Solar cells based on polymeric materials offer ease of fabrication and potentially low cost, but suffer from low solar energy conversion efficiency relative to inorganic semiconductor based solar photovoltaic devices. The most efficient organic photovoltaic (OPV) solar cells to date are made from blending conjugated polymers (donors) with fullerene molecules (acceptors) to form nanostructured bulk heterojunction (BHJ) active layers. The nanoscale morphology of the blend determines the charge generation and transport characteristics, and hence dictates the device performance. However, it is difficult to accurately correlate morphology to device performance, or to understand how processing conditions control the formation of desirable morphologies, due to the lack of in situ characterization tools. Scanning probe microscopy (SPM) is a potentially versatile tool for the characterization organic photovoltaic cells in the ambient state. However, current SPM approaches are not able to differentiate between the donor and acceptor domains at the nanoscale, and are also not able to observe in situ morphological changes during bulk heterojunction formation, particularly during annealing.
The objective of this proposed research is to develop relationships between nanoscale morphology, OPV device performance, and processing conditions used to form the BJH through the use of Kelvin Probe Force Microscopy (KPFM). Kelvin probe force microscopy has the potential to differentiate between phase separated donor and acceptor domains, and can be adapted to observe the in situ morphological evolution of bulk heterojunctions in real time during material processing, with high sensitivity and nanometer scale resolution. Specifically, a cross-section of the bulk heterojunction will be exposed to the scanning probe and characterized by KPFM. The surface of the cross-section is expected to offer in situ morphological details that are more representative of the actual BJH fine structure. By using a scanning probe with an ultra-sharp tip and a shape-tailored cantilever, KPFM can be enhanced to achieve both high resolution and high sensitivity in the ambient state. The proposed studies with KPFM are designed to gain a better understanding of the fundamental physics of OPV devices, and to elucidate the processing conditions that create organic bulk heterojunctions which give rise to enhanced solar energy conversion efficiency.
The proposed education and outreach activities focus on bringing solar energy topics into graduate and undergraduate curricula, as well as to local high schools. Course materials based on the research will be developed for graduate students in the areas of scanning probe microscopy and photovoltaics. Undergraduate students will participate in senior design projects involving hands-on learning with solar cell device characterization and scanning probe microscopy. The PI will work with the University of Pittsburgh Robotics Club to develop a mobile robot driven by solar power for use at an annual robot show for high school students, and will visit local high schools to give demonstrations on flexible organic solar cells.