Assessing the Photovoltaic Quality of Vacuum-Thermal Evaporated Organic Semiconductor Blends

Vacuum-thermal evaporation (VTE) is a highly relevant fabrication route for organic solar cells (OSCs), especially on an industrial scale as proven by the commercialization of organic light emitting diode-based displays. While OSC performance is reported for a range of VTE-deposited molecules, a comprehensive assessment of donor:acceptor blend properties with respect to their photovoltaic performance is scarce. Here, the organic thin films and solar cells of three select systems are fabricated and ellipsometry, external quantum efficiency with high dynamic range, as well as OTRACE are measured to quantify absorption, voltage losses, and charge carrier mobility. These parameters are key to explain OSC performance and will help to rationalize the performance of other material systems reported in literature as the authors' methodology is applicable beyond VTE systems. Furthermore, it can help to judge the prospects of new molecules in general. The authors find large differences in the measured values and find that today's VTE OSCs can reach high extinction coefficients, but only moderate mobility and voltage loss compared to their solution-processed counterparts. What needs to be improved for VTE OSCs is outlined to again catch up with their solution-processed counterparts in terms of power conversion efficiency.

Azadipyrromethene-based Zn(II) complexes as nonplanar conjugated electron acceptors for organic photovoltaics

The effectiveness of new a electron acceptor for organic solar cells is demonstrated. The acceptor is a homoleptic zinc(II) complex of 2,6-diphenylethynyl-1,3,7,9-tetraphenylazadipyrromethene. The high power-conversion efficiency obtained is attributed to the acceptor's 3D structure, which prevents crystallization and promotes a favourable nanoscale morphology, its high Voc , and its ability to contribute to light harvesting at 600-800 nm.

Signatures of Multiphase Formation in the Active Layer of Organic Solar Cells from Resonant Soft X-ray Scattering

Resonant soft X-ray scattering (RSOXS) is a complementary tool to existing reciprocal space methods, such as grazing-incidence small-angle X-ray scattering, for studying order formation in polymer thin films. In particular, RSOXS can exploit differences in absorption between multiple phases by tuning the X-ray energy to one or more resonance peaks of organic materials containing carbon, oxygen, nitrogen, or other atoms. Here, we have examined the structural evolution in poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C₆₁-butyric acid methyl ester mixtures by tuning X-rays to resonant absorption energies of carbon and oxygen. Our studies reveal that the energy dependence of RSOXS profiles marks the formation of multiple phases in the active layer of organic solar cells, which is consistent with elemental maps obtained through energy-filtered transmission electron microscopy.