| Owing to the direct and adjustable band gap,strong light absorption ability,low preparation cost,and simple preparation process,organometal trihalide perovskite,described by CH3NH3MX3?M=Pb,or Sn,X=Cl,Br,or I?,has a wide range of application prospects in photovoltaic devices like solar cells,photodetectors,photodiodes,lasers and phototransistors.Since 2009,organometal trihalide perovskites are widely studied as semiconductors in optoelectronic devices,especially solar cells.After only several years,the highest power conversion efficiency?PCE?of the perovskite solar cell is reported to be 22.1%in 2017.Meanwhile,how to optimize the properties of the structures,the parameters and other aspects of perovskite based solar cells has also been deeply analyzed.Compare to the solar cell devices,the progress of other perovskite photovoltaic devices?such as photodetectors?are relatively slow.What's more,there exist movable positive and negative ions in the perovskite films,which will cause current hysteresis,repetitive photovoltage effects,degradation of transistor mobilities,huge dielectric constant and other problems.The specific analysis of what kind of polar ions are responsible for the electrical properties and how to effectively suppress the ionization problem are remained to be solved.Finally,current perovskite optoelectronic devices are focused on the improvement of device parameters and the performance optimization,while the increasing energy consumptions are ignored.Based on previous research,the perovskite Metal-Oxide-Semiconductor?MOS?capacitor detectors are prepared and studied.According to the unipolar transport properties of the MOS capacitors,the polar of the ions which contribute to the electrical properties are analyzed.[6,6]-Phenyl-C61-butyric Acid Methyl Ester?PCBM?is doped to inhibit the effect of the ions.The interfacial characteristics and current transport mechanism of the perovskite MOS capacitors are studied after the doping of PCBM.Finally,the advantages of perofskite MOS detectors relative to conventional structure detectors are analyzed.The main research results are as follows:1.The perovskite based MOS capacitors were prepared and analyzed.The polarity of ions in the MOS capacitors were discussed,and the influence of the ions on the parameters of the photovoltaic devices were studied.The electrical model of the MOS capacitor was given to prove that the capacitance of the heavily doped Si can be neglected with respect to that of SiO2.Under low frequency,the accumulation capacitance is the capacitance of the insulation layer.However,when it occurs to the high frequency?>1 M?condition,the total accumulation capacitance is calculated to be 110 pF,which is consistent with the result of the capacitance-voltage?C-V?measurements.The perovskite is found to be a weak p-type semiconductor according to C-V curves,and the role of ions mainly reflects under dark condition.In the dark,hysteresis capacitance curves were found in the inversion zone.However,the hysteresis curves are disappeared under light illumination,indicating that a large number of photogenerated carriers can shield the ions.Same effects can also be found in the current-voltage?I-V?curves.The energy band diagrams of Au/PVK/SiO2/Si/Al MOS capacitors under different gate voltages were given.Perovskite is in a near-intrinsic state when no voltage is applied,and the C-V curves do not change under negative gate voltages,proving that the effect of positive ions(MA+or Pb2+)are not considered.After positive gate voltage is applied,the ideal perovskite MOS capacitors will enter the inversion zone,and the capacitance value will increase as the voltage increases without considering the ions effect.But the truth is that the actual measured C-V curves go through the inversion zone only under light condition,but will be in the deep exhaustion state in the dark.This is due to the fact that the negative ions move to the interface between the perovskite and the insulation layer under the action of the electric field,thus the space charge region is broadened and the C-V curves are kept in a deep depletion state.When the C-V curves are reversely scanned,the electrons in the depletion region are rapidly filled and the total capacitance rapidly increases.According to the analysis above,the relationship between the moving ions charge amount and the applied volgate can be calculated through the broadening of the deep-depleted space charge region.It is found that more ions affects the electrical characteristics of the perovskite photovoltaic devices under lower frequencies.2.