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a b s t r a c t We reported a low band-gap conjugated polymer, poly(2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,7- di(thiophen-2-yl)thieno(3,4-b)pyrazine) (PDTTP), was studied for the near infrared (NIR) photo-detector application. PDTTP shows intense absorption in NIR wavelength (to 1000 nm) and the estimated optical and electrochemical band-gaps of PDTTP are quite small around 1.15 eV and 1.08 eV, respectively. The low band-gap and the extended long wavelength absorption originates from the introduction of alter- nating TP units when its parent polythieno(3,4-b)pyrazine shows excellent narrow band-gap properties. Therefore, the relatively low band-gap and intense absorption in long wavelength of PDTTP make itself a promising candidate for near-infrared photo-detector. The hole mobility of the PDTTP measured from the bottom contact field effect transistor is around 1.40 × 10 −3 cm 2 /V s with a on/off ratio of 2100. The photo-detector based on bulk hetero-junction PDTTP and (6,6)-phenyl-C61-butyric acid methyl ester blend (PCBM) has the incident photon-to-electron conversion efficiency 28.9% at 1000 nm (−5 V) and 6.2% at 1100 nm (−5 V). This photo-detector can be operated at a high-speed of 1 MHz. The experimen- tal result suggests the potential applications of low band-gap conjugated polymers on near-infrared photo-detectors.

Solution-processed electron transport layers (ETL) have been fabricated by solution process and applied in multilayer polymer light-emitting diodes with tris(2-(p-tolyl)pyridine)iridium(III) blended in poly(vinylcarbazole) as the emissive layer. Three kinds of small molecular electron transport materials,including 2,2� ,2�� -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 3-(4- biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), and 4,7-diphenyl-1,10-phenanthroline (BPhen), are tested and dissolved in methanol to form electron transport layers by blade coating. Such electron transport layer provides efficient electron injection and electron transport ability in the devices. The efficiency of the devices with the combination of ETL and LiF/Al cathode reaches 21.5 cd/A at 10 V (4050 cd/m 2 ). The efficiency of the devices without ETL are 3.5 cd/A (13 V) for LiF/Al cathode and 17 cd/A

A series of anthracene-based conjugated copolymers containing 9,10-bis(6-bromonaphthalen2-yl)-2-tert-butylanthracene (BNA) and 2,7-diphenyl substituted fluorene (DPPF) moieties are prepared via a palladium-catalyzed Suzuki polymerization. All of the synthesized polymers emit blue light at around 450nm and show good thermal and color stability. Their electroluminescence spectra remain unchanged at high driving voltage. The double-layer polymer light-emitting diode (PLED) fabricated with ITO/PEDOT:PSS/DPPFBNA3/CsF/Al, produces a +

A uniform dispersion of highly soluble phosphorescent dendrimer emitters is achieved by blending with a polymer host poly(9-vinylcarbazole) (PVK) containing N,N′-diphenyl-N,N′-(bis(3-methylphenyl)-[1,1-biphenyl]-4,4′-diamine (TPD) and 2-(4-biphen-4′-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). No visible aggregation or self-quenching was observed for guest-to-host weight ratios of up to 33:67. The dendrimers contain a fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)3] core, first generation biphenyl-based dendrons, and 2-ethylhexyloxy surface groups. The guest–host blend is used for all solution processed organic light-emitting diodes. A maximum external and current efficiency of 10.2% and 38 cd/A (at 5 V and a brightness of 50 cd/m2), and a maximum brightness of 27,000 cd/m2 (at 14.5 V), were obtained when a CsF/Al cathode was used. Blade coating was used to fabricate a multi-layer structure that also contained an electron-transport layer. The device that had a LiF/Al cathode had a maximal efficiency of 40 cd/A corresponding to an external quantum efficiency of 10.8% (at 5 V and a brightness of 19 cd/m2). The maximum brightness of the second device was 17,840 cd/m2 at 14 V.

Efficient red light polymer light-emitting diodes are obtained by energy transfer from host and guest blend system. Two kinds of blend systems, SPB02:(10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1] benzopyrano [6,7,8-ij]quinolizin-11-one)(C545T):4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and SuperYellow (SY):poly(3,3 ′′′ -didodecylquaterthiophene) (PQT-12), are used as the active layers. C545T is a green emitter, acting as an assistant dopant to improve the energy transfer from blue SPB02 host to red DCJTB. The efficiency can achieve to 3.3 cd/A for DCJTB and 3.5 cd/A for PQT-12 system by multilayer device structure with 2,2,2 ′′ -(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TPBi) as electron transport layer. The SuperYellow:PQT-12 device shows great luminance of 16,930 cd/m 2 .

