Isomeric Poly(benzophenone)s: synthesis of Highly Crystalline Poly(4,4'-benzophenone) and Amorphous Poly(2,5-benzophenone), a Soluble Poly(p-phenylene) Derivative

Nickel-catalyzed polymerization which employs the coupling of isomeric dichlorobenzophenones is described. The polymerization utilizes inexpensive readily available monomers, 4,4-dichlorobenzophenone (4,4'-DCBP) and 2,5-dichlorobenzophenone (2,5-DCBP). The poly(4,4'-benzophenone) can be derivatized to be soluble during the synthesis by the use of a ketimine precursor that is subsequently hydrolyzed to give the target material. The polymerization of 2,5-dichlorobenzophenone yields a soluble derivative of poly(p-phenylene). The resulting polymers were characterized to confirm the composition, molar mass, and thermal properties. The Ni(O) catalyzed route proves to be facile and economically feasible and opens the way to a large variety of heterocyclic and phenyl-based homo- and copolymers

Minimizing interfacial losses in inverted organic solar cells comprising Al-doped ZnO

We demonstrated a 35% enhancement in the efficiency of inverted solar cells as a result of increased open-circuit voltage and fill factor by adsorbing an ultrathin layer of a ruthenium dye N719 on an aluminum-doped zinc oxide (ZnO-Al) electron collecting interfacial layer. The interface modification with N719 changes the charge injection levels as indicated by ultraviolet photoemission spectroscopy. The efficiency of inverted solar cells comprising a bulk heterojunction photo-active film of poly(3-hexylthiophene) and phenyl-C 61-butyric acid methyl ester has increased from ∼2.80% to 3.80% upon employing the dye modification of the electrode interface

Nonideal parasitic resistance effects in bulk heterojunction organic solar cells

A common assumption in both experimental measurements and device modeling of bulk heterojunction (BHJ) organic solar cells is that parasitic resistances are ideal. In other words, series resistance (Rsr) is near zero while shunt resistance (Rsh) approaches infinity. Relaxation of this assumption affects device performance differently depending on the chosen BHJ material system. Specifically, the impact of nonideal Rsr is controlled by the electric field dependence of the probability of charge transfer (CT) state dissociation (PCT). This is demonstrated by evaluating the experimental current density versus voltage response within the framework of a drift/diffusion model for two BHJ systems that strongly differ in PCT. Second, light intensity measurements of devices with nonideal Rsr and Rsh are shown to convolute the scaling of short-circuit current and open-circuit voltage with light intensity, which is a common technique to study BHJ device physics. Finally, we show the connection between the drift/diffusion and equivalent circuit model with regard to each model's treatment of CT state dissociation. In particular, the equivalent circuit model utilizes a light intensity dependent Rsh to describe this dissociation process and predicts a photocurrent under reverse bias that exceeds the photocurrent permitted by light absorption

Electrophotonic enhancement of bulk heterojunction organic solar cells through photonic crystal photoactive layer

We present one- (1D) and two-dimensional (2D) periodic nanostructured designs for organic photovoltaics where a photonic crystal is formed between blended poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and nanocrystalline zinc oxide. Absorption enhancements over the full absorption range of P3HT:PCBM of 20% (one polarization) and 14% are shown for the 1D and 2D structures, respectively. These improvements result in part from band edge excitation of quasiguided modes. The geometries are also shown to create excitons 26% (1D) and 11% (2D) closer to P3HT:PCBM exit interfaces indicating further photovoltaic improvement

Absorption and quasiguided mode analysis of organic solar cells with photonic crystal photoactive layers

We analyze optical absorption enhancements and quasiguided mode properties of organic solar cells with highly ordered nanostructured photoactive layers comprised of the bulk heterojunction blend, poly-3- hexylthiophene/[6,6]-phenyl- C61-butyric acid methyl ester (P3HT:PCBM) and a low index of refraction conducting material (LICM). This photonic crystal geometry is capable of enhancing spectral absorption by ∼17% in part due to the excitation of quasiguided modes near the band edge of P3HT:PCBM. A nanostructure thickness between 200 nm and 300 nm is determined to be optimal, while the LICM must have an index of refraction ∼0.3 lower than P3HT:PCBM to produce absorption enhancements. Quasiguided modes that differ in lifetime by an order of magnitude are also identified and yield absorption that is concentrated in the P3HT:PCBM flash layer

Analyzing local exciton generation profiles as a means to extract transport lengths in organic solar cells

