Solution-processed small-molecule solar cells with 6.7% efficiency


Organic photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost-effective devices1,2. Most of the attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells3,4,5,6,7. A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies (PCEs) reaching the 6–8% range for conjugated polymer/fullerene blends8,9. Solution-processed small-molecule BHJ (SM BHJ) solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts10. Here, we report efficient solution-processed SM BHJ solar cells based on a new molecular donor, DTS(PTTh2)2. A record PCE of 6.7% under AM 1.5 G irradiation (100 mW cm−2) is achieved for small-molecule BHJ devices from DTS(PTTh2)2:PC70BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25% v/v of 1,8-diiodooctane, DIO) during the film-forming process, which leads to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.

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Figure 1: Molecular structures, ultraviolet absorption spectra and energy levels in SM BHJ solar cells.
Figure 2: Device performance of SM BHJ solar cells.
Figure 3: High-magnification TEM images.


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The design, synthesis and characterization of DTS(PTTh2)2 was supported by the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences of the US Department of Energy (DE-DC0001009). The solar-cell fabrication and optimization was supported by the Air Force Office of Scientific Research (FA9550-11-1-0063). The TEM studies were supported by the National Science Foundation (DMR-0856060). The authors thank M. Chabinyc (UCSB) for useful discussions on the TEM analysis, C. Hoven (UCSB) for contributions to the development of the device fabrication methodology and B.B.Y. Hsu (UCSB) for the thickness measurement. W.L.L. thanks the Agency for Science Technology and Research (A*Star) of Singapore for a postdoctoral fellowship.

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G.C.B. and A.J.H. proposed the research. G.C.W. designed, synthesized and characterized the materials. Y.S. and W.L.L. carried out the fabrication and measurement of electronic devices. C.J.T. carried out the TEM characterization and image analysis. All authors contributed to the writing of the manuscript.

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Correspondence to Guillermo C. Bazan or Alan J. Heeger.

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Sun, Y., Welch, G., Leong, W. et al. Solution-processed small-molecule solar cells with 6.7% efficiency. Nature Mater 11, 44–48 (2012).

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