Preparation Of High-efficiency Perovskite Solar Cells By Interface Treatment

Surface and interface play a very critical role in improving the performance of perovskite solar cell devices.It is well known that metal halide perovskite solar cells based on p-i-n junctions have a complex multilayer interface structure,which makes the interface a key factor that limits device performance and stability.The crystal structure,surface morphology,and chemical composition of the perovskite light-absorbing layer and electron transport layer affect the electronic structure of the interface,which will change the behavior of photogenerated carriers at the interface,thereby affecting the performance of the solar cell device.Therefore,the surface modification of the light-absorbing layer or the electron transport layer is very important for the improvement of the device performance.Based on the above,methods for controlling the behavior of photo-generated carriers and improving device performance by adjusting the surface properties of semiconductor crystals at the interface are discussed in this paper.The specific research content is as follows:（1）The optimal alignment of semiconductor’s electronic energy level with respect to hole-transporting materials greatly affects the photovoltaic conversion efficiency of perovskite solar cells.The energy level of perovskite can be greatly affected by the crystal surface termination state formed in the preparation process of perovskite polycrystalline film.Perovskite featured with Pb-X termination always bears deep energy level which cannot match well HTM.Herein,to regulate the surface termination state of the perovskite crystals,a controlled solvent atmosphere of dimethyl sulfoxide and N,N-dimethylformamide was employed to induce the regrowth of perovskite crystals.Through the experimental results,it is proved that the mixed solvent of DMSO and DMF can induce the transformation of perovskite crystals from Pb-X termination to A-X termination.Density functional theory calculations show that the terminated conversion moves the valence and conduction band edge（VBE and CBE）of perovskite crystal to a shallower energy level,which does great favor to match the energy level of HTM,and then contributes to extracting charges.In addition,the defect density of the optimized perovskite polycrystalline film is decreased from 1.258×10¹⁶ to1.022×10¹⁶ cm^-3,and the depletion width of PSCs increased from 48.9 to 56.1nm.The open circuit voltage of the PSC is increased from 0.99 to 1.09 V,and the conversion efficiency is boosted from 17.8%to 20.3%.（2）The lattice mismatch at the contact interface between the perovskite absorber layer and the Sn O₂ electron transport layer in the PSC device accumulates high-density trap states.They not only affect the charge transport at the interface,but may also cause ion migration and make the perovskite film easy to interact with water and oxygen to cause device degradation.The inorganic compound magnesium potassium chloride（KMg Cl₃）was introduced into the Sn O₂ water-soluble colloidal dispersion to prepare the electron transport layer Sn O₂-KMg Cl₃.The presence of KMg Cl₃ improves the dispersion state of Sn O₂ colloid and enhances the conductivity of the Sn O₂-KMg Cl₃electron transport layer.At the same time,K⁺and Cl^-have a certain passivation effect on the defects at the bottom of the perovskite film,which greatly reduces the grain boundaries of the high defect concentration in the perovskite crystal(from 3.54×10¹⁵cm^-3 to 3.05×10¹⁵ cm^-3),and the interface carrier recombination is reduced.In addition,KMg Cl₃ improves the energy level alignment at the perovskite/ETL interface.Finally,the device based on KMg Cl₃ modified Sn O₂ can achieve an open circuit voltage of 1.09V and a photoelectric conversion efficiency of up to 20.11%.（3）The high concentration of defects on the interface between the perovskite and the Sn O₂ electron transport layer limits the marginal improvement,increases the open circuit voltage loss,and reduces the photoelectric conversion efficiency of PSC devices.By modifying Sn O₂ with KI as the electron transport layer of the planar perovskite solar cell,I^-and K⁺will diffuse to V_I-and V_MA+on the interface of the perovskite crystal,passivating defects,and make the electron trap density of the perovskite film change from 3.84×10¹⁵ cm-3 to 3.15×10¹⁵ cm-3,which greatly inhibits the interface charge recombination.At the same time,the presence of KI improves the energy level alignment between the conduction band of perovskite and Sn O₂ and reduces the energy barrier for electron extraction.Eventually,the open circuit voltage of the PSC device was increased to 1.13 V due to these two effects,and the PCE was increased from 19.4%to 22.2%.