattice orientation through correlated small- and wide-angle grazing-incidence diffraction. Small-angle scattering is used to determine the phase of the nanorod superlattices and their preferential growth directions from the subphase. Wide-angle diffraction is used to quantify the orientations of nanorods within the superlattices and with respect to the substrate. Not only are the nanorod long axes aligned within the structures, but truncation of the short axes also coaligns the crystal axes of the nanorods with the zone axes in assembled smectic B crystals. Three dimensional orientational alignment of nanocrystals in superlattices is highly desirable in device applications. Depending on the subphase used for self-assembly, the films range from nearly quantitative vertical to horizontal alignment. Controlling for other variables, we find that the surface tension of the subphase is strongly correlated with the orientational ordering of the nanorod superlattices. The microstructure of nanorod superlattices shows many classic defects of atomic and liquid crystalline systems. The nature of defect structures supports a mechanism of nuclei formation on the subphase–solvent interface rather than in solution. Last, we demonstrate the relationship between optical absorption polarization and superlattice structure using correlated optical spectroscopy and electron microscopy

Spectrally-Resolved Dielectric Functions of Solution-Cast Quantum Dot Thin Films

Quantum confinement is the divergence, at small crystallite size, of the electronic structure of semiconductor nanocrystals, or quantum dots, from the properties of larger crystals of the same materials. Although the extinction properties of quantum dots in the dispersed state have been extensively studied, many applications for quantum dots require the formation of a solid material which nonetheless retains a size-dependent electronic structure. The complex index of refraction (or complex dielectric function), including the extinction coefficient, is critical information for interpretation of optoelectronic measurements and use of quantum dot solids in optoelectronic devices. Here, spectroscopic ellipsometry is used to provide an all-optical method to determine the thickness, complex index, and extinction coefficient of thin films made of quantum-confined materials through the visible and near-infrared spectral ranges. The characteristic, size-dependent spectral features in the absorption of monodisperse quantum dots are readily translated into spectral variations of the index of refraction. The complex indices of refraction of CdSe and PbS quantum dot solids depend strongly on quantum dot size and the processing conditions of the thin film, including ligand exchange and annealing. The dielectric functions of quantum dot solids are dominated by the fill fraction of quantum dots, with only secondary influence from interparticle interaction

Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells

The competition between exciton dissociation and charge transport in organic solar cells comprising poly­(3-hexylthiophene) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] is investigated by correlated scanning confocal photoluminescence and photocurrent microscopies. Contrary to the general expectation that higher photoluminescence quenching is indicative of higher photocurrent, microscale mapping of bulk-heterojunction solar-cell devices shows that photoluminescence quenching and photocurrent can be inversely proportional to one another. To understand this phenomenon, we construct a model system by selectively laminating a PCBM layer onto a P3HT film to form a PCBM/P3HT planar junction on half of the device and a P3HT single junction on the other half. Upon thermal annealing to allow for interdiffusion of PCBM into P3HT, an inverse relationship between photoluminescence quenching and photocurrent is observed at the boundary between the PCBM/P3HT junction and P3HT layer. Incorporation of PCBM in P3HT works to increase photoluminescence quenching, consistent with efficient charge separation, but conductive atomic force microscopy measurements reveal that PCBM acts to decrease P3HT hole mobility, limiting the efficiency of charge transport. This suggests that photoluminescence-quenching measurements should be used with caution in evaluating new organic materials for organic solar cells

Thiocyanate-Capped PbS Nanocubes: Ambipolar Transport Enables Quantum Dot Based Circuits on a Flexible Substrate

We report the use of thiocyanate as a ligand for lead sulfide (PbS) nanocubes for high-performance, thin-film electronics. PbS nanocubes, self-assembled into thin films and capped with the thiocyanate, exhibit ambipolar characteristics in field-effect transistors. The nearly balanced, high mobilities for electrons and holes enable the fabrication of CMOS-like inverters with promising gains of ∼22 from a single semiconductor material. The mild chemical treatment and low-temperature processing conditions are compatible with plastic substrates, allowing the realization of flexible, nonsintered quantum dot circuits

Plasmonic Enhancement of Nanophosphor Upconversion Luminescence in Au Nanohole Arrays

Arrays of subwavelength holes (nanoholes) in Au films were computationally designed, fabricated, and used as templates to localize and enhance the luminescence of upconversion nanophosphors (UCNPs)hexagonal phase NaYF₄ doped with Yb³⁺ and Er³⁺. The dimensions of nanohole Au arrays were designed to accept only a single UCNP upon particle filling and with a periodicity to be resonant with the excitation wavelength of the upconversion. Frequency-dependent luminescence enhancements of up to 35-fold and a concomitant shortening of the UCNP luminescence rise time were observed, consistent with simulations of plasmonic enhancement of the UCNP absorption

Large Exciton Polaron Formation in 2D Hybrid Perovskites via Time-Resolved Photoluminescence

We find evidence for the formation and relaxation of large exciton polarons in 2D organic–inorganic hybrid perovskites. Using ps-scale time-resolved photoluminescence within the phenethylammonium lead iodide family of compounds, we identify a red shifting of emission that we associate with exciton polaron formation time scales of 3–10 ps. Atomic substitutions of the phenethylammonium cation allow local control over the structure of the inorganic lattice, and we show that the structural differences among materials strongly influence the exciton polaron relaxation process, revealing a polaron binding energy that grows larger (up to 15 meV) in more strongly distorted compounds

Increased Carrier Mobility and Lifetime in CdSe Quantum Dot Thin Films through Surface Trap Passivation and Doping

Passivating surface defects and controlling the carrier concentration and mobility in quantum dot (QD) thin films is prerequisite to designing electronic and optoelectronic devices. We investigate the effect of introducing indium in CdSe QD thin films on the dark mobility and the photogenerated carrier mobility and lifetime using field-effect transistor (FET) and time-resolved microwave conductivity (TRMC) measurements. We evaporate indium films ranging from 1 to 11 nm in thickness on top of approximately 40 nm thick thiocyanate-capped CdSe QD thin films and anneal the QD films at 300 °C to densify and drive diffusion of indium through the films. As the amount of indium increases, the FET and TRMC mobilities and the TRMC lifetime increase. The increase in mobility and lifetime is consistent with increased indium passivating midgap and band-tail trap states and doping the films, shifting the Fermi energy closer to and into the conduction band

Bistable Magnetoresistance Switching in Exchange-Coupled CoFe₂O₄–Fe₃O₄ Binary Nanocrystal Superlattices by Self-Assembly and Thermal Annealing

Self-assembly of multicomponent nanocrystal superlattices provides a modular approach to the design of metamaterials by choosing constituent nanocrystal building blocks with desired physical properties and engineering the interparticle coupling. In this work, we report the self-assembly of binary nanocrystal superlattices composed of magnetically hard CoFe₂O₄ nanocrystals and magnetically soft Fe₃O₄ nanocrystals. Both NaZn₁₃- and MgZn₂-type CoFe₂O₄–Fe₃O₄ binary nanocrystal superlattices have been formed by the liquid–air interfacial assembly approach. Exchange coupling is achieved in both types of binary superlattices after thermal annealing under vacuum at 400 °C. The exchange-coupled CoFe₂O₄–Fe₃O₄ binary nanocrystal superlattices show single-phase magnetization switching behavior and magnetoresistance switching behavior below 200 K. The NaZn₁₃-type CoFe₂O₄–Fe₃O₄ binary nanocrystal superlattices annealed at 500 °C even exhibit bistable magnetoresistance switching behavior at room temperature constituting a simple nonvolatile memory function