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Study On Efficiency Optimization And Degradation Properties Of Small-molecule Organic Solar Cells

Posted on:2013-01-31Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y ZouFull Text:PDF
GTID:1112330371478795Subject:Optical Engineering
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This work focus on the efficiency enhancement and/or initial degradation properties of small-moleculue organic solar cells (OSCs). By optimizing OSCs device structure or control the evaporation condition of organic thin films, we can get higher power conversion efficiency (PCE) and/or higher operational stability of OSCs. Some of the interesting results have been obtained as follows.1. The effect of bulk-heterojunction thickness of photoactive layer, grade structure of bulk-heterojunction and substrate heating technology on the photovoltaic properties of small-molecule OSCs based on Aluminum phthalocyanine chloride (AlPcCl) as donor and C6o as acceptor has been systematicly studied. We found that PCE of OSCs can be enhanced by using bulk-heterojunction structure within an optimized thickness. While continuesly enhanced the bulk-heterojunction thickness, the series resistance of OSCs would also increase obviously which lead to a decreased PCE of the OSCs. By heating the substrate to390K during AlPcCl thin film evaporation, the quantum efficiency and PCE of AlPcCl/C6o planar heterojunction can be effectively improved, which is attributed to higher charge carrier transport efficiency inside the OSCs though a face-on molecule orientation of AlPcCl moleculars and a more effective exciton separation efficiency via a rougher surface of390K evaporated AlPcCl films. By heating the substrate to390K during grade structrure of AlPcCl:C60bulk heterojunction evaporation,we obtain significantly enhanced PCE of AlPcCl/C6o heterojunction OSCs, from about-2%to3.1%.2. The initial degradation properties of small-molecule OSCs under continue illumination has been investigated systematically. The impact of MoO3hole extraction buffer layer,4,4',4"-tri(N-carbazolyl)triphenylamine (TCTA) donor layer thickness, and MoO3-doped4,4'-bis[N-(1-napthyl)-N-phenyl-amino] biphenyl (a-NPD) on the initial degradation properties of OSCs have been investigated. We found that by using MoO3as a hole extraction buffer layer, the degradation properties of OSCs under continuous illumination can be greatly inhibited. After a systematical ananlisis though UV/vis/NIR absorption, XRD, AFM and HOMO-level measurements, we found that MoO3buffer layer has no impact on film morphology of AlPcCl donor layer which was exluded to be the origin of degradation. We confirm from XPS measurement results that MoO3buffer layer has great effect on inhibiting the oxygen diffusion from ITO to organic active layer, which is suggested to be one of the important mechanisms for the OSCs. degradation. Although increasing TCTA donor layer thickness decreases PCE of OSCs, it can lead to an obviously improved stability of the device. We suppose that the diffused oxygen from ITO anode has great effect on donor/acceptor interface or C60acceptor bulk layer, which lead to the degradation. While increasing the donor layer thickness, the diffuse of oxygen from ITO anode would be partly inhibited inside donor layer. Inserting a thin layer of α-NPD between doped MoO3:α-NPD structure and C60acceptor layer can approach higher device stability without significantly decreasing the PCE of the OSCs based on a-NPD/C6o heterojunction structure. In addition, we found that BCP electron extraction buffer layer would also lead to OSCs degradation.3. OSCs based on AlPcCl as donor and C60as acceptor with a multi-tandem structure were fabricated. We demonstrated very high open-circuit voltage (Voc) and enhanced power conversion efficiency (PCE) for the multi-tandem OSCs though the using of effective BCP/Ag/MoO3intermediate connecting electrode layer. By using fivefold structure, we obtained a PCE of2.49%with a high VOC of3.50V, in comparison with PCE of-2%and Voc of0.72-0.81V for the single device. Further, we fabricated a tenfold stacked OSC showing an extremely high VOC of5.89V. The internal optical electrical field distribution inside the multi-tandem OSCs has been simulated. We also optimized the cell performance though a series theoretical calculation. The multi-tandem OSCs with very high Voc are suggested to provide potential application in area-limited low-power electronics.
Keywords/Search Tags:Organic semiconductor, Solar cell, Photoelectric effect, Powerconversion efficiency, Tandem structure, Lifetime, Stability, Exciton
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