CUPID (CUORE Upgrade with Particle ID) is a proposed tonne-scale $0\nu\beta\beta$ experiment that will use arrays of low-temperature calorimeters to probe the Majorana nature of neutrinos. CUORE has already shown the power and scalability of tonne-scale array at low temperatures (~10 mK), and CUPID aims to improve on its search sensitivity by reducing backgrounds by at least two orders of magnitude. CUPID will do background rejection by reading out phonon and photon signals simultaneously from a scintillating crystal or a Cherenkov light-emitting crystal. We are developing sensitive low temperature calorimeters that can measure tiny amounts of scintillation/Cherenkov light. The detectors use a novel Ir/Pt bilayer superconducting transition-edge-sensor (TES) that can be operated below 40 mK. We will present an overview of the fabrication, characterization, and performance of TES based large area photon detectors for CUPID.