Cyclobutadiene–C60 Adducts: N‐Type Materials for Organic Photovoltaic Cells with High VOC

New tetraalkylcyclobutadiene–C₆₀ adducts are developed via Diels–Alder cycloaddition of C₆₀ with in situ generated cyclobutadienes. The cofacial π‐orbital interactions between the fullerene orbitals and the cyclobutene are shown to decrease the electron affinity and thereby increase the lowest unoccupied molecular orbital (LUMO) energy level of C₆₀ significantly (ca. 100 and 300 meV for mono‐ and bisadducts, respectively). These variations in LUMO levels of fullerene can be used to generate higher open‐circuit voltages (V_OC) in bulk heterojunction polymer solar cells. The tetramethylcyclobutadiene–C₆₀ monoadduct displays an open‐circuit voltage (0.61 V) and a power conversion efficiency (2.49%) comparable to the widely used P3HT/PCBM (poly(3‐hexylthiophene/([6,6]‐phenyl‐C61‐butyric acid methyl ester) composite (0.58 V and 2.57%, respectively). The role of the cofacial π‐orbital interactions between C₆₀ and the attached cyclobutene group was probed chemically by epoxidation of the cyclobutene moiety and theoretically through density functional theory calculations. The electrochemical, photophysical, and thermal properties of the newly synthesized fullerene derivatives support the proposed effect of functionalization on electron affinities and photovoltaic performance.     

Film Morphology of High Efficiency Solution‐Processed Small‐Molecule Solar Cells

Morphological control over the bulk heterojunction (BHJ) microstructure of a high‐efficiency small molecule photovoltaic system is demonstrated using both thermal treatment and solvent additive processing. Single crystal X‐ray diffraction is utilized to understand molecular interactions in the solid state and the BHJ morphology is examined using bright field, high‐resolution, and cross‐section transmission electron microscopy techniques. Controlling the domain size, while maintaining good molecular order within the semiconducting donor material, is found to be crucial in achieving high performance and over 90% internal quantum efficiency exhibited under the optimized conditions.     

Resolving the Three‐Dimensional Microstructure of Polymer Electrolyte Fuel Cell Electrodes using Nanometer‐Scale X‐ray Computed Tomography

The electrodes of a polymer electrolyte fuel cell (PEFC) are composite porous layers consisting of carbon and platinum nanoparticles and a polymer electrolyte binder. The proper composition and arrangement of these materials for fast reactant transport and high electrochemical activity is crucial to achieving high performance, long lifetimes, and low costs. Here, the microstructure of a PEFC electrode using nanometer‐scale X‐ray computed tomography (nano‐CT) with a resolution of 50 nm is investigated. The nano‐CT instrument obtains this resolution for the low‐atomic‐number catalyst support and binder using a combination of a Fresnel zone plate objective and Zernike phase contrast imaging. High‐resolution, non‐destructive imaging of the three‐dimensional (3D) microstructures provides important new information on the size and form of the catalyst particle agglomerates and pore spaces. Transmission electron microscopy (TEM) and mercury intrusion porosimetry (MIP) is applied to evaluate the limits of the resolution and to verify the 3D reconstructions. The computational reconstructions and size distributions obtained with nano‐CT can be used for evaluating electrode preparation, performing pore‐scale simulations, and extracting effective morphological parameters for large‐scale computational models.     

Fabrication of Multicomponent Microsystems by Directed Three‐Dimensional Self‐Assembly

