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Issue 4, 2015
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High open-circuit voltage small-molecule p-DTS(FBTTh2)2:ICBA bulk heterojunction solar cells – morphology, excited-state dynamics, and photovoltaic performance

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Abstract

The photovoltaic performance of bulk heterojunction solar cells using the solution-processable small molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2 in combination with indene-C60 bis-adduct (ICBA) as an acceptor is systematically optimized by altering the processing conditions. A high open-circuit voltage of 1 V, more than 0.2 V higher than that of a p-DTS(FBTTh2)2:PC70BM blend, is achieved. However, the power conversion efficiency remains around 5% and thus is lower than ∼8% previously reported for p-DTS(FBTTh2)2:PC70BM. Transient absorption (TA) pump–probe spectroscopy over a wide spectral (Vis-NIR) and dynamic (fs to μs) range in combination with multivariate curve resolution analysis of the TA data reveals that generation of free charges is more efficient in the blend with PC70BM as an acceptor. In contrast, blends with ICBA create more coulombically bound interfacial charge transfer (CT) states, which recombine on the sub-nanosecond timescale by geminate recombination. Furthermore, the ns to μs charge carrier dynamics in p-DTS(FBTTh2)2:ICBA blends are only weakly intensity dependent implying a significant contribution of recombination from long-lived CT states and trapped charges, while those in p-DTS(FBTTh2)2:PC70BM decay via an intensity-dependent recombination mechanism indicating that spatially separated (free) charge carriers are observed, which can be extracted as photocurrent from the device.

Graphical abstract: High open-circuit voltage small-molecule p-DTS(FBTTh2)2:ICBA bulk heterojunction solar cells – morphology, excited-state dynamics, and photovoltaic performance

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Publication details

The article was received on 18 Nov 2014, accepted on 18 Nov 2014 and first published on 27 Nov 2014


Article type: Paper
DOI: 10.1039/C4TA06256F
Citation: J. Mater. Chem. A, 2015,3, 1530-1539
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    High open-circuit voltage small-molecule p-DTS(FBTTh2)2:ICBA bulk heterojunction solar cells – morphology, excited-state dynamics, and photovoltaic performance

    A. K. Ko Kyaw, D. Gehrig, J. Zhang, Y. Huang, G. C. Bazan, F. Laquai and T. Nguyen, J. Mater. Chem. A, 2015, 3, 1530
    DOI: 10.1039/C4TA06256F

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