Regulating Interfacial Charge Transport Of Perovskite Solar Cells

Solar energy,as a clean energy source,has attracted wide attention in recent years.As one of solar energy conversion devices,organic-inorganic hybrid perovskite solar cells(PSCs)have gained rapid development during the past decade due to their advantages of simple preparation,high efficiency and low cost,and compatibility with flexible substrates.The current certified power conversion efficiency(PCE)of single-junction perovskite solar cells is 25.5%,which is almost comparable to the PCE of commercial crystalline silicon cells.Even so,several critical issues such as PCE,stability,toxicity and so on still hamper the commercialization road of PSCs.The interfaces between perovskite/charge transport layer determine energy level alignment,the charge transfer as well as the defect passivation,which has an important influence on the efficiency and stability of the device.Therefore,to regulate the interface has proved necessary in improving device performance.On the other hand,considering hazard to human and environment caused by lead leakage,to exploit lead-free perovskites solar cells is highly desired.Hence,we carried out the following researches focusing on improving the efficiency and stability of cell devices by regulating the interface charge transfer of organic-inorganic hybrid and lead-free perovskite solar cells.:1.Titanium oxide(TiO2)is one of the most commonly used electron transport layers of regular-structure planar heterojunction(PHJ)PSCs,but TiO2 also suffers from some inherent shortcomings such as low carrier mobility,high density of electron defect states and high catalytic activity under ultraviolet light.Herein,the amino-functionalized TiO2 nanoparticles(abbreviated as NH2-TiO2 NPs)were successfully in situ synthesized at low temperature by doping urea as a precursor during the process of preparing TiO2 nanoparticles via a simple one-step non-hydrolytic method.A series of characterizations shows that the introduction of urea can accelerate the nucleation and growth of TiO2 during the alcoholysis process,thus improving the crystallinity of TiO2.At the same time,the amino group(-NH2)bonding onto the surface of TiO2 NPs via Ti-N bonds can passivate the oxygen vacancy defect of TiO2,reduce the work function of TiO2 and improve the interface contact between perovskite and TiO2 layer.Moreover,the amino coordinated with Pb2+of perovskite may increase the nucleation site of perovskite growth,improve the quality of perovskite film,and effectively passivate the defects of perovskite.As a result,Cs0.05FA0.83MA0.12-PbI2.55Br0.45 PHJ PSCs based on NH2-TiO2 ETL obtain the best PCE of 21.33%,which is significantly improved compared with 19.82%PCE of the control devices.In addition,the passivation of perovskite defects upon amino functionalization also inhibits the hysteresis of current-voltage curves and improves the environmental stability of the devices.2.We prepared in-situ surface fluorinated TiO2(F-TiO2)nanoparticles(NPs)with TiF4 as a fluorine source,and incorporated F-TiO2 NPs as ETL in n-i-p PHJ PSCs,affording PCE of 22.68%,which is among the highest PCEs for PHJ PSCs based on low-temperature solution-processed TiO2 ETLs.Fluorine ion(F-)bonded to the(001)surface of anatase TiO2 by the Ti-F bond increases the electron mobility,reduces the density of the oxygen vacancy defect,and inhibits the photocatalytic activity of TiO2.The bonded F-on the surface of TiO2 interacts with perovskite to form Pb-F bonds and hydrogen bonds,reinforcing interface binding of K0.025Cs0.05FA0.83MA0.12PbI2.55Br0.45 perovskite layer with TiO2 ETL and passivating the defect states on the surface of perovskite film.In addition to improving the PCE of the device,fluorinated TiO2 can also improve the environmental and thermal stability of the device.In particular,the illumination stability of the device under ultraviolet light is enhanced due to the inhibited photocatalytic activity of F-TiO2.3.We constructed novel lead-free bulk heterojunction(BHJ)Bi-based solar cells with in-situ phase-separated Cs3Bi2I9 and Ag3Bi2I9 components as the photoactive layer,affording 3.59%PCE and best Voc reaching 0.89 V.Compared to the single-component CS3Bi2I9 or Ag3Bi2I9 devices,the performance of BHJ devices is significantly improved.A series of characterizations show that the formation of BHJ not only increases the grain size of Cs3Bi2I9,but also optimizes grain orientation of Ag3Bi2I9.More importantly,Cs3Bi2I9 and Ag3Bi2I9 form Type-II energy level structure,promoting exciton separation and accelerating carrier interface transport.In addition to the significant improvement of PCE,the BHJ device also shows excellent thermal stability,remaining～90%of the initial PCE after 450 h heating at 85 ℃ in a nitrogen atmosphere.Moreover,the universality of BHJ concept in boosting device performance of perovskite solar cells based on other reported AgxBiyIx+3y light-absorbers is verified.