^{+}to CsPbI

_{3}(110) Surface States: From the First Principles Calculations

^{+}to CsPbI

_{3}(110) Surface States: From the First Principles Calculations

^{+}to CsPbI

_{3}(110) Surface States: From the First Principles Calculations

This work investigates the effect of passivation on the electronic properties of inorganic perovskite CsPbI_{3} materials by using first-principles calculations with density functional theory (DFT). The passivation effect after the addition of Phenylethylamine (PEA^{+}) molecule to CsPbI_{3} (110) surface is studied. The results of density of states (DOS) calculations show that the CsPbI_{3} (110) surface model with I atom terminated reveals new electronic DOS peaks (surface states) near the Fermi level. These surface states are mainly due to the contribution of I-5_{3}-based solar cells because they reduce the photoelectric conversion efficiency. The surface states near the Fermi level are significantly reduced, and the decline rate reaches 38.8% with the addition with PEA^{+} molecule to the CsPbI_{3} (110) surface.

After more than ten years of development, perovskite solar cells (PSCs) have made amazing progress, whereas how to further increase device stability and efficiency of PSCs is still a matter of concern. Since the perovskite crystal growth requires high temperature and the crystallization is very fast, the existence of defects in the polycrystalline perovskite prepared by the solution method is inevitable, and it is harmful to the performance of the devices. How to passivate the defects of the perovskite absorber layer is very important to improve the performance of the device, which is of great significance for the mechanism research and further development of PSCs [

According to the type and location of the defect states, many methods have been adopted to reduce the defect effects in the body, grain boundary and surface of the perovskite layer. The defects in the perovskite form shallow level defect states and deep level defect states between the band gaps of the perovskite, which affect the transport behavior of carriers and reduce the efficiency of the device. At the same time, defects act as channels for ion migration, causing the hysteresis of PSCs. In addition, the water and oxygen molecules in the air enter the perovskite layer through the defect position. This promotes the decomposition of the perovskite, which does harm to the stability of the device. For the sake of further improving the efficiency and stability of the PSCs, passivating the defect states of the perovskite is a very effective method [

In recent years, in order to passivate the perovskite grain boundary or surface defects, various types of additives have been introduced into the perovskite. These additives can bond at the perovskite defect position to achieve the passivation effect [_{n+1}Ru_{n}O_{3n+1} (n = 1, 2) and found the modification of the structural parameters, which was successful in tuning the electrical and magnetic properties of perovskites. It also was instrumental in adjusting the surface chemistry and reactivity of these promising materials [^{16} cm^{−3} to 8.83 × 10^{15} cm^{−3}, and the efficiency increased from 18.86% to 19.96%. Yang et al. used imidazole sulfonate zwitterion, 4-(1H-imidazol-3-ium-3-yl) butane-1-sulfonate (IMS) as an additive to passivate I^{−} ions defects, then the efficiency increased from 18.77% to 20.84%, and the open-circuit voltage (V_{oc}_{3}^{−} ions in the two-step method and found that they can passivate the deep-level defects of perovskite and increased the device efficiency to 22.1% [^{+} molecule to modify the surface of CsPbI_{3} and found that the organic cation surface terminated α-CsPbI_{3} perovskite not only shows moisture resistance and enhanced phase stability, but also exhibits passivated effects with higher V_{oc}_{3} have been researched in this work. We investigated and calculated these surfaces separately, and the results show that there are surface states on the (110) surface. Next, we examine the passivation effect of PEA^{+} molecule to the surface states of CsPbI_{3} (110) and explore the microscopic mechanism after modification by the first-principles calculations.

