Cascaded band alignments of PbS heterojunction layers for improved performance of PbS quantum dot solar cells

摘要

Efficient energy band alignment of heterojunction layers in colloidal quantum dot (CQD)-based solar cells is a crucial factor to govern the charge transport characteristics and device performance. In this work, we develop novel cascaded junctions of lead sulfide (PbS) CQD triple layers consisting of an alkylammonium iodide (AMI)-treated PbS bilayer and a 1,3-propanedithiol (PDT)-treated single layer. The two AMIs, i.e. triethylamine hydroiodide (tri-EAHI) and tetraethylammonium iodide (TEAI), are less hindered and have superior passivation performance compared to tetrabutylammonium iodide (TBAI), the most commonly used AMI. In addition, the band positions of the PbS-TEAL and PbS-tri-EAHI layers are deeper by 0.26 and 0.46 eV, respectively, than those of the PbS-PDT layer, and hence the sequential stacking of these three layers enable an effective cascaded band alignment. The various benefits of the improved band alignment such as increased built-in potential, reduced trap states, widened depletion region, enhanced charge transport and suppressed charge recombination lead to a significant improvement in the device parameters, and the best power conversion efficiency of 10.46% is obtained for the cascaded PbS-tri-EAHI/PbS-TEAI/PbS-PDT-based device.