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.
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