Increasing Photovoltaic Performance of an Organic Cationic Chromophore by Anion Exchange

Abstract

A symmetrical cyanine dye chromophore is modified with different counteranions to study the effect on crystal packing, polarizability, thermal stability, optical properties, light absorbing layer morphology, and organic photovoltaic (OPV) device parameters. Four sulfonate‐based anions and the bulky bistriflylimide anion are introduced to the 2‐[5‐(1,3‐dihydro‐1,3,3‐trimethyl‐2H‐indol‐2‐ylidene)‐1,3‐pentadien‐1‐yl]‐1,3,3‐trimethyl‐3H‐indolium chromophore using an Amberlyst A26 (OH− form) anion exchanger. Anionic charge distribution clearly correlates with device performance, whereby an average efficiency of 2% was reached in a standard bilayer organic solar. Evidence is given that the negative charge of the anion distributed over a large number of atoms is significantly more important than the size of the organic moieties of the sulfonate charge carrying group. This provides a clear strategy for future design of more efficient cyanine dyes for OPV applications.

Superweak Coordinating Anion as Superstrong Enhancer of Cyanine Organic Semiconductor Properties

“…Cyanine dyes are able to exhibit unusual thermal behaviour when weakly coordinating anions are used. As already known, the melting point of the different pentamethine based cyanine salts presented in previous work lays above 150 °C . In principle, weakly coordinating anions are expected to lower the melting point mainly by preventing strong Coulomb interactions and closed packed stacking in the solid state.…”

“…A direct comparison of the bond length alternation of the Cy5[Al(pftb) 4 ] compound with different crystalized salts of the pentamethine chromophore, evaluated previously, shows surprisingly high values for the Cy5[Al(pftb) 4 ] compound (Figure C). While previous results suggested a clear trend towards the ideal polymethine state with increasing anion size, [Al(pftb) 4 ] − breaks the expected trend and introduces a similar bond length alternation as observed for the point charge like sulfonates or also other investigated anions (Figure C) ,,. Particularly the C9‐C10 bond length possesses a pronounced single bond character including Cy5O 3 SMe and Cy5TFSI.…”

“…However, this expected behaviour has to compete against the boosted molecular weight of the cyanine salt exceeding 1300 g mol −1 . Indeed the melting point for Cy5[Al(pftb) 4 ] approaches 215 °C which is an increase of 70 °C compared to previous investigated salts . The melting point is followed by two sharp peaks, an exothermic one at 269 °C and an endothermal at 279 °C (Figure ).…”

“…Cy5[Al(pftb) 4 ] exhibits a high molar extinction coefficient of 3.55×10 5 L mol −1 cm −1 extracted from an absorption band ranging from 500 to 700 nm in ethanol solution, proving that the anion does not influence the optical properties; the latter being very similar to those in other salt solutions of this chromophore (Figure A, S3, Table S5, S6) . Major differences lie in the UV‐Vis solid state spectrum of Cy5[Al(pftb) 4 ], which is free of aggregates and approaches the monomeric state regarding the shoulder to peak ratio (1 : 0.72, 1353 cm −1 ) and bandwidth of 5793 cm −1 , which is only about 1669 cm −1 wider than the solution band width of 4124 cm −1 .…”

“…According to literature the aluminium core of the [Al(pftb) 4 ] − likewise increases the tendency in crystallization ,,. Indeed, while the solubility in various solvents remained, substrate coating trials from various solvents revealed a strong tendency in either dendritic polycrystallite formation or pronounced dewetting when compared to the previous salts of the pentamethine chromophore (Figure , S5). Usual hole transport layers (HTL) such as MoO 3 or PEDOT:PSS seem to provide a strong surface energy mismatch for the Cy5[Al(pftb) 4 ].…”