סמינר בננו: From Nanostructure to High Efficiency in Organic Photovoltaics
Stephen Forrest, University of Michigan
It has recently been shown that the fundamental physical origins of the photogeneration process can be simply understood in the context of the kinetics of polaron-pair dissociation and recombination at donoracceptor heterojunctions1, 2. This analysis has given rise to the derivation of an ideal diode equation that simply describes the dark current and photoresponse characteristics of organic photovoltaics. In the context of this framework, we discuss methods of controlling the nanostructure of organic thin films that follow the design principles inferred from the ideal diode theory. Nanostructure control demands that the properties of the donor-acceptor interface and that of the bulk thin films be independently optimized, whereby there needs to be disorder at the interface to reduce the polaron-pair dissociation rate and order in the film bulk to reduce series resistance3. We show that this morphological control using both solution and vapor phase deposition technologies can create nanostructures with solar conversion efficiencies of over 8% based on squaraine donors and electron conducting buffer layers. Indeed, using a combination vapor deposited blends with nanocrystalline morphology, tandem cells have achieved efficiencies of 11% in our lab. Recently we have extended our analysis to understand polaron and exciton dynamics at organic/inorganic semiconductor junctions. We will discuss these and other aspects of the photon-toelectron conversion process that lead from nanostructural control to high efficiency organic and hybrid solar cells.
1. N. C. Giebink, B. E. Lassiter, G. P. Wiederrecht, M. R. Wasielewski and S. R. Forrest, Phys. Rev. B 82, 155306 (2010).
2. N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski and S. R. Forrest, Phys. Rev. B 82, 155305 (2010).
3. J. D. Zimmerman, X. Xiao, C. K. Renshaw, S. Wang, V. V. Diev, M. E. Thompson and S. R. Forrest, Nano Lett. 12, 4366 (2012).