Study On Control Of Perovskite Crystallization Process By Solvent Engineering And Photoelectric Properties Of Devices

Organometallic halide perovskites have emerged as one of the most promising materials for low-cost,high-efficiency solar cells during the past few years,because of their advantages of direct band gap,large absorption coefficient,ambipolar diffusion,weakly bound excitons and long carrier diffusion length.The efficiency of perovskite solar cells has rapidly increased from 3.8% to more than 22.1% within nine years.Despite the inherent advantages of perovskite materials and tremendous progress of perovskite solar cells,it is urgent to find a facile and controllable preparation technology to deposit perovskite films with high quality and reproducibility.This thesis has done a series of experiments and analysis focused on the perovskite light absorbing layer in perovskite solar cells.The main works and achievements are as follows:(a)To avoid the generation of uncontrollable perovskite film morphology with poor surface coverage by the conventional spin-coating method,we demonstrated a new method to produce high-quality perovskite films with controlled morphology by a solvent engineering in one-step process for planar heterojunction perovskite solar cells.First,we deposited relative dense but not full covered MAPb I3 perovskite thin-films on a cp-Ti O2/FTO substrate by inclusion of HI aqueous solution in a MAPb I3/DMF precursor solution because the MAPb I3 in DMF+HI solution would retard the nucleation time during the spin-coating process due to the better solubility than MAPb I3 in DMF solution.More-concentrated MAPb I3 in DMF+HI solution improved the quality of perovskite film with higher surface coverage.In order to obtain a dense MAPb I3 thin-film with full surface coverage,we further controlled the perovskite crystallization process through antisolvent(such as chlorobenzene,CB)dropping during the spin-coating process.The morphology of MAPb I3 film can be fine-tuned,leading to an optimization of the perovskite active layer.(b)In the standard two-sequential deposition method,the Pb I2 film prepared by a 1.0 M Pb I2 precursor solution was dipped in CH3NH3 I solution for 30 s to obtain sufficient deposition of MAPb I3 to absorb light.We examined the dipping time dependence of the conversion of Pb I2 into MAPb I3 during the two-step deposition process by using UV-vis and XRD measurements.Besides,the effects of the dipping time on the photovoltaic performance of mesostructured perovskite solar cells were also studied.The results of the experiments indicate that more and more Pb I2 convert to MAPb I3 with increasing of dipping time,but the degeneration of perovskite solar cell was also found with a too long dipping time.(c)Although the two-step sequential deposition method provides an efficient route to fabricate high performance perovskite solar cells(PSSCs)with increasing reproducibility,the inefficient and incomplete conversion of Pb I2 to perovskite is still quite a challenge.Following pioneering works,we found that the conversion process from Pb I2 to perovskite mainly involves diffusion,infiltration,contact and reaction.In order to facilitate the conversion from Pb I2 to perovskite,we demonstrate an effective method to regulate supersaturation level(the driving force to crystallization)of Pb I2 by solventing-out crystallization combining with subsequent time-delay thermal annealing of Pb I2 wet film.Enough voids and spaces in resulting porous Pb I2 layer will be in favor of efficient diffusion,infiltration of CH3NH3 I solution,and further enhance the contact and reaction between Pb I2 and CH3NH3 I in the whole film,leading to rapid,efficient and complete perovskite conversion with a conversion level of about 99.9%.Enhancement of light harvesting ranging from visible to near-IR region was achieved for the resultant high-quality perovskite.Upon this combined method,the fabricated mesostructured perovskite solar cells show tremendous power conversion efficiency(PCE)improvement from 3.2% to about 12.3% with less hysteresis owing to the simultaneous enhancement of short-circuit photocurrent density(Jsc),open-circuit voltage(Voc)and fill factor(FF).