I know that the maximum open circuit voltage of a perovskite solar cell increases with a deeper HOMO level of a hole transport layer, but what happens when it is too deep?
The homolevel of the HTL must slightly lie above that homolevel of the perovskite.
This is required such that the holes easily slides down from the perovskite to the HTL. If the HTL homolevel shifted down the holes will not easily slide to the HTL they will need activation to cross the barrier if it lies under the homolevel of perovskite. The highest contact difference of potential will be when the homolevel of the HTL is completely aligned to that of the perovskite.
For more discussion about this issue please refer to the paper:
Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells