K. Pakbaz

Semiconducting polymer diodes: Large size, low cost photodetectors with excellent visible‐ultraviolet sensitivity

Photodiodes fabricated from conjugated polymers exhibit excellent sensitivity to visible‐UV radiation. The photosensitivity increases with reverse bias voltage. The photoresponse of diodes fabricated from poly(3‐octyl thiophene) is relatively flat in the visible and near UV; for wavelengths shorter than 550 nm, the absolute sensitivity is greater than 0.3 A/W under reverse bias of 15 V, larger than that of commercial UV‐enhanced Si photodiodes. Photodiodes made from poly[2‐methoxy‐5‐(2’‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene], MEH‐PPV, sensitized with C60, show similar sensitivity. The ease of fabrication into large size, arbitrary shapes, and even onto flexible substrates makes the polymer photodiode a novel photodetector with potential for use in a wide range of applications.

Blue electroluminescent diodes utilizing blends of poly(p-phenylphenylene vinylene) in poly(9-vinylcarbazole)

Abstract We report blue light emission from diodes made from polymer blends composed of poly( p -phenylphenylene vinylene) (PPPV) in a hole-transporting polymer, poly(9-vinylcarbazole) (PVK). The soluble PPPV and PVK allow fabrication of light-emitting diodes (LEDs) by spin-casting the electroluminescent polymer blend from solution at room temperature with no subsequent processing or heat treatment required. The initial devices utilized calcium as the electron-injecting (rectifying) contact on the front surface of a PPPV/PVK film spin-cast onto a glass substrate partially coated with a layer of indium/tin oxide (ITO) as the hole-injecting contact. The LEDs turn on at ∼30 V and have a peak emission wavelength in the blue at 495 nm (at room temperature). The quantum efficiency is measured as a function of PPPV content in the blend; the maximum efficiency is approximately 0.16% photons/electron at a concentration of only 2% PPPV in PVK.

Improved efficiency in green polymer light-emitting diodes with air-stable electrodes

By dispersing an electron transporting molecular dopant into the active semiconducting luminescent polymer, we have achieved improved efficiencies for green light-emitting diodes (LEDs). These green emitting LEDs were fabricated by adding an electron transporting molecular dopant, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (PBD), into the semiconducting luminescent polymer as the emitting layer in the polymer LEDs. The devices used poly(2-cholestanoxy-5-thexyldimethylsilyl-l,4-phenylene vinylene) (CS-PPV), a new soluble green light emitter, as the semiconducting luminescent polymer and either aluminum or indium as the electron injection electrodes. Quantum efficiencies of LEDs with the electron transporting molecular additive in the luminescent polymer and an Al electrode are about 0.3% photons per electron, better by a factor of 18 than similar devices made without the addition of the electron transport molecular dopant; quantum efficiencies of similar LEDs fabricated with an In electrode are 0.23% photons per electron, better by a factor of 16 than devices without the electron transport molecular additive.

Yellow luminescent diodes utilizing poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene)

We report visible yellow light emission from diodes made from a new derivative of poly(p-phenylene vinylene). The soluble di-cholestanol derivative, poly(2,5-bis(cholestanoxy)-l, 4-phenylene vinylene), allows fabrication of the light emitting diodes by spinning the electroluminescent polymer film from solution prepared at room temperature with no subsequent processing or heat treatment required. These initial devices turn on at >8 V and have a peak emission wavelength of 570 nm at room temperature with quantum efficiencies of approximately 0.3% photons per electron.

Optocoupler made from semiconducting polymers

Optocouplers (optoisolators) were fabricated using semiconducting polymers. The input unit is a polymer light emitting diode with an external quantum efficiency of ∼1% photons/electron. The output unit is a polymer photodiode with a quantum yield of ∼35% electrons/photon at 590 nm. Both units can be operated at bias voltages sufficiently low to be compatible with TTL and complementary metal-oxide semiconductor logic circuits. Since the transfer characteristic is nearly linear, the polymer optocoupler can be used in analogue circuits as well. The current transfer ratio reaches 2 × 10−3 under-10V reverse bias, comparable to that of commercial inorganic optocouplers.

Recent progress in conducting polymers: Opportunities for science and opportunities for technology

Abstract The remarkable electrical and mechanical properties of conducting polymers formed the initial (and continuing) basis of interest in the field. Recent results have demonstrated progress in the development of conducting polymer blends as composite materials without a percolation threshold.

