Crystallinity,Dimensionality,and Phase Modulation Of Perovskite Light-Absorbing Layer In Perovskite Solar Cells

Solar cell is one of the most effective way to convert solar energy into electricity.Among the third-generation solar cells,perovskite solar cells(PSCs)have attracted lots of attention due its high power conversion efficiency(PCE)and easy fabrication and so on.As the most commonly used light absorber layer in PSCs,CH3NH3PbI3(abbreviate as MAPbI3)owns several unique advantages such as high carrier mobility,long carrier diffusion length,large extinction coefficient and tunable band gap,etc.,rendering fascinating photovoltaic properties of PSCs,which fulfills the requirement of future industrialization of solar cells.After ten years of development,the commercialization of PSCs is finally accessible,however,in order to meet the requirement of practical application,the PCE and stability of PSCs demands further enhancement.From the viewpoint of carrier generation,PCE is highly related to the quality of light-absorbing layer.By improving the quality of MAPbI3 film,passivating trap-states and suppressing carrier recombination,PCE of PSCs can be effectively enhanced.On the other hand,the inferior stability of PSCs mainly originates from the intrinsic vulnerability of 3-dimensional(3 D)perovskite layer to H2O,through introducing hydrophobic functional groups into perovskite layer to increase the hydrophobicity of perovskite is supposed to be an effective way to promote device stability.Considering the present research status and future trend of PSCs,we focused on enhancing the PCE and stability of PSCs through modulating crystallinity,dimensionality and phase composition of perovskite layer and carried out the following works:(1)TiO2 is the most commonly used electron transporting layer(ETL)in n-i-p planar heterojunction(PHJ)PSCs,still it has inherent drawbacks such as low electrical conductivity and electron mobility,high electronic trap states density,etc.,leading to inefficient charge extraction.Meanwhile,as the substrate of perovskite film,the wettability of perovskite precursor atop TiO2 is crucial for the final crystallinity of perovskite film.In this work,by successively modifying the TiO2 compact layer with hydrophobic fullerene derivative[6,6]-phenyl-C61-butyric acid methyl ester(PC6,BM)and another hydrophilic ethanolamine(ETA)-functionalized fullerene derivative(C60-ETA),the PCE of the MAPbI3-based PHJ-PSC devices increased from 15.52%to 18.49%.PC61BM layer could passivate the traps on the TiO2 surface,while the subsequent C60-ETA layer improved the wettability of the perovskite precursor atop TiO2/PC6iBM substrate,thus promoted the crystallinity of perovskite film and facilitated electron transport across the interface between the perovskite and the TiO2 ETL.As a consequence,based on the synergistic effect of these two fullerene derivatives led to enhanced PCE of PSCs with dramatically suppressed current-voltage hysteresis,(2)Solution-processed MAPbI3 film usually suffers from high trap density with unbalanced carrier mobility.Herein,we reported the synthesis of a novel pyridine-functionalized fullerene derivative called C60-PyP via a 1,3-dipolar cycloaddition reaction and employed it as additive in n-i-p PSCs,Upon incorporating C60-PyP with an optimized doping ratio of 0.13 wt%into the MAPbI3 perovskite precursor,PCE of PSC devices can be enhanced from 17.61%to 19.82%with suppressed J-V curve hysteresis and improved ambient stability.During the crystallization process of perovskite film,C60-PyP could act as heterogeneous nucleation sites and decreased the nucleation Gibbs free energy,leading to improved crystallization and orientation of MAPbI3 film with enlarged grain size.Meanwhile,due to the lone pair electrons pyridine owns,pyridine moiety of C60-PyP could coordinate with un-coordinated Pb2+within MAPbI3 film,leading to passivated trap states and suppressed non-radiative recombination process.Besides,owing to the strong electron-accepting ability of C60-PyP,the electron mobility of perovskite film can be effectively enhanced after incorporation of C60-PyP,leading to facilitated electron transfer and transport,thus contributed to improved device performance.