Effect of Cation Occupancy Ordering in Double Perovskites To Overcome Hurdles in Carrier Transport: Cs2AgBiBr6 as a Case Study

In spite of environmental stability and toxic-free merits, the performance of double perovskite solar cells (Cs2AgBiBr6 to be specific) has not excelled due to their large indirect band gap, low absorption of solar spectrum, and more importantly hurdles in carrier transport processes. A large number of intrinsic defects and self-trapped excitons (STE) appear in the double perovskite due to a disordered arrangement of Ag/Bi octahedra in the perovskite lattice. In this work, we investigate the role of cation ordering in Cs2AgBiBr6 through the use of phenethylammonium bromide (PEABr) in the precursor toward suppression of STEs and deep-level defect states, such as antisite defects of Ag+/Bi3+, for an improved carrier conduction process. While cation ordering has been verified from X-ray diffraction studies and Raman spectroscopy, a reduction in the defect density and inhibition of STE formation have been witnessed from optical spectroscopy results. The transport gap, obtained from scanning tunneling spectroscopy and density of states spectra thereof, has been found to increase, suggesting the defect states to become suppressed. The effect of cation ordering toward an efficient carrier transport in the ordered lattice of Cs2AgBiBr6 has been demonstrated by Kelvin probe force microscopy and conductive atomic force microscopy. Finally, a fair improvement in photovoltaic parameters was observed in devices based on the ordered double perovskite.