| Molecular organic thin films have been used as active materials in organic light-emitting devices (OLEDs), photovoltaic cells, photodetectors, and thin-film transistors. In this thesis, we study the properties of pure and mixed organic thin films and various heterojunction (HJ) structures, and their influences on the performance of organic optoelectronic devices.; In Part I, we study the thin film growth of a planar-stacking molecule, BTQBT. Charge transport in BTQBT films is revealed by studying the field-effect mobility of thin-film transistors using such films as the channel. Mixing BTQBT with another molecule, PTCDA, leads to enhanced electrical conductivity, and the unique "bipolar doping" between the organic donor-acceptor (D-A) couple is demonstrated.; Part II is devoted to multilayer organic photodetectors with an alternating donor and acceptor layers as the active region. We study the carrier transport within the multilayer D-A HJ as well as at its contact with the anode. Charge carriers tunneling through the thin (≤30A thick) individual active layers are responsible for the high quantum efficiency and bandwidth of these devices, whereas thermally-assisted tunneling injection of electrons from the anode into the organic layers dominates the dark current. Furthermore, optical bistability is achieved when such a photodetector is integrated on top of an OLED, extending the functionality of this type of device.; Organic photovoltaic cells have the potential for low-cost solar energy conversion, and are studied in Part III. We demonstrate that cells with very low series resistances can be fabricated, and their performance can be described using conventional p-n junction theory. The photovoltaic characteristics of various D-A HJ structures are studied, including a planar HJ between pure layers, a mixed HJ with donor and acceptor molecules mixed on the molecular scale, and a hybrid planar-mixed HJ (PM-HJ) with a mixed layer sandwiched between pure layers. A power conversion efficiency of 5.0% under 1 sun (simulated AM1.5) is demonstrated in a CuPC-C60 hybrid PM-HJ cell, which is further improved to a record-high 5.7% by stacking two thin hybrid PM-HJ cells in series. |