Photonic devices made with semiconducting conjugated polymers: New developments

Abstract We present recent progress on polymer devices developed at UCSB. We have fabricated polymer devices with dual functions: light emission and photodetection. The same Ca/MEH-PPV/ITO devices can be used as light emitting devices under forward bias with external quantum efficiency of ∼1% photons/electron, and as photodiodes under reverse bias with photosensitivity of −90mA/Watt and quantum yield of −20% electrons/photon (∼ 1μW/cm 2 , 430nm). We have also fabricated polymer photodiodes with P3OT and MEH-PPV:C 60 composites with excellent visible-UV sensitivity. For wavelengths shorter than 540 nm, the sensitivity is over 0.3A/Watt under reverse bias of 15 V, greater even than that of commercial UV-enhanced Si photodiodes. By matching MEH-PPV LEDs with P3OT photodiodes, we have demonstrated polymer optocouples with current transfer rate of ∼2×10 −3 , comparable to that of commercial optocouplers made with inorganic semiconductors.

Photosensitivity of MEH-PPV Sandwich Devices and Its Implication to Polymer Electronic Structure

Abstract Thin film devices made with Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, are known to be efficient light emitting diodes. The same devices, under reverse bias, exhibit excellent photosensitivity. Large photovoltages were observed in devices with selected metals as the cathode and anode electrodes. The open-circuit voltage, Voc, varies with the work functions of the electrodes; in most cases, Voc approaches the work function difference between the two electrodes at high excitation levels. The photosensitivity increases significantly under reverse bias. At −10 V, the sensitivity in Ca/MEH-PPV devices reaches 45 mA/Watt at 20 mW/cm2 (quantum yield of ∼ 13% electrons/photon). The action spectra of the photovoltaic signal at zero bias, and the photocurrent at 2V forward bias follow the absorption spectrum at the band edge. Thus, mobile carriers can be generated either by charge injection at the contacts or by interband photoexsitation, as observed in inorganic semiconductors.

Polarized electroabsorption spectroscopy of MEH-PPV oriented by gel processing in polyethylene: polarization anisotropy, the off-axis dipole moment and excited state delocalization

Summary form only given. We present the polarized electric field modulation spectra of dilute blends of MEH-PPV oriented in ultra-high molecular weight polyethylene, PE. The polarized electroabsorption spectra of the oriented blend demonstratesa field-induced nonlinear refractive index anisotropy in excess of 150:1, with preferred polarization parallel to the draw axis. Because of the orientation, the enhanced order and the dilution of the MEH-PPV conjugated polymer in the polyethylene matrix, the MEH-PPV/PE blend is an ideal system in which to investigate the intrinsic linear and nonlinear optical properties of oriented conjugated polymer chains. The absorption is directly proportional to the imaginary part of X/sup (10)/(w), and the electroabsorption is directly proportional to the imaginary part of X/sup (3)/(w, 0, 0) of the ordered MEH-PPV /spl pi/-electron system. The optical data obtained from the oriented blends is therefore directly relevant to existing theoretical models which do not account for disorder. Using a model for the off-axis transition dipole moment of the conjugated polymer, which is based on the measured off-axis transition dipole moment of transstilbene and the geometric structure of MEH-PPV, we demonstrate that the observed anisotropy of 150:1 in the field-induced absorption requires that the instantaneous excited state wavefunctions are delocalized over a minimum of 50 unit cells (400 /spl Aring/).

Photonic devices made from conducting polymers: new developments

Summary form only given. We present recent progress on polymer devices developed at UCSB. We have fabricated polymer devices with dual functions: light emission and photodetection. The same Ca/MEH-PPV/ITO devices can be used as light emitting devices under forward bias with an external quantum efficiency of -I% photons/electron, and as photodiodes under reverse bias with a dc sensitivity of 90mA/Watt and quantum yield of -20% electrons/photon (~/spl mu/W/cm/sup 2/, 430nm). We have also fabricated P3OT photodiodes with excellent visible-UV sensitivity. For wavelengths shorter than 550 nm, the sensitivity is over 0.3A/Watt under reverse bias of 15 V, larger than that of commercial UV-enhanced Si photodiodes. By matching MEH-PPV LEDs with P30T photodiodes, we have demonstrated polymer optocouplers with a current transfer ratio of over 10/sup -3/, comparable to that of commercial optocouplers made with inorganic semiconductors.