Performance Enhancement Mechanism Of Functionalizing Perovskite Layer In Solar Cells

Recently,organic and inorganic hybrid perovskite solar cells?PSCs?have attracted widespread attention because of their intrinsic advantages such as adjustable band gap,long carrier diffusion distance?^?m?,high carrier mobility,long carrier lifetime and high extinction coefficient for perovskite materials.The efficiency of PSCs increases from the initial 3.8%in 2009 to the current 22.1%.In order to obtain highly efficient PSCs,many researches are focus on the selection of types of materials,structural design and assembly of the devices,and the fabrication technique in PSCs.Additionally,morphological control of perovskite films and interface modification between each functional layer also play a vital role in improving the performance of PSCs.The effective contact among perovskite layer,carrier extraction layer and electrode can prevent energy loss at the interfaces,achieving an effective charge transport and extraction,which is beneficial to improving the short-circuit current density of the device.In the thesis,we initially review the recent development in PSCs,then introduce the experimental procedure and reaction principle with the strategies for improved PSCs photovoltaic performance.Planar heterojunction perovskite solar cells with a configuration of FTO/Compact TiO₂/Perovskite/Spiro-OMeTAD/Ag are fabricated.The main contents of this thesis include three aspects as follows:1.We investigate the effects of thiocyanate anion content on crystallization of perovskite film by using anti-solvent engineering procedure,which arise a fast crystallization process to better tailor the crystal grain sizes for higher performance than previous approaches.As Pb?SCN?₂ firstly reacts with CH₃NH₃I to form PbI₂,the process will retard the perovskite crystal nucleation.Then,the product of HSCN would quickly evaporate in the crystallization process by the anti-solvent and annealing process,which speeds up the growth of perovskite crystal and enlarges the perovskite grain sizes.The combined effect of Pb?SCN?₂ and anti-solvent engineering on perovskite grains information realizes a balance between crystal nucleation rate and crystal growth rate.In comparison to the pristine precursor solution,the incorporation of Pb?SCN?₂ offers larger grain sizes?¹?m vs.⁰.09-0.36?m?,less grain boundary,less holes and more uniform film.The Pb?SCN?₂-tailored optimal planar perovskite solar cells deliver a much higher power conversion efficiency of 15.89%and an average efficiency of 14.59?0.94%with remarkable reproducibility.The greatly improved photocurrent density by 17%leads to significantly enhanced power conversion efficiency by 44%over the pristine one.In addition,Pb?SCN?₂-tailored optimal one shows a better conductivity measured by conductive AFM and passivation of the trap-states with less non-radiation indicated from steady-state photoluminescence test.AC impedance analysis evidences that the large grain sized perovskite film could significantly suppress the charge recombination for improvement of charge separation,transportation and collection.This work proves that the anti-solvent engineering procedure to incorporate Pb?SCN?₂ can promote growth of larger grains of perovskite crystals to greatly enhance solar cell performance.2.In this work,fullerene derivatives?ICBA and PCBM?are dissolved into perovskite precursors,and the corresponding perovskite solar cells are fabricated.Compared to PCBM,ICBA has better solubility in perovskite precursor solution?DMF?.Morphological study shows that the dissolved ICBA improves film quality.Steady-state and time-resolved photoluminescence,together with the impedance measurements confirm the enhanced electron transportation property in perovskite-ICBA film.An excellent PCE of 18.14%is achieved in CH₃NH₃PbI_3-xCl_x-ICBA-based solar cells with significantly suppressed hysteresis.Poor solubility of PCBM leads to a low concentration of PCBM in perovskite precursor,and results in a moderate PCE of 16.54%in device containing PCBM.By contrast,device based on pristine perovskite shows an inferior efficiency of 15.49%with large hysteresis.PSCs prepared from perovskite-ICBA with large area?1.12 cm²?are also fabricated,exhibiting a PCE of13.69%.Results demonstrate that the strategy of dissolving ICBA into perovskite precursor directly is an efficient and facile way to improve PSCs performance as well as to reduce hysteresis.3.A perovskite layer containing hole transport material Spiro-OMeTAD with small organic molecules in anti-solvent chlorobenzene is fabricated,which enables good contact between perovskite and hole transport material to form a continuous and efficient interface of perovskite/hole transport layer facilitating hole extraction and significantly shortening the charge transport distance.Time-of-flight secondary ion mass was used to investigate the specific distribution of Spiro-OMeTAD inside the perovskite film and the distribution gradually decreases as the surface to the interior of perovskite.In addition,the infiltrated organic molecules enhance water-resistibility of the perovskite layer for improved stability.As a consequence,the planar perovskite solar cell with rational gradient Spiro-OMeTAD exhibits a high power conversion efficiency of 17.49%with reduced hysteresis,corresponding to 20%increase than the control device for greatly improved photovoltaic performance.The enhancement mechanism is discussed to explore the fundamental insight.In conclusion,we have shown the effects of functionalizing for perovskite layer to enhance solar cells photovoltaic performance.They are all favorable to charge extraction while suppressing charge recombination for a high photovoltaic performance.