Spatial control of cocatalysts and elimination of interfacial defects towards efficient and robust CIGS photocathodes for solar water splitting

Chalcopyrite thin film absorbers such as Cu(In,Ga)Se 2 (CIGS) exhibit excellent solar energy conversion efficiency, particularly when coupled with CdS to form an excellent p–n junction. To advance CIGS towards an efficient photoelectrochemical (PEC) hydrogen evolution reaction (HER), a protective overlayer (typically TiO 2 ) is needed to prevent the corrosion of CIGS and CdS in the electrolyte, and a HER catalyst (typically Pt) is required to promote the surface reaction. However, it is a great challenge to realize delicate spatial control of the HER catalyst on the surface of the protective overlayer using the traditional deposition method. The charge transport through the CdS/overlayer interface is also of vital importance but is rarely considered. Herein, through a new two-step platinization strategy, the dispersion and particle size of Pt nanoparticles are independently controlled to realize high HER catalytic activity. Moreover, defects at the CdS/TiO 2 interface are passivated via an ultrathin Al 2 O 3 insertion layer. Consequently, we have obtained a robust CIGS/CdS/Al 2 O 3 /TiO 2 /Pt photocathode for PEC hydrogen evolution, which yields an applied bias photon-to-current efficiency (ABPE) of 6.6% in neutral electrolyte with a long-term stability up to 8 h (4.5% drop), and an unprecedented ABPE of 9.3% in acidic electrolyte that is the highest among chalcopyrite-based photocathodes. When paired with a BiVO 4 photoanode to form a PEC tandem cell, an unbiased solar-to-hydrogen conversion efficiency of 1.01% is achieved.