Tailored fabrication of quasi-isoporous and double layered α-Fe2O3 thin films and their application in photovoltaic devices

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Highlights

  • Double layered α-Fe2O3 thin film is applied as the hole blocking modification layer.

  • Synergy of suppressed charge carrier recombination and enhanced light transmittance.

  • Morphology-structure–property relationship of perovskite solar cells is determined.

Abstract

A series of α-Fe2O3 thin films with distinct morphologies are prepared via a facile polystyrene-block-polyethylene oxide templated sol–gel method. By tailoring the poor solvent contents and FeCl3-to-polymer weight ratio in the sol–gel solutions, quasi-isoporous α-Fe2O3 thin films with different substructures and thicknesses are obtained. Via a thermal annealing post-treatment, double layered structures are induced by a synergistic dewetting and Oswald ripening effect. Special focus is set on the α-Fe2O3 thin films prepared with no annealing/annealing-medium FeCl3 concentration, as they possess uniform periodic structures, which is suitable to be used as hole blocking modification layer of perovskite solar cells (PSCs). An improved power conversion efficiency (PCE) is obtained when the double layered α-Fe2O3 thin film is applied as the hole blocking modification layer for PSCs. The improved PCE primarily originates from the increased VOC, which probably benefits from the synergistic effect of the suppressed charge carrier recombination at the interfaces, the enhanced light transmittance as well as the superior electron extraction capacity.

Graphical abstract

Series of iron oxide thin films with different morphologies are prepared via a facile block copolymer templated wet chemical method and the large area homogeneity of all prepared films are characterized with GISAXS and GIWAXS technique. Compared with the quasi-isoporous nanostructure, improved photoelectric property is obtained when double layered α-Fe2O3 thin film is used as the electron transport modification layer.

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Introduction

Iron (III) oxide, α-Fe2O3, as the most thermodynamically stable polymorph, has attracted large attention in photovoltaic devices [1], [2], lithium-ion batteries [3], photoelectrochemical conversions [4], gas sensors [5], and high-density nanoelectronic technology applications [6], during the past several decades. Especially when used as hole blocking layer in perovskite solar cells (PSCs), it is favorable for enhancing the UV stability due to its low photocatalytic activity. Literature reported preparation methods for nanostructured α-Fe2O3 thin films including thermal evaporation [7], spray pyrolysis [8], [9], [10], [11], electron beam deposition [12], [13], chemical vapor deposition [14], [15], pulsed laser deposition [16], [17], sol–gel method [18], [19], [20], liquid phase deposition [21], [22], electrodeposition [23], [24], and sputter deposition [25], [26], [27]. Among the various preparation methods, the solution-processable sol–gel method features the advantages of low costs and good compatibility with the large-scale spray coating and printing techniques being both of high interest for upscaling PSC fabrication [28], [29]. Furthermore, with the assistance of a templating effect via a use of diblock copolymers, diverse periodic metal oxide nanostructures can be achieved [30], [31], [32], [33]. The phase separation behavior of the diblock copolymer in the solvent or cosolvent is easily affected by the stretching degree of the core-forming polymer chains and the repulsive interactions among corona-forming chains [34]. Therefore, poor solvent, acid, base, or salt addition are commonly adopted strategies for tuning the phase-separated structures of the diblock copolymer thin films. Distinguished from the phase separation behavior of the pure diblock copolymer, the introduction of the metal oxide precursors into the sol–gel solution makes the phase separation process of the diblock copolymer more sophisticated. For example, Cheng et al. reported in a series of research works the tuning of the morphology of polystyrene-block-polyethylene oxide (PS-b-PEO) templated TiO2 thin films by controlling the proportion of each component in the sol–gel solution and the corresponding formation mechanism of the different morphologies were addressed. They demonstrated that the phase separation behavior of the TiO2 thin film can be affected by the component ratio in the solution, the aging time of the sol–gel solution, and the calcination temperature for removing the polymeric template amongst other parameters [30], [35]. In our previous works, we demonstrated the solvent effect in the diblock copolymer templated TiO2, SnO2 and α-Fe2O3 thin film synthesis process [32], [33], [36]. Sarkar et al. investigated the effect of the weight fractions of the solvents and ZnO precursors on PS-b-PEO templated ZnO nanostructures and constructed a ternary phase diagram for showing the compositional boundaries of the investigated morphologies [37]. Moreover, thermal annealing was testified to be an efficient strategy for reconstructing the morphology of the diblock copolymer thin films by affecting the interfacial interactions between the copolymers and substrate as well as the competition between dewetting and microphase separation [38], [39]. Park et al. fabricated a wafer--level antireflective nanostructure array via the combined action of dewetting and Oswald ripening of the spin-coated multilayer of nanosilver colloids [40]. These studies validated the possibility for fabricating α-Fe2O3 thin films via a facile solution processed method and tuning the morphology of thin films via different strategies.

