D. Braun

Visible light emission from semiconducting polymer diodes

We report visible light emission from Shottky diodes made from semiconducting polymers, confirming the discovery by the Cambridge group [Nature 347, 539 (1990)]. Our results demonstrate that light‐emitting diodes can be fabricated by casting the polymer film from solution with no subsequent processing or heat treatment required. Electrical characterization reveals diode behavior with rectification ratios greater than 104. We propose that tunneling of electrons from the recitifying metal contact into the gap states of the positive polaron majority carriers dominates current flow and provides the mechanism for light emission.

Semiconducting Polymer-Buckminsterfullerene Heterojunctions: Diodes, Photodiodes, and Photovoltaic Cells

The characterization of rectifying heterojunctions (diodes) fabricated from a semiconducting polymer, a soluble derivative of poly(phenylene‐vinylene), and buckminsterfullerene, C60, are reported. Rectification ratios in the current versus voltage characteristics exceed 104. When illuminated, the devices exhibit a large photoresponse as a result of photoinduced electron transfer across the heterojunction interface from the semiconducting polymer (donor) onto C60 (acceptor). The photodiode and photovoltaic responses are characterized. Photoinduced electron transfer across the donor‐accepted rectifying heterojunction offers potential for photodetector and for solar cell applications.

Electroluminescence and Electrical Transport in Poly(3-Octylthiophene) Diodes

We report visible light emission from diodes made from poly(3‐octylthiophene). Use of a soluble derivative of polythiophene allows fabrication of the light emitting diodes by casting the polymer film from solution with no subsequent processing or heat treatment required. The devices emit dim red‐orange light with relatively low external quantum efficiencies, below 2.5×10−5 photons per electron at room temperature. Electrical characterization reveals diode behavior with rectification ratios greater than 102. The temperature dependence indicates that tunneling phenomena dominate the charge injection.

Improved efficiency in semiconducting polymer light-emitting diodes

We report visible light emission from metal-polymer diodes made from semiconducting polymers, with indium-tin oxide as the “ohmic” contact, and a variety of metals as the barrier metal. Our results, which confirm the discovery by the Cambridge group [Nature347, 539 (1990)], demonstrate that light-emitting diodes can be fabricated by casting the polymer film on indium-tin oxide from solution with no subsequent polymer processing or heat treatment required. Electrical characterization reveals diode behavior with rectification ratios greater than 105 at sufficiently high voltages. Use of an electrode material with low work function leads to more than an order of magnitude improvement in the room-temperature efficiency of the devices. For example, the most efficient devices made with calcium as the rectifying contact display efficiencies of 0.01 photons per electron.

Metal-polymer Schottky barriers on cast films of soluble poly(3-alkylthiophenes)

Abstract We present the results of a study of strong rectification by metal-polymer (Schottky) diodes made by evaporating metal contacts onto films of a soluble semiconducting polymer cast from solution. Poly(3-hexylthiophene), a soluble alkyl derivative of poly(thiophene), and indium contacts from the Schottky diodes. Current-voltage characteristics exhibit rectification ratios in the range 100:1 to 1000:1; the forward current increases exponentially over several decades, whereas a relatively small leakage current flows under reversed bias. Capacitance-voltage data indicate nearly uniform dopant concentration over a depth of about 1500 A.

Light‐emitting diodes from partially conjugated poly(p‐phenylene vinylene)

We report the effect of conversion conditions on the device characteristics of poly(p‐phenylene vinylene) (PPV) light‐emitting diodes. Both electroluminescence and photoluminescence intensities decrease with increasing degree of conversion. Partial conjugation enhances the electroluminescence intensity and gives an efficiency (with Ca as electron‐injecting contact) as high as 0.75% photons per electron, about two orders of magnitude more efficient than from similar devices prepared from fully converted PPV. The results of constant current stress measurements suggest that the partially conjugated PPV diode is relatively stable at room temperature.

Nanosecond transient electroluminescence from polymer light‐emitting diodes

The transient electroluminescence from polymer light‐emitting diodes is reported. When the devices are mounted on a microstrip transmission line, the temporal response is limited by the electrode geometry, with rise and fall times below 50 ns. With low duty‐cycle pulses (0.5%) the electroluminescence intensity remains proportional to the current at values up to 10 A/cm2, two orders of magnitude greater than possible under direct current operation. Since the spectral blue‐shift observed at high current levels (with power dissipation above 1 W/cm2) indicates significant sample heating, still higher levels should be possible with proper thermal management.

Metal-polymer Schottky barriers on processible polymers

Abstract We present the results of a study of strong rectification by metal-polymer (Schottky) diodes made by evaporating metal contacts onto films of a soluble semiconducting polymer cast from solution. Poly(3-hexylthiophene), a soluble alkyl derivative of poly(thiophene), and indium contacts form the Schottky diodes. Current-voltage characteristics exhibit rectification ratios in the range 100:1 to 1000:1; the forward current increases exponentially over several decades, whereas a relatively small leakage current flows under reverse bias. Temperature dependence of the current-voltage characteristics differs from that of conventional Schottky diodes and suggests transport processes other than conventional thermionic emission.

Transient electroluminescence from polymer light emitting diodes

Abstract The transient electroluminescence from polymer light emitting diodes is reported. When the devices are mounted on a microstrip transmission line, the temporal response is limited by the electrode geometry, with rise and fall times below 50 ns. With low duty-cycle pulses (0.5%) the electroluminescence intensity remains proportional to the current at values up to 10 A/cm 2 , two orders of magnitude greater than possible under direct current operation. Since the spectral blue-shift observed at high current levels (with power dissipation above 1 W/cm 2 ) indicates significant sample heating, still higher levels should be possible with proper thermal management.

Observation of a photoinduced electron transfer from a conducting polymer (MEHPPV) onto C60

Abstract Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto Buckminsterfullerene, C 60 , is reported. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C 60 anion. Since the photoluminescence in the conducting polymer is quenched by interaction with C 60 , the data imply that charge transfer from the excited state occurs on a picosecond time scale.