Role of Carrier Mobility and Band Alignment Engineering on the Efficiency of Colloidal Quantum Dot Solar Cells

We investigate the 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 is quantitatively explored as a function of subbandgap defect density (N
_t
) in the bulk CQD. For N
_t
≤ 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 μm) CQD layers. For the thicker (>1 μm) 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
-10
⁰
cm
²
/V·s and shows variation as a function of CQD layer thickness and the interfacial band offset. For N
_t
≈ 1014cm
^-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 the quantum dot solar cell design that is needed for improving the efficiency for this emerging low-cost solar cell technology.