Recycled or Bio-Based Solvents for the Synthesis of ZnO Nanoparticles: Characterization and Validation in Organic Solar Cells

Abstract

Among solution-processable metal oxides, zinc oxide (ZnO) nanoparticle inks are widely used in inverted organic solar cells for the preparation, at relatively low temperatures (<120 °C), of highly efficient electron-transporting layers. There is, however, a recent interest to develop more sustainable and less impacting methods/strategies for the preparation of ZnO NPs with controlled properties and improved performance. To this end, we report here the synthesis and characterization of ZnO NPs obtained using alternative reaction solvents derived from renewable or recycled sources. In detail, we use (i) recycled methanol (r-MeOH) to close the loop and minimize wastes or (ii) bioethanol (b-EtOH) to prove the effectiveness of a bio-based solvent. The effect of r-MeOH and b-EtOH on the optical, morphological, and electronic properties of the resulting ZnO NPs, both in solution and thin-films, is investigated, discussed, and compared to an analogous reference material. Moreover, to validate the properties of the resulting materials, we have prepared PTB7:PC₇₁BM-based solar cells containing the different ZnO NPs as a cathode interlayer. Power conversion efficiencies comparable to the reference system (≈7%) were obtained, validating the proposed alternative and more sustainable approach.

Keywords: ZnO nanoparticles; electron-transporting layer; organic solar cells; recycled or bio-based solvents; solution processing.

Scheme 1

Schematic illustration of ZnO synthesis.

Figure 1

(a) UV–vis absorption spectra in solution and (b) DLS plots of the ZnO NP inks.

Figure 2

Optical transmission spectra of the different ZnO NP thin-films deposited on glass/ITO substrates.

Figure 3

AFM images, 1 × 1 μm², of the b-ZnO (a), r-ZnO, (b) and Ref-ZnO (c) thin-films. Insets are 30 × 30 μm² images of the same samples at a lower magnification to investigate film homogeneity.

Figure 4

KPFM images of the ITO surface (a) and of the b-ZnO (b), r-ZnO, (c) and Ref-ZnO (d) thin-films.

Figure 5

Band diagram of the glass/ITO/ZnO NP thin-films using KPFM measurements. The ITO WF is (4.75 ± 0.08) eV, obtained by averaging the maximum, 4.83 eV, and the minimum, 4.67 eV, values in the literature. The absolute error is the discrepancy, i.e., (4.83–4.67)/2. The bottom of the conduction band (E_CB) of the ITO is 4.4 eV [53], while the bang gap E_G is in the range [3.5, 4.3] eV [54], i.e., on average (3.9 ± 0.4) eV. The WF and E_CB are considered equal to 4.5 eV [55], whereas the E_G ≈ 3.37 eV [56], which brings the top of valence band to E_VB ≈ 7.87 eV (all energy values in the band diagram are negative since the vacuum level is fixed to zero).

Figure 6

J-V curves of the inverted PTB7:PC₇₁BM solar cells based on the different ZnO NP thin-films as an ETL.