Showing papers by "Alan J. Heeger published in 2010"

Recombination in polymer-fullerene bulk heterojunction solar cells

Abstract: Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current $({J}_{sc})$ and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature-dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first-order recombination at short circuit to bimolecular recombination at open circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The ``missing 0.3 V'' inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open-circuit voltage at room-temperature results from the temperature dependence of the quasi-Fermi levels in the polymer and fullerene domains---a conclusion based on the fundamental statistics of fermions.

Semiconducting polymers: the Third Generation

Abstract: There has been remarkable progress in the science and technology of semiconducting polymers during the past decade. The field has evolved from the early work on polyacetylene (the First Generation material) to a proper focus on soluble and processible polymers and co-polymers. The soluble poly(alkylthiophenes) and the soluble PPVs are perhaps the most important examples of the Second Generation of semiconducting polymers. Third Generation semiconducting polymers have more complex molecular structures with more atoms in the repeat unit. Important examples include the highly ordered and crystalline PDTTT and the ever-growing class of donor–acceptor co-polymers that has emerged in the past few years. Examples of the latter include the bithiophene–acceptor co-polymers pioneered by Konarka and the polycarbazole–acceptor co-polymers pioneered by Leclerc and colleagues. In this tutorial review, I will summarize progress in the basic physics, the materials science, the device science and the device performance with emphasis on the following recent studies of Third Generation semiconducting polymers: stable semiconducting polymers; self-assembly of bulk heterojunction (BHJ) materials by spontaneous phase separation; bulk heterojunction solar cells with internal quantum efficiency approaching 100%; high detectivity photodetectors fabricated from BHJ materials.

Colorimetric detection of DNA, small molecules, proteins, and ions using unmodified gold nanoparticles and conjugated polyelectrolytes

Abstract: We have demonstrated a novel sensing strategy employing single-stranded probe DNA, unmodified gold nanoparticles, and a positively charged, water-soluble conjugated polyelectrolyte to detect a broad range of targets including nucleic acid (DNA) sequences, proteins, small molecules, and inorganic ions. This nearly "universal" biosensor approach is based on the observation that, while the conjugated polyelectrolyte specifically inhibits the ability of single-stranded DNA to prevent the aggregation of gold-nanoparticles, no such inhibition is observed with double-stranded or otherwise "folded" DNA structures. Colorimetric assays employing this mechanism for the detection of hybridization are sensitive and convenient--picomolar concentrations of target DNA are readily detected with the naked eye, and the sensor works even when challenged with complex sample matrices such as blood serum. Likewise, by employing the binding-induced folding or association of aptamers we have generalized the approach to the specific and convenient detection of proteins, small molecules, and inorganic ions. Finally, this new biosensor approach is quite straightforward and can be completed in minutes without significant equipment or training overhead.

Effect of processing additive on the nanomorphology of a bulk heterojunction material.

Abstract: The bulk heterojunction (BHJ) material Si-PDTBT:PC70BM is sensitive to the use of a small amount of 1-chloronaphthalene (CN) as a processing additive; CN as a cosolvent (e.g., 4% in chlorobenzene) causes in a factor of 2 increase in the power conversion efficiency of BHJ solar cells.1 The morphology of the BHJ material, prepared with and without the CN additive is studied with top-down transmission electron microscopy, cross-sectional transmission electron microscopy, and atomic force microscopy. The improved performance is the result of changes in the nanoscale morphology. Field-effect transistor measurements are consistent with the observed changes in morphology.

An Electrochemical Supersandwich Assay for Sensitive and Selective DNA Detection in Complex Matrices

Abstract: In a traditional sandwich assay, a DNA target hybridizes to a single copy of the signal probe. Here we employ a modified signal probe containing a methylene blue (a redox moiety) label and a "sticky end." When a DNA target hybridizes this signal probe, the sticky end remains free to hybridize another target leading to the creation of a supersandwich structure containing multiple labels. This leads to large signal amplification upon monitoring by voltammetry.

Exciton Formation, Relaxation, and Decay in PCDTBT

Abstract: The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband π−π* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes ...

A thermally stable semiconducting polymer.

