We are developing copper indium diselenide (CuInSe2, CIS) thin films as the absorber layer for the bottom cell of a monolithically grown perovskite/CuInSe2 tandem solar cell. Although the perovskite top cell behaves as nearly defect-free, CIS shows defect-dominated behavior which severely limits carrier lifetime, and therefore also limits efficiency. We report here on studies of the composition-dependent defect optical emission as studied at low temperatures for copper-poor CIS thin films. The goal of the project is to develop a correlation between defect emission and device performance. Achieving this will also require understanding and control of surfaces and interfaces. In support of these efforts, we are working to correlate PL measurements with spectroscopic ellipsometry (SE) studies of the electronic and crystalline structure. In this report, low temperature photoluminescence (PL) measurement has been applied to study native and impurity defects in several compositions of Cu-poor (i.e. Cu/In <1) CIS films deposited on soda-lime glass via a co-evaporation process. The compositions of the films studied to date range from Cu/In = 0.47 to Cu/In = 0.80. Laser excitation power dependence of PL emission from these CIS films indicates the existence of Donor-Acceptor-Pair (DAP) recombination, with the DAP peak energy varying by +/- 10 meV near 0.85 eV depending on Cu/In ratio. We also observed defect emission at ~0.78 eV on some CuInSe2 films which has not been explored to date. Temperature dependent PL studies will be used to calculate the donor and acceptor binding energies contributing to DAP recombination, and the activation energies of other observed point defect states. Lastly, we report on initial results of correlating PL results with composition and results of spectroscopic ellipsometry (SE) modeling and analysis to understand the relationship between the complex optical constants and the observed radiative recombination.