Martin Knipper

Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells

Electrical and optical properties of poly(3-hexylthiophene-2.5diyl) (P3HT) used as the main component in a polymer/fullerene solar cell were studied. From the study of space-charge limited current behavior of indium-tin-oxide (ITO)/P3HT/Au hole-only devices, the hole mobility and density were estimated to range from 1.4×10−6 cm2/V s and 5.3×1014 cm−3 at 150 K to 8.5×10−5 cm2/V s and 1.1×1015 cm−3 at 250 K, respectively. The highest occupied to lowest occupied molecular orbital energetic difference was estimated from absorption spectrometry to be about 2.14 eV. Strong quenching of photoluminescence when the polymer was mixed with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), provided evidence of photoinduced charge transfer from P3HT to PCBM. Characterization of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells was done by analyzing the dependence of current density–voltage characteristics on temperature and illumination intensity. The main solar cell characteristics recorded at 300 K under 100 mW/cm2 white-ligh...

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.

Electrical and optical design and characterisation of regioregular poly(3-hexylthiophene-2,5diyl)/fullerene-based heterojunction polymer solar cells

Electrical and optical properties of poly(3-hexylthiophene-2,5diyl) (P3HT-2,5diyl) used as the main component in a bulk heterojunction polymer/fullerene solar cell were investigated. The HOMO level of the polymer was estimated at about 4.7–5.1 eV, from the observed space charge limited current (SCLC) studies in ITO/P3HT-2,5diyl/Au hole-only devices, which confirmed the formation of ohmic contacts between the polymer and the Au and ITO electrodes. The values calculated for hole mobility and density range from 1:4 � 10 � 6 cm 2 /(V s) and 5:3 � 10 14 cm � 3 at 150 K to 8:5 � 10 � 5 cm 2 /(V s) and 1:1 � 10 15 cm � 3 at 250 K, respectively. A HOMO–LUMO gap of 2.14 eV was estimated from an absorption spectrum of the polymer. Photoinduced charge transfer from polymer to PCBM was evidenced by strong photoluminiscence quenching, which was observed when the polymer was mixed with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Charge carrier transport properties of the polymer/fullerene solar cells were studied by analysing the dependence of J–V characteristics on temperature and illumination intensity. A linear decrease of the open-circuit voltage with increasing temperature, with a local maximum around 320 K, was observed. The short-circuit current density increased with temperature, having a maximum around 300 K and decreased thereafter. Efficiency and fill factor presented maxima around 3 mW/cm 2 white light intensity, and this was attributed to the poor bulk transport properties of the active layer. Typical values recorded for the solar cell at 300 K under white light of 100 mW/cm 2 intensity were: open-circuit voltage 0.48 V, and current density 1.28 mA/cm 2 , with an efficiency of 0.2% and fill factor of 30.6%.

1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells

1,2,4-Triazolium perfluorobutanesulfonate (1), a novel, pure protic organic ionic plastic crystal (POIPC) with a wide plastic crystalline phase, has been explored as a proof-of-principle anhydrous proton conductor for all-solid-state high temperature hydrogen/air fuel cells. Its physicochemical properties, including thermal, mechanical, structural, morphological, crystallographic, spectral, and ion-conducting properties, as well as fuel cell performances, have been studied comprehensively in both fundamental and device-oriented aspects. With superior thermal stability, 1 exhibits crystal (phase III), plastic crystalline (phase II and I) and melt phases successively from −173 °C to 200 °C. Differential scanning calorimetry and temperature-dependent powder X-ray diffraction (XRD) measurements together with polarized optical microscopy and thermomechanical analysis reveal the two solid–solid phase transitions of 1 at 76.8 °C and 87.2 °C prior to the melting transition at 180.9 °C, showing a wide plastic phase (87–181 °C). Scanning electron microscopy displays the morphology of different phases, indicating the plasticity in phase I. Single-crystal XRD studies reveal the molecular structure of 1 and its three-dimensional N–H⋯O hydrogen bonding network. The influence of the three-dimensional hydrogen bonding network on the physicochemical properties of 1 has been highlighted. The temperature dependence of hydrogen bonding is investigated by variable-temperature infrared spectroscopy. The sudden weakening of hydrogen bonds at 82 °C seems to be coupled with the onset of orientational or rotational disorder of the ions. The temperature dependence of ionic conductivity in the solid and molten states is measured via impedance spectroscopy and current interruption technique, respectively. The Arrhenius plot of the ionic conductivity assumes a lower plateau region (phase I, 100–155 °C) with a low activation energy of ∼36.7 kJ mol−1 (i.e. ∼0.38 eV), suggesting likely a Grotthuss mechanism for the proton conduction. Variable-temperature infrared analysis, optical morphological observations, and powder XRD patterns further illustrate the structural changes. Electrochemical hydrogen pumping tests confirm the protonic nature of the ionic conduction observed in the lower plateau region. Finally, measurements of the open circuit voltages (OCVs) and the polarization curves of a dry hydrogen/air fuel cell prove the long-range proton conduction. At 150 °C, a high OCV of 1.05 V is achieved, approaching the theoretical maximum (1.11 V).

Impedance Spectroscopy on Polymer-Fullerene Solar Cells

Impedance spectroscopy is used for studying the electrical transport properties of bulk heterojunction solar cells. A replacement circuit is needed to translate the frequency response of the circuit to the individual interfaces and layers of the solar cell. As a material combination and device architecture, composites of P3HT and PCBM, sandwiched between a transparent ITO front electrode and an aluminum back electrode, as well as a polymer buffer layer were investigated. By varying the film thickness we identified an equivalent circuit capable to fit our experimental data. We found a dielectric constant for the P3HT and for the P3HT:PCBM bulk.