We have developed a directed self‐assembly process for the fabrication of three‐dimensional (3D) microsystems that contain non‐identical parts and a statistical model that relates the process yield to the process parameters. The self‐assembly process uses geometric‐shape recognition to identify different components, and surface tension between liquid solder and metal‐coated areas to form mechanical and electrical connections. The concept is used to realize self‐packaging microsystems that contain non‐identical subunits. To enable the realization of microsystems that contain more than two non‐identical subunits, sequential self‐assembly is introduced, a process that is similar to the formation of heterodimers, heterotrimers, and higher aggregates found in nature, chemistry, and chemical biology. The self‐assembly of three‐component assemblies is demonstrated by sequentially adding device segments to the assembly solution including two hundred micrometer‐sized light‐emitting diodes (LEDs) and complementary metal oxide semiconductor (CMOS) integrated circuits. Six hundred AlGaInP/GaAs LED segments self‐assembled onto device carriers in two minutes, without defects, and encapsulation units self‐assembled onto the LED‐carrier assemblies to form a 3D circuit path to operate the final device. The self‐assembly process is a well‐defined statistical process. The process follows a first‐order, non‐linear differential equation. The presented model relates the progression of the self‐assembly and yield with the process parameters—component population and capture probability—that are defined by the agitation and the component design.     

Three‐Dimensional Programmable Assembly by Untethered Magnetic Robotic Micro‐Grippers

Mobile sub‐millimeter micro‐robots have demonstrated untethered motion and transport of cargo in remote, confined or enclosed environments. However, limited by simple design and actuation, they lack remotely‐actuated on‐board mechanisms required to perform complex tasks such as object assembly. A flexible patterned magnetic material which allows internal actuation, resulting in a mobile micro‐gripper which is driven and actuated by magnetic fields, is introduced here. By remotely controlling the magnetization direction of each micro‐gripper arm, a gripping motion which can be combined with locomotion for precise transport, orientation, and programmable three‐dimensional assembly of micro‐parts in remote environments is demonstrated. This allows the creation of out‐of‐plane 3D structures and mechanisms made from several building blocks. Using multiple magnetic materials in each micro‐gripper, the addressable actuation of gripper teams for parallel, distributed operation is also demonstrated. These mobile micro‐grippers can potentially be applied to 3D assembly of heterogeneous meta‐materials, construction of medical devices inside the human body, the study of biological systems in micro‐fluidic channels, 3D micro‐device prototyping or desktop micro‐factories.     

Direct Transcription of Two‐Dimensional Colloidal Crystal Arrays into Three‐Dimensional Photonic Crystals

A simple protocol for the fabrication of three‐dimensional (3D) photonic crystals in silicon is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, sequential passivation reactive ion etching, is a one‐step processing protocol relying on sequential passivation and reactive ion etching reactions using C₄F₈ and SF₆ plasma chemistries. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the two‐dimensional (2D) colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. 3D photonic crystal lattices exhibiting optical stop‐bands in the infrared spectral region are demonstrated, proving the potential of SPRIE for fast, simple, and large‐scale fabrication of photonic structures.     

Charge Transport and Photocurrent Generation in Poly(3‐hexylthiophene): Methanofullerene Bulk‐Heterojunction Solar Cells

The effect of controlled thermal annealing on charge transport and photogeneration in bulk‐heterojunction solar cells made from blend films of regioregular poly(3‐hexylthiophene) (P3HT) and methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10^–8 m² V^–1 s^–1 in as‐cast devices to ≈3 × 10^–7 m² V^–1 s^–1 after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to ≈ 2 × 10^–8 m² V^–1 s^–1 after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red‐shift, improving the spectral overlap with solar emission, which results in an increase of more than 60 % in the rate of charge‐carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non‐annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short‐circuit conditions the dissociation efficiency of bound electron–hole pairs at the donor/acceptor interface is close to 90 %, which explains the large quantum efficiencies measured in P3HT:PCBM blends.     

Fabrication of Organic Thin‐Film Transistors on Three‐Dimensional Substrates Using Free‐Standing Polymeric Masks Based on Soft Lithography

Here, a novel fabrication technique for integrated organic devices on substrates with complex structure is presented. For this work, free‐standing polymeric masks with stencil‐patterns are fabricated using an ultra‐violet (UV) curable polyurethaneacrylate (PUA) mixture, and used as shadow masks for thermal evaporation. High flexibility and adhesive properties of the free‐standing PUA masks ensure conformal contact with various materials such as glass, silicon (Si), and polymer, and thus can also be utilized as patterning masks for solution‐based deposition methods, such as spin‐coating and drop‐casting. Based on this technique, a number of integrated organic transistors are fabricated simultaneously on a cylindrical glass bottle with high curvature, as well as on a flat silicon wafer. It is anticipated that these results will be applied to the development of various integrated organic devices on complex‐structured substrates, which can lead to further applications.     

