To enhance the performance of organic devices, doping and graded mixed-layer structures, formed by co-evaporation methods, have been extensively adopted in the formation of organic thin films. Among the criteria for selecting materials systems, much attention has been paid to the materials' energy-band structure and carrier-transport behavior. As a result, some other important characteristics may have been overlooked, such as material compatibility or solubility. In this paper, we propose a new doping method utilizing fused organic solid solutions (FOSSs) which are prepared via high-pressure and high-temperature processing. By preparing fused solid solutions of organic compounds, the stable materials systems can be selected for device fabrication. Furthermore, by using these FOSSs, doping concentration and uniformity can be precisely controlled using only one thermal source. As an example of application in organic thin films, high-performance organic light-emitting diodes with both single-color and white-light emission have been prepared using this new method. Compared to the traditional co-evaporation method, a FOSS provides us with a more convenient way to optimize the doping system and fabricate relatively complicated organic devices.