Investigation On The Electrical Transport And Photoelectric Properties Of Hybrid Halide Perovskites Under High Pressure

The emergence of organic-inorganic hybrid perovskite?HOIP?solar cell has revolutionized the photovoltaics technology.In a short span of ten years,the power conversion efficiency was improved from initial 3.8%to 23.9%.This rapid progress has created a situation where the technological advances measured by power conversion efficiencies have proceeded much more quickly than the basic physics,chemistry,and materials science necessary to understand the materials and their composites from the perspective of synthesis,structure and photoelectric properties.Therefore,it is critical to understand the relationship between structure and properties of the HOIPs.In particular,understanding the relationship among the ion transport properties,photoelectric properties and structure.Pressure as an important means which can alter the structure and electronic structure of the materials,thereby changing the electrical transport properties and photoelectric properties of the materials.In this paper,we choose the FAPbBr₃ and CsPbBr₃ as subjects,by combining in situ high pressure alternative current impedance spectroscopy and photocurrent measurements,we have studied the electrical transport and photoelectrical properties of hybrid organic-inorganic perovskites based on the diamond anvil cell?DAC?.We found that the pressure can dramatically tune their electrical transport properties and improve their photoelectrical properties.In these measurements,the detailed results of study as follows:1 By in situ high pressure impedance spectroscopy measurements,we have systematically studied the electrical transport properties of FAPbBr₃ polycrystalline powder.We found that there are three physical process in FAPbBr₃ through analyzing the Nyquist plots of the impedance spectra:electronic conduction,ionic conduction and inductive loop.Comparing the impedance spectra with those of MAPbBr₃,the inductive loop of FAPbBr₃ is related to the larger mobile FA ions.In order to generate the inductive loop in the impedance spectra,the parameter B=(??3?????must fufill the condition of B>0.X denotes the unknown variable that can cause the inductive loop.We suggest that the number of escaped FA cations,n,is defined as the variable X.It can be known that the Faradaic electric current I_F is proportional to the number of escaped FA cations???3=?9?>0?.The potential E is proportional to the number of escaped FA ions per unit time(/=??99???9?/>0?.Hence leading to the appearence of the inductive loop in the impedance spectra.2 Under high pressure,the electrical parameters of FAPbBr₃ include electronic and ionic resistances have two discontinuous changes at 0.7 and 2.1 GPa,which is attributed to the pressure-induced phase transition.The pressure has an effect on the ion migration and can tune the inductive loop dramatically.In phase I,all of the FA can participate in the migration freely in their channels and the inductive loop is not obvious.In phase II,the number of FA passing through the channel is strongly depend on the electrode potential,and the inductance value increases with the increasing pressure.In phase III,the FA ion migration is seriously hindered by volume compression and strengthened H-bonding.Nevertheless,the FA ions migration in the channel is still depend on the electrode potential,but the dependence is weaker than that in phase II.When the FAPbBr₃ became amorphous,the ion migrate channel is closed and then there is no longer any ions migration in the sample and no inductance is present.3 From ambient to 2 GPa,the photocurrent of FAPbBr₃ increases with increasing pressure,which indicates that pressure can improve the photoelectronic performance of FAPbBr₃.At 1.3 GPa,its photoelectrical response is optimal.With further compression up to 3.5 GPa,the photocurrent decreased significantly and almost disappeared.By comparing with MAPbBr₃ and MAPbI₃,the maximum photocurrent value in FAPbBr₃?¹.2?A?is nearly 10 times higher than that in MAPbBr₃?⁰.1?A?and 3 times larger than that in MAPbI₃?⁰.4?A?.4 By in situ high pressure impedance spectroscopy measurements,we have systematically studied the electrical transport properties of CsPbBr₃ powder and single crystal.At pressures below 2.3 GPa,the semicircles of impedance spectra are highly symmetrical,and we cannot distinguish the contributions of bulk and grain boundaries accurately.At 2.3 GPa,the semicircles turns into two arcs which demonstrate the bulk?at high frequencies?and grain boundary?at low frequencies?contributions,respectively.With the applied pressure exceeding 2.3 GPa,a mixed ionic/electronic conduction to pure electronic conduction transition occur.5 By fitting he impedance spectra with the equivalent circuit,we obtained the pressure dependence of electrical parameters include electronic resistance,ionic resistance,bulk resistance,grain boundary resistance and relative diffusion coefficients.By comparing the pressure-dependent impedance spectra of CsPbBr₃ powder and single crystal found that the pressure-induced ionic-electric to pure electronic conduction transition also occur at 2.3 GPa,and then it's the intrinsic properties in CsPbBr₃.6 The photocurrents of CsPbBr₃ powder under pressure are significantly higher than the value measured under ambient conditions.At 1.4 GPa,CsPbBr₃ powder exhibits the strongest photoresponsiveness.An extremely sharp and needle-like peak emerges at every moment of light irradiation and then transforms into a gradual decrease in photocurrent until the light is off,which can be attributed to the ionic conduction within CsPbBr₃.All the ions appearing in the sample include pre-existing ions?in dark?and photo-excited ions?under illumination?.The pre-existing ions will migrate to the perovskite-electrode interface and form a built-in electric field,which will consume the external electric field because its direction is opposite to the external electric field.And then the photocurrent decreases gradually.