Photochromic spiro-indoline naphthoxazines and naphthopyrans in dye-sensitized solar cells

Abstract

Photochromic dyes possess unique properties that can be exploited in different domains, including optics, biomedicine and optoelectronics. Herein, we explore the potential of photochromic spiro-indoline naphthoxazine (SINO) and naphthopyran (NIPS) for application in photovoltaics. We designed and synthesized four new photosensitizers with a donor-pi-acceptor structure embedding SINO and NIPS units as photochromic cores. Their optical, photochromic and acidochromic properties were thoroughly studied to establish structure-properties relationships. Then, after unravelling the possible forms adopted depending on the stimuli, their photovoltaic properties were evaluated in DSSCs. Although the photochromic behavior is not always preserved, we elucidate the interplay between photochromic, acidochromic and photovoltaic properties and we demonstrate that these dyes can act as photosensitizers in DSSCs. We report a maximum power conversion efficiency of 2.7% with a NIPS-based dye, a tenfold improvement in comparison to previous works on similar class of compounds. This work opens new perspectives of developments for SINO and NIPS in optical and photovoltaic devices, and it provides novel research directions to design photochromic materials with improved characteristics.

Scheme 1. General structure of the SINO (X = N) spiro[indoline-2,3′-[3H]naphth[2,1-b][l,4]oxazine and NIPS (X = CH) spiro[indoline-2,3′-[3H]naphtho[2,1-b]pyran derivatives synthesized in the present work as well as the interconversion reaction between their close (CF) and open quinoidal and merocyanine forms (MC).

Scheme 2. Synthetic pathway followed to obtain precursors 1, 5 and 6. DIPEA: N,N-diisopropylethylamine; Tf: trifluoromethylsulfonyl, dba: benzylideneacetone.

Scheme 3. Synthetic pathway used to obtain the new dyes SINO-1, SINO-2, NIPS-1 and NIPS-2. dppf: (diphenylphosphino)ferrocene.

Fig. 1. UV-vis absorption spectra of (a) SINO-1, (b) SINO-2, (c) NIPS-1 and (d) NIPS-2 in 2 × 10⁻⁵ M DMF solutions at 25 °C in the dark and under light using a 200–600 nm/200 W xenon lamp.

Fig. 2. Normalized discoloration curves of SINO-1, SINO-2 and NIPS-1, example given in DMF in 2 × 10⁻⁵ M solutions at 25 °C in the dark after irradiation 60 s with a 200–600 nm/200 W xenon lamp.

Scheme 4. Interconversion between the close (CF) and open protonated forms (MCH) of SINO (X = N) and NIPS (X = CH) derivatives in the presence of strong acids.

Fig. 3. Absorption titration of (a) SINO-1, (c) NIPS-1 (5 × 10⁻⁵ M), (b) SINO-2 and (d) NIPS-2 (2.5 × 10⁻⁵ M) with increments of 70 and 140 eq. of HCl respectively in THF solutions in the dark at 25 °C.

Fig. 4. Coloration kinetics of NIPS-1 (2 × 10⁻⁵ M in the dark at 25 °C after illuminating with a 200–600 nm/200 W xenon lamp) (a) after the addition of 1 drop of different acids in THF and (b) in THF, DMF and toluene after the addition of 1 drop of HCl.

Fig. 5. DFT-calculated energy levels (versus the vacuum level) of the frontier orbitals of the four new dyes and their spatial localizations in closed (CF), merocyanine (MC) and protonated merocyanine forms (MCH) (full lines). Experimental values of the CF obtained by CV are shown in dashed lines. The potential of the I₃⁻/I⁻ pair (ca. −4.95 eV) and the TiO₂ conduction band (ca. −4.10 eV) are shown in yellow and grey dashed lines respectively.

Fig. 6. J–V curves of the 13 μm DSSCs using (a) SINO-1, (b) SINO-2, (c) NIPS-1, and (d) NIPS-2 as sensitizers under standard irradiation conditions AM 1.5 G, 1000 W m⁻²; 25 °C (active area = 0.36 cm²) using our homemade electrolyte (black) and Iodolyte (red) and the pictures of the respective devices in the absence of illumination.

Fig. 7. UV-Vis spectra and pictures of the respective devices of NIPS-2-based DSSCs on 2 μm transparent TiO₂ electrodes using Iodolyte and our homebased electrolyte in the absence of illumination.

Fig. 8. Charge transfer resistances comparisons at the same quasi-Fermi level of 13 μm DSSCs made of NIPS-2 and both electrolytes (a) in the dark and (b) under operating conditions.

Fig. 9. (a) J–V curve of the 13 μm NIPS-2/Iodolyte-based DSSCs under standard irradiation conditions AM 1.5 G, 1000 W m⁻²; 25 °C (active area = 0.36 cm²) and (b) respective transmittance spectrum together with a picture of the actual device.