Recent work with high-efficiency solar cells has led to improvements in optical efficiency to the point where short-circuit currents of 40.8 mA/cm2 have been measured under terrestrial sunlight in 380 micron thick cells; this is over 90% of the theoretical maximum for silicon. Three parts of the design of these cells contribute to the high currents: First, careful grid design, surface texture, and a double-layer antireflection coating reduce the reflection loss to approximately 4% over the entire solar spectrum. Second, the use of high-purity float-zone silicon, coupled with an effective back-surface-field structure and an aluminum back surface reflector, is responsible for the efficient collection of carriers generated by infrared light, which can penetrate through the entire cell. Internal quantum efficiencies of .85 at 1050 nm and .56 at 1100 nm have been achieved. Finally, the use of a thin emitter, the optimization of the emitter doping level and the passivation of the front surface have allowed internal quantum efficiencies exceeding 0.95 over the entire visible range. The possibility of achieving quantum efficiencies greater than 1 for ultraviolet light is also discussed.