Abstract: A bs or pt io n RT 120 170 200 250 300 350 400 Early research on polymer electronic devices successfully demonstrated function and performance adequate for specific applications. As a result, the performance of devices fabricated from semiconducting polymers has improved to the point where ‘‘plastic’’ electronics are now expected to develop into a significant industry with a large market opportunity. However, the limited stability of polymer-based devices continues to hinder the path toward commercialization. Because stability in air is critical to the commercialization of polymer electronic devices, discussions concerning the stability of semiconducting polymers have focused on degradation caused by reaction with oxygen and water vapor. Conjugated polymers are, however, generally believed to be incapable of withstanding high temperatures (i.e., temperatures well above the glasstransition temperature, Tg), [6,7] thus, stability at high temperatures has received less attention. The availability of semiconducting polymers that can survive exposure to elevated temperatures would open a variety of new possibilities. For example, since inorganic electronic devices typically require process steps that must be carried out at high temperature (often over 300 8C), semiconducting polymers capable of withstanding high temperatures will enable the fabrication of novel organic–inorganic hybrid devices. Here, we report the remarkable stability of the poly(2,7carbazole) derivative, poly[N-900-hepta-decanyl-2,7-carbazole-alt5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)], (PCDTBT; see the inset of Fig. 1a). Prior to this report, there was no known example of a semiconducting polymer that is both stable in air at (and above) room temperature and capable of withstanding high temperatures for extended periods of time. PCDTBT is one of a relatively large class of ‘‘donor–acceptor’’ polycarbazole co-polymers. Recently, polymer bulkheterojuction solar cells fabricated with phase-separated blends of PCDTBT and PC71BM were demonstrated with internal quantum efficiency approaching 100%, power conversion efficiency of 17% in response to monochromatic radiation within the absorption band, and power conversion efficiency of 6.1% in response to solar radiation. To investigate the stability of PCDTBT, we have carried out spectroscopic studies on PCDTBT thin films and transport studies using the field-effect transistor (FET) architecture with PCDTBTas the semiconductor material in the channel. Figure 1 shows UV–visable (UV–vis) absorption spectra of PCDTBT thin films annealed for 15 minutes at various temperatures in air (Fig. 1a) and under N2 atmosphere (Fig. 1b). In air, the p–p* absorption spectrum is not affected after exposure to temperatures up to 150 8C. Under N2 atmosphere (Fig 1b), the electronic band structure of PCDTBT is stable after exposure to temperatures as high as 350 8C.

Semiconducting and Metallic Polymers

Abstract: Conducting and semiconducting (conjugated) polymers have a unique set of properties, combining the electronic properties of metals and semiconductors with the processing advantages and mechanical properties of polymers. Now, thirty-five years after their discovery, metallic conducting polymers have been demonstrated in the laboratory to have electrical conductivities approaching that of copper, and mechanical strengths exceeding that of steel, a remarkable achievement.A wide variety of electrical and optical devices have been demonstrated using semiconducting polymers. Light-emitting devices have been made which are as bright as fluorescent lamps at applied voltages of only a few volts; photovoltaic solar energy conversion using conjugated polymer composites is in industrial production; conjugated polymer transistors, circuits and chips have been demonstrated. Indeed, semiconducting and metallic polymers can be thought of as electronic 'inks'. The advances in printing technology (ink-jet printing, off-set printing, etc) combined with the science and technology of conducting polymers will revolutionize the way in which electronic devices are manufactured. In addition, semiconducting and metallic polymers can be used in applications which require special mechanical properties such as flexibility.The field of semiconducting and conducting polymers has become one of the most attractive areas of interdisciplinary materials science and technology. Ranging from physics, chemistry, electrical and electronic engineering to the optical sciences, this field covers a wide range of competences and interdisciplinary knowledge.Readership: Graduate students and postdoctoral researchers in polymer physics, as well as interested specialists in industry.

Experimental test for geminate recombination applied to organic solar cells

Abstract: We show that a clear experimental test can distinguish between geminate and nongeminate recombination in low mobility semiconductors. For the particular case of the organic solar cell, the relative contribution of geminate recombination can be determined by measuring transient photoconductivity versus applied voltage. Measurements carried out at room temperature and 200 K on bulk heterojunction organic solar cells fabricated with two different semiconducting polymers show that neither exhibits significant geminate recombination.

