Manipulating Depletion Region of Aqueous-Processed Nanocrystals Solar Cells with Widened Fermi Level Offset

Abstract

Water soluble nanocrystals (NCs) are promising materials in aqueous-processed solar cells because of their high extinction coefficient, low-cost, and favorable photoelectric characteristics. However, the power conversion efficiency (PCE) of the present aqueous-processed NC solar cells is restricted by the short depletion region of the active layer and limited Fermi level offset between NCs and the electron transport layer. Herein, these issues are effectively addressed by preparing Cd_x Zn_{1- x} Te NCs capped with 2-aminoethanethiol hydrochloride. The introduction of Zn²⁺ into CdTe NCs widens the Fermi level offset from 0.68 to 0.74 eV, lengthens the depletion region from 130 to 137 nm, and hence brings obvious improvement in the open circuit voltage (V_oc ) and fill factor. Especially, the depletion region is successfully tuned from 137 to 171 nm, and even lengthened to a record thickness of 200 nm based on aqueous-processed solar cells. As a result, a champion thickness ratio (74%) of depletion region to active layer (200/270 nm) is achieved. A champion PCE of 5.96% and short-circuit current (J_sc ) of 21.2 mA cm^-2 are achieved among aqueous-processed NC solar cells. This work provides a simple way to prepare polynary NCs and highlights a prospective method to develop more efficient and cost-effective solution-processed environment friendly solar cells.

Keywords: Fermi level offset; MA-capped CdxZn1-xTe; aqueous-processed solar cells; bulk heterojunction; depletion region.