All-solution-processed multilayer blue small molecular organic light-emitting diodes are fabricated by blade coating method. Fluorescent blue host,1-(7-(9,9′-bianthracen-10-yl)- 9,9-dioctyl-9H-fluoren-2-yl)pyrene, and blue dopant, 4,4′-(1E,1′E)-2,2′-(naphthalene-2,6-diyl)bis(ethene-2,1-diyl)bis(N,N-bis(4-hexylphenyl)aniline), are used to achieve good solubility and pinhole-free thin film by solution process. The multilayer device structure with hole/electron transport layer is achieved by blade coating method without the dissolution problem between layers. The efficiency of the all-solution-processed device is 4.8 cd/A at 1200 cd/m2, close to that by thermal deposition in high vacuum chamber. The device performance is optimized with the annealing temperature of TPBi layer at 50 °C.

Polymer bulk hetero-junction solar cells of poly(3-hexylthiophene) (P3HT) donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) acceptor are fabricated by blade coating in toluene solution. Desired donor–acceptor self-organization is achieved without the slow drying process and high boiling point solvent. Power conversion efficiency is 3.8%, much higher than the 2.6% obtained by spin coating in toluene solution. The blade coating method has nearly 100% material usage and can be integrated in the roll-to-roll process with high throughput production.

The relation between charge balance and electroluminescent efficiency for polymers is studied systematically in this work. Light-emitting diodes of several kinds of polymers with very different efficiency are compared. The electron and hole currents are measured in the uni-polar devices and fitted by the theoretical model to get carrier mobilities. The universal features of the high efficiency polymers are that the electron and hole mobilities are comparable and the electron current is larger than the hole current due to the higher hole barriers. The electron current is more crucial than hole current to determine the efficiency in the bipolar light-emitting diode. The purity of polymers and low electron injection barriers both play significant roles in producing stronger electron currents. These results give the physical rules for designing new polymers and the device structures to achieve highly electroluminescent devices.

A highly efficient deep blue polymer light-emitting diode based on poly(9,9-dioctylfluorene) is demonstrated. The performance is found to increase significantly with the molecular weight. Two different molecular weights are compared, one is 71,000 and the other is 365,000. The electroluminescent efficiency and color stability are improved by slightly doping hole traps into the emission layer and bilayer structure. The maximum efficiency is 3.8 cd/A with the corresponding external quantum efficiency of 3.7% at deep blue with Commission Internationale de L’Eclairage (CIE) coordinate at (0.15, 0.09). Stable blue emission is maintained up to 6600 cd/m2 without growth of green shoulder in emission spectrum.

Efficient phosphorescent polymer light-emitting diode with poly(vinylcarbazole) (PVK) doped by two or three iridium complexes in single and bilayer structures are studied. With (tris-(2-4(4-toltyl) phenylpyridine) (Ir(mppy) 3 ) as the green emitter and (1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq) 2 ) as the red emitter the efficiency is as high as 23 cd/A with broad band emission from 500 nm to 720 nm. For white emission a second layer is added with blue emitter ((III) bis[(4,-6-di-fluorophenyl-pyridinato)N,C 2 ] picolinate) (FIrpic) doped in PVK. White light containing three spectral peaks results with efficiency 8.1 cd/A. As the second blue layer is replaced by the fluorescent (poly(9,9-dioctylfluorene)) (PFO) white emission with high color rendering index 86 is achieved. The efficiency is 5.7 cd/A with peak luminance 8900 cd/m 2 . For a given iridium complexes ratio the relative intensity of the green and red emission depends sensitively on the second blue layer.

A bright, highly efficient blue polymer light-emitting diode is fabricated by spin-coating with multilayer structure. By introducing a liquid buffer layer, the dissolution problem is overcome successfully. This general method can be applied to fabricate arbitrary polymer multilayer structure, in particular the addition of the carrier blocking layers to balance the electron and hole currents and improve the device efficiency. By adding an electron-blocking layer, the external quantum efficiency for blue emission can be increased to 5.5% (9.1 cd/A) which is even higher than the theoretical limit of 5%. The luminance reaches 26,530 cd/m 2 and the Commission Internationale de L’Eclairage (CIE) coordinates is (0.15, 0.23). Further more the lifetime is increased by three times relative to the single layer device.

We study systematically the underlying property which determines the electroluminescence efficiency of blue polymers based on polyfluorene and its copolymers. The photoluminescence efficiency is high (40–50%) for all the selected polymers, but the electroluminescence efficiency varies in a wide range. The hole mobility is measured by transient electroluminescence. The electron mobility is obtained from the current in electron-only device using space-charge-limited current equation. All polymers share roughly the same results, whereas electron mobility is the critical factor for electroluminescence. In general, low efficiency results from the low electron mobility.

