Elsevier

Applied Surface Science

Volume 477, 31 May 2019, Pages 131-136
Applied Surface Science

Full length article
Effect of TiO2 particle size and layer thickness on mesoscopic perovskite solar cells

https://doi.org/10.1016/j.apsusc.2017.11.124Get rights and content

Highlights

  • Electron injection and transport properties of mesoporous layer containing TiO2 nanoparticles are investigated.

  • The photovoltaic properties are poor with small TiO2 particles and thick mesoporous layers.

  • This poor performance is due to the increase of the area of the TiO2/TiO2 interface.

  • The TiO2/TiO2 interfacial resistance largely influences the photovoltaic parameters.

Abstract

Mesoporous TiO2 (mp-TiO2) layers are commonly used as electron transport layers in perovskite solar cells, which help to extract electrons from the perovskite light-absorbing layer and transport them to the electrodes. We investigated the effects of the layer thickness of mp-TiO2 and particle size of TiO2 on photovoltaic properties, in terms of the surface area of the mp-layer and the interfacial areas of the TiO2 nanoparticles in the mp-layer. Various mp-TiO2 layers with thicknesses of 150, 250, and 400 nm and particle sizes of 25 nm and 41 nm were prepared to compare the photovoltaic properties of such layer-containing perovskite solar cells. Time-resolved photoluminescence decay and impedance studies showed that interfacial resistance as well as perovskite-to-TiO2 charge injection are important factors affecting photovoltaic performance. The deterioration of the photovoltaic parameters with increasing TiO2/TiO2 interfacial area also confirms that the interfacial series resistance that arises from these connections should be reduced to enhance the performance of mesoscopic perovskite solar cells.

Introduction

Perovskite solar cells (PSCs), based on organic–inorganic halide perovskite light-absorbing materials, are one of the most promising photo-energy conversion devices, having shown a rapid development in power conversion efficiency (PCE) of up to 22.1% during the last several years [1]. A great deal of effort has been concentrated on PSCs to improve the device performance and/or understand the underlying principles. Focus has been placed on their varied chemistries, such as doping, substitution, and interfacial treatments; physical analyses, such as charge carrier generation, separation, recombination, and transport; as well as device engineering, such as adopting mesoscopic/planar- and normal/inverted-type structures [2]. The highest PCEs have been achieved with mesoscopic PSCs [3], [4], [5], in which a mesoporous metal oxide layer plays important roles as the charge-transport channel, scaffold for loading the light-absorbing materials, and electron–hole separator [6].

TiO2 is one of the most studied mesoporous layer (mp-layer) materials due to its suitable electronic band levels and ease of nanoparticle (NP) fabrication [7], [8]. Moreover, it is known that the n-type semiconducting nature of a TiO2 layer aids electron injection and transport in PSCs, as such layers help the generated charge carriers in the perovskite to be well separated [9]. Therefore, the photovoltaic performance of mesoscopic PSCs might be largely influenced by electron injection at the perovskite/mp-TiO2 interface and the electron transport in the mp-TiO2 layer. The charge injection from the perovskite to the TiO2 layer can be improved by increasing the perovskite/mp-TiO2 interfacial area, that is, the surface area of the TiO2 NPs, which is expected to be beneficial to the PSC performance. However, enlarging the surface area does not always give rise to an improvement in PSC performance, because electron transport should be considered. For example, decreasing the TiO2 NP size would increase the TiO2/TiO2 interfacial area, which could diminish the electron transport ability of the mp-layer.

In this study, we investigate the charge-injection and transport properties of mp-TiO2 layers in PSCs by varying the size of the TiO2 NPs and the thickness of the mp-TiO2 layer. The particle size and layer thickness are directly related to the surface area (i.e., perovskite/TiO2 interface) and the area of the TiO2/TiO2 interfaces, which can affect the electron injection and transport properties, respectively. We compared the electron injection, electron transport, and interfacial resistance of mp-TiO2 layers in PSCs, with the variation of the layer thickness (150 nm, 250 nm, and 400 nm) and NP diameter (25 nm and 41 nm) of TiO2.

Section snippets

Device fabrication

All the devices were fabricated via a similar process to that given in our previous report [10]. Fluorine-doped tin oxide (FTO) substrates (Pilkington TEC15) were cleaned sequentially using acetone, ethanol, and deionized water in an ultrasonic bath for 15 min. A compact TiO2 (c-TiO2) layer was spin-coated (3000 rpm, 20 s) on the cleaned FTO substrate using 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt.% in isopropanol, Sigma-Aldrich) in anhydrous 1-butanol (Tokyo Chemical

Results and discussion

To investigate the effect of particle size on the photovoltaic properties of PSCs, we used two TiO2 pastes with NP sizes of 25 nm and 41 nm. Fig. 1a shows the TEM images of these two kinds of TiO2 NPs. The diameters were measured from the TEM images and the diameter distributions are shown in Fig. S1 (Supplementary materials). The mean diameters of the smaller and the larger NPs are 21.7 nm and 45.4 nm. The XRD patterns (Fig. 1b) of the NPs confirm that both pastes contain anatase-phase TiO2

Conclusion

Mesoporous TiO2 layers as an electron transport layer are known to be very important components of PSCs. However, their various effects, for example, on electron injection and transport, have not been clearly identified despite recent intensive studies. We investigated the electron injection and transport properties of mp-TiO2 layers by varying the layer thickness and the size of the TiO2 NPs in the layer. For our PSC structure comprised of a thick (∼600 nm) perovskite layer and a thin

Acknowledgements

This work was supported by the Technology Development Program to Solve Climate Changes (NRF-2015M1A2A2056827), and the Global Frontier R&D Program of the Center for Multiscale Energy System (NRF-2012M3A6A7054855). This research was also supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B2013087 and 2017R1A4A1015022).

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