Controllable Fabrication Of Organolead Halide Perovskite Thin Films For Solar Cells

Solar cells are the effective strategy to resolve the increasingly serious energy and environmental problems,which can convert solar energy into electric energy.Design and preparation of new photovoltaic materials with high photovoltaic performance,low cost and long stability are the key to the fabrication of high-performance solar cells.Organolead halide perovskites is one kind of high-performance photovoltaic materials due to its tunable narrow band gap,high light harvesting coefficient,low photo-exciton binding energy,high carrier mobility and long balanced carrier diffusion length.Perovskite solar cells（PSCs）have become a hot topic worldwide.As the light-harvester in PSCs,perovskite thin film not only absorbs sunlight,but also completes photogenerated carrier separation and transport.Therefore,high-quality perovskite thin film is extremely important to enhance photovoltaic performance of PSCs.However,in the early works,the quality of perovskite thin film restrict the device performance.Thus,it has become a hotspot to prepare perovskite thin film with pure phase structure,high crystallinity,large grain size and high surface coverage.We follow closely the frontier of international research in this work,which investigated the influence of the process conditions and PbI2 precursor microstructure and crystallinity,and a new BaSnO3 electron transporting material with high carrier mobility on the controllable fabrication of perovskite thin film and its photovoltaic properties from the standpoint of film growth and morphology control,and optimized electron transporting material to fabricate high-performance PSCs.Finally,through controlling the morphology and quality of perovskite thin film and optimizing electron transporting layer,power conversion efficiency（PCE）of PSCs has been significantly improved,which paves the way for the fabrication and mechanism research of high-performance PSCs.Four major conclusions are summarized as below:（1）Following by the two-step dipping method,the growth and morphology of perovskite thin film can be well controlled through the introduction of polar solvent DMF into CH₃NH₃I（MAI）solution.The addition of polar solvent DMF can“activate”the PbI2 precursor and facilitate the conversion of the compact PbI2 precursor to MAPbI3,leading to the formation of uniform,pinhole-free and pure phase structured CH3NH3PbI3（MAPbI3）thin film with no PbI2 residual.Manipulating the ratio of DMF and dipping time can effectively tune the MAPbI3 film growth and morphology.In addition,the dissolution-recrystallization mechanism of MAPbI3 thin film formation during polar solvent DMF assisted fabrication has been revealed.High-quality perovskite thin film with no PbI2 residual,large grain size and complete surface coverage has been prepared with 1%DMF and 2 min dipping time.PCE of this DMF-based device increased nearly by 42%with respect to that of the device without DMF addition.（2）In order to further implement the control of film growth and morphology and the controllable fabrication of high-quaility MAPbI3 thin film,following by two-step spin-coating method,the addition of polymer P（VDF-TrFE）into PbI2 precursors can successfully regulate the final perovskite film morphology and crystalline.The polymer P（VDF-TrFE）can effectively retard crystallization of PbI2 and control the microstructure of PbI2 thin film,leading to the formation of low crystallinity PbI2precursor with tiny-holes surface.This PbI2 precursor can be further converted into high-quality MAPbI3 thin film with large grain size,interconnected crystallites structure,high crystallinity and uniform pinhole-free surface.MAPbI3 thin film prepared from 1.0 mg/mL P（VDF-TrFE）addition exhibits prolong carrier lifetime,carrier lifetime increase from 37.3 ns（no P（VDF-TrFE））to 60.6 ns（with P（VDF-TrFE））.Analysis of TR-PL also show that the perovskite thin film prepared with the addition of P（VDF-TrFE）exhibits more effective charge separation and extraction character.PCE of the champion PSCs fabricated from P（VDF-TrFE）additives can reach up to 13.24%with stable power output of 12.83%.（3）Apart from the strategies mentioned above,an annealing-free,wet PbI2 thin film（w-PbI2）can serve as effective precursor to be converted into high-quality perovskite thin film,the corresponding PSCs has best PCE of 15.49%with high stable power output of 15.08%.The formation mechanism of MAPbI3 from wet PbI2 can be explained as follow:due to the higher binding capacity of MAI to Pb²⁺than that of DMF solvent molecules,MAI can in-situ substitute DMF molecules in w-PbI2 to assemble MAPbI3.Such formation mechanism of in-situ substitution can not only facilitate the complete conversion of PbI2 into MAPbI3,but also well retain the uniform surface of PbI2 precursor,leading to the controllable formation of MAPbI3.Meanwhile,compared with MAPbI3 prepared from regular compact and dry PbI2 precursor,w-PbI2-based MAPbI3 exhibits high crystallinity,large light harvesting,no PbI2 residual and ultra-smooth surface structure.In addition,analysis of TR-PL also show that the MAPbI3 thin film prepared with w-PbI2 precursor exhibits longer slower carrier lifetime（16.4 ns）,indicating its lower non-radiative recombination along with high film quality.（4）For the fabrication of high-performance PSCs,well-dispersed BaSnO3nanoparticles（BSO NPs）has been designed and synthesized,and first solely used as electron transporting material in planar PSCs,replacing traditional electron transporting material of TiO2.BaSnO3 is an indirect band gap semiconductor with a highly dispersed band structure,high charge mobility and an inferior UV photocatalytic ability.The result shows that high coverage of BSO NPs on FTO substrate and suitable thickness of BSO layer are the key to fabricate high-performance BSO-based planar PSCs.High coverage of BSO NPs on FTO substrate can obviously restrict the contact of MAPbI3with FTO substrate,leading to the reduction of shunting path and current leakage,and the enhancement of fill factor in PSCs;suitable thickness of BSO layer can effectively reduce the series resistance of PSCs,and lead to improving short-circuit photocurrent.Compared with the conventional TiO2,due to the lower conduction band position and high carrier mobility of BSO material,the photogenerated carrier can more easily inject into conduction band of BSO and fastly transport in BSO layer,which further leading to higher photovoltaic performance of BSO-based planar PSCs.PCE of best TiO2-based planar PSC is only 9.36%,PCE of BSO-based planar PSC can reach up to 10.96%in this work.To the best of our knowledge,this is so far the best reported conversion efficiency of BSO-based planar MAPbI3 perovskite solar cells.