D. Vanderzande

Disclosure of the nanostructure of MDMO-PPV:PCBM bulk hetero-junction organic solar cells by a combination of SPM and TEM

Abstract The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is studied by atomic force microscopy (AFM) to image the surface morphology and by means of transmission electron microscopy (TEM) to disclose the bulk nanostructure of the film. Typical thin films, as used for state-of-the-art organic bulk hetero-junction solar cells consist of a 1:4 ratio by weight of MDMO-PPV as electron donating polymer and PCBM, a soluble electron accepting C 60 derivative. For these films it is found, using both TEM an AFM, that phase separation occurs. A two-phase system is observed that consists of PCBM-rich domains that are embedded in a matrix consisting of a mixture of MDMO-PPV and PCBM. By combining planar and cross-sectional views, three-dimensional information is obtained on the phase separated PCBM-rich regions, formed during spincoating. Changing the solvent is found to influence the size of the phase separated PCBM-rich domains. But not only the dimensions of the phase separated regions are affected by changing the solvent. Also the composition of the matrix is found to be determined by the choice of solvent. This was studied by changing the ratio of PCBM compared to MDMO-PPV. Since it is commonly believed that the morphology of the active layer influences electrical properties and photovoltaic performance, the nanostructural information obtained with the presented analytical techniques will attribute to a better understanding and improvement of present organic photovoltaic devices.

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.

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.

Poly (5,6-dithiooctylisothianaphtene), a new low band gap polymer : spectroscopy and solar cell construction

To enhance the efficiency of polymer photovoltaics, much effort is put into synthesis of novel compounds which show a better harvesting of solar light. In this respect, a new low band gap polymer, namely, poly(5,6-dithiooctylisothianaphtene), was synthesised. This work focusses on the spectroscopic characterisation of the material. The dynamics of the photoexcitations were studied by monitoring the dependence of the photoinduced absorption band on the laser modulation frequency and indicated dispersive recombination kinetics in this material. An appropriate model was used to describe the observed behaviour. To investigate the nature of the photogenerated species more profound, photoinduced absorption spectroscopy in the infrared was performed, showing an infrared active vibrational pattern (IRAV). Solar cells were constructed with an active layer consisting of the pristine material or a mixture with an electron accepting moiety. For this purpose, (6,6)-phenyl-C61-butyric-acid (PCBM) in 1:1 a (w/w) ratio with respect to the polymer was used. A clear improvement of the diode behaviour was observed going from the pristine material to the mixture. Photocurrent action spectra of the solar cells with the polymer:PCBM-mixture indicated an active contribution of the polymer to the photocurrent.

Synthesis of a Processible High Molecular Weight Poly(thienylene vinylene). Polymerisation and Thin-Film Transistor Properties

High molecular weight Poly(2,5-thienylene vinylene) derivatives were prepared by use of two precursor routes: the Sulphinyl route developed within our group and the Bis-xanthate route. These polymers were tested in organic metal-insulator-semiconductor field-effect transistors MISFET structures. The results are discussed.

Synthesis of electron-rich versus electron-poor poly{[1,4-phenylene]-[1-(n-alkylsulfinyl)ethylene]}s via the sulfinyl precursor route in different organic solvents

In this paper new insights in the mechanism of the sulfinyl precursor route towards poly(phenylene vinylene) (PPV) derivatives are presented by studying the polymerisation reaction in various solvents and by evaluating the influence of both electron donating and withdrawing substituents. A strong indication is presented that in alcohols the expulsion of the leaving group is involved in the rate determing step of the reaction, the formation of the p-quinodimethane intermediate, which further also depends on the type of solvent and substituents. Moreover, two polymerisation mechanisms can occur simultaneously: a radical mechanism that results in high molecular weight polymers and an anionic mechanism yielding low molecular weights. Competition between both mechanisms strongly depends on the solvent and type of substituents.

Electrical field induced ageing of polymer light-emitting diodes in an oxygen-rich atmosphere studied by emission microscopy, scanning electron microscopy and secondary ion mass spectroscopy

Abstract Polymer light-emitting diodes (PLEDs) made with poly( p -phenylene vinylene) (PPV) using a non-ionic precursor route with indium—tin oxide (ITO) as anode and Al as cathode have been examined during continuous electrical stress in an oxygen-rich atmosphere. Three distinct regions in the time evolution of the equivalent electrical resistance and the light output of PLEDs are identified. Various electrical and analytical measurement results are presented to explain the main failure mechanisms. The most severe degradation mode can be identified as dielectric breakdown, resulting in ‘hot spots’ and ohmic leakage paths. The inhibition of the ohmic path formation by oxidation under ambient conditions results in a local delamination of the electrode, shrinking the active area of the device. This loss of active area caused by these oxidative burn-outs can clearly be observed by scanning electron microscopy (SEM) and is consistent with secondary ion mass spectroscopy (SIMS) results. Emission microscopy (EMMI) inspection provides evidence for electric field induced ageing at defects present in the device. These defects are already present in the as-produced samples, e.g. particle impurities, interface roughness and structural weakness (edges of the Al electrodes).

The influence of the microstructure upon the photovoltaic performance of MDMOPPV: PCBM bulk hetero-junction organic solar cells

In this paper, a clear view on the bulk microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is obtained by means of TEM (Transmission Electron Microscopy). Using TEM, 3-dimensional information is acquired on phase separated regions, formed during casting. Particle statistics illustrate quantitatively that a.o. drying conditions and choice of solvent dramatically influence the blend structure. More information about the lateral blend structure and distribution is obtained in cross-sectional view. Since blend morphology is strongly related to photovoltaic performance, TEM can be a powerful tool for understanding todays photovoltaic performances and screening new sets of materials.

A general synthetic route towards soluble poly(1,3-dithienylisothianaphthene) derivatives

Abstract ‘Formal’ copolymers consisting of bithiophene and aromatic isothianaphthene units, based on 1,3-dithienylisothianaphthene (DTI) derivatives, have been synthesized. The obtained materials have low band gaps and show high intrinsic conductivities. The polymers are obtained after oxidative polymerization of the corresponding DTI monomers with FeCl 3 . Solubilizing alkyl and thioalkyl side chains where introduced on different locations of the DTI skeleton. The substituents also influence the stability of the DTI monomers and the band gap of the polymers.

A new precursor to electroconducting conjugated polymers: Synthesis and opto-electrical properties of luminescent devices based on these PPV derivatives

Abstract A variety of novel conjugated PPV-related polymers have been synthesised using a synthetic approach that allows the tailoring of the chemical structure of the polymer backbone. In this way we have been able to prepare not only poly(p-phenylene vinylene) (PPV) (1), but also some electron-rich PPV derivatives such as poly(2,5-dimethyl-1,4-phenylene vinylene) (PDMetPV) (2) and poly(2,5-dimethoxy-1,4-phenylene vinylene) (PDMeoPV) (3) and PPV derivatives with an enlarged aromatic system, poly(4,4′-bisphenylene vinylene) (PBPV) (4) and poly(2,6-naphtaln vinylene) (PNV) (5). These materials were characterised in terms of molecular weight (GPC), elimination temperature (TGA, FT-IR), optical properties (UV-VIS), photoluminescence (PL) and device characteristics of the LEDs.