Evan Williams - Academia.edu (original) (raw)

Papers by Evan Williams

Research paper thumbnail of Solution processable nanocomposites based on silsesquioxane cores for use in organic light emitting diodes (OLEDs)

MRS Proceedings, 2004

ABSTRACTA new family of materials that synergistically combine the attributes of both organic and... more ABSTRACTA new family of materials that synergistically combine the attributes of both organic and inorganic properties for use in organic light emitting diodes (OLEDs) is presented. The hybrid materials are based on 3-D inorganic cores of octavinylsilsesquioxanes (OVS). The resultant materials have high Tg's (120–210°C), are formed from minimal step/high yield reactions and readily available starting reagents, are monodisperse (PDI<1.1), can be highly purified via common chroma-tographic techniques, and form defect-free amorphous films via spin-dip coating. For example TPD is known for its good hole injection/transport properties in OLED applications but suffers from a low Tg (65°C). TPD-OVS hybrid material has a Tg of 142°C while maintaining similar injection/transport properties to TPD. Photoluminescence analysis of the hybrid thin film reveals: 1) a 30 nm blue shift versus their dilute solution counterparts; and 2) 5 hour annealing cycles to within 10°C of their Tg show no...

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

Thin Solid Films, 2003

We have developed a p-type, crystalline Si-based solar cell using hot-wire chemical vapor deposit... more 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 mm 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 ntype emitter was formed at 220 8C by depositing 50 A a-Si:H and then a 100 A mc-Si n-layer. The total deposition time to formt he emitter was less than 1 min. The top contact of the device is a lithograph defined and isolated 1=1 cm and 780 A indium 2t in oxides (ITO) with metal fingers on top. Our best solar cell conversion efficiency is 13.3% with V of 0.58 V, FF of 0.773, oc and J of 29.86 mA cm under one-sun condition. Quantum efficiency (QE) measurement on this solar cell shows over 90% y2 sc 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 needs to be increased. sc Better emitter and light trapping will be developed in future work. The cell shows no degradation after 1000 h of standard light soaking.

Research paper thumbnail of A benzothiadiazole end capped donor–acceptor based small molecule for organic electronics

Physical Chemistry Chemical Physics, 2013

All the solvents, catalysts and reagents were purchased from Sigma-Aldrich, Strem, Acros and used... more All the solvents, catalysts and reagents were purchased from Sigma-Aldrich, Strem, Acros and used without further purification. Most of the reactions were conducted using Schlenk techniques in an argon or nitrogen atmosphere with anhydrous solvents. Compounds 1, 2 and 3 were synthesized according to earlier reported procedures. Characterization 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy measurements were conducted on a Bruker DPX 300 MHz or 400 MHz spectrometer using solvent CDCl 3. The chemical shifts were recorded in ppm using TMS as an internal standard. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were obtained on a Bruker Autoflex TOF/TOF

Research paper thumbnail of Solution processable high band gap hosts based on carbazole functionalized cyclic phosphazene cores for application in organic light‐emitting diodes

Journal of Polymer Science Part B: Polymer Physics, 2011

A new class of solution processable dendrimers based on cyclic phosphazene (CP) cores have been p... more A new class of solution processable dendrimers based on cyclic phosphazene (CP) cores have been prepared and used as host materials for blue and green organic light emitting diodes (OLEDs). The dendrimers are prepared in high yield from minimal step reactions, are soluble in common solvents for solution processing, are amorphous, and have excellent thermal properties necessary for application in OLEDs. OLED efficiencies of 10.3 cd/A (4.2 lm/W) and 35.3 cd/A (33.5 lm/W) were achieved using commercially available FIrpic and Ir(mppy) 3 as blue and green phosphorescent emitters, respectively. These efficiencies were 2Â higher than control devices prepared using poly(N-vinylcarbazole), the most commonly used host material in solution processed phosphorescent OLEDs. V

Research paper thumbnail of Heck coupling of haloaromatics with octavinylsilsesquioxane: solution processable nanocomposites for application in electroluminescent devices

Chemical Communications, 2005

A new solution processable nanocomposite material has been prepared via the Heck coupling of octa... more A new solution processable nanocomposite material has been prepared via the Heck coupling of octavinylsilsesquioxane with a selected bromoaromatic hole transport compound. Resultant electroluminescent devices show an 18% improvement in external quantum efficiencies over their small molecule analogues.