Higher Molecular Weight Leads to Improved Photoresponsivity, Charge Transport and Interfacial Ordering in a Narrow Bandgap Semiconducting Polymer

Abstract: Increasing the molecular weight of the low-bandgap semiconducting copolymer, poly[(4,4-didoecyldithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], Si-PDTBT, from 9 kDa to 38 kDa improves both photoresponsivity and charge transport properties dramatically. The photocurrent measured under steady state conditions is 20 times larger in the higher molecular weight polymer (HM{sub n} Si-PDTBT). Different decays of polarization memory in transient photoinduced spectroscopy measurements are consistent with more mobile photoexcitations in HM{sub n} Si-PDTBT relative to the lower molecular weight counterpart (LM{sub n} Si-PDTBT). Analysis of the current-voltage characteristics of field effect transistors reveals an increase in the mobility by a factor of 700 for HM{sub n} Si-PDTBT. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence small angle X-ray scattering (GISAXS) measurements demonstrate that LM{sub n} Si-PDTBT forms a disordered morphology throughout the depth of the film, whereas HM{sub n} Si-PDTBT exhibits pronounced {pi}-{pi} stacking in an edge-on configuration near the substrate interface. Increased interchain overlap between polymers in the edge-on configuration in HM{sub n} Si-PDTBT results in the higher carrier mobility. The improved optical response, transport mobility, and interfacial ordering highlight the subtle role that the degree of polymerization plays on the optoelectronic properties of conjugated polymer based organic semiconductors.

Enhanced diode characteristics of organic solar cells using titanium suboxide electron transport layer

Abstract: The (dark) diode characteristics of the organic bulk heterojunction solar cell based on the phase separated blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thenyl-2′,1′,3′-benzothiadiazole)] with [6,6]-phenyl C70-butyric acid methyl ester have been analyzed with a focus on the effect of the titanium suboxide (TiOx) electron transport layer. The addition of the TiOx layer into the device structure causes the saturation current density to decrease by a factor of 26 and the shunt resistance to increase by a factor of 12. The diode ideality factor and series resistance are, respectively, almost the same for diodes made with and without the TiOx layer. The results indicate that the TiOx layer increases the energy barrier for hole transport and reduces the minority carrier density.

Semiconducting polymer photodetectors with electron and hole blocking layers: high detectivity in the near-infrared.

Abstract: Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications Photodetectors with broad spectral response, from 300 nm to 1,100 nm, were fabricated using a narrow-band gap semiconducting polymer blended with a fullerene derivative By using both an electron-blocking layer and a hole-blocking layer, the polymer photodetectors, operating at room temperature, exhibited calculated detectivities greater than 1013 cm Hz1/2/W over entire spectral range with linear dynamic range approximately 130 dB The performance is comparable to or even better than Si photodetectors

Detection of telomerase activity in high concentration of cell lysates using primer-modified gold nanoparticles

Abstract: Although the telomeric repeat amplification protocol (TRAP) has served as a powerful assay for detecting telomerase activity, its use has been significantly limited when performed directly in complex, interferant-laced samples. In this work, we report a modification of the TRAP assay that allows the detection of high-fidelity amplification of telomerase products directly from concentrated cell lysates. Briefly, we covalently attached 12 nm gold nanoparticles (AuNPs) to the telomere strand (TS) primer, which is used as a substrate for telomerase elongation. These TS-modified AuNPs significantly reduce polymerase chain reaction (PCR) artifacts (such as primer dimers) and improve the yield of amplified telomerase products relative to the traditional TRAP assay when amplification is performed in concentrated cell lysates. Specifically, because the TS-modified AuNPs eliminate most of the primer-dimer artifacts normally visible at the same position as the shortest amplified telomerase PCR product apparent on agarose gels, the AuNP-modified TRAP assay exhibits excellent sensitivity. Consequently, we observed a 10-fold increase in sensitivity for cancer cells diluted 1000-fold with somatic cells. It thus appears that the use of AuNP-modified primers significantly improves the sensitivity and specificity of the traditional TRAP assay and may be an effective method by which PCR can be performed directly in concentrated cell lysates.

On the Binding of Cationic, Water-Soluble Conjugated Polymers to DNA: Electrostatic and Hydrophobic Interactions

Abstract: Water-soluble, cationic conjugated polymer binds single-stranded DNA with higher affinity than it binds double-stranded or otherwise "folded" DNA. This stronger binding results from the greater hydrophobicity of single-stranded DNA. Upon reducing the strength of the hydrophobic interactions, the electrostatic attraction becomes the important interaction that regulates the binding between the water-soluble conjugated polymer and DNA. The different affinities between the cationic conjugated polymer and various Forms of DNA (molecular beacons and its open state; single-stranded DNA and double-stranded DNA and single-stranded DNA and complex DNA folds) can be used to design a variety of biosensors.

