Single-junction GaAs solar cells have been studied extensively recently, and have reached over 28% efficiency. Further improvement requires an optically thick but physically thin absorbing layer to provide not only high open-circuit voltage but also large short-circuit current. This paper studies various optical designs, such as light scattering and reflection at various surfaces. It is concluded that thin GaAs solar cells with hundreds of nanometers thickness and the combination of textured surfaces and reflective back surfaces can potentially offer efficiencies greater than 30% under one sun AM1.5G solar spectrum. A practical approach is proposed to realize these structures by roughening a lattice-matched ZnSe layer grown on the device. The achievable efficiencies of all these structures are discussed.