Described herein are materials and systems for efficient upconversion of photons. The materials may be disposed in a system comprising two semiconductor materials with an interface therebetween, the interface comprising a valence and/or conduction band offset between the semiconducting materials of about −0.5 eV to about 0.5 eV, including 0, wherein one of the semiconductor materials is a material with discrete energy states and the other is a material with a graded composition and/or controlled band gap. The system can upconvert photons by: a) controlling energy levels of discrete energy states of a semiconducting material in a system to direct tunneling and exciton separation; b) controlling a compositional profile of another semiconducting material in the system to funnel charges away from an upconversion region and into a recombination zone; and c) utilizing the discrete energy states of the semiconducting material in the system to inhibit phonon relaxation.