Two-Photon Sub-Bandgap Photocurrent in Surface-Nanotextured Black Diamond Films for Solar Energy Conversion

Two-photon sub-bandgap photocurrent (TPPC) is demonstrated in surface-nanotextured black diamond films fabricated by a two-step femtosecond laser treatment at an optimal total accumulated laser fluence of 5.0 kJ cm^-2, unequally split between the two steps with a split ratio of 2:1. A broad intermediate band of electrically active deep-level defects, located at an energy distance of 1.83 eV from the conduction band, is experimentally observed by sub-bandgap spectral response evaluation in the 190-1000 nm wavelength range, allowing for a significant enhancement of quantum efficiency under photovoltaic conditions. This work represents the first experimental demonstration of TPPC in bulk materials with deep-level impurities conceived for intermediate-band solar cells and provides a solid physical interpretation of the operating principle of defect-engineered black diamond-based devices for solar energy conversion.