David Bilby

Effect of Polymer Aggregation on the Open Circuit Voltage in Organic Photovoltaic Cells: Aggregation-Induced Conjugated Polymer Gel and its Application for Preventing Open Circuit Voltage Drop

To investigate the structure-dependent aggregation behavior of conjugated polymers and the effect of aggregation on the device performance of conjugated polymer photovoltaic cells, new conjugated polymers (PVTT and CN-PVTT) having the same regioregularity but different intermolecular packing were prepared and characterized by means of UV-vis spectroscopy and atomic force microscopy (AFM). Photovoltaic devices were prepared with these polymers under different polymer-aggregate conditions. Polymer aggregation induced by thermal annealing increases the short circuit current but provides no advantage in the overall power conversion efficiency because of a decrease in the open circuit voltage. The device fabricated from a pre-aggregated polymer suspension, acquired from ultrasonic agitation of a conjugated polymer gel, showed enhanced performance because of better phase separation and reduced recombination between polymer/PCBM.

Recovering lost excitons in organic photovoltaics using a transparent dissociation layer

In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that “reflect” excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.

Design considerations for electrode buffer layer materials in polymer solar cells.

Electrode buffer layers in polymer-based photovoltaic devices enable highly efficient devices. In the absence of buffer layers, we show that diode rectification is lost in ITO/P3HT:PCBM/Ag (ITO = indium tin oxide; P3HT = poly(3-hexylthiophene); PCBM = phenyl C61-butyric acid methyl ester) devices due to nonselective charge injection through the percolated phase pathways of a bulk heterojunction active layer. Charge-selective injection, and thus rectification and device function, can be regained by placing thin, polymeric buffer layers that break the direct electrode-active layer contact. Additionally, we show that strong active layer-buffer layer interactions lead to unwanted vertical phase separation and a kinked current-voltage curve. Device function is regained, increasing power conversion efficiency from 3.6% to 7.2%, by placing a noninteracting layer between the buffer and active layer. These results guide the design and selection of future polymeric electrode buffer layers for efficient polymer solar cell devices.

A Novel Optical Ozone Sensor Based on Purely Organic Phosphor

An optical ozone sensor was developed based on the finding that a purely organic phosphor linearly loses its phosphorescence emission intensity in the presence of varying concentration of ozone gas and ozonated water. Compared to conventional conductance-based inorganic sensors, our novel sensory film has many advantages such as easy fabrication, low-cost, and portability. NMR data confirmed that phosphorescence drop is attributed to oxidation of the core triplet generating aldehyde group of the phosphor. We observed that linear correlation between phosphorescence and ozone concentration and it can detect ozone concentrations of 0.1 ppm that is the threshold concentration harmful to human tissue and respiratory organs. Like a litmus paper, this ozone sensor can be fabricated as a free-standing and disposable film.

High-Performing Thin-Film Transistors in Large Spherulites of Conjugated Polymer Formed by Epitaxial Growth on Removable Organic Crystalline Templates

Diketopyrrolopyrrole (DPP)-based conjugated polymer PDTDPPQT was synthesized and was used to perform epitaxial polymer crystal growth on removable 1,3,5-trichlorobenzene crystallite templates. A thin-film transistor (TFT) was successfully fabricated in well-grown large spherulites of PDTDPPQT. The charge carrier mobility along the radial direction of the spherulites was measured to be 5.46-12.04 cm(2) V(-1) s(-1), which is significantly higher than that in the direction perpendicular to the radial direction. The dynamic response of charge transport was also investigated by applying a pulsed bias to TFTs loaded with a resistor (∼20 MΩ). The charge-transport behaviors along the radial direction and perpendicular to the radial direction were investigated by static and dynamic experiments through a resistor-loaded (RL) inverter. The RL inverter made of PDTDPPQT-based TFT operates well, maintaining a fairly high switching voltage ratio (Vout(ON)/Vout(OFF)) at a relatively high frequency when the source-drain electrodes are aligned parallel to the radial direction.

Reduction of open circuit voltage loss in a polymer photovoltaic cell via interfacial molecular design: Insertion of a molecular spacer

Loss to the open circuit voltage (Voc) in organic photovoltaic cells is a critical bottleneck to achieving high power conversion efficiency. We demonstrate that the insertion of multilayers of a poly(phenylene ethynylene) spacer into the planar heterojunction between poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester incrementally escalates the Voc of a polymer solar cell from 0.43 V to 0.9 V. Through a combination of light intensity and temperature dependent measurements, we show that this control over the molecular structure local to the interface increases Voc by raising the polaron pair energy and by suppressing the dark-diode current.

Recent design strategies for polymer solar cell materials

Recent design tools for tuning the properties of conjugated polymers for efficient polymer solar cells (PSCs) are briefly reviewed. Based on limitations in the solar-to-electric energy conversion process imposed by material properties, recent research has focused on lowering the highest occupied molecular orbital (HOMO) level, reducing the bandgap, and controlling the molecular conformation and donor–acceptor phase separation. Additionally, the stability of PSCs can be improved through molecular design. Finally, a few less-conventional material design strategies for device improvement through polymer–polymer blends and triplet utilization are introduced. Molecular design has been an invaluable tool in controlling these material properties.

Design principles of chemiluminescence (CL) chemodosimeter for self-signaling detection: luminol protective approach

Chemiluminescence (CL) sensors can provide convenience and high sensitivity because they do not require an external excitation light source to produce a fluorescence signal. However, most CL based detection systems do not have a built-in self-signaling process, leading to inefficient and complex protocols due to the required multistep cascade reactions. Here, we develop a CL based sensory system with a built-in self-signaling feature by adapting the chemodosimeter concept. We found that a masking group incorporated to luminol efficiently suppresses the CL of luminol and that selective removal of the masking group by a target analyte can turn on the CL process, generating a sensitive fluorescence turn-on signal. Through systematic studies on newly devised TBS-luminol and TIPS-luminol, we optimized the molecular design parameters to achieve a highly sensitive and selective CL chemodosimeter. The optimized conditions rendered highly sensitive (Limit of Detection (LOD) = 18 nM) and selective fluoride sensing in aqueous environments. We anticipate that our new sensor system offers an efficient way to achieve highly sensitive, selective, and convenient CL turn-on detection of various important analytes.