Research On Perovskite Solar Cells With The Lead-Alkaline Earth Metal Composite Perovskite Materials

In recent years,organic-inorganic metal halide perovskite solar cells(PSCs)have made impressive progress with maximum power conversion efficiencies(PCEs)evolving from 3.8% to a certified 22.1%.The most popular and archetypical metal halide perovskite,methylammonium lead triiodide(CH3NH3PbI3),possesses specific photoelectrical properties including appropriate and direct band gap,low excition binding energy,high molar extinction coefficient,excellent dipolar charge transfer property and simple solution processing at low temperature,enabling the PSCs a promising PV technology for realizing high performance and cost-effective solar cells in the future.However,currently the large-scale commercial applications of these Pb-based perovskites are vastly limited by the toxicity of Pb,control of crystal growth and thin film morphology and intrinsic instability of materials.Therefore,it is imperative to find environmentally friendly metal ions to replace lead ion and form new-typed perovskite materials for the further development of PSCs.Previous work has demonstrated that Sn2+,Ge2+,Sb3+,Bi3+ and other transition metal cations could be used as alternative cations in perovskite configurations to form a new lead-metal composite perovskite structure.In this paper,II A group ions have been employed to replace Pb2+,e.g.,Sr2+,Ca2+,Ba2+.The main contents of this thesis are listed as following:Firstly,a series of Pb-Sr mixed metal perovskite films(CH3NH3SrxPb1-xI3,x=0,0.2%,0.5%,1%,2%),prepared by one-step solution spin-coating method,have been successfully applied as light absorption layers in the devices based on inverted planar architecture FTO/NiMgLiO/Perovskite/PCBM/BCP/Ag.As evidenced in the results from X-ray diffraction(XRD)/ scanning electron microscopy(SEM)and Time-resolved photoluminescence measurements,doping a small quantity of Sr2+(0.2%,0.5% molar)in CH3NH3PbI3 was significant to acquire high-quality perovskite films,with larger crystal size,flatter and denser films,and longer carrier lifetimes.According to the current density-voltage(J-V)curves,devices using these films showed an effective improvement of photovoltaic performance,yielding an overall power conversion efficiency of 17.7% with a greatly increased open circuit voltage(Voc).Electrochemical impedance spectroscopy(EIS)and Mott-Schottky(MS)characterization have revealed that Sr2+ ions(0.2%,0.5% molar)not only efficiently improved the built-in electric field but also increased the interfacial recombination resistance in the construction of inverted planar perovskite solar cells.These significantly retard charge recombination at the interface and accelerate charge extraction to the external circuit.Secondly,CH3NH3CaxPb1-xI3-2xCl2 x and CH3NH3BaxPb1-x I3-2xCl2 x composite perovskite films were deposited by solution processing and used as the absorbers in p-i-n type inverted planar perovskite solar cells.Ca2+-doped and Ba2+-doped(0.5% molar)thin films possessed less crystal defects due to the greatly improved perovskite film quality and coverage with well-ordered and relatively large grain size.It is significant to find that doping CH3NH3 Pb I3 by inclusion of very small amounts(0.5% molar)of Ca2+ and Ba2+ improved the built-in electric field and suppressed charge recombination via Electrochemical impedance spectroscopy and Mott-Schottky characterization.A dramatic improvement of the photovoltage and performance were achieved in Ca2+-doped and Ba2+-doped perovskite solar cells.