Studying the effect of Eu3+ ion doping in perovskite solar cells

Abstract

Perovskite solar cells have been attracting a lot of attention recently due to rapid improvements in its power conversion efficiency over the last decade. Their application in building integrated photovoltaics (BIPV) is advancing with time and highly relevant for countries like Singapore where the land area available for solar farms is limited. Therefore, making semi-transparent solar cells that can be put on building windows, facades and walls will alleviates their limitation of area utilization. The aim of this work is to fabricate semi-transparent perovskite absorber and study its device performance. Research efforts have been devoted into the triple cation based organic-inorganic hybrid perovskite formulation of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 as it is able to address the chemical and thermal stability of mixed cation perovskite. Despite this stabilized composition, the problem with incomplete formation of PbI2 perovskite backbone structure persists. The conversion of elemental Pb0 and I0 to their ionic form Pb2+ and I- is kinetically limited at room temperature. To address these defects, Eu3+/Eu2+ redox shuttle is introduced into the perovskite to help reduce the defect density in the material. Eu3+ would oxidise Pb0 into Pb2+ while generating Eu2+. This Eu2+ would then reduce I0 into I- while regenerating back Eu3+. Thus, this process can continue repeating in a cycle.

In this paper, Eu(acac)3 is used to produce the Eu3+ trivalent ions. However, Eu(acac)3 has two conflicting effects onto the perovskite material. While Eu(acac)3 helps reduce the defect density in perovskite, Eu(acac)3 contains H2O ligands. Perovskite is highly susceptible to degradation in the presence of H2O molecules. Thus, having Eu(acac)3 concentration beyond the optimum concentration would lead to a lower power conversion efficiency due to an increased degradation of the absorber perovskite material. The downconversion effect of incorporating Eu3+ ions using Eu(TTA)3Phen onto the electron transport material is also investigated. Downconversion is a quantum-cutting process where the downconversion material splits a high energy incident photon into two or more lower energy photons. This would increase the overall photon absorption by the solar cell.

Photovoltaic measurements together with other optical and structural characterizations were conducted to investigate for any improvements. The concentration of Eu(acac)3 in perovskite was optimized to achieve higher power conversion efficiency.