In the present work, the morphology control of the PS-b-PEO templated α-Fe2O3 thin films is realized from multi-dimensional approaches. ·H2O, as a poor solvent, is added into the sol–gel solution for tuning the stretching degree of the core- and corona-forming polymer segments. Different FeCl3-to-PS-b-PEO ratios are designed for checking the templating effect of the PS-b-PEO block copolymer on the α-Fe2O3 nanostructures. Thermal annealing post-treatment is performed for reconstructing the nanostructures of the α-Fe2O3. The structural information of the as-deposited and calcined thin films is characterized by scanning electron microscope (SEM) and grazing-incidence small-angle X-ray scattering (GISAXS) measurements. The crystallinity, crystal orientation, and composition of the calcined α-Fe2O3 thin films are detected by X-ray diffraction (XRD), grazing-incidence wide-angle X-ray scattering (GIWAXS) and X-ray photoelectron spectroscopy (XPS) measurements.

Two representative α-Fe2O3 thin films with quasi-isoporous and double layered structure are used as hole blocking modification layer for investigating their morphological effect on the photovoltaic properties of PSCs. The photovoltaic performance characterization indicates that the open-circuit voltage (VOC), as well as the power conversion efficiency (PCE) values of the devices are significantly improved by capitalizing the unique double layered α-Fe2O3 nanostructures as the hole blocking modification layer of the PSCs. This improvement is supposed to be induced by the enhanced light transmittance and electron extraction ability of the hole blocking layer. Moreover, the suppressed charge carrier recombination at the interfaces could also be the reason for the observed increase in VOC values [2], [41], [42], [43].

Section snippets

Thin film preparation and device fabrication

Fig. 1 schematically shows the thin film preparation process used in the present work based on the materials as detailed in the Supporting Information. To prevent moisture-induced deliquesce, the FeCl3 powder used for the stock solution preparation was weighted in an N2 glove box. Specifically, 21.3 mg, 42.7 mg, and 85.3 mg FeCl3 were weighed for preparing the stock solution with low, medium, and high FeCl3 concentrations. After taking out from the glovebox, the FeCl3 powder was dissolved into

Surface morphology of the M-α-Fe2O3 thin films

Fig. 2a shows the SEM images of the M-α-Fe2O3-N thin films. With the variation of the H2O content added to the solution, different substructures including spheres, cylinders and sac-like structures are detected in the calcined α-Fe2O3 thin films. The corresponding micellar structures for forming these porous nanostructures are schematically shown in Fig. 2b.

Due to the presence of DMF and H2O molecules in the cosolvents, the main form of the Fe-associated species are supposed to be FeCl2DMF4+,

Conclusion

In this work, a facile method involving block copolymer templated wet-chemistry and thermal annealing induced thin film dewetting is presented to synthesize ordered and double layered α-Fe2O3 nanostructures. By tuning the FeCl3 to PS-b-PEO ratio, the poor solvent content in the sol–gel solution and the annealing temperature, various quasi-isoporous and double layered α-Fe2O3 thin films are fabricated. The structure information within the local and large area is systematically investigated by

Author contributions

S. Yin and Y. Zou contributed equally to this paper.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germanýs Excellence Strategy – EXC 2089/1 – 390776260 (e-conversion) and the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS), TUM.solar in the context of the Bavarian Collaborative Research Project Solar Technologies Go Hybrid (SolTech) and the Center

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    Shanshan Yin and Yuqin Zou contributed equally to this work.

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