Preparation Of High Quality Organic-Inorganic Perovskite Films And Solar Cells

In recent years,organic-inorganic perovskite solar cells have attracted much attention due to their superior performance.The main advantages are as follows:solution preparation,low production cost,high photoelectric conversion efficiency(PCE)and so on.High-quality perovskite film is the key to prepara the high-performance solar cells.Large-grained perovskites facilitate the rapid transfer of electrons and hole,reducing its energy loss and prolonging the carriers lifetime.Thus the PCE of perovskite solar cells is improved effectively.Solution spin-coating with adding some additives is a simple and effective method for improving the quality of perovskite films.Commonly used additives such as Dimethylformamide(DMF),N,N-dimethyl sulfoxide(DMSO),N-Methyl pyrrolidone(NMP),hexamethylphosphoramide(HMPA),Pyridine,etc.,have certain toxicity,which is very unfavorable to human health and the environment.Moreover,the solvent can be used not only to dissolve the solute,but also to regulate the crystallization of the perovskite and to modify the morphology of the perovskite.Therefore,we have carried out a series of investigation on the key issues of preparing high-quality perovskite films by selecting green and low-toxic solvents and studying the role of solvents in perovskite film formation.The main research contents are as follows:(1)Compared with DMF,a low volatility and environmental friendly solvent of 1,3-dimethyl-2-imidazolidinone or N-ethyl-2-pyrrolidone can form a more stable Lewis acid-base adduct of PbI2·Sol(Sol=DMI,NEP).This compound is a kind of PbI2 crystal pre-inserted with the solvent molecules,which provides a diffusion channels for CH3NH3I or CH(NH2)2I.By adding two different solvent of NEP and DMI in the PbI2/DMF precursor solution separately,high-quality perovskite films with a flat and dense surface,and non-residual PbI2 are obtained.At the same time,in order to further promote the growth of the crystal grains,the annealing temperature is increased.By adding 10%DMI in the precursor solution,the average grain size of the perovskite increases from?216 to 375 nm,when annealing temperature increasing from 100 to 130 ?.As a result,the efficiency of the PSCs increases from 10.7 to 14.5%.(2)An easy-to-operate closed-steam annealing(CSA)method has been developed.Compared with the traditional direct annealing method,CSA method can greatly improve the quality of the perovskite film with columnar crystals throughout the entire section of film.The method prolongs the recrystallization time of the perovskite film by confining the solvent vapor between the perovskite precursor film and the substrate,and effectively promotes grain coarsening and the conversion of PbI2 to perovskite.This phenomenon is related to the Oswald ripening effect,in which smaller grains are preferentially dissolved by the solvent vapor.And then the dissolved small grains continuously deposit on the large grains,thus promoting the grain growth.The columnar crystals enhance the efficient transport of electrons and holes with low recombination rate,and the efficiency of the perovskite solar cells improves from 12.3%to 16.8%.(3)High quality perovskite film can be fabricated by combining two methods based on the Lewis acid-base theory and dissolution-recrystallization theory.First,a low-toxic NEP solvent is added into the Pbl2/PbBr2/DMF solution in the first step of the two-step solution spin coating.A Lewis acid-base adduct of PbI2·Sol(Sol=DMF,NEP)are formed at the first step,and then the adduct react with CH(NH2)2I and CH3NH3Br to form an intermediate phase of perovskite.PbI2·NEP not only provides the channels for FAI/MABr diffusion,but also delays the reaction between PbI2 and FAI/MABr due to the strong interaction between NEP and PbI2.The slow reaction is helpful for grain growth to form perovskite film with improved surface morphology.Second,the residual solvent embedded in the intermediate phase is used to dissolve and recrystallize the perovskite film by the CSA method.and the crystal grains grow larger.Based on the dual roles of the solvent,the average grain size of the perovskite film increases from 205 nm to 718 nm,and the average carrier lifetime increases from 155 ns to 561 ns.We finally fabricated a high-quality perovskite film with good crystallinity,fewer defects,longer carrier lifetime and columnar crystal shape,and the photoelectric conversion efficiency(PCE)of the solar cells increases from 10.8%to 17.0%.