A low-bandgap alternating copolymer containing the dimethylbenzimidazole moiety

Abstract: A new acceptor unit containing dimethyl-2H-benzimidazole was synthesized and utilized for the synthesis of a conjugated polymer containing electron donor–acceptor pairs for OPV devices. The dimethyl-2H-benzimidazole unit was designed to act as a substitute for the BT unit of PCDTBT. This novel dimethyl-2H-benzimidazole unit has two methyl groups which can supply higher solubility than those of the BT series. A thin film of PCDTMBI, containing the dimethyl-2H-benzimidazole unit, shows two broad absorption bands with maxima at 400 and 636 nm and an absorption onset of 756 nm, corresponding to a band gap of 1.64 eV. The oxidation onset of PCDTMBI was estimated to be 0.67 V, which corresponds to a HOMO energy level of −5.47 eV. The LUMO energy level of the polymer was thus determined to be −3.82 eV. A device with a PCDTMBI:PC71BM blend had a VOC value of 0.65 V, a JSC value of 10.0 mA cm−2, and a FF of 0.48, leading to an efficiency of 3.12%. The enhanced efficiency of PCDTMBI was caused by the higher IPCE value between 400 and 700 nm and high mobility (2.2 × 10−3 cm2 V−1 s−1).

Bulk heterojunction solar cells based on a low-bandgap carbazole-diketopyrrolopyrrole copolymer

Abstract: Bulk heterojunction (BHJ) solar cells fabricated with a phase separated nanomaterial comprising a carbazole-diketopyrrolopyrrole copolymer (PCBTDPP) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) are demonstrated with power conversion efficiency>3.5%. The PCBTDPP:PC70BM BHJ nanomorphology was controlled by changing the length of the alkyl side-chain of the polymer and by utilizing processing additives.

Semiconducting Polymers: The Third Generation

Abstract: There has been remarkable progress in the science and technology of semiconducting polymers during the past decade. The field has evolved from the early work on polyacetylene (the First Generation material) to a proper focus on soluble and processible polymers and co-polymers. The soluble poly(alkylthiophenes) and the soluble PPVs are perhaps the most important examples of the Second Generation of semiconducting polymers. Third Generation semiconducting polymers have more complex molecular structures with more atoms in the repeat unit. Important examples include the highly ordered and crystalline PDTTT and the ever-growing class of donor–acceptor co-polymers that has emerged in the past few years. Examples of the latter include the bithiophene–acceptor co-polymers pioneered by Konarka and the polycarbazole–acceptor co-polymers pioneered by Leclerc and colleagues. In this tutorial review, I will summarize progress in the basic physics, the materials science, the device science and the device performance with emphasis on the following recent studies of Third Generation semiconducting polymers: stable semiconducting polymers; self-assembly of bulk heterojunction (BHJ) materials by spontaneous phase separation; bulk heterojunction solar cells with internal quantum efficiency approaching 100%; high detectivity photodetectors fabricated from BHJ materials.

Split-gate organic field effect transistors: control over charge injection and transport.

Abstract: A split-gate field effect transistor containing four electrodes, source, drain, two gates allows enhanced transport for specific carrier species and separate control of carrier polarity over two gate regimes. The device can be operated as a transistor or a diode by controlling gate biases.

Conjugated polyelectrolytes for organic light emitting transistors

Abstract: We report on solution-processed light emitting field-effect transistors (LEFETs) that incorporate symmetric high work function (WF) source and drain metal electrodes. A key architectural design is the incorporation of a conjugated polyelectrolyte (CPE) electron injection layer atop the emissive layer. The device structure also comprises a hole-transporting layer underneath the emissive layer. Both holes and electrons are injected from stable, high WF metal though the CPE layer leading to electroluminescence near the electron-injecting electrode. With the benefits of the simplicity in device fabrication, the LEFETs incorporating CPEs are interesting structures for integrated organic optoelectronic devices.

“Plastic” Solar Cells: Self‐Assembly of Bulk Heterojunction Nanomaterials by Spontaneous Phase Separation

Abstract: As the global demand for low-cost renewable energy sources intensifies, interest in new routes for converting solar energy to electricity is rapidly increasing. Although photovoltaic cells have been commercially available for more than 50 years, only 0.1% of the total electricity generated in the United States comes directly from sunlight. The earliest commercial solar technology remains the basis for the most prevalent devices in current use, namely, highly-ordered crystalline, inorganic solar cells, commonly referred to as silicon cells. Another class of solar cells that has recently inspired significant academic and industrial excitement is the bulk heterojunction (BHJ) “plastic” solar cell. Research by a rapidly growing community of scientists across the globe is generating a steady stream of new insights into the fundamental physics, the materials design and synthesis, the film processing and morphology, and the device science and architecture of BHJ technology. Future progress in the fabrication of ...

