- Synthesis and photovoltaic properties of two-dimensional benzodithiophene-thiophene copolymers with pendent rational naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole side chains
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A series of new two-dimensional copolymers, PBDTT-TABT, PBDTT-TANT and PBDTT-TSNT, with conjugated benzodiathiazole (BT) or naphthobisthiadiazole (NT) side chains were successfully synthesized and characterized for high performance polymer solar cells (PSCs). The NT unit showed a stronger electron-withdrawing ability and a larger conjugation than the BT unit, which induced a stronger ICT process between the benzodithiophene (BDT)-thiophene (T) backbone and the conjugated side chain for NT containing copolymers. As a result, PBDTT-TANT and PBDTT-TSNT showed lower-lying bandgaps and more red-shifted absorption than PBDTT-TABT. The alkylthio modification effect of conjugated side chains was also investigated in this work. This effect showed a positive role in lowering the HOMO energy level, and a negative role in elevating carrier mobility and molecular stacking properties in our two-dimensional polymeric systems. Bulk heterojunction (BHJ) PSCs were fabricated using these copolymers as the donor materials to evaluate their photovoltaic properties. The PBDTT-TABT, PBDTT-TANT and PBDTT-TSNT devices exhibited PCEs of 4.60%, 5.65% and 4.01%, respectively. In spite of the Voc values, the highest Jsc, FF and PCE were achieved for the PBDTT-TANT device, which was attributed to its red-shifted absorption, improved carrier mobility and well-defined phase separation. It is interesting that the Jscs, FFs and PCEs of all these devices were elevated significantly when made using the solvent vapor annealing (SVA) method. The THF-SVA process would provide a driving force to facilitate the formation of a much more well-defined surface morphology, resulting in the enhanced Jsc and FF values. The PBDTT-TANT device showed the highest PCE of 8.04%, which is the top efficiency for this type of two-dimensional copolymer with donor (D)-donor (D) polymer backbones and donor (D)-acceptor (A) conjugated side chains. Our design strategy would give an instructive guide to developing high performance two-dimensional polymer donors to be used in organic photovoltaic applications.
- Xu, Xiaopeng,Feng, Kui,Li, Kai,Peng, Qiang
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- Tuning optical and electronic properties of star-shaped conjugated molecules with enlarged π-delocalization for organic solar cell application
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Three structurally related conjugated molecules (BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2) in star shape have been designed and synthesized as donor materials for small molecule based bulk heterojunction (BHJ) solar cells. The structural features of these molecules include a planarized benzo[1,2-b:3,4-b′:5,6- b″]trithiophene (BTT) with a C3h symmetry as the central core and three conjugated arms incorporating electron deficient benzo[2,1,3] thiadiazole (BTD) units, with arms being linked to the core via different number of thiophene connecting units (e.g., 0, 1, 2 corresponding to BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2, respectively). Comparative analyses of optical and electronic properties indicate that the molecules bearing more thiophene units between the BTT core and the BTD arms possess higher-lying HOMO levels while their LUMO levels remain almost unchanged. The improvement of BHJ device performance, with [6,6]-phenyl-C61-butyric acid methyl ester (PC 61BM) as the acceptor, is observed with increasing number of thiophene units between the BTT core and BTD arms, from BTT-BTD-0 to BTT-BTD-1 and BTT-BTD-2. The BTT-BTD-2:PC61BM based BHJ devices show the highest power conversion efficiency (PCE) of 0.74%, with an open-circuit voltage (Voc) of 0.69 V, a short-circuit current density (Jsc) of 2.93 mA cm-2, and a fill factor (FF) of 0.37 under 1 sun (100 mW cm-2) AM 1.5G simulated solar illumination. The PV performance of BTT-BTD-2 is further improved when [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is used as the electron acceptor, yielding the best device performance with Jsc of 4.13 mA cm-2, V oc of 0.72 V, FF at 0.46 and PCE of 1.36%. The effect of the different number of thiophenes linking the BTT core and the conjugated BTD arms has been clearly demonstrated on regulating optical and electrochemical properties of the three molecules and their BHJ device performances.
- Jiang, Youyu,Yu, Di,Lu, Luhua,Zhan, Chun,Wu, Di,You, Wei,Xie, Zhizhong,Xiao, Shengqiang
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p. 8270 - 8279
(2013/07/26)
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