Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. In particular, Cs2AgBiBr6 has been the subject of many investigations in photovoltaic devices. This thesis focuses on understanding and controlling the optoelectronic properties of Cs2AgBiBr6 double perovskite thin films, focussing on three main areas: 1) understanding the role of grain boundaries on device performance, 2) lowering the bandgap through alloying with Sb3+, and 3) understanding the potential and limitations of bismuth-antimony double perovskite alloys for photovoltaic applications. For the first area, I show through cathodoluminescence measurements that grain boundaries are the dominant non-radiative recombination sites. I also demonstrate through field-effect transistor and temperature-dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, I find a positive correlation between carrier mobility and temperature, which resembles the hopping mechanism often seen in organic semiconductors. These findings account for the discrepancy between the long diffusion lengths found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts, where the diffusion length of the minority carrier (electrons) can be as low as 30 nm. For the second area, I show through photothermal deflection spectroscopy measurements that mixed alloys of Cs2Ag(SbxBi1-x)Br6 (x between 0.5 and 0.9) demonstrate smaller bandgaps than the pure Sb- or Bi-based compounds. The reduction in the bandgap of Cs2AgBiBr6 achieved through alloying (170 meV) is larger than if the mixed alloys had obeyed Vegard's law (70 meV). Through in-depth computations, I propose that bandgap lowering arises from the type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch between the Bi and Sb s and p atomic orbitals, coupled with their non-linear mixing, results in the alloys adopting a smaller bandgap than the pure compounds. For the third area, I show that despite Sb alloying lowering the bandgap, there is a strong decrease in the power conversion efficiency in photovoltaic devices. Through photothermal deflection spectroscopy and steady-state photoluminescence measurements, I demonstrate that Sb alloying introduces sub-bandgap states, especially a weak luminescent state at 1.55 eV, thus deteriorating the performances of the mixed alloy films in solar cells, especially by causing a substantial reduction in the open-circuit voltage. These results indicate that addressing the effects of grain boundaries of Cs2AgBiBr6 double perovskite and mitigating the sub-bandgap states in Cs2Ag(SbxBi1-x)Br6 alloy are essential to improve their optoelectronic properties and further facilitate their application in optoelectronic devices.