Functionalization of boron-doped diamond with a push-pull chromophore via Sonogashira and CuAAC chemistry

Abstract

Improving the performance of p-type photoelectrodes represents a key challenge toward significant advancement in the field of tandem dye-sensitized solar cells. Herein, we demonstrate the application of boron-doped nanocrystalline diamond (B:NCD) thin films, covalently functionalized with a dithienopyrrole-benzothiadiazole push-pull chromophore, as alternative photocathodes. First, a primary functional handle is introduced on H-terminated diamond via electrochemical diazonium grafting. Afterwards, Sonogashira cross-coupling and Cu(i) catalyzed azide-alkyne cycloaddition (CuAAC) reactions are employed to attach the chromophore, enabling the comparison of the degree of surface functionalization and the importance of the employed linker at the diamond-dye interface. X-ray photoelectron spectroscopy shows that surface functionalization via CuAAC results in a slightly higher chromophore coverage compared to the Sonogashira cross-coupling. However, photocurrents and photovoltages, obtained by photoelectrochemical and Kelvin probe measurements, are approximately three times larger on photocathodes functionalized via Sonogashira cross-coupling. Surface functionalization via Sonogashira cross-coupling is thus considered the preferential method for the development of diamond-based hybrid photovoltaics.

Fig. 1. Working principle of a p-type dye DSSC. Under solar illumination, an electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the dye. This excited electron will result in a reduction of the redox electrolyte, which will diffuse to the counter electrode, where it is re-oxidized. Finally, the electron is transferred back to the dye via an external circuit, which produces a current and leads to regeneration of the dye. (VBM = valence band maximum)

Scheme 1. General functionalization strategy of boron-doped nanocrystalline diamond for the design of p-type photoelectrodes.^{a a}(i) In situ diazotization of 4-iodoaniline or 4-azidoaniline; (ii) electrochemical reduction and subsequent grafting of the corresponding diazonium salt; (iii) Sonogashira cross-coupling (a) and Cu(i) catalyzed azide–alkyne cycloaddition (CuAAC) (b) using an alkyne functionalized dithieno[3,2-b:2′,3′-d]pyrrole-2,1,3-benzothiadiazole (alkyne-DTP-BT) chromophore.

Scheme 2. Synthetic route toward the alkyne functionalized dithieno[3,2-b:2′,3′-d]pyrrole-2,1,3-benzothiadiazole (alkyne-DTP-BT) chromophore.^{a a}(i) LDA, CuCl₂, THF; (ii) t-BuONa, hexylamine, Pd₂(dba)₃, BINAP, toluene; (iii) n-BuLi, Me₃SnCl, THF; (iv) Pd₂(dba)₃, P(o-tolyl)₃, toluene, DMF; (v) NIS, CHCl₃, AcOH; (vi) TMS-acetylene, Pd(PPh₃)₂Cl₂, CuI, THF, TEA; (vii) TBAF, THF.

Fig. 2. Cyclic voltammograms showing the electrochemical grafting of in situ generated 4-iodobenzenediazonium chloride (top) and 4-azidobenzenediazonium chloride (bottom) on H-terminated B:NCD electrodes at a scan rate of 100 mV s⁻¹ for 8 scans.

Fig. 3. High resolution I 3d and N 1s XPS spectra for iodophenyl (top) and azidophenyl (bottom) functionalized B:NCD electrodes, respectively. The N 1s peak was deconvoluted into three peaks at 399.0, 400.1 and 404.1 eV.

Scheme 3. (i) Oxidative addition, (ii) transmetallation, and (iii) reductive elimination for the Sonogashira cross-coupling on iodophenyl functionalized B:NCD surfaces.

Scheme 4. Simplified mechanism for the Cu(i) catalyzed azide–alkyne cycloaddition (CuAAC) on azidophenyl functionalized B:NCD surfaces.

Fig. 4. Normalized XPS survey spectra for the surface functionalization of H-terminated B:NCD via (a) Sonogashira cross-coupling (Table 1, entry 4) or (b) CuAAC chemistry (Table 2, entry 2). Deconvoluted high resolution S 2p (c and e) and N 1s (d and f) signals after attachment of the dye via Sonogashira or CuAAC are provided below the XPS survey spectra.

Fig. 5. KPFM images of (a) a bare H-terminated B:NCD thin film, (b) a B:NCD film with the chromophore coupled via CuAAC chemistry, and (c) a B:NCD film with the chromophore coupled via Sonogashira. The Z scale is set to 20 mV in all images to visualize subtle potential differences. (d) Surface potential RMS roughness from the KPFM images. (e) Macroscopically measured surface photovoltages and (f) surface potential profiles in time in dark (shaded area) and under illumination (white area) for the three sample types. The potential profiles are stacked along the y-axis for better visibility. For Cu(i)AAC, CuI was used as the catalyst, while for Cu(Br)AAC, CuBr was used instead (according to Table 2).

Fig. 6. Photocurrent densities (nA cm⁻²) of H-terminated (B:NCD-H), iodophenyl (B:NCD-I), azidophenyl (B:NCD-N₃) and DTP-BT functionalized (B:NCD-DTP-BT) diamond electrodes in a 5 mM methyl viologen solution (in 0.1 M Na₂SO₄) under white light illumination (90 mW cm⁻²; simulated AM 1.5G solar spectrum, 10 s 1 : 1 dark/light interval) at a bias of −0.2 V. Surface functionalization was performed via Sonogashira cross-coupling (top) and CuAAC (bottom).

Fig. 7. Photocurrent densities (nA cm⁻²) of DTP-BT functionalized (B:NCD-DTP-BT) diamond electrodes in a 5 mM methyl viologen solution (in 0.1 M Na₂SO₄) under white light illumination (90 mW cm⁻²; simulated AM 1.5G solar spectrum, 10 s 1 : 1 dark/light interval) at a bias of −0.3 V. Surface functionalization was performed via Sonogashira cross-coupling (top) and CuAAC (bottom).