In this work, we determine the carrier-transport lengths of electrons and holes (Le,h) for bulk heterojunction (BHJ) organic solar cells using a method applicable to functional devices. By linking the local exciton generation profile [G (x)] in the photoactive layer to photocurrent losses, we are able to determine the onset of bimolecular recombination, which is the dominate loss process of free carrier transport. Even though many factors affect photocurrent generation, we single out bimolecular recombination by measuring the scaling of photocurrent with light intensity as a function of applied voltage. For the common BHJ system, annealed poly-3-hexylthiophene:[6,6]-phenyl- C61-butyric acid methyl ester (P3HT:PCBM), a minimum for Le in PCBM is found to be 340 nm while Lh is estimated to be 90 nm for P3HT. The relationship between G (x) and carrier transport is further exemplified by demonstrating a scaling exponent below that for traditional space-charge-limited photocurrent. Likewise, by incorporating a drift/diffusion model, an intuitive link between G (x) and charge transport is established where recombination is shown to occur in regions of the photoactive layer far from the electrode of the slowest carrier species. Finally, the consequences of Le,h on device design for operation under 1 Sun conditions are described

Minority carrier transport length of electrodeposited Cu2O in ZnO/Cu2O heterojunction solar cells

The minority carrier transport length is a critical parameter limiting the performance of inexpensive Cu2 O-ZnO photovoltaic devices. In this letter, this length is estimated to be ∼430 nm for electrochemically deposited Cu2 O by linking the cell's carrier generation profile with back and front incident photon-to-electron conversion efficiency measurements to a one-dimensional transport model. This critical length explains the losses typically presented by these devices and appears to correlate well with the microcrystalline film structure. The consequences of the magnitude of the length on device design with the aim of improving solar cell performance are described

Thiophene-based poly(arylene ether ketone)s: 2. Thermal and mechanical properties of amorphous systems using bis(p-fluorobenzoyl)aryl monomers

A series of high molar mass and controlled molar mass poly(aryl ether ketone)s were synthesized based on bis(p-fluoro-benzoyl)aryl monomers and 4,4'-isopropylidenediphenol. The central aromatic unit of the activated bishalide was varied to include 1,4 phenylene, 2,5 thiophene, and 1,3 phenylene to systematically change the exocyclic bond angle from 180˚ to 148˚ to 120˚, respectively. The thermal, dynamic mechanical and mechanical properties of the three polymers were determined for the controlled molar mass materials. The glass transition temperature of the controlled molar mass 2,5-thiophene based polymer was 147˚ C compared to 149˚ C for 1,3-phenylene based polymer and 162˚ C for the 1,4-phenylene based polymer. The thermal stability of poly(BFTh-BisA) was similar to that of poly(1,3-BFBBisA) and poly(1,4-BFB-BisA). Two sub T loss dispersions were measured for each of the three polymers. The pronounced y relaxation for poly(BFTh-BisA), poly(1,4- BFB-BisA), and poly(1,3-BFB-BisA), has an activation energy of 7.5, 9.0, and 10 kcal/mole, respectively. The activation energy of β dispersion for poly(BFTh-BisA) is 16 kcal/mole. The Young's modulus is 2.9 GPa, 3.1 GPa and 3.6 GPa for poly(1,4-BFB-BisA), poly(2,5-BFTh-BisA) and poly(1,3-BFB-BisA), respectively

Large area nanofabrication of butterfly wings three dimensional ultrastructures

The authors report a simple method for the artificial fabrication of the complex three-dimensional (3D) ultrastructures of butterfly wing scales. This method uses chemical vapor deposition, UV lithography, and chemical etching to create the ultrastructures over a large area surpassing previously used focused ion beam techniques that are limited to microscopic areas. Furthermore, this method shows flexibility to modify nanostructure types and can precisely control shapes and dimensions and periodicity. Fabricated 3D ultrastructures are also replicated using a nanoimprint method into soft polymer materials. Reflectivity measurements and simulations of the master and polymer replicas show selective UV reflection consistent with the length scales used in such butterfly-like nanostructures

Benzene at 1GHz. Magnetic field-induced fine structure

The deuterium NMR spectrum of benzene-d6 in a high field spectrometer (1 GHz protons) exhibits a magnetic field-induced deuterium quadrupolar splitting ??. The magnitude of ?? observed for the central resonance is smaller than that observed for the 13C satellite doublets ???. This difference, ?(??) = ??? ? ??, is due to unresolved fine structure contributions to the respective resonances. We determine the origins of and simulate this difference, and report pulse sequences that exploit the connectivity of the peaks in the 13C and 2H spectra to determine the relative signs of the indirect coupling, JCD, and ??. The positive sign found for ?? is consonant with the magnetic field biasing of an isolated benzene molecule—the magnetic energy of the aromatic ring is lowest for configurations where the C6 axis is normal to the field. In the neat liquid the magnitude of ?? is decreased by the pair correlations in this prototypical molecular liquid