Defect Passivation And Interface Energetics Modulation In Perovskite Solar Cells

As an emerging and very promising photovoltaic technology,perovskite solar cells have received extensive attention from all walks of life.After intensive research in material design,preparation process,device structure and interface engineering in recent years,perovskite solar cells have achieved great improments.At present,the certified efficiency has reached 25.7%,standing out among many photovoltaic technologies.Despite the rapid development of perovskite solar cells,the unavoidable large number of defects in perovskite and the undesired interfacial properties in the device will cause severe recombination losses,which still seriously restrict the progress of the device efficiency to the theoretical limit.Moreover,they also have far-reaching influence on the device stability.Therefore,to realize the full thermodynamic potential and commercial application of perovskite solar cells,we focus on the defect passivation and interface energetics modulation of perovskite photovoltaic devices.(1)A strategy to improve the interface properties of PCBM/Al cathode by using low-cost and air-stable dye molecule(Isatin)and its derivatives(Isatin-Cl)as the interlayer was proposed.Isatin and Isatin-Cl can adjust the work function of the back-metal electrode and modify the interface energetics of PCBM/Al,thereby improving the electron transport and collection efficiency at the cathode interface and reducing the interfacial recombination loss.Besides,Isatin and Isatin-Cl can improve the hydrophobicity of PCBM,which slows the invasion of water into the device interior and the degradation of perovskite.Therefore,the efficiency and stability of devices are significantly improved.(2)The mechanism of the dye molecule(Isatin-Cl)as perovskite precursor additive to improve the device peroformance was revealed.Isatin-Cl can share lone pairs of electrons with perovskite through its carbonyl group,which passivates uncoordinated Pb²⁺and leads to more n-type perovskite film,greatly reducing the probability of defect-induced recombination and improving all performance parameters of perovskite photovoltaic devices simultaneously.Moreover,the hydrophobic Isatin-Cl enhances the water resistance of the perovskite film and suppresses ions migration through hydrogen bonding,which strengthens the crystal structure of perovskite and further increases the operating lifetime of the device.(3)The importance of developing green and non-toxic additives was emphasized,in line with the environmental-friendly and sustainable development goals of solar cells.Betulin,a woody material,was used as additive to achieve the improvement of both efficiency and stability of devices.It is found that Betulin can effectively regulate the crystallization kinetics of perovskite while passivating defects through coordination effect,which slows down the growth rate of polycrystalline perovskite and realizes the formation of high-quality perovskite film with strong crystallization,large grain size and few defects.Hence,the carrier recombination losses are greatly reduced.In addition,Betulin inhibits the invasion of water and oxygen molecules into the perovskite and strengthens the perovskite crystal structure,attributed to its strong hydrophobicity,passivation effect and hydrogen bonding with perovskite.Consequently,the device achieves a high efficiency of 21.15%and excellent stability.(4)A complete transformation of perovskite surface region energetics from p-to n-type during defect passivation caused by natural additive capsaicin was realized.This is originated from the charge transfer during defect passivation by Capsaicin,which induces the spontaneous formation of a p-n homojunction on the perovskite surface.The synergistic effects of defect passivation and energetics transformation promote charge transport in bulk perovskite layer and at perovskite/PCBM interface,suppressing both defect-assisted recombination and interface recombination.Consequently,the device achieves an efficiency of 21.88%and a fill factor of 83.81%,both values are the highest records for polycrystalline MAPb I₃ based p-i-n PSCs when reported.At the same time,the stability of perovskite devices is also significantly improved due to the improved hydrophobicity,reduced defect density,and enhanced crystal structure of perovskite films.(5)A strategy to induce p-type modification of surface energetics while passivating perovskite defects using a single additive,pentafluorophenol acrylate(PFPA),was reported.The surface-aggregated PFPA can increase the work function of the perovskite and form an upward band-bending at the perovskite surface,which promotes the interfacial energy level alignment between perovskite and hole transport layer,thus facilitating hole extraction efficiency and suppressing interface carrier recombination.Meanwhile,the strong electronic interaction between PFPA and perovskite passivates the defects in perovskite and enhances the crystallinity and grain size of perovskite films,reducing the traps-mediated recombination losses.In addition,the hydrogen bonding effect of PFPA further inhibits the volatilization and migration of MA⁺/FA⁺ions and enhances the stability of the perovskite crystal structure.further reducing trap-assisted recombination losses.Therefore,the device achieves a PCE of 22.42%and a V_oc of 1.193 V,which are among the highest efficiency and the highest photovoltage of polycrystalline Cs FAMA based n-i-p PSCs using low-cost silver electrode when reported.Moreover,PFPA additive shows similar synergistic effects in the MAPb I₃perovskite,demonstrating the universality of this strategy.