Modular TiO₂‐Squaraine Dyes/Electrolyte Interface for Dye‐Sensitized Solar Cells with Cobalt Electrolyte

Abstract

Strategies to diminish both charge recombination and aggregation of dyes on the photoanode by functionalizing the sensitizer with alkyl groups is the best approach to achieve high dye‐sensitized solar cell (DSSC) efficiency. Development of such a photoanode with NIR‐active dyes which is compatible with a cobalt electrolyte is important to enhance the photovoltaic performance. In this report, alkyl‐group‐wrapped donor‐acceptor‐donor (D‐A‐D) based unsymmetrical squaraine dyes have been used for DSSC device characterization with a cobalt electrolyte. Surface passivation of photoanode was varied systematically by the extent of functionalization with alkyl groups to avoid charge recombination. DSSC device performance of 5.92 % was achieved for an alkyl‐group‐wrapped squaraine dye with a cobalt electrolyte. Hence, appending the alkyl groups on the donor unit of squaraine dyes helps passivating the photoanode, whereas introducing hydrophilic groups provides a leaky surface where oxidized electrolyte species reach the titanium‐metal‐oxide surface which promotes the charge recombination process.

Effect of Position of Donor Units and Alkyl Groups on Dye-Sensitized Solar Cell Device Performance: Indoline–Aniline Donor-Based Visible Light Active Unsymmetrical Squaraine Dyes

“…A DSSC device performance of 37% has been achieved under the dim light condition (6000 l×) and 10.0% for DSSC-based small modules. , Among them, organic sensitizers have attracted great attention of researchers due to diversity in the tuning of donor, acceptor, and π-spacer moieties that absorb solar light from visible to near-infrared regions. , Organic sensitizers with the dye architectures of donor–acceptor (D–A), donor–π–acceptor (D–π–A), donor–acceptor–π–acceptor (D–A–π–A), and donor–donor–π–acceptor (D–D–π–A) for harvesting most of the solar light have been employed. Further, the effect of the position of donor, acceptor, and π-spacer moieties has been studied in DSSCs. − Further, charge injection and dye regeneration processes of a DSSC device are strongly affected by the distance and interactions between the anchoring group and the acceptor unit of the dyes. − The alkyl group-wrapped far-red and visible light active unsymmetrical squaraine dyes have been explored for DSSCs. − Squaraine dyes belong to the family of polymethine chromophores and exhibit a sharp transition with high extinction coefficient (>10 5 M –1 cm –1 ) in the far-red region of the solar spectrum and also form well-defined H- and J-aggregated structures both in solution and on the metal oxide surface. , The self-assembled structures of the squaraine dyes have the variety of applications, such as in organic photovoltaic, sensors, and DSSCs. , In our previous work, we have systematically studied the effect of alkyl-group-wrapping, position of anchoring group, best sensitizing solvents/electrolytes, and effect of changing the donor moieties for unsymmetrical squaraine on the DSSC performance. − ,− Recently, it has been reported that aniline (An) and indoline (In) donors containing unsymmetrical squaraine dyes absorb in the visible region centered at 520 nm with an extinction coefficient of >10 5 M –1 cm –1 . Furthermore, the alkyl group-wrapped visible light active squaraine dye, AK4 (η = 7.73%), and the dye with methyl groups, AK2 (η = 7.34%), showed good device performance, and the effect of position of donor units within the squaraine dye framework has not been studied.…”

Solvent-Dependent Functional Aggregates of Unsymmetrical Squaraine Dyes on TiO2 Surface for Dye-Sensitized Solar Cells

“…Further, concerted companion dyes are good alternatives to co-sensitization for enhancing DSSC J SC and efficiency. − Squaraine-based dyes are very impressive compared with other metal-free organic dyes, because of their high molar extinction coefficient in the far-red region, and show aggregation properties in both solution and bulk, which have been explored in various areas of research. − Aggregation of dyes is a facile process on a semiconducting metal oxide surface and found as a detrimental factor for device performance in DSSCs due to the self-quenching of photo-excited sensitizers. , However, controlling the aggregation of dyes on the TiO 2 surface helps enhance the light-harvesting efficiency of the dyes with improved photocurrent generation from the self-assembled structures, aside from reducing the charge recombination processes by passivating the photoanode. , Charge recombination at the dye–TiO 2 /electrolyte interfaces has been found to decrease photocurrents and photovoltages through inner- and outer-path recombinations, respectively . These interfacial recombination can be prevented by introduction of hydrophobic alkyl chains within the dye backbone. ,,, Hence, the device efficiency of DSSCs can be enhanced by enhancing the photocurrent and photovoltage values with the in-built structural features of the sensitizers . The maximum efficiency achieved by an alkyl group wrapped NIR-light-active unsymmetrical squaraine dye was 7.7% without a co-absorbent because of the well-packed controlled aggregated structures on the surface of TiO 2 …”

“…2,10,11,37 Hence, the device efficiency of DSSCs can be enhanced by enhancing the photocurrent and photovoltage values with the in-built structural features of the sensitizers. 38 The maximum efficiency achieved by an alkyl group wrapped NIR-light-active unsymmetrical squaraine dye was 7.7% without a co-absorbent because of the well-packed controlled aggregated structures on the surface of TiO 2 . 34 The solvent systems used during the sensitization of dyes on the surface of TiO 2 play a tremendous role in achieving better J SC , V OC , and device efficiency 39 aside from sensitizers, 14 electrolytes, 40 and counter electrodes.…”