| Polymer:fullerene bulk heterojunction solar cells have been the subject of intense research since their discovery more than 10 years ago. The polymer:fullerene bulk heterojuction is the most efficient organic device structure due to the intimate contact between the hole (polymer) and electron (fullerene) transporting materials throughout the bulk. The morphology of the bicontinuous networks affects exciton separation, carrier transport, charge recombination, and energy conversion efficiency of the bulk device. Despite the popularity of bulk heterojuction devices, probing the morphology of the donor and acceptor networks, both at the surface of the blended film and throughout the interior of the bulk heterojuntion, has proven to be a challenge. To image the internal networks of the films, we developed a method to prepare a cross section of the bulk heterojunction device and subsequently studied it using phase imaging atomic force microscopy. The cross section was fabricated by focused ion beam and placed on an indium tin oxide (ITO) coated glass substrate for imaging. To measure the nanoscale carrier mobilities through the thickness of the film, we used conductive atomic force microscopy (C-AFM). In C-AFM, the conducting tip acts as a nanoelectrode and is able to measure the electrical properties of regions approximately 10 nm in diameter. C-AFM measures current as a function of applied voltage either at specific points on a surface or maps out the current image. We selectively probed the hole (donor) or electron (acceptor) transporting networks by controlling the work functions of the substrate and the conducting tip. By taking a large number of mobility measurements at different locations across the surface of the film, we were able to determine the homogeneity of the film's electrical properties. In addition to measuring mobilities, this technique in current imaging mode was used to identify surface structures as being composed of either hole or electron transporting material. Coupled with an inverted optical microscope, nanoscale photocurrent images and I-V curves can be obtained that help to understand the charge generation process and surface morphology.;The above techniques were used to study several different systems, including poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PC 61BM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiopene)- alt-4,7-(2,1,3-bezothiadiazole)] (PCPDTBT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) bulk heterojunctions, and P3HT- b-P(S89BAz11)-C60, a rod-coil block copolymer composed of a P3HT block and fullerene containing block. The effect of processing conditions on the surface and internal morphology and on the charge transport characteristics of the film were studied. The processing conditions examined included thermal annealing, the use of solvent additives, and film casting technique. In general, it was found that these processing techniques can have a large impact on the morphology of the donor and acceptor networks of the device, which in turn can dramatically affect the ability of free carriers to move through the film, and thus on the overall energy conversion efficiency of the device. |
