钙钛矿/NiO异质结结构和性质的第一性原理研究

Structures and properties of perovskite/NiO heterojunctions:a first-principles investigation

空穴传输层与钙钛矿吸光层形成的异质结界面处的能带排列以及点缺陷的存在是影响钙钛矿太阳能电池光电转换效率的2个重要因素.采用基于密度泛函理论的第一性原理计算研究了钙钛矿/NiO异质结的结构和性质,探讨了异质结中的点缺陷、价带和导带的能级排列、碱金属K掺杂钙钛矿层和过渡金属Zn掺杂NiO层对电子-空穴的分离与输运的影响.计算的晶格失配度和结合能表明MAPbI_(3)和MASnI_(3)的(001)面与NiO(100)面能够形成稳定的异质结,且形成NiO/PbI_(2)和NiO/SnI_(2)异质结的可能性更大.形成异质结后钙钛矿层中的绝大多数空位缺陷形成能都有所降低,说明缺陷在异质结的界面处更容易形成.异质结中的主要缺陷是在富碘条件下产生的VPb,VSn和VMA,这些缺陷对异质结中空穴的分离和传输十分不利,建议在富铅条件下构建NiO/MAPbI_(3)和NiO/MASnI_(3)异质结.用碱金属K原子对异质结的钙钛矿层进行掺杂,可以使钙钛矿层和空穴传输层有更匹配的能带排列,提高NiO/PbI_(2)和NiO/SnI_(2)异质结的空穴分离和运输效率,降低载流子的复合.对异质结中NiO层进行Zn取代掺杂能够使NiO/PbI_(2)和NiO/SnI_(2)异质结获得更合适的价带偏移(VBO),有利于空穴的分离和传输,提高电池的光电转换性能.本研究可为新型高效钙钛矿太阳能电池的设计开发提供理论指导.

Point defects and band alignment at the interface of heterojunction constructed by hole transport layer and perovskite light-absorbing layer are the key factors of affecting the power conversion efficiency of perovskite solar cells.In this paper,the structures and properties of perovskite/NiO heterojunctions were studied by First-principles calculations based on density functional theory.The influence of electron-hole separa-tion and transportation in heterojunction by the existence of point defects,energy level alignment of valence and conduction bands,alkali metal K doping perovskite layer and transition metal Zn doping NiO layer are discussed.MAPbI_(3)(001),MASnI_(3)(001)and NiO(100)surface slabs were employed to build NiO/MAPbI_(3) and NiO/MASnI_(3) heterojunctions.The calculated lattice mismatch ratio and binding energy indicate that the(001)surface of MAPbI_(3) and MASnI_(3) can form stable heterojunctions with the NiO(100)surface,and the possibility of forming NiO/PbI_(2) and NiO/SnI_(2) heterojunctions is larger due to the formation of O-Pb or O-Sn bond.Charge density rearrangement occurs near the interface of heterojunction.NiO/PbI_(2) and NiO/MAI(MAPbI_(3))heterojunctions belong to typeⅡband alignment,which is conducive to the effective separation of electrons and holes.NiO/SnI_(2) has a near zero valence band offset(VBO)and a large conduction band offset(CBO),which also facilitates the extraction of holes and prevents the flow of electrons from perovskite to NiO layer.NiO/MAI(MASnI_(3))is typeⅠsemiconductor.Holes cannot be transferred to the NiO layer and accumulate at the interface,which will increase the non-radiative recombination of carriers.The formation energy of most vacancy defects in the perovskite layer decreases after the formation of heterojunction,indicating that defects are more easily formed at the interface of heterojunction.The dominant defects in heterojunctions are V_(Pb),V_(Sn)and V_(MA)produced under I-rich conditions,which are very unfavorable to the separation and transport of holes in heterojunctions.It is recommended to construct NiO/MAPbI_(3) and NiO/MASnI_(3) heterojunctions under Pb-rich condition.Doping the perovskite layer of heterojunction with alkali metal K atom can make the perovskite layer and the hole transport layer have more matching band arrangement,improve the hole separation and transport efficiency of NiO/PbI_(2) and NiO/SnI_(2) heterojunction,and reduce the recombination of carriers.The Zn-substituted doping of the NiO layer in the heterojunction can make the NiO/PbI_(2) and NiO/SnI_(2) heterojunction obtain more suitable VBO,which is beneficial to the separation and transportation of holes,and improve the photoelectric conversion performance of the solar cell device.This study can provide theoretical guidance for the design and development of new high-efficiency perovskite solar cells.