wangzhaowei

Stability studies of organic solar cells have recently revealed an initial light-induced burn-in process that calls into question the intrinsic stability of these devices. In article number 1400848, Eszter Voroshazi and co-workers demonstrate that multiple concurrent failure mechanisms occur during the burn-in, indicating that burn-in is not intrinsic, but rather material dependent. This understanding allows the proposing of mitigation routes that are tested with the aim of solving this recognized issue.

The microstructure of the polymer PBDTTT-EFT and blends with the fullerene derivative PC₇₁BM that achieve solar conversion efficiencies of over 9% is comprehensively investigated. A combination of synchrotron techniques are employed including surface-sensitive near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and bulk-sensitive grazing-incidence wide angle X-ray scattering (GIWAXS). A preferential “face-on” orientation of PBDTTT-EFT is observed in the bulk of both pristine and blend thin films, with π–π stacking largely normal to the substrate, which is thought to be beneficial for charge transport. At the surface of the blend, a slight “edge-on” structure of the polymer is observed with side-chains aligned normal to the substrate. The effect of the solvent additive 1,8-diiodooctane (DIO) on solar cell efficiency and film microstructure is also investigated, where the addition of 3 vol% DIO results in an efficiency increase from ≈6.4% to ≈9.5%. GIWAXS studies indicate that the addition of DIO improves the crystallization of the polymer. Furthermore, atomic force microscopy and transmission electron microscopy are employed to image surface and bulk morphology revealing that DIO suppresses the formation of large PC₇₁BM aggregates.

The morphology of a novel and highly efficient polymer solar cell is comprehensively investigated using surface sensitive near-edge X-ray absorption fine structure spectroscopy and bulk-sensitive grazing incidence wide angle X-ray scattering. The solvent additive 1,8-diiodooctane in particular is shown to be effective at controlling fullerene aggregation and enhancing polymer ordering, facilitating efficiencies of over 9%.