Manganese oxide phases and morphologies: A study on calcination temperature and atmospheric dependence.

Summary Manganese oxides are one of the most important groups of materials in energy storage science. In order to fully leverage their application potential, precise control of their properties such as particle size, surface area and Mnx + oxidation state is required. Here, Mn3O4 and Mn5O8 nanoparticles as well as mesoporous α-Mn2O3 particles were synthesized by calcination of Mn(II) glycolate nanoparticles obtained through an economical route based on a polyol synthesis. The preparation of the different manganese oxides via one route facilitates assigning actual structure–property relationships. The oxidation process related to the different MnOx species was observed by in situ X-ray diffraction (XRD) measurements showing time- and temperature-dependent phase transformations occurring during oxidation of the Mn(II) glycolate precursor to α-Mn2O3 via Mn3O4 and Mn5O8 in O2 atmosphere. Detailed structural and morphological investigations using transmission electron microscopy (TEM) and powder XRD revealed the dependence of the lattice constants and particle sizes of the MnOx species on the calcination temperature and the presence of an oxidizing or neutral atmosphere. Furthermore, to demonstrate the application potential of the synthesized MnOx species, we studied their catalytic activity for the oxygen reduction reaction in aprotic media. Linear sweep voltammetry revealed the best performance for the mesoporous α-Mn2O3 species.

Synthesis and electrochemical performance of surface-modified nano-sized core/shell tin particles for lithium ion batteries

Tin is able to lithiate and delithiate reversibly with a high theoretical specific capacity, which makes it a promising candidate to supersede graphite as the state-of-the-art negative electrode material in lithium ion battery technology. Nevertheless, it still suffers from poor cycling stability and high irreversible capacities. In this contribution, we show the synthesis of three different nano-sized core/shell-type particles with crystalline tin cores and different amorphous surface shells consisting of SnOx and organic polymers. The spherical size and the surface shell can be tailored by adjusting the synthesis temperature and the polymer reagents in the synthesis, respectively. We determine the influence of the surface modifications with respect to the electrochemical performance and characterize the morphology, structure, and thermal properties of the nano-sized tin particles by means of high-resolution transmission electron microscopy, x-ray diffraction, and thermogravimetric analysis. The electrochemical performance is investigated by constant current charge/discharge cycling as well as cyclic voltammetry.

In situ X-ray diffraction study on the formation of α-Sn in nanocrystalline Sn-based electrodes for lithium-ion batteries

In situ X-ray diffraction (XRD) was performed to study the formation of the α-Sn structure in nanocrystalline Sn-based electrodes during electrochemical lithium insertion and extraction at room temperature. Therefore, pure β-Sn nanoparticles were synthesised and further processed into electrodes. The lithiation and de-lithiation process of the β-Sn nanoparticles follows the formation of discrete lithium–tin phases which perfectly fits the voltage plateaus in the charge/discharge diagram. However, unlike bulk electrodes, where no α-Sn is formed, we observed the formation of the semiconducting α-modification at 870 mV vs. Li within the first de-lithiation process. This observation explains earlier reports of an increasing internal resistance of such an electrode. Additionally, our study supports earlier suggestions that predominantly small tin crystallites are transformed from the β-Sn phase into the α-Sn phase, while larger crystallites retain their metallic β-Sn structure.

Critical size for the β- to α-transformation in tin nanoparticles after lithium insertion and extraction

Tin nanoparticles can be transformed from the metallic β-Sn structure to the semiconducting α-Sn structure after electrochemical lithiation and delithiation at room temperature. Here, we studied the influence of the size of the crystallites on the β- to α-transformation in Sn nanoparticles. Differently sized Sn/SnOx nanoparticles were synthesized, processed in electrodes and cycled ten times in a lithium-ion cell at room temperature. X-ray diffraction (XRD) patterns before and after electrochemical lithium insertion/extraction reveal that samples with small particles contain the α-Sn structure. The critical size for this transformation is 17(4) nm. Smaller particles were transformed into the α-Sn structure while particles larger than 17 nm retain the β-Sn structure. Temperature dependent XRD measurements show that this α-Sn structure is stable up to 220 °C before its reflections disappear. The formation of the α-Sn structure at room temperature in small particles and the unexpected high transition point can be explained by the substantial contribution of the surface energy (facilitating formation of alloys not observed in the bulk), lithium impurities in the α-Sn structure and the Li2O shell which is formed during lithium insertion.

Influence of Vanadium Ions on the Degradation Behavior of Platinum Catalysts for Oxygen Reduction Reaction

The vanadium air redox flow battery is a combination of a redox flow battery and a reversible fuel cell. For the oxygen reduction during discharge, platinum (Pt) catalysts are common. During operation, vanadium (V) cations can penetrate through a proton exchange membrane into the water/air half-cell. The aim of the present work is to study whether V compounds are deposited on the Pt surface under operation conditions or whether the V ions influence the stability of Pt in any other way. Thereby, bulk platinum electrodes are compared as a simple model system to carbon-supported Pt nanoparticles via cyclic voltammetry. In the case of bulk platinum, electrochemical quartz crystal microbalance measurements showed no deposition of vanadium compounds but indicated the decrease of the (hydr)oxide layer on Pt above V3+ and VO2+ redox potentials. Cycling 100 times between oxygen reduction and oxygen evolution potentials with and without a heavy V contamination did not lead to significant degradation of the model catalyst and shows no influence of V ions. On the contrary, the nanoparticle-based catalyst significantly degraded during the same stability protocol. The V contamination lowered the degradation in this case.