Ludwig Goris

Steric control of the donor/acceptor interface: implications in organic photovoltaic charge generation

The performance of organic photovoltaic (OPV) devices is currently limited by modest short-circuit current densities. Approaches toward improving this output parameter may provide new avenues to advance OPV technologies and the basic science of charge transfer in organic semiconductors. This work highlights how steric control of the charge separation interface can be effectively tuned in OPV devices. By introducing an octylphenyl substituent onto the investigated polymer backbones, the thermally relaxed charge-transfer state, and potentially excited charge-transfer states, can be raised in energy. This decreases the barrier to charge separation and results in increased photocurrent generation. This finding is of particular significance for nonfullerene OPVs, which have many potential advantages such as tunable energy levels and spectral breadth, but are prone to poor exciton separation efficiencies. Computational, spectroscopic, and synthetic methods were combined to develop a structure-property relationship that correlates polymer substituents with charge-transfer state energies and, ultimately, device efficiencies.

Microstructural Origin of High Mobility in High-Performance Poly(thieno-thiophene) Thin-Film Transistors

Adv. Mater. 2010, 22, 697–701 2010 WILEY-VCH Verlag Gm In recent years, semiconducting polymers have been widely studied for their potential applications in low-cost, printed, and flexible electronic devices. Carrier mobility in these materials has been steadily increasing, approaching that of hydrogenated amorphous silicon. The best-performing polymeric semiconductors exhibit a high degree of order and are typically semicrystalline. Charge transport in semicrystalline polymers is controlled at several length scales. In the ordered regions of the material, conjugated polymer chains stack in lamellar sheets with p–p interactions between neighboring chains. In addition to ordered crystallites, the microstructure of semicrystalline polymers comprises disordered regions. The spatial arrangement of the crystallites and the disordered regions affect transport via trapping at defects and the percolation properties of the crystalline and disordered networks. Therefore, in order to develop accurate models of charge transport, it is important to understand the relationship between the morphology of the film, its microstructure and its electronic properties. Identifying transport mechanisms and bottlenecks is of particular relevance to the design of materials with improved performance. In an effort to improve mobility by controlling the microstructure of the polymer, a family of poly(2,5bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes, PBTTT) was recently synthesized. When processed on self-assembled monolayers such as octyltrichlorosilane (OTS) and heated into the liquid crystalline regime, thin films of PBTTT can achieve high room-temperature mobilities up to 0.7–1 cm V 1 s . X-ray diffraction (XRD) strongly suggests that crystallites in PBTTT films are significantly more ordered than those found in thin films of other regio-regular poly(thiophenes) such as poly(3hexyl-thiophene) (P3HT) or poly(3-30 0 0-didodecylquarterthiophene) (PQT-12) The superior electronic performance of annealed PBTTT films is attributed to a highly organized mesoscale morphology that arises from annealing the material through its liquid-crystalline phase. Indeed, atomic force microscopy (AFM) reveals the existence of large (a few hundred nm) terraces that are interpreted as crystalline grains. As a result, transport across the film is thought to be greatly enhanced due to the lower areal density of grain-boundaries, which are known to impede charge transport. Furthermore, because of the liquid-crystalline nature of PBTTT, the regions between the crystalline domains are likely to have a more ordered morphology compared to that of other polymers. To first order, transport in semicrystalline polymers can be understood as a combination of transport through a network of crystallites separated by defects that trap charge. If the regions between the crystallites are more ordered, mobility should be increased due to the correspondingly improved intergranular transport. Thus, according to the previous discussion, it is expected that PBTTT films exhibit a much lower trap density than films of P3HT or PQT-12, and such reduced trap density is the reason for the higher mobility in PBTTT. In this work, we investigate explicitly factors that limit charge transport in PBTTT transistors. We combine a study of charge transport in PBTTT thin films by thin-film transistor (TFT) measurements with structural andmorphological characterization performed by AFM and transmission electronmicroscopy (TEM). Charge transport is analyzed using the mobility edge (ME) model, which has been successfully applied to other semicrystalline polymers before. The great advantage of the ME model is that it allows us to deconvolute the effect of traps and estimate the mobility of the mobile charge in the film, thereby providing means to compare structure–property relationships between different polymers. TFTs were prepared in the bottom-gate staggered configuration. Highly doped silicon wafers with 200 nm of thermal oxide were used as substrates and were cleaned prior to undergoing UV irradiation in an ozone furnace for 20min. Substrates were submerged in octadecyltrichlorosilane (OTS) to form amonolayer on the dielectric surface. Semiconducting polymers solutions, 0.5wt% for both PBTTTwith a C14 alkyl chain (Mw1⁄4 70 kDa) and P3HT (Mw1⁄4 64 and 158 kDa) in 1,2-dichlorobenzene (DCB), were deposited on the substrates via spin-coating. Films of PBTTT were annealed at 180 8C for 10min and, then, cooled down slowly through the mesophase region. To fabricate transistors, 80-nm-thick gold contacts were thermally evaporated

