Jürgen Parisi

Impact of the Incorporation of Au Nanoparticles into Polymer/Fullerene Solar Cells †

The addition of small amounts of dodecylamine-capped Au nanoparticles into the active layer of organic bulk heterojunction solar cells consisting of poly(3-octylthiophene) (P3OT) and C(60) was recently suggested to have a positive impact on device performance due to improved electron transport. This issue was systematically further investigated in the present work. Different strategies to incorporate colloidally prepared Au nanoparticles with a narrow size distribution into organic solar cells with the more common donor/acceptor system consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) were pursued. Au nanoparticles were prepared with either P3HT or dodecylamine as ligands. Additionally, efforts were undertaken to incorporate nearly ligand-free Au nanoparticles into the system. Therefore, a procedure was successfully developed to remove the dodecylamine ligand shell by a postpreparative ligand exchange with pyridine, a much smaller molecule that can later partly be removed from solid films by annealing. However, for all types of nanoparticles studied here, the performance of the P3HT/PCBM solar cells was found to decrease with the Au particles as an additive to the active layer, meaning that adding Au nanoparticles is not a suitable strategy in the case of the P3HT/PCBM system. Possible reasons are discussed on the basis of detailed investigations of the structure, photophysics and charge transport in the system.

Size and shape control of colloidal copper(I) sulfide nanorods.

Many physical and chemical properties of semiconducting nanocrystals strongly depend on their spatial dimensions and crystallographic structure. For these reasons, achieving a high degree of size and shape control plays an important role with respect to their application potential. In this report we present a facile route for the direct colloidal synthesis of copper(I) sulfide nanorods. A high reactivity of the starting materials is essential to obtain nanorods. We achieve this by using a thiol that thermally decomposes easily and serves as the sulfur source. The thiol is mixed in a noncoordinating solvent, which acts as the reaction medium. Adjustment of the nucleation temperature makes it possible to tailor uniform nanorods with lengths from 10 to 100 nm. The nanorods are single crystalline, and the growth direction is shown to occur along the a-axis of djurleite. The growth process and character of the nanorods were investigated through UV-vis and NIR absorption spectroscopy, transmission electron microscopy, and powder X-ray diffraction measurements.

Light induced changes in the electrical behavior of CdTe and Cu(In,Ga)Se2 solar cells

Abstract The electrical properties of CdTe and Cu(In x ,Ga 1− x )Se 2 (CIGS) solar cells change depending on their illumination conditions. The dark and light current–voltage (I–V) curves of both solar cells exhibit a distinct cross over. The reason for the cross over of the CIGS solar cells is located in the CdS buffer layer and its interface to the absorber. Light which is absorbed in the CdS buffer layer results in a steeper I–V curve in contrast to the dark curve. The buffer layer of the CdTe solar cell has a similar influence on the I–V curve. In this case, however, the effect described above overlaps with a second feature. Many CdTe solar cells have a Schottky contact as their back contact. This is expressed in the I–V curve as a current limitation at positive voltage bias (roll over). However, under light bias even this diode can be influenced. By means of quantum efficiency (QE) measurements under voltage bias it is possible to analyze both effects. All experimental observations can be explained by traps in CdS and/or CdTe assuming strongly different capture cross sections for holes and electrons. A model describing the influence of the illumination on the I–V and QE curves is presented.

Femtosecond up-conversion technique for probing the charge transfer in a P3HT: PCBM blend via photoluminescence quenching

We report on an experimental study of the charge transfer dynamics in a P3HT : PCBM blend by means of a femtosecond fluorescence up-conversion technique. Using two-photon excitation we probe the exciton dynamics in P3HT and a P3HT : PCBM blend with a weight ratio of 1 : 1 at excitation densities of up to 6 × 1018 cm−3. In both samples we find strongly nonexponential decay traces compatible with (i) diffusion-limited exciton–exciton annihilation and (ii) diffusion-limited donor–acceptor charge transfer in the polymer blend. Additionally, our results indicate that in the P3HT : PCBM blend about 50% of the photogenerated excitons undergo a prompt charge transfer process on a time scale of about 150 fs. Our study shows that fluorescence spectroscopy with femtosecond time resolution is a powerful technique for probing ultrafast charge transfer processes in solar cell materials.

