van Jkj Jeroen Duren

Synthesis and Characterization of a Low Bandgap Conjugated Polymer for Bulk Heterojunction Photovoltaic Cells

Low optical bandgap conjugated polymers may improve the efficiency of organic photovoltaic devices by increasing the absorption in the visible and near infrared region of the solar spectrum. Here we demonstrate that condensation polymerization of 2,5-bis(5-trimethylstannyl-2-thienyl)-N-dodecylpyrrole and 4,7-dibromo-2,1,3-benzothiadiazole in the presence of Pd(PPh3)2Cl2 as a catalyst affords a novel conjugated oligomeric material (PTPTB), which exhibits a low optical bandgap as a result of the alternation of electron-rich and electron-deficient units along the chain. By varying the molar ratio of the monomers in the reaction and fractionation of the reaction product, two different molecular weight fractions (PTPTB-I and PTPTB-II, see Experimental section) were isolated, containing 5–17 and 13–33 aromatic units respectively, as inferred from matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Thin films of PTPTB-I and PTPTB-II exhibit an optical bandgap of 1.60 and 1.46 eV, respectively. Photoinduced absorption (PIA) and photoluminescence spectroscopy of blends of PTPTB-I and a methanofullerene (1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61, PCBM) gave direct spectral evidence of the photoinduced electron-transfer reaction from PTPTB-I as a donor to the fullerene derivative as an acceptor. Thin PTPTB-I:PCBM composite films were sandwiched between indium tin oxide/poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (ITO/PEDOT:PSS) and Al electrodes to prepare working photovoltaic devices, which show an open circuit voltage of 0.67 V under white-light illumination. The spectral dependence of the device shows an onset of the photocurrent at 1.65 eV (750 nm).

Injection-limited electron current in a methanofullerene

The dark current of bulk-heterojunction photodiodes consisting of a blend of a methanofullerene (PCBM) as n-type electron acceptor and a dialkoxy-(p-phenylene vinylene) (OC1C10−PPV) as a p-type electron donor sandwiched between electrodes with different work functions has been investigated. With ohmic contacts for hole and electron injection, the dark current appears completely dominated by the electron current in the PCBM, as a result of a much higher electron mobility. This electron current is bulk space-charge limited. With Au as a high work function metal, the electron current becomes injection limited. The injection-limited electron current from the Au electrode into PCBM is explained within a thermally assisted hopping model. In spite of the presence of an injection barrier of about 0.76 eV, the injection-limited electron current from a Au electrode into PCBM still exceeds the bulk-limited hole current in OC1C10−PPV.

Design and synthesis of processible functional copolymers

Novel tailor-made AB, ABAC, and ABAA type heteroaromatic copolymers, in which B represents an N-alkylpyrrole unit, were synthesized via Pd-catalyzed Stille polymerization and characterized by a variety of analytical techniques. MALDI-TOF mass spectrometry allowed the detailed characterization of polymer weight and determination of end groups. By varying the nature of the repeating monomer units and their sequence, copolymers with promising properties for photovoltaic devices could be obtained.

Stimulation of electrical conductivity in a π-conjugated polymeric conductor with infrared light

Irradiation with infrared light is found to stimulate the electrical conductivity of a film of an organic polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in conductivity is found to be linear in the intensity of the irradiation (4–400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradiation, relaxes according to ΔR(t)∝(1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, we model the diffusion of heat from the polymer film to the supporting glass substrate. Assuming that the change in resistance is linear with the raise in temperature caused by the infrared irradiation, one predicts a ΔR(t)∝(1/t)0.5 dependence. The similarity between the model and experimental behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Electrical characteriza...

Imaging individual chains and aggregates on conjugated polymer films

We demonstrate a novel method to visualize individual molecular chains in the surface layer of organic semiconducting polymer films of arbitrary thickness. The method is based on scanning force mic ...

Measuring the potential distribution inside soft organic semiconductors with a scanning-tunneling microscope

For the first time, we directly measured the potential distribution inside organic semiconductors. Combined spectroscopic measurements are performed on MDMO-PPV layers on Au and Yb substrates, using a scanning-tunneling microscope. The results are analyzed with a model that treats both current injection and bulk transport in detail. It is found that tip height-bias curves, which are taken by following the height of the STM tip as a function of bias, while the STM feedback system is active, reflect the potential distribution between tip and sample electrode.