Single-Particle Spectroscopy on Conducting Polymer-Fullerene Composite Materials for Application in Organic Photovoltaic Devices

Mar 31, 2008

The study of the photophysical and optoelectronic properties of a functioning conducting polymer device is extremely complicated and is hampered by the complex nanostructure and overall morphology of the conducting polymer materials applied in these devices. Here we discuss a novel approach to investigate this issue spectroscopically in terms of bulk-heterojunction organic photovoltaic devices. Novel composite nanoparticles of the conjugated polymers MEH-PPV and P3HT blended with the fullerene PCBM were fabricated and are observed to be excellent simplified model systems for the study of molecular-scale morphology effects at play in these complex nanostructured materials. Single-particle spectroscopy reveals the extent to which variations in polymer-chain folding and interactions between polymer chains and fullerenes affect material morphology, spectral properties, and optoelectronic properties, providing a detailed molecular scale insight into the morphological effects at play in the active layers of bulk-heterojunction organic photovoltaic devices that otherwise would be masked by the presence of the bulk.

Figure 1
Conducting polymers have been at the forefront of materials research on novel semiconductors given the exciting prospect of plastic electronics that can be built from solution-processed materials with low-cost manufacturing processes. The photophysical and optoelectronic properties of these materials have been investigated extensively for potential application in organic light-emitting diodes (OLED) (1,2), which for conducting polymer devices also are referred to as polymer light-emitting diodes (PLED), organic field effect transistors (OFET) (3–5), and organic photovoltaic (OPV) devices (Figure 1) (6–9). Despite intense and sustained research on conducting polymer materials and devices, there are no products slated for commercialization within the next few years that are based upon conducting polymers, in contrast to small-molecule organics that have been commercialized successfully in organic flat-panel displays. Unfortunately, the study of the photophysical and optoelectronic properties of a functioning conjugated polymer device is extremely complicated. Several factors contribute to difficulties with developing a detailed understanding of conducting polymer materials and devices. First, in a functioning device, there are a number of different excitations that exist at the same time, such as singlet excitons, triplet excitons, and polarons. These all interact with each other and are distributed heterogeneously throughout the device. Second, conducting polymer materials are enormously heterogeneous. This is complicated further by the fact that conducting polymers can fold into complex nanostructured particles for which the different resulting morphologies can lead to varying photophysical and optoelectronic properties. This was demonstrated clearly by characterization of MEH-PPV/C60 OPVs in relation to the morphology of the active layer (10). Third, there are particular issues related to charge transport through conducting polymers. Deep charge trapping in itself leads to unexpected observations when characterizing conducting polymer devices and has been cumbersome, particularly in terms of developing suitable device models (11,12). The result of these issues is that device behavior and performance incorporating these materials is unpredictable at best. In particular, the low photostability of conducting polymers and the ease with which they participate in electrochemical processes lies at the basis of the latter issue.