This is a depth-profile cross-section, not a 3D tank: the horizontal axis is depth into the semiconductor, from the electrolyte-facing surface (left) to the field-free bulk (right). An n-type photoanode bends its conduction/valence bands upward near the surface, carving out an electron-depleted space-charge layer of width W (Mott-Schottky relation):
W = sqrt( 2·εr·ε₀·(Vbi + Vapp) / (q·Nd) )
Photons entering at the surface are absorbed with depth per Beer-Lambert law, generating electron-hole pairs at rate G(x) = α·exp(−α·x) — drawn here as the filled amber curve. Only pairs born inside W (left of the dashed line) get swept apart by the field before recombining; the rest fade in place. Integrating G(x) over 0..W against the total gives the same Gärtner collected fraction the dots are sampled from:
fcollected ≈ 1 − exp(−α·W)
Jphoto ≈ Jmax · (suns) · fcollected
Collected holes drift left to the surface and oxidize water (2H₂O + 4h⁺ → O₂ + 4H⁺, left column); collected electrons drift right to the back contact, travel the top wire to the cathode, and reduce protons there (4H⁺ + 4e⁻ → 2H₂, right column) — twice as fast as O₂ by Faraday's law.
- Applied bias — widens W (band-bending line reaches further into the bulk), pulling the dashed collection boundary right and letting more of the amber generation curve fall inside it.
- Illumination — scales how often new electron-hole pairs appear along the depth axis.
- Donor density Nd — heavier doping compresses W at fixed bias.
- Material — sets the bandgap, absorption coefficient α (how quickly the amber curve decays with depth) and a representative saturation photocurrent Jmax: TiO₂'s curve barely decays across this window (very weak absorber), Fe₂O₃'s decays fast, BiVO₄ sits between — illustrative, order-of-magnitude values for teaching the trend.