“…The p-n homo-junction on the front is formed by highly-doped boron diffusion. With regards to the top perovskite cell structure, although theoretically a band gap near 1.7 eV has been recommended in the absence of any optical loss, 29,30 parasitic absorption of short wavelengths exists in the hole transport materials (HTM) commonly used in state of the art solar cells 4,7,9,16,31 such as poly[bis(4-phenyl)(2,5,6-trimethylphenyl)]amine (PTAA), 4,9 and 2,2 0 ,7,7 0 -tetrakis(N,N-di-pmethoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD). 4,31 Parasitic absorption of short wavelength light also exists in the ITO/MoO 3 stack, which is also the most commonly used transparent conductive stack for the top of the semi-transparent perovskite in a tandem.…”