Thomas Heiser

High-Performance Solution-Processed Solar Cells and Ambipolar Behavior in Organic Field-Effect Transistors with Thienyl-BODIPY Scaffoldings

Green-absorbing dipyrromethene dyes engineered from bis-vinyl-thienyl modules are planar molecules, exhibiting strong absorption in the 713-724 nm range and displaying comparable electron and hole mobilities in thin films (maximum value 1 × 10(-3) cm(2)/(V·s)). Bulk heterojunction solar cells assembled with these dyes and a fullerene derivative (PC(61)BM) at a low ratio give a power conversion efficiency as high as 4.7%, with short-circuit current values of 14.2 mA/cm(2), open-circuit voltage of 0.7 V, and a broad external quantum efficiency ranging from 350 to 920 nm with a maximum value of 60%.

A New Supramolecular Route for Using Rod-Coil Block Copolymers in Photovoltaic Applications

A new polymer blend formed by poly(3-hexylthiophene)-poly(4-vinylpyridine) (P3HT-P4VP) block copolymers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. The P4VP and PCBM are mixed together by weak supramolecular interactions, and the resulting materials exhibit microphase separated morphologies of electron-donor and electron-acceptor rich domains. The properties of the blend, used in photovoltaic devices as active layers, are also discussed.

Electrical and Recombination Properties of Copper‐Silicide Precipitates in Silicon

Copper-silicide precipitates in silicon obtained after copper diffusion and quench in different liquids were studied by transmission electron microscopy and capacitance spectroscopy techniques. A correlation between the quenching rate, geometric size, and deep level spectra of the copper-silicide precipitates was established. The unusually wide deep level spectra are shown to be due to a defect-related band in the bandgap. The parameters of the band are evaluated using numerical simulations. a positive charge of copper-silicide precipitates in p-type and moderately doped n-type Si is predicted by simulations and confirmed by minority carrier transient spectroscopy measurements. Strong recombination activity of the precipitates due to attraction of minority carriers by the electric field around the precipitates and their recombination via the defect band is predicted and confirmed by the experiments. The pairing of copper with boron is shown to be an important factor determining the precipitation kinetics of the interstitial copper at room temperature.

Absorption tuning of monosubstituted triazatruxenes for bulk heterojunction solar cells.

A series of triazatruxene (TAT)-functionalized Bodipy dyes were prepared by a sequence of reactions involving either cross-coupling reactions promoted by Pd complexes or a Knoevenagel reaction leading to a vinyl linker. The new dyes show large absorption coefficients and fluorescence quantum yields as well as interesting electrochemical properties. The blue dyes of this series exhibit interesting photovoltaic effects (V(OC) = 0.83 V, J(SC) = 3.6 mA/cm(2), efficiency 0.9%) in bulk heterojunction solar cells, due to the good hole mobility imported by the TAT entity.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

Solution-processed bulk heterojunction solar cells have experienced a remarkable acceleration in performances in the last two decades, reaching power conversion efficiencies above 10%. This impressive progress is the outcome of a simultaneous development of more advanced device architectures and of optimized semiconducting polymers. Several chemical approaches have been developed to fine-tune the optoelectronics and structural polymer parameters required to reach high efficiencies. Fluorination of the conjugated polymer backbone has appeared recently to be an especially promising approach for the development of efficient semiconducting polymers. As a matter of fact, most currently best-performing semiconducting polymers are using fluorine atoms in their conjugated backbone. In this review, we attempt to give an up-to-date overview of the latest results achieved on fluorinated polymers for solar cells and to highlight general polymer properties’ evolution trends related to the fluorination of their conjugated backbone.

Room temperature ZnO growth by rf magnetron sputtering on top of photoactive P3HT: PCBM for organic solar cells

In this work, we use rf magnetron sputtering to deposit at room temperature a ZnO thin film on top of a photoactive material based on P3HT:PCBM blend for organic photovoltaic devices. We investigate the influence of the sputtering conditions on the ZnO crystallinity, on the substrate layer nanoscale morphology and on the photovoltaic device performances. We show that, under appropriate sputtering conditions, the modification of the P3HT:PCBM film causes no functional damage of the photoactive layer, and leads to an improved performance of the photovoltaic devices.

Gettering of iron by oxygen precipitates

In order to better understand and model internal gettering of iron in silicon, a quantitative investigation of iron precipitation in silicon containing different oxygen precipitate densities was performed. The number of iron precipitation sites was obtained from the iron precipitation kinetics using Ham’s Law. At low temperatures, the iron precipitate density corresponded to the oxygen precipitate density. A strong temperature dependence of the iron precipitate density was observed for the samples with larger oxygen precipitate densities. These data were used to simulate iron precipitation during a slow cool. From those simulations, optimal cooling rates were obtained for different silicon materials assuming various iron precipitation site densities in the epitaxial layer.

Interstitial copper-related center in n-type silicon

n-type silicon samples were measured by deep level transient spectroscopy (DLTS) immediately (within one hour of storage at room temperature, required for the preparation of Schottky-diodes) after copper diffusion and quench. A donor level at Ec-(0.15±0.01) eV with a concentration of up to 1013 cm−3 was detected. The amplitude of the DLTS peak decreased with the time of storage at room temperature, and stabilized at a concentration (4 to 7)×1011 cm−3 after 15–20 h. The activation energies and prefactors of the decay of the DLTS peak in n-type Si and the reactivation of copper-compensated boron in p-type Si concur. This correlation suggests that the deep level is interstitial copper itself or a complex of interstitial copper.

Transient ion drift detection of low level copper contamination in silicon

The transient ion drift (TID) method was used to measure quenched interstitial copper concentrations in both copper plated and copper implanted silicon. Comparison with existing literature data allows one to conclude that, contrary to the general expectation, it is possible to quench in most of the Cu dissolved at temperatures of 600 °C and below. This result suggests that the TID technique could be an excellent means to detect copper contamination in p-type silicon. The expected detection limit, on the order of 1011 cm−3, makes the method a potentially interesting tool to use in gettering or in-diffusion barrier studies.

Electronic Properties and Photovoltaic Performances of a Series of Oligothiophene Copolymers Incorporating Both Thieno[3,2-b]thiophene and 2,1,3-Benzothiadiazole Moieties.

A series of donor-acceptor alternated conjugated copolymers, composed of thiophene, bithiophene, thieno[3,2-b]thiophene, and 2,1,3-benzothiadiazole units and differing from each other by the nature and the number of 3-alkylthiophene in the backbone, have been synthesized by Stille cross-coupling polymerization. The materials optical and electrochemical properties, in solution and in thin films, have been investigated using UV-Visible absorption and cyclic voltammetry. Bulk heterojunction solar cells using blends of the newly synthesized copolymers, as electron donor, and C60-PCBM or C70-PCBM, as electron transporting material, have been elaborated. A maximum power conversion efficiency of 1.8% is achieved with a 1:4 PPBzT(2) -C12:C70-PCBM weight ratio.