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.
Subject terms:
- Molecular electronics
- Optical, photonic and optoelectronic materials
- Materials for energy
- Design synthesis and processing
Figures at a glance
Figure 1: Molecular structures, ultraviolet absorption spectra and energy levels in SM BHJ solar cells. 
a, Molecular structures of DTS(PTTh2)2 and PC70BM. b, Normalized ultraviolet–visible absorption spectra of DTS(PTTh2)2 in CHCl3 solution and as a thin film on quartz substrate. c, Energy-level diagram of the components in solution-processed DTS(PTTh2)2:PC70BM solar cells.
Figure 2: Device performance of SM BHJ solar cells. 
a, Current–voltage (J–V) characteristics of SM BHJ solar cells based on a DTS(PTTh2)2:PC70BM active layer with different blend ratios. The films were cast from 4% w/v chlorobenzene solution. b,c, J–V characteristics (b) and IPCE spectra (c) of solar cells with a DTS(PTTh2)2:PC70BM (70:30 w/w) active layer as a function of DIO content in mixed solutions (by volume relative to chlorobenzene).
Figure 3: High-magnification TEM images. 
