Effect of wettability of substrate on metal halide perovskite growth

Highlights:

• Effect of wettability of substrate on grain size of perovskite layer is studied.

• Hydrophobic recovery after UV exposure used to vary wetting properties.

• Wetting properties are not the main contributor to grain size of perovskite.

Abstract

In the last decade, inorganic–organic hybrid perovskites have demonstrated a dramatic increase in photoconversion efficiency by engineering the perovskite composition, as well as adjusting the properties of the charge transporting materials. The nucleation and growth of the perovskite layer are governed by the surface properties of the substrate and depend on its wetting properties defined by the roughness and the species adsorbed. Fixing all fabrication conditions of a PSC but only changing the wettability of the substrate is practically difficult because varying the surface hydrophilicity can contribute side effects such as chemical identity change or adsorption of new species on the surface. Herein, we studied a double-layered TiO₂/SnO₂ ETL for perovskite deposition and we changed the hydrophilicity of the samples using hydrophobic recovery after UV exposure. Thus, the effect of wettability of the substrate on nucleation and growth of the perovskite layer was elucidated under minimized interference. It was found that the influence of the wetting properties of the substrate on the growth of perovskite can be exaggerated in the literature since the increase in the grain size of the perovskite layer is just 14% bigger if the water contact angle of the substrate increases from 3° to 72°.

Introduction

One of the most promising candidates for commercial solar cells is organic/inorganic perovskite solar cells (PSCs) in terms of both efficiency and cost [1]. Currently, significant progress in both theoretical simulations and fabrication process of PSC has been made and the certified efficiency of PSCs has reached 25% [2]. The high performance was achieved by compositional engineering [3], defect passivation [4], and constant development of electron and hole transporting layers [5], [6].

Despite the large number of works on PSCs, there are still unanswered and controversial questions in the field including the influence of hydrophilicity of charge- transporting layers on the growth of the perovskite layer. To investigate this, a planar PSC architecture in which a TiO₂/SnO₂ bi-layer was used as an electron- transporting layer (ETL) to eliminate the influence of the mesoporous scaffold on the hydrophilicity. This TiO₂/SnO₂ ETL improves the charge extraction properties of ETL and provides an optimal band alignment with a mixed cation perovskite [7].

In [8], [9], [10], [11], [12], [13] it was claimed that more hydrophilic ETLs lead to bigger grains of perovskite and thus to a higher PCE. Moreover, the hydrophilicity of ETLs is not extensively studied in the references and not supported by the data. Probably, it is due to a well- known fact that a UV treatment of an ETL layer before deposition of the perovskite layer leads to an increased PCE [14] and it is partially due to an increase in the wettability of the substrate after the UV treatment.

This conflicts with [15], [16], [17], [18], [19], where the opposite was claimed: less hydrophilic ETLs help to form larger grains of perovskite.

In [17] different hole- transporting layers (HTL) with different wetting properties were used to fabricate inverted PSC devices. The grain size of perovskite deposited on PCDTBT with water contact angle (WCA) of 108° was 2.7 μm, which was 800% bigger than in the case of using PEDOT:PSS substrate with WCA of 12°. It was found that non-wetting substrate suppresses heterogeneous nucleation and thus results in less dense nuclei and consequently larger grain size of the perovskite layer. Nevertheless, the grain size of the perovskite layer was 1.1 μm on the PTAA substrate that showed similar to PCDTBT WCA of 105°.

Thus, the grain size of perovskite can be affected by other properties of the substrates such as anchoring to perovskite [9] or morphology. For example, in [16], [19], [20], [21], [22] different species on TiO₂ or SnO₂ ETLs led to an increased perovskite grain size. However, the modifications used (doping or self-assembled monolayers(SAM)) resulted in different morphologies or altered surface chemical identity when compared to the original substrates. These can affect the nucleation and growth of the perovskite layer due to altered anchoring properties, as well as charge transport parameters of PSCs.

Fixing all fabrication conditions of a PSC but only changing the wetting properties of the substrate is practically difficult because most methods used to vary the surface hydrophilicity contribute side effects such as chemical identity change or adsorption of new species on the surface.

Herein, we studied a double-layered TiO₂/SnO₂ ETL for perovskite deposition and we changed the hydrophilicity of the samples using hydrophobic recovery after UV exposure. Thus, the effect of wettability of the substrate on nucleation and growth of the perovskite layer was elucidated under minimized interference.