Modulating PCBM-Acceptor Crystallinity and Organic Solar Cell Performance by Judiciously Designing Small-Molecule Mainchain End-Capping Units.

In this article, we report that the bulk-size and electron-donating/electron-accepting nature of moieties, which are end-capping onto small-molecule donor mainchain, not only modulate the donor's absorption, molecular frontier orbitals, and phase ordering, but also effectively tune the PC71BM-acceptor phase crystallinity. Compared to the electron-deficient trifluoromethyl (SM-CF3) units on the diketopyrrolopyrrole (DPP) small molecule mainchain ends, the electron-rich methoxyl (SM-OCH3) units ending on the same mainchain help improve the PC71BM-acceptor phase short-range ordering. As a result, the -OCH3 capping small-molecule displays larger short-circuit current density (Jsc) when blended with PC71BM (10.72 ± 0.22 vs. 16.15 ± 0.53 mA/cm2). However, the electron-donating nature of -OCH3 raises the donor HOMO level, which leads to a quite small open-circuit voltage (Voc) (0.624 vs. 0.881 V). Replacement of the -OCH3 with the large and weak electron-donating aromatic carbazolyl (SM-Cz) ones affords the small molecule of SM-Cz. The SM-Cz:PC71BM system affords a high Voc of 0.846 V and a large Jsc of 13.33 ± 0.34 mA/cm2 after thermal annealing, and hence gives a larger power conversion efficiency (PCE) of 6.26 ± 0.13%, which is among the top values achieved so far from the DPP molecules. Taken together, these results demonstrate that engineering the end-capping units on small-molecule donor mainchain can effectively modulate the organic solar cell performance.