Dear Hassan,

Hope you are well!

The perovskite solar cells can be built such that are pin diode, pi diode or ni diode.

The conventional structure is that where the active layer is sandwiched between the p hole transport layer HTL and the electron transport layer ETL.

There are the solar cell that can built by a HTL making a junction with either intrinsic or n-type perovskite or vice verse.

The other side of the absorber is just supplied with a metal forming an ohmic contact.

The pin diode solar cell may realize more photo conversion efficiency. as one can increase the opencurcuit voltage and external quantum efficiency.

Best wishes

Dear Hasan Abbas .

Although the mechanism you mentioned is still unclear, the perovskite layer in HTL-free seems to act as hole transport material as well as light harvester, simultaneously. Moreover, the work function, band alignment and the Schottky contact at the metal-semiconductor interface between Perovskite layer and back contact can also be considered as significant issues. You can find more information in details at the articles below.

1. Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells

DOI: 10.1021/ja307789s

2. Modeling and analysis of HTM-free perovskite solar cells based on ZnO electron transport layer

DOI: 10.1016/j.spmi.2017.02.028

3. A simple way to engineer metal-semiconductor interface for enhanced performance of perovskite organic lead iodide solar cells

DOI: 10.1021/am5001773

4. Depletion region effect of highly efficient hole conductor free CH3NH3PbI3 perovskite solar cells

DOI: 10.1039/c4cp00460d

5. Optimal design of efficient hole transporting layer free planar perovskite solar cell

DOI: 10.1007/s40843-016-5108-4

Milad Fallah , thanks a lot; I will study these articles.

I'll recommend you this brilliant book by etgar for a clear depiction of whole mechanism.

https://bibliotecas.cio.mx/ebooks/pdf/e_26.pdf

Thanks @ Zulqarnain Ar, I'll study this book..........