Stacking multiple connecting functional materials in tandem organic light-emitting diodes

Tandem device is an important architecture in fabricating high performance organic light-emitting diodes and organic photovoltaic cells. The key element in making a high performance tandem device is the connecting materials stack, which plays an important role in electric field distribution, charge generation and charge injection. For a tandem organic light-emitting diode (OLED) with a simple Liq/Al/MoO3 stack, we discovered that there is a significant current lateral spreading causing light emission over an extremely large area outside the OLED pixel when the Al thickness exceeds 2 nm. This spread light emission, caused by an inductive electric field over one of the device unit, limits one’s ability to fabricate high performance tandem devices. To resolve this issue, a new connecting materials stack with a C60 fullerene buffer layer is reported. This new structure permits optimization of the Al metal layer in the connecting stack and thus enables us to fabricate an efficient tandem OLED having a high 155.6 cd/A current efficiency and a low roll-off (or droop) in current efficiency.

Two types of transfer matrix are used in calculation: Transmission matrix L, which indicates the phase change in the same layer and is related to the thickness of material and to the angle between the surface normal and the wave vector; Interface matrix I, which indicates the reflection and refraction at interface of two media and is obtained by Fresnel reflection and transmission coefficients. Optical electric field E coming out of the OLED can be calculated by the methods descripted in reference 1 .
To investigate the weak microcavity effect in OLED devices, microcavity effect factor is introduced 2 in calculation. The output optical electric field E out can be described by equation (1) (1) R t and R b are the reflectivity of cathode and anode respectively, n source is the refractive index of emitting layer, and d is the distance between dipole source and cathode. ∆φ is the total phase shift in the cavity, is the phase shift at cathode. So, the relative spectral power distribution P(λ) is calculated by:      Figure S5. The schematic diagram of a test structure for measuring connecting electrode lateral resistance. The red dotted lines with arrows denote the current direction in the working structure.
To elucidate the impact of lateral conductivity by inserting C 60 between Al and MoO 3 layers on CGL, we investigated the lateral conductivity of the devices [with a test structure: Si substrate/ CBP(20 nm)/ CBP:Ir(ppy) 2 (acac) (8 wt.%, 30 nm)/ TPBi(65 nm)/ Al(100 nm)/ Liq(1 nm)/Al(2 nm)/with or without C 60 (2 nm)/ MoO 3 (10 nm)/NPB(50 nm)]. The total lateral resistance on CGL was measured by a four point probe measurement technique at ambient temperature. The schematic diagram of the test structure for measuring connecting electrode lateral resistance is shown in Figure S5. The measurement technique uses four finger-electrodes where the outer two electrodes supply a constant current and the voltage drop over the two inner electrodes is measured. The spacing of these finger electrodes is 1 mm.
The total resistance R of the CGL can be calculated by the Ohm's law: where and are the measured electric potential and the measured current.

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The lateral resistivity can be calculated using: where and are the width and the thickness of the CGL layer, is the spacing between the two inner electrodes. Values of the lateral electric conductivity were obtained by simply inverting the corresponding values of the lateral resistivity as follows: In our test structure, the width w of the films is 16 mm and the electrode spacing l is 1 mm.
The measured electric potential V of the CGL films without/with a C 60 layer is 44.25 mV and 57.03 mV at a constant current of 4.459 µA and 0.4459 µA, respectively. Due to the reduction reaction between Al and MoO 3 leading to formation of a laterally conducting interfacial MoO 3-x layer, the thickness of the lateral conducted layers could vary from 2 nm (just Al layer) to 12 nm (Al+MoO 3 layer). Thus the lateral electric conductivity of the CGL films without a C 60 layer is in the range 5.25-31.50 S/cm, while the CGL lateral conductivity with a C 60 layer is in the range 0.41-2.44 S/cm.