Many potential solar cell materials suffer from a tradeoff with device thickness: while thinner cells have improved collection, higher open circuit voltages, and reduced costs, they also show reduced photocurrents due to decreased absorption. Plasmonic nanoparticles integrated into the back metal contact of an ultrathin film solar cell can enhance absorption via controlled coupling of incident free space optical modes into localized resonant modes and propagating guided wave modes. We have fabricated ultrathin film amorphous Si cells that exhibit a 10% enhancement in photocurrent over randomly textured back reflectors, and an improvement of 50% over flat back reflectors. Conformal deposition with both top and backside texturing leads to broadband enhancement over the solar spectrum. Electromagnetic modeling confirms the observations, and is used to optimize the pattern. Coupling to waveguide modes is confirmed via angle resolved photocurrent measurements. We will discuss the role of nanoparticle shape, size, and arrangement on the photocurrent, as well as applications to other materials.