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

We simulate the interchain polaron recombination process in conjugated polymer systems using a nonadiabatic molecular dynamics method, which allows for the coupled evolution of the nuclear degrees of freedom and multiconfigurational electronic wavefunctions. Within the method, the appropriate spin symmetry of the electronic wavefunction is taken into account, thus allowing us to distinguish between singlet and triplet excited states. It is found that the incident polarons can form an exciton, form a bound interchain polaron pair, or pass each other, depending on the interchain interaction strength and the strength of an external electric field. Most importantly, we found that the formation of singlet excitons is considerably easier than triplet excitons. This shows that in real organic light emitting devices, the electroluminescence quantum efficiency can exceed the statistical limitation value of 25%, in agreement with experiments.

/pdf/spin-dependent-polaron-recombination-in-conjugated-polymers-42wycmpija.pdf

Spin-dependent polaron recombination in conjugated polymers.

Polaron stability in molecular crystals

Polaron dynamics in a system of coupled conjugated polymer chains

Changes in the geometric and electronic structure of polyenes upon sodium doping have been studied in detail. A short polyene containing three double bonds, hexatriene, forms a charge transfer complex with a maximum of one sodium ion. The interaction results in the formation of a polaron (radical anion) on the polyene chain. This type of charge storage state is shown to be unstable relative to the formation of a hexatriene dimer when interaction between two polyenes is allowed for; the two polyenes are then bound to each other via a carbon–carbon single bond. The charge storage states in this case correspond to two singly charged, spinless defects, one on each polyene segment of the dimer molecule. Decapentaene, which contains five carbon–carbon double bonds, is shown to be able to accept two electrons from a pair of sodium atoms. Similar to the case of doped trans‐polyacetylene, the charges are stored as a soliton–antisoliton pair. However, when there is an excess of the number of polyene molecules relat...

Charge storage states in polyenes

As a model molecule for a specific part of a polyimide, pthalimide has been studied adsorbed upon copper substrates at low temperatures using angle‐dependent x‐ray photoelectron spectroscopy. A stable adsorbate is found to exist in the approximate range −30 °C≤T≤−5 °C. The pthalimide molecule lies approximately flat upon the polycrystalline copper surface. Charge transfer from the copper to the adsorbate is observed. The present results are in contrast with those obtained when copper is vapor deposited upon a polyimide, as reported in the literature.

Sven Stafström

Papers

Spin-dependent polaron recombination in conjugated polymers.

Polaron stability in molecular crystals

Polaron dynamics in a system of coupled conjugated polymer chains

Charge storage states in polyenes

Pthalimide on copper: A model system to address certain site-specific interactions at the polyimide-copper interface