Zachariah A. Page

Fulleropyrrolidine interlayers: Tailoring electrodes to raise organic solar cell efficiency

Layering on solar cell power and stability Solar cells made from carbon-based polymers are helpfully flexible. However, theres been a frustrating tradeoff between cell stability and efficiency when converting solar power to electrical power. Page et al. offer a strategy to partially resolve this dilemma by inserting a layer of polar organic compound (a fullerene derivative) between the cathode (the positive pole in the circuit) and the rest of the cell. Aluminum is an efficient cathode material but is prone to oxidative degradation. The easily applied polar layer enables the use of more stable metals, such as silver and copper, for the cathode, while counteracting their tendencies to diminish power conversion efficiency. Science, this issue p. 441 A polar fullerene derivative layered on the cathode of a polymer solar cell fosters simultaneous stability and efficiency. A major challenge in organic solar cell design is the trade-off between oxidative stability and work function of the metal cathode. We found that in single-junction polymer solar cells, this problem can be surmounted by solution-based incorporation of fulleropyrrolidines with amine (C60-N) or zwitterionic (C60-SB) substituents as cathode-independent buffer layers. Specifically, a thin layer of C60-N reduced the effective work function of Ag, Cu, and Au electrodes to 3.65 electron volts. Power conversion efficiency values exceeding 8.5% were obtained for organic photovoltaics independent of the cathode selection (Al, Ag, Cu, or Au). Such high efficiencies did not require precise control over interlayer thickness, as devices prepared with C60-N and C60-SB layers ranging from 5 to 55 nanometers performed with high efficiency.

Hierarchical Helical Assembly of Conjugated Poly(3-hexylthiophene)-block poly(3-triethylene glycol thiophene) Diblock Copolymers

We report on the solution-state assembly of all-conjugated polythiophene diblock copolymers containing nonpolar (hexyl) and polar (triethylene glycol) side chains. The polar substituents provide a large contrast in solubility, enabling formation of stably suspended crystalline fibrils even under very poor solvent conditions for the poly(3-hexylthiophene) block. For appropriate block ratios, complexation of the triethylene glycol side chains with added potassium ions drives the formation of helical nanowires that further bundle into superhelical structures.

Conjugated Polymeric Zwitterions as Efficient Interlayers in Organic Solar Cells

Conjugated polymeric zwitterions, when utilized as interlayer materials in bulk heterojunction organic solar cells, lead to significantly enhanced power conversion efficiencies. The electrostatic model of self-aligning dipolar side groups in the vicinity of the metal surface rationalizes the effects of reduced cathode work function, a key factor behind the observed enhanced efficiency.

Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency.

Three conjugated polymer zwitterions (CPZs), containing thiophene-, diketopyrrolopyrrole- (DPP), and naphthalene diimide (NDI) backbones, were synthesized with pendant zwitterions, specifically sulfobetaine groups. Diboronate-ester-functionalized bithiophene and benzothiadiazole monomers were copolymerized with zwitterion-substituted dibromothiophene, DPP, and NDI monomers by A2 + B2 Suzuki polymerization. The CPZs were incorporated into polymer solar cells (PSCs) as interlayers between the photoactive layer and Ag cathode. The thiophene-based CPZs gave power conversion efficiencies (PCEs) of about 5%, while the narrow-energy-gap DPP- and NDI-based CPZs performed exceptionally well, giving PCEs of 9.49% and 10.19%, respectively. The interlayer thickness had only a minor impact on the device performance for the DPP- and NDI-CPZs, a finding attributed to their electron-transport properties. Ultraviolet photoelectron and reflectance spectroscopies, combined with external quantum efficiency measurements, provided structure-property relationships that lend insight into the function of CPZ interlayers in PSCs. NDI-based CPZ interlayers provide some of the best performing organic solar cells reported to date, and prove useful in conjunction with high-performing polymer-active layers and stable, high-work-function, metal cathodes.

PEGylated silicon nanoparticles: synthesis and characterization.

Silicon nanoparticles were prepared and functionalized with alkene-terminated poly(ethylene oxide) to impart amphiphilic solution properties to the particles.

High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer.

