Organic solar cells (OSCs) have been demonstrated a great potential to become an efficient solar energy technology, achieving maximum power conversion efficiencies (PCEs) around 10-12%, [1-3] and presenting unique advantages not observed on the inorganic contraparts, as lightweight and mechanical flexibility. However, one of the main causes to the still moderate performance of OSCs is related with the incomplete harvesting of solar photons. The narrow absorption spectra of the polymer donors and the low absorption of fullerenes on the visible region result in an important energy loss in the devices. To overcome such limitation, the so-called multi-donor solar cells, incorporating multiple organic polymers with complementary properties, have been explored. The concept of ternary bulk heterojunctions (BHJ) blend has been successful implemented, however, the mechanisms sustaining such optimizations are often difficult to undertstand due to the more complex nature of the ternary system, i.e. in diversity of materials properties and morphological features.
Here, two efficient ternary blends based on two polymers donors, with complementary absorption spectrum, and the acceptor PC61BM are presented (Figure 1): one is composed of PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]), and F8T2 (poly[9,9’-dioctyl-fluorene-alt-bithiophene). At a certain blend composition, a PCE as high as 4.93% was achieved, this representing an improvement of 30% in comparison with the control devices. The other ternary system includes pBTTT (Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene), a polymer exhibiting high hole mobility, and PTB7, as the donor polymers. In this system an improvement of 18% of PCE was obtained. We discuss such improvements on the basis of morphological studies and of cells characteristics.