Application of hybrid blocking layers in solid-state dye-sensitized solar cells.

A hybrid blocking layer consisting of a conducting TiO2 network embedded in a ceramic matrix is implemented in a solid-state dye-sensitized solar cell. This novel type of blocking layer is thinner than the classical blocking layer films as shown with SEM and XRR measurements, and thereby the conductivity of the hybrid film is increased by 110%. A percolating TiO2 network, proven by TEM/ESI and GISAXS measurements, allows for the charge transport. Although being thinner, the hybrid film completely separates the rough electrode material from the hole-transport medium in solar cells to avoid the recombination of charge carriers at this interface. In total, the power conversion efficiency of solar cells is improved: the application in photovoltaics shows that the efficiency of devices with the hybrid blocking layer is increased by 6% compared to identical solar cells employing the conventional blocking layer.

Development of the morphology during functional stack build-up of P3HT:PCBM bulk heterojunction solar cells with inverted geometry.

Highly efficient poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells are achieved by using an inverted geometry. The development of the morphology is investigated as a function of the multilayer stack assembling during the inverted solar cell preparation. Atomic force microscopy is used to reveal the surface morphology of each stack, and the inner structure is probed with grazing incidence small-angle X-ray scattering. It is found that the smallest domain size of P3HT is introduced by replicating the fluorine-doped tin oxide structure underneath. The structure sizes of the P3HT:PCBM active layer are further optimized after thermal annealing. Compared to devices with standard geometry, the P3HT:PCBM layer in the inverted solar cells shows smaller domain sizes, which are much closer to the exciton diffusion length in the polymer. The decrease in domain sizes is identified as the main reason for the improvement of the device performance.

Nano- and microstructures of magnetic field-guided maghemite nanoparticles in diblock copolymer films.

The control over the alignment of nanoparticles within a block copolymer matrix was investigated for different external magnetic fields with respect to producing well-aligned, highly oriented metal-oxide-polymer nanopatterns. Hybrid films were prepared by solution casting under a range of external magnetic fields. The nano- and microstructure of maghemite nanoparticles within poly(styrene-b-methyl methacrylate) diblock copolymer films as a function of the nanoparticle concentration was studied using optical microscopy, atomic force microscopy, scanning electron microscopy, and grazing incidence small-angle X-ray scattering. Because of a polystyrene (PS) coating, the nanoparticles are incorporated in the PS domains of the diblock copolymer morphology. At higher nanoparticle concentrations, nanoparticle aggregates perturb the block copolymer structure and accumulate at the films surface into wire-shaped stripes. These wire-shaped nanoparticle aggregates form mainly because of the competition between nanoparticle-polymer friction and magnetic dipolar interaction. The magnetic behavior of the hybrid films was probed at different temperatures for two orthogonal directions (with the line-shaped particle aggregates parallel and perpendicular to the magnetic field). The hybrid film systems show superparamagnetic behavior and remarkable shape anisotropy that render them interesting for magnetic applications.

A direct evidence of morphological degradation on a nanometer scale in polymer solar cells.

In situ measurement of a polymer solar cell using micro grazing incidence small angle X-ray scattering (µGISAXS) and current-voltage tracking is demonstrated. While measuring electric characteristics under illumination, morphological changes are probed by µGISAXS. The X-ray beam (green) impinges on the photo active layer with a shallow angle and scatters on a 2d detector. Degradation is explained by the ongoing nanomorphological changes observed.

Infiltration of polymer hole-conductor into mesoporous titania structures for solid-state dye-sensitized solar cells.

The degree of filling of titania nanostructures with a solid hole-conducting material is important for the performance of solid-state dye-sensitized solar cells (ssDSSCs). Different ways to infiltrate the hole-conducting polymer poly(3-hexylthiophene) (P3HT) into titania structures, both granular structures as they are already applied commercially and tailored sponge nanostructures, are investigated. The solar cell performance is compared to the morphology determined with scanning electron microscopy (SEM) and time-of-flight grazing incidence small-angle neutron scattering (TOF-GISANS). The granular titania structure, commonly used for ssDSSCs, shows a large distribution of particle and pore sizes, with porosities in the range from 41 to 67%, including even dense parts without pores. In contrast, the tailored sponge nanostructure has well-defined pore sizes of 25 nm with an all-over porosity of 54%. Filling of the titania structures with P3HT by solution casting results in a mesoscopic P3HT overlayer and consequently a bad solar cell performance, even though a filling ratio of 67% is observed. For the infiltration by repeated spin coating, only 57% pore filling is achieved, whereas filling by soaking in the solvent with subsequent spin coating yields filling as high as 84% in the case of the tailored titania sponge structures. The granular titania structure is filled less completely than the well-defined porous structures. The solar cell performance is increased with an increasing filling ratio for these two ways of infiltration. Therefore, filling by soaking in the solvent with subsequent spin coating is proposed.

Low-temperature sol-gel synthesis of nanostructured polymer/titania hybrid films based on custom-made poly(3-alkoxy thiophene).

