This document summarizes research on using nanoimprint lithography to pattern the active layers in organic solar cells to improve their efficiency. Key findings include: 1) Nanoimprinting polymer and acceptor layers led to up to a 1.35% power conversion efficiency for P3HT/PCBM solar cells compared to 0.9% for a bilayer device. 2) Nanoimprinting the donor polymer created well-defined patterns that improved exciton dissociation and charge collection. 3) Imprinting dehydrated PEDOT:PSS enabled high aspect ratio patterns needed for electrode applications with improved device performance. 4) Ongoing work involves imprinting low bandgap polymer P

1. N anoimprinted Organic S olar C ells T owards High E fficiency Yi Yang , Anvar Zakhidov, Walter Hu Nanoscale Integration Laboratory Nanotech Institute Department of Materials Science and Engineering The University of Texas at Dallas

3. OPV Working P rinciple P. Peumans, S. Uchida, and S. R. Forrest, Nature 425, 158 (2003 ) Exciton creation Diffusion Transfer Collection Processes: C 60 Solar cell device Acceptor: Donor: PCBM P3HT PCPDTBT PC 70 BM Short diffusition length (Ld~10 nm) Low mobility (μ h ~10 -4 cm 2 /v.s)

5. Polymeric Nanostructure by NIL Polymer patterning by nanoimprint lithography (NIL) Si nanolined mold Imprinted P3HT nanostructure (Invited) Yi Yang et al., ACS Nano (to submit).

6. Acceptor Deposition Spin coating Oblique angle evaporation P3HT nanostructure defined by NIL Transfer imprinting

7. Spincoating of PCBM M . Aryal et al., J. Vac. Sci. Technol. B, 2 6 (20 08 ) M. Zhou et al . , J. Vac. Sci. Technol. B, 28 (2010)

8. Oblique Deposition of C 60 C oated from one side C oated from two sides Y i Yang et al. , J. Vac. Sci. Technol. B 28 (6), C6M104 (2010) Bilayer Imprinted Voc (V) Jsc (mA/cm2) FF PCE (%) Bilayer 0.32 6.98 0.41 0.90 Imprinted 0.32 9.46 0.45 1.35

9. Nanoimprinted Organic Electrode active layer electrode Patterned polymer nanostructures Improve exciton separation Lower electrode resistance h + e -

10. Challenge for Nanoimprinted PEDOT:PSS Low for PEDOT:PSS Cohesion For successful de-molding, Aspect ratio Demolding ~10 -2 aspect ratio Broken PEDO:PSS nanogratings Adhesion Y i Yang et al. , Nanotechnology 22 (48), 485301 (2011). : interface surface energy : polymer surface energy

11. Nanoimprint of dehydrated PEDOT:PSS PEDOT:PSS spincoating Nanoimprinting at 100 o C/2 MPa D ehydrated at 15% r H for 24 hr Si nanograting mold PEDOT:PSS nanogratings (h=60 nm, w=p=70 nm) Y i Yang et al. , Nanotechnology 22 (48), 485301 (2011).

13. Low Bandgap PCPDTBT OPV J.Young et al., Science 317 , 222 (2007) PCPDTBT PC 71 BM P3HT PC 61 BM

14. Ongoing Work: I mprinted PCPDTBT/C 70 OPV PCPDTBT nanogratings PCPDTBT/C 70 Steffen et al., Appl. Phys. Lett. 94 (22), 223307 (2009). Y i Yang et al., manuscript in preparation