At an applied potential of 0 vs RHE an increase over the whole wavelength range by a factor of 2.2 can be observed in the IPCE (see also Figure ). At -0.1 V vs RHE, however, a significant increase in the charge collection of blue photons (~400 nm) is evident. This observation can be explained by considering the initial band bending at the heterojunction and the wavelengthdependent penetration depth of the photons. Generally, the low energy photons are absorbed near the back contact of the absorbing film while the higher energy photons are absorbed closer to the heterojunction interface. From the band alignment (Figure ) and the Fermi level position of the ALD TiO2, an unfavorable band bending is predicted after equilibration of the Fermi levels (Figure ), which arises due to the near intrinsic, weakly n-type Fermi level position of the Sb2S3. Due to the weak doping, the width of the space charge layer is large and extends deep into the film. Nearly no band bending is expected in the TiO2 layer, as the films grown by ALD are highly doped (10 20 cm -3 ). XPS results also confirm the high doping concentration as evidenced by the high fermi level position of TiO2 (Figure ). The barrier for electrons at the heterojunction interface can be overcome with a small bias (-0.1 V vs RHE, considering that the TiO2 Fermi level is pinned to the electrolyte at ~0 V vs RHE) (Figure ). The initial unfavorable band bending would trap photogenerated holes that are produced close to the Sb2S3/TiO2 interface (generated from high energy photons) and lead to an increased recombination before collection. In contrast, the inversion of the unfavorable band bending by the small applied negative bias would drive the photogenerated holes from the heterointerface to the back contact (Figure ). This phenomenon can explain the larger impact of the applied bias for the blue photons.