Start Date: September 2010
Michael McGehee, and Alan Sellinger, Department of Materials Science and Engineering, Stanford University
The objectives of the proposed work are to design, prepare and characterize a family of new advanced electron transport materials from simple, minimal-step, high-yield, and inexpensive synthetic processes for application in organic photovoltaics (OPV). These novel materials will be chemically prepared by linking together conjugated electron deficient moieties that may include, but are not limited to, phenyl- and naphthyl-imides, benzothiadiazoles, dicyanoimidazoles, and diketo-pyrrolo-pyrroles. These new materials will be used to fabricate OPV devices with efficiencies and lifetimes exceeding those of state-of-art OPVs that typically use fullerene-based electron transport materials.
Organic photovoltaics (OPV) have recently reached power conversion efficiencies (PCE) of 7.7%1 and extrapolated lifetimes of >30 years2 bringing this area of technology closer to commercialization. Common to both of these reports are the use of fullerenes as the electron acceptor/transport materials. Fullerenes tend to have low absorption in the visible range, produce devices with relatively low open circuit voltages (Voc), are very difficult to synthesize and purify, and are very expensive.
New acceptor materials that could address and solve the issues of fullerenes could push PCE levels to >10%. For example, replacing the weakly absorbing fullerenes with more strongly absorbing acceptors should lead to efficient light harvesting below 700nm (Figure 1) and increased short circuit current densities (Jsc). Assuming a peak external quantum efficiency (EQE) of 0.75, Jsc can potentially be improved from 15.2 mA/cm2 in the record device to 17.2 mA/cm2 (Figure 2).
The general strategy of how the materials will be designed to address important properties necessary for OPV acceptors is shown in Figure 3. The materials will have two different electron deficient moieties linked together in an A-B-A type arrangement. Furthermore the A-B-A moieties will be linked by alkene, alkyne and/or phenyl linkages (sp2 hybridization) that will allow for electron delocalization throughout the molecule, thus stabilizing the negative charge over the entire molecule. Although the A-B-A moieties will be electron deficient in nature, they will have different degrees of deficiency so as to have partial push-pull character. This is known to impart low band gap properties, important for absorbing a greater amount of the visible spectrum.
The proposed research is designed to be very interdisciplinary involving synthetic organic chemistry, materials characterization, and device processing/characterization. The research is aimed towards the goal of OPVs with PCEs >10% by the end of the project and will be divided into two sections – chemical synthesis and device physics. Design and synthesis of new electron deficient molecules will be carried out to achieve crucial fundamental properties such as: proper electronic energy levels; strong visible light absorption; side chain attachments for solution or vacuum processing; and solid-state packing for optimized charge transport. Charge carrier mobilities and solid state morphologies of neat and blend thin films using various microscopy and x-ray techniques will be examined. Energetic loss pathways due to triplet and charge transfer states will be explored using spectroscopic techniques. The physical, thermal, optical, and electronic properties of the materials are designed to be tunable so that they may be used with a large number of corresponding electron donor materials for device fabrication.
- Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. “Polymer solar cells with enhanced open-circuit voltage and efficiency.” Nature Photonics 2009, 3 (11), 649-653.
- Schwartz, G.; Maennig, B.; Uhrich, C.; Gnehr, W.; Sonntag, S.; Erfurth, O.; Wollrab, E.; Walzer, K.; Pfeiffer, M. “Efficient and long-term stable organic vacuum deposited tandem solar cells.” Kafafi, Z. H., Lane, P. A., Eds.; SPIE: San Diego, CA, USA, 2009; 74160K-741611.