Evan L. Williams

Organic light-emitting diodes having exclusive near-infrared electrophosphorescence

Near-infrared (NIR) emission is demonstrated from phosphorescent organic light-emitting diodes containing blends of polymeric host and heavy metal complex, iridium(III) bis(1-pyrenyl-isoquinolinato-N,C′) acetylacetonate. The devices exhibit exclusive NIR emission with a peak value at 720nm. Forward light output exceeds 100μW∕cm2, and the external quantum efficiency is nearly 0.1%. These values are shown to increase upon using a hole blocking layer in the device architecture.

Furan containing diketopyrrolopyrrole copolymers: synthesis, characterization, organic field effect transistor performance and photovoltaic properties

In this work, we report design, synthesis and characterization of solution processable low band gap polymer semiconductors, poly{3,6-difuran-2-yl-2,5-di(2- octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-phenylene} (PDPP-FPF), poly{3,6-difuran-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1, 4-dione-alt-naphthalene} (PDPP-FNF) and poly{3,6-difuran-2-yl-2,5-di(2- octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-anthracene} (PDPP-FAF) using the furan-containing 3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBF) building block. As DBF acts as an acceptor moiety, a series of donor-acceptor (D-A) copolymers can be generated when it is attached alternatively with phenylene, naphthalene or anthracene donor comonomer blocks. Optical and electrochemical characterization of thin films of these polymers reveals band gaps in the range of 1.55-1.64 eV. These polymers exhibit excellent hole mobility when used as the active layer in organic thin-film transistor (OTFT) devices. Among the series, the highest hole mobility of 0.11 cm 2 V -1 s -1 is achieved in bottom gate and top-contact OTFT devices using PDPP-FNF. When these polymers are used as a donor and [70]PCBM as the acceptor in organic photovoltaic (OPV) devices, power conversion efficiencies (PCE) of 2.5 and 2.6% are obtained for PDPP-FAF and PDPP-FNF polymers, respectively. Such mobility values in OTFTs and performance in OPV make furan-containing DBF a very promising block for designing new polymer semiconductors for a wide range of organic electronic applications.

Conducting polymer and hydrogenated amorphous silicon hybrid solar cells

An organic-inorganic hybrid solar cell with a p-i-n stack structure has been investigated. The p-layer was a spin coated film of PEDOT:PSS [poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate)]. The i-layer was hydrogenated amorphous silicon (a-Si:H), and the n-layer was microcrystalline silicon (μc-Si). The inorganic layers were deposited on top of the organic layer by the hot-wire chemical vapor deposition technique at 200°C. These hybrid devices exhibited open circuit voltages (VOC) as large as 0.88V and solar conversion efficiencies as large as 2.1%. Comparison of these devices with those incorporating a-SiC:H:B p-layers indicates that the organic layer is acting as an electrically ideal p-layer.

Thiophene–benzothiadiazole–thiophene (D–A–D) based polymers: effect of donor/acceptor moieties adjacent to D–A–D segment on photophysical and photovoltaic properties

New push-pull copolymers based on thiophene (donor) and benzothiadiazole (acceptor) units, poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co- thiophene] (PT3B1) and poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co-benzothiadiazole] (PT2B2), are designed and synthesized via Stille and Suzuki coupling routes respectively. Gel permeation chromatography shows the number average molecular weights are 31100 and 8400 g mol-1 for the two polymers, respectively. Both polymers have shown absorption throughout a wide range of the UV-vis region, from 300 to 650 nm. A significant red shift of the absorption edge is observed in thin films compared to solution of the copolymers; the optical band gap is in the range of 1.7 to 1.8 eV. Cyclic voltammetry indicates reversible oxidation and reduction processes with HOMO energy levels calculated to be in the range of 5.2 to 5.4 eV. Upon testing both materials for organic field-effect transistors (OFETs), PT3B1 showed a hole mobility of 6.1 × 10-4 cm2 V-1 s -1, while PT2B2 did not show any field effect transport. Both copolymers displayed a photovoltaic response when combined with a methanofullerene as an electron acceptor. The best performance was achieved when the copolymer PT3B1 was blended with [70]PCBM in a 1:4 ratio, exhibiting a short-circuit current of 7.27 mA cm-2, an open circuit voltage of 0.85 V, and a fill factor of 41% yielding a power conversion efficiency of 2.54% under simulated air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm-2). Similar devices utilizing PT2B2 in place of PT3B1 demonstrated reduced performance with a short-circuit current of 4.8 mA cm -2, an open circuit voltage of 0.73 V, and a fill factor of 30% resulting in a power conversion efficiency of roughly 1.06%.

