Transition Metal Dichalcogenide Multijunction Solar Cells Toward the Multicolor Limit

Transition metal dichalcogenides (TMDs) and other van der Waals (vdW) semiconductors enable transfer‐printed, lattice–mismatch‐free stacking of photovoltaic junctions, motivating a thermodynamic assessment of multijunction designs under realistic material and optical constraints. We develop a detailed‐balance framework that combines bandgap‐window optimization, optical boundary conditions, external radiative efficiency, luminescent coupling, excitonic absorptance, and implementation‐level derating. Applying it to a conservative TMD window of 1.0–2.1 eV, we find that full‐concentration efficiency is limited by the accessible spectral range: Unconstrained 50‐junction ladders approach 84.5%, whereas TMD‐window ladders plateau near 63.4%. The practical gain therefore saturates after about five junctions, for which an experimentally motivated ladder, eV, is mapped to candidate vdW/TMD absorbers. We quantify penalties from finite radiative quality, two‐sided emission, and downward luminescence and separate these thermodynamic ceilings from losses caused by interface recombination, parasitic absorption, exciton collection, contact resistance, and power electronics. We further compare reciprocal and idealized nonreciprocal optical boundary conditions to estimate multijunction efficiency headroom. The framework defines thickness, optical management, and bottom‐cell requirements for transfer‐printed TMD multijunction photovoltaics approaching the multicolor limit.