Investigation Of Perovskite Deposition And Materials For Organic-Inorganic Hybride Perovskite Solar Cells

The organic-inorganic hybrid perovskite solar cells based on CH3NH3PbI3 have attracted tremendous attentions in recent years with power conversion efficiencies over 22%,which was recognized by the world economic forum as one of the Ten New Technology in 2016.The orgenic-inorganic hybrid perovskites show some unique properties,such as high Molar extinction coefficient,tunable band gap,as well as long charge diffusion life time.These,together with the low cost and easier preparation techniques,make these kinds of materials to be attractive candidates for photovoltaic application.In order to realize the large-scale commercialization,improving the quality and stability of perovskite films,and decreasing the overall fabrication cost of the device are still challenging.This thesis focuses on these basic problems,together with some reported peer research results,aiming to give our own solutions.The main research results are summarized as follows:To improve the quality of perovskite film fabricated by traditional two-step solution method,a modification has been introduced by adding a little of additional CH3NH3I into the PbI2 precursor solution in the first step.Besides capable of adjusting the phase composition,crystallization and surface morphology of PbI2 film,it can also acts as a reaction precursor,reserving expansion space for the next conversion from PbI2 to CH3NH3PbI3.It can be seen that with adding more CH3NH3I in advance,the surface morphology changes smaller in the followed process,and the CH3NH3PbI3 film will be smoother.We also found that when the molar ratio of CH3NH3I in the mixture is 0.2,the final CH3NH3PbI3 film exhibited the best quality and light capture ability,and the weakest recombination of photo-generated carriers.After modification,the PCE of our solar cell increased from 11.1%to 13.4%.A new solvent-solvent extraction method for preparation of "amorphous" Pbl2 film was developed,which completely solved the problem of PbI2 residue in the traditional two-step method of perovskite film deposition.Though amorphous PbI2 film has solved the problem of PbI2 residue,it introduced new problem of poor crystallinity of CH3NH3PbI3 film.In order to solve this problem,we abandoned the"immersing conversion" process in the second step of the traditional two-step method,and developed a new conversion technique of "spin-spray".Compared to the traditional static conversion process,"spin-spray" method takes advantage of the dynamic process of relative high-speed motion between the PbI2 film and CH3NH3I droplet.The reaction time of conversion process is shortened from 30 minutes to several seconds.Additionally,the crystallization of CH3NH3PbI3 film increased within the high-speed rotation process.With the influences of various dynamic factors,the problem of low crystallization of CH3NH3PbI3 film is solved,and the flatness of the film is improved.Finally,high-quality CH3NH3PbI3 films with no PbI2 residues were prepared.The efficiency of the device based on this film was increased to 14.3%.In order to further improve the stability of tetragonal phase CH3NH3PbI3,this paper constructed a stable highly symmetric cubic phase system after reducing the inner stress of crystal lattice by regulating the organic cation radius within[PbI6]three-dimensional skeleton.The dimethyl amine(DMA+)with slightly larger ion radius was selected to make a partial substitution of methylamine cations,and it was found in the study that dimethyl amine could enter the[PbI6]octahedral gap connected by common apex angles with methylamine cations.The perovskite lattice expanded with the increase in the content of dimethyl amine,thereby making the tetragonal phase transform to a more stable cubic phase structure at room temperature.Simultaneously,the introduction of 5%DMA+ could inhibit nonradiative recombination,so as to improve the device efficiency from 17.1%to 18.6%,and make hysteresis effect almost disappear.Meanwhile,the unpackaged devices could keep the decay rate of energy conversion efficiency under 85℃ lowering significantly,thus achieving the construction of stable and efficient cubic phase organic-inorganic perovskite system.In perovskite solar cells,using noble metals as back electrodes increases the overall fabrication cost of the devices.In this paper,the transition metal dichalcogenides(TMDs)2H-NbS2 films were introduced to be alternatives to noble metal electrodes.By using the original solid-phase stripping method for TMDs materials,this paper firstly obtained the micron-sized less-layer NbS2 nano sheets,and then the high-purity 2H-NbS2 flexible thin film by stacking and assembling the nano sheets.The film conductivity could reach 8.7×103 S cm-1,and the work function was 5.20 eV,which is well matched with the hole transporting material-spiro-OMeTAD(5.20 eV).This flexible thin film can be directly attached on the surface of organic hole transport layer through "cold isostatic pressing" method,so as to avoid the high-vacuum and high-energy consumption thermal evaporation of precious metal electrode.The efficiency of the meso-structured perovskite solar cell based on 2H-NbS2 back electrode can reach 8.3%preliminarily,and it is expected to get further improvement.