Reducing trap density and carrier concentration by a Ge additive for an efficient quasi 2D/3D perovskite solar cell

Abstract

We report that doping with hydrophobic bulky 2D phenylethylammonium (PEA⁺) is desirable to stabilize the perovskite matrix and enhance its stability. The addition of PEA⁺ alters the crystal growth orientation and improves the connectivity of the crystal grains. However, solely adding the PEA⁺ material cannot fully passivate the severe bulk recombination sites/interior defects due to Sn vacancies, leading to an efficiency of 3.96% (V_oc of 0.36 V) for a Ge-free device. In contrast, we find that the addition of smaller-sized Ge ions with an optimum doping concentration effectively reduces the leakage current and suppresses the carrier density of the perovskite material. From the perspective of traps, the addition of Ge reduces the traps, typically deep traps, and its effectiveness (Ge) in trap passivation is further deduced from the thermally stimulated current (TSC) profile. The total trap density was doubly reduced to 4.14 × 10²⁰ cm⁻³ when 7.5 mol% Ge was added, which led to a photo-conversion efficiency of 7.45% with a high V_oc of 0.46 V. In addition, defect healing by the Ge additive significantly enhanced the stability of the unencapsulated device for 192 h. This work shows that Ge is an effective additive to suppress the recombination sites (trap state passivation), leading to the establishment of an efficient tin-based perovskite solar cell.