Photoelectric Properties of Lead Iodine-Based Perovskite Doped with Methylammonium/Formamidinium/Cesium

Building a clean, low-carbon, safe, and efficient energy system and implementing renewable energy substitution actions become important issues to promote sustainable energy utilization. As a representative of clean and sustainable energy, the use of solar energy can greatly reduce carbon emissions, while meeting the growing demand for energy consumption. Since 2009, CH₃NH₃PbI₃ perovskite has been used as a light absorbing layer, achieving a photoelectric conversion efficiency of 3.8%. The organic–inorganic hybrid perovskite material has attracted much attention as a solar cell light absorbing material. However, lead halide perovskite materials are highly susceptible to various external environmental factors such as dopants, heat, light, humidity, etc., which can cause failures in the chemical or physical structure during their application, greatly restricting the commercial application of the devices. It is thus of great significance for the practical application of lead halide based perovskite devices to investigate the intrinsic relationship among the structure, properties and stability. This study employed methylammonium (CH₃NH₃⁺, MA), formamidinium (CH (NH₂)₂⁺, FA), and Cesium (Cs) as A-site ions to construct three perovskite configurations (i.e., MA_1–xFA_xPbI₃, MA_1–xCs_xPbI₃, and Cs_1–xF A_xPbI₃). The structure, stability, electronic and optical properties were investigated, and the theoretical basis for further experimental optimization and exploration of efficient and stable perovskite solar cell materials was also discussed.

The structure and optoelectronic properties of A-site modified lead halide A'_1–XA''_XPbI₃ (where A' and A'' are any of MA, FA, and Cs) perovskite configurations were investigated based on density functional theory by using CASTEP module under the Perdew Burke Ernzerhof exchange correlation functional of generalized gradient approximation via optimizing the structure and calculating the formation energy, band structure, density of states and absorption coefficient. After structural convergence testing, the energy cutoff of the plane wave was selected as 600 eV, and the Monkhorst–Pack scheme was used to mark the Brillouin zone by using a 2×3×2 k-point grid. The energy convergence standard of the system was set at 10^–5 eV/atom. The maximum force was limited to 0.03 eV/Å with the maximum stress of 0.05 GPa, and the maximum displacement of 0.01 Å.

The optimized crystal structures of MA_1–xCs_xPbI₃, MA_1–xFA_xPbI₃ and Cs_1–xFA_xPbI₃ (x=0.125, 0.250, 0.500) perovskite configurations show that the cell volume of Cs_1–xFA_xPbI₃ and MA_1–xFA_xPbI₃ increases while MA_1–xCs_xPbI₃ decreases with increasing the doping content due to the atomic distance of FA group with 3.996 A, MA group with 3.009 A and the radius of Cs⁺ only 1.88 A. Meanwhile, the doping ions at A-site can cause the distortion of Pb–I octahedron, resulting in variations on bond length, bond angle and torsion of octahedron, and MA_0.875FA_0.125PbI₃ demonstrates a slight distortion. The formation energy of MA_1–xCs_xPbI₃, MA_1–xFA_xPbI₃ and Cs_1–xFA_xPbI₃ configurations is negative. As the Cs/MA, FA/MA, and FA/Cs ratios increase, the formation energy becomes more negative, indicating the doped groups can enter the lattice spontaneously and smoothly. The formation energy of MA_1–xFA_xPbI₃ is minimum (i.e., –521.77, –1042.79 eV and –1824.49 eV correspond to x=0.125, 0.250 and 0.500), compared to that of MA_1–xCs_xPbI₃ (i.e., –492.58, –984.86 eV and –1722.95 eV) and Cs_1–xFA_xPbI₃ (–28.87, –57.79 eV and –101.61 eV), due to the large energy difference in MA and FA groups and the resulting strong force between atomic groups. This indicates that FA groups can enter MAPbI₃ lattice more easily and form a more stable structure. The energy band structure calculated shows that the conduction band minimum (CBM) and valance band maximum (VBM) of MA_1–xCs_xPbI₃, MA_1–xFA_xPbI₃ and Cs_1–xFA_xPbI₃ are located at the same high symmetric point G(0, 0, 0), thus determining direct band gap semiconductors. Furthermore, Pb 6p orbital and I 5p orbital dominate in CBM and VBM, respectively. Except that, the electron orbital of FA contributes to CBM significantly, mainly situating at 3.33 eV. The contribution of MA and Cs is primarily at –4.60 eV and 5.35 eV away from the Fermi surface, hence leading to a relatively large band gap in MA_1–xCs_xPbI₃. As for Cs_1–xFA_xPbI₃, Cs and FA orbital overlap at the conduction band (CB, 2.0–4.0 eV), and a great covalent bond can be formed between C–Cs, which facilitates the new energy orbital formed and thus more stronger orbital hybridization with Pb and I, and eventually in turn causes CB to shift upward and the band gap increases. However, compared with Cs, the interaction of MA and FA electron orbital at CB reduces, and MA_1–xFA_xPbI₃ shows a relatively small band gap. MA_0.875FA_0.125PbI₃ exhibits a decent band gap and a decreased energy band dispersion at CBM, promoting the transition of charge carriers and improving light absorption. Also, the real part of the dielectric functions ε₁(ω) peak for MA_0.875FA_0.125PbI₃ demonstrates a red shift from 533 nm to 572 nm, and ε₁(ω) is optimal with the increase of wavelength. The widened wavelength response range and high ε₁(ω) endow MA_0.875FA_0.125PbI₃ with an optimal light absorption capacity in the visible light range of 380–780 nm.

The optoelectronic properties of lead iodide-based perovskite A'_1–XA''_XPbI₃ were predicted by using first-principles via the structural optimization and the calculation of formation energy, band structure, density of states, absorption coefficient. The results indicated that MA, FA groups, and Cs ions could spontaneously enter the A-site of lead iodide A'_1–XA''_XPbI₃ perovskite to form a stable structure. Compared with MA_1–xCs_xPbI₃ and Cs_1–xFA_xPbI₃, MA_1–xFA_xPbI₃ configuration displayed a minimum distortion of Pb–I octahedral and a more stable structure. In addition to the contribution of I 5p orbitals to VBM and Pb 6p orbitals to CBM, the electronic orbitals of FA in the CBM resulted in a dramatical decrease in band dispersion, which was conducive to light absorption. The absorption peak of MA_0.875FA_0.125PbI₃ at 500nm showed a significant red shift. The widened wavelength response range and increased light absorption coefficient endowed MA_0.875FA_0.125PbI₃ with an optimal light absorption ability in the visible light range, making it a promising solar cell photovoltaic material.