Solution-processed cross-linkable hole selective layer for polymer solar cells in the inverted structure

Abstract: Solution-processed cross-linkable tetraphenyldiamine-containing material (TPD-BVB) as a highly efficient hole selective transport layer was demonstrated. Polymer solar cells (PSCs) with an inverted structure fabricated with a thin cross-linked TPD-BVB film show comparable efficiency and superior long-term air stability when compared to devices fabricated with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Thus, solution-processed TPD-BVB is an attractive alternative to PEDOT:PSS as a hole extraction layer in inverted structure PSCs.

Organic light emitting complementary inverters

Abstract: We show that p- and n-type light emitting field-effect transistors (LEFETs) can be made using “superyellow” as a light-emitting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as a p-type material and a naphthalene di-imide as an n-type material. By connecting two of these LEFETs, we have demonstrated a light emitting complementary inverter (LECI). The LECI exhibited electrical and optical characteristics in the first and third quadrant of the transfer characteristics with voltage gain of 6 and 8, respectively.

Increased open-circuit voltage in bulk-heterojunction solar cells using a C60 derivative

Abstract: The fullerene derivative C60-fused N-methyl-2-(3-hexylthiophen-2-yl)pyrrolidine (C60-TH-Hx) is used as the acceptor material in bulk-heterojunction (BHJ) solar cells fabricated with the low band-gap polymer poly[(4,4′-bis(2-ethylhexyl)dithiene[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl]. Direct comparison with BHJ solar cells based on [6,6]-phenyl-C61-butyric methyl ester and Si-PCPDTBT indicates that the C60-TH-Hx acceptor yields a larger open-circuit voltage because of higher lowest unoccupied molecular orbital energy level of C60-TH-Hx.

On the Binding of Cationic, Water-Soluble Conjugated Polymers to DNA: Electrostatic and Hydrophobic Interactions [J. Am. Chem. Soc. 2010, 132, 1252-1254]

Abstract: Water-soluble, cationic conjugated polymer binds single-stranded DNA with higher affinity than it binds double-stranded or otherwise \"folded\" DNA. This stronger binding results from the greater hydrophobicity of single-stranded DNA. Upon reducing the strength of the hydrophobic interactions, the electrostatic attraction becomes the important interaction that regulates the binding between the water-soluble conjugated polymer and DNA. The different affinities between the cationic conjugated polymer and various forms of DNA (molecular beacons and its open state; single-stranded DNA and double-stranded DNA and single-stranded DNA and complex DNA folds) can be used to design a variety of biosensors.

New Developments in the Photonic Applications of Conjugated Polymers

Abstract: I. Semiconducting Polymers as Materials for “Plastic” Photonics Devices Solid-state photonic devices are a class of devices in which the quantum of light, the photon, plays a role. Because the interband optical transition (absorption and/or emission) is involved in photonic phenomena and because photon energies from near-infrared to near-ultraviolet are of interest, the relevant materials are semiconductors with band gaps in the range from 1 to 3 eV. Typical inorganic semiconductors used for photonic devices are Si, Ge, and Group III-V and Group II-VI alloys.1 Conjugated polymers are a novel class of semiconductors that combine the optical and electronic properties of semiconductors with the processing advantages and mechanical properties of polymers. Important examples of polymers within this class include poly(p-phenylenevinylene) (PPV), poly(p-phenylene) (PPP), and polyfluorene (PF) derivatives whose molecular structures are shown in Figure 1. The relative simplicity with which high photoluminescence (PL) efficiency polymers of different colors can be achieved is in stark contrast to inorganic semiconductors, where, for example, bright blue light emitting diodes (LEDs) were not available until recently because of the difficulties in growing InGaN films.2 Most of the photonic phenomena known in conventional inorganic semiconductors have been observed in these semiconducting polymers. The dream of using such materials in high-performance “plastic” photonic devices is rapidly becoming reality: high-performance photonic devices fabricated from conjugated polymers have been demonstrated, including diodes,3 light-emitting diodes,4 photodiodes,5 field-effect transistors,6 polymer grid triodes,7 light-emitting electrochemical cells,8 and optocouplers,9 i.e., all the categories that characterize the field of photonic devices. These polymer-based devices have reached performance levels comparable to or even better than those of their inorganic counterparts. For a recent review of progress in plastic photonic devices fabricated with semiconducting polymers, see ref 10.