Probing the electrical properties of highly-doped Al: ZnO nanowire ensembles

The analysis of transparent conducting oxide nanostructures suffers from a lack of high throughput yet quantitatively sensitive set of analytical techniques that can properly assess their electrical properties and serve both as characterization and diagnosis tools. This is addressed by applying a comprehensive set of characterization techniques to study the electrical properties of solution-grown Al-doped ZnO nanowires as a function of composition from 0 to 4 at. % Al:Zn. Carrier mobility and charge density extracted from sensitive optical absorption measurements are in agreement with those extracted from single-wire field-effect transistor devices. The mobility in undoped nanowires is 28 cm2/V s and decreases to ∼14 cm2/V s at the highest doping density, though the carrier density remains approximately constant (1020 cm−3) due to limited dopant activation or the creation of charge-compensating defects. Additionally, the local geometry of the Al dopant is studied by nuclear magnetic resonance, showing the...

Transmission electron microscopy of solution-processed, intrinsic and Al-doped ZnO nanowires for transparent electrode fabrication

A solution‐based chemistry was used to synthesize intrinsic and Al‐doped (1% and 5% nominal atomic concentration of Al) ZnO nanostructures. The nanowires were grown at 300°C in trioctylamine by dissolving Zn acetate and Al acetate. Different doping conditions gave rise to different nanoscale morphologies. The effect of a surfactant (oleic acid) was also investigated. An electron microscopy study correlating morphology, aspect ratio and doping of the individual ZnO wires to the electrical properties of the spin coated films is presented. HRTEM revealed single crystalline [0001] wires.

Transport and structural characterization of solution-processable doped ZnO nanowires

The use of ZnO nanowires has become a widespread topic of interest in optoelectronics. In order to correctly assess the quality, functionality, and possible applications of such nanostructures it is important to accurately understand their electrical and optical properties. Aluminum- and gallium-doped crystalline ZnO nanowires were synthesized using a low-temperature solution-based process, achieving dopant densities of the order of 1020 cm-3. A non-contact optical technique, photothermal deflection spectroscopy, is used to characterize ensembles of ZnO nanowires. By modeling the free charge carrier absorption as a Drude metal, we are able to calculate the free carrier density and mobility. Determining the location of the dopant atoms in the ZnO lattice is important to determine the doping mechanisms of the ZnO nanowires. Solid-state NMR is used to distinguish between coordination environments of the dopant atoms.

Relating microstructure to transport in organic semiconductor transistors

As organic semiconductors approach commercialization, there is a need to better understand the relationship between charge transport and microstructure, in particular to identify the inherent bottlenecks to charge transport. The highest field-effect mobility is found in semicrystalline polymers and in solubilized small molecules that crystallize readily. No consistent correlation however is found between grain size and mobility. This observation suggests the importance of the role of intergrain charge transport in semicrystalline and polycrystalline organic thin films.