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

Perovskites solar cells（PSCs）have made remarkable achievements in the past decade with certificated power conversion efficiency（PCE）of 25.5%in small-area（～0.1 cm²）laboratory devices.High quality perovskite active layer and effective charge transport are the key factors to realize high performance PSCs.In addition,to achieve the goal of commercial application of PSCs,the long-term stability of PSCs and the lead toxicity issue of perovskites need to be solved urgently.This thesis focuses on optimization of active layer materials and clarifying charge transport processes of PSCs.A series of works have been carried out in the fields of preparation low defect density perovskite active layer,designing of electron transport layer and optimization of active layer for flexible PSCs,structural design of novel graded heterojunction inorganic PSCs,development of low toxicity Sn/Pb binary metal inorganic perovskites.Various testing and characterization methods,including electrochemical impedance spectroscopy,transient photocurrent/photovoltage decay and transient absorption spectroscopy have been carried out to systematically study the charge transport and reconbination processes of PSCs.The main research contents and conclusions are as follows:（1）High-quality perovskite active layers are critical for achieving highly efficient and stable PSCs.This study introduces I₃^-ions into MAI/MACl organic cation dripping solution through chemical reaction of^-+₂=₃^-to regulate the nucleation and crystal growth processes of MAPbI_3-xCl_x perovskite,as well as decreasing the concentration of defects,therefore the PCE and stability of resulting PSCs can be improved.Under AM 1.5G solar irradiation,the PSCs based on I₃^-ions achieved high PCE of 18.7%,and the PSCs can be stably stored for more than 400 hours in ambient air 20%-30%humidity condition.（2）In order to improve PCE and mechanical flexibility of flexible PSCs,the designing of low-temperature interconnected SnO₂ electron transport layer and the optimization of（FAPbI₃）_0.85（MAPb Br₃）_0.15 active layer were systematically studied.Charge carriers dynamics characterization of the PSCs shows that the interconnected SnO₂ electron transport layer enables efficient electron extraction and retards nonradiative charge carrier recombination at the perovskite/electron transport layer interface.Consequently,the flexible PSCs based on the low-temperature interconnected SnO₂ electron transport layer achieved a PCE up to 16.29%as well as excellent mechanical flexibility.After 200 bending cycles at a curvature radius of 10 mm,the PSC device can maintain 90%of the initial efficiency.（3）Inorganic perovskites have outstanding photoelectric conversion characteristic and excellent structural stability.This study reports on CsPbI_2.5Br_0.5/fluorine doped carbon quantum dots（FCQDs）graded heterojunction for photovoltaic application,effectively solved the poor interfacial contact as well as mismatched energy level alignment between perovskite layer and carbon-electrode,facilitating the separation and extraction of photogenerated charge carriers.The CsPbI_2.5Br_0.5/FCQDs graded heterojunction in conjunction with low-temperature processed carbon-electrode achieved impressive PCE of13.53%in 1 cm² device at AM 1.5G solar irradiation.To the best of our knowledge,this is the first report of 1 cm² carbon electrode based inorganic PSC with PCE exceeding 13%.More importantly,the solar cells based on CsPbI_2.5Br_0.5/FCQDs graded heterojunction exhibited excellent long-term stability.The unencapsulated devices maintained over 90%of their initial efficiency under thermal aging at 85℃for 1056 h.（4）The substitution of Pb with Sn in inorganic perovskites has multi-advantages of reducing the bandgaps,enhancing the phase stability,as well as mitigating the toxicity issue.In this study,Sn was used to partially replace Pb in CsPbI₂Br to prepare low toxicity Sn/Pb binary metal inorganic perovskite photovoltaics.The all-inorganic mesoporous solar cell based on CsPb_0.5Sn_0.5I₂Br perovskite light-absorption material achieved a PCE of 7.63%,and maintained 81.8%of its initial efficiency after 500 hours of thermal aging at 85℃.