First Report of Chenodeoxycholic Acid–Substituted Dyes Improving the Dye Monolayer Quality in Dye‐Sensitized Solar Cells

Abstract

Chenodeoxycholic acid (CDCA) is the most used antiaggregation additive in dye‐sensitized solar cells since its introduction to the field in 1993. However, effective suppression of dye aggregation comes at the cost of reduced dye loading, a lower open‐circuit voltage, and limited control of dye/additive distribution when cosensitizing with free CDCA. To combat this, herein, a novel dye design concept that uses the covalent attachment of a CDCA moiety to triarylamine sensitizers is reported. The CDCA substituents do not affect the photophysical or electrochemical properties of the sensitizers but have a positive effect on the photovoltaic performance with [Cu+/2+(tmby)2](TFSI)1/2 electrolyte (tmby = 4,4′,6,6′‐tetramethyl‐2,2′‐bipyridine, TFSI = bis(trifluoromethanesulfonyl)imide). By ensuring a one‐to‐one ratio of dye and CDCA, paired with isotropic distributions of each component, this approach results in a higher‐quality dye monolayer. Compared with the reference system, the novel approach reported herein gives a higher open‐circuit voltage and power conversion efficiency (PCE). The best device is fabricated with the dye C6–CDCA, delivering a PCE of 6.84% (8 μm TiO2, 1 mm CDCA, JSC = 8.64 mA cm−2, VOC = 1007 mV, and FF = 0.77).

Adamantyl Side Chains as Anti‐Aggregating Moieties in Dyes for Dye‐Sensitized Solar Cells

“…[22] In our previous study on anti-aggregation measures for DSSC, see Figure 1, we prepared two dyes with CDCA tethered to their π-spacer (C 3 -CDCA and C 6 -CDCA), and found this to be beneficial for photovoltaic performance compared to the reference dyes (C 3 and C 6 ). [23] Our previous study also highlighted the anti-aggregational effect of the alkoxy chains on the donor side of the molecules, where superior performance was seen for the dyes with longer side chains (C 6 and C 6 -CDCA vs. C 3 and C 3 -CDCA). Some of the improvements seen for this structural modification is likely due to another closely related aspect of solar cell performance, namely the recombination of electrons in TiO 2 with the oxidized species of the electrolyte.…”

“…The synthesis of the dyes is based on the route used for the CDCA‐dyes previously reported by our group [23] . However, in contrast to CDCA, the adamantyl‐moiety bears no additional heteroatoms and this simplified the synthesis somewhat as it made protection chemistry redundant.…”

“…It is also quite common to add anti‐aggregation additives to the staining solution, the most common additive for DSSC is chenodeoxycholic acid (CDCA) [22] . In our previous study on anti‐aggregation measures for DSSC, see Figure 1, we prepared two dyes with CDCA tethered to their π‐spacer ( C 3 ‐CDCA and C 6 ‐CDCA ), and found this to be beneficial for photovoltaic performance compared to the reference dyes ( C 3 and C 6 ) [23] . Our previous study also highlighted the anti‐aggregational effect of the alkoxy chains on the donor side of the molecules, where superior performance was seen for the dyes with longer side chains ( C 6 and C 6 ‐CDCA vs. C 3 and C 3 ‐CDCA) .…”

Synthetic Efforts to Investigate the Effect of Planarizing the Triarylamine Geometry in Dyes for Dye-Sensitized Solar Cells

“…The advanced triarylamine fragment 1 was prepared as previously described by our group. 41 Then, a Suzuki cross-coupling with (5-formylthiophen-2-yl)boronic acid and (5-formylfuran-2-yl)boronic acid was carried out. These boronic acids are rather unstable, 42 so we selected the very active XPhos palladium third-generation precatalyst for the transformation.…”

“…To ensure sufficient amounts of the target dye to work with, we set out to improve the first step of the synthesis route. This was done by protecting the N -10 position of 4 as N -acetyl, allowing for a decent Suzuki cross-coupling between phenothiazine 5 and pinacol boronate ester 6 , 41 giving 7 in 65% yield. A facile deprotection then gave the key intermediate 8 ( Scheme 2 ).…”