| Description |
The present dissertation is the result of our studies of the optical and electrical properties of self-assembled monolayer (SAM) diodes and bulk heterojunction organic photovoltaic (BOPV) devices. In our studies of SAM diodes, we fabricated solid-state mixtures of two dierent kinds of molecules; 1,4 benzene-dimethane-thiol (MeBDT) and 1-pentanethiol (PT). By varying the concentration r of MeBDT with respect to PT, we can go from a regime of isolated molecular wires (10ˉ8 < r < 10ˉ4) to a regime of aggregated molecular wire (r > 10ˉ3). For r = 0, we found that a potential barrier dominated the transport properties of the device. In the isolated molecules regime, the conductance of MeBDT dominates the transport. In this regime, because of the linearity of the conductance with respect to r, we were able to obtain a \single molecule resistance" at V = 0:1V of RM = 6 10ˉ9. In the aggregated molecules regime, an ohmic response in the current-voltage (I-V) characteristics was observed for bias voltages 0:5V with the appearance of a new band in the dierential conductance around V = 0 along with a new double band in the optical gap at 2:4eV resulting in yellow/red photoluminescence emission. Opto-electrical studies of BOPV devices reveal that there are very few similarities between these types of solar cells and conventional solar cells. From simulations and experiemental measurements of the I-V characteristics, we found that while the open voltage circuit (Voc) is important for engineers, it carries no intrinsic information of the device. It cannot exceed the built-in potential of the device (Vbuiltˉin ). The later origin was found to be dependent on electrode work function dierence for a non-Ohmic contact conguration and on the active layer's blend in an Ohmic contact conguration. In a bid to improve BOPV device performance, we added to the blend spin 1=2 radical molecules. At concentration ( 2%), an increase in device performance was observed. The principal cause for this increase was the increase in the carrier's mobility as a function of the concentration of radicals. |
