Improvement Strategies For Open-circuit Voltage And Stability In Tin Halide Perovskite Solar Cells

Tin halide perovskites（THPs）have been extensively investigated in the fields of photovoltaics,lighting,and displays due to their excellent semiconductor properties and low biotoxicity.Tin perovskite solar cells（TPSCs）have garnered significant attention for their near-ideal optical bandgap,small exciton binding energy,and high carrier mobility,leading to a remarkable increase in power conversion efficiency（PCE）from an initial 5-6%to over 15%.According to the Shockley-Queisser（S-Q）limit theory,the theoretical PCE can reach 33%with a bandgap of 1.4 e V in TPSCs,thus leaving plenty room for improvement in PCE.However,tin（Sn）in THPs,lacking lanthanide contraction,exhibits higher reactivity due to the higher oxidation potential of Sn²⁺compared to Pb²⁺,making Sn²⁺more prone to oxidation and generating decomposition process,thus creating defects/trap centers at grain boundaries,surfaces,and within crystal phases.Moreover,the rapid crystallization process in THPs affects the film crystallinity,crystal orientation,and phase distribution,adversely impacting the enhancement of PCE and stability of TPSCs.Therefore,investigating defect types and crystallization mechanisms in TPSCs is of crucial importance for advancing their photovoltaic performance and prolonged stability.Based on this,this study extensively investigates defect passivation and crystallization kinetics of THPs.Specifically,it includes methods such as the modulation of different ion defect types at various energy levels using benzoic acid derivatives,pyridine derivatives,π-conjugated compounds,and borohydride derivatives as passivating agents,as well as the control of crystallization and phase distribution using organic spacer molecules containing halogen anions.These approaches aim to further enhance the photovoltaic performance and open-circuit voltage(V_oc)output of TPSCs.The research findings are as follows:（1）The use of a novel Lewis base,4-amino-3-hydroxybenzoic acid（4A3HA）,effectively mitigates structural disorder and Sn²⁺defects in THPs.Density functional theory（DFT）calculations reveal that 4A3HA passivation increases the formation energy of tin vacancies(V_Sn).Devices passivated with 4A3HA exhibit improved PCE from 9.44%to 13.43%,with V_oc increasing from 842 m V to 897 m V.These devices show decent stability after aging in the N₂ atmosphere for 2000 h at 85°C,retaining over 80%of the initial PCE.（2）Inspired by the dimensionality control of bulky organic cations and the crystallization regulation mechanism of halogen anions,phenethylammonium thiocyanate（PEASCN）is utilized to induce dimensionality and crystallization control in THPs.The strong anchoring of halogen SCN^-ions with Sn²⁺and the orientation growth of tin-based crystals by PEA⁺improve the dimensional structure of THPs,alleviate Sn²⁺oxidation,and enhance stability.Inverted devices with PEASCN achieve a high PCE of12.88%with V_oc increasing from 733 m V to 863 m V.These devices exhibit exceptional stability under an N₂ atmosphere for over 2000 h.（3）Inspired by the defect passivation principle of Lewis bases,pyridine derivatives,namely dipicolylamine（DP）and 4-bromo-2,6-dipicolylamine（4Br DP）,are developed to control defect chemistry,oxidation behavior,and carrier dynamics in THPs.DP and4Br DP not only fix under-coordinated Sn-related defects through coordination bonds but also compensate for halide-related defects through N-H···I hydrogen bonding,reducing lattice distortions and defect/trap centers in corresponding Sn perovskite films.4Br DP devices exhibit enhanced carrier mobility and charge transfer,and suppressed trap density,resulting in a PCE of 13.40%and V_oc of 881 m V.These devices show significant stability improvements,retaining over 90%of the initial PCE after storage in the N₂ atmosphere for 2000 h and at 85°C for 1000 h.（4）Further investigation usingπ-conjugated 1H-pyrrolo[2,3-b]pyridine-6-amine（1H6An）as a multifunctional antioxidant reveals its interaction with Sn halide perovskites.The electron delocalization of 1H6An from the conjugated system to functional groups promotes anchoring effects with THPs.Coordination interactions and hydrogen bonding between 1H6An and perovskites precisely anchor Sn and I-related defects,reducing Sn⁴⁺content and improving charge extraction and non-radiative recombination.Device doped with 1H6An achieve a PCE of 13.28%and a V_oc of 907 m V,with excellent stability,retaining over 90%of the initial PCE after storage in an N₂atmosphere for over 2500 h.（5）Sodium borohydride with three symmetric acetoxyl groups（BNa3Ac）is further utilized to regulate defect states and oxidation issues in THPs and TPSCs.It exhibits strong multifunctional passivation effects,simultaneously passivating Sn-related and A-site cation defects.The Sn²⁺oxidation behavior in THPs was significantly suppressed.Meanwhile,the electron mobilities of BNa3Ac-passivated devices were enhanced accordingly.BNa3Ac-modified devices show a significant increase in V_oc output from857 m V to 906 m V,with a PCE of 13.33%.BNa3Ac film and passivated device demonstrates enhanced antioxidation properties and stability,retaining over 90%of the initial PCE after storage in the N₂ atmosphere for over 2200 h.