Photovoltaic solution to global energy demands has attracted the efforts to get the low-cost and efficient photon-to-electron conversion inorganic material systems for solar cells thanks to the high charge mobility and good stability of inorganic semiconductors. The bulk/nano-planar heterojunction (BnPHJ), consisting of an inorganic bulk p-type semiconductor layer of grain sizes comparable to layer thickness and an inorganic n-type nanoparticle film, is preferred in efficient solar cells. However, solution-processed growth of BnPHJ is still challenging. Moreover, heterojunctions are normally formed epitaxially between two single-crystals even with lattice-misfits. While the growth of large single crystals on polycrystalline surface has been observed experimentally, the nucleation/growth mechanism of such single-crystals is so far not clear yet. Therefore, to develop the solution-processing strategy to prepare the BnPHJ and get insight into its formation mechanism is of a great importance to the fabrication of efficient and low-cost solar cells.

Antimony trisulfide (Sb₂S₃) and titania (TiO₂) are the important p-type photon-harvesting material and n-type electron transporting material for solar cells, respectively. In the present paper, we develop an oriented seed-assisted solution-processing method to prepare Sb₂S₃/TiO₂-BnPHJ, where the monolayer of large, preferentially oriented and closely packed stibnite Sb₂S₃ single-crystals in the form of truncated cuboids is in situ grown over polycrystalline anatase TiO₂ nanopartilce film surface. It is found that the growth features of Sb₂S₃ single-crystals are: (i) Sb₂S₃ crystal orientation is exclusively governed by the beneath TiO₂ nanoparticle film, (ii) one Sb₂S₃ single-crystal is grown over many TiO₂ nanoparticles, (iii) the heterogeneously nucleation follows the epitaxial relationship of Sb₂S₃ (101) // TiO₂ (101), and (iv) a growth transition zone exists at the Sb₂S₃/TiO₂ interface, different from the sharp interface in the cases of single-crystalline/single-crystalline heterojunctions. Based on experimental evidences and theoretical calculations, an orientation-competing-epitaxial nucleation/growth mechanism is proposed for understanding the growth of the Sb₂S₃ single-crystals on polycrystalline TiO₂ surface.

The Sb₂S₃/TiO₂-BnPHJ possesses the following distinctive advantages for photovoltaics: it has a preferred material structure and a high absorption coefficient, it is stable and easily solution-processed on a large scale without cracks, and the [221]-oriented Sb₂S₃ single-crystals provide the highly effective charge transport channels formed by the covalently bounded (Sb₄S₆)_n ribbons for the photogenerated charge carriers inside the Sb₂S₃ layer to reach collection electrode. With an organic hole transporting material (i.e., Spiro-OMeTAD), the resulting solar cell of the heterojunction yields a certificated and considerable power conversion efficiency of 5.12%. We believe that the present study provides a facile solution-processing strategy to prepare the high-quality Sb₂S₃/TiO₂ planar heterojunctions for photovoltaic and other optoelectronic applications, and offers an understanding of the heterogeneously epitaxial growth of large single-crystals on polycrystalline surface. We are attempting to obtain the more efficient solar cell based on Sb₂S₃/TiO₂-BnPHJ. Please pay your attention to our future works. 

Our paper “Preferentially oriented large antimony trisulfide single-crystalline cuboids grown on polycrystalline titania film for solar cells” published in Communication Chemistry can be read from here