  We investigate physics based design of colloidal quantum dot (CQD) solar
cells using self-consistent computational modeling. The significance of band
alignment engineering and optimized carrier mobility are quantitatively
explored as a function of sub bandgap defect densities (N_t) in the bulk CQD.
For $N_t \leq 10^{15} cm^{-3}$, band alignment engineering near the interface
of CQD and the metal contact could significantly improve open circuit voltage
by suppressing the forward bias dark current. This effect could enhance cell
efficiency up to ~37% for thinner $(< 1 \mu m)$ CQD layers. For thicker $(> 1
\mu m)$ CQD layer, the effect of band engineering is diminished as the forward
bias dark current becomes diffusion-limited and less dependent on the
interfacial band offsets. An optimal carrier mobility in CQD lies in the range
~ 10^{-2} cm^2/Vs - 10^0 cm^2/Vs and shows variation as a function of CQD layer
thickness and the interfacial band offset. For $N_t \approx 10^{14} cm^{-3}$,
an optimally designed cell could provide ~20% efficiency under AM1.5G solar
spectrum without employing advanced structural optimizations such as the
nanostructured electrodes. These physical insights contribute to a better
understanding of quantum dot solar cell design, allowing a step further towards
a highly efficient and a low cost solar cell technology.
