Dimensional Control And Molecularly Engineered Interfaces In Metal Halide Perovskite Solar Cells

As a kind of direct band gap semiconductor with ionic properties,metal halide perovskites have been widely studied owing to their excellent optoelectronic properties and low-temperature processing ability.The single-junction perovskite solar cells（PSCs）have achieved a certified world record power conversion efficiency（PCE）of more than 25%,which is thus regarded as the most potential photovoltaic technology for the third-generation solar cells.To gain the full potential of device performance,an in-depth understanding of surface/interface science is an urgent need.The design of low-cost and efficient molecularly interfacial materials is an imperative research direction to reduce processing cost,improve device performance and stability of PSCs,and therefore accelerate their commercial application.In spite of it,this thesis will focus on the research around the aspects of design and synthesis of the efficient charge transporting materials as well as the preparation of key materials of the low-dimensional PSCs.The effects of molecular modulation on the interfacial properties,charge dynamics,optoelectronic properties,device efficiency,and degradation mechanism will be systematically investigated.The detailed work contained in this thesis is divided into the following four parts:In Chapter 3,we modified the quasi-two-dimensional（quasi-2D）BDAFA₃Pb₄I₁₃perovskite by introducing different ammonium chloride salts with varied cationic sizes.It was found that ammonium chloride can dramatically affect the self-assembly process of quasi-2D perovskite films.With the assistance of Cl^-,the grain size and crystallinity of quasi-2D perovskite can be significantly improved.In addition,the size difference of ammonium ions influences their diffusion rate in the precursor solution and intermediate-phase formation ability,which then determine the nucleation rate and the QW orientation and phase purity in final films.By combining the efficient ammonium chloride additives with IPA post-treatment method,the champion efficiency of 19.5%was achieved for the quasi-2D PSCs（n=4）.In addition,the optimized film quality and QW arrangement of decorated devices ensure good long-term stability under humidity and continuous illumination environments.In Chapter 4,we sought to gain insight into the structure–property relationships in diammonium-based 2D/3D perovskite hierarchical heterostructures.It was found that the chemical structure and concentration of the spacers can dramatically affect the characteristics of the 2D capping layers,including phase purity and orientation of quantum wells（QWs）.Ultraviolet photoelectron spectroscopy and density functional theory calculations revealed the effects of ligand structures on electronic structures.The spacer-chemistry design can regulate the charge transport and extraction properties at the 2D/3D interface and therefore benefit the improved charge collection efficiency in decorated devices.Benefiting from the strong intermolecular interactions between the 2,2-（ethylenedioxy）bis（ethylammonium）（EDBE）cations and inorganic[Pb I₆]^4-slabs,the controlled deposition of a phase-pure QW structure with a horizontal orientation was achieved,leading to better surface passivation and carrier extraction.These benefits endow EDBE-based 2D/3D devices with a high PCE of 22.6%.The superior long-term stability of the 2D/3D devices under different aging scenarios further demonstrates the robustness of the 2D phase against environmental stimuli.In Chapter 5,a low-cost and efficient dopant-free hole transporting material（HTM）named DTB-FL with a D-A-π-A-D molecular configuration was proposed.The rational donor/acceptor moieties in molecular structure and the extendedπ-conjugation length of DTB-FL ensure good solubility,film quality,and efficient charge transport properties of the HTM.More importantly,it was found that the well-matched band alignment and surface passivation effects endow the DTB-FL layer with great benefits in charge transport and extraction property at the perovskite/HTM interface with reduced energetic barrier and interfacial recombination loss.As a result,the DTB-FL based PSCs deliver the champion PCEs of 21.5%and DTB-FL also presents good compatibility with large-scale fabrication and all-inorganic PSCs.Furthermore,significant improvements in long-term stability under different aging environments are achieved in the dopant-free DTB-FL based PSCs.In Chapter 6,three carboxylic acid-based molecules,namely,propandioic acid（PA）,glutaric acid（GA）,and citric acid（CA）,were employed to complex with the Sn O₂ electron transporting layer（ETL）,and the effects of molecular configurations on the chemical binding properties and related interfacial properties of Sn O₂ were investigated.It was found that the carboxylic acid molecules can dramatically affect the morphology quality,oxygen-vacancy density,electron mobility,and energy levels of the modified Sn O₂ films,which determines the charge collection and recombination loss at the electron extraction interface.In addition,the acid-modified Sn O₂ films can improve the surface wettability,which facilitates a better perovskite crystallization process,leading to perovskite films with smooth surface coverage and improved grain size and crystallinity.Among the three tested molecules,we found that the CA molecules provided the strongest chemical coordination effect to the Sn O₂ ETL（CA-Sn O₂）,resulting in the optimal charge transport efficiency,reduced interfacial energy offset,and minimized nonradiative recombination loss of the PSCs.Combining these advantages,the CA-Sn O₂-based PSCs achieved a champion efficiency of 23.1%with good shelf life and stability under one sun-equivalent illumination.