Design And Optimization Of Low Bandgap Sn-Pb Inorganic Perovskite Solar Cells

In recent years,owing to the rapid improvement of power conversion efficiency(PCE)of organic-inorganic hybrid perovskite solar cells,it has shown a broad commercial application prospect,which has attracted much attention.The organic cation CH₃NH₃⁺or HNCH(NH₃)⁺in the organic-inorganic hybrid perovskites with characteristics of poor thermal stability,easy degradation and volatilization,which will result in the attenuation of device efficiency and the decrease of device stability.Furthermore,the perovskite materials contain heavy metals of Pb,which will greatly pollute the environment.These shortcomings seriously restrict the further application and development of perovskite solar cells.Therefore,it is essential to develop low toxicity,high efficiency and high stability photovoltaic materials.In this work,aiming at perovskite optoelectronic materials with low toxicity,narrow band gap and long-term stability and combining theoretical simulation calculation with material synthesis/device process optimization,a novel all-inorganic Sn-Pb alloy perovskite material was prepared by one-step spin-coating green antisolvent method,namely CsPb_1-xSn_xI₂Br(x=0?1).Compared to pure Pb-based CsPb I₂Br perovskite materials with a bandgap of?1.91 eV,these Sn-Pb alloy perovskite materials have a tunable narrow band gap closed to ideal band gap width(1.40 eV)and a wide spectral response range.Meanwhile,through the bulk doping and surface passivation,the optical-wet-thermal stability of the materials and the performance of devices are improved.The conclusions are as follows:1.The design and preparation of narrow bandgap inorganic Sn-Pb alloy perovskite film materials:All inorganic CsPb_1-xSn_xI₂Br is predicted as a perovskite material with ideal band gap,but no stable perovskite phase has been reported.In this report,through precursor design and optimization,a series of CsPb_1-xSn_xI₂Br(x=0,0.15,0.30,0.45,0.60,0.80,1.00)perovskites with tunable bandgaps(1.92?1.38 eV)are successfully prepared at low annealing temperature(60^oC)for the first time.With the increasing of Sn content,the absorption edge of perovskite films greatly moves from 650 nm to 900 nm.Meanwhile,due to the Sn²⁺with a smaller ionic radius(112pm),the tolerance factor of Sn-Pb alloy perovskites is closer to 1,leading to a more stable perovskite crystal structure.The perovskite solar cells with the CsPb_0.55Sn_0.45I₂Br(E_g=1.50 eV)as active layer have the best device performance through Sn-Pb proportional regulation and energy level matching2.Stability optimization of all-inorganic Sn-Pb alloy perovskite film materials:i)Bulk doping:the new similar halogen ions(Cl^-)is introduced into inorganic Sn-Pb alloy perovskite to reduce the bulk phase defects.On the one hand,it can effectively delay the crystallization rate,regulate the nucleation and growth process of perovskite,increase the grain size and dense grain boundary.On the other hand,Cl^-can interact with Sn²⁺to inhibit the Sn⁴⁺oxidation and occupy the iodine ion vacancy,which effectively reduces the defect sites in the perovskite phase and improves the quality of the perovskite crystal.ii)Surface coordination:the dense protective layer(Pb SO₄)is formed by the coordination reaction of the surface lead suspension bond(Pb²⁺)with the introduced sulfate ions(SO₄^2-).It resists the erosion of inorganic Sn-Pb perovskite layer by external water vapor and oxygen.It not only delays the oxidation of the Sn²⁺in the perovskite film,but also largely inhibits the migration of iodine ions and further stabilizes the crystal structure of the perovskite.The first principle calculation shows that Cl ions can occupy I ion vacancies and effectively reduce bulk phase defects.Moreover,the dense charge clouds around well-bonded Pb SO₄can be serve as a“bridge”to promote charge transfer.As a result,The PCE of all-inorganic Sn-Pb alloy perovskite solar cells with an inverted structure of ITO/PEDOT:PSS/Perovskite/Sulfate/PCB₆₁M/BCP/Ag was optimized to 10.39%.The unencapsulated perovskite devices retained over 92.5%of its initial PCE after being stored in a N₂atmosphere for 2000 h.Meanwhile,the devices also exhibited excellent light,humidity(40?50%)and thermal stability(85^oC).In summary,this work enriches the all-inorganic Sn-Pb alloy perovskite material system.The stable CsPb_1-xSn_xI₂Br(x=0?1)perovskite phase with adjustable bandgap was prepared at low temperature.Moreover,It also provides a feasible process route for the fabrication of flexible all-inorganic perovskite solar cells and perovskite-perovskite laminated solar cells.