Solar energy conversion via internal photoemission in aluminum, copper, and silver: Band structure effects and theoretical efficiency estimates

Internal photoemission (IPE) across an n-type Schottky junction due to standard AM1.5G solar illumination is quantified with practical considerations for Cu, Ag, and Al under direct and fully nondirect transitions, all in the context of the constant matrix element approximation. Under direct transitions, photoemitted electrons from d bands dominate the photocurrent and exhibit a strong dependence on the barrier energy ΦB but are less sensitive to the change in the metal thickness. Photocurrent is shown to be nearly completely contributed by s-state electrons in the fully nondirect approximation that offers nearly identical results as in the direct transition for metals having a free-electron-like band structure. Compared with noble metals, Al-based IPE has the highest quantum yield up to about 5.4% at Φ_B = 0.5 eV and a maximum power conversion efficiency of approximately 0.31% due mainly to its relatively uniform and wide P_exc energy spectral width. Metals (e.g., Ag) with a larger interband absorption edge are shown to outperform those with shallower d-bands (e.g., Cu and Au).