Patent Application: US-90188897-A

Abstract:
in general terms , the present invention includes a light emitting polymeric material the light emitting polymeric material capable of producing electroluminescence upon being provided with a flow of electrons , the light emitting polymeric material comprising a plurality of polymeric chains comprising polymeric chains each having substituent moieties of sufficient number and size and extending from the polymeric chain and about a substantial portion of the circumference about the polymer chain so as to maintain the polymeric chains in a sufficiently deaggregated state , so as to substantially prevent the redshifting of the electroluminescence and the lowering of light emission efficiency of the electroluminescence .

Description:
in accordance with the foregoing summary of the invention , the following presents a detailed description of the preferred embodiment of the invention which is presently considered to be its best mode . the synthesis of the ppyvpv copolymers is described elsewhere [ 22 ]. fig1 ( a ) shows the molecular structure of poly ( pyridyl vinylene phenylene vinylene ) ( ppyvpv ). we report results for copolymers with side groups r = cooc 12 h 25 and c 12 h 25 . fig1 ( b ) shows the same copolymer with a “ strap ” across the phenyl ring in alternate ppv segments . for the second copolymer the side groups are r 1 = oc 16 h 33 with a strap r 2 = c 10 h 22 . the copolymers are soluble in common organic solvents such as tetrahydrofuran ( thf ), xylene , and chloroform . the pvk ( fig1 ( c )) was purchased from aldrich chemical co . the pl and el measurements were made using a pti qm 1 luminescence spectrometer . the absorption measurements were made using a perkin elmer lambda 19 uv / vis / nir spectrometer . the current - voltage characteristics were measured using two keithley 195a multimeters . the voltage was applied using a fip 6218a dc power supply . quantum efficiency measurements were made using a calibrated qth lamp and a united detector technologies silicon photodiode ( uv - 100 ). the samples for pl and absorption measurements were spin cast on quartz . pvk was spin cast (˜ 3000 rpms ) from a 10 mg / ml solution in thf . the copolymers were all spin cast (˜ 1000 rpms ) from xylene (˜ 10 mg / ml ) so the underlying pvk layer was not dissolved . for devices the polymers were spin cast on indium tin - oxide ( ito ) substrates , which had previously been cleaned . the pvk layer in the bilayer configurations was dried for ˜ 30 seconds in a laminar flow hood before the copolymer layer was spin cast . all fabrication steps for the devices were conducted in an ambient atmosphere in a class 100 cleanroom . aluminum electrodes were thermally evaporated onto the copolymer surface at low pressures (& lt ; 10 − 6 torr ). the active area of the devices was 7 mm 2 . fig2 shows the absorbance for single and bilayer systems . each of the plots shows the absorbance of a single layer of pvk , a single layer of a copolymer , and a bilayer configuration of pvk and the corresponding copolymer . the onset of pvk absorption is at 3 . 5 ev and shows two spectral features at 3 . 6 and 3 . 75 ev similar to previous reports [ 6 , 7 ]. in each of the three cases ( a ), ( b ) and ( c ) of fig2 the absorbance of the bilayer configuration is the sum of the absorbance of each of the individual layers . no new ground to excited state transitions are present . photoluminescence excitation ( ple ) results ( not shown ) for each of the systems confirm these results . the photoluminescence for the single layer and bilayer configurations are shown in fig3 . the pl ( excited at 3 . 6 ev ) of a pvk film is shown in fig3 a , 3 b and 3 c by the open circles (◯) and peaks at 3 . 05 ev similar to previous reports [ 6 , 7 ]. the dashed line is the pl of a single layer film of each copolymer , ( a ) ppyvp ( c 12 h 25 ) 2 v , ( b ) ppyvp ( cooc 12 h 25 ) 2 v and ( c ) the strapped copolymer . the spectra are similar with each of the peaks at ˜ 2 . 1 ev with the exception of the strapped copolymer which also has a significant shoulder at 2 . 25 ev . the pl of the copolymer films , which peak near 2 . 6 ev [ 23 ], are significantly redshifted from that of the solution pl ( not shown ). the pl redshift from solution to film is clue to aggregation in the copolymer films [ 23 ]. the solid lines in fig3 are the pl spectra for the bilayer configurations of pvk and each of the copolymers . in each case the bilayer films were excited at 3 . 6 ev an energy that is greater than the band gap of pvk . in each case , more prominently in fig3 b and 3 c , there is pl emission at the same energy as the pvk pl emission ( 3 . 1 ev ). however , the main feature in the pl of the bilayer films is located at 2 . 5 ev for ( a ) ppyvp ( c 12 h 25 ) 2 v and ( b ) ppyvp ( cooc 12 h 25 ) 2 v and at 2 . 4 ev for the ( c ) strapped copolymer . emission at these energies is not observed for individual films of either pvk or the copolymers indicating that the emission is due to a completely different species , the exciplex . when the excitation energy is lowered below 3 . 4 ev ( band gap of pvk ) the emission due to the exciplex is drastically reduced . in addition , varying the concentration or thickness of the copolymer or pvk films in the bilayer configuration will change the relative strengths of the exciplex peak and pvk peak . ppyvp ( c 12 h 25 ) 2 v and ppyvp ( cooc 12 h 25 ) 2 v have nearly identical pl results , which is expected since the side chains tend to perform the same function in both copolymers . the single layer pl results for the strapped copolymer in fig3 c show a completely new feature , a high energy shoulder . the high energy shoulder is closer to the solution pl and is attributed to unaggregated sites in the film . the c 10 h 20 strap around every other phenyl ring tends to disturb the aggregation that occurs in the other copolymers of this family . the same shoulder ( now on the low energy side ) also appears in the bilayer film , indicating the pl has contributions from exciplex sites and from unaggregated regions of the strapped copolymer . the bilayer devices have turn - on voltages ˜ 12 - 16 volts with current densities between 0 . 1 and 0 . 5 ma / mm 2 . the devices can easily be seen in a brightly lit room and have internal quantum efficiencies ˜ 0 . 1 - 0 . 5 %. fig5 shows the current - voltage (◯) and voltage - brightness ( solid line ) characteristics for a typical ito / pvk / ppyvp ( cooc 12 h 25 ) 2 vl bilayer device . the inset of fig5 shows a comparison between a single layer device ( ito / ppyvp ( cooc 12 h 25 ) 2 v / al ) and the bilayer device shown in the main plot . the bilayer device is ten times brighter at an order of magnitude lower current density which means the bilayer device is ˜ 100 times more efficient than the single layer device . in the devices the electrons are injected from the al electrode into the conduction band of the copolymer , but they are confined at the pvk / copolymer interface due to a large barrier . the holes injected from the ito also may be confined at the interface by a somewhat smaller barrier . the increased number of electrons and holes in the interface region increase the probability of recombination via exciplex emission . in addition the buried interface severely reduces the non - 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