Laurence Lutsen

Charge transport and recombination in bulk heterojunction solar cells studied by the photoinduced charge extraction in linearly increasing voltage technique

Charge carrier mobility and recombination in a bulk heterojunction solar cell based on the mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61 (PCBM) has been studied using the novel technique of photoinduced charge carrier extraction in a linearly increasing voltage (Photo-CELIV). In this technique, charge carriers are photogenerated by a short laser flash, and extracted under a reverse bias voltage ramp after an adjustable delay time (tdel). The Photo-CELIV mobility at room temperature is found to be μ=2×10−4cm2V−1s−1, which is almost independent on charge carrier density, but slightly dependent on tdel. Furthermore, determination of charge carrier lifetime and demonstration of an electric field dependent mobility is presented.

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

Efficient formation, isolation and characterization of poly(3-alkylthiophene) nanofibres: probing order as a function of side-chain length

Efficient fibre formation for all regioregular poly(3-alkylthiophene)s (P3ATs) with alkyl chain lengths (A) between 3 and 9 carbon atoms has been accomplished in several solvents. It was observed that for the aliphatic and (chlorinated) aromatic hydrocarbon solvents used, the solvent refractive index offers some rationale to predict the feasibility of a solvent for fibre formation. The fibres were separated from remaining non-organised polymer by centrifugation. This enabled the characterisation of the isolated fibres in function of alkyl chain length (A) with TEM, AFM, XRD and UV-Vis spectroscopy. The fibres are 20 ± 5 nm wide and 0.5 to >4 µm long and mainly crystallize in the common type I crystal phase. The order within the fibres was probed with XRD, SAED, and UV-Vis and was found to strongly improve with increasing alkyl chain length in going from P33T to P35T, resulting in a longer conjugation length. For P35T to P39T the improvement in order is only marginal. Fibres from P37T were found to mainly crystallize in a crystal phase slightly different from type I that we refer to as type I′. This new crystal structure has a lattice constant a that is marginally shorter than that of phase I and a slightly longer lattice constant b of 4.0 A and thus in XRD can hardly be distinguished from phase I. It is furthermore characterized by a blue-shifted absorption band in UV-Vis spectroscopy. The type I′ fibres were converted into normal type I fibres in the solid state at 70 °C and in solution around 50 °C.

Varying polymer crystallinity in nanofiber poly(3-alkylthiophene): PCBM solar cells: Influence on charge-transfer state energy and open-circuit voltage

The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (ECT) and on the open-circuit voltage (Voc) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between ∼0.1 and ∼0.9 by temperature control. ECT, as obtained by Fourier-transform photocurrent spectroscopy decreased with f as ECT=ECT0−0.2f eV. Alkyl side-chain length only influences ECT0. Voc relates to ECT as Voc=ECT/q−0.6 V.

Nanoscale electrical characterization of organic photovoltaic blends by conductive atomic force microscopy

Conductive atomic force microscopy (CAFM) is introduced to perform electrical characterization of organic photovoltaic blends with high spatial resolution. Reference blends used in organic bulk heterojunction solar cells are investigated. The ability of CAFM to electrically evidence phase separated donor and acceptor regions is demonstrated. Furthermore, local spectroscopy is performed to analyze charge transport mechanisms in the blends. Significant modifications of the electrical properties of the semiconducting polymers are shown to occur after blending with fullerene derivatives. Finally, the sensitivity of CAFM to photoelectrical phenomena is revealed. Current variations of few picoamperes are locally observed under illumination of P3HT:PCBM.

