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Fluid dynamics and mass transport in organic vapor jet printing
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color) (a) Diagram of the OVJP source cell. Organic vapor evaporates at a rate , dependent solely on the equilibrium vapor pressure of the organic material , and a kinetic constant k. Material re-condenses at rate k, where is the organic vapor pressure near the source. Organic vapor is transported at a rate determined by the mass transport coefficient A. Organic vapor has a partial pressure of Pv in the carrier gas at pressure P1 . (b) Radial cross-section of an organic vapor source cell modeled by using COMSOL Multiphysics® finite element analysis software. Carrier gas travels around a vented organic source capsule, from the top inlet to the bottom outlet, along streamlines shown in red. Color grading shows Pv at points within the cell. The location of condensed organic material is labeled. The capsule is 6 mm in diameter and encased in an 8 mm diameter heated tube.

Image of FIG. 2.
FIG. 2.

(Color) Dimensionless evaporation rate, η, vs dimensionless flow rate q and pressure, p. Due to pressure driven flow through the print head downstream of the source, p2 scales with q. Lines showing this relationship are indicated. For a given p, η is maximized when q = 1, as indicated by the dashed white line.

Image of FIG. 3.
FIG. 3.

Equivalent circuit for gas flow through the print head. Organic vapor and carrier gas are fed into the source channels with molar rates of QS1 and QS2 . The source channels, mixing channel, nozzle array, and nozzle-to-substrate gap correspond to elements S, M, N, and G, respectively. (b) Layout of the microchannels transporting vapor from the organic source cells to the nozzle array, N. The lengths of source channels, S, and mixing channel, M, are lS  = 10 mm and lM  = 3 mm, respectively. The channel width, wC  = 1 mm is constant for all channels. The channel depth h = 100 μm extends normal to the diagram.

Image of FIG. 4.
FIG. 4.

(Color) (a) Cross section of a single nozzle modeled by the DSMC method. Gas pressure, P, is indicated by colors, and flow speed and direction by arrows. The right side shows the flow constriction induced by proximity to the substrate. Model dimensions of g, a = 10 μm, and lG = 70 μm are noted. Aperture width wN  = 200 μm extends normal to the diagram. (b) Temperature, T, of the carrier gas within the nozzle cross section determined using the direct simulation Monte Carlo method. The walls of the nozzle array are at 300 °C, and the substrate surface is at 25 °C. (c) Heat flux, φ, from the nozzle to the substrate vs horizontal distance from the nozzle centerline, x. The solid and dashed lines show φ for P4  = 15 Torr and 26 Torr for g = 10 μm. The dotted line shows φ for g = 25 μm and P4  = 15 Torr.

Image of FIG. 5.
FIG. 5.

(Color) Deposition rate and doping ratio as functions of host and dopant source cell carrier gas flows, QS1 and QS2 . Pressures within the print head are calculated using the closed form model in Fig. 2 for the case of anozzle-to-substrate gap of g = 10 μm. Source geometry is assumed to be asin Fig. 1(b). Both the host and dopant have identical thermodynamic properties. The upper left section of the plot shows the total deposition rate of organic material, J1 + J2 , from both sources. This half of the plot is symmetric about line QS1 = QS2 . The lower right section shows the mole fraction of material 2, J2/(J1 + J2 ) in the deposited film.

Image of FIG. 6.
FIG. 6.

(Color online) (a) Nozzle array viewed through its borosilicate glass upper surface. Microchannels and nozzle inlets are highlighted. (b) Scanning electron micrograph of the nozzle inlets. Each inlet tapers from 130 × 320 μm at its entrance down to an aperture of 20 × 200 μm at the nozzle outlet. (c) Scanning electron micrograph of the nozzle array. Dark rectangular apertures are centered inside raised rectangular nozzles that provide pressure relief to the escaping gas. A detail of the corner and straight sidewall of the nozzle is shown in the inset.

Image of FIG. 7.
FIG. 7.

Pressure, P1 , of the carrier gas in the organic vapor source cells as a function of total carrier gas flow rate, QM , through the nozzle array. Experimentally determined pressures within the nozzle-to-substrate gap g = 10, 25, 50, 100 μm, and ∞ are indicated by data points marked by solid squares, open squares, solid circles, open circles, and crosses, respectively. Pressure and flow data for the nozzle and substrate system calculated by a DSMC model are indicated by dotted lines. Pressures calculated using the analytical model at g = 10 μm and g → ∞ are indicated by solid lines. Predicted back pressure for a nozzle array with tapered sidewalls such that lG  = 20 μm is shown by the dashed line for g = 10 μm.

Image of FIG. 8.
FIG. 8.

(a) Current density, j, and luminance, l, vs applied voltage for a PHOLED with an emissive layer grown using OVJP, compared to an analogous device grown entirely by VTE (b) External quantum efficiency, η, and power efficiency, PE, of OVJP and VTE-grown PHOLEDs vs j. Inset: Electroluminescence spectra of OVJP and VTE-grown PHOLEDs.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Fluid dynamics and mass transport in organic vapor jet printing