Organic polymer-based photovoltaic (OPV) cells are a potentially low-cost sunlight-to-electricity conversion technology with unique advantages compared to other solar cell technologies, such as flexibility, light weight, and facile processing. However, OPV devices suffer from low solar energy conversion efficiency and still face many technical challenges. For example, OPV devices suffer from resistive losses at the semiconductor-electrode interface. Building organic solar cells with ohmic contacts ? where the interface is non-rectifying with negligible resistive losses ? has the potential for improved fill factors, short-circuit currents, and perhaps higher open-circuit voltages, all of which have the potential to increase the efficiency of organic photovoltaics toward their theoretical limit of 15%. Current contact methodologies for organic photovoltaics rely on tuning the work function of the electrodes to match the transport energy levels of the electron donors and acceptors which make up the photoactive layer. In contrast, inorganic devices rely on site-specific doping of the semiconductor near the electrode interface to promote tunneling through the charge extraction barrier by reducing the barrier width. The proposed research will engineer organic semiconductor-electrode interfaces in organic solar cells, focusing specifically on developing a methodology analogous to site-specific doping in inorganic semiconductors by covalently linking macromolecular dopants at sub-monolayer coverage to electrode surfaces.
The proposed research will synthesize p-type and n-type semiconducting polymers that can anchor to electrode materials. Though this approach, it is hypothesized that the electrical properties of organic semiconductors near the cathode and anode can be tuned to promote efficient charge extraction and explore the consequences of building ohmic contacts on solar cell device performance. By tuning the molecular structure of polymer dopants, the wetting behavior of organic semiconductors on electrode surfaces can be also controlled to control polymer phase aggregation and hence prevent shunt paths and promote further charge extraction. It is proposed that the localization of dopants at the electrode contacts may become a widely-applicable strategy for the minimization of losses at the semiconductor-electrode interface of organic solar cells.
Broader Impacts
The proposal education and outreach activities seek to launch a new initiative at Pennsylvania State University entitled ?Sunlight, Energy, Polymers? (Sun-E-Poly), which will serve as a nucleation point for current and future efforts in research, education, and outreach centered on OPV across campus. Undergraduate students, women, and under-represented minorities will be integrally involved in the proposed research activities through the current Penn State Soft Materials and Chemical Energy Storage and Conversion NSF Research Experiences for Undergraduates (REU) site programs.
Organic electronic circuits and devices are becoming a reality. New organic light emitting diode (OLED) devices and flexible solar cells rely on controlling the properties of organic molecules to conduct electricity. The major scientific outcome from this work is a new way to produce plastic solar cells with less material and improved efficiency. We have focused our efforts on improving the junctions or interfaces in a solar cell to increase the current flow. Controlling the interaction between two layers – the semiconductor layer and the electrode – helps to move charges across the interface to improve the efficiency. We have demonstrated our approach in a number of materials and have tested hundreds of solar cells to prove our new scientific concept. This idea for improved interfaces in solar cells is finding commercial importance in OLED devices and other plastic electronic circuits. While more research and development is required, companies are becoming interested in this technology to improve their products as new applications of electronic devices emerge. The intellectual merit of the work revolves around the molecular interactions at the interface. We have performed experiments to measure the interactions of the molecules at the electrode interface and have connected this interaction to the solar cell performance. We have designed specific polymers that can improve performance at the electrode interface and consequently solar cell efficiency (Figure 1). Our major broader impact has been the direct education of more than 8 students and researchers that have gained hands-on experience with this project and over 100 other students who have had some contact with these topics in courses. We have also had outreach to industry through this work and have engaged companies who are interested in learning more about the work occurring at the University. The outcome of the work is a new approach for interface junction refinement for plastic electronics. This invention is applicable to solar cells and OLEDs – two major applications of conducting polymers. We have also educated a number of students and performed significant outreach to increase awareness of the important role that science plays in promoting the development of new high-tech devices.
