Impact of Structural Polymorphs on Charge Collection and Nongeminate Recombination in Organic Photovoltaic Devices

Abstract

The formation of different types of structural polymorphs of poly(3-hexylthiophene) (P3HT) affects the performance of organic photovoltaic (OPV) devices that use thermally-annealed P3HT:PCBM[60] blend films as photoactive layer. Here it is demonstrated that, when densely-packed and non-densely packed P3HT polymorphs co-exist in the P3HT:PCBM[60] layer, non-geminate charge recombination is fast; however, in a device nongeminate recombination is effectively overruled by efficient and fast charge carrier extraction. In stark contrast, when only a less-densely packed P3HT polymorph is present in the blend, non-geminate charge recombination losses are less pronounced, and the charge carrier extraction efficiency is lower. The antagonistic non-geminate charge recombination and charge carrier extraction processes in these systems are monitored by time-delayed-collection field (TDCF) and ultrafast transient absorption (TA) experiments. Furthermore, resonance Raman spectroscopy reveals that in the absence of the densely-packed P3HT polymorph, the energetic disorder present in the P3HT:PCBM[60] blend is higher. High-resolution atomic force microscopy imaging further identifies pronounced differences in the layer morphology when the polymorph distribution varies between unimodal and bimodal. These results indicate that less-densely packed P3HT polymorphs increase disorder and impede charge collection, leading to a reduction of the device fill factor.

Influence of the dispersity and molar mass distribution of conjugated polymers on the aggregation type and subsequent chiral expression

“…Therefore, the presence of type I aggregates benefits applications in which a good charge transfer is essential, such as organic photovoltaic devices (oPVs), thermo-electronics and organic field effect transistors (oFETs). [13][14][15][16] In type II, the longer interlamellar distance increases fluorescence as photoluminescence quenching is diminished. This aids applications in which the optical aspects are important, such as organic light emitting devices (oLEDs).…”

Readily Accessible Benzo[d]thiazole Polymers for Nonfullerene Solar Cells with >16% Efficiency and Potential Pitfalls

“…Overall these changes suggest an expanded unit cell. Two possibilities for the new packing motif are (1) a metastable Y6 polymorph with a different unit cell , or (2) a polymer– Y6 cocrystal with an expanded Y6 lattice. Although published GIWAXS data for Y6 are limited, there is no evidence of polymorphism.…”

Introducing Molecular Asymmetry in Copolymer Structures to Favor Herringbone-Like Aggregation and Tune the Chiral Expression

“…In general, type I is favored when a strong π–π stacking between the main chains is present and the π–π stacking distance is therefore short. − Type II aggregation, on the other hand, is a result of van der Waals–London dispersion forces between interdigitating side chains, leading to a short interlamellar spacing (see Figure ). , Most experimental techniques (e.g., X-ray diffraction) focus on these differences in stacking distance to determine the aggregation type. − Type I and type II promote different material properties, namely efficient charge transfer and photoluminescence, respectively. , CPs exhibiting type I aggregation are therefore mostly employed in organic photovoltaic devices or organic field-effect transistors, , while CPs aggregating according to type II are beneficial for applications such as organic light-emitting diodes . Moreover, the chiral expression of both aggregates differs significantly, which is essential for the development of, e.g., metamaterials and chiral photosensors. , …”

“…6−8 Type I and type II promote different material properties, namely efficient charge transfer and photoluminescence, respectively. 9,10 CPs exhibiting type I aggregation are therefore mostly employed in organic photovoltaic devices 11 or organic field-effect transistors, 12,13 while CPs aggregating according to type II are beneficial for applications such as organic light-emitting diodes. 14 Moreover, the chiral expression of both aggregates differs significantly, which is essential for the development of, e.g., metamaterials and chiral photosensors.…”