Our calculation works are performed using first-principles calculations based on the generalized gradient approximation (GGA) of the density functional theory (DFT) generalized in the Vienna ab-initio simulation package (VASP) [^{+} to adsorb on CsPbI_{3} (110) surface. The electronic configurations of H, N, C, Cs, I and Pb atoms in the crystal model are H1^{1}, N2^{2}2^{3}, C2^{2}2^{2}, [Xe]6^{1}, [Kr]5^{2}5^{5} and [Xe]5^{10}6^{2}6^{2}. The cut-off energy is 340 eV in the calculation of the system. The _{3} (110) surface and PEA^{+}-CsPbI_{3} (110) system is the same as 4 × 3 × 1. The passivation model structure is shown in ^{+} molecule, and the relaxed CsPbI_{3} (110) plane with 7 atomic layers on the left. In order to avoid atomic interaction between adjacent periodic units along the Z axis, we added a 20 Å vacuum layer to the adsorption system. During the relaxation of the structure, the three-layer atoms which are far away from surface CsPbI_{3} are fixed at the left end, and the remaining atoms and PEA^{+} molecule are released. The passivation system contains 3 Cs atoms, 3 Pb atoms and 11 I atoms, and a passivation molecule. The lattice parameters of the passivation system are ^{−2} eV/Å and 10^{−5} eV, respectively, and the calculation results can meet the accuracy requirements.

The PEA^{+}-CsPbI_{3} passivation structure has two different binding methods: I (layer 1) is used as the end face to combine with PEA^{+} molecule, and the I-Pb-Cs (layer 2) is used as the end face to combine with PEA^{+} molecule. For the convenience of explanation, we call these two different combinations as model A and model B (as shown in ^{+} molecule and I atoms in the perovskite surface. According to this definition, the optimal passivation distance can be obtained by using the single-point energy calculation method. After many tests, it is found that when the interface distances are 3.72 and 3.11 Å, model A and model B reach the lowest point of total interface energy, respectively (as shown in ^{+}-CsPbI_{3} passivation structure is derived by using the following

In this expression _{atom/CsPbI3} and _{CsPbI3} are the total energy of the adsorption system and surface CsPbI_{3} (110), respectively. The total energy of the adsorbed material PEA^{+} molecule is _{atom}, where ^{+} molecule) and −0.43 eV (each PEA^{+} molecule), respectively, it can be seen from the calculation results that model A is more stable than model B. Therefore, the electronic properties of model A are studied in the subsequent calculation.

The total density of states (TDOS) of the clean surface CsPbI_{3} (110), and the local density of states (LDOS) of layers 1 to 7, and the partial density of states (PDOS) of the first layer are shown in ^{st}, 3^{rd}, 5^{th}, and 7^{th} layers, each of which is occupied by only the same number of I atoms, and the 2^{nd}, 4^{th}, and 6^{th} layers are occupied by the same type and number atoms in each layer, which are Cs, Pb and I atoms. In the surface model, released layers are the first to fourth layers, and fixed layers are the fifth to seventh layers. By comparing DOS of each layer, it can be found that the LDOS in the first and seventh layers are similar, and their contribution to the surface states is the largest at the Fermi level, which is mainly due to the contribution of unsaturated bonds at the end faces. From the 2^{nd} to the 6^{th} layer, every layer contributes to the surface states at the Fermi level, but the contribution to the surface states is smaller than that of layer 1 and layer 7. Through the analysis of PDOS, it is found that the contribution of I-5_{3} (110), which may have a detrimental effect on the optoelectronic properties of the device. Therefore, it is particularly important to find a surface passivation substance to weaken or even eliminate the surface states.

^{+}-CsPbI_{3} passivation system. From the TDOS, it can be found that the adsorption of PEA^{+} greatly weakens the surfaces states of surface CsPbI_{3} (110). Compared with that of before passivation, its value has reduced from 18.3 to 11.2 (relative value), and the passivation rate is 38.8%. In addition, the band gap of surface CsPbI_{3} (110) is regulated by PEA^{+}, compared with that of clean surface CsPbI_{3} (110), the surface band gap value of the PEA^{+}-CsPbI_{3} passivation system will increase from 0.98 eV to about 1.06 eV. From the ^{+}, it can be seen that there are no electronic states near the Fermi level, indicating that the material is an ideal passivation material. From the PDOS of the I atom of the first layer, it can be seen that the electronic states of the I-5^{+}-CsPbI_{3} system in the vicinity of Fermi level mainly originate from unsaturated bonds. To sum up, the calculation results show that PEA^{+} substance can reduce and eliminate the surface states of the CsPbI_{3} (110) surface. The passivation effect of PEA^{+} to the CsPbI_{3} (110) surface is effective.

