Fabrication Of High-efficiency Quasi-2D Perovskite Solar Cells Based On Band Gap Engineering

In recent years,perovskite solar cell has attracted great attention at home and abroad due to high mobility,strong light absorption,long carrier diffusion distance,various fabrication methods,changeable bandgap,and flexible cell fabrication.Perovskite solar cells have now achieved tremendous development,and the power conversion efficiency has increased from the initial 3.8%to 25.2%.Due to the low cost and superior photoelectric properties of perovskite materials,perovskite solar cells have extremely broad commercial prospects.However,perovskite materials have a fatal shortcoming of poor long-term stability,which limits the commercialization of perovskite solar cells.It is reported that reducing the dimensionality of perovskite materials from a three-dimensional structure to a two-dimensional/quasi-two-dimensional structure can significantly improve long-term stability.However,because the carrier transmission in the two-dimensional/quasi-two-dimensional perovskite structure is severely hindered accompanied with wide bandgap,the device efficiency of the two-dimensional/quasi-two-dimensional perovskite materials is lower than that of three-dimensional perovskite materials.Therefore,how to effectively improve the device efficiency of two-dimensional/quasi-two-dimensional perovskite solar cells has become a key issue.This article mainly focuses on the research of planar inverted(p-i-n)quasi-two-dimensional perovskite solar cells and improving the device efficiency of quasi-two-dimensional perovskite through band gap engineering.The specific work content is shown as follows:(1)Using fluorophenethylamine ions(FPEA⁺)as organic spacer cations to fabricate pure methylamine(MA)quasi-two-dimensional perovskite solar cells and and reducing the band gap of quasi-two-dimensional perovskite by introducing formamidine cation(FA⁺)to improve device performance.FPEA⁺cation is selected as organic spacer cation to fabricate quasi-two-dimensional perovskite solar cell(FPEA)₂MA₄Pb₅I₁₆.Finally,a perovskite solar cell with power conversion efficiency of 17.3%is obtained.Based on this,formamidine hydroiodide(FAI)and Formamidine hydrochloride(FACl)are used to introduce formamidine cation(FA⁺)into pure MA quasi-two-dimensional perovskite (FPEA)₂MA₄Pb₅I₁₆to form MAFA mix cation quasi-two-dimensional perovskite.The experimental results show that:(1)to FAI,when the molar ratio of FAI to MAI(methyl ammonium iodide)is 3:7,the obtained quasi-two-dimensional perovskite solar cell(FPEA)₂(FA_0.3MA_0.7)₄Pb₅I₁₆has an optimal device efficiency of 20.21%.It is proved that the improvement of power conversion efficiency originates from narrower band gap of the quasi-two-dimensional perovskite after the introduction of FA⁺ions.(2)to FACl,when the molar ratio of FACl additive and MAI is 3:10,the obtained quasi-two-dimensional perovskite solar cell has an optimal device efficiency of of 20.37%.Experimental results show that the Cl^-anions in FACl can optimize the crystal growth orientation during the perovskite crystallization process and improve the carrier transmission;the FA⁺cations in FACl can replace MA⁺cations in(FPEA)₂MA₄Pb₅I₁₆and form a FAMA mixed cation quasi-two-dimensional perovskite with a narrower band gap,the process of which is called cation exchange.Furthermore,the molar ratio of FA⁺ions in the FACl quasi-two-dimensional perovskite is determined by comparing the photoelectric performance and band gap of the FACl-based quasi-two-dimensional perovskite solar cell with the FAI-based quasi-two-dimensional perovskite solar cell and nuclear magnetic resonance is used to prove the rationality of this method.In addition,the optimal quasi-two-dimensional perovskite solar cells based on FACl have been tested for long-term stability under the condition of 80±5?and relative humidity RH=80%±5%,and can maintain more than 90%of the initial efficiency after 1200 hours,which proves the excellent long-term stability.(2)Reducing the thickness of the organic spacer cation layer to reduce the band gap of the quasi-two-dimensional perovskite and improving the power conversion efficiency by introducing short-chain guanidine cations(GA⁺).The short-chain organic ammonium salt guanidine iodide(GAI)is introduced into pure phenethylamine(PEA)quasi-two-dimensional perovskite(PEA)₂MA₄Pb₅I₁₆to form a quasi-two-dimensional perovskite with mixed organic spacer cations.By optimizing the molar ratio of GA⁺cations and PEA⁺ions,an optimal quasi-two-dimensional perovskite solar cell with power conversion efficiency of 15.27%is obtained under the condition of GA⁺:PEA⁺=0.2:0.8.In contrast,the power conversion efficiency of the optimal quasi-two-dimensional perovskite solar cell based on PEA⁺single organic spacer cations is only 11.58%.The experimental results show that the increase of power conversion efficiency after the introduction of GA⁺ions comes from the following two causes:(1)because the size of GA⁺cations is smaller than phenethylamine ions(PEA⁺),the introduction of GA⁺cations can reduce the thickness of the organic spacer cation layer to narrow the band gap of the quasi-two-dimensional perovskite;(2)compared with the quasi-two-dimensional perovskite with single PEA⁺spacer cation,the introduction of GA⁺cations can increase the absorption coefficient of the quasi-two-dimensional perovskite in the absorption wavelength range.In addition,the optimal quasi-two-dimensional perovskite solar cell based on PEA⁺and GA⁺mixed organic spacer cations can maintain more than 80%of the initial efficiency after 800 hours under the condition of relative humidity RH=55%?60%,which proves the excellent long-term stability.