Crystallization Regulation And Optoelectronic Properties Of Tin-based Perovskites

Exploring stable and efficient lead-free perovskite solar cells(PSCs)is critical to solving the environmental concerns caused by lead counterparts.Recently,tin halide perovskites have become a promising candidate due to its low toxicity and similar electronic configuration to lead counterparts.Currently,the power conversion efficiency(PCE)of tin-based perovskite solar cells has been pushed over 14%.However,there is still a considerable gap compared to the lead perovskite solar cells mainly due to the fast crystallization rate,poor film quality,easy oxidation of divalent tin,and tin vacancies in tin-based perovskite,which generate more trap states in the film,thus inducing non-radiative recombination,causing the open-circuit voltage loss and the degradation of perovskite films.Therefore,it is important to reduce the defect sites and the non-radiative composite center in the perovskite films to improve the device performance and stability.In this thesis,the quality of perovskite films is optimized by the introduction of additives,and reduced the non-radiative composite centers in the films.By characterizing the films and devices,their effects on the PCE and stability of tin-based PSCs were investigated.(1)A small molecule Lewis base thiourea(TU)was used to incorporate into the perovskite precursor solution,and the results showed that the introduction of TU improved the quality of perovskite films by slowing down the crystallization rate of perovskite through interacting with SnI₂.Meanwhile,the introduction of TU suppressed the trap-state assisted non-radiative recombination and prolonged the carrier lifetime in the perovskite films.In addition,an increase in the built-in potential facilitated the charge separation and extraction of PSCs.As a result,p-i-n-type PSCs with an open-circuit voltage of 0.63 V and a PCE of 10.07%were obtained after TU modification,and 90%of the initial efficiency was retained after storage under N₂ atmosphere for more than 1600 h.(2)Benzoic acid(BA)and the monofunctional substituted molecule 4-chlorobenzoic acid(4-CBA)were used to investigate the effect of functional groups in perovskite films.It was shown that the two molecules modulated the lattice stress by interacting with Sn²⁺in perovskite films which induced the orientation of(h00)to gain high quality perovskite films.The substitution of Cl increases the electron density of the molecule,which generates a large dipole moment and improves the base of the molecule.The improved performance is mainly attributed to the suppression of nonradiative recombination,the reduction of defect density,the improvement of carrier transport and collection efficiency.The devices with 4-CBA obtained a PCE of 9.26%.Therefore,high-quality perovskite films were successfully fabricated by introducing additives into perovskite precursor.Efficient and stable PSCs were prepared and the the molecular mechanism of interaction was discussed.