Plastic Solar Cells Tobin J. Marks Department of Chemistry and the Materials Research Center Northwestern University, Evanston IL 60208-311 USA The ability to fabricate molecularly tailored interfaces with nanoscale precision can selectively modulate charge transport across hard matter-soft matter interfaces, facilitating transport of the “correct charges” while blocking transport of the “incorrect charges.” This interfacial tailoring can also control defect densities at such interfaces and stabilize them with respect to physical/thermal decohesion. In this lecture, challenges and opportunities are illustrated for two specific and related areas of research: 1) charge transport across hard matter-soft matter interfaces in organic electroluminescent devices, 2) charge transport across hard matter-soft matter interfaces in organic photovoltaic cells. For the latter, rational interface engineering along with improved bulk-heterojunction polymer structures leads to solar power conversion efficiencies as high as 5.6% - 7.3%, along with far greater cell durability. Photovoltaic References Motiei, L.; Yao, Y.; Choudhury, J.; Marks, T.J.; van der Boom, M.E.; Facchetti, A. Self-Propagating Molecular Assemblies as Interlayers for Efficient Inverted Bulk-Heterojunction Solar Cells, J. Am. Chem. Soc., 2010, 132, 12528. Bagnis, D.; Beverina, L.; Huang, H.; Silvestri, F.; Zheng, Y.; Pagani, G.A.; Marks, T.J.; Facchetti, A. Alkenyl vs. Alkyl Substituent Effects in Squaraine-Based Bulk Heterojunction Solar Cells, J. Am. Chem. Soc., 2010, 132, 4074. Liu, J.; Hains, A.W.; Marks, T.J. Double-Layer Thin Films as Low Indium Content Anodes for Organic Photovoltaic Cells, Thin Solid Films, 2010, 518, 3694. Hains, A.W.; Ramanan, C.; Irwin, M.D.; Liu, J.; Wasielewski, M.R.; Marks, T.J. Designed Bithiophene- Based Interfacial Layer for High-Efficiency Bulk-Heterojunction Organic Photovoltaic Cells. Importance of Interfacial Energy Level Matching, ACS Appl. Mater. Interfaces, 2010, 2, 175. Servaites, J.D.; Yeganeh, S.; Marks, T.J.; Ratner, M.A. Efficiency Enhancement in Organic Photovoltaic Cells: Consequences of Optimizing Series Resistance, Adv. Funct. Mater. 2010, 20, 97. Hains, A.W.; Liu, J.; Martinson, A.B.F.; Irwin, M.D.; Marks, T.J. Anode Interfacial Tuning via Electron- Blocking/Hole-Transport Layers and Indium Tin Oxide Surface Treatment in Bulk-Heterojunction Organic Photovoltaic Cells, Adv. Funct. Mater., 2010, 20, 595. Silvestri, F.; Irwin, M.D.; Beverina, L.; Facchetti, A.; Pagani, G.A.; Marks, T.J. Efficient Squaraine- Based Solution Processable Bulk Heterojunction Solar Cells, J. Am. Chem. Soc., 2008, 130, 17640. Irwin, M.D.; Buchholz, D.B.; Hains, A.W.; Chang, R.P.H.; Marks, T.J. p-Type Semiconducting Nickel Oxide as an Efficiency-Enhancing Anode Interfacial Layer in Polymer Bulk-Heterojuction Solar Cells, Proc. Nat. Acad. Sci, 2008, 105, 2783.