We report on two types of single-junction GaAs solar cells that have achieved a record efficiency. We achieved an NREL-confirmed conversion efficiency of 27.5%@1cm2 in substrate-type GaAs solar cell under AM1.5G illumination at 1 sun concentration, which exceeds the previously published records for devices on singlecrystalline substrates. In addition, we also achieved 28.7%@1cm2 of conversion efficiency in ELO-type GaAs solar cell under AM1.5G illumination, which is very close to the world record 28.8% of Alta Devices. In this study, the ptype Al0.3Ga0.7As heterojunction emitter with 1.82 eV bandgap was applied to increase Voc and conversion efficiency. In order to relieve the valence band offset between p-Al0.3Ga0.7As (emitter) and n-GaAs (base) in the p-n junction, the Eg-graded buffer layer was applied to the p-n junction region. In addition, the thickness and Al composition of the Eggraded layers were optimized considering band bending in the carrier depletion region. A simulation program ‘AFORS-HET’, designed especially for the solar cells with heterojunction structure, was used to understand the subtle differences in the band bending of the band diagrams and the performance of the solar cells.