Examining the Heterojunction between Active Layers in Poly(3-hexylthiophene-2,5-diyl) / [6,6]-Penyl-C61-butyric-acid-methyl-ester Photovoltaic Devices
Organic photovoltaic devices consist of materials that are cheap, flexible, and easy to process, yet typically low in efficiency. A primary method of improving the efficiencies of these devices is to mix of the active layers of the device into a single solution, forming a bulk heterojunction when deposited. This method is easier to accomplish than depositing each layer separately, and increases the surface area of the interface between the materials while decreasing the distance required for an exciton to travel before disassociating across the heterojunction, contributing to an increase in device efficiency. However, some drawbacks of this design include limited control over nanoscale features in the device, and the increase in interfacial recombination and leakage currents leading to lower open circuit voltage. We present an alternative to the bulk heterojunction device structure by demonstrating the success of a recently developed method of depositing solution processed molecular materials. Our method employs stamping of the electron acceptor layer, [6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM), on top of a spin-coated hole acceptor layer, poly(3-hexylthiophene-2,5-diyl) (P3HT). This fabrication process results in a planar heterojunction geometry. Solar cells fabricated in this way have a smaller donor-acceptor interfacial area for exciton dissociation, but better defined pathways for charge collection, potentially resulting in decreased short circuit current but improved shunt resistance. Additionally, these cells are better suited for fundamental studies of organic-organic interfaces, because of the inherent control over the thickness and concentration of each organic layer. We analyze the morphology of the active layers in our device through the use of an atomic force microscope and find the film's RMS roughness to be 15.1nm. We conclude that the roughness of the P3HT surface should contribute to a large contact area between the P3HT and PCBM films, and possibly a mixed layer of material in the interface between the two layers. We present the current-voltage characteristics of our bi-layer devices and compare their performance to bulk heterojunction devices fabricated within the same laboratory conditions. Our highest efficiency P3HT:PCBM bulk heterojunction devices have JSC=4.8mA with VOC=0.54V and FF=0.585, while our most efficient P3HT/PCBM planar heterojunction devices have JSC = 3:45x10^3mA, VOC=0.1V and FF=0.021. These results demonstrate great promise for our laboratory, which has only just begun to fabricate bulk heterojunction devices, and is now generating new devices with cutting-edge fabrication techniques. We compare our results to an analytical model for bi-layer devices based on the diffusion equation for exciton generation and the drift-diffusion equations for charge transport within the device. This model shows good agreement with experiments. Additionally, we ran monte carlo simulations for charge generation and transport in the organic materials and analyzed the results of these simulations. Our analysis found physical inconsistencies in these simulations, and further work will be necessary to understand the errors in this model before it can be compared to our experimental results.