David Braun

Bio: David Braun is an academic researcher from California Polytechnic State University. The author has contributed to research in topics: Electroluminescence & Light-emitting diode. The author has an hindex of 21, co-authored 35 publications receiving 4706 citations. Previous affiliations of David Braun include University of California & Philips.

Visible light emission from semiconducting polymer diodes

Abstract: 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

Abstract: 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

Abstract: 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.

Visible light emitting diodes fabricated from soluble semiconducting polymers

Abstract: Visible light LEDs are produced having a layer of conjugated polymer which is cast directly from solution or formed as a gel-processed admixture with a carrier polymer The LEDs can be formed so as to emit polarized light

Improved efficiency in semiconducting polymer light-emitting diodes

Abstract: 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.

Polymer photovoltaic cells : enhanced efficiencies via a network of internal donor-acceptor heterojunctions

Abstract: The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.

Conjugated polymer-based organic solar cells

Abstract: The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

Electroluminescence in conjugated polymers

Abstract: Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

The path to ubiquitous and low-cost organic electronic appliances on plastic

Abstract: Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

Polymer–Fullerene Composite Solar Cells

Abstract: Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Polymer-based organic photovoltaic systems hold the promise for a cost-effective, lightweight solar energy conversion platform, which could benefit from simple solution processing of the active layer. The function of such excitonic solar cells is based on photoinduced electron transfer from a donor to an acceptor. Fullerenes have become the ubiquitous acceptors because of their high electron affinity and ability to transport charge effectively. The most effective solar cells have been made from bicontinuous polymer–fullerene composites, or so-called bulk heterojunctions. The best solar cells currently achieve an efficiency of about 5 %, thus significant advances in the fundamental understanding of the complex interplay between the active layer morphology and electronic properties are required if this technology is to find viable application.