有機金屬鹵化物鈣鈦礦太陽能電池之界面工程

Abstract

本論文著重在探討鈣鈦礦太陽能電池的界面改質工程。首先我們利用多種胺基酸來改質二氧化鈦(TiO2)電子傳輸層，結果顯示改質明顯地影響TiO2/MAPbI3界面的電荷轉移效率。其中，利用L-alanine改質的元件在光電轉換效率的表現上有30%的提升。我們也發現MAPbI3鈣鈦礦晶粒的(110)結晶面會傾向延著有胺基酸改質之TiO2基板垂直方向來排列，此現象可經由對MAPbI3薄膜進行低掠角X光散射量測的結果來證實。我們透過電化學阻抗分析儀的量測結果發現經由胺基酸改質過後，TiO2/MAPbI3界面的電荷轉移電阻下降，且從螢光光譜訊號強度的減少及螢光生命週期的減短也證實了此界面改質方法可提升電荷轉移效率。此外，透過螢光光譜對試片不同面的激發量測結果顯示胺基酸改質面的電荷陷阱現象較輕微，也可能因此提升界面電荷的轉移效率。透過這些研究結果，我們說明了鈣鈦礦結晶面的擇優取向與界面缺陷鈍化是提高鈣鈦礦太陽能電池效能的關鍵之一。 接著，我們探討帶電載子和離子在TiO2/MAPbI3界面的累積與電極極化之間的關聯性，並且全面性的討論利用不同厚度PCBM嵌入TiO2/MAPbI3界面對鈣鈦礦太陽能電池在照光時開路電壓上升，與閉光時下降的影響。同時藉由電化學阻抗分析的結果，我們提出了一個可能的模型來描述界面帶電載子和離子對於時間及頻率關係的動態行為。我們的結果說明開路電壓上升與下降之延遲現象與電極極化有關，而此電極極化來自於TiO2中束縛電荷的生成。這些束縛電荷被鈣鈦礦層中累積於界面處的帶電載子和離子與缺陷所吸引。此外，我們發現PCBM作為TiO2/MAPbI3界面的鈍化層除了可有效地減少元件中電流-電壓遲滯效應之外，也有效地消弱電極極化的程度，這個現象可經由電化學阻抗分析儀的量測結果中，低頻率介電係數的下降來證實。透過此研究成果，我們提出了對於鈣鈦礦太陽能電池界面的研究方法及展示富勒烯分子在界面的鈍化作用。此研究成果有助於對離子遷移如何影響鈣鈦礦太陽能電池的效能有更深入地瞭解。 接著我們更進一步說明了鈣鈦礦內累積的帶電載子和離子及缺陷與TiO2中累積的束縛電荷是造成低頻電容上升的原因，也因此導致了在效能量測時出現電流電壓遲滯的現象。此遲滯現象會對元件效能量測造成不準確的影響，但可經由PCBM作為鈍化層或是透過電池結構設計來減小。此外，於效能量測中，銀奈米粒子鑲嵌於TiO2緻密層中可幫助電荷收集且有效地降低電容所造成的遲滯現象。我們的研究結果也指出外加電壓與光強會影響離子與缺陷的生成，此兩項外加因素在進行鈣鈦礦太陽能電池的效能量測時需要納入考量。另外，我們也於鈣鈦礦太陽能電池元件中發現奇異的負電容現象。此負電容現象的程度可透過PCBM鈍化層、電池結構設計、與銀奈米粒子鑲嵌於TiO2中等方法來減輕。此結果表示負電容現象可能與鈣鈦礦內的離子或缺陷的重新分佈及TiO2中束縛電荷的釋放有關。

In this dissertation, we demonstrate the effects of interfacial engineering on the performance of perovskite solar cells (PSCs). Modification of TiO2 electron transporting layer with various amino acids strikingly affects the charge transfer efficiency at the TiO2/MAPbI3 interface, among which L-alanine modified cell exhibits the best power conversion efficiency with 30% enhancement. This study also shows that (110) plane of MAPbI3 organometal halide perovskite (OHP) crystallites tends to align in the direction perpendicular to the amino acid-modified TiO2 as observed from the grazing-incidence wide-angle X-ray scattering of thin MAPbI3 perovskite film. The results of electrochemical impedance spectroscopy (EIS) reveal less charge transfer resistance at the TiO2/MAPbI3 interface after modified with amino acids, which is also supported by the lower intensity of steady state photoluminescence (PL) and the reduced PL lifetime of perovskite. In addition, based on the PL measurement with excitation from different side of the sample, amino acid-modified samples show less surface trapping effect compared to the sample without modification, which may also facilitate the charge transfer efficiency at the interface. The results suggest that the preferential orientation of perovskite crystallites in the interface and the trap-passivation are the niche for better perovskite photovoltaic performance. Next, we report the behavior of charged species at compact TiO2/MAPbI3 interface with respect to electrode polarization in PSC devices, and comprehensively discuss the results obtained from open-circuit voltage (VOC) buildup and VOC decay measurements under illumination and in the dark, respectively, for the PSCs with PCBM inserting at TiO2/MAPbI3 interface with varied thickness. By combining with EIS, we propose a justified mechanism in an attempt to elucidate the dynamics of interfacial species with respect to the time and frequency domains. Our results demonstrate that the retarded VOC buildup and decay observed in PSC devices is related to the electrode polarization taking place as a consequence of bound charge formation in TiO2. Such bound charge is attracted by oppositely charged ion/vacancy accumulating at the OHP side. Besides, inserting a layer of PCBM at TiO2/MAPbI3 interface as a passivation layer can efficiently reduce current-voltage hysteresis of devices and alleviate the electrode polarization, which can be verified by the less dielectric constant response observed in low frequency regime from EIS analysis. This work opens up another window to investigate the interfacial issue of PSCs associated with passivation effect of fullerene, and help to further understand the impact of ion migration on photovoltaic performance. We further report the accumulated charged species in OHP and piled-up bound charges at TiO2 side are the origin of the increased capacitive response in low frequency regime for PSC device and thereby cause the current-voltage hysteresis during performance evaluation. Such hysteretic response detrimentally affects the reliability of performance evaluation but can be well suppressed by PCBM passivation and structural design. In addition, Ag nanoparticles can be embedded into compact TiO2 to facilitate the charge extraction and efficiently suppress the capacitance-induced current-voltage hysteresis during performance evaluation. Moreover, externally applied voltage and light power are found to influence the ion/vacancy generation. Hence these two factors should be taken into account during the performance evaluation for PSCs. Furthermore, an anomalous negative capacitance is observed from PSCs. By well interfacial engineering for PSC device such as PCBM passivation, structural design, and embedding Ag nanoparticle for TiO2 modification, the negative capacitance is remarkably suppressed. This implies that the negative capacitance should correlate with the back redistribution of accumulated ions/vacancies in OHP layer and the release of bound charges at TiO2.