Here we discuss the design, fabrication, and simulation of ultrathin film n-i-p a-Si:H solar cells incorporating light trapping plasmonic back reflectors which exceed the performance of n-i-p cells on randomly textured Asahi substrates. The periodic patterns are made via an inexpensive and scalable nanoimprint method, and are structured directly into the metallic back contact. Compared to reference cells with randomly textured back contacts and flat back contacts, the patterned cells exhibit higher short-circuit current densities and improved overall efficiencies than either reference case. Angle-resolved photocurrent measurements confirm that the enhanced photocurrents are due to coupling to waveguide modes of the cell. Electromagnetic modeling is shown to agree well with measurements, and used to understand further details of the device.