Research paper thumbnail of Water-based nanoparticulate solar cells using a diketopyrrolopyrrole donor polymer

Physical Chemistry Chemical Physics, 2014

Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active... more Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active layers have been prepared from a 1 : 2 blend of (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) (PDPP-TNT) and the fullerene acceptor, ([6,6]-phenyl C71-butyric acid methyl ester) (PC70BM). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to investigate the morphology of the active layers of the two approaches. Mild thermal treatment of the NP film is required to promote initial joining of the NPs in order for the devices to function, however the NP structure is retained. Consequently, whereas gross phase segregation of the active layer occurs in the BHJ device spin cast from chloroform, the nanoparticulate approach retains control of the material domain sizes on the length scale of exciton diffusion in the materials. As a result, NP devices are found to generate more than twice the current density of BHJ devices and have a substantially greater overall efficiency. The use of aqueous nanoparticulate dispersions offers a promising approach to control the donor acceptor morphology on the nanoscale with the benefit of environmentally-friendly, solution-based fabrication.

Research paper thumbnail of Water-based nanoparticulate solar cells using a diketopyrrolopyrrole donor polymer

Physical Chemistry Chemical Physics, 2014

Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active... more Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active layers have been prepared from a 1 : 2 blend of (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) (PDPP-TNT) and the fullerene acceptor, ([6,6]-phenyl C71-butyric acid methyl ester) (PC70BM). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to investigate the morphology of the active layers of the two approaches. Mild thermal treatment of the NP film is required to promote initial joining of the NPs in order for the devices to function, however the NP structure is retained. Consequently, whereas gross phase segregation of the active layer occurs in the BHJ device spin cast from chloroform, the nanoparticulate approach retains control of the material domain sizes on the length scale of exciton diffusion in the materials. As a result, NP devices are found to generate more than twice the current density of BHJ devices and have a substantially greater overall efficiency. The use of aqueous nanoparticulate dispersions offers a promising approach to control the donor acceptor morphology on the nanoscale with the benefit of environmentally-friendly, solution-based fabrication.

Research paper thumbnail of Solution processable nanocomposites based on silsesquioxane cores for use in organic light emitting diodes (OLEDs)

MRS Proceedings, 2004

ABSTRACTA new family of materials that synergistically combine the attributes of both organic and... more ABSTRACTA new family of materials that synergistically combine the attributes of both organic and inorganic properties for use in organic light emitting diodes (OLEDs) is presented. The hybrid materials are based on 3-D inorganic cores of octavinylsilsesquioxanes (OVS). The resultant materials have high Tg's (120–210°C), are formed from minimal step/high yield reactions and readily available starting reagents, are monodisperse (PDI<1.1), can be highly purified via common chroma-tographic techniques, and form defect-free amorphous films via spin-dip coating. For example TPD is known for its good hole injection/transport properties in OLED applications but suffers from a low Tg (65°C). TPD-OVS hybrid material has a Tg of 142°C while maintaining similar injection/transport properties to TPD. Photoluminescence analysis of the hybrid thin film reveals: 1) a 30 nm blue shift versus their dilute solution counterparts; and 2) 5 hour annealing cycles to within 10°C of their Tg show no...

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

Thin Solid Films, 2003

We have developed a p-type, crystalline Si-based solar cell using hot-wire chemical vapor deposit... more 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 mm 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 ntype emitter was formed at 220 8C by depositing 50 A a-Si:H and then a 100 A mc-Si n-layer. The total deposition time to formt he emitter was less than 1 min. The top contact of the device is a lithograph defined and isolated 1=1 cm and 780 A indium 2t in oxides (ITO) with metal fingers on top. Our best solar cell conversion efficiency is 13.3% with V of 0.58 V, FF of 0.773, oc and J of 29.86 mA cm under one-sun condition. Quantum efficiency (QE) measurement on this solar cell shows over 90% y2 sc 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 needs to be increased. sc Better emitter and light trapping will be developed in future work. The cell shows no degradation after 1000 h of standard light soaking.

