Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.

https://yylab.seas.ucla.edu/papers/yy-pv-7-slow growth-nm-05.pdf

High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with significant sp3 bonding. DLC is a semiconductor with a high mechanical hardness, chemical inertness, and optical transparency. This review will describe the deposition methods, deposition mechanisms, characterisation methods, electronic structure, gap states, defects, doping, luminescence, field emission, mechanical properties and some applications of DLCs. The films have widespread applications as protective coatings in areas, such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMs).

/pdf/diamond-like-amorphous-carbon-3pczopr0z7.pdf

Diamond-like amorphous carbon

2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

THE recent fabrication of light-emitting diodes (LEDs) from conjugated polymers1,2demonstrates the technological potential of this class of electronic materials. A variety of colours are possible, because the wavelength of luminescence emission can be chemically tuned during synthesis1–4. In addition, the mechanical properties of polymers suggest that light-emitting structures can be made that are more flexible than their inorganic counterparts, provided appropriate materials can be found for the substrate and electrodes. Here we report the fabrication of a fully flexible LED using poly(ethylene terephthalate) as the substrate, soluble poly-aniline as the hole-injecting electrode, a substituted poly(1,4-phenylene-vinylene) as the electroluminescent layer and calcium as the electron-injecting top contract. The structure is mechanically robust and may be sharply bent without failure. The LED is easily visible under room lighting and has an external quantum efficiency of about 1%. With a turn-on voltage for light emission of 2–3 V, the 'plastic' LED demonstrates that this unique combination of optical, electrical and mechanical properties can be used to make novel structures that are compatible with conventional devices.

Flexible light-emitting diodes made from soluble conducting polymers

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

The electronic structure of polymide and several related compounds was investigated theoretically and experimentally. The compounds include pyromellitic dianhydride, oxydianiline, and polyamic acid. Experimental electronic-structure determinations for poly(methyl phenylene oxide) and poly(vinyl methyl ketone) are also reported. The theoretical approach employed valence--effective-Hamiltonian calculations. Photoelectron spectroscopy (x-ray photoelectron spectroscopy, soft-x-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy) was used to experimentally measure the total valence-band density of states (VBDOS) from thin films of the above compounds. The theoretical VBDOS's were cross-section modulated to facilitate comparison with experiment. Very good agreement is found between the theoretical results and the experimental VBDOS's.

Electronic structure of polyimide and related monomers: Theory and experiment

Ab initio calculations show that the energy cost for incorporating lattice defects such as pentagons and heptagons is significantly reduced for BCN compared to BN, thus promoting bending of basal planes in these compounds. Boron–carbon–nitride (B C N) thin films with a fullerene-like (FL) microstructure were then deposited by dual cathode magnetron sputtering from C and B 4 C targets. Up to 1 μm thick films were grown at a total gas pressure of 3 mTorr (0.4 Pa) in varying Ar/N 2 ratios, and substrate temperatures between 225 and 350 °C. Compositional and microstructural studies were performed using RBS, SEM and HREM, respectively. Depending on the deposition condition, ternary B x C y N z films with fullerene-like microstructure could be prepared in agreement with the calculations within the composition range 0 ≤  x  ≤ 53, 15 ≤  y  ≤ 62, and 24 ≤  z  ≤ 50 at.%. Fullerene-like structures also tend to form at lower temperatures in the case of BCN compared to CN. Nanoindentation measurements show that all B x C y N z films exhibited a highly elastic response independent of elemental composition. In addition, the calculations suggest a driving force for C and BN phase separation.

Fullerene-like BCN thin films: a computational and experimental study

The structural and electronic properties of heavily doped trans-polyacetylene are studied using a total Hamiltonian consisting three terms: 1) the Su-Schrieffer-Heeger Hamiltonian, 2) an external potential term due to electrostatic interactions between electrons of the polymer chain and counterions and charges on neighboring chains, and 3) a term describing the intra-chain electron-electron interactions. The effects of weak disorder in the positions of the counterions are also studied. At all doping levels included in this study, the ground state configuration is a soliton lattice. The energy gap around the Fermi energy and the density of states at the Fermi energy are calculated. A low bandgap system is obatined in the case of weak disorder at doping levels above 8%. The electronic properties of the polaron lattice are also calculated. Previously reported experimental results and theoretical models for the metallic state of heavily doped trans-polyacetylene are reviewed and discussed in comparison with the results obtained in this study.

Electronic Properties of Heavily Doped Trans-Polyacetylene

The electronic structure of tetracyanoethylene (TCNE) has been studied both in its pristine state and upon stepwise rubidium intercalation, by UV and x-ray photoelectron spectroscopy as well as with theoretical calculations. The intercalated system may serve as a model for TCNE-based organometallic magnets, of which the electronic structure remains largely unknown. Rubidium is found to n-dope the TCNE molecules forming ${\mathrm{Rb}}^{+}{\mathrm{TCNE}}^{\ensuremath{-}}$ with almost complete charge transfer. Calculations show a spin splitting of the former highest occupied molecular orbital level upon Rb doping. We see no evidence for the formation of doubly charged TCNE molecules. A gap opens up at the Fermi energy for ${\mathrm{Rb}}^{+}{\mathrm{TCNE}}^{\ensuremath{-}}$ due to on-site Coulomb interactions. We estimate the on-site Coulomb interaction of amorphous TCNE doped with Rb to be \ensuremath{\sim}2 eV.

Sven Stafström

Papers

Electronic structure of polyimide and related monomers: Theory and experiment

Fullerene-like BCN thin films: a computational and experimental study

Electronic Properties of Heavily Doped Trans-Polyacetylene

Coulomb interactions in rubidium-doped tetracyanoethylene: A model system for organometallic magnets

Formation of C60 dimers: A theoretical study of electronic structure and optical absorption.