Perovskite-containing tandem solar cells are attracting attention for their potential to achieve high efficiencies. We demonstrate a series connection of a ∼ 90 nm thick perovskite front subcell and a ∼ 100 nm thick polymer:fullerene blend back subcell that benefits from an efficient graded recombination layer containing a zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO3). This methodology eliminates the adverse effects of thermal annealing or chemical treatment that occurs during perovskite fabrication on polymer-based front subcells. The record tandem perovskite/polymer solar cell efficiency of 16.0%, with low hysteresis, is 75% greater than that of the corresponding ∼ 90 nm thick perovskite single-junction device and 65% greater than that of the polymer single-junction device. The high efficiency of this hybrid tandem device, achieved using only a ∼ 90 nm thick perovskite layer, provides an opportunity to substantially reduce the lead content in the device, while maintaining the high performance derived from perovskites.

A Versatile and Highly Selective Colorimetric Sensor for the Detection of Amines

The utility of Meldrums activated furan (MAF) for the colorimetric detection of sub ppm levels of amines in solution, on solid supports, and as vapors is reported. MAF is synthesized in one step from inexpensive and commercially available starting materials and exhibits high selectivity for primary and secondary amines in the presence of competing nucleophiles. The reaction between activated furans and amines results in a distinct color change, discernable by the naked eye. UV/Vis absorption spectroscopy was utilized to monitor reactions in solution and determine detection limits. Additionally, solutions of MAF were useful as stains for thin layer chromatography and for monitoring solid-phase synthesis of peptides and peptidomimetics. Finally, MAF was used to detect volatile amines released from fish samples, demonstrating potential for food spoilage applications.

Raising efficiency of organic solar cells with electrotropic additives

Incorporation of electrotropic additives with large molecular dipole moments into the bulk heterojunction layer of organic photovoltaic devices followed by electric field poling led to an increase of power conversion efficiency up to 7.97% from 7.17% for devices that did not utilize the additives and from 5.18% for devices with additives prior to poling. The improvement is due to more efficient extraction of photogenerated charge carriers, resulting in higher short circuit current density and fill factor. The observed effects are proposed to arise from a re-orientation of additive molecules in the external electric field, i.e., electrotropism, leading to a macroscopic alignment of their dipole moments. This leads to an increased built-in electrostatic potential difference in the device active layer post-poling. The dependence of device performance on the polarity of poling bias and reversibility of the effect are demonstrated, further supporting the proposed mechanism.

Rapid, facile synthesis of conjugated polymer zwitterions in ionic liquids

Ionic liquids (ILs) were utilized for the rapid air-stable Suzuki polymerization of polar zwitterionic thiophene monomers, precluding the need for volatile organic solvents, phosphine ligands and phase transfer catalysts typically used in conjugated polymer synthesis. Ten different ILs were examined, demonstrating the scope and limitations of their utility as solvents in this Suzuki polymerization. Imidazolium, pyridinium and pyrrolidinium ILs proved effective for these polymerizations, with top performance achieved using pyrrolidinium triflate, affording polymers with molecular weight values >30 kDa. The enhanced solubility of these conjugated polymer zwitterions (CPZs) in ILs, relative to organic solvents, led to higher molecular weight polymers than obtained using previously reported methods. CPZs synthesized in ILs proved effective as cathode modification layers in solar cells, giving rise to a power conversion efficiency (PCE) of 7.57% in bulk heterojunction devices.

Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications

A light-mediated methodology to grow patterned, emissive polymer brushes with micron feature resolution is reported and applied to organic light emitting diode (OLED) displays. Light is used for both initiator functionalization of indium tin oxide and subsequent atom transfer radical polymerization of methacrylate-based fluorescent and phosphorescent iridium monomers. The iridium centers play key roles in photocatalyzing and mediating polymer growth while also emitting light in the final OLED structure. The scope of the presented procedure enables the synthesis of a library of polymers with emissive colors spanning the visible spectrum where the dopant incorporation, position of brush growth, and brush thickness are readily controlled. The chain-ends of the polymer brushes remain intact, affording subsequent chain extension and formation of well-defined diblock architectures. This high level of structure and function control allows for the facile preparation of random ternary copolymers and red–green–blue arrays to yield white emission.