A low-temperature route to directly obtain polymer/titania hybrid films is presented. For this, a custom-made poly(3-alkoxy thiophene) was synthesized and used in a sol-gel process together with an ethylene-glycol-modified titanate (EGMT) as a suitable titania precursor. The poly(3-alkoxy thiophene) was designed to act as the structure-directing agent for titanium dioxide through selective incorporation of the titania precursor. The nanostructured titania network, embedded in the polymer matrix, is examined with atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. By means of the scattering technique grazing incidence wide-angle X-ray scattering (GIWAXS), a high degree of crystallinity of the polymer as well as successful transformation of the precursor into the rutile phase of titania is verified. UV/Vis measurements reveal an absorption behavior around 500 nm which is similar to poly(3-hexyl thiophene), a commonly used polymer for photoelectronic applications, and in addition, the typical UV absorption behavior of rutile titania is observed.

Low-temperature route to crystalline titania network structures in thin films.

A low temperature route to crystalline titania nanostructures in thin films is presented. The synthesis is performed by the combination of sol-gel processes, using a novel precursor for this kind of application, an ethylene glycol-modified titanate (EGMT), and the structure templating by micro-phase separation of a di-block copolymer. Different temperatures around 100 °C are investigated. The nanostructure morphology is examined with scanning electron microscopy, whereas the crystal structure and thin film compositions are examined by scattering methods. Optoelectronic measurements reveal the band-gap energies and sub-band states of the titania films. An optimum titania thin film is created at temperatures not higher than 90 °C, regarding sponge-like morphology with pore sizes of 25-30 nm, porosity of up to 71% near the sample surface, and crystallinity of titania in the rutile phase. The low temperature during synthesis is of high importance for photovoltaic applications and renders the resulting titania films interesting for future energy solutions.

Influence of nanoparticle surface functionalization on the thermal stability of colloidal polystyrene films.

The installation of large scale colloidal nanoparticle thin films is of great interest in sensor technology or data storage. Often, such devices are operated at elevated temperatures. In the present study, we investigate the effect of heat treatment on the structure of colloidal thin films of polystyrene (PS) nanoparticles in situ by using the combination of grazing incidence small-angle X-ray scattering (GISAXS) and optical ellipsometry. In addition, the samples are investigated with optical microscopy, atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM). To install large scale coatings on silicon wafers, spin-coating of colloidal pure PS nanoparticles and carboxylated PS nanoparticles is used. Our results indicate that thermal annealing in the vicinity of the glass transition temperature T(g) of pure PS leads to a rapid loss in the ordering of the nanoparticles in spin-coated films. For carboxylated particles, this loss of order is shifted to a higher temperature, which can be useful for applications at elevated temperatures. Our model assumes a softening of the boundaries between the individual colloidal spheres, leading to strong changes in the nanostructure morphology. While the nanostructure changes drastically, the macroscopic morphology remains unaffected by annealing near T(g).

Fabrication and characterization of nanostructured titania films with integrated function from inorganic-organic hybrid materials.

Nanostructured titania films are of growing interest due to their application in future photovoltaic technologies. Therefore, a lot of effort has been put into the controlled fabrication and tailoring of titania nanostructures. The controlled sol-gel synthesis of titania, in particular in combination with block copolymer templates, is very promising because of its high control on the nanostructure, easy application and cheap processing possibilities. This tutorial review gives a short overview of the structural control of titania films gained by using templated sol-gel chemistry and shows how this approach is extended by the addition of further functionality to the films. Different expansions of the sol-gel templating are possible by the fabrication of gradient samples, by the addition of a homopolymer, by the combination with micro-fluidics and also by the application of novel precursors for low-temperature processing. Moreover, hierarchically structured titania films can be fabricated via the subsequent application of several sol-gel steps or via the inclusion of colloidal templates in a one-step process. Integrated function in the block copolymer used in the sol-gel synthesis allows for the fabrication of an integrated blocking layer or an integrated hole-conductor. Both approaches grant a one-step fabrication of two components of a working solar cell, which make them very promising towards a cheap solar cell production route. Looking to the complete solar cell, the top contact is also of great importance as it influences the function of the whole solar cell. Thus, the mechanisms acting in the top contact formation are also reviewed. For all these aspects, characterization techniques that allow for a structural investigation of nanostructures inside the active layers are important. Therefore, the characterization techniques that are used in real space as well as in reciprocal space are explained shortly as well.

Comparative study of conventional and hybrid blocking layers for solid-state dye-sensitized solar cells.

In hybrid solar cells a blocking layer between the transparent electrode and the mesoporous titanium dioxide is used to prevent short-circuits between the hole-conductor and the front electrode. The conventional approach is to use a compact film of titanium dioxide. This layer has to be of optimum thickness: it has to cover the rough surface of the anode material completely while keeping it as thin as possible since the layer acts as an ohmic resistance itself. A competitive alternative arises when using an amphiphilic diblock copolymer as a functional template to produce thin, hybrid films containing a conducting titanium dioxide network embedded in an insulating ceramic material. These hybrid films can be produced much thinner compared to the conventional approach and, hence, they possess a 32% higher conductivity. The conventional and the hybrid blocking layer are characterized by conductive scanning probe microscopy and macroscopic conductance measurements. Additionally, the functionality of both blocking layers in solid-state dye-sensitized solar cells, as tested with current-voltage measurements, is verified.