Study On Optimization Of Perovskite Layer And Charge Transport Materials For Perovskite Solar Cells

Hybrid organic-inorganic lead halide perovskite solar cells（PSCs）have attracted tremendous interest in the past 10 years due to their unique properties such as direct optical band-gap,high absorption coefficients,modest carrier mobility,and long charge-carrier diffusion length.A power conversion efficiency（PCE）of 3.8%was first demonstrated in2009,with the most recent validated performance of 25.2%PCE for a single junction cell,and 28%efficiency for a perovskite/silicon tandem cell achieved in 2018.Such rapid evolution makes PSCs the leading contender for next-generation thin-film and for combination with mainstream Si solar cells.But for further developments toward commercialization of perovskite solar cells,there are still a number of challenges,such as how to scale up the progress of fabrication and increase intrinsic and environmental stability further.The state-of-the-art PSCs use mixed-cation and mixed-halide perovskite compositions with excess PbI_2.Nevertheless,there are also reports showing that the residual PbI₂ can lead to degradation and fragile film structure with long-term device stability concerns.Tin（IV）oxide（SnO₂）is emerging as an ideal inorganic electron transport layer in n-i-p perovskite devices,due to superior electronic and low-temperature processing properties.However significant differences in current-voltage performance and hysteresis phenomena arise as a result of the chosen fabrication technique.This indicates enormous scope to optimise the electron transport layer（ETL）,however,to date our understanding of the origin of these phenomena is lacking.In planar perovskite solar cells with n-i-p structure,2,2′,7,7′-tetrakis（N,N-di-p-methoxyphenylamine）-9,9′-spirobifl uorene（spiro-OMeTAD）has been the most employed as hole transport layer（HTL）.However,many reports have pointed out that spiro-OMeTAD requires two additives which cause the issue of degradation and the high cost is not beneficial to the commercialization.This thesis systeymatical investigated and optimised the funcational layers（perovskite layer,electron transport layer and hole transport layer）in perovskite solar cells.We believe this study will provide new insight and stratagy for further optimization of perovskite solar cells.The main thoughts and conclusions are summarized as follows:The effect of FACl treatment on perovskite films and devices:in Chapter 3,we propose a facile bulk recrystallization process by applying formadinium chloride（FACl）on perovskite to remove excess PbI₂ in the formed crystal.We are able to demonstrate bulk recrystallization,proved and observed by the Grazing incidence XRD（GIXRD）to analyze the crystal structure as a function of depth profiling.The reconstructed crystal displays improved optoelectronic qualities with reduced interfacial recombination as well as enhanced device stability.When measured under AM 1.5G spectral conditions,the champion device reached a maximum PCE of 20.2%.In addition,we measured the stability of devices stored in the dark（T²5℃and RH²0%）.The devices with FACl recrystallization show negligible degradation in 3 months.Strategically constructed bilayer tin（Ⅳ）oxide（SnO₂）as electron transport layer in perovskite solar cells:in Chapter 4,we first compare two common SnO₂ ETLs with contrasting performance and hysteresis phenomena and develop a strategy to combine the beneficial properties in a bilayer SnO₂（B-SnO₂）architecture.In doing so,we eliminate room-temperature hysteresis whilst simultaneously attaining impressive PCE of 20.3%,V_occ of 1.13 V in small area（0.08 cm²）device and PCE of 18.5%,V_occ of 1.17 V in large area（1cm²）device.Applied Fe（Ⅱ）/（Ⅲ）complexes as efficient and highly stable hole transport materials for perovskite solar cells:in Chapter 5,we synthesized a series of cost effective Fe complex as potential hole transporting materials and and pick up the most suitable complex Fe²⁺-DA/Fe³⁺-DA based on energy level alignment.Tuning the ratio of Fe²⁺/Fe³⁺,we achieved suitable highest occupied molecular orbital（HOMO）level and successfully applied it as a HTL in our PSCs.PCE=10.5%,V_oc=0.81 V,J_sc=20.7 mA/cm²,FF=63%in champion device with free hysteresis was obtained.In addition,the device showed impressive stability.After 600 h of operation at maximum power point（MPP）under AM1.5G one sun illumination,the device retained 90%of its initial PCE.