For advanced organic thin film photovoltaic cell, stacked structure of single cells, tandem
structure, would be a key issue. Many kinds of tandem structure have been already reported.
When an appropriate intermediate layer was inserted between the single cells, open circuit
voltage (Voc) can be doubled compared with the single cell. For small molecules, vacuum
evaporation can be applicable for fabrication. Systematic investigations have been made to
reveal the requirement for the intermediate materials. Quite thin, a few nm thick, metal layer
can act as intermediate layer[1]. The metal cannot form continuous layer but island lake
structure in such small amount. On the other hand, the combination of the metal oxide
(such as ZnO, TiO2 and ITO) and PEDOT:PSS are used for the intermediate layer. We need
to reveal minimum requirement for the intermediate materials for polymer based
bulkhererojunction cells for low-cost high performance organic photovoltaic cells.
We have developed a polymer thin film preparation technique, Evaporative Spray
Deposition using Ultradilute Solution (ESDUS)[2].
This method has enabled fabricating organic thin films
applicable to polymer light-emitting diodes, organic
photovoltaics and organic field-effect transistors13
from highly diluted solutions of 1-10 ppm. Moreover,
it has been exhibited that a successive polymer layer
can be deposited without damaging the preceding
polymer layer by use of a same solvent for each layer
deposition.
We conduct the systematic investigation of the
intermediate materials. Onto the bottom cell/intermediate layer, top cell can be deposited by use of ESDUS.
A distribution-controlled bulk heterojunction (dc-BHJ) photovoltaic cell where the distribution of an electron donor and
acceptor is controlled across the film was fabricated by Evaporative Spray Deposition using Ultradilute Solution
(ESDUS) method, which can build a layered structure of polymer semiconductors soluble in a same solvent, and the
energy conversion efficiency, PCE, of the dc-BHJ was significantly improved compared with a conventional BHJ cell.
The dc-BHJ cells were fabricated by changing the ratio of a donor, regioregular poly(3-hexyl-thiophene-2,5-diyl)
(P3HT), and an acceptor, a fullerene derivative (PCBM), from 2:1 to 1:2. The short circuit current, Jsc, and PCE of a dc-BHJ cell having P3HT-rich BHJ/PCBM-rich BHJ structure was 6.06 mA/cm2 and 2.15 %, respectively, while those of a
conventional BHJ cell having P3HT:PCBM=1:1 layer were 5.26 mA/cm2, 1.71 %. In the dc-BHJ cells, the introduction
of composition gradient brought about the increase in conversion efficiency. The transportation of photogenerated charge
carriers to the collecting electrodes was improved because the better pathways should be formed in the dc-BHJ cells.
KEYWORDS: Photovoltaics, Electrodes, Absorption, Gold, Polarization, Organic photovoltaics, Thin film devices, Thin film solar cells, Anisotropy, Thin films
We report the fabrication of polarization-sensitive photovoltaic devices made of hetero-junction type vacuum-sublimed multilayer films composed of aligned 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (aligned-PTCBI) and titanyl phthalocyanine (TiOPc). The PTCBI layer was successfully made to be well aligned without losing high photovoltaic power-conversion efficiency. High polarization sensitivity was achieved at around 540 nm. The device configuration was ITO/In/aligned-PTCBI/TiOPc/PEDOT:PSS/Au and the thickness of each layer was optimized for polarization-sensitive photo-detection. The power-conversion efficiencies under the polarized white light parallel and perpendicular to the molecular-orientation axis, through the ITO electrode were 0.78% and 0.45%, respectively. The ratio of short-circuit current, parallel to perpendicular, was 1.66. This device can be used as transparent photo-detectors, because the transmittance of the Au electrode was about 40% at 500-600nm. The short-circuit current ratio was increased to 3.0, when 510nm monochromatic polarized light through the Au electrode was used.