(3)Solution-processed fabrication method usually affording MAPbI3 films with random crystal orientation and high trap density caused by un-coordinated Pb2+ and free I-.Herein,we synthesized an imidazole sulfonate zwitterion[4-(1H-imidazol-3-ium-3－yL)butane-1-sulfonate,IMS]through reaction between imidazole and 1,4-butane sultone.After incorporating IMS as an additive into perovskite precursor,the crystallization rate of MAPbI3 film can be effectively slowed down due to the coordination effect between Pb2+ within perovskite precursor and SO3-of IMS,yielding highly ordered MAPbl3 film with improved morphology and crystallinity.Moreover,the positive-charged imidazole of IMS could passivate the free-ion of MAPbI3 via electrostatic interaction,while negative-charged SO3-of IMS could coordinated with Pb2+ ion thus passivate iodine vacancy defects,fulfilling a dual-effect of crystallization regulation and defect passivation of IMS.As a result,under an optimized IMS doping ratio of 0.5 wt%,the PSC device exhibited a significant increase in PCE from 18.77%to 20.84%with suppressed current-voltage hysteresis and promoted ambient stability.Moreover,a high Voc of 1.208 V can be achieved under a higher IMS doping ratio of 1.2 wt%.(4)The stability of PSCs has been a big obstacle for its commercialization.The inferior stability of PSCs mainly originates from the intrinsic vulnerability of 3D perovskite layer to H2O,so the stability of PSC devices can be effectively promoted through increasing the hydrophobicity of perovskite film.In this work,we synthesized a bis-adducted amino-functionalized fullerene derivative named bis-C60-PN through 1,3-dipolar cycloaddition,then we incorporated it as A-site ion into MAPbl3 after iodine-ionized with HI and in-situ formed 2D/3D hybrid perovskite.By employing bis-C60-PNI/chlorobenzene solution as anti-solvent,we incorporated bis-C60-PNI into MAPbI3 layer through drop-casting process and formed(bis-C60-PN)MAn-iPbnI3n+i 2D/3D hybrid perovskite with gradient-distributed n value after thermal annealing.For the formed perovskite film,2D/3D perovskite mainly located at the upper part while the underlying layer is still 3D perovskite.The carrier concentration difference between 2D and 3D perovskite led to the increase of built-in electric field in PSCs,facilitating charge separation and transport process.Besides,bis-C6o-PN2+ could combine two[PbI6]4-octahedral through C60,promoting charge transport process and electron extraction from perovskite to PC61BM,thus improved device performance.Meanwhile,the upper 2D/3D perovskite with hydrophobic bis-C60-PN2+ could protect the underlying 3D perovskite from H2O and O2,leading to enhanced device stability.Consequently,the PCE of 2D/3D-3D PSCs can be promoted from 17.60%for 3D PSCs to 19.44%with～10.5%enhancement.We further stored the unencapsulated PSC under 40%humidity for 1700 hours,the PCE of 3D PSCs dropped by～50%after 1700 h storage,while the 2D/3D-3D device showed a PCE drop of only～20%under the same condition,indicating the greatly improved ambient stability of 2D/3D-3D PSCs through dimensionality modulation.(5)Two-step-fabricated MAPbI3 absorber layer usually suffers from small crystalline grain size with high trap density.It is reported that incorporation of chlorine(Cl)could assist the crystallization of perovskite film,however,in most cases,a predominant MAPbI3 phase with trace of Cl-is obtained ultimately.Herein,we developed a low-cost and facile method,named hydrochloric acid vapor annealing(HAVA)post-treatment,and realized a rapid conversion of MAPbI3 to phase-pure MAPbCl3.Under the optimized HAVA post-treatment time(2 min),we achieved a significant enhancement of PCE of MAPbI3-based PHJ-PSC device from 14.02 to 17.40%(the highest PCE reached 18.45%)with greatly suppressed hysteresis of current-voltage response.The average grain size of perovskite film after HAVA post-treatment increased remarkably through Ostwald ripening process,leading to a denser and smoother perovskite film with reduced trap states and enhanced crystallinity.More importantly,the generation of MAPbCl3 secondary phase via phase engineering was beneficial for improving the carrier mobility with a more balanced carrier transport rate and enlarging the band gap of perovskite film along with optimized energy level alignment,contributing to enhanced Voc and Jsc.The existence of MAPbCl3 further suppressed the decomposition of MAPbl3 perovskite and improved device stability.