Patterned Luminescence of Organic Light-Emitting Diodes by Hot Microcontact Printing (HμCP) of Self-Assembled Monolayers

Abstract

Modification of the anode (usually Sn-doped In 2 O 3 , ITO)-organic interface in organic light-emitting diode (OLED) structures by chemisorption/self-assembly of nanoscopic adsorbate layers can effect dramatic enhancements in device performance for reasons that are variously associated with balancing holeelectron injection fluences, 1 altering the anode work function, 2 electric field effects, 3 anode chemical passivation, 1a improving anode wetting by the hole transport layer, 4 and smoothing interlayer HOMO energetic discontinuities. 5 In principle, the nanoscale charge-blocking modulation possible via such interfacial effects combined with soft lithographic techniques such as microcontact printing (µCP) 6 should offer an easily implemented alternative/complement to/improvement over current approaches 7-13 for patterning OLEDs. The attraction versus several current approaches includes smaller feature sizes, parallel rather than serial fabrication, nanoscale interface tailorability, and potential molecular recognition characteristics. However, achieving rapid, contiguous anode coverage requires addressing poorly understood features of ITO surface chemistry which appear to limit efficient chemisorption. 2,14 We report here an approach to OLED anode patterning using high-temperature microcontact printing (or hot microcontact printing, HµCP) which readily affords pixel features down to 1.0 µm dimensions 15 and which, by virtue of the length scale-dependent carrier tunneling through SAM structures, 16 affords tunability in luminescent patterns.The mold for the PDMS µCP "stamp" was fabricated using standard photolithographic techniques and a vapor-deposited, CF 3 (CF 2 ) 5 CH 2 CH 2 SiCl 3 -derived release SAM. 6b,17 The test pattern consisted of arrays of cylinders with diameters ranging from 1 to 40 µm and with 100 µm center-to-center distances. A series of freshly distilled RSiCl 3 reagents (R ) n-CH 3 (CH 2 ) n -(n ) 21, 17, 7), n-CF 3 (CF 2 ) 5 CH 2 CH 2 -) as solutions in dry hexane were used as "inks", and were applied under rigorously anaerobic conditions to the PDMS stamp, followed by spinning at 2000 rpm for 60 s. After application of the test patterned SAMs to cleaned, 3-4 nm rms roughness, 2.5 cm 2 ITO-glass substrates (vide infra), standard OLED structures were fabricated in a double evaporator-interfaced glovebox as described elsewhere 1a,18 using vapor deposited layers of gradient sublimed TPD (50 nm) and Alq 3 (60 nm) as hole transport and electron transport/emission layers, respectively. Vapor deposition of a 70-100 nm Al cathode completed the device. Device characterization was carried out in the continuous dc mode using a Tektronix PS281 power supply and a calibrated Si photodiode. I-V responses were recorded using a Keithley 2400 current source.Initially, all attempts to transfer an RSiCl 3 -derived pattern to ITO substrates using standard µCP methodologies (stamping performed in ambient or in a glovebox; ink in various solvents at various concentrations; stamping for short or long times using an ...

Spatially Modulating Interfacial Properties of Transparent Conductive Oxides: Patterning Work Function with Phosphonic Acid Self‐Assembled Monolayers

“…Examples range from thiols on gold, [3][4][5][6][7][8] to silanes on hydroxyl-terminated surfaces, [11,12,15,17] to mixtures of SAMs containing different terminal functional groups on metal oxide buffer layers. [13] A number of groups have incorporated SAMs into OLEDs, [1][2][3][4]6,[10][11][12]18,19] with many focusing on the use of phosphonic acid SAMs to modulate interfacial properties and improve device performance. [20][21][22][23][24][25][26] Given the strong influence SAM modifiers can have on the performance of organic electronic devices, the ability to microcontact print SAMs with large work function contrast is both scientifically interesting from the standpoint of creating model systems to explore the role of barriers and energy level offsets on charge injection in OLEDs, and technologically useful in the context of applications including low-cost illuminated signs and displays.…”

“…Second, while silanes can be microcontact printed onto oxides, they can often exhibit spreading and poor fidelity unless they are microcontact printed onto hot substrates. [18,19] Fujihira and co-workers shifted the ITO work function using benzoyl-and phosphoryl-chlorides, [28,29] and microcontact printed these molecules to give patterned electroluminescence. [27] However, the resulting contact potential difference (CPD) contrast between patterned and unpatterned SAM regions of the ITO substrate was significantly lower than the CPD shift measured on ITO that had been soaked in a SAM solution, suggesting poorer ordering or lower density of the microcontact printed SAMs relative to solution-deposited SAMs.…”

Preparation, Photo‐luminescence and Electro‐Luminescence Behavior of Langmuir−Blodgett Films of Bipyridylrhenium(I) Surfactant Complexes

“…[9] In some circumstances LB films may produce films of higher quality than the general vacuum deposited or spin-coat films and may have potential applications in molecular electronic devices. [9,10] Employment of transition metal complexes as the emissive layer in OLEDs has become one of the important subjects in recent years, [5,11] since many of them have excellent photoluminescence properties and potential advantages of achieving a maximum internal quantum efficiency of 100%. [2f] The formation efficiency of the triplet state will be triple that of the singlet state in the non-geminate pair electron combination (as in ELs), thus an improvement in the EL yield should be predicted by using triplet state materials as the emitting layer, which makes them promising candidates for OLEDs, and a number of related studies have been reported.…”

“…[2f] The formation efficiency of the triplet state will be triple that of the singlet state in the non-geminate pair electron combination (as in ELs), thus an improvement in the EL yield should be predicted by using triplet state materials as the emitting layer, which makes them promising candidates for OLEDs, and a number of related studies have been reported. [5,11,12] Amongst the transition metal complexes studied, those of the bipyridylruthenium() systems, which show 3 MLCT emission, have been extensively studied. [5a,5b] Other relatively less studied systems involving 3 MLCT emissive states include the well-known tricarbonyl(diimine)rhenium() complexes.…”

Selective Deposition of Rod-like Phthalocyanine Aggregates on Au Surfaces Patterned with a Combination of Microcontact Printing and Electropolymerization

“…[15±26] We have recently shown that Pcs with benzyl-terminated ethylene oxide chains (i.e., (2,3,9,10,16,17,23,24-oktakis((2-benzyloxy)ethoxy)phthalocyaninato) copper, 1 (Fig. 1a)) adopt cofacial aggregate geometries in dilute solution.…”