The use of doped wide-gap charge transport layers with high conductivity and low absorption in the visible range enables one to achieve high internal quantum efficiencies and to optimize the devices with respect to optical interference effects. Here, it is shown that this architecture is particularly useful for stacking several cells on top of each other. The doping eases the recombination of the majority carriers at the interface between the cells, whereas the recombination centers are hidden for excitons and minority carriers. By stacking two p-i-n cells both with a phthalocyanine-fullerene blend as photoactive layer, a power efficiency of up to 3.8% at simulated AM1.5 illumination as compared to 2.1% for the respective single p-i-n cell has been achieved. Numerical simulations of the optical field distribution based on the transfer-matrix formalism are applied for optimization. The concept paves the way to even higher efficiencies by stacking several p-i-n cells with different photoactive materials that together cover the full visible spectrum.
We demonstrate low-voltage inverted transparent vacuum deposited organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The devices comprise an intrinsic 8-tris-hydroxyquinoline aluminum (Alq3) emitting layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers. They exhibit low driving voltages (∼4 V for a luminance of ∼100 cd/m2). The devices are about 50% transparent in the Alq3 emission region and emit green light from both sides with a total external current efficiency of about 2.5 cd/A.
We demonstrate high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs) with double light-emitting layers (D–EMLs) by doping both hole and electron transport hosts with fac tris(2-phenylpyridine)iridium [Ir(ppy)3] simultaneously. The D–EMLs PHOLEDs show significantly improved efficiency (peak external quantum efficiency of about 12.6%, corresponding to a current efficiency of 44.3 cd/A) compared to the conventional PHOLEDs with a single EML and either hole or electron transport host doped with Ir(ppy)3. We attribute this improvement mainly to reduced losses of triplet excitons into regions that are not doped by phosphorescent emitter molecules.
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