Research paper thumbnail of A benzothiadiazole end capped donor–acceptor based small molecule for organic electronics

Physical Chemistry Chemical Physics, 2013

All the solvents, catalysts and reagents were purchased from Sigma-Aldrich, Strem, Acros and used... more All the solvents, catalysts and reagents were purchased from Sigma-Aldrich, Strem, Acros and used without further purification. Most of the reactions were conducted using Schlenk techniques in an argon or nitrogen atmosphere with anhydrous solvents. Compounds 1, 2 and 3 were synthesized according to earlier reported procedures. Characterization 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy measurements were conducted on a Bruker DPX 300 MHz or 400 MHz spectrometer using solvent CDCl 3. The chemical shifts were recorded in ppm using TMS as an internal standard. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were obtained on a Bruker Autoflex TOF/TOF

Research paper thumbnail of Solution processable high band gap hosts based on carbazole functionalized cyclic phosphazene cores for application in organic light‐emitting diodes

Journal of Polymer Science Part B: Polymer Physics, 2011

A new class of solution processable dendrimers based on cyclic phosphazene (CP) cores have been p... more A new class of solution processable dendrimers based on cyclic phosphazene (CP) cores have been prepared and used as host materials for blue and green organic light emitting diodes (OLEDs). The dendrimers are prepared in high yield from minimal step reactions, are soluble in common solvents for solution processing, are amorphous, and have excellent thermal properties necessary for application in OLEDs. OLED efficiencies of 10.3 cd/A (4.2 lm/W) and 35.3 cd/A (33.5 lm/W) were achieved using commercially available FIrpic and Ir(mppy) 3 as blue and green phosphorescent emitters, respectively. These efficiencies were 2Â higher than control devices prepared using poly(N-vinylcarbazole), the most commonly used host material in solution processed phosphorescent OLEDs. V

Research paper thumbnail of Heck coupling of haloaromatics with octavinylsilsesquioxane: solution processable nanocomposites for application in electroluminescent devices

Chemical Communications, 2005

A new solution processable nanocomposite material has been prepared via the Heck coupling of octa... more A new solution processable nanocomposite material has been prepared via the Heck coupling of octavinylsilsesquioxane with a selected bromoaromatic hole transport compound. Resultant electroluminescent devices show an 18% improvement in external quantum efficiencies over their small molecule analogues.

Research paper thumbnail of Water-based nanoparticulate solar cells using a diketopyrrolopyrrole donor polymer

Physical Chemistry Chemical Physics, 2014

Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active... more Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active layers have been prepared from a 1 : 2 blend of (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) (PDPP-TNT) and the fullerene acceptor, ([6,6]-phenyl C71-butyric acid methyl ester) (PC70BM). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to investigate the morphology of the active layers of the two approaches. Mild thermal treatment of the NP film is required to promote initial joining of the NPs in order for the devices to function, however the NP structure is retained. Consequently, whereas gross phase segregation of the active layer occurs in the BHJ device spin cast from chloroform, the nanoparticulate approach retains control of the material domain sizes on the length scale of exciton diffusion in the materials. As a result, NP devices are found to generate more than twice the current density of BHJ devices and have a substantially greater overall efficiency. The use of aqueous nanoparticulate dispersions offers a promising approach to control the donor acceptor morphology on the nanoscale with the benefit of environmentally-friendly, solution-based fabrication.

Research paper thumbnail of Water-based nanoparticulate solar cells using a diketopyrrolopyrrole donor polymer

Physical Chemistry Chemical Physics, 2014

Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active... more Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active layers have been prepared from a 1 : 2 blend of (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) (PDPP-TNT) and the fullerene acceptor, ([6,6]-phenyl C71-butyric acid methyl ester) (PC70BM). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to investigate the morphology of the active layers of the two approaches. Mild thermal treatment of the NP film is required to promote initial joining of the NPs in order for the devices to function, however the NP structure is retained. Consequently, whereas gross phase segregation of the active layer occurs in the BHJ device spin cast from chloroform, the nanoparticulate approach retains control of the material domain sizes on the length scale of exciton diffusion in the materials. As a result, NP devices are found to generate more than twice the current density of BHJ devices and have a substantially greater overall efficiency. The use of aqueous nanoparticulate dispersions offers a promising approach to control the donor acceptor morphology on the nanoscale with the benefit of environmentally-friendly, solution-based fabrication.