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Home > Indolo[3,2-b]indole donor-based D–p–A dyes for DSCs: investigating the role of p-spacers towards recombination
Indolo[3,2-b]indole donor-based D–p–A dyes for DSCs: investigating the role of p-spacers towards recombination
P. V. Santhini, Jayadev V., Sourava C. Pradhan, Sivasankaran Lingamoorthy, Nitha P. R., Chaithanya M. V., Rakesh K. Mishra, Narayanan Unni K. N., Jubi John and Suraj Soman
The increasing energy demand worldwide is as a result of the growing human population and necessitates the need to investigate new sustainable energy sources that are capable of reducing the growing consumption of fossil fuels. Of the existing renewable energy technologies, photovoltaics (PV) occupy a prominent position.1 Among the various third gene- ration photovoltaic technologies, DSCs represent a unique, emerging area of PV research that has been gaining widespread interest among research communities in recent years based on their ease of fabrication, relatively low cost, flexibility of integration into a range of substrates, colour tunability, trans- parency and, above all, the ability to harness indoor/diffused light and to convert it into useful power with very high efficiencies as compared to silicon and thin film technologies.2,3 Recently, Michael Gr¨atzel and Anders Hagfeldt reported power conversion efficiencies above 30% in indoor light harvesting conditions, which resulted in a paradigm shift in this area of photovoltaic research.
In DSC, the dye plays the major role in harnessing the light. Conventionally, Ru complexes, Zn porphyrins, and metal-free organic dyes used to serve as the most promising candidates in DSCs.6–13,63,64 Even though Ru-based sensitizers have been proved to be highly efficient and stable, they lack the opportunity for higher structural modifications in addition to the scarcity of Ru metal, which has resulted in the introduction of stable organic dyes that can be easily synthesized, having higher molar extinc- tion coefficients, lower cost and low toxicity.7,14 Metal-free organic sensitizers are compatible with alternative redox electrolytes like cobalt and copper, achieving impressive power conversion effi- ciencies of 12–15% and higher voltages (41 V from a single junction device) in full sun conditions.15–20 Organic metal-free dyes have been made in different architectures such as D–p–A, D–A–p–A and D–D–p–A, featuring improved performance.
Herein, we report the successful synthesis and characterization of three metal-free D–p–A organic dyes (IID-1, IID-2, and IID-3), where we have incorporated a fused ring N-hetero aromatic indolo[3,2-b]indole donor functionality as shown in Scheme 1. Indoloindoles are an interesting class of N-hetero- acenes of which indolo[3,2-b]indoles are considered to be good electron donors.33 This donor character of the indolo[3,2-b]indole moiety can be attributed to the small atomic radius of the N-atom, which in turn results in a more dense assembly of molecules. In addition, these molecules are also known to possess better charge carrier mobility due to the bidirectional electronic coupling enabled by NH–p interactions. These fused N-heteroacenes have been used in the development of organic photovoltaics, organic field-effect transistors, heterojunction solar cells and high-spin organic polymers.36–41 However, the lack of competent synthetic routes in making indolo[3,2-b]- indoles has limited its widespread material applications.38,42–48 Very recently, we introduced a facile route involving a sequen- tial MCR-oxidation approach to the synthesis of these fused indole heteroacenes.49,50 The present methodology involves an
initial MCR of an enolizable ketone (cyclohexanone), amine, and N-tosyl-3-nitroindole furnishing the intermediate pyrrolo[3,2-b]indole, which on further oxidation with chloranil affords indolo[3,2-b]indole moieties in good yields. The advantage of our method as compared existing methods was clearly demonstrated by the generation of symmetrical and unsymmetrical indolo[3,2-b]- indoles, which are highly relevant for material applications.
The strategically synthesized indolo[3,2-b]indole dyes (IID series) were found to offer many promising features, considering the functionalities that are required for a stable and efficient DSC dye, which include (1) extended p-conjugation that enhances the intermolecular interaction; a symmetric conjugated structure leading to a more rigid backbone; (3) properly placed alkyl groups for increased solubility, to prevent the approach of triiodide in getting closer to TiO2 and also to prevent aggregation. These interesting skeletal characteristics make indolo[3,2-b]indole a potential building block for use in DSC. Recently, Nopporn and co-workers also showed the potential of this nitrogen-bridged phenylenevinylene unit to be used in DSCs.62 Our focus was to explore its potential as a donor in a D–p–A dye architecture and to study in detail how the variation in p-spacers linking the indolo[3,2-b]indole donor to the cyanoacrylic acid acceptor affects the physical properties and in particular, the PV performance. Unlike the previous reports on D–p–A dyes that showed improved current density and photovoltaic performance employing conju- gated thiophene and furan heterocyclic spacers, in the present work, incorporating these p-linker groups led to more recombina- tion and lower lifetimes, resulting in lower PV performance. The enhanced performance of the benzene-substituted dye, IID-1, is attributed to reduced interception (recombination), which was studied in detail using open-circuit voltage decay (OCVD), intensity modulated photovoltage spectroscopy (IMVS) and electro- chemical impedance spectroscopy (EIS) measurements and was correlated to the structural properties of dyes obtained from detailed theoretical calculations.
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