Nanoimprint with a large-scale nanoporous Si mold is developed to fabricate high density periodic nanopillars (similar to 10(10)/cm(2)) in various functional polymers. A anodic alumina membrane is first obtained using electrochemical anodization. The membrane is used as a mask for a two-step plasma etching process to obtain a Si mold of 50-80 nm wide and 100-900 nm deep pores. The mold is used in nanoimprint lithography to fabricate ordered and high density polymer nanopillars and nanopores in SU-8, hydrogen silsesquixane, polymethylmethacrylate, poly(3-hexylthiophane) (P3HT), and phenyl-C61-butyric acid methyl ester (PCBM). Then, the imprinted P3HT nanopillars were used to make bulk heterojunction solar cells by depositing PCBM on top of the pillars. Imprinting provides a way to precisely control the interdigitized heterojunction morphology, leading to improved solar cell performance.