Effect of vacuum treatment in diketopyrrolopyrrole (DPP) based copolymer with ratio controlled toluene- and benzene- functional groups for efficient organic photovoltaic cells: Morphological and electrical contribution

Solution-processed organic bulk-heterojunction (BHJ) photovoltaic cells using random copolymeric donor materials have been extensively reported due to their suitable film-forming characteristics and phase-separated nano-morphology. Here, ratio-controlled toluene-versus benzene-chemical group based diketopyrrolopyrrole (DPP) donor polymers mixed with a fullerene acceptor were investigated to fabricate an efficient photovoltaic active layer with improved electrical properties through a vacuum treatment. The vacuum process leads to an increase in the phase-separation with a low surface roughness and nanoscale-distributed crystallinity due to securing the dry time of the residual solvent and solvent additive within the active layer. Moreover, the optimized DPP-based donor with toluene (T) versus benzene (B) linkers and electron transporting layer leads to an improvement in the power conversion efficiencies of up to 6.31% under AM 1.5G illumination due to the contributions of an efficient charge transfer and reduced series resistance. Therefore, the organic semiconductor obtained with the ratio-controlled molecular structure and proper solvent drying process plays an important role in increasing the electrical and morphological properties to produce efficient organic solar cells.     

The influence of nanostructure on polymer-based optoelectronic device functioning: A computer simulation study

During the last years it has been clear that it is important to understand and control the nanostructure of the active polymer layer used in optoelectronic devices, like polymer diodes, solar cells or field effect transistors. Several experimental studies have shown that the nanostructure of polymer thin films used in these optoelectronic devices depends on the conditions used to deposit the polymer layer between the electrodes. As a result, in solid state conjugated polymer chains tend to be stacked and aligned relative to the electrodes creating domains with different sizes that influence the performance of these devices. To understand how the spatial arrangement of polymer chains affects the functioning of optoelectronic devices, we performed computer simulations using our mesoscopic model based on a generalized dynamic Monte Carlo method. We focus our study on the influence of the nanomorphology on the electric properties of polymer light emitting diodes. Our results show that for a pristine polymer layer and in the presence of ohmic contacts between the electrodes and the polymer layer, the electric properties of the device, namely current density, charge density, internal electric field and the number of charges that undergo recombination strongly depend on the polymer morphology at nanoscale.     

Combined Langmuir-Blodgett and sol-gel coatings

Optical properties and in-depth structure of double-layer coatings on glass substrates were investigated. One of the layers was prepared by dip coating either from silica or titania sol, the other layer was made from ca. 130 nm Stöber silica particles by the Langmuir-Blodgett (LB) technique. Two different types of combined coatings were prepared: (1) nanoparticulate LB films coated with sol-gel (SG) films and (2) nanoparticulate LB films drawn onto SG films.Scanning electron microscopy and optical methods, i.e. UV-vis spectroscopy and scanning angle reflectometry were applied for analyzing the structure and thickness of coatings. These measurements revealed that the precursor sols could not penetrate into the particulate LB film completely in case of coating type (1). For coating type (2) very little overlap between the SG and LB layers was found resulting in significant improvement of light transmittance of combined coatings compared to single SG films.To show some possible advantages of the combination of these techniques additional studies were carried out. Surface morphology of combined coatings (1) was studied by atomic force microscopy. Surfaces with different roughness were developed depending on the thickness of the sol-gel film (titania: ca. 70 nm; silica: ca. 210 nm). The adhesive peel off test revealed improved mechanical stability of combined coatings (2), in comparison to LB films which makes them good candidates for further applications.