J. Drechsel

Efficient organic solar cells based on a double p-i-n architecture using doped wide-gap transport layers

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 tha...

Low-voltage inverted transparent vacuum deposited organic light-emitting diodes using electrical doping

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.

High-efficiency electrophosphorescent organic light-emitting diodes with double light-emitting layers

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.

High-efficiency low-voltage stable inverted transparent electrophosphorescent organic light-emitting diodes: combining electrically doped carrier transport layers and iridium-complex doped emissive layer

We demonstrated high-efficiency low-voltage stable inverted transparent clectrophosphorescent 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 structure contains an iridium-complex doped emissive layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers to form a nip structure. The devices are about 50% transparent and emit green light from both sides with peak external quantum efficiency (EQE) of 4.08% (14.3 cd/A). At 100 cd/m 2 , the EQE is 2.8% (13 cd/A) at an operating voltage of 4.3 V. The devices exhibit a lifetime of above 50 hours under continuous constant-current driving for the initial luminance of about 9000 cd/m 2 in vacuum, which project a lifetime of ∼5000 hours for 100 cd/m 2 .

N-type doping of organic electron transport materials for solar cells

To realize organic solar cells with high performance, we developed a novel way of stable n-doping using cationic dyes in electron transport materials. In our approach, the volatile donors are created in-situ from stable precursor compounds. Using the cationic dye pyronin B (PyB) as a model precursor, we carried out conductivity and field effect measurements to characterize the properties of doped naphtalene tetracarboxylic dianhydride (NTCDA) thin film. The results show a strong increase in n-type conductivity. Combined FTIR, UV/VIS/NIR and mass spectroscopic measurements suggest the formation of leuco pyronin B during sublimation of pyronin B chloride, and a subsequent charge transfer between dopant and matrix providing free electrons, which increase the n-type conductivity.