Photovoltaic light absorber with spatial energy band gradient using PbS quantum dot layers

•Graded bandgap absorber made of lead sulfide QDs of different sizes is fabricated.•Solar cells with graded absorber exhibited improvements both in Jsc and Voc.•Interfacial large bandgap layer working as electron energy booster for higher Voc.

Using a combination of quantum dots (QDs) of different sizes, and thus different bandgaps, to extend the light-harvesting spectrum of a photovoltaic device in tandem architecture is a promising strategy for increasing the solar cell efficiency. In this study, we propose a new architecture for a solar cell device consisting of a graded-bandgap active layer made of lead sulfide QDs of different sizes and based on the structure of the Schottky junction. Colloidal PbS QDs with bandgaps of 1.55, 1.44, and 1.36 eV were synthesized and used to construct a series of Schottky junction solar cells. Cells with a graded-bandgap active layer exhibited an increase in short-circuit current density (Jsc) but yielded lower open-circuit voltage (Voc) compared with cells having a uniform bandgap. We found that adding a thin electron energy-boosting layer (EEB) made from 1.55-eV-bandgap QDs can partially compensate for the thermalization loss, and thus enhance Jsc and recover Voc. Consequently, this study provides a conceptual basis for further improvement in colloidal QD-based solar cells.

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