Effect of oligothiophene spacer length on photogenerated charge transfer from perylene diimide to boron-doped diamond electrodes

https://doi.org/10.1016/j.solmat.2022.111984Get rights and content

Highlights

  • Variable donor-acceptor chromophore/diamond system designed in-silico and fabricated.

  • Oligothiophene donor length adjusts valence band and charge transport with diamond.

  • Intermediate oligothiophene length (3 units) yields the maximum photovoltage.

  • Up to 0.87 electron per molecule transferred to diamond across 44 Å under excitation.

Abstract

Organic-based photovoltaic devices emerged as a complementary technology to silicon solar cells with specific advantages in terms of cost, ease of deployment, semi-transparency, and performance under low and diffuse light conditions. In this work, thin-film boron-doped diamond (B:NCD) electrodes are employed for their useful optical, electronic, and chemical properties, as well as stability and environmental safety. A set of oligothiophene perylene diimide (nT-PDI) donor-acceptor chromophores is designed and synthesized in order to investigate the influence of the oligothiophene spacer length when the nT-PDI molecule is attached to a B:NCD electrode. The chromophores are anchored to the diamond surface via diazonium grafting followed by Sonogashira cross-coupling. X-ray photoelectron spectroscopy shows that the surface coverage decreases with increasing oligothiophene length. Density functional theory (DFT/TDDFT) calculations reveal the upright nT-PDI orientation and the most efficient photogenerated charge separation and injection to diamond for elongated oligothiophene chains (8T-PDI). Yet, the maximum photovoltage is obtained for an intermediate oligothiophene length (3T-PDI), providing an optimum between decreasing transport efficiency and increasing efficiency of charge separation and reduced recombination with increasing oligothiophene length. Holes transferred from nT-PDI to diamond persist there even after the illumination is switched off. Such features may be beneficial for application in solar cells.

Introduction

The scientific community has been seeking for a cost-effective and more versatile alternative to conventional silicon-based solar cells for decades. Organic-based systems emerged as a suitable option due their relatively low cost, ease of deployment, variation in form factors, and performance advantages under low and diffuse light. Among such technologies, organic photovoltaic cells (OPVs) and dye-sensitized solar cells (DSSCs) have been highlighted as they can be produced in a laboratory using simple equipment and easily obtainable materials [[1], [2], [3]].

Standard p-type DSSCs are constituted by five main components: (1) a transparent conducting oxide (TCO) glass substrate (2) a semiconductor (e.g. NiO), operating as a working electrode, (3) a dye sensitizer (an organic chromophore) anchored to the semiconductor, (4) a redox electrolyte (commonly I/I3), and finally (5) a counter electrode, typically a platinum coated TCO. The working principle by which a p-DSSC can generate energy can be briefly summarized as follows [4,5]. The photon is absorbed by the dye, that injects a hole to the semiconductor and then to the working electrode. On the other hand, the chromophore reduces the electrolyte, which then diffuses to the counter electrode where it is regenerated. With this last step, the circuit is closed and completed.

Substitution of the standard NiO working electrode by boron-doped diamond [[6], [7], [8], [9], [10]] (BDD) has been proposed due to the relevant advantages it shows [[11], [12], [13]]. BDD demonstrates excellent chemical inertness and stability, [14] thermal conductivity, [15] high hardness, [16] a high hole diffusion coefficient, [17] optical transparency, [18] a wide electrochemical potential window, [19] biocompatibility, [20] tunable electron affinity, [21] and simplicity of surface chemical functionalization [22,23].