Stacked devices that consisted of transparent organic photodiodes (TOPDs) and organic electroluminescence devices (OELs) were demonstrated. TOPDs were prepared by poly-(2-methoxy-5- (2'-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films as an active layer and transparent Au cathode (10 nm thick). These TOPDs showed about 45 % transmittance on average in visible light region (380-780 nm) and good correlation between incident light intensity and output photocurrent. Based on these results, the stacked devices were prepared by introducing OELs on TOPDs through a SiO insulating layer. The structure of OELs was ITO/Carbon/TPD/Alq3/LiF/Al. These stacked devices work as light emitting devices and also photo diodes. Since TOPDs have transparency, OELs can illuminate a paper put on the glass substrate through TOPDs and TOPDs can receive reflective light from the paper. Although the TOPDs also absorb light from OELs directly, the output signals from TOPDs changed according to the black and white pattern of the paper. These results show that the devices act as an image sensor having light emitting layer and light receiving layer in a same area.
The spray deposition method has been developed as a new way of polymer ultra-thin film preparation for organic optoelectronic devices such as an organic photovoltaic cell (OPC). In this method, a highly diluted solution of an organic material is nebulized into air and concentrated under a controlled evaporation condition. The resulting aerosol is transported by a carrier gas and deposited onto a solid substrate. This method has substantial advantages that an almost insoluble and non-evaporative material can be fabricated into a thin film, and that a separate area coating and layer-by-layer structure of polymer materials can be performed. An OPC was prepared from highly diluted THF solutions (below 1 ppm) of fullerene/ poly-phenylenevinylene (PPV) derivative mixture. The device configuration, ITO/PEDOT-PSS/fullerene-PPV/LiF/Al, is known as a bulk hetero-junction PC. By control of temperature, we obtained homogeneous active layer by spray deposition. In addition, we can achieve power conversion efficiency of 0.63% using the active layer consist of fullerene: PPV(1:1). This method has significant potential to provide a structural control of the active layer and to reduce the cost of the OPCs drastically in spite that the power conversion efficiency is slightly lower than a PC prepared in same ratio by spin-coating.
High field-effect hole mobility of (formula available in paper)and threshold voltage is -3.2 V) in organic-inorganic layered perovskite film (formula available in paper)prepared by a vapor phase deposition technique have been demonstrated through the octadecyltrichlorosilane treatment of substrate. Previously, the (formula available in paper)films prepared on the
octadecyltrichlorosilane-covered substrates using a vapor evaporation showed not only intense exciton absorption and photoluminescence in the optical spectroscopy but also excellent crystallinity and large grain structure in X-ray and atomic force microscopic studies. Especially, the (formula available in paper)structure in the region below few nm closed to the surface of octadecyltrichlorosilane monolayer was drastically improved in comparison with that on the non-covered substrate. Though our initial (formula available in paper)films via a same sequence of preparation of (formula available in paper)and octadecyltrichlorosilane monolayer did
not show the field-effect properties because of a lack of spectral, structural, and morphological features. The unformation of favorable
(formula available in paper)structure in the very thin region, that is very important for the field-effect transistors to transport electrons or holes, closed to the surface of non-covered (formula available in paper)dielectric layer was also one of the problems for no observation of them. By adding further optimization and development, such as deposition rate of perovskite, substrate heating during deposition, and tuning device architecture, with hydrophobic treatment, the vacuum-deposited (formula available in paper)have achieved above-described high
performance in organic-inorganic hybrid transistors.
A flexible insulator film would be one of the most important elements of flexible organic field-effect transistors (OFETs). It should be produced from a soft organic material rather than a stiff inorganic material. Many polymeric materials were spin-coated from the solution and the resulted films have to be baked or cured to obtain a good insulator. Since those procedures impose a restriction on the OFETs, a fabrication process without using a solvent has been desired. Poly-p-xylylene derivatives have been made into an insulator film by a non-solvent procedure, chemical vapor deposition (CVD). The insulator film has additional advantages, pinhole-free, resistance to many solvents and no thermal stress to a material beneath. We have fabricated and characterized OFETs with the inverted staggered geometry, substrate/ gate electrode/ poly-p-xylylene derivatives/ organic semiconductor/ source-drain electrodes. And the CVD enables to form an insulator film even above the organic semiconductor. So we fabricated the staggered type configuration, substrate/ source-drain electrodes/ organic semiconductor/ poly-chloro-p-xylylene/ gate electrode. The device performance of a staggered type transistor indicated that the molecular arrangement of organic semiconductor at the insulator interface is more dominant than the damage or chemical deterioration due to the attack of the radicals during the CVD procedure.
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