simulation – Transfer matrix formalism

I have a Mathematica code for Perovskite solar cell simulation based on TMF. The code already have 6 layers works with 6 layers. I would like to know how to include a buffer layer between PCBM and Aluminum. The layers stack goes from Glass/ITO/PEDOT:PSS/Active Layer/PCBM/Buffer Layer/Aluminium.

The parameters to be calculated include the reflectance, transmittance, number of absorbed photons etc