The organic systems used to functionalize the surface of DSSCs show a rich chemistry, which allows a broad variety of chemical modifications. Due to its unique negative electron affinity, functional aromatic molecules can be easily attached to the BDD surface via diazonium grafting, a mechanism that holds several advantages over the photochemical approach [23]. In the second step, this functional handle can then be employed to introduce a chromophore. The simplicity by which the organic dyes can be modified has also a paramount importance for the tunability and performance of those photovoltaic devices. For instance, the synthesis of longer molecular chains yields, besides an effect on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) alignment, [24] π-π stacking interactions between the aromatic rings, contributing to the formation of denser layers [25]. Additionally, fully conjugated systems are thought to be beneficial for charge transfer [26]. On the other hand, it should be noted that increasing the length of a conjugated system decreases the charge carrier transport [27] and red-shifts the absorption maximum. Consequently, the elongation of the conjugated system might introduce both beneficial (denser surface functionalization and better optical properties) and disadvantageous features (slower charge kinetics).

In this work we aimed to determine the optimal length of a donor system attached to a boron-doped nanocrystalline diamond (B:NCD) electrode. A set of oligothiophene perylene diimide chromophores with an increasing number of thiophene units (from two to eight) are immobilized onto the diamond surface via diazonium grafting and subsequent Sonogashira cross-coupling. Each chromophore is identified with the label “nT-PDI”, in which the “n” represents the number of thiophene (T) units, while PDI denotes “perylene diimide”. The oligothiophene represents the donor part of the chromophore, serving also as a linker to diamond and a spacer between diamond and the acceptor part (hereafter hence simply denoted as “spacer”). Experimental and computational methods are combined to characterize the resulting structure and evaluate the influence of the oligothiophene spacer length on charge separation and transfer to the diamond electrode. Thereby we identify an optimal oligothiophene chain length, balancing the effect of various factors.

Section snippets

Chromophore synthesis and characterization

All chemicals purchased had the highest quality and no purification was required. All solvents used for rinsing were of HPLC grade. Preparative (recycling) size exclusion chromatography (prep-SEC) was performed on a JAI LC-9110 NEXT system equipped with JAIGEL 1H, 2H, and 3H columns (eluent CHCl3, flow rate 3.5 mL min−1). Nuclear magnetic resonance (NMR) chemical shifts (δ, in ppm) were determined relative to the residual CHCl3 (7.26 ppm)/CD2Cl2 (5.32 ppm) absorption or the 13C resonance shift

Properties of the nT-PDI chromophores before and after grafting to diamond

The electrochemical and optical features of the nT-PDI chromophores are gathered in Table S1 in the Supplementary Information. The experimental absorption spectra for the nT-PDI series are shown in Fig. 3a. They are composed of a high-energy absorption band (e.g. at 325 nm for the smallest 2T-PDI chromophore) assigned to the oligothiophene part, a second absorption between 450 and 500 nm from the PDI unit [49], and a higher wavelength absorption due to intramolecular charge transfer (ICT) from

Conclusions

With a view on solar cell applications, we designed and synthesized a set of oligothiophene perylene diimide (nT-PDI) chromophores with an increasing number of thiophene units (from two to eight). The addition of thiophene units shifts the HOMO energy upwards, whereas the LUMO energy remains similar. Thereby, the thiophene spacer length enabled us to adjust the HOMO energy vs. the valence band maximum of diamond, as confirmed by cyclic voltammetry and DFT computing.

The oligothiophene donor part

Notes

The authors declare no competing financial interest.

CRediT authorship contribution statement

Diego López-Carballeira: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis. Jorne Raymakers: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Conceptualization. Anna Artemenko: Visualization, Investigation, Formal analysis. Ruben Lenaerts: Investigation. Jan Čermák: Visualization, Investigation, Formal analysis. Jaroslav

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank Hasselt University, the Research Foundation Flanders (FWO Vlaanderen), and the European Regional Development Fund project CZ.02.1.01/0.0/0.0/15_003/0000464 (CAP) for financial support. J. Raymakers thanks the FWO for his PhD fellowship. This work was also supported by The Ministry of Education, Youth and Sports through the e-INFRA CZ (ID:90140) project, the Large Research Infrastructures IT4Innovations, and Czech Nano Lab. The authors also thank Dr. Jasper Deckers for his

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