Highly efficient organic devices based on electrically doped transport layers

In this review, we have discussed the controlled doping of organic semiconductors by coevaporation with suitable dopant molecules and its application for highly efficient devices, such as organic LED and organic solar cells. The experimental data show that the conductivities can be raised many orders of magnitude above the conductivity of nominally undoped materials. Due to low mobilities, the conductivity of the materials is still much lower than those of inorganic semiconductors but sufficient for many devices that do not need too high current densities, such as organic light-emitting diodes and solar cells. Although some basic doping effects like Fermi level shifts can be well compared to the standard behavior of inorganic semiconductors, there are deviations that cannot be explained by the simple models used for crystalline inorganic semiconductors. A detailed understanding of the dependence of conductivity on doping concentration requires models that take effects like localization and percolation into account. While molecular p-type doping has been available for some time, impressive progress has recently also been made for n-type doping, which is more difficult since electrons have to be transferred into rather high-lying orbitals. We have further discussed that doped organic semiconductors are well suited for device applications. For OLEDs, the conductivities achieved are high enough to avoid significant voltage drops even in thicker layers. A key effect of doping is the generation of Ohmic contacts by tunneling through a thin barrier formed by space charge layers, an effect which works in organic semiconductors very well. This is particularly important for OLED devices where the nominally undoped transport layers have required extensive measures to achieve low barriers at the interfaces and have made the devices very sensitive to the contact properties. It has been demonstrated that doped transport layers allow realization of very efficient inverted top-emitting and transparent OLED devices. The application of doped transport layers to organic solar cells has progressed much less than that for OLEDs. Again, one key advantage is the decoupling of the electrical and optical optimization, which allows the placement of the active region of the solar cells at the regions where the optical field is the largest. Other points are that the use of doped window layers allows the extension of the quasi-Fermi level splitting from the active layers in the most efficient way toward the contacts, thus allowing maximum open-circuit voltage independent of the detailed nature of the contact materials. Many of the aspects of doped organic layers that we have discussed here are directly taken from inorganic semiconductors. It is thus easily predictable that the multitude of device principles that have developed over decades in the field of inorganic semiconductors can be explored as well for organic semiconductors, with some modifications. We thus believe that in the future, there will be ample space for further investigations of organic devices with doped layers. From a materials perspective, the progress on new organic semiconductors is rather rapid since the commercial application in devices like OLEDs has spurred large interest from industry, and a systematic search for new materials with improved properties has begun. Part of these investigations should also address new dopants, since the experiments and materials being reported here are still a very small part of what is possible. © 2007 American Chemical Society.