We have investigated properties of poly(9-vinylcarbazole) (PVK) doped with 30% wt 4-dibutylamino-4'-nitrostilbene
(DBANS), depending on orientation of the polar DBANS molecules. Appearance of the orientation-induced built-in
electric field was proven optically by the solid electric field induced second harmonic generation and electrically by the
current-voltage characterization. Modification of optical properties was evidenced by the spectral dependencies of the
absorption coefficient. The thermally stimulated current spectra had demonstrated that carrier transport and trapping are
affected, too.
Among the class of conjugated polymers, polythiophenes and in particular 3-alkyl-substituted thiophenes seem to focus all the attention in the domain of photovoltaic conversion. At CEA, we are working on the optimization of bulk heterojunction solar cells made of poly-3-hexylthiophene (P3HT)and [6,6]-phenyl C61 butyric acid methylester (PCBM) blend. First we will describe the influence of the ratio of P3HT and PCBM blend on the efficiency of the resulting bulk heterojunction solar cells. Best cells based on 1:1 in weight ratio yield 3.6 % power conversion efficiency under air-mass 1.5, 100 mW/cm2 illumination. Then we will compare the efficiency and lifetime of different cells by changing the nature and thickness of cathode (Aluminum or Calcium/Silver). On the optimized cells, we have proceeded to ageing and accelerated lifetime measurements on devices with Ca/Ag cathode. It shows that the current densities decrease less than 3 % and that efficiency is still higher than 1.7 %, after 400 hours under AM 1.5, 100 mW/cm2 illuminations and at high temperature (60°C).
Though being much less efficient than silicon cells, organic solar cells exhibit a unique combination of interesting properties: low cost, flexibility, and the possibility of large surface coverage. Large progresses have been made over the last years using MDMO-PPV (Poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene) reaching efficiencies of 2.9% and recently efficiencies over 3%, using poly(3-hexyl thiophene). A great deal of research however has still to be invested to improve the current state of the art. Among the main key-points to be addressed are namely the stability and lifetime of such devices.
We are currently working on bulk heterojunction solar cells made from MDMO-PPV and PCBM (methano-fullerene[6,6]-phenyl C61-butyric acid methyl ester). Different batches of MDMO-PPV, originating from different synthesis modes (classical "Gilch" synthesis and "Sulphinyl" synthesis led by IMEC-IMOMEC) have been tested. Evolution of the power efficiency following continuous illumination (AM1.5, 80 mW.cm-2) was characterized under controlled atmosphere of nitrogen. In parallel, photodegradation studies are also investigated and electrical modeling is under way in order to get a better understanding of the relations between photochemical and electrical parameters of the diode that can be deduced from I/V curves.
Though being much less efficient than silicon cells, organic solar cells exhibit a unique combination of interesting properties: low cost, flexibility, and the possibility of large surface coverage. Large progresses have been made over the last years using MDMO-PPV (Poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene) reaching efficiencies of 2.9% and recently efficiencies over 3%, using poly(3-hexyl thiophene). A great deal of research however has still to be invested to improve the current state of the art. Among the main key-points to be addressed are namely the stability and lifetime of such devices.
We are currently working on bulk heterojunction solar cells made from MDMO-PPV and PCBM (methano-fullerene[6,6]-phenyl C61-butyric acid methyl ester). Different batches of MDMO-PPV, originating from different synthesis modes (classical "Gilch" synthesis and "Sulphinyl" synthesis led by IMEC-IMOMEC) have been tested. Evolution of the power efficiency following continuous illumination (AM1.5, 80 mW.cm-2) was characterized under controlled atmosphere of nitrogen. In parallel, photodegradation studies are also investigated and electrical modeling is under way in order to get a better understanding of the relations between photochemical and electrical parameters of the diode that can be deduced from I/V curves.
In order to improve the photogeneration efficiency and charge transport in polymer photovoltaic cells, we orient diode like molecules inside a polymeric monolayer. Previous results gave experimental evidence of the induction of a rectification behavior as well as an increase in the charge mobility through polar orientation. We present here the first experimental realization where the photovoltaic conversion efficiency is increased by two orders of magnitude in a semiconducting polymer blend.
In the field of organic semiconductor devices, such as electroluminescent diodes or photovoltaic cells, a rectifying junction is mandatory to improve their efficiency. In order to build single polymer film photovoltaic cells, we realize an equivalent distributed p-n junction in a polymeric monolayer. For this purpose, we propose an inter-disciplinary principle derived from the application of nonlinear optical techniques to the technology of polymer semiconductors. We give experimental evidence that molecular rectification induced in an oriented amorphous polymer bearing polar chromophores improves efficiency of polymeric semiconductor device such as photovoltaic cells. Orientation is performed through DC- field ordering of the polar molecules contained in the polymer. Second harmonic generation is used as a probe of the molecular order and permits optimization of the sample orientation parameters, i.e., orientation-field, sample temperature, poling duration and cooling conditions to achieve the largest stable orientation. Additionally, to establish a model which describes charge injection and transport through such device, we propose to use Solid Electric Field Induced Second Harmonic Generation (SEFISHG) as an in situ internal field measurement technique. In this way, before orientation of the molecules, SEFISHG provides the electron work function difference between the two electrode materials, and after orientation, SEFISHG permits the measure of the internal field stored in the structure.
In an attempt to build single polymer film photovoltaic cells, we underline the intrinsic rectifying nature of an oriented polymeric material. Orientation of the initially symmetric structure is performed through DC-field ordering of the polar molecules contained in the polymer. The internal field induced in the polymeric material is evidenced by the induction of a rectifying current-voltage characteristic. Second harmonic generation is used as a probe of the molecular order. Orientation induced rectification is proportional to the molecular order. This preparation technique opens a new route for the improvement of organic-semiconductor devices efficiency.
Light emitting diodes made of a single spin-coated layer of poly(9-vinylcarbazole) doped with coumarin-515 dye have been studied. The influence of dye concentration on emission and electrical characteristics is evidenced. Two different regimes are identified. At low concentrations, apparent hole injection barrier raises, holes are trapped and mobility decreases. External quantum efficiency increases with concentration. At concentrations larger than 10%wt, coumarin aggregates, photoluminescence yield drops and electroluminescence yield increases up to 0.1%-photons per electron. We show that using coumarin dye in a single-layer diode improves electron-hole injection and recombination balance more than using an additional hole-blocking layer.
We developed a new type of light-emitting-diodes in which the emission layer is confined inside an aluminium channel. A 1200 angstrom aluminium layer was first deposited on glass by vacuum evaporation and then carved using microlithography techniques. Channels were typically 1.5 micrometers width. A 2000 angstrom thick electroluminescent polymer film was spin coated on top of the Al channel. The diode emits light under alternative voltages. It can be seen under room illumination. We studied current-voltage characteristics and LED quantum efficiency. Electroluminescence results from a field ionization and recombination process. No charge injection at electrodes takes place in such device.
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