Alkoxyphenylthiophene Linked Benzodithiophene Based Medium Band Gap Polymers for Organic Photovoltaics: Efficiency Improvement upon Methanol Treatment Depends on the Planarity of Backbone

Title
Alkoxyphenylthiophene Linked Benzodithiophene Based Medium Band Gap Polymers for Organic Photovoltaics: Efficiency Improvement upon Methanol Treatment Depends on the Planarity of Backbone
Authors
Kranthiraja, KakaparthiGunasekar, Kumarasamy조우섬송명관박영근이진용신유림강인남김아정김현정김봉수진성호
Keywords
organic photovoltaic cells; medium band gap polymer; power conversion efficiency; methanol treatment; photoactive layer; blend morphology
Issue Date
2014-10
Publisher
Macromolecules
Citation
VOL 47, 7060-7069
Abstract
Two donor–acceptor (D–A) medium band gap polymers, P1 and P2, alkoxyphenylthiophene (APTh) linked benzodithiophene (BDT) as an electron-rich unit and 1,3-di(2′-bromothien-5′-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (TPD) (A1) or [5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (BT) (A2) as an electron-deficient unit, have successfully been synthesized via microwave-assisted Stille polymerization and utilized for bulk heterojunction (BHJ) polymer solar cells (PSCs). P1 shows a well-distinguished absorption shoulder between 590 and 620 nm attributed to the π–π stacking of a polymer backbone; such kind of absorption shoulder is not observed in P2, indicating that the P1 has more planar structure than that of P2. This is due to the fact that the sulfur atom of thiophene spacer and the oxygen atom of carbonyl groups in TPD have more pronounced intramolecular noncovalent interactions (INCI) in P1 than that of the sulfur atom of thiophene spacer and the oxygen atom of alkoxy groups of BT in P2. The bulk heterojunction polymer solar cells (BHJ PSCs) were fabricated with the configuration of ITO/PEDOT:PSS/polymer (P1 or P2):PC71BM/LiF/Al. The P1 device shows better photovoltaic performance with open-circuit voltage (Voc) of 0.91 V and the power conversion efficiency (PCE) of 4.19% than the P2 device (Voc: 0.71 V; PCE: 1.88%) in neat blend films under the illumination of AM 1.5G (100 mW/cm2). Upon treating the active layers containing P1 and P2 with methanol, the PCE of the P1 device is increased from 4.19 to 7.14%. In contrast, the PCE of the P2 device is decreased from 1.88 to 1.82%. Space charge limited current mobility, atomic force microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and impedance spectroscopy studies strongly support the enhanced PCE for the P1 device is attributed to the increased mobility, nanoscale morphology,
URI
http://pubs.kist.re.kr/handle/201004/48383
ISSN
00249297
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KIST Publication > Article
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