Sustainable A₂B^ⅠB^ⅢX₆ based lead free perovskite solar cells: The challenges and research roadmap for power conversion efficiency improvement

The stability issues in the widely known CH₃NH₃PbI_3, lead to the development of alternative halide double perovskite materials, which has received great attention in recent times. Although the stability issue of double halide perovskite seems promising, their device performance remains far inferior to CH₃NH₃PbI₃ and with challenges for further improvements. Furthermore, the power conversion efficiency of single junction organic-inorganic halide perovskite is now 24.2% and 29.15% for the textured monolithic perovskite/silicon tandem solar cell; however, for the all-inorganic halide perovskite solar cell, it is 7.11%, and halide double perovskite solar cells are based on A₂B^ⅠB^ⅢX₆ (A = monocation, B = cation or vacancy, X = halide) such as Cs₂AgBiBr₆, Cs₂TiBr₆, Cs₂AgTlBr₆ and Cs₂Ag(Bi_1−xIn_x)Br₆, being 2.8% and 3.3%, respectively. This creates big questions and concerns about the performance improvement of A₂B^ⅠB^ⅢX₆-based perovskite solar cells. Not only is this a concern, but there are many other big challenges faced by halide double perovskite solar cells. Such big challenges include: (a) geometric constraints and limited integration with interfacial materials; (b) dynamic disorder, a wide band gap, and a localized conduction band caused by a cubic unit cell that restrains the interactions of orbitals; (c) high processing temperature which may limit the diverse applications; and (d) low electronic dimensionality that makes them less appropriate for single junction solar cell purpose, etc. Moreover, the origin of electronic and optical properties such as the polarizability, the presence of molecular dipoles, and their influence on the dynamics of the photo-excitations remain bottleneck concerns that need to be elucidated. We roadmap performance sustainable improvement, which is suggested with a particular focus on engineering material surface and bulk, band gap, interfacial, composition, doping, device architectural, polar, and domain order. The reason that this review was developed was to forward great contributions to the readers and commercial ventures.