超音波輔助化學水浴法製備 AgInS2 薄膜之電化學阻抗頻譜分析; Electrochemical Impedance Spectroscopic Analysis of AgInS2 Thin Films Prepared by Using Ultrasonic Assisted Chemical Bath Deposition

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Title:	超音波輔助化學水浴法製備 AgInS2 薄膜之電化學阻抗頻譜分析;Electrochemical Impedance Spectroscopic Analysis of AgInS2 Thin Films Prepared by Using Ultrasonic Assisted Chemical Bath Deposition
Authors:	李芳紜;Li,Fang-Yun
Contributors:	化學工程與材料工程學系
Keywords:	光觸媒;半導體薄膜;電化學阻抗頻譜分析;載子傳輸;photocatalyst;semiconductor thin films;EIS;carrier transfer
Date:	2013-07-29
Issue Date:	2013-08-22 11:39:24 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究利用超音波輔助化學水浴法製備不同厚度之AgInS2半導體薄膜，並分析此材料之物性與光電性質，探討載子傳輸機制。從實驗結果可發現AgInS2薄膜為Orthorhombic phase，使用吸收光譜推測直接能隙值約為1.93~1.98 eV。光電性質量測的結果顯示，薄膜隨著鍍膜次數增加，可有效降低基材裸露與暗電流上升問題，其中鍍膜兩次之AgInS2光電極具有較佳的光電流值，在偏壓 1 V vs. SCE 下為 1.8 mA/cm2。由開環電位法量測薄膜於犧牲試劑( Na2S+K2SO3 )之費米能階(平帶電位)為 -0.845 V~ -1.020 V v.s. SCE，而Electrochemistry Impedance Spectroscopy（EIS）分析得知薄膜於相同犧牲試劑之費米能階為 -0.8　V~ -1.2 V vs. SCE，兩者數值相近。暗反應中，於犧牲試劑量測之EIS，可發現隨著鍍膜次數增加，R1(溶液電阻及薄膜電阻)也隨著增加；隨著偏壓較負，由於空乏區厚度變薄，會使得半導體電容越來越大；於照光情況下使用犧牲試劑為電解質所量測出來之EIS，可推測照光下，光激發生成之載子在不同偏壓下的傳輸機制，隨著光強增加，R2(電荷傳輸阻抗)會變小，使得載子傳輸較為容易。而不同光強下，皆於 -1.0 V ( vs. S.C.E.) 電容有最大值且載子存活時間最長。未來的研究可調整不同銀銦比所製備之薄膜，由 EIS 分析技術得知薄膜於照光下之表面態電容、表面態時間，以了解此三成分硫化物光電極薄膜的光電化學行為。 In our previous studies, we have prepared photocatalyst thin films using Ultrasonic Chemical Bath Deposition (UCBD). By controlling [Ag]/[In] molar ratios in the precursors, we can obtain a single phase AgIn5S8, mixtures of AgIn5S8 and AgInS2 and a single phase AgInS2 thin films. Our studies focused on preparing AgInS2 films with different thickness and studying their electrochemical properties. All the AgInS2 films after 400 °C thermal treatment have the orthorhombic structure and the direct energy band gap in the range of 1.93 to 1.98 eV. In order to understand the photoelectrochemical properties, AgInS2 films with different coatings were prepared. Xe lamp with an intensity of 100 mW/cm2 was then used to illuminate our samples. The photocurrent densities as a function of applied potential were measured. It was found that homogeneous AgInS2 films were obtained with increasing coatings. In addition, these dense films can effectively suppress the the dark current. In particular, the AgInS2 thin film of deposition two times (485.2 ± 28.2 nm) has the highest photocurrent density of 1.8 mA/cm2 under a bias of 1 V vs. SCE. The fermi level (flat band potential) of films can be estimated from open circuit potential (OCP) measurements, as well as electrochemical impedance spectroscopic (EIS) analysis. The fermi levels of films in the sacrificial reagent consisted of Na2S and K2SO3 measured using OCP and EIS were varied from -0.845 V~ -1.020 V and -0.8 V~ -1.2 V, respectively. More information, such as charge transfer resistance and capacitance, can be retrieved from EIS analysis by fitting the experimental data to the model. In fact, Randle’ s model fitted the data better than other complicated models, which suggested that carriers transfer to the electrolyte directly from valence band under illumination. When depositing times increase, the resistance R1 (solution resistance and film resistance) will increase. When the applied potential decreases, the capacitance of the semiconductor will increase due to the thinner depletion layer. R2 (charge transfer resistance) will decrease dramatically under illumination, perhaps due to much higher carrier density. At -1.0 V vs. SCE, the AgInS2 film (D3) has the highest capacitance and the logest lifetime. In the future, the EIS analysis can be used to investigate Ag-In-S thin film photoelectrode with different [Ag]/[In] molar ratios, to realize the physical original of charge transfer process of such materials.
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