(Invited) Highly Efficient Solar-to-Hydrogen Conversion: Design, Implementation, and Validation

This synthesis integrates spectral design, precise metrology, and advanced device engineering into a unified framework for unassisted solar-to-hydrogen operation. The interplay between light absorption by the electrolyte and optical utilization is addressed by employing representative reference spectra and spectral budgeting to quantify realistic STH prospects and to guide electrolyte-compatible concepts. By linking absorption losses, photovoltage requirements, and photocurrent generation to concrete material choices and junction configurations, a pathway toward high efficiency under practical conditions is delineated.
The full implementation of the derived design implication requires an advanced device architecture. Immersed multi-junction photoelectrochemical electrodes produced by inverted metamorphic (IMM) epitaxy with a transparent graded buffer permit independent bandgap tuning of each junction, reducing interfacial voltage losses by approximately 0.55 V relative to traditional uniformly p-doped photocathodes.
However, illumination conditions and measurement practices shape reported performance. Artificial light and imperfect metrology can skew benchmarks and hinder cross-study comparability. Spectrally corrected benchmarking and standardized measurement protocols are advocated to establish credible performance standards that reflect real operating conditions, enabling apples-to-apples comparisons across studies and devices.
Testing with spectrally corrected metrics yields solar-to-hydrogen efficiencies exceeding 16% for GaInP/GaInAs IMM tandems, representing a substantial improvement over prior high-performance configurations.