  We present a physics based multiscale materials-to-systems model for
polycrystalline $PbSe$ photodetectors that connects fundamental material
properties to circuit level performance metrics. From experimentally observed
film structures and electrical characterization, we first develop a
bandstructure model that explains carrier-type inversion and large carrier
lifetimes in sensitized films. The unique bandstructure of the photosensitive
film causes separation of generated carriers with holes migrating to the
inverted $PbSe|PbI_2$ interface, while electrons are trapped in the bulk of the
film inter-grain regions. These flows together forms the 2-current theory of
photoconduction that quantitatively captures the $I-V$ relationship in these
films. To capture the effect of pixel scaling and minority carrier blocking, we
develop a model for the metallic contacts with the detector films based on the
relative workfunction differences. We also develop detailed models for various
physical parameters such as mobility, lifetime, quantum efficiency, noise etc.
that connect the detector performance metrics such as responsivity
$\mathcal{R}$ and specific detectivity $\mathcal{D^*}$ intimately with material
properties and operating conditions. A compact Verilog-A based SPICE model is
developed which can be directly combined with advanced digital ROIC cell
designs to simulate and optimize high performance FPAs which form a critical
component in the rapidly expanding market of self-driven automotive, IoTs,
security, and embedded applications.
