Well Al2O3 is pretty transparent, but if you want it to absorb as much as possible make it as thick as possible.

If you want the Si or FTO underneath to absorb as much as possible, make the Al2O3 the thickness of an antireflective coating - lambda/4n, where lambda is the main wavelength you want to absorb below the Al2O3, and n is the Al2O3 refractive index.

Well Al2O3 is pretty transparent, but if you want it to absorb as much as possible make it as thick as possible.

If you want the Si or FTO underneath to absorb as much as possible, make the Al2O3 the thickness of an antireflective coating - lambda/4n, where lambda is the main wavelength you want to absorb below the Al2O3, and n is the Al2O3 refractive index.

Agreeing with Prof. Manuel Schnabel's answer, I think we should also consider the diffusion length of the carriers which should reach the semiconductor-electrolyte interface. when increasing the thickness for maximum light absorption the number charge carriers reaching to the surface reduces and thus decreasing the efficiency of the cell.

I agree with Artur that Al2O3 is not used as an absorber layer since it is a wide gap material. So it is used as a transparent window layer. In case of Die sensitized solar cells it is used as a scaffold layer to host the die material which is the active material. So its thickness must be sufficiently large to absorb the incident solar radiation. Since the die has high absorption coefficient the thickness required is of the order of few tenths of a micrometer. In case of compact structure for use with peovskites as a die, the aluminum oxide layer works as a hole transport layer. So, its thickness must be made as thin as possible to pass the incident solar radiation to the active layer and to reduce its resistance to the hole transport.

Otherwise the conceptual answers introduced by the colleagues are okay.

Best wishes