Multilayer polymer light-emitting diode (PLED) and solar cell are fabricated by blade coating. Multilayer of semiconducting polymers can be easily deposited by blade coating without layer mixing, as confirmed by the total thickness and cross section view in scanning electron microscope. The film thickness variation is only 3 % in 10 cm scale and the film roughness is about 0.3 nm in the micron scale. In addition blade coating has the advantages of low material waste and easy scaling up to the meter scale. For blue fluorescent emission, the current efficiency of 5 cd/A is achieved for a three layer structure with hole transport, emissive, and electron transport layers. Only soluble small molecules are used for all the three layers. For green emission phosphorescent emission of 45 cd/A is achieved for a combination of polymer and small molecules. The efficiency for deep red is 4 cd/A based on polymers and small molecules. In all devices efficient electron injection is realized by a blade coated electron transport layer combined with the relatively stable LiF/Al cathode, in contrary the unstable CsF/Al cathode conventionally used for high efficiency PLED. Patterned multi-color PLED is achieved by an epoxy mask during blade coating.

The microscopic states and performance of organic solar cell are investigated theoretically to explore the effect of the carrier mobility. With Ohmic contacts between the semiconductor and the metal electrodes there are two origins of carriers in the semiconductor: the photocarriers generated by photon absorption and the dark carriers diffused from the electrodes. The power efficiency of the solar cell is limited by the recombination of a carrier with either the photocarrier or a dark carrier. Near the short-circuit condition the photocarrier recombination in the semiconductor bulk decreases as the mobility increases. Near the open-circuit condition the dark carrier recombination increases with the mobility. These two opposite effects balance with one another, resulting in an optimal mobility about 10−2 cm2/V s which gives the highest power conversion efficiency. The balance of the electron and hole mobilities are not necessary to maintain the optimal efficiency also because of the balance of the photocar...

Polymer photodetectors with voltage-adjustable photoresponse from visible to near infrared range are demonstrated. Poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) blend is used as the active layer. The photoresponse can be continuously adjusted by the thickness of the active layer as well as the applied voltage bias. The thickness of the active layer is varied from 250 nm to 16.2 μm. The mechanism for the photoresponse adjusted by the thickness can be attributed to the absorption of the photons in the infrared range by thick PCBM layer. The mechanism for the photoresponse adjusted by the applied bias can be attributed to the carrier recombination reduction when the applied bias increases. The adjustable photodetector also has high operating speed up to 10 kHz.

Multilayer polymer light-emitting diodes fabricated by blade coating are presented. Multilayer of polymers can be easily deposited by blade coating on a hot plate. The multilayer structure is confirmed by the total thickness and the cross section view in the scanning electron microscope. The film thickness variation is only 3.3% in 10cm scale and the film roughness is about 0.3nm in the micron scale. The efficiency of single layer poly(para-phenylene vinylene) copolymer Super Yellow and poly(9,9-dioctylfluorene) (PFO, deep blue) devices are 9 and 1.7cd∕A, respectively, by blade coating. The efficiency of the PFO device is raised to 2.9cd∕A with a 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) hole-blocking layer and to 2.3cd∕A with a poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] elec-tron-blocking layer added by blade coating.

The carrier recombination in organic solar cells is investigated by numerical modeling to understand the weak dependence of the open-circuit voltage on the workfunction of the electrodes. In Ohmic contact structures, photocarriers recombine predominantly with dark carriers diffused from the electrode into the semiconductor. Such dark carrier recombination becomes the main limit of power conversion efficiency and open-circuit voltage. For a given semiconductor decreasing the workfunction difference of the electrodes reduces simultaneously the dark carrier recombination and the flat band voltage. The balance between these two opposite factors gives a nearly constant open-circuit voltage. In an ideal bilayer structure there is no dark carrier recombination and the efficiency is demonstrated to be 60% higher than single layer blend.

A proximity sensor that combines a polymer light-emitting diode and a polymer photodiode is presented. The operation wavelength is in the near infrared from 700to850nm. The infrared emission is obtained by adding a color conversion film of polyvinylpyrrolidone polymer matrix blended with infrared dye 1,1-diethyl-2,2-dicarbocyanine iodide to a red polymer light-emitting diode. The photodetector relies on the direct charge-transfer exciton generation in a donor-acceptor polymer blend of poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester. The detection distance is up to 19cm for objects with various colors and roughness under ambient indoor lighting.

A highly efficient blue polymer light-emitting diode based exclusively on commercial poly(9,9-dioctylfluorene) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl)) diphenylamine)] is demonstrated. High electroluminescent efficiency is achieved by enhancing electron currents and making devices in multilayered structures. CsF∕Al is used as the efficient electron injection cathode, and the fabrication process is in the glove box to enhance electron mobility by reducing oxygen adsorption. The multilayer structure is prepared by the liquid buffer layer technique. The maximum efficiency is 2.5 cd∕A at deep blue with the corresponding external quantum efficiency of 2%.

An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer conjugated polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin coated on the common p-type emissive polymer layers. One green- and two blue-emitting polymers are chosen as the emissive layers. The electron-hole balance and efficiency are significantly improved by the addition of hole-blocking layer. The electroluminescence efficiency can be increased up to nine times, while the luminance up to seven times. In particular, 1.5cd∕A is obtained for deep blue emission from poly(9,9-dioctyl-fluorene) with 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene spin coated as the hole-blocking material.