This study pinpoints the advantages of ultrathin electron 15 selective layers (ESL) of SnO2 fabricated by atomic layer deposition (ALD). These layers recently caught attention in planar perovskite solar cells and appear as powerful alternatives to other oxides such as TiO2. Here, we carry out a thorough characterization of the nature of these ultrathin ALD SnO2 layers providing a novel physical insight for the design of various photoelectrodes in perovskite and dye-sensitized solar cells and in photoelectrochemical water splitting. We use a combination of cyclic voltammetry, electrochemical impedance spectroscopy, Hall measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and electron microscopy to analyze the blocking behavior and energetics of as-deposited (low-temperature) and also calcined ALD SnO2 layers. First, we find that the low-temperature ALD-grown SnO2 layers are amorphous and perfectly pinhole-free for thicknesses down to 2 run. This exceptional blocking behavior of thin ALD SnO2 layers allows photoelectrode designs with even thinner electron selective layers, thus potentially minimizing resistance losses. The compact nature and blocking function of thin SnO2 films are not perturbed by annealing at 450 degrees C, which is a significant benefit compared to other amorphous ALD oxides. Further on, we show that amorphous and crystalline ALD SnO2 films substantially differ in their Hatband (and conduction band) positions a finding to be taken into account when considering band alignment engineering in solar devices using these high-quality blocking layers.