Study On Perovskite Solar Cells Based On TiO₂ Modified With BaTiO₃

With the development of economy and the acceleration of industrialization,environmentpollution is becoming more and more serious.Development and utilization of clean energy,which includes wind energy,water energy,solar energy,tidal energy,geothermal energy,and so forth,is an effective way to alleviate environment pollution.Solar energy has the greatest potential in renewable resources,and it has the advantages of universality,longevity,and large content.Meanwhile,the use of solar energy is also varied.Solar cell is a device that converts luminous energy into electric energy.In 2012,organic-inorganic hybrids perovskite material?MAPbI₃?was reported on all-solid-state solar cells used as light absorber,which yielded a power conversion efficiency?PCE?of 9%.Perovskite material is an excellent light absorbing material due to its suitable band gap?1.3-2.3eV?,long carrier diffusion length?>100nm?,wide light absorption?350-1100nm?and ambipolar charge transport.Over the past five years,PCE of perovskite solar cells?PSCs?has been increased rapidly from 9%to 22.1%.Generally speaking,PSCs are always composed of electron transport layer?ETL?,perovskitelayer and hole transport layer?HTL?.The structure of PSCs can be divided into:mesoporous structure,planar structure?n-i-p and p-i-n?and without HTL structure and so forth.In our paper,we used the structure of mesoporous structure:FTO/compact TiO₂ layer/mesoporous TiO₂ layer?mp-TiO₂?/perovskite layer/HTL?spiro-OMeTAD?/Au.The early studies show that carrier recombination inside the cells seriously affect PCE of PSCs.As is reported,the compact layer not only transport the electrons generated from perovskite layer but also prevent the direct contact between holes and FTO,which can effectively suppress the carrier recombination between FTO and perovskite/HTL.Therefore,it is particularly vital to fabricate high-quality compact layer.Furthermore,it is widely recognized that interface engineer is also an effective way to retard carrier recombination and enhance performance of the cells.In response to the issues discussed above,we have done the following works:?1?optimizing the compact layer;?2?modifying the mesoporous layer with BaTiO₃.The detailed contents and conclusions are as follows:?1?optimizing the compact layer:We selected three kinds of commonly used titanium precursorsolutions to fabricate compact layers,namely,c-TBOT,c-TTIP,and c-TTDB.Through a series of tests,the optimal compact layer and its precursor solution were obtained.AFM and SEM show that the TiO₂ layer has been successfully deposited on the FTO glass,and the surface roughness of the compact layers is also significantly different.The DC I-V curves imply that conductivity of c-TBOT is best among three samples.UV-vis absorption spectra indicate that both c-TBOT and c-TTDB have excellent optical properties.Steady-state photoluminescence?PL?and time-resolved photoluminescence?TRPL?spectra display that c-TBOT has higher charge extraction and faster electron transport among the three samples.Electrochemical impedance spectroscopy?EIS?suggests that the device based on c-TBOT has a larger recombination resistance(R_rec)and a smaller series resistance?R_s?,indicating that c-TBOT has a stronger ability to suppress carrier recombination.Consequently,the device based on c-TBOT has higher short-circuit current density(J_sc),open-circuit voltage(V_oc),and smaller curve hysteresis.In sum,c-TBOT is the best compact layer.?2?modifying the mesoporous layer with BaTiO₃:Firstly,we fabricated BaTiO₃ by coatingBa?NO₃?₂ on the surface of mesoporous TiO₂ layer to enhance performance of PSCs.Secondly,the optimum concentration of Ba?NO₃?₂ was obtained by comparing a series of concentrations.Finally,we explore the mechanism of the effect of BaTiO₃ modification layer on performance of cells after a series of tests and characterizations.AFM and SEM show that BaTiO₃ modification layer has no obvious influence on the morphology of mesoporous layer other than the surface roughness.XRD and XPS indicate that an ultra-thin BaTiO₃ has been formed on the surface of mesoporous layer.The DC I-V curves imply that the conductivity of ETL changes with the concentration of Ba?NO₃?₂ solution,and the sample of mp-TiO₂/BaTiO₃?0.9wt%?shows the best conductivity among the samples.UV-vis absorption spectra display that the transparency of device has no significant change after BaTiO₃ modification.PL and TRPL spectra suggest that charge extraction and electron transfer are enhanced after BaTiO₃ modification.EIS result exhibits that the device based on mp-TiO₂/BaTiO₃?0.9wt%?has a larger R_rec,which indicates it has a stronger ability to retard carrier recombination compared with that of mp-TiO₂.Therefore,compared with mp-TiO₂,the device based on mp-TiO₂/BaTiO₃?0.9wt%?has larger J_sc,V_oc,and smaller curve hysteresis,yielding an increased PCE from 16.13%to 17.87%.In sum,we can conclude that the introduction of BaTiO₃?0.9wt%?modification layer can effectively reduce carrier recombination and dramatically improve performance of PSCs.