FASnI₃ Lead-free Perovskite Solar Cells Passivated With Amino Acid

Perovskite solar cells have advantages of easy fabrication,lower cost,flexibility and so on,becoming one of the most promising next generation photovoltaic devices.Since 2009,the power conversion efficiency（PCE）of perovskite solar cells has increased from the initial 3.8%to 25.2%.Although the PCE is close to the commercial standard,the lead element contained in perovskite still limits its large-scale commercial application.Therefore,it is urgent to find alternative lead-free perovskite materials.Among them,tin-based perovskite has been widely concerned because its ion radius,extinction coefficient and exciton binding energy are similar to lead-based counterpart.Compared with Pb analog,tin-based perovskite has narrower optical band gap and higher carrier mobilities,but its film morphology is difficult to control and the film is easy to oxidize.With the joint efforts of researchers all over the world,the PCE of tin-based perovskite solar cells has reached 12.4%.（1）The quality of perovskite active layer is the primary factor affecting the performance of the device.In this thesis,FASnI₃ perovskite is chosen as the absorption layer.In order to solve the problem of poor film quality caused by too fast crystallization of tin-based perovskite,the ratio of precursor solvent and the type of antisolvent are adjusted,respectively,and perovskite thin films with high quality and good crystallinity are obtained.Then,the structure of the device is improved with a stable PCE of 4-5%achieved by replacing the electron transport layer of[6,6]-phenyl-C₆₁-butyric acid methyl ester(PC₆₁BM)with fullerene(C₆₀),.（2）Defects at film surface and grain boundaries induce a high density of trap states which dramatically impair the efficiency and stability of perovskite solar cells.Therefore,passivation of electronic surface trap states is necessary to reduce the charge recombination rate and enhance the efficiency of perovskite solar cells.After investigating the common passivation materials,we introduce aminopropionic acid hydrochloride（APAC）into the precursor solution to passivate the surface defects of FASnI₃ perovskite.APAC passivated FASnI₃ via a hydrogen bond interaction（Cl^-···FA⁺）,a coordination interaction(N-H···Sn²⁺)and an electrostatic attraction(-COO^-···Sn I₆^4-with I vacancy or Sn²⁺).With the introduction of APAC,the quality,crystallinity and optical properties of FASnI₃ perovskite films have been improved.Compared with the control device,the APAC-passivated FASnI₃ perovskite solar cells have increased their PCEs from 5.13%to 6.80%.（3）We introduce 2,3-diaminopropionic acid hydrochloride（2,3-DAPAC）with both diammonium cation and carboxyl group into FASnI₃ perovskite solar cells.Different from APAC,the diammonium cations provide more bonding sites and may passivate charged defects in FASnI₃perovskite more effectively.The analysis of grazing incidence wide angle X-ray scattering（GIWAXS）shows that after doping 2,3-DAPAC,the perovskite crystal orientation is tuned from nearly random to～45°with respect to the substrate,which promotes the charge transport in the perovskite film from bottom to the top surface.In addition,the incorporation of 2,3-DAPAC into FASnI₃ enables dense and smooth high-quality perovskite films with less charged defects and lower Sn⁴⁺content.Compared with control device,the 1.5%2,3-DAPAC-doped FASnI₃ perovskite solar cells have significantly improved their PCEs from 5.27%to 7.23%and the storage stability of both perovskite films and devices increases significantly with the improvement of film quality.The performance and stability of FASnI₃ perovskite solar cells in this thesis have been investigated and improved.We hope that the as-mentioned methods can provide some valuable ideas for the development and their commercialization of tin-based perovskite solar cells.