Study On Key Materials And Interfaces For Efficient And Stable SnO₂-based Perovskite Solar Cells

The hybrid organic-inorganic perovskite solar cells（PSCs）show huge potential applications in the new-generation photovoltaics,due to their excellent optoelectronic properties,low fabrication cost,and high power conversion efficiency（PCE）.Although a certified PCE of 24.2%is achieved,there still remains some challenges,such as device stability,scalable fabrication of high-quality perovskite films and its corresponding device,repeatability of high-efficiency device.In this dissertation,we aim at fabricating highly efficient and stable PSCs,mainly involving modification of charge（electron and hole）transport layers,improving the quality of perovskite layer via surface-interface engineering strategy.The typical research achievements are summarized as following:（1）Yttrium-doped tin dioxide（Y-SnO₂）electron transport layer（ETL）synthesized by an in situ hydrothermal growth process can signicantly reduce the hysteresis and improve the performance of PSCs.Comparison studies reveal two main effects of Y doping of SnO₂ ETLs:（1）it promotes the formation of well-aligned and more homogeneous distribution of SnO₂ nanosheet arrays（NSAs）,which allows better perovskite infiltration,better contacts of perovskite with SnO₂ nanosheets,and improves electron transfer from perovskite to ETL;（2）it enlarges the band gap and upshifts the band energy levels,resulting in better energy level alignment with perovskite and suppressed charge recombination at relevant interfaces.As a result,PSCs employing Y-SnO₂ ETL show a PCE of 17.29%without J-V hysteresis.（2）3-aminopropyltriethoxysilane（APTES）self-assembled monolayer（SAM）was used to modify the SnO₂ ETL/perovskite layer interface.This APTES SAM demonstrates multiple functions:（1）it can increase the surface energy and enhance the affinity of the SnO₂ ESL,which induce the formation of high quality perovskite films with a better morphology and enhanced crystallinity.（2）Its terminal functional groups form dipoles on the SnO₂ surface,leading to a decreased work function of SnO₂ and enlarged built-in potential of SnO₂/perovskite heterojunctions.（3）The terminal groups can passivate the trap states at the perovskite surface via hydrogen bonding.（4）The thin insulating layer at the interface can hinder electron back transfer.With these desirable properties,the best-performing cell employing APTES SAM modified-SnO₂ESL achieved a PCE over 18%and a steady-state efficiency of 17.54%.Impressively,to the best of our knowledge,the obtained V_OC of 1.16 V is the highest value reported for the CH₃NH₃PbI₃（MAPbI₃）system.（3）A facile yet effective two-step method,i.e.,room-temperature colloidal synthesis and low-temperature removal of additive（thiourea）,to control the carrier concentration of SnO₂ quantum dot（QD）ETLs to achieve high-performance PSCs is developed.By optimizing the electron density of SnO₂ QD ETLs,a champion stabilized power output of 20.32%for the planar PSCs using triple cation perovskite absorber and a 19.73%of PCE for those using CH₃NH₃PbI₃ absorber is achieved.The superior uniformity of low-temperature processed SnO₂ QD ETLs also enables the fabrication of¹9%efficiency PSCs with an aperture area of 1.0 cm² and 16.97%efficiency flexible device.（4）We report a new HTM of copper（II）phthalocyanine with octamethyl-substituted function groups（CuMe₂Pc）.Unlike the normal edge on orientation of pristine copper（II）phthalocyanine（CuPc）,we found that CuMe₂Pc could form face-on molecular alignment when deposited on perovskite via vacuum thermal evaporation,resulting in higher hole mobility,more condense thin film structure and more hydrophobic surface.These properties are more favorable for hole transport and moisture resistance applications in PSCs.PSCs with planar structure were fabricated and tested,utilizing different phthalocyanines and Spiro-OMeTAD as HTMs.PSCs with CuMe₂Pc showed 25%higher PCE compared with those with CuPc.Furthermore,beneficial from the hydrophobic nature of CuMe₂Pc,the devices with CuMe₂Pc as HTM show improved stability and retained over 95%of their initial efficiencies even after storage in the humidity about 50%for 2000 h without encapsulation.（5）An organic semiconducting nonfullerene acceptor（NFA）molecule IT-M is introduced as Lewis base to improve perovskite interfaces.This facile Lewis base-assistedstrategyeffectivelyenhancestheelectronicpropertiesof Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ thin films,passivates the perovskite surface defects and further boosts the performance of perovskite solar cells.With the reduced surface defects,this approach significantly enhances the charge transport properties and boosts the photogenerated photoluminescence lifetime from 1.46 to 2.20?μs.As a result,the n-i-p planar perovskite solar cell champion efficiency reaches 20.5%with a high FF of 81%.Due to the hydrophobic property of IT-M,the IT-M passivated perovskite device show enhanced stability under moisture and thermal stress.In this thesis,we aim at improving both the performance and stability of perovskite solar cells via systematically optimizing the properties of each functional layers and their relative interfaces.In addition,we also investigated their corresponding enhancement mechanism,which will provide guidance to further improve the performance and stability of perovksite solar cells.