The concept of tandem organic light-emitting devices (OLEDs) provides a pathway for developing highly stable and efficient OLEDs. The connecting structure that bridges adjacent light-emitting units, substantially affects the device performance of tandem OLEDs. In this letter, we introduce an effective connecting structure in which an ultrathin middle metal layer is sandwiched between efficient electron- and hole-injection layers for the tandem OLEDs, which in essence, avoids the use of reactive metals during fabrication. Two-unit tandem OLEDs with such connecting structure exhibit less than double the driving voltage, yet more than double the efficiency, more saturated emission color, and longer operational lifetime compared to those of single-unit devices. A model based on a hypothesis of energy level pinning effect has been proposed as the mechanism of the connecting structure in the tandem devices. This model is also consistent with the results obtained from the photovoltaic effect measurements in tandem OLEDs.
In this paper, we report an effective thermal transfer process for performing controllable and selective doping of polymer films for multicolor organic light-emitting devices (OLEDs) or color pixels in OLED displays. In this process, the polymer receiver film is placed in direct contact with the dye-dispersed polymer donor film (with patterns) to permit direct dye-diffusion thermal transfer. It permits controllable lateral and vertical modulations of the dopant distributions of a polymer film for organic devices. Furthermore, it is the intention to combine the selectively doped (patterned) polymer layers with the vacuum deposited small-molecule films, such that both the heterostructure versatility of small-molecule materials and the patterning advantage of polymeric materials could be conserved in the hybrid polymer/small molecule heterostructure OLEDs. The rather thermally stable polymer poly(N-vinylcarbazole) (PVK), which is hole-transport and deep-blue emitting, in combination with several electron-transport molecules has been studied to demonstrate the feasibility of these concepts.
Color integration in organic LEDs (OLEDs) on a substrate has always been a challenge due to the incompatibility of OLED materials with the conventional photolithography. In this paper, we report a process for the fabrication of large-area multicolor OLEDs of arbitrary patterns by combination of thermal-transfer printing and screen-printing. Thermal transfer printing is used to introduce color-tuning dyes into a thermally stable OLED polymer layer from a dye- dispersed polymer layer on the donor plate. Such a process permits controllable and uniform doping of a polymer layer over large areas. By using a patterned color donor plate, color integration in OLEDs could be accomplished with a single thermal transfer step. In this work, the source plate containing multicolor patterns is fabricated by screen- printing. The RGB color patterns were printed sequentially by using RGB inks prepared by dispersing nile red,C6 and perylene into a commercial screen-printing paste. Based on these printing approaches, we have successfully fabricated multicolor single-layer and heterostructure OLEDs.
An effective process of performing controllable doping of polymer films in organic light-emitting devices is reported. In this approach, a film to be doped is brought into direct contact with a dye-dispersed polymer donor film to permit direct dye-diffusion thermal transfer. Theoretical and experimental studies indicate that this doping process can be modeled by Fick's diffusion theory and that a desired dopant distribution from shallow to flat profiles may be obtained in a single transfer step by adjusting the diffusion conditions. Doped-polymer light-emitting devices made by this process and the conventional blending process exhibited same device characteristics. Along with patterned color donor plates, we demonstrated multicolor OLEDs of arbitrary patterns over large areas with a single thermal transfer step.
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