Preparations And Performances Of Semitransparent Perovskite Films And Solar Cell Devices

Photovoltaic power technology as a new energy,which converts solar energy into electricity,has attracted worldwide attention.Perovskite have been nominated as the outstanding materials due to high absorption,long carrier diffusion length,low exciton binding energy and solution-processability.Moreover,perovskite solar cell devices with simple structure are low cost.Conventional perovskite solar cells need a thicker perovskite film to achieve the highest power conversion efficiency.But semitransparent perovskite solar cells are based on semitransparent perovskite film as the photoactive layer,and the other layers are composed of transparent materials.As a result,semitransparent perovskite solar cells have the characteristics of both daylighting and power conversion,which can be assembled tandem solar cells to improve energy density,or design multi-functional glass for buildings,vehicles or phones.Therefore,the development of semitransparent perovskite solar cells has become a hotspot.The thickness of semitransparent perovskite film is very thin to ensure transmittance,so the film quality including compactness,surface coverage and defects,needs to be higher,and thus the film preparation conditions are harsh.Both the quality and preparation conditions of semitransparent perovskite films have decisive impacts on the performance of devices.Therefore,we focused on the preparation of high-quality semitransparent perovskite films with general conditions by solvent engineering,defect passivation and nitrogen assistance strategies.Annealing process is usually requires to crystallize or remove residual solvents for preparation of semitransparent perovskite films.Therefore,rigid or flexible FTO and ITO conductive glasses with good thermal stability are mainly substrates for cell devices.If perovskite films can be formed at room temperature,the thermal stability of the substrates is beyond concern.We found that,high-quality semitransparent perovskite films with high compactness,mirror and semitransparent can be prepared by one step at room temperature through synergistic effect of mixed solvents and antisolvents.The power conversion efficiency reached 16.1%for conventional planar device structure based on the semitransparent films.The power conversion efficiency is still up to 12.6%for the planar structure without electron collection layer.In addition,by comparing the removal ways of residual solvents,it is proved that annealing can enhance the film roughness,increase the contact area between interfaces,and facilitate the carriers transport between interfaces.However,annealing also leads to increase of crystal defects,and electron trap density,improve carrier recombination loss probability.The power conversion efficiency of planar device without electron collection layer based on semitransparent perovskite film with annealing-free was still over 10%.The preparation method of the high-quality semitransparent film is value for expanding the scope of the electrode substrate or constructing stacked batteries.The quality of perovskite film prepared by two-step is determined by the lead iodide films,which is more benefical to large-scale production.Semitransparent perovskite films with optimum thickness can be obtained by regulating the concentration of lead iodide and the speed of spin-coating.However,the obtained films with loose grains and more defects at grain boundaries,are sensitive to solvent atmosphere,water and oxygen.These molecules can invade into the crystal structure at the boundaries and lead to lower performance and poor stability of cells.Therefore,recycling glove box is necessary for preparing perovskite films via two-step.We prepared graphite-like C3N4 nanoparticles with chemical stropping and plan to passivate boundaries defects caused by the abnormal electronic states and distortion of bonds.The polar sites and N-H bonds at the perovskite grain boundaries can form hydrogen bonds with N-H,O-H,C-O and C=O bonds at the surface of graphite-like C3N4 nanoparticles.Semitransparent perovskite films were prepared in a simple closed system.The power conversion efficiency of solar cell based on semitransparent perovskite films modified with graphite-like C3N4 nanoparticles is 35%higher than the reference device.In addition,with the aid of steady-state fluorescence spectroscopy,transient fluorescence spectroscopy and electrochemical impedance spectroscopy,we have confirmed that graphite-like C3N4 nanoparticles can recognize and effectively passivate deep electron trapping centers at the grain boundaries,reduce the non-radiative recombination of carriers,improve the power conversion efficiency and stability.The defects passivation of grain boundaries provides a feasible scheme for the preparation of large-scale perovskite film in a simple closed system.The function of the antisolvent is to remove solvent for forming intermediate adduct or for rapid crystallization of perovskite.Is there any other way for fast solvent removal?We found that blowing nitrogen gas during spin coating can also rapidly remove solvent,increase nucleation sites,and and obtain perovskite films with fast crystallization.Moreover,perovskite defects caused by solvent atmosphere can be reduced,and destruction of perovskite by water and oxygen molecules can be prevented.More importantly,the preparation of perovskite films can be operated in the air atmosphere.The experiments show that the mirror-like semitransparent perovskite films with good compact and high surface coverage were successfully prepared by the method in the air atmosphere.Solar cells with planar structure are assembled with the obtained semitransparent perovskite films and the power conversion efficiency is up to 16.6%,which is 1.3 times higher than that of the reference device.The power conversion efficiency of planar device without electron collection layer based on semitransparent perovskite film with annealing-free is up to 12.3%.Note that the reference device is composed of perovskite films which are prepared under the same conditions without blowing nitrogen gas.The method provides a feasible scheme for preparation of high quanlity semitransparent perovskite films from the limitations of harsh preparation conditions.