Dicyanovinyl–Substituted Oligothiophenes: Structure‐Property Relationships and Application in Vacuum‐Processed Small Molecule Organic Solar Cells

Efficient synthesis of a series of terminally dicyanovinyl (DCV)‐substituted oligothiophenes, DCVnT 1–6, without solubilizing side chains synthesized via a novel convergent approach and their application as electron donors in vacuum‐processed m‐i‐p‐type planar and p‐i‐n‐type bulk heterojunction organic solar cells is described. Purification of the products via gradient sublimation yields thermally highly stable organic semiconducting materials in single crystalline quality which allows for X‐ray structure analysis. Important insights into the packing features and intermolecular interactions of these promising solar cell materials are provided. Optical absorption spectra and electrochemical properties of the oligomers are investigated and valuable structure–property relationships deduced. Photovoltaic devices incorporating DCVnTs 4–6 showed power conversion efficiencies up to 2.8% for planar and 5.2% for bulk heterojunction organic solar cells under full sun illumination (mismatch corrected simulated AM 1.5G sunlight). The 5.2% efficiency shown here represents one of the highest values ever reported for organic vacuum‐deposited single heterojunction solar cells.     

Improved Performance of Molecular Bulk‐Heterojunction Photovoltaic Cells through Predictable Selection of Solvent Additives

Solvent additives provide an effective means to alter the morphology and thereby improve the performance of organic bulk‐heterojunction photovoltaics, although guidelines for selecting an appropriate solvent additive remain relatively unclear. Here, a family of solvent additives spanning a wide range of Hansen solubility parameters is applied to a molecular bulk‐heterojunction system consisting of an isoindigo and thiophene containing oligomer as the electron donor and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) as the electron acceptor. Hansen solubility parameters are calculated using the group contribution method and compared with the measured solubilities for use as a screening method in solvent additive selection. The additives are shown to alter the morphologies in a semipredictable manner, with the poorer solvents generally resulting in decreased domain sizes, increased hole mobilities, and improved photovoltaic performance. The additives with larger hydrogen bonding parameters, namely triethylene glycol (TEG) and N‐methyl‐2‐pyrrolidone (NMP), are demonstrated to increase the open circuit voltage by ~0.2 V. Combining a solvent additive observed to increase short circuit current, poly(dimethylsiloxane), with TEG results in an increase in power conversion efficiency from 1.4 to 3.3%.     

Identifying a Threshold Impurity Level for Organic Solar Cells: Enhanced First‐Order Recombination Via Well‐Defined PC84BM Traps in Organic Bulk Heterojunction Solar Cells

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap‐assisted recombination rate when [6,6]‐phenyl C₈₄ butyric acid methyl ester (PC₈₄BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]‐phenyl C₆₁ butyric acid methyl ester (PC₆₀BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley–Read–Hall recombination; the latter is dramatically enhanced by the addition of the PC₈₄BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap‐induced recombination loss.     

Three‐Dimensional Nanostructures for Photonics

Recent progress in direct laser writing of three‐dimensional (3D) polymer nanostructures for photonics is reviewed. This technology has reached a level of maturity at which it can be considered as the 3D analogue of planar electron‐beam lithography. Combined with atomic‐layer deposition and/or chemical‐vapor deposition of dielectrics—the 3D analogues of planar evaporation technologies, the 3D polymer templates can be converted or inverted into 3D high‐refractive‐index‐contrast nanostructures. Examples discussed in this review include positive and inverse 3D silicon‐based woodpile photonic crystals possessing complete photonic bandgaps, novel optical resonator designs within these structures, 3D chiral photonic crystals for polarization‐state manipulation, and 3D icosahedral photonic quasicrystals. The latter represent a particularly complex 3D nanostructure.     

