Organic-semiconductor-based devices have been attracting a large research and development community due to their advantages of flexibility, low cost, and ease of fabrication. They also open up new realms of applications such as spintronic devices. Based on experience from numerous experiments, a heterostructure is a key component for enabling the performance of many types of organic devices such as light-emitting diodes, photovoltaic cells, and field-effect transistors. Theoretical work aims at better understanding the physics as well as providing guidance for future device development. In this dissertation, device theories, models, and calculations for various types of organic devices are developed and presented. Organic light-emitting and photovoltaic devices have similarity in their structures. In both types of devices, heterostructures are employed. We develop a unified device model combining both the microscopic processes at the heterojunction interface and the macroscopic transport in the bulk. By tuning the parameters, the model can simulate both the light-emitting and photovoltaic devices. The study of carrier density profiles and current-voltage characteristics for different cases provides insight on how different processes affect the device properties. Model calculations show that the microscopic processes at the heterostructure interface are critical for the efficiency of organic photovoltaic cells. By inserting a thin tunnel barrier at the interface, these processes can be controlled and the device efficiency can be improved. We study the effect of the interfacial layer on the enhancement/ suppression of the processes and incorporate the results into the unified device model. The calculated short-circuit current and open-circuit voltage agree well with experimental observations. In organic heterojunction light-emitting diodes, it has been observed that the electroluminescence can be improved significantly (~ 10%) by an external magnetic field. This is caused by the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules, as well as the Zeeman effect due to an external magnetic field. The ratio of singlet/triplet excited states in the polaron pair states at the interface indicates the dominant process. We develop an analytical model using a density matrix approach and rate equations based on quantum statistics, with different types of spin correlation functions. The model calculations agree well with experimental results. In organic field-effect transistors, a layer of organic ionic liquid has recently been introduced to substitute for a conventional dielectric in order to improve the channel charge density and reduce the operating voltage. The interface between an ionic liquid and an organic semiconductor has interesting physics which is not yet fully understood. We also incorporate this interface into the “heterojunction” family. One important property of this heterojunction is that there exists a coupling effect between the channel conductance and the gate-to-channel capacitance. We propose an equivalent-circuit based model which describes the physical mechanism and explains experimental results.