Colloidal synthesis and structural control of PtSn bimetallic nanoparticles.

PtSn bimetallic nanoparticles with different particle sizes (1-9 nm), metal compositions (Sn content of 10-80 mol %), and organic capping agents (e.g., amine, thiol, carboxylic acid and polymer) were synthesized by colloidal chemistry methods. Transmission electron microscopy (TEM) measurements show that, depending on the particle size, the as-prepared bimetallic nanocrystals have quasi-spherical or faceted shapes. Energy-dispersive X-ray (EDX) analyses indicate that for all samples the signals of both Pt and Sn can be detected from single nanoparticles, confirming that the products are actually bimetallic but not only a physical mixture of pure Pt and Sn metal nanoparticles. X-ray diffraction (XRD) measurements were also conducted on the bimetallic particle systems. When compared with the diffraction patterns of monometallic Pt nanoparticles, the bimetallic samples show distinct shifts of the Bragg reflections to lower degrees, which gives clear proof of the alloying of Pt with Sn. However, a quantitative analysis of the lattice parameter shifts indicates that only part of the Sn atoms are incorporated into the alloy nanocrystals. This is consistent with X-ray photoelectron spectroscopy (XPS) measurements that reveal the segregation of Sn at the surfaces of the nanocrystals. Moreover, short PtSn bimetallic nanowires were synthesized by a seed-mediated growth method with amine-capped bimetallic particles as precursors. The resulting nanowires have an average width of 2.3 nm and lengths ranging from 5 to 20 nm.

Role of copper sulfide seeds in the growth process of CuInS2 nanorods and networks.

CuInS2 nanorods and networks are interesting candidates for applications requiring efficient charge transport, such as solar energy conversion, because of the increased electrical conductivity in elongated or interconnected nanocrystals, compared to isolated, quasi-spherical ones. However, little is known about the growth mechanisms involved in the formation of this kind of nanostructures, yet. Here, CuInS2 nanorods and networks were synthesized through a facile low-cost and phosphine-free method. Copper and indium sources were added together in the presence of oleylamine and oleic acid. Changing the amount of oleic acid present in the reaction solution influenced the reactivity of the monomers, and consequently, the size of copper sulfide seeds formed in situ after the injection of tert-dodecanethiol, serving as the source of sulfur. Two different growth mechanisms of CuInS2 nanorods were observed, depending on the size of the copper sulfide seeds. Larger seeds (8 nm), which were generated with relatively small amounts of oleic acid, resulted in the formation of hybrid copper sulfide-copper indium disulfide nanocrystals as intermediates in the growth process of the nanorods, while smaller seeds (4 nm) obtained with relatively large amounts of oleic acid were gradually converted to copper indium sulfide nanorods. At longer reaction times, these nanorods formed network structures. The reaction between oleylamine and oleic acid at high temperature turned out to be the crucial factor to induce the attachment of nanorods to multipods and networks.

Role of Oxygen Adsorption in Nanocrystalline ZnO Interfacial Layers for Polymer−Fullerene Bulk Heterojunction Solar Cells

Colloidal zinc oxide (ZnO) nanoparticles are frequently used in the field of organic photovoltaics for the realization of solution-producible, electron-selective interfacial layers. Despite the widespread use, there is still lack of detailed investigations regarding the impact of structural properties of the ZnO particles, like the particle size and shape on the device performance. In the present work, ZnO nanoparticles with varying surface-area-to-volume ratio were synthesized and implemented into indium tin oxide free polymer–fullerene bulk heterojunction solar cells featuring a gas-permeable top electrode. By comparison of the electrical characteristics before and after encapsulation from the ambient atmosphere, it was found that the internal surface area of the ZnO layer plays a crucial role under conditions where oxygen can penetrate into the solar cells. The adsorption of oxygen species at the nanoparticle surface is believed to cause band bending and electron depletion next to the surface. Both eff...

