Dear Umesh,

The good hole transport layer must possess the following characteristics:

- It must have high hole conductivity

- It must energetically match the perovskite layer. In the sense its valence band must be lie above the valence band of the perovskite such that the collected holes can be easily extracted from the active layer while its conduction band edge must lie above the conduction band of the perovskite layer to reflect back the electrons toward the electron transport layer and thereby reduce the recombination of electron-holes at the its interface with the active layer. Also if it contacts the conductive oxide it must build ohmic contact with it or with any other metal contacting it and it must have high energy gap to transfer the most of the photons without absorption.

- It must be technologically compatible with other fabrication steps.

So, you can compare the materials on the given criteria and you can classify them according to them.

Best wishes