Impact of Blend Morphology on Interface State Recombination in Bulk Heterojunction Organic Solar Cells

This work is a reinvestigation of the impact of blend morphology and thermal annealing on the electrical performance of regioregular‐P3HT:PC₆₀BM bulk heterojunction organic solar cells. The morphological, structural, and electrical properties of the blend are experimentally investigated with atomic force microscopy, X‐ray diffraction, and time‐of‐flight measurements. Current–voltage characteristics of photodiode devices are measured in the dark and under illumination. Finally, the existence of exponential electronic band tails due to gap states is experimentally confirmed by measuring the device spectral response in the subband gap regime. This method reveals the existence of a large density of gap states, which is partially and systematically reduced by thermal annealing. When the band tails are properly accounted for in the drift and diffusion simulations, experimentally measured charge transport characteristics, under both dark and illuminated conditions and as a function of annealing time, can be satisfactorily reproduced. This work further confirms the critical impact of tails states on the performance of solar cells.     

Temperature‐Dependent Aggregation Donor Polymers Enable Highly Efficient Sequentially Processed Organic Photovoltaics Without the Need of Orthogonal Solvents

The conventional method to prepare bulk‐heterojunction organic photovoltaics (OPVs) is a one‐step method from the blend solution of donor and acceptor materials, known as blend‐casting (BC). Recently, an alternative method was demonstrated to achieve high efficiencies (13%) comparable to state‐of‐the‐art BC devices. This two‐step‐coating method, known as “sequential processing,” (SqP) involves sequential deposition of the donor and then the acceptor from two orthogonal solvents. However, the requirement of orthogonal solvents to process the donor and acceptor constrains the choice of materials and processing solvents. In this paper, an improved version of SqP method without the need for using orthogonal solvents is reported. The success is based on donor polymers with strong temperature‐dependent aggregation properties whose solution can be processed at a high temperature, but the resulting film becomes completely insoluble at room temperature, which allows for the processing of overlying acceptors from a wide range of nonorthogonal solvents. With this approach, efficient SqP OPVs is demonstrated based on a range of donor/acceptor materials and processing solvents, and, in every single case, SqP OPVs can outperform their BC counterparts. The results broaden the solvent choices and open a much larger window to optimize the processing parameters of SqP method.     

Inverted‐Colloidal‐Crystal Hydrogel Matrices as Three‐Dimensional Cell Scaffolds

Successful engineering of functional tissues requires the development of three‐dimensional (3D) scaffolds that can provide an optimum microenvironment for tissue growth and regeneration. A new class of 3D scaffolds with a high degree of organization and unique topography is fabricated from polyacrylamide hydrogel. The hydrogel matrix is molded by inverted colloidal crystals made from 104 μm poly(methyl methacrylate) spheres. The topography of the scaffold can be described as hexagonally packed 97 μm spherical cavities interconnected by a network of channels. The scale of the long‐range ordering of the cavities exceeds several millimeters. In contrast to analogous material in the bulk, hydrogel shaped as an inverted opal exhibits much higher swelling ratios; its swelling kinetics is an order of magnitude faster as well. The engineered scaffold possesses desirable mechanical and optical properties that can facilitate tissue regeneration while allowing for continuous high‐resolution optical monitoring of cell proliferation and cell–cell interaction within the scaffold. The scaffold biocompatibility as well as cellular growth and infiltration within the scaffold were observed for two distinct human cell lines which were seeded on the scaffold and were tracked microscopically up to a depth of 250 μm within the scaffold for a duration of up to five weeks. Ease of production, a unique 3D structure, biocompatibility, and optical transparency make this new type of hydrogel scaffold suitable for most challenging tasks in tissue engineering.     