Influence of particle size in hybrid solar cells composed of CdSe nanocrystals and poly(3-hexylthiophene)

Inorganic semiconductor nanoparticles, such as CdSe quantum dots, are considered to be a promising alternative to fullerene derivates for application as electron acceptors in polymer-based bulk heterojunction solar cells. The main potential advantage is the strong light absorption of CdSe nanoparticles with a spectral bandwidth, which can even be tuned, due to the quantum size effect. However, the impact of the particle size on the performance of polymer/CdSe solar cells has remained largely unexplored so far. Therefore, the influence of particle size in hybrid solar cells using a blend of poly(3-hexylthiophene) (P3HT) and quasi-spherical CdSe nanoparticles on relevant cell parameters and the overall solar cell performance is systematically studied in the present work. As the most important result, an increase of the open-circuit voltage (VOC) can be found for smaller nanoparticles and can be explained by an “effective bandgap” model. In contrast, no significant changes of the short-circuit current densit...

Colloidally Prepared Pt Nanoparticles for Heterogeneous Gas-Phase Catalysis: Influence of Ligand Shell and Catalyst Loading on CO Oxidation Activity

In contrast to conventional methods, colloidally prepared heterogeneous supported metal catalysts are excellent systems to study the catalytic properties as a function of metal loading, monodispersity, particle shape, or the type of support without changing the other parameters, as will be demonstrated herein. Colloidal, ligand‐capped Pt nanoparticles deposited on oxide supports are investigated for CO adsorption and oxidation. Dodecylamine and different alkanethiols are used as ligands. IR spectroscopic experiments reveal that small molecules, such as CO, can pass through the ligand shell and can adsorb on the particle surface, even if the ligand shell is not removed by a special pretreatment. The ability to penetrate the shell was found to depend on the type of ligand used which renders ligand‐capped nanoparticles potentially interesting for reaction and selectivity control. In the case of CO oxidation, high activity is detected only at temperatures at which a partial loss of ligands has already occurred, resulting in a rather similar catalytic behavior independent on the type of ligand. However, there are no indications for poisoning of the catalysts by decomposition of the ligand shell. Simple purification procedures of the Pt nanoparticles are sufficient to avoid further poisoning effects. Depositing nanoparticles with the same size in different amounts on a support enabled a detailed study of the influence of metal loading on the activity. The activity per gram metal increases with the metal loading. Local autothermal heating is responsible for this effect, which is also detected for a reference system consisting of Pt nanoparticles prepared without a ligand shell.

Semitransparent Polymer-Based Solar Cells with Aluminum-Doped Zinc Oxide Electrodes

With the use of two transparent electrodes, organic polymer-fullerene solar cells are semitransparent and may be combined to parallel-connected multijunction devices or used for innovative applications like power-generating windows. A challenging issue is the optimization of the electrodes, to combine high transparency with adequate electric properties. In the present work, we study the potential of sputter-deposited aluminum-doped zinc oxide as an alternative to the widely used but relatively expensive indium tin oxide (ITO) as cathode material in semitransparent polymer-fullerene solar cells. Concerning the anode, we utilized an insulator-metal-insulator structure based on ultrathin Au films embedded between two evaporated MoO3 layers, with the outer MoO3 film (capping layer) serving as a light coupling layer. The performance of the ITO-free semitransparent polymer-fullerene solar cells was systematically studied as dependent on the thickness of the capping layer and the active layer as well as the illumination direction. These variations were found to have strong impact on the obtained photocurrent densities. We performed optical simulations of the electric field distribution within the devices using the transfer-matrix method, to analyze the origin of the current density variations in detail and provide deep insight into the device physics. With the conventional absorber materials studied here, optimized ITO-free and semitransparent devices reached 2.0% power conversion efficiency and a maximum optical transmission of 60%, with the device concept being potentially transferable to other absorber materials.