Development of a hot-wire chemical vapor deposition n-type emitter on p-type crystalline Si-based solar cells

Abstract We have developed a p-type, crystalline Si-based solar cell using hot-wire chemical vapor deposition (HWCVD) n-type microcrystalline Si to form an n-p junction (emitter). The CVD process was rapid and a low substrate temperature was used. The p-type Czochralski (CZ) c-Si wafer has a thickness of 400 μm and has a thermally diffused Al back-field contact. Before forming the n-p junction, the front surface of the p-type c-Si was cleaned using a diluted HF solution to remove the native oxides. The n-type emitter was formed at 220 °C by depositing 50 A a-Si:H and then a 100 A μc-Si n-layer. The total deposition time to form the emitter was less than 1 min. The top contact of the device is a lithograph defined and isolated 1×1 cm 2 and 780 A indium tin oxides (ITO) with metal fingers on top. Our best solar cell conversion efficiency is 13.3% with V oc of 0.58 V, FF of 0.773, and J sc of 29.86 mA cm −2 under one-sun condition. Quantum efficiency (QE) measurement on this solar cell shows over 90% in the region between 540 and 780 nm, but poor response in the blue and deep red. We find that the ITO top contact that acts as an antireflection layer increases the QE in the middle region. To improve the device efficiency further, J sc needs to be increased. Better emitter and light trapping will be developed in future work. The cell shows no degradation after 1000 h of standard light soaking.

Nanoscale phase domain structure and associated device performance of organic solar cells based on a diketopyrrolopyrrole polymer

We investigate the blend morphology and performance of bulk heterojunction organic photovoltaic devices comprising the donor polymer, pDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1, 4-dione-alt-naphthalene}) and the fullerene acceptor, [70]PCBM ([6,6]-phenyl C71-butyric acid methyl ester). The blend morphology is heavily dependent upon the solvent system used in the fabrication of thin films. Thin films spin-coated from chloroform possess a cobblestone-like morphology, consisting of thick, round-shaped [70]PCBM-rich mounds separated by thin polymer-rich valleys. The size of the [70]PCBM domains is found to depend on the overall film thickness. Thin films spin-coated from a chloroform:dichlorobenzene mixed solvent system are smooth and consist of a network of pDPP-TNT nanofibers embedded in a [70]PCBM-rich matrix. Rinsing the films in hexane selectively removes [70]PCBM and allows for analysis of domain size and purity. It also provides a means for investigating exciton dissociation efficiency through relative photoluminescence yield measurements. Devices fabricated from chloroform solutions show much poorer performance than the devices fabricated from the mixed solvent system; this disparity in performance is seen to be more pronounced with increasing film thickness. The primary cause for the improved performance of devices fabricated from mixed solvents is attributed to the greater donor-acceptor interfacial area and resulting greater capacity for charge carrier generation.

Hybrid organic-inorganic photoconductive diode

A hybrid organic-inorganic diode has been fabricated by hot-wire chemical vapor deposition of low process temperature (<140 °C) hydrogenated amorphous silicon (a-Si:H) onto a spin-coated layer of poly(N-vinylcarbazole) (PVK). The hybrid device was found to be a photoconductive diode with a high photosensitivity of near 700, a high photogain of over 100, and a response time of 500 μs. The high photosensitivity of the a-Si:H intrinsic layer and the large barrier for carrier injection at the PVK/a-Si:H interface are responsible for the good performance.

Amorphous ruthenium nanoparticles for enhanced electrochemical water splitting

This paper demonstrates an optimized fabrication of amorphous Ru nanoparticles through annealing at various temperatures ranging from 150 to 700 °C, which are used as water oxidation catalyst for effective electrochemical water splitting under a low overpotential of less than 300 mV. The amorphous Ru nanoparticles with short-range ordered structure exhibit an optimal and stable electrocatalytic activity after annealing at 250 °C. Interestingly, a small quantity of such Ru nanoparticles in a thin film on fluorine-doped tin oxide glass is also effectively driven by a conventional crystalline silicon solar cell that has excellent capability for harvesting visible light. Remarkably, it achieves an overall solar-to-hydrogen efficiency of 11.3% in acidic electrolyte.

Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films

Embedding metal nanoparticles in the active layer of organic solar cells has been explored as a route for improving charge carrier generation, with localized field enhancement as a proposed mechanism. However, embedded metal nanoparticles can also act as charge recombination sites. To suppress such recombination, the metal nanoparticles are commonly coated with a thin insulating shell. At the same time, this insulating shell limits the extent that the localized enhanced electric field influences charge generation in the organic medium. It is presumed that there is an optimal thickness which maximizes field enhancement effects while suppressing recombination. Atomic Layer Deposition (ALD) was used to deposit Al2O3 layers of different thicknesses onto silver nanoparticles (Ag NPs), in a thin film of P3HT. Photoinduced absorption (PIA) spectroscopy was used to study the dependence of the photogenerated P3HT(+) polaron population on the Al2O3 thickness. The optimal thickness was found to be 3-5 nm. This knowledge can be further applied in the design of metal nanoparticle-enhanced solar cells.

Delayed onset of photochromism in molybdenum oxide films caused by photoinduced defect formation

Abstract We report the photochromic properties of amorphous MoO3 films deposited by dc sputtering with different O2 flow rates. The kinetics of film coloration under UV light irradiation is determined using optical transmission spectroscopy. Changes in the absorbance and refractive index were derived from the analysis of transmittance spectra. The absorbance spectra exhibited a growing broad peak centered around 830 nm, which was induced by the UV irradiation. In the early stages of irradiation, the absorbance of the films did not change but their refractive indices did change. This induction time was correlated with the O2 partial pressure during the film deposition, which was controlled by the O2 flow rate. The origins of this observation are discussed.