Dimer‐Based Three‐Dimensional Photonic Crystals

The self‐assembly of polystyrene dimer‐ and spherocylinder‐shaped colloids is achieved via controlled drying on glass and silicon substrates. 3D monoclinic colloidal crystal structures are determined from scanning electron microscopy images of sections prepared using focused ion‐beam (FIB) milling. Full photonic bandgaps between the eighth and ninth bands are found for a systematic range of colloidal dimer shapes explored with respect to the degree of constituent lobe fusion and radius ratio. The pseudogap between bands 2 and 3 for spherocylinder‐based monoclinic crystals is also probed using normal incidence reflection spectroscopy.     

Hydrothermal Growth of Manganese Dioxide into Three‐Dimensional Hierarchical Nanoarchitectures

Novel three‐dimensional (3D) hierarchical nanoarchitectures of ?‐MnO₂ have been synthesized by a simple chemical route without the addition of any surfactants or organic templates. The self‐organized crystals consist of a major ?‐MnO₂ dipyramidal single crystal axis and six secondary branches, which are arrays of single‐crystal ?‐MnO₂ nanobelts. The growth directions of the nanobelts are perpendicular to the central dipyramidal axis, which shows sixfold symmetry. The shape of the ?‐MnO₂ assembly can be controlled by the reaction temperature. The morphology of ?‐MnO₂ changes from a six‐branched star‐like shape to a hexagonal dipyramidal morphology when the temperature is increased from 160 to 180 °C. A possible growth mechanism is proposed. The synthesized ?‐MnO₂ shows both semiconducting and magnetic properties. These materials exhibit ferromagnetic behavior below 25 K and paramagnetic behavior above 25 K. The ?‐MnO₂ system may have potential applications in areas such as fabrication of nanoscale spintronic materials, catalysis, and sensors.     

Ambient Layer‐by‐Layer ZnO Assembly for Highly Efficient Polymer Bulk Heterojunction Solar Cells

The use of metal oxide interlayers in polymer solar cells has great potential because metal oxides are abundant, thermally stable, and can be used in flexible devices. Here, a layer‐by‐layer (LbL) protocol is reported as a facile, room‐temperature, solution‐processed method to prepare electron transport layers from commercial ZnO nanoparticles and polyacrylic acid (PAA) with a controlled and tunable porous structure, which provides large interfacial contacts with the active layer. Applying the LbL approach to bulk heterojunction polymer solar cells with an optimized ZnO layer thickness of ≈25 nm yields solar cell power‐conversion efficiencies (PCEs) of ≈6%, exceeding the efficiency of amorphous ZnO interlayers formed by conventional sputtering methods. Interestingly, annealing the ZnO/PAA interlayers in nitrogen and air environments in the range of 60–300 °C reduces the device PCEs by almost 20% to 50%, indicating the importance of conformational changes inherent to the PAA polymer in the LbL‐deposited films to solar cell performance. This protocol suggests a new fabrication method for solution‐processed polymer solar cell devices that does not require postprocessing thermal annealing treatments and that is applicable to flexible devices printed on plastic substrates.     

Large‐Scale Fabrication of Three‐Dimensional Surface Patterns Using Template‐Defined Electrochemical Deposition

A new strategy to achieve large‐scale, three‐dimensional (3D) micro‐ and nanostructured surface patterns through selective electrochemical growth on monolayer colloidal crystal (MCC) templates is reported. This method can effectively create large‐area (>1 cm²), 3D surface patterns with well‐defined structures in a cost‐effective and time‐saving manner (<30 min). A variety of 3D surface patterns, including semishells, Janus particles, microcups, and mushroom‐like clusters, is generated. Most importantly, our method can be used to prepare surface patterns with prescribed compositions, such as metals, metal oxides, organic materials, or composites (e.g., metal/metal oxide, metal/polymer). The 3D surface patterns produced by our method can be valuable in a wide range of applications, such as biosensing, data storage, and plasmonics. In a proof‐of‐concept study, we investigated, both experimentally and theoretically, the surface‐enhanced Raman scattering (SERS) performance of the fabricated silver 3D semishell arrays.