Investigation Of Triphenylamine Based Covalent Organic Frameworks As Hole Transport Layers For Perovskite Solar Cells

Interfacial recombination is a key factor in the performance loss of perovskite solar cells.Many works have attempted to minimize the interfacial recombination between the electron/hole transport layer and the perovskite layer by optimizing existing materials or developing new ones.From this perspective,a variety of novel molecules and polymers have been synthesized as transport layers to accommodate the energy levels of the perovskite layer and to create optimized interfaces for high performance.For an“ideal”transport layer material,various basic requirements should be considered,such as suitable energy levels and high hole/electron mobility.Processability is another key factor in these requirements.Covalent organic frameworks（COFs）are stable frameworks that do not dissolve in any solvent after formation,which forms the theoretical basis for targeting COFs as charge transport layers.Moreover,the designability of COFs offers additional possibilities.In the first part of this work,a series of hole transport layer molecules were designed and synthesized with triphenylbenzene,triphenylamine and tristhiophenebenzene as core and di（4-methoxy-phenyl）amine and di（4-methoxy）carbazole as terminals to investigate relations between structural features and optoelectronic physicochemical properties,as well as the photovoltaic performance of these small molecules as hole transport materials in dopant-free invert（p-i-n）perovskite solar cells.The best photovoltaic performance of perovskite solar cells presented here are devices with TPA-DPAOMe as hole transport layer,which show up to 18.00%of photoelectric conversion efficiency（PCE）,77.20%of fill factor（FF）,1.07 V of open-circuit voltage(V_OC)and 21.80 m A/cm² of short circuit current density(J_SC).In the second part of this work,a series of COFs based on the cores of the first part are designed and synthesized as films directly on top of ITO substrates,which serve as hole transport layer in perovskite solar cells.The main goal was to design polymers with suitable hole mobilities in dopants-free p-i-n perovskite solar cells,which can skip solution processing to overcome solubility issues of general hole transport layers using polymeric materials.The best photovoltaic performances were shown by perovskite solar cells using TPA-NC-T1 as hole transport layer,showing12.28%of PCE,65.12%of FF,0.99 V of V_OC and 19.04 m A/cm² of J_SC.Considering the ongoing deficiencies in device performance improvement attributed to COFs structures,we ventured into regulating interfacial carrier dynamics of conventional inorganic and organic hole transport layers,thereby guiding subsequent refinements in COFs design.This exploration led to the highly dispersed fabrication of oil-soluble nickel oxide（Oil-Ni O_X）nanoparticles through the use of a mixed solvent comprised of oleic acid and oleylamine.Furthermore,the introduction of the additive F4TCNQ was instrumental in boosting optoelectronic properties.The formation of robust chemical interactions between the fabricated nanoparticles and perovskite crystals,including the establishment of novel halogen bonds and coordination bonds,played a pivotal role in inhibiting the formation of halide vacancies and optimizing the interface contact between the Oil-Ni O_X layer and the perovskite.Consequently,printed perovskite solar cells（PSCs）with active areas of0.1 cm²and 1.01 cm²respectively achieved remarkable PCE of 25.17%and 24.36%.These PSCs also demonstrated exceptional thermal stability when subjected to heating at 85℃,maintaining 90%of their initial performance over an extended period of 2800 hours.This study underscores the importance of future COFs material design strategies that carefully consider the impact of bonding interactions on the optimization of perovskite layers and the exploration of printable properties,thereby paving the way for advancements in the field.