異質接面共軛高分子/奈米顆粒光伏及記憶體元件之電性與形態分析

Abstract

本論文乃利用同步幅射X光研究予體–受體異質接面奈米級相分離結構與高分子太陽能電池元件效率的關聯性，而予體–受體異質接面因可以大量且快速的分離電子電洞，因此除了可應用在太陽能電池外，亦可將其概念製作成光寫入電抹除的電晶體式記憶體。 論文的第一部分，我們在同一時間利用同步幅射小角度X光散射及廣角X光繞射技術分別觀察到聚噻吩及奈米碳球在經由不同熱處理溫度下其內部結構分離結果，再搭配太陽能電池的光電特性量測找到最佳化的元件製程條件，利用廣角X光繞射觀察到聚噻吩的玻璃轉移溫度附近做持溫(100度及120度)可以有效增加聚噻吩的自組裝結晶，其中120度是聚噻吩的最佳結晶條件，但溫度升到150度時晶體大小開始變小，在小角度X光散射的觀察下發現持溫100及120度時奈米碳球的聚集大小與未持溫條件類似，其散射半徑約16奈米，但當持溫達150度時其散射半徑增加到23奈米，此時有最佳的元件光電轉換效率。在太陽能電池的應用上，聚噻吩的結晶大小及分子的排列方面，還有奈米碳球聚集的情況決定了元件特性，因此在我們找出熱處理對聚噻吩及奈米碳球聚集的影響後，更進一步分析不同奈米碳球含量對聚噻吩分子堆疊排列的影響，由實驗中發現，奈米碳球的聚集大小及在聚噻吩中的含量會破壞聚噻吩垂直於元件方向的分子排列長度，但增進聚噻吩分子水平於元件方向的排列長度。在搭配電子、電洞的速率量測下得到當聚噻吩結晶大小與奈米碳球聚集有一適合的大小時會有最佳的電子、電洞傳導路徑，進而使得太陽能電池的元件有最佳的效率。 我們用聚噻吩–硒化鎘奈米級相分離結構製作了光寫入電晶體式的記憶體，因為硒化鎘混入聚噻吩後即形成予體–受體異質接面奈米相分離結構，此結構有利於電子、電洞的分離，比單純聚噻吩製成的記憶體有更高的光電流及記憶特性，因為聚噻吩–硒化鎘結構除了增加電子、電洞的分離外，硒化鎘本身也提供了一個電子侷限的空間，電子會被侷限在硒化鎘中達數小時之久，甚至在不需要閘極偏壓下也會有記憶體的效應，而此結果可以重複經由光寫入、電抹除來達到記憶重複使用的特性。我們更進一步的導入量子井的概念於硒化鎘中，並更進一步的增加電子停留在硒化鎘裡的時間，實驗發現量子井結構的核殼硒化鎘/硒化鋅量子點混入聚噻吩後電流的開關比更高達到2700倍之多，並且可以更穩定的侷限電子於量子點中，在實驗中其電流保持時間超過8000秒，當對元件施以負10伏特的閘極電壓時，硒化鎘中的電子同樣的可以被抹除，證明量子井結構之量子點更適合應用於電晶體式的記憶體元件中。

In this dissertation, we studied the nanoscale phase separation of donor-acceptor bulk heterojunction structure (BHJ) in thin film polymer solar cells by using synchrotron based grazing-incidence small-angle X-ray scattering (GISAXS) and wide-angle X-ray diffraction (GIWAXD) simultaneously and fabricated the BHJ photoresponsive memory device incorporating conjugated polymer as donor and quantum dots as acceptor. In donor-acceptor BHJ morphological studies, first, we used GISAXS and GIWAXD techniques to analyze the nanoscale phase separation of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) after annealing, this study investigated the effects of the sizes of the PCBM clusters and P3HT crystallites on the power conversion efficiency of BHJ solar cells. Second, we used simultaneous GISAXS and GIWAXD to elucidate the overall morphologies of BHJ thin film solar cells containing phase-separated P3HT and PCBM domains. Specifically, the dimensions and orientation of the P3HT crystallites and the sizes of the PCBM aggregates in BHJ thin films were determined. The appropriate PCBM aggregate size and density required for an optimum performance of the film in the photovoltaic device resulted in deteriorated ordering in the out-of-plane direction, but improved the in-plane packing of the P3HT lamellae. When the P3HT crystallites and PCBM aggregates had comparable domain sizes and number densities, the inter-percolated networks for electron- and hole-transport were optimized in the film. This new understanding of the underlying mechanism of carrier mobility in BHJ thin films might be crucial in improving the efficiency of future solar cells. In BHJ photoresponsive memory devices, we describe the optical responses and memory effects of P3HT/CdSe quantum dot (QD) thin-film transistors (TFTs). TFTs incorporating P3HT/CdSe QD BHJ blends as the active layer exhibited higher photocurrents than did the corresponding P3HT-only devices because the heterojunction between P3HT and the CdSe QDs enhanced the separation of excitons. Moreover, the CdSe QDs served as trap centers so that the memory effect was maintained for several hours, even when the device was operated without a gating voltage. Herein, we demonstrate the potential applicability of such P3HT/CdSe QD TFTs through repeated optical programming and electrical erasing. An optical programming/electrical erasing memory device was fabricated by adopting organic thin film transistors incorporating core/shell CdSe@ZnSe QDs and P3HT as active layers. After illumination, the presence of quantum well-structured core/shell CdSe@ZnSe QDs within the P3HT film enhanced the maximum ON/OFF ratio substantially to 2700; this value was maintained for 8000 s without noticeable decay. The ON state current could be erased effectively when using a single pulse of the gate voltage (–10 V). This fabrication approach opens up the possibility of improving the memory performance of polymeric materials prepared at low cost using simple processes.