| Because of its excellent photoelectric properties,the power conversion efficiency(PCE)of perovskite solar cells(PSCs)has been increased to 25.2%in just a few years,which is close to the certified efficiency of the best monocrystalline silicon cells(26.7%),and the efficiency of silicon/perovskite tandem cells has exceeded 29.1%,e showing great potential for the perovskite materials..At present,perovskite solar cells with high efficiency are based on organic-inorganic hybrid films.At the module level,high effificiencies,low manufacturing costs,excellent stability against humidity,heat and l ight soaking need to be fully considered.However,organic cations in organic-inorganic hybrid perovskite films can be readily decomposed by H2O in the atmosphere nm,resulting in the decomposition of films.In addition,high temperature duration could also lead to a collapsed perovskite structure.Recently,all-inorganic CsPbX3(X=Cl,Br,and I)perovskite filmshave received great attention due to its excellent stability against H2O and heat duration.Among them,the CsPbI2 material is promising to fabricate higher PCE of inorganic solar cells due to its suitable band-gap(?1.73 eV).However,the photovoltaic property of CsPbI3 is cubic phase,which only be prepared at more than 300?,and it can transform to a thermodynamic ?-CsPbI3 at ambient temperature.For the above reasons,the development of CsPbI3 PSCs is relatively lagging behind.Hence,a higher CsPbI3 PCE with stabilized output is an urgent issue to be solved.Theoretical calculations shows that there are two metastable phases of CsPbI3 named ?and y phase formed at 260? and 175?,respectively.The two phases are black with excellent photovoltaic properties and can be stable at room temperature.But the metastable phase-based CsPbI3 solar cells are less reported in the experiment.Therefore,if the ?-CsPbI3 and ?-CsPbI3 thin films can be successfully prepared,it may be a breakthrough in compatibility of high PCE and phase stability,and this dissertation is structured in 4 sections,with the following details:(1)Fabrication of metastable CsPbI3 phase and used in perovskite solar cells.Herein,we first obtain the "HPbI3+x" intermediate with faint yellow by reaction of PbI2 and HI solution in dimethylformamide(DMF)solvent.Then metastable black-phase CsPbI3 film is successfully prepared through one-step coating method.We found the band gap of metastable CSPbI3 is about1.69 eV,which lower than that of ?-CsPbI3(?1.73 eV).The PCE of metastable CsPbI3 PSCs is 11.5%and demonstrated poor stability with PCE dEgradation to 0.65%after storage in ambient enviro nment for 8 days.Subsequently,PEAI additive was added into CsPbI3 precursor solution to optimize the CsPbI3 film,which contributes to the oriented growth of crystals and can improve the stability of the CsPbI3 films.Finally,a energy level matched solar cell was designed and corresponding PCE is up to 15.07%,which was one of the highest PCE of CsPbI3 solar cells at the time.Surprisingly,the devicemaintained 92%of initial PCE after storing in air for 60 days and the light soaking and thermal stability were allsignificantly improved.The reported metastable CsPbI3 phase has injected new ideas and dynamics into the development of inorganic perovskite solar cells.(2)We exploited PbCl2 as an effective additive to improve the qulity of ?-CsPbI3 film for high efficiency inorganic perovskite solar cell.The results show that Cl-can promote the nucleation and crystallization of ?-CsPbI3 crystals.The increased dopping content lead to the oriented growth and crystal size of ?-CsPbI3 films.By doping 3 mol%chlorine into the ?-CsPbI3 film,its trap density is minimized with enhanced electrical conductivity,long carrier mobility and reduced interfacial recombination.Consequently,the optimized?-CsPbI3 PSCs can yield an impressive PCE of 16.07%.More importantly,the PCE of the optimized device shows 94%of its initial PCE after storage in air with a relative humidity of?30%and only 0.45%dEgradation after continuous light soaking.(3)We used PEAI as Ruddlesden-Popper(RP)components to modify 3D ?-CsPbI3 thin film by prescuor engineering.The effects of PEAI on the crystal growth and photovoltaic properties of ?-CsPbI3 films were investigated.Firstly,the dimensionality of 2D,2D/3D and 3D ?-CsPbI3 microcrystalline films were obtained by adjusting stoichiometric ratio of PEA+ and Cs+.The crystal orientation,surface morphology and humidity stability of ?-CsPbI3 were improved by an increased PEAI content.The obvious phase separation in 2D structure resulted in low conductivity,carrier recombination,which arised from the layered quantumwell structure.Interestingly,the 2D/3D ?-CsPbI3 films exhibit similar carrier dynamics,charge extraction and carrier lifetimes compared to 3D films.As a result,the class of 2D/3D ?-CsPbI3 PSCs yielded a PCE of 13.56%,and their humidity stability and thermal stability were significantly improved.It retains 88%of initial PCE after 12 days when exposed in air with relative humidity of?30%without encapsulation.(4)We designed achair-beach-shaped(CSB)energy band alignment for high performance inorganic ?-CsPbI3 solar cells.The energy level gradients is introduced by adopting CsPbI3 quantum dots(QDs)layer and PMABr passivation layer at both sides of ?-CsPbI3 thin films to establish additional internal electrical fields.The CSB band structure is favorable to improve charge extraction with enhanced carrier diffusion length and improved electron-hole splitting.In addition,the favorable energy level can reduce recombination at the transport layers and block the opposite charge carriers and excitons.As a result,the PCE of CSB band structure based solar cells is increased to 17.12%whereas 15.07%for pristine davice and the VOC is as high as 1.15 V.In addition,the optimized device retains 90.4%of its initial PCE after being stored for 1 month without encapsulation. |
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