溶液相法合成之硒化鉛及硒化鎘半導體量子點薄膜光伏元件之電性及形態分析

Abstract

此論文主旨為研究溶液相法合成之IV-VI族硒化鉛(PbSe)與II-VI族硒化鎘(CdSe)半導體量子點材料，並製備PbSe量子點薄膜元件以及CdSe/予體-受體共軛高分子混摻薄膜元件，接著針對其表面修飾，結構形態以及元件最佳化製程做一系列之探討。 在PbSe半導體量子點材料方面，我們分別以三種不同長碳鏈之官能基－18碳之油酸(OA)，4碳之正丁胺(BA)以及2碳之1,2-乙二硫醇(EDT)，來修飾PbSe量子點表面並探討其薄膜形態、結構與光伏元件之性質。我們利用同步輻射中心XRR分析得知，以BA及EDT修飾PbSe量子點表面，會提高量子點薄膜之平整度並造成薄膜平均密度增加。尤其以EDT修飾之後，會更有效地縮短PbSe量子點垂直方向之距離－此現象有利於光伏元件之載子傳輸，並具有更好之光電轉換效率。因此，我們以逐層塗佈法製備EDT修飾之PbSe量子點薄膜光伏元件，並研究發現結合PEDOT:PSS電洞傳輸層於元件中，會改善量子點薄膜之平整度以及元件界面接合，進而大幅地增加光電轉換效率(從1.5%提升至2.4%)。此外，結合PEDOT:PSS電洞傳輸層會具有較長之元件壽命等特性，其中光電流響應範圍從可見光波段至近紅外光波段。 在CdSe半導體量子點材料方面，我們分以四角結構之CdSe與噻吩并雙酮吡咯衍伸之予體-受體共軛高分子(PDTTTPD)為n、p型材料，並製備CdSe/PDTTTPD混摻之薄膜元件，並探討製程中熱退火對元件電性及薄膜形態之影響。在元件電性方面，光電轉換效率經由熱退火之後會大幅地增加 (從1.0%提升至2.9%)。在薄膜形態方面，由於熱退火會使得CdSe表面之吡啶官能基脫附，因此會造成薄膜平均密度之增加以及CdSe聚集等特性－均利於光伏元件之載子傳輸，進而提升元件光電轉換效率。

In this dissertation, thin films of PbSe quantum dots (QDs) featuring three different ligands, oleic acid (OA), butylamine (BA), and 1,2-ethanedithiol (EDT), which have pronounced effects on the arrangement and photovoltaic performance of the PbSe QDs in the thin films. Using a synchrotron X-ray reflectivity (XRR) probe, we determined that the roughnesses decreased and the average densities increased for PbSe QD thin films capped with BA and EDT, relative to those of the OA-capped PbSe QD film. In particular, the PbSe QDs’ vertical packing density, which is critical for charge transport, increased substantially for the system incorporating EDT ligands. As a result, devices containing the EDT-treated PbSe QD film displayed much improved power conversion efficiencies (PCEs) relative to those of corresponding devices featuring either the OA- or BA-capped PbSe QD films. Furthermore, we fabricated the EDT-capped PbSe QD devices adopting a layer-by-layer technique. We found that a thin poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole transport layer enhances the PCE of a PbSe QD device to 2.4%, from 1.5% for a standard PbSe QD device, a relative increase of 60%. The device life time under continuous simulated AM1.5 irradiation, measured in terms of the time required to reach 80% of the normalized efficiency, for the PbSe QD device incorporating the hole transport layer was six times longer than that of the standard device. Finally, we reported the photovoltaic devices based on blend films of CdSe tetrapods and the donor/acceptor conjugated polymer PDTTTPD, which comprises 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene and thieno[3,4-c]pyrrole-4,6-dione units. The PCE of a photovoltaic device had experienced thermal annealing was three times greater than that of the corresponding device incorporating the as-prepared device (2.9% vs. 1.0%). Thermal annealing enhanced the degree of aggregation of the CdSe tetrapods and induced denser morphologies relative to that of the as-prepared blend film, leading to substantially increased charge transport, which enhanced the PCE of the device.