The electronic orbital characteristics at the band edges plays an important role in determining the electrical, optical and defect properties of perovskite photovoltaic materials. It is highly desirable to establish the relationship between the underlying atomic orbitals and the optoelectronic properties as a guide to maximize the photovoltaic performance. Here, using first-principles calculations and taking anion ordered Ruddlesden-Popper (RP) phase halide perovskites Cs$_{n+1}$Ge$_n$I$_{n+1}$Cl$_{2n}$ as an example, we demonstrate how to rationally optimize the optoelectronic properties (e.g., band gap, transition dipole matrix elements, carrier effective masses, band width) through a simple band structure parameter. Our results show that reducing the splitting energy $|Delta c|$ of p orbitals of B-site atom can effectively reduce the band gap and carrier effective masses while greatly improving the optical absorption in the visible region. Thereby, the orbital-property relationship with $Delta c$ is well established through biaxial compressive strain. Finally, it is shown that this approach can be reasonably extended to several other non-cubic halide perovskites with similar p orbitals characteristics at the conduction band edges. Therefore, we believe that our proposed orbital engineering approach provides atomic-level guidance for understanding and optimizing the device performance of layered perovskite solar cells.
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