The Bader atomic charges [^{+}-CsPbI_{3} system after passivation were calculated to analyze the scale of surface electron transfer. The Charge density difference after passivation can be calculated using

In _{PEA/CsPbI3} is the total charge density of the PEA^{+}-CsPbI_{3} (110) system._{CsPbI3} are the calculated charge densities of the PEA system alone and the CsPbI_{3} (110) system alone in the same passivation system, respectively. When the charge density of PEA^{+} is calculated, PEA^{+} is retained and the CsPbI_{3} (110) part is replaced by vacuum layer. In the same way, when the charge density of CsPbI_{3} (110) is calculated in passivation system, CsPbI_{3} (110) remains and PEA^{+} is replaced by vacuum layer.

^{+}-CsPbI_{3} passivation system. ^{+}-CsPbI_{3} system has a strong bonding effect at the adsorption site. The electron concentration is the highest at the adsorption site, and the further away from the adsorption site, the lower the electron concentration.

The simple and intuitive schematic figure of charge density difference cannot accurately analyze the specific charge transfer amount at the adsorption site. Therefore, we calculated the Bader charges of the clean CsPbI_{3} (110), PEA^{+} and the adsorption system PEA^{+}-CsPbI_{3}, respectively. The surface states of CsPbI_{3} (110) chiefly originate from the contribution of I, N, and C atoms of non-benzene ring groups with PEA^{+} make a greater contribution to eliminating surface states. Therefore, only Bader charges of the I atoms of clean CsPbI_{3} (110), and those of the N and C atoms of non-benzene ring groups with PEA^{+}, and those of I, N and C atoms in the corresponding adsorption system PEA^{+}-CsPbI_{3} are given in _{3} (110) are in the range of −0.46 to −0.45 ^{+} are 0.07 to 0.40 and −3.54 ^{+}-CsPbI_{3} system, the Bader charges of the corresponding I atoms are in the range of −0.63 to −0.59 _{3} form bonds with the N and C atoms of the adsorbent PEA^{+}, respectively, and the I atoms can obtain electrons from the N and C atoms bonded to it, thereby reducing the CsPbI_{3} (110) surface states.

Systems | I | C | N |
---|---|---|---|

CsPbI_{3} layer1 |
−0.46 to −0.45 | ||

PEA^{+} |
0.07 to 0.40 | −3.54 | |

PEA^{+}-CsPbI_{3} |
−0.63 to −0.59 | 0.02 to 0.32 | −2.95 |

This paper uses the first principles calculation of density functional theory to study electronic properties of CsPbI_{3} (110) surface. Based on this, the surface states with the CsPbI_{3} (110) surface near the Fermi level are passivated by PEA^{+} molecule. The micro mechanism is deeply discussed by investigating the stability and electronic structure of the passivation system. The calculated results of adsorption energy show that the model with I atom at the end faces to combine with PEA^{+} molecule is stable. The results of surface electronic structure calculation reveal that surface states on the CsPbI_{3} (110) surface near the Fermi level are mainly contributed by I-5_{3} (110) surface near the Fermi level. Electronic structure of the PEA^{+}-CsPbI_{3} system calculations demonstrates that the value of DOS has reduced from 18.3 to 11.2, and the passivation rate is 38.8%. The material PEA^{+} can improve surface states on the CsPbI_{3} (110) surface, and it is a potential passivation material. Bader charge analysis shows that electrons are transferred from passivation material to perovskite, which reduces the hole concentration in perovskite. Considering the experimental application of PEA^{+}, our theoretical study is expected to inspire the experimental study of PEA^{+} in perovskite materials, leading to the development of high-performance perovskite solar cells.

The authors would like to thank the Gansu Supercomputer Center and School of Chemical Engineering, Northwest University.

_{3}NH

_{3}PbI

_{3}perovskite film delivers a high open circuit voltage exceeding 1.2 V

_{3}N nanoribbons as anode materials for Li-ion batteries

_{4}NBr passivation

_{3}photovoltaics with surface terminated organic cations

_{2}pb