A comparison between state-of-the-art ‘gilch’ and ‘sulphinyl’ synthesised MDMO-PPV/PCBM bulk hetero-junction solar cells

Abstract To obtain photovoltaic devices based on electron donating conjugated polymers with a higher efficiency, a major breakthrough was realised by mixing the polymers with a suitable electron acceptor, thereby enhancing the rate for photo-induced charge generation by several orders. State-of-the-art organic bulk hetero-junction photovoltaic cells are based on an interpenetrating donor–acceptor network in the bulk to form efficient nanostructured p–n junctions in the organic materials. Devices made with ‘Gilch’ poly(2-methoxy-5-(3′,7′-dimethyl-octyloxy))- p -phenylene vinylene, (MDMO-PPV), as an electron donor and (6,6)-phenyl-C 61 -butyric-acid (PCBM) (a soluble C60 derivative) as an electron acceptor yielded the highest efficiency until now in this class of devices. A power conversion efficiency of approximately η e ≥2.5% (electrical power out/incident light power) under AM 1.5 illumination was reported. The ‘gilch’ route is a direct synthetic route. The ‘sulphinyl’ route is a promising, indirect precursor-route towards MDMO-PPV. Due to the non-symmetric monomer, so-called ‘head-to-head’ and ‘tail-to-tail’ additions are excluded to a higher level in comparison to the ‘gilch’ route. This difference between both materials makes them interesting candidates to compare them in the state-of-the-art photovoltaic devices. Preliminary results indicate that the ‘sulphinyl’ MDMO-PPV/PCBM bulk hetero-junction solar cells attain a power conversion efficiency of nearly η e =3% (electrical power out/incident light power), have a higher fill factor, incident photon per converted electron value (IPCE) and short circuit current. It is indicated that the observed solar cell characteristics are related to the defect level of the conjugated polymer used.

Synthesis of poly(p-phenylene vinylene) materials via the precursor routes

Poly(p-phenylene vinylene)s (PPVs) are an important class of conjugated polymer materials that have in the last few years gained significant interest in the polymer community. (Precursor) Synthesis routes to obtain high-molecular weight PPVs are reviewed with respect to the applicability of the reactions towards specific synthesis goals and materials as well as structural integrity of the obtained polymers, which affect the applicability of these compounds in electronic devices.

Conjugated polymers based on new thienylene – PPV derivatives for solar cell applications

Two p-conjugated monomers based on bis-(1-cyano-2-thienyl-vinylene)phenylene derivatives were synthesized by Knoevenagel condensation. Both monomers are found to form electroactive polymers upon electrochemical oxidation. The withdrawing effect due to the cyano-substituent allows for the reversible n-doping of the polymer. Thus, the band gap Eg was measured using electrochemical techniques and compared with that obtained by UV–VIS–NIR spectroscopy. Based on the measured band gap of 1.87 and 1.58 eV, these polymers appear to be interesting candidates for solar-cell applications. 2002 Elsevier Science B.V. All rights reserved.

In situ conductivity measurements on polyethylenedioxythiophene derivatives with different counter ions

Abstract 3,4-Ethylenedioxythiophene (EDT) and thieno-(3,4-b)-2,3-dihydro-2-tetradecyl-1,4-dioxane (EDT-C14) were electrochemically deposited onto Pt using different electrolytes in acetonitrile. The resulting PEDT films containing different counter ions were subsequently characterized and their in situ conductivity values were measured using a two band electrode. The latter showed that the highest conductivities were obtained with ClO4− and BF4− as counter ions. Furthermore, the morphology, and thus the conductive properties, of the films seem to be strongly dependent on the applied polymerization potential.

Charge dissociation in polymer:fullerene bulk heterojunction solar cells with enhanced permittivity

The dissociation efficiency of bound electron-hole pairs at the donor-acceptor interface in bulk heterojunction solar cells is partly limited due to the low dielectric constant of the polymer:fullerene blend. We investigate the photocurrent generation in blends consisting of a fullerene derivative and an oligo(oxyethylene) substituted poly(p-phenylene vinylene) (PPV) derivative with an enhanced relative permittivity of 4. It is demonstrated that in spite of the relatively low hole mobility of the glycol substituted PPV the increase in the spatially averaged permittivity leads to an enhanced charge dissociation of 72% for these polymer:fullerene blends.