Efficient electron-hole (e-h) pair multiplication could lead to highly sensitive photodetectors, electroluminescent emitters, and improved-efficiency photovoltaic devices. In this thesis, using advanced optoelectronic measurements, I discuss the discovery of highly efficient multiplication of interlayer electron-hole pairs at the interface of ultrathin tungsten diselenide / molybdenum diselenide integrated into fieldeffect heterojunction devices. Electronic transport measurements of the interlayer current-voltage characteristics indicate that interlayer electron-hole pairs are generated by hot electron impact excitation at temperatures near T = 300 K. By exploiting this highly efficient interlayer e-h pair multiplication process, we demonstrated near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels. We extend our nderstanding of these materials by conducting spatially and spectrally resolved imaging of the device photoresponse at low temperatures. Under carefully tuned experimental conditions, we observe phonon assisted anti-stokes absorption near the interlayer exciton edge of the van der Waals semiconductor heterostructure composed of tungsten diselenide and molybdenum diselenide. At low photon energies near 1 eV, we observed a strong photocurrent peak with several low energy echoes spaced by 30 meV below the fundamental absorption feature. We attribute this highly unusual absorption to high-harmonic anti-stokes absorption; The alignment of the exciton dipole moment to the atomic displacement of the out-of-plane optical phonon modes gives rise to resonant absorption features akin to vibronic transitions in molecules. The anti-stokes process is the first and most critical step toward laser cooling of atomic layer semiconductors. Moreover, it could enhance the efficiency of next generation photovoltaics, since it converts vibrational energy into electronic excitations using photons with energies that are lower than the band gap.