Fabrication Of High Efficient And Stable FAPbI₃ Perovskite Solar Cells Based On Polyamidoamine Dendrimers

In recent years,formamidinium lead iodide（FAPbI₃）has been considered to be one of the most promising materials in the photovoltaic field due to its excellent photoelectric properties,such as the narrow bandgap,high absorption coefficient,low exciton binding energy,high carrier mobility,and long carrier diffusion length.At present,the certified power conversion efficiency（PCE）of FAPbI₃ perovskite solar cells（PSCs）has reached up to 25.7%.Despite the impressive progress made in the PCE of FAPbI₃ PSCs,there are still several challenges on their way toward commercialization.Firstly,the enriched defects at the surface and poor-connected grain boundaries in solution-processed polycrystalline perovskite films are considered as trapping sites for carriers,thus affecting the PCE and stability of PSCs.Secondly,the blackα-FAPbI₃ perovskite phase tends to convert to the yellowδ-FAPbI₃non-perovskite phase at room temperature,which makes it difficult to prepare high-quality pure-phaseα-FAPbI₃ perovskite films,eventually deteriorating the long-term stability of devices.Therefore,the PCE of state-of-the-art FAPbI₃ PSCs is still lower than the Shockley-Queisser theoretical limit for single-junction PSCs and the long-term stability of FAPbI₃ PSCs is poor.To address these issues,the interface engineering and additive engineering have been adopted to regulate the crystal growth,passivate defects and inhibit the formation ofδ-FAPbI₃,thus improving the crystal quality,micro-morphology,phase purity and stability ofα-FAPbI₃ perovskite films,ultimately enhancing the device performance.However,the current recipes are still difficult to regulate the crystal growth and passviate defects of FAPbI₃ perovskite films on a large functional range and at full dimensions.Therefore,it is urgent to develop a kind of novel three-dimensional functional interfacial modifiers and additives for high-performance FAPbI₃ PSCs.In this scenario,in this thesis,the three-dimensional（3D）functional polyamidoamine（PAMAM）dendrimers are used as the interface modifiers and additives for FAPbI₃ PSCs.The effects of PAMAM dendrimer applications on the photovoltaic performance and stability of FAPbI₃ PSCs are investigated in details.The main contents are as follows:1.The surface of FAPbI₃ perovskite films are treated using PAMAM dendrimers with different generations and concentrations,and the influences of PAMAM surface treatment on the morphologies,optical and electrical properties of FAPbI₃ perovskite films,as well as the performance of photovoltaic devices,are investigated.It is found that ultrathin and uniform PAMAM interlayer can be formed on the surface of FAPbI₃perovskite films,which not only can effectively passivate the surface defects and optimize the interface energy level alignment,but also can prevent the invasion of water,thus improving the photovoltaic performance and stability of FAPbI₃ PSCs.As a result,FAPbI₃ PSC treated with the 2-generation PAMAM dendrimer（PAMAM-G₂）exhibits a champion PCE of 21.08%,along with greatly improved stability when subjected to humidity,thermal and light stresses.2.PAMAM dendrimers are employed as a kind of novel multifunctional additives for preparation of FAPbI₃ perovskite films using the sequential two-step solution process.The effects of PAMAM dendrimers with different generations on the crystal nucleation and growth,as well as the phase stability of FAPbI₃ perovskite films are studied.The mechanisms of PAMAM inclusion on defect passivation,photophysical properties,lattice strain and phase stability of FAPbI₃ perovskite films are discussed.The results show that the introduction of PAMAM dendrimer additives can promote the crystal nucleation and growth ofα-FAPbI₃ at room temperature,effectively inhibiting the formation ofδ-FAPbI₃.Resultantly,high-quailtyα-FAPbI₃perovskite films with high crystallintiy and large grain size are fabricated.Moreover,the inclusion of 3D PAMAM dendrimers can effectively crosslink FAPbI₃ perovskite grains on a large functional range and at full dimensions,thus inhibiting the ion migration,passivating the defects,and then reducing carrier non-radiative recombination significantly.As a consequence,the FAPbI₃ PSC with PAMAM-G₂exhibits a champion PCE of 21.89%.Moreover,the unencapsulated device with PAMAM-G₂ shows improved stability and can keep 70%of initial PCE after storage for 30 days under dark conditions at 25°C and relative humidity（RH）of 50-60%.3.A series of PAMAM dendrimers with different surface functional groups（amino,hydroxyl and carboxyl groups,denoted as PAMAM-NH₂,PAMAM-OH and PAMAM-COOH,respectively）are introduced as additives into the precursors for the fabrication of FAPbI₃ perovskite films.The regulation mechanisms on the nucleation and growth of perovskite crystals,as well as microstructure,photophysical properties,defect passivation and phase stability of FAPbI₃ perovskite films through using PAMAM dendrimers with different surface functional groups,are investigated.It is found that the PAMAM-COOH additive can effectively regulate the crystal growth behavior and passivate the uncoordinated Pb²⁺in FAPbI₃ perovskite films,thus reducing the carrier non-radiative recombination dramatically.Consequently,the FAPbI₃ PSC with PAMAM-COOH achieves a PCE of 23.15%with the enhanced stability.More strikingly,the optimized device with PAMAM-COOH delivers an open-circuit voltage(V_OC)as high as 1.14 V,which is the the highest V_OC for the inverted planar Ni O_x-based FAPbI₃ PSC to the best of our knowledge.In this work,taking full advantages of the unique 3D structure and rich functional groups in PAMAM dendrimers,the crystal growth behavior of FAPbI₃perovskite films is effectively regulated,thus improving the phase stability ofα-FAPbI₃ and realizing the efficient defect-passivation and grain crosslinking on a large functional range and at 3D dimensions.Our work provides a new strategy for preparing the high-quality phase-pureα-FAPbI₃ films,thereby realizing efficient and stable FAPbI₃ PSCs.