Aggregation-dependent photovoltaic properties of squaraine/PC61BM bulk heterojunctions

Abstract

In this study, we investigated bulk-heterojunction solar cells composed of PC(61)BM and squaraine dyes with different donor subunits. Both, H- and J-aggregates, have been observed for the squaraine dyes in the mixture, even for the same dye depending on the annealing conditions of the blends. Supramolecular organization of the squaraine dyes noticeably affects solar cell performance. The J-aggregate exhibits a red-shift of the absorption maximum that results in a significant increase in the short-circuit current and decrease in the open-circuit voltage compared to the as-cast device. The H-aggregate shows a blue-shift in the absorption maximum and concomitantly only a moderate increase in the short-circuit current as well as an increase in the open-circuit voltage. The course of domain growth during annealing was monitored by local photocurrent mapping.

Molecular aspects of squaraine dyes aggregation and its influence on spectroscopic properties

“…The obtained computational effects of aggregation on the absorption spectrum comply well with the aggregation manifestation in the experiment: the signal shift and broadening [11,[18][19][20].…”

“…Moreover, the given squaraine dye can be forced into the H-or J-aggregates by the proper choice of material preparation conditions. Additionally, the squaraines have been obtained with simultaneous coexistence of both types of aggregates in different domains [11]. In this case, the absorption band splitting is observed and the two parts appear on the spectrum: one that is bathochromically and the other-hypsochromically shifted.…”

“…Squaraine dyes have attracted attention for their unique properties [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A one-dimensional molecule stacking observed in squaraine solutions is of similar nature as that recognized for cyanine or merocyanine dyes.…”

“…A thorough investigation of squaraines in various media has revealed that they can form either H-or J-aggregates depending on their structure and parameters of microenvironment [11,18,19]. Moreover, the given squaraine dye can be forced into the H-or J-aggregates by the proper choice of material preparation conditions.…”

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…[238,239] Several of these squaraine dyes were shown to reorganize in the solid state (thin films) from H-aggregates (stacks) into J-aggregates (brick wall) upon thermal annealing. [240] This structural transition has not only a tremendous influence on the optical properties of films but also leads to aggregation-dependent photo voltaic performance when these squaraine monomers are mixed with PC 61 BM in order to form bulk heterojunction solar cells. Thus, for some squaraine dyes thermal annealing leads to the formation of J-aggregates which posses a much higher exciton mobility thereby increasing the short-circuit current and the overall power conversion efficiency.…”