[6,6]-Phenyl-C61-butyric Acid Methyl Ester,abbreviated as PCBM,is a substance that can effectively inhibit the effect of ions in the perovskite films and is widely used as electron transport layers in perovskite solar cell structures.In this thesis,1.0 wt%PCBM was doped in the perovskite solution,and the characteristics of the perovskite MOS capacitor with an without PCBM were analyzed by material and electrical measurements.The photoluminescence spectrum?PL?shows that the intensity is reduced after doping of PCBM,which is due to the extraction of the photogenerated carriers.X-ray diffraction?XRD?proves that PCBM has no effect on the size and the crystal form of the perovskite.C-V curves indicate that the incorporation of PCBM makes the hysteresis area smaller,especially under1 MHz frequency,where the hysteresis disappears under dark condition.The total carrier concentration which is responsive to the electrical characteristics can be fitted by C-2-V curves.It is observed that a large amount of photogenerated carriers are generated under illumination relative to the dark state.In the dark,the carrier concentration of the MOS capacitor with PCBM is reduced,which is due to the ion suppression.The carrier concentration decreases more under light illumination,indicating that PCBM not only inhibits ions,but also the photogenerated carriers.The density of interface state between the perovskite and the insulating layer was calculated by the conductance method.The results show that doping of PCBM will increase the density of the interface state.The leakage current of the MOS capacitor under different scanning steps were analyzed.It was found that the currents of the two MOS capacitors in the dark state become larger as the scan step length increases.This is because the larger the step size,the more the ions cannot respond to the change of the voltage,and the smaller the ions impact on the total current.Therefore,the total current increase.3.The characteristics of the perovskite based Metal-Oxide-Semiconductor?MOS?tunneling capacitors were analyzed.Based on the tunneling current-voltage curves,the electrical model is proposed.The model indicates that the main current loop is the vertical loop,while the secondary loop is the horizontal loop.The gate leakage current curves were measured and founded to be coincide with the vertical current curves under VD=0 V.The change of horizontal voltage increases the current hysteresis of the forward and reverse sweep curves,with little effect on the total current magnitude.This is because that VD mainly affects the secondary current loop,which passes through the perovskite layer and influenced by the ions in the perovskite.The perovskite and electrodes were etched away,and Au electrodes with the same thickness and fabrication process were grown on the insulation layer to test whether SiO2 is damaged by depositing perovskite.The results show that the leakage current of SiO2capacitor is the order of 10-12 A,indicating that the grown perovskite do not destroy the insulating properties of the SiO2 layer.4.The tunneling device can be used as a MOS capacitor photodetector.C-V curves prove that the charge can accumulate at the interface though there exists leakage current.By I-V and current-time?I-T?measurements,the rise and fall times of the perovskite based MOS photodetectors are obtained as 7 ms and 11.1 ms,respectively.The responsibility under the voltage of 30 V is 2.17×10-3 A/W,which is relatively small compared to other reported values.This is because the photogenerated carriers are partially confined by the insulation layer.The dark noise current value is tested to be in the order of 10-15 A HZ-0.5,and the detectively is calculated to be 1.2×1014 Jones at 10 V and 2 mW/cm2 light illumination.The power consumption was 3.6×10-11 under 1 V.Compared with other literatures,the perovskite MOS photodetector has many advantages such as low power consumption,wide operating voltage range,and high detectivity.Current transport mechanisms were studied here.Under low voltages,the space charge limited current?SCLC?mechanism is not satisfied in the dark.Under illumination,the primary current mechanism is Ohm's Law,where the thermal excitation current plays the main role.When the voltage increases,the current mechanism changes from Ohm's Law to trap filling limited?TFL?emission.The conduction band offset between perovskite and SiO2 is 0.87 eV by XPS measurements.In this way,the main leakage current mechanism under large electric field is the Schottky emission?SE?mechanism.By fitting SE curves,the barrier height was calculated to be 0.857 eV in the dark and 0.74 eV in the light.This is due to a large number of photogenerated carriers generated under illumination.The carriers move to the boundary of the SiO2/perovskite interface under the action of the electric field.These carriers enhance the energy level at the interface.As a result,the conductor band bias is reduced,resulting in a higher swiching ratio and detectivity.5.By using the high-k material Y2O3 instead of the insulation layer SiO2,a lower power consumption perovskite MIS capacitance detector was obtained.The XPS measurements indicate that the bandgap of Y2O3 is 5.2 eV,lower than the theoretical value?5.5 eV?.This is because Y2O3 and perovskite contact and react with each other.I-V curves prove that the perovskite MIS capacitors show very low dark current.Replacing SiO2 by Y2O3,the current value is reduced from 4×10-10 A to 2.7×10-10 A under the same electric field.The operating current of the perovskite MIS capacitor photodetector gradually increases as the light intensity increases.The ratio of light/dark current is 225 under 2.5 mW/cm2 and 10 V gate voltage,proving